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An upper Paleogene shallowing-upward sequence in the southern Sandino Forearc Basin (NW Costa Rica): response to tectonic uplift


Abstract and Figures

The Sandino Forearc Basin of western Nicaragua and northwestern Costa Rica (Central America) provides a Campanian to Pliocene sedimentary record. The study of the onshore part of the basin in northwestern Costa Rica reveals for the first time the occurrence of upper Paleogene shallow-marine siliciclastic and carbonate sequences. These sequences have remained undescribed so far and are grouped herein into two new lithostratigraphic units—the upper Eocene Junquillal Formation (Fm.) and the upper Oligocene Juanilla Fm. The upper Eocene Junquillal Fm. is characterized by storm-related, arenitic to conglomeratic deposits comprised in metric, massive amalgamated beds. The shallow shelfal environment of deposition is attested by the presence of hummocky and swaley cross-stratifications. The lithologies of the Junquillal Fm. were previously considered to be part of the underlying, deep-water turbiditic deposits of the Eocene Descartes Fm. The deposition of the Junquillal Fm. is indicative of tectonic uplift that forced regression, which affected the southeastern part of the Sandino Forearc Basin during the late Eocene. The upper Oligocene Juanilla Fm. unconformably overlies the Junquillal Fm. and occurs as a 25-m-thick, 700-m-wide outcrop on Isla Juanilla. It is composed essentially of nodular, coral framestones exhibiting massive, closely packed corals in growth position that are associated with coralline red algae and Larger Benthic Foraminifera (LBF). A late Oligocene age of the reef is attested by LBF assemblages occurring in two different facies. The Juanilla Fm. coral reef is a unique exposure, characterized by extensive constructed coral framework, and which has no equivalent in the Oligocene geological record of Central America. The reef grew on a short-lived, siliciclastic-poor tectonic high, which developed in relation to a lower Oligocene, basin-scale folding event in the Sandino Forearc Basin.
a New geological map of the Santa Elena-Punta Descartes area (Sandino Forearc Basin; modified after Dengo 1962; Baumgartner et al. 1984; Astorga 1987; Denyer and Alvarado 2007; Escuder-Viruete et al. 2015). The mapped area encompasses the 1:50’000 topographic map sheets 3048 I (Murciélago), 3048 IV (Santa Elena) and 3049 II (Bahia Salinas) of the National Geographic Institute of Costa Rica. The detailed description of the mapped lithostratigraphic units is presented in the Fig. 3. We introduce two new lithostratigraphic units, the Junquillal Fm. and the Juanilla Fm., which have been previously mapped as Descartes Fm. The localities mentioned in the text and the position of the cross-section (A–A’, b of the same figure) are indicated. b NNE–SSW-oriented cross-section (A–A’, no vertical exaggeration) set perpendicular to the main tectonic structures of the Santa Elena-Punta Descartes area. The cross-section displays one informal unit (Kas) which is not exposed in the mapped area shown in a. This unit corresponds to Upper Cretaceous arc-derived, tuffaceous sediments, which underlie the upper Campanian–upper Maastrichtian Piedras Blancas Fm. and are only known from the Ostional-1 and Rivas-1 boreholes drilled in Nicaragua (see Fig. 1 for borehole locations, Ranero et al. 2000). These Upper Cretaceous arc-derived sediments overlie a basaltic terrane possibly associated to the Mesquito Composite Oceanic Terrane (Baumgartner et al. 2008) and which was recovered from the Rivas-1 borehole (Ranero et al. 2000). Such sediments are not known to overly the Santa Elena tectonic pile (in the field), possibly due to the Campanian tectonic event (see text). We induce a subcrop contact between the basaltic terrane (Mesquito Composite Oceanic Terrane) and the Santa Elena ultramafic nappe. This terrane boundary is tentatively indicated on the cross-section beneath the Bahia Cuajiniquil, given the absence of data on this subject
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Facies (2016) 62:9
DOI 10.1007/s10347-016-0463-y
An upper Paleogene shallowing‑upward sequence in the southern
Sandino Forearc Basin (NW Costa Rica): response to tectonic
Goran Andjic´1 · Claudia Baumgartner‑Mora1 · Peter O. Baumgartner1
Received: 8 September 2015 / Accepted: 18 January 2016
© Springer-Verlag Berlin Heidelberg 2016
growth position that are associated with coralline red algae
and Larger Benthic Foraminifera (LBF). A late Oligocene
age of the reef is attested by LBF assemblages occurring in
two different facies. The Juanilla Fm. coral reef is a unique
exposure, characterized by extensive constructed coral
framework, and which has no equivalent in the Oligocene
geological record of Central America. The reef grew on a
short-lived, siliciclastic-poor tectonic high, which devel-
oped in relation to a lower Oligocene, basin-scale folding
event in the Sandino Forearc Basin.
Keywords Coral reef · Larger Benthic Foraminifera ·
Tectonic uplift · Eocene · Oligocene · Sandino Basin ·
N-Costa Rica
Forearc basins represent one of the several geotectonic
features developed in arc-trench systems of active mar-
gins (Dickinson and Seely 1979; Dickinson 1995). Forearc
sedimentary sequences record the tectonic and volcanic
activity of convergent margins and often remain the only
source of information of its past evolution, especially if
neotectonics and recent volcanism have obliterated part
of the geological record. More specifically, the short-lived
occurrence of shallow-marine carbonate and siliciclastic
facies in predominantly deep-water forearc basins provides
a record of tectonic uplift that interfered with eustatic sea-
level changes, paleoclimate, and other paleoenvironmental
conditions (Dorobek 2008). Ephemeral carbonate buildups
can grow in siliciclastic–volcaniclastic settings if they are
temporarily isolated from a direct input of detrital sedi-
ments (Braga and Martin 1988; Wilson and Lokier 2002;
Wilson 2005; Bosence 2005; Dorobek 2008). The isolation
Abstract The Sandino Forearc Basin of western Nicara-
gua and northwestern Costa Rica (Central America) pro-
vides a Campanian to Pliocene sedimentary record. The
study of the onshore part of the basin in northwestern Costa
Rica reveals for the first time the occurrence of upper Paleo-
gene shallow-marine siliciclastic and carbonate sequences.
These sequences have remained undescribed so far and are
grouped herein into two new lithostratigraphic units—the
upper Eocene Junquillal Formation (Fm.) and the upper
Oligocene Juanilla Fm. The upper Eocene Junquillal Fm.
is characterized by storm-related, arenitic to conglomeratic
deposits comprised in metric, massive amalgamated beds.
The shallow shelfal environment of deposition is attested
by the presence of hummocky and swaley cross-stratifica-
tions. The lithologies of the Junquillal Fm. were previously
considered to be part of the underlying, deep-water turbid-
itic deposits of the Eocene Descartes Fm. The deposition of
the Junquillal Fm. is indicative of tectonic uplift that forced
regression, which affected the southeastern part of the San-
dino Forearc Basin during the late Eocene. The upper Oli-
gocene Juanilla Fm. unconformably overlies the Junquillal
Fm. and occurs as a 25-m-thick, 700-m-wide outcrop on
Isla Juanilla. It is composed essentially of nodular, coral
framestones exhibiting massive, closely packed corals in
* Goran Andjić
Claudia Baumgartner-Mora
Peter O. Baumgartner
1 Institut des Sciences de la Terre, Université de Lausanne,
Bâtiment Géopolis, Bureau 3623, 1015 Lausanne,
Author's personal copy
Facies (2016) 62:9
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from a siliciclastic supply in a forearc context is achieved
by the development of tectonically induced structural highs
or sheltering from clastics in areas with reliefs such as shelf
margins, abandoned lowstand delta margins, and lobes
(Bosence 2005; Wilson and Hall 2010; Saller et al. 2011).
The Sandino Forearc Basin (SFB) corresponds to one of
the many Upper Cretaceous–Tertiary forearc basins formed
along the western margin of the Caribbean Plate (Dengo
1985; Alvarado et al. 2007). The SFB stratigraphy has been
studied in northwestern Costa Rica and western Nicaragua
for more than a century in the framework of several refer-
ence studies (Hayes 1899; Vaughan 1918; Dorr 1933; Har-
rison 1953; Hoffstetter et al. 1956; Zoppis Bracci and Del
Giudice 1958; Dengo 1962; McBirney and Williams 1965;
Kuang 1971; Schmidt-Effing 1974; Weyl 1980; Baum-
gartner et al. 1984; Astorga 1987, 1988; Kolb and Schmidt
1991; Winsemann 1992; Weinberg 1992; Krawinkel and
Kolb 1994; Elming et al. 1998; Ranero et al. 2000; Walther
et al. 2000; McIntosh et al. 2007; Struss et al. 2007, 2008;
Funk et al. 2009; Fig. 1). There are also numerous, infor-
mal studies and reports done for the petroleum industry
(Petronic) and several Nicaraguan government institutes
never been published and remain mostly inaccessible (for
example: Auer 1942; Wilson 1942; Williams 1972; Parsons
Corporation Report 1972; Barboza et al. 1993; RECOPE-
INE 1993). The present study focuses on the description,
interpretation, and definition of two new, shelfal lithostrati-
graphic units, the upper Eocene Junquillal Fm. and the
upper Oligocene Juanilla Fm., which crop out in the north-
ern Costa Rican part of the SFB (Fig. 2). The existence of
these new units represents a striking feature of the studied
segment of the SFB. Previous studies of this area recog-
nized only deep-water, turbiditic lithologies in the Tertiary
record. The facies study of the Eocene–Oligocene strati-
graphic succession reveals important paleoenvironmental
and relative sea-level changes that shed light on the sedi-
mentary and tectonic evolution of the SFB. The present
CLIP-derived fragments
200 km
Nicarag. Depression
Santa Elena accreted arc
Mesquito Composite
Oceanic Terrane
Panama Microplate
Manzanillo Terrane
Nicoya Complex
CLIP-derived fragments
North America-derived
continental terranes
Caribbean Large Igneous
Province (CLIP)
Mesquito Composite
Oceanic Terrane
Chortis Block s.s.
Sandino Forearc Basin
Middle America Trench
Gulf of Fonseca
CLIP s.s.
Studied area: Fig. 2
South American
Cocos Ridge
Nazca Pl.
500 km
North American Plate
CLIP s.s.
Santa Elena
Fig. 1 Terrane map displaying the major structural features of the
Central America arc-trench system, with the general tectonic plate
setting of the Caribbean Plate in inset (upper right; modified after
Baumgartner et al. 2008; Bandini et al. 2008; Flores 2009; Funk
et al. 2009; Buchs et al. 2010). The studied area (black rectangle) is
situated at the southeastern end of the Sandino Forearc Basin, in the
Santa Elena Peninsula-Punta Descartes area (Costa Rica northwest-
ernmost corner). The Rivas-1 and Ostional-1 boreholes are indicated.
NPDB North Panama Deformed Belt, CLIP Caribbean Large Igneous
Province, GA Guatemala, ES El Salvador, HS Honduras, NI Nicara-
gua, CR Costa Rica, PA Panama, Nicarag. Depression Nicaraguan
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Facies (2016) 62:9
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study is part of a basin-scale investigation of the SFB and
represents the first paper of a series that will adopt a similar
approach for outcrops located in the Nicaraguan part of the
Geological setting
The Sandino Forearc Basin (SFB) corresponds to a latest
Cretaceous–Neogene forearc basin. The SFB presents both
offshore and onshore parts that extend over a distance of
300 km from the Santa Elena Peninsula (NW Costa Rica)
to the Gulf of Fonseca, which is situated at the intersection
of the Nicaragua, Honduras and El Salvador borders (Ran-
ero et al. 2000; Funk et al. 2009; Fig. 1). The offshore part
of the SFB lies between the Nicaraguan Pacific coast and
the Middle America Trench off Nicaragua (Ranero et al.
2000, 2007; Fig. 1). The depocenter of the basin is located
in the vicinity of the coastline and is filled with ~10 km
of forearc sequences that progressively thin towards an
outer high (Ranero et al. 2000, 2007). The onshore geol-
ogy of the SFB crops out in a 160-km-long/30-km-wide,
slightly folded belt, exposed along the Nicaraguan Isthmus
(Hodgson 1998; Darce and Duarte 2002; Figs. 1, 2). This
300-km-long isthmus is bordered to the east by normal and
strike-slip faults that mark the limit with the Nicaraguan
Depression (Fig. 1).
Classically, the Chortis Block s.l. was considered as
the basement of the SFB (Dengo 1985). More recently, it
became evident from few outcrops, geophysics and vol-
canic geochemistry that most of Nicaragua is underlain by
the Mesquito Composite Oceanic Terrane (MCOT), which
represents Franciscan-type, accreted terranes with oce-
anic remnants in a serpentinitic matrix (Rogers et al. 2007;
Baumgartner et al. 2008; Fig. 1). To the south, the MCOT
is bounded by a complex puzzle of Mesozoic plateau-like
oceanic terranes that crop out in northwestern Costa Rica
(Baumgartner and Denyer 2006; Baumgartner et al. 2008;
Bandini et al. 2008; Buchs et al. 2010; Fig. 1).
Escuder-Viruete and Baumgartner (2014) proposed that
the Santa Elena tectonic pile corresponds to an intraoce-
anic arc that became accreted to the MCOT during Late
Cretaceous time. Escuder-Viruete et al. (2015) revalidate
the term “ophiolite” for the Santa Elena ultramafic nappe,
based on the observation of the paleo-Moho and overlying
layered and massive gabbros (Isla Negritos gabbros; Fig. 2)
along the top of the main body.
The SFB deposits overlap this area of terrane collage
rendering its internal limits unobservable. In the southeast-
ernmost segment of the SFB, the forearc sequences depos-
ited over the essentially ultramafic, tectonic pile exposed
in the Santa Elena Peninsula (Baumgartner et al. 1984;
Escuder-Viruete and Baumgartner 2014). In the Nicaraguan
part of the SFB, the underlying basement is poorly known.
Supposedly it is part of the MCOT. In one of two onshore
drill holes (Rivas-1; Fig. 1) the oldest lithologies of the
SFB consist of volcaniclastic to tuffaceous deep-water sed-
iments underlain by a basaltic basement of unknown origin
(Ranero et al. 2000; Fig. 2).
During Campanian to Oligocene, predominantly deep-
water pelagic, hemipelagic and turbiditic sequences are
successively replaced by platform siliciclastics and car-
bonates at different steps of the basin evolution (Zoppis
Bracci and Del Giudice 1958; Elming et al. 1998). The ini-
tial stage of basin development only crops out in the Santa
Elena Peninsula (NW Costa Rica; Fig. 2), where the base-
ment of the southeastern part of the basin is overlain by a
veneer of shallow-water limestones followed by deep-water
sedimentary sequences deposited in Latest Cretaceous–
Eocene times (Zoppis Bracci and Del Giudice 1958; Dengo
1962; Baumgartner et al. 1984; Astorga 1987, 1988; Win-
semann 1992). The Santa Elena Peninsula consists mainly
of an ultramafic nappe made of serpentinized peridotites
cut by mafic dykes and overlain by massive and sheeted
gabbros, interpreted as a piece of a supra-subduction zone
mantle and crust of unknown age (Santa Elena ultramafic
nappe; Harisson 1953; Azéma and Tournon 1980, 1982;
Tournon 1994; Hauff et al. 2000; Baumgartner and Denyer
2006; Gazel et al. 2006; Zaccarini et al. 2011; Escuder-
Viruete and Baumgartner 2014; Santa Elena ophiolite,
Escuder-Viruete et al. 2015). The Santa Elena ultramafic
nappe thrusted onto a relative autochthonous accretionary
complex, known as the Santa Rosa Accretionary Com-
plex (Baumgartner and Denyer 2006; Escuder-Viruete and
Baumgartner 2014), built up by the stacking of several tec-
tonic units (Tournon 1984; Baumgartner and Denyer 2006).
A gap in the dated sedimentary record exists between the
youngest rocks observed in the nappe edifice (Albian;
Bandini et al. 2011) and the oldest rocks (upper Campa-
nian) of the sedimentary cover overlapping the ultramafic
nappe. Intraoceanic accretion must have occurred since
the Albian, while a hypothetical arc–arc collision and final
overthrust of the Santa Elena ultramafic nappe must have
occurred shortly prior to the late Campanian (Escuder-
Viruete and Baumgartner 2014). Tectonic uplift resulted in
local emersion and the deposition of the shallow-water El
Viejo Formation, characterized by upper Campanian rud-
ist biostromes (Schmidt-Effing 1974, 1980; Seyfried and
Sprechmann 1985) and associated periplatform to slope
deposits (e.g., Bahia Santa Elena outcrops; Di Marco et al.
1995; Baumgartner-Mora and Denyer 2002; Figs. 2, 3).
Reworked ultramafic and mafic clasts of the underlying
nappe are observed in all the lithologies of the El Viejo Fm.
and suggest an original sedimentary contact with the ultra-
mafic/mafic basement, which we have mapped in 2015.
This observation is in contradiction with the existence of a
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22 25
37 19
34 57
Bahía Playa
Bahía Cuajiniquil
Bahía Salinas
Isla Juanilla
Isla Despensa
Isla Bolaños
Punta Blanca
Peninsula de
Santa Elena
5 km
Costa Rica
B. El Jicote
Islas Los Muñecos
B. La Virgen
Punta Zacate
Playa El Jobo
Playa El Coco
Isla Los Cabros Cuajiniquil
Nicaraguan Isthmus
4 km to Ostional-1 onshore well
Punta Castilla
85°45' 85°40'
El Viejo Fm.- Peña Bruja facies (upper Campanian)
Piedras Blancas Fm. (upper Campanian
upper Maastrichtian)
Curú Fm. (upper Maastrichtian
upper Paleocene)
Descartes Fm. (lower
upper Eocene)
Isla Negritos gabbros
Juanilla Fm. (upper Oligocene)
Anticline (Oligocene)
Inclined bedding
Horizontal bedding
Lithostratigraphic units
Junquillal Fm. (upper Eocene)
Buenavista Fm. (upper Paleocene
lower Eocene)
AA' Cross-section orientation
New lithostratigraphic units
Undifferentiated deposits (Quaternary)
Serpentinized peridotites
Santa Elena ultramafic nappe
Upper Cretaceous)
Punta Descartes
Peninsula de Santa Elena
Isla Juanilla
5 km
Islas Los Muñecos
Nicaraguan Isthmus
Projection of the Ostional-1 well
Arc-derived and hemipelagic sediments (Upper Cretaceous)
Lithostratigraphic units: same as shown in a, and includes also:
Mesquito Composite Oceanic Terrane ?
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Facies (2016) 62:9
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major fault, prolongating the trace of the Hess Escarpment,
between the Santa Elena serpentinites and the overlying
sediments, as drawn for instance by Meschede and Frisch
(1998, “Santa Elena Fault”) and Giunta et al. (2006). The
El Viejo Fm. represents the oldest formation in this part of
the SFB (Figs. 2, 3).
The Campanian shallow-water and periplatform deposits
are overlain by a pluri-kilometric forearc sequence exposed
in large-scale, open folds with WNW–ESE-oriented axes
exposed along the northern coast of the Santa Elena Pen-
insula and in Punta Descartes (Zoppis Bracci and Del Giu-
dice 1958; Dengo 1962; Baumgartner et al. 1984; Astorga
1987, 1988; Winsemann 1992; Denyer and Alvarado 2007;
Fig. 2). Similar deposits are known from the Nicoya Pen-
insula and the Nicoya Gulf (Dengo 1962; Schmidt-Eff-
ing 1979; Lundberg 1982; Rivier 1983; Astorga 1987,
1988; Flores et al. 2003a, b). The base of the deep-water
sequence consists of hemipelagic, globotruncanid-rich
limestones of the upper Campanian–upper Maastrichtian
Piedras Blancas Fm. (Baumgartner et al. 1984; Astorga
1987, 1988; Di Marco et al. 1995; Figs. 2, 3). The upper
Maastrichtian–upper Paleocene Curú Fm. (=Rivas Fm.
in Nicaragua) conformably overlies the Piedras Blancas
Fm. and marks the beginning of detrital forearc sedimen-
tation in the area, with distal turbidites of mafic composi-
tion (Baumgartner et al. 1984; Astorga 1987, 1988). The
overlying upper Paleocene–lower Eocene siliceous pelagic
limestones of the Buenavista Fm. represent a break in the
turbiditic sedimentation (Baumgartner et al. 1984). This
300-m-thick formation is exposed on the northern shore of
the Santa Elena Peninsula. The Eocene Descartes Fm. indi-
cates the transition to a more evolved volcanic arc activity
of its source, underlined by an acidic composition of the
forearc turbidites (Astorga 1987, 1988).
Due to kilometric, open folding, the Descartes Fm.
(=Brito Fm. in Nicaragua) covers the whole area between
the northern coast of the Santa Elena Peninsula and the
southeastern coast of Nicaragua, with several repetitions.
The northern shore of the Santa Elena Peninsula and the
southern coast of the Punta Descartes are composed of the
most ancient and most distal sequences of the Descartes
Fm. and correspond to distal turbidites (Baumgartner
et al. 1984; Astorga 1987, 1988; Figs. 2, 3). On the other
hand, the western-northwestern Punta Descartes and the
area south of Bahia Junquillal show more recent and more
proximal deposits (Dengo 1962; Baumgartner et al. 1984;
Astorga 1987). In this paper, these deposits are defined as
a new, upper Eocene formation, the Junquillal Fm. This
formation corresponds to shelfal, storm-influenced detrital
lithologies that were previously considered as deep-water,
turbiditic deposits attributed to the Eocene Descartes Fm.
The overlying Isla Juanilla Fm. forms an island (~0.2 km2),
which represents the youngest deposits preserved in a syn-
form cropping out in Bahia Junquillal (Fig. 2). The Juanilla
Fm. represents a striking feature in comparison to the sur-
rounding geological setting, as it is the only Tertiary shal-
low-water limestone outcrop in the northwesternmost cor-
ner of Costa Rica. Furthermore, the geology of this small
island has remained until now undescribed. Field observa-
tions indicate that it consists mainly of a reef buildup of
corals and red algae with local occurrences of Larger Ben-
thic Foraminifera-rich outcrops.
Here, we reexamine the outcrops of the deep-water
Descartes Fm. of the studied area, to clarify its differences,
as compared to the newly established, shelfal Junquillal Fm.
Additionally, we present the facies description and interpre-
tation of the new Junquillal and Juanilla fms. (Figs. 3, 4, 5,
6, 7, 8). We dated the discussed lithostratigraphic units with
Larger Benthic Foraminifera (LBF) recovered from several
outcrops (Figs. 9, 10, 11). The samples that yielded valu-
able biostratigraphic data are mentioned within brackets in
the text and in the figures (Figs. 3, 4, 7). The location and
the extension of the described lithostratigraphic units are
shown in Fig. 2. Their significance for the tectonic inter-
pretation will be treated in the discussion (see below). The
stratigraphic terminology used in the following paragraphs
is based on Murphy and Salvador (1999).
Fig. 2 a New geological map of the Santa Elena-Punta Descartes
area (Sandino Forearc Basin; modified after Dengo 1962; Baum-
gartner et al. 1984; Astorga 1987; Denyer and Alvarado 2007; Escu-
der-Viruete et al. 2015). The mapped area encompasses the 1:50,000
topographic map sheets 3048 I (Murciélago), 3048 IV (Santa Elena)
and 3049 II (Bahia Salinas) of the National Geographic Institute of
Costa Rica. The detailed description of the mapped lithostratigraphic
units is presented in the Fig. 3. We introduce two new lithostrati-
graphic units, the Junquillal Fm. and the Juanilla Fm., which have
been previously mapped as Descartes Fm. The localities mentioned in
the text and the position of the cross section (AA’, b of the same fig-
ure) are indicated. b NNE–SSW-oriented cross section (AA’, no ver-
tical exaggeration) set perpendicular to the main tectonic structures
of the Santa Elena-Punta Descartes area. The cross section displays
one informal unit (Kas), which is not exposed in the mapped area
shown in a. This unit corresponds to Upper Cretaceous arc-derived,
tuffaceous sediments, which underlie the upper Campanian–upper
Maastrichtian Piedras Blancas Fm. and are only known from the
Ostional-1 and Rivas-1 boreholes drilled in Nicaragua (see Fig. 1 for
borehole locations, Ranero et al. 2000). These Upper Cretaceous arc-
derived sediments overlie a basaltic terrane possibly associated to the
Mesquito Composite Oceanic Terrane (Baumgartner et al. 2008) and
which was recovered from the Rivas-1 borehole (Ranero et al. 2000).
Such sediments are not known to overly the Santa Elena tectonic pile
(in the field), possibly due to the Campanian tectonic event (see text).
We induce a subcrop contact between the basaltic terrane (Mesquito
Composite Oceanic Terrane) and the Santa Elena ultramafic nappe.
This terrane boundary is tentatively indicated on the cross section
beneath the Bahia Cuajiniquil, given the absence of data on this sub-
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Facies (2016) 62:9
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Descartes Formation: a formal definition
The Descartes Fm. corresponds to a commonly used
informal unit name, introduced by Astorga (1987) in an
unpublished work. A formal definition and a revision of
this unit is necessary, since: (1) Astorga (1987) represents
Camp. Ba. Pria. RupelianChattianTh.
Juanilla Formation (new lithostratigraphic unit)
Junquillal Formation (new lithostratigraphic unit)
Descartes Formation
Buenavista Formation
Curú Formation
Piedras Blancas Formation
El Viejo Formation
Santa Elena ultramafic nappe (pre-Late Cretaceous)
Lithology: 25 m thick coralgal reef composed of nodular, beige
framestones. The corals display massive, domal, decimetric
morphologies. Occurence of Lepidocyclinid-rich facies within the
reef and in associated platform deposits.
Index fossils: Lepidocyclina undosa, L. favosa, L. foresti, L.
vaughani, L. canellei, Miolepidocyclina cf. panamensis,
Miogypsinoides sp. (IJ5, IJ18) = late Oligocene (this study).
Lithology: shelf deposits consisting of fine- to coarse-grained
lithic-arkosic wackes and arenites alternating in places with pebble
and cobble, matrix-supported, polymict conglomerates. Occurrence
of hummocky and swaley cross-stratifications.
Index fossils: Lepidocyclina chaperi, L. macdonaldi, L. ariana, L.
pustulosa, Nummulites willcoxi, N. striatoreticulatus, N. floridensis,
Asterocyclina asterisca (1529, COC5) = Priabonian (this study).
Lithology: thin-bedded turbidites consisting of siliceous
hemipelagic mudstones and lithic-arkosic, tuffaceous wackes and
arenites. Index fossils: a) uppermost part: Nummulites floridensis,
N. trinitatensis, Amphistegina parvula, Lepidocyclina pustulosa, L.
ariana, L. chaperi (CR14-07, MZO1) = Priabonian (this study);
b) lowermost part: Morozovella subbotinae (Clr26), Globanoma-
lina luxorensis (Clr27) = latest Th.
middle Ypresian (Clerc 1998).
Lithology: siliceous, pelagic, beige limestones displaying
occasionally red cherts and alternating in places with yellow to
green tuffaceous turbidites. Index fossils: a) uppermost part:
Morozovella subbotinae, Turborotalia praecentralis (Clr24) =
middle Ypresian (Clerc 1998); b) lowermost part: Globanomalina
imitata, Morozovella pasionensis. M. acuta, M. velascoensis, M.
subbotinae (Clr7) = latest Thanetian
earliest Ypresian (Clerc 1998).
Lithology: deepening-upward sequence exhibiting thin- to thick-
bedded turbidites with increasing occurrence of distal turbidites
upsection. The distal turbidites alternate with hemipelagic
limestones in the uppermost 120 m of the formation.
Index fossils (uppermost Curú Fm.): Morozovella velascoensis,
Subbotinae triloculinoides, Pseudomenardella ehrenbergi (652,
653) = late Selandian
middle Thanetian (Azéma et al. 1981).
Lithology: red to greyish hemipelagic limestones. The transition
to the Curú Fm. is progressive with an increasing detrital input.
Index fossils: a) lowermost Piedras Blancas Fm.: Rosita fornicata,
Globotruncanita calcarata, Globotruncana linneiana, G.
ventricosa (Site 36) = late Campanian (DiMarco et al. 1995); b)
upper Piedras Blancas Fm.: Globotruncana spp. (late
Maastrichtian, Baumgartner et al. 1984).
Lithology: rudist-orbitoid biostromes containing reworked clasts
of the underlying serpentinitic massif. The Peña Bruja periplatform
facies overlies and/or interfingers laterally with the biostromes.
(Peña Bruja Islet, Bahia Santa Elena and Rio Nisperal outcrops).
Index fossils : Pseudorbitoides israelski, Pseudorbitoides rutteni,
Sulcoperculina globosa (Peña Bruja Islet sample) = late
Maastrichtian (Baumgartner-Mora and Denyer 2002).
Santa Elena - Punta Descartes
upper Campanian
upper Eocene
generalized stratigraphic log
IJ5 IJ18
Site 36
P. Bruja Islet
Lithostratigraphic units
Isla Juanilla reduced
upper Eocene
Rudist colony
Colonial coral
Solitary coral
Peysonnelid alga
Encrusting red alga
Larger Foraminifera
Sample with index
fossils (this study)
Sample with index
fossils (previous
1 km
Fig. 3 General stratigraphy of the Sandino Forearc Basin units
exposed in the northwesternmost corner of Costa Rica. The figure
is divided into three distinct parts, from left to right: a chronostrati-
graphic column of the lithostratigraphic units (left), a three-column
chart presenting the general description of the lithostratigraphic
units (middle), and two stratigraphic logs of the studied area (right).
The three parts are correlated with continuous lines representing the
boundaries of the lithostratigraphic units. The three-column chart
(middle) displays the characteristics of each lithostratigraphic unit.
This include short lithological descriptions (=legend for the litholo-
gies depicted in the left and right columns), lists of index fossils with
corresponding authors and sample names, and indications on depo-
sitional environments and fossil contents. The fossil contents are not
exhaustive and only the most common fossils are mentioned. The
lists of index fossils (planktonic and Larger Benthic Foraminifera)
are used to constrain the chronostratigraphic column (left) and origi-
nate from the present study (sample names with black rectangles)
and from previous studies (sample names with white rectangles).
The general stratigraphic logs (right) exhibit the thickness of each
lithostratigraphic unit and the stratigraphic position of the samples
containing index fossils
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a work that was not published in a recognized scientific
medium (see Murphy and Salvador 1999); (2) the litholo-
gies of the upper part of the Descartes Fm. (sensu Astorga
1987) are excluded from the latter and grouped in a new,
distinct unit herein defined as the Junquillal Fm. (see
below; Figs. 2, 3, 4). The siliceous pelagic limestones crop-
ping out on the northern shore of the Santa Elena Penin-
sula (Fig. 2) were also included with the lower Descartes
Fm. (Astorga 1987). Here, we consider these as a sepa-
rate formation, the Buenavista Fm., as it was defined by
0 m
Siltstone (St) or mudstone (M)
Sandstone (S)
Matrix-supported granule (g)
conglomerate (C)
PCS Planar cross-stratification
Hummocky cross-stratification
LCS Low-angle cross-stratification
SCS Swaley cross-stratification
Platform carbonates
Pebble (p) conglomerate (C)
Clast- or matrix-supported
Upper Oligocene (Juanilla Formation)
Upper Eocene
Intraformational correlation
Unit boundary
IJ5 Dated sample
N 11°00'25.2"
W 085°43'52.9"
N 11°00'09.3"
W 085°44'46.7"
N 10°59'00.4"
W 085°42'57.8"
N 11°02'49.0"
W 085°44'00.8"
N 11°02'39.3"
W 085°43'22.2"
N 11°02'53.8"
W 085°43'37.8"
Juanilla Fm.
Junquillal Fm.
Descartes Fm.
Playa El Coco
Isla Juanilla
Isla Despensa
Playa Rajada
Punta Zacate
P. Manzanillo
Transition to shoreface
ansition to shorefac
Junquillal Fm.
Descartes Fm.
Fig. 4 Stratigraphic logs of the Punta Descartes-Bahia Junquil-
lal area evidencing a transition from deep-water, turbiditic facies
(Descartes Fm.) to siliciclastic (Junquillal Fm.) and carbonate
(Juanilla Fm.) shallow shelf facies. The limits between the three
lithostratigraphic units are depicted with continuous lines, whereas
the dashed lines highlight the correlation within the units. The Playa
Manzanillo section represents the lectostratotype of the Descartes
Fm. (see text). For the newly established Junquillal Fm., the Playa
Despensa section corresponds to the holostratotype (see text),
whereas the Playa El Coco, the Playa Rajada and the Punta Zacate
sections represent parastratotypes (see text). The samples that yielded
biostratigraphic data are indicated within black rectangles. The GPS
coordinates indicate the precise location of each section. The locali-
ties where the six sections were studied are shown on the geological
map in Fig. 2. P. Manzanillo Playa Manzanillo
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Baumgartner et al. (1984; Figs. 2, 3). The general definition
of the Descartes Fm. remains unchanged. The aspects of
the Descartes Fm. examined hereafter are essentially based
on our observations in the Santa Elena Peninsula-Punta
Descartes area that represents the type area of the Descartes
Fm. (Fig. 2).
Stratotype and reference sections
The type locality was informally defined in the Punta
Descartes Peninsula by Astorga (1987, unpublished work).
However, he omitted to establish and localize a holostra-
totype. The well-preserved exposures located at the Playa
Manzanillo (N 11°0028.5/W 085°4357.2; Figs. 2, 4,
5a) and in the vicinity (southern Punta Descartes) consti-
tute a continuous 160-m-thick section that is regarded here
as a lectostratotype section. The coastal outcrops exposed
along the western Punta Descartes are excluded from the
Descartes Fm. and included into the Junquillal Fm.
Numerous characteristic, well-preserved coastal expo-
sures of the Descartes Fm. can be observed on the north-
ern shore of the Santa Elena Peninsula. The Punta Castilla
section (N 10°5611.1/W 085°4233.8; Fig. 2) represents
the best preserved and the most easily accessible outcrop of
this area and is here considered as a hypostratotype section.
Formation boundaries
The lower boundary is defined at the beach located 240 m
southwest of the Isla Los Cabros (N 10°5627.0/W
085°4857.4; Fig. 2) by an abrupt change from the beige,
siliceous pelagic limestones of the Buenavista Fm. to the
Fig. 5 Field photographs of the Eocene Descartes Fm. See Fig. 2 for
locations. a Thin-bedded turbidites presenting centimetric alterna-
tions of silty/hemipelagic (bright color) and sandy (dark color) beds
(Playa Manzanillo, N 11°0025.2/W 085°4352.9). Hammer length
31 cm. b Close-up view of a decimetric bed consisting of several tur-
biditic events (Punta Castilla, N 11°5611.1/W 085°4233.8). Note
the double meter stick for scale. c Thin-section photomicrograph
of mm- to cm-scale turbidites, which grade from very fine sand to
radiolarian-bearing mud. Scale bar 5 mm. Same locality as in b. d
Erosive contact between a LBF-rich, graded arenitic turbidite and the
underlying mudstone (Playa Manzanillo). Note the double meter stick
for scale. e Heterolithic facies (F1 in Fig. 7) consisting of centimetric
to decimetric silty mudstones and centimetric, arkosic wackes (Isla
Juanilla, N 10°5858.3/W 085°4253.8). GPS device length 14 cm.
f Thin-section photomicrograph of the facies shown in e. Scale bar
1 mm. g Conformable contact between the silty-sandy lithologies of
the uppermost Descartes Fm. and the sandy lithologies of the lower-
most Junquillal Fm. (Isla Despensa, N 11°0007.6/W 085°4448.8).
Hammer length 31 cm
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conformably overlying, brown and green, fine-grained tur-
bidites of the Descartes Fm.
The upper boundary is defined on the Isla Despensa (N
11°0007.6/W 085°4448.8; Figs. 2, 4, 5g) by an abrupt
change from a heterolithic facies of the Descartes Fm.,
consisting of silty-sandy alternations, to the conformably
overlying, amalgamated medium-grained sandstones of the
Junquillal Fm. The boundary between the two formations is
also evident from the outcrop morphology. The silty litholo-
gies of the uppermost Descartes Fm. were strongly scoured
by wave action in comparison to the competent, sandy lith-
ologies of the overlying Junquillal Fm. The upper bound-
ary of the formation is also observed on the northern shore
of the Bahia Cuajiniquil (N 10°5711.1/W 085°4255.8;
Fig. 2) with the transition from thin- to thick-bedded tur-
bidites of the Descartes Fm. to the thick- to massive-bed-
ded, amalgamated arenites of the Junquillal Fm.
Derivation of the name and synonymy
The formation is named after the Punta Descartes Penin-
sula (Fig. 2). The synonymy includes the following, previ-
ously used terms: Brito Fm., defined by Dengo (1962), and
Cuajiniquil unit (Baumgartner et al. 1984).
Lithology and sedimentary structures
Originally, the Descartes Fm. was described as consisting
of thin-bedded, tuffaceous, silts and fine- to coarse-grained
sandstones alternating with breccias and conglomerates
(Astorga 1987). However, reexamination in the study area
revealed that the Descartes Fm. is composed of mm- to
cm-sized turbiditic alternations. They consist of siliceous,
hemipelagic mudstones and siltstones and lithic-arkosic,
tuffaceous wackes and arenites that present a total thickness
of 1800 m (Figs. 2, 3, 4, 5). The planktonic/pelagic frac-
tion in the hemipelagic rocks is low (<20 %) and includes
moderately preserved planktonic Foraminifera and radio-
larians. The most common turbidites correspond to cm- to
dm-sized, subtly graded, plane-parallel beds exhibiting Ta
and Te interval couplets (Bouma 1962; Fig. 5). Generally,
each bed contains several amalgamated turbiditic events
(Fig. 5b, c). The grain size of the Ta intervals is generally
comprised between silt and medium-grained sand, whereas
the Te intervals correspond to siliceous hemipelagic mud-
stone and siltstone. Within one couplet, the Ta and Te inter-
vals show different thicknesses, with one of the interval
being systematically thicker than the other one. However,
at the scale of the formation, the detrital-tuffaceous (Ta)
and the hemipelagic (Te) sediments are present in equal
proportions. Turbidites consisting of a more complete
Bouma sequence are scarce and occur generally in 10 to
20-cm-thick beds.
In the upper part of the formation, a few turbidites con-
tain small LBF that mark the base of Ta intervals (MZO1
and CR14-07; Figs. 4, 5d, 9). The input of these shallow-
water organisms is concomitant with a general increase in
the turbidite bed thickness. This trend is particularly well
defined on the northern shore of the Bahia Cuajiniquil (N
10°5711.1/W 085°4255.8; Fig. 2), where dm- to m-bed-
ded turbidites crop out below the limit with the overlying
Junquillal Fm. These turbidites contain in places pebbles of
volcanic rocks and shallow-water limestones with red algae
and LBF that also occur in metric debris flows (roadside
outcrop, now covered, located 2 km east of the Cuajiniquil
village; Baumgartner et al. 1984).
On the Isla Juanilla (Fig. 2), the Descartes Fm. con-
sists of a heterolithic facies (facies F1; Figs. 4, 5e, 7). This
facies of the uppermost Descartes Fm. represents the old-
est lithology observed on the Isla Juanilla. It crops out over
a distance of 200 m (at low tide) and up to 50 m above
sea level on the northern slope of the island, with an esti-
mated thickness of 25 m. This brittle lithology is made of
cm- to dm-bedded, greenish brown, silty mudstones alter-
nating with centimetric, beige, arkosic wackes (Fig. 5e, f).
The mudstones represent more than 80 % of the lithology.
They are composed of hemipelagic, carbonate-bearing mud
containing a low fraction of detrital minerals consisting of
anhedral plagioclase crystals and opaque minerals (<10 %,
0.03–0.05 mm). The arkosic wackes are subtly graded and
present a higher content in detrital minerals (15–17 % anhe-
dral plagioclases; 7–9 % opaques; 0.04–0.2 mm, for both)
and a carbonate matrix. Very rare planktonic Foraminifera
are the only visible bioclasts. The sandy mudstone facies
crops out also on the Isla Despensa (Figs. 2, 4, 5g) with a
thickness of ~10 m and marks the transition from the fine-
grained, turbidite facies of the Descartes Fm. to the amal-
gamated sandstones of the conformably overlying Junquil-
lal Fm.
The submarine fan features interpreted from outcrops of
the Punta Descartes (Astorga 1987) are actually tempestite
deposits, now included with the overlying Junquillal Fm.
Typical, coarse-grained, submarine channel complexes
within the Descartes Fm. can be observed elsewhere in
Costa Rica (SE Nicoya Peninsula; Astorga 1987) and Nica-
ragua (SW Nicaraguan Pacific coast; Struss et al. 2007).
Age of the Descartes Formation
The lowermost part of the formation exposed in Isla Los
Cabros (Fig. 2) yielded planktonic Foraminifera species
such as Morozovella subbotinae and Globanomalina luxo-
rensis, which indicate a latest Thanetian–middle Ypresian
age (sample Clr26 of Clerc 1998; Fig. 3). The co-occur-
rence of Morozovella subbotinae (range: latest Thanetian–
middle Ypresian; Pearson et al. 2006) and Turborotalia
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praecentralis (range: middle Ypresian–late Lutetian; Blow
1979) indicates a middle Ypresian age for the underlying,
uppermost Buenavista Fm. (sample Clr24 of Clerc 1998;
Fig. 3). Consequently, the base of the Descartes Fm. is
middle Ypresian (early Eocene) in age (by stratigraphic
The uppermost part of the formation (Playa Manza-
nillo; Fig. 2) yielded a LBF assemblage (samples MZO1
and CR14-07; Figs. 3, 4, 9) with the following species:
Nummulites floridensis Heilprin, Nummulites trinitatensis
(Nutall), Amphistegina parvula (Cushman), Lepidocyclina
(Nephrolepidina) chaperi Lemoine and Douvillé, Lepido-
cyclina pustulosa H. Douvillé, and Lepidocyclina ariana
Cole and Ponton. According to Frost and Langenheim
(1974), Butterlin (1981), and Robinson and Wright (1993),
this assemblage indicates a Priabonian (late Eocene) age.
Recent investigations by Molina et al. (2016) found Num-
mulites floridensis, Lepidocyclina chaperi, L. pustulosa
and orthophragminids in deep water sediments of Cuba,
associated with earliest Oligocene (Rupelian) planktonic
Foraminifera and nannofossils (Ol, P18, NP21/CP16).
Based on a review of data from Florida, Jamaica and the
Tethyan realm, these authors conclude that the extinction of
the orthophragminids may be diachronous and occur in the
earliest Oligocene of the Caribbean and Tethyan realms.
However, reworking of latest Priabonian faunas into
lower Rupelian deep-water turbiditic sediments cannot be
excluded, neither in the Cuban section or in the Descartes
Fm., which therefore, may or may not reach into the low-
est Oligocene. In the Brito Fm. (Nicaraguan equivalent of
the Descartes Fm.), an early late Eocene age was obtained
with planktonic Foraminifera and nannofossils (Struss et al.
Hence, the Descartes Fm. is of middle Ypresian to Pria-
bonian (Eocene), perhaps to early Rupelian (Oligocene)
age (Fig. 3).
Depositional environment
The lithologies of the Descartes Fm. were interpreted as
basin plain volcaniclastic turbidites interstratified with
submarine fan complexes, channel overbank deposits and
slumps (Astorga 1987). In its type area, the Descartes Fm.
shows exposures of thin-bedded, tuffaceous, fine-grained
turbidites, which alternate with hemipelagic siliceous
rocks containing planktonic Foraminifera and radiolar-
ians. These thin alternations are indicative of a constant
interplay between turbiditic and hemipelagic sedimenta-
tion in a deep-water environment. The distal character of
the turbidites is attested by their limited thickness (mainly
mm- to cm-thick), their grain-size (silt to medium-grained
sand) and the high content of hemipelagic sediment, which
is present in the same proportions as the turbiditic detrital
fraction. Regarding our study, there are no paleobathym-
etric (faunal) indicators that would allow to ascertain pre-
cisely the water depth at which the turbidites deposited.
Based on paleobathymetric data from well reports, Struss
et al. (2008) estimate a paleo-water depth of 3000 m for
the Brito Fm. (Nicaragua). Mutti et al. (2007) indicate that
most of the modern turbidite systems develop at depths in
excess of 1000 m. Moreover, such depths are commonly
inferred for ancient examples.
In the uppermost part of the formation, the input of
platform-derived clasts associated with a thickening- and
coarsening-upwards of the turbidites evidence an increas-
ing proximality of the turbiditic system that precedes the
onset of shelf sedimentation. Similarly, the heterolithic lith-
ologies of the Descartes Fm., which are exposed on the Isla
Juanilla (Fig. 5e) and on the Isla Despensa (Figs. 4, 5g) rep-
resent a rather proximal, offshore facies. These sediments
deposited in a low-energy environment that marks a regres-
sion between the deep-water deposits of the Descartes Fm.
Fig. 6 Field photographs of the upper Eocene Junquillal Fm. The
continuous lines represent unconformities whereas the dashed lines
underline the stratification. See Fig. 2 for locations. a Low-angle
cross-stratification occurring in coarse-grained arenites (Playa El
Coco, N 11°0250.2/W 085°4352.4). Scale bar 1 m. b Channel fill
deposits unconformably overlain by tempestitic deposits (115 m SE
of Punta Zacate, N 11°0253.8/W 085°4337.8). The channel fills
show a basal lag deposit and unconformably overly planar-stratified
arenites. The channel fill deposits present planar cross-stratifications
prograding mainly to the right. The tempestite exhibits a basal clast-
supported pebble conglomerate that grades into fine-grained arenites
presenting hummocky cross-stratifications (HCS). Double meter stick
1 m. c Organic matter debris in a silty facies (Playa El Coco). Scale
bar 1 cm. d Nummulitid shell (Bahia El Jobo southern shore). Scale
bar 1 mm. e Thin-section photomicrograph of a centimetric bivalve
shell (southern Punta Descartes). Scale bar 1 cm. f Gastropod shell
(Playa Rajada). Scale bar 1 cm. g Finning-upward set of cross-strata
(300 m SE of Punta Zacate, N 11°0249.3/W 085°4330.0). The
lower, coarse-grained part shows low-angle cross-stratifications. The
upper, fine-grained part displays swaley cross-stratifications (SCS)
with concave-up strata. Length of the GPS device is 10 cm. h Hum-
mocky cross-stratifications (HCS) developed in fine-grained aren-
ites (Bahia El Jicote, N 10°5736.3/W 085°4217.1). GPS device
length 14 cm. i Wavy-bedded heterolithic facies consisting of very
fine-grained arenites (bright color) and silts (dark color), and encom-
passed between coarser deposits (390 m SE of the Punta Zacate, N
11°0248.7/W 085°4329.1). GPS device length 10 cm. j Cross-
stratified, arkosic-lithic arenite (facies F2 in Fig. 7) occurring as
boulders on the northern shore of the Isla Juanilla (N 10°5900.5/W
085°4256.8). Hammer length is 31 cm. The lower left insight
shows a thin-section photomicrograph of the arenitic facies. Width
of the image is 3.1 mm. k Unorganized deposits consisting of clast-
supported, polymict conglomerates (Playa Rajada, N 11°0239.5/W
085°4319.8). Scale bar 1 m. l Submarine dunes comprised in mas-
sive, metric, stacked beds unconformably overlying centimetric silt-
stones (Playa El Jobo, N 11°0219.2/W 085°4419.8). Scale bar
5 m. m Close-up view of the outcrop shown in l. An erosive surface
separates the submarine dunes from the underlying silty beds. The
dunes grade from coarse-grained to fine-grained arenites and are
characterized by low-angle cross-stratifications. Double meter stick
length 0.8 m
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F1 F2 F7
F3 F4 F5
Junquillal Fm.
F3 F4 F5
U2 U4
5 m
Transition to reef facies
Isla Juanilla
Coral Reef
Coral in life position
Encrusting coral
Coral rubble
Solitary coral
Red algae rhodolith
Dated sample
Disconformity (inferred)
Unit boundary (unconformity)
Red algae fragment
Planktonic Foraminifera
50 m
U1 U2 U3 U4 U6
~1.5:1 VE
Slumped unit (dip slope) F4
Isla Juanilla
200 m F3
Lepidocyclina packstone
Sandy coral rudstone
Rhodolith-coral rubble rudstone
Arkosic-lithic arenite
Sandy mudstone
Lepidocyclina-encrusting coral bindstone
Nodular coral framestone
fold axis
41 70
10 m
50 m
50 m
5 m
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and the siliciclastic shelf deposits of the Junquillal Fm. The
latter is marked by the appearance of massive, amalga-
mated arenites.
Junquillal Formation: a new lithostratigraphic unit
The Junquillal Fm. is herein defined as a new formal
lithostratigraphic unit which consists of outcrops located
between the Bahia Cuajiniquil northern shore and the
Bahia Salinas northern shore (type area; Fig. 2). The defini-
tion of the Junquillal Fm. proposed hereafter includes the
Junquillal unit of Baumgartner et al. (1984). Its definition
is based on a sedimentologic, stratigraphic and biostrati-
graphic study of the exposures described below. These out-
crops were previously considered as part of the Descartes
Fm. (Astorga 1987). However, the detailed study of the
outcrops reveals shallow shelfal characteristics that are
incompatible with an assignment to the deep-water, turbid-
itic Descartes Fm.
Stratotype and reference sections
The holostratotype section is located on the Isla Despensa
(Figs. 2, 4). The base of the 25-m-thick section is situated
on the southern end of the island’s inner beach (Fig. 5g).
The Isla Despensa section is established as the holostrato-
type section for the following reasons: (1) the exposed lith-
ologies are characteristic of the newly defined lithostrati-
graphic unit, (2) the clear stratigraphic relationship with the
underlying Descartes Fm., and (3) the conspicuous facies
difference with the underlying Descartes Fm., as defined
Well-preserved outcrops of the Junquillal Fm. are con-
tinuously exposed on the western shore of Punta Descartes,
and between the northern shore of the Bahia Cuajiniquil
and the southern shore of the Bahia Junquillal (Fig. 2).
Within these exposures, three parastratotype sections are
established in order to additionally characterize the Jun-
quillal Fm. (Figs. 4, 6): (1) a 40-m-thick section located at
the Playa El Coco, (2) a 20-m-thick section located at the
Playa Rajada, and (3) a 7-m-thick section located 115 m
southeast of the Punta Zacate.
Formation boundaries
The lower boundary is equivalent to the Descartes Fm.
upper boundary defined above.
A cryptic upper boundary with the Juanilla Fm. is
tentatively established on the northern Isla Juanilla (N
10°5900.5/W 085°4256.8; Figs. 2, 7a). However, the
Juanilla Fm. rests unconformably on the deeply eroded
Junquillal Fm., in view of its reduced thickness outcrop-
ping on the island (Figs. 4, 7c). The contact has not been
observed directly, but it is deduced from the occurrence of
unconformable outcrops of Junquillal lithologies between
the Descartes and the Juanilla outcrops (Fig. 6j).
Derivation of the name and synonymy
The formation is named after the Bahia Junquillal (Fig. 2).
The synonymy includes the following, previously used
terms: part of Brito Fm., defined by Dengo (1962), Junquil-
lal unit (Baumgartner et al. 1984), and part of Descartes
Fm. (Astorga 1987; Denyer and Alvarado 2007).
Lithology and sedimentary structures
General features The Junquillal Fm. is defined as a silici-
clastic shelf deposit which crops out on the coasts of the
Bahia Junquillal and the Punta Descartes, as well as on
the Isla Despensa and Isla Juanilla (Fig. 2). The forma-
tion consists mainly of medium- to coarse-grained, amal-
gamated sandstones which alternate in places with lenses
and beds of granule to pebble and locally boulder conglom-
erates with a sandy matrix. These lithologies are typically
comprised in large-scale, low-angle to high-angle cross-
stratified beds (Fig. 6). The Junquillal Fm. exhibits numer-
ous fining-upwards, metric tempestites. They consist of a
basal, lag deposit, composed of massive conglomerates
with rip-up clasts resting in general on a scoured surface
and grading upsection into coarse-grained arenites. The
basal lag deposits are overlain by fine-grained arenites
showing hummocky cross-stratification (HCS) and which
can be in turn overlain by very thin-bedded, fine-grained
sand- and siltstones. Large-scale, low-angle cross-stratifi-
cation is observed in the basal coarse-grained deposits of
the tempestites. These features are present in the strato-
type sections (Fig. 4). Pelites are almost absent from the
Junquillal Fm. and correspond to sandy/silty mudstones
when they crop out. Based on the geological map (Fig. 2),
Fig. 7 Geology, structure and stratigraphic architecture of the upper
Oligocene Juanilla Formation. a Facies map of the Isla Juanilla dis-
playing the seven facies recognized in the field and described in the
text (F1 = Descartes Fm.; F2 = Junquillal Fm.; F3F7 = Juanilla
Fm.). The cross-section transect positions are indicated by AA’, B
B’, CC’ and DD’, and shown in b. b Cross sections of the Isla Jua-
nilla. AA’: NE–SW-oriented cross-section (no vertical exaggeration)
of the Isla Juanilla northwestern corner exhibiting a recumbent fold.
BB’: NE–SW-oriented cross section (no vertical exaggeration) of the
Isla Juanilla central part. CC’: NE–SW-oriented cross section (no
vertical exaggeration) of the Isla Juanilla southern part. DD’: NW–
SE-oriented cross section (1.5:1 vertical exaggeration) of the WSW
Isla Juanilla shore. This cross section shows the Isla Juanilla Coral
Reef internal architecture, which consists of six distinct units (U1
U6). c Stratigraphic logs of the six coral reef units (U1U6) depicted
in b (DD’). The fossil content is not exhaustive and only the most
common fossils are depicted. The unit U1 represents the oldest part
of the Isla Juanilla Coral Reef
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the apparent thickness of the Junquillal Fm. is comprised
between 2000 and 2200 m. The described sections share
common lithological characteristics, therefore microfacies
details are given only for one locality, the Playa El Coco
section (Fig. 4).
Isla Despensa section (holostratotype) The Isla
Despensa (Fig. 2) displays a 20-m-thick section consist-
ing of cm-bedded, fine- to coarse-grained arenites alternat-
ing with dm- to m-bedded granule and pebble conglomer-
ates (Fig. 4). The base of the section is made of 10 m of
fine- to medium-grained arenites which show occurrences
of HCS. These arenites are unconformably overlain by a
fining-upward, 5-m-thick tempestite which grades from a
basal pebble conglomerate into a hummocky cross-strati-
fied arenitic bed at the top. The tempestite is unconform-
ably overlain by a massive, 8-m-thick succession of met-
ric, stacked channel deposits made of granule and pebble
Playa El Coco section The Playa El Coco (Fig. 2) west-
ern shore displays a 40-m-thick section (Fig. 4) composed
of cm- to dm-bedded sandstones. The sandstones corre-
spond to lithic-arkosic wackes and arenites compounded
of moderately to moderately well sorted, subrounded to
rounded elements. Volcaniclastic lithoclasts represent
generally the dominant rock constituent (25–45 %, 0.5–
1.5 mm), although they are almost absent from the most
fine-grained lithologies. Scarce sedimentary lithoclasts
include sand- to pebble-sized clasts of radiolarite, pelagic
limestone and platform limestone. In few beds, pebbles
and rare cobbles appear in ungraded, granule conglomer-
ate lenses. Detrital minerals correspond to fine- to coarse-
grained sands. Anhedral to subeuhedral plagioclase crystals
are the most common mineralogical elements (15–30 %).
Anhedral opaque minerals also account for a significant
rock fraction (10–12 %). Green pyroxene and amphibole
minerals correspond to a minor component (<5 %). Bio-
clasts are rare and include mm- to cm-sized LBF, mollusk
shells and corals. The sample COC5 yielded datable LBF
(Fig. 9a). Wood-rich layers are commonly encountered
(Fig. 6c). Sedimentary structures are observed in several
places and comprise HCS, swaley cross-stratifications
(SCS) and high-angle cross-stratifications. HCS and SCS
developed in fine-grained sandstones and are composed of
low-angle, centimetric, curved beds. The angle between the
beds and erosional truncations is low, with the overlying
beds being parallel to the erosional surface. The structures
have amplitudes of 0.3–0.4 m and wavelengths between
0.9 and 2 m. A 3-m-thick conglomerate deposit rests with
an erosive base on finer lithologies in the upper part of the
section. Well-preserved nummulitids and lepidocyclinids as
well as cm-sized bivalve shells are widespread in the con-
glomerate matrix.
Punta Zacate area Cross-stratification features are vis-
ible in the outcrops located 115 m southeast of the Punta
Zacate (Fig. 2). There, large-scale, planar cross-stratifica-
tion structures developed in coarse arenites and matrix-
supported, polymict, pebble conglomerates interpreted
as stacked channel fill deposits (Figs. 4, 6b). The base of
the 5-m-thick coset consists of a structureless, lag deposit
containing cobbles and rare boulders. A part of the coset
locally overlies a centimetric bed which shows occurrence
of wavy bedding structures. Generally, the conglomeratic
channel deposits are stacked and no finer-grained depos-
its are preserved. The dm- to m-thick sets are separated
by truncation surfaces and the different sets show variable
strata angles between them, sometimes with opposite dip
directions. Strata can be followed laterally over distances
up to 10 m and are generally well defined in cases where
different grain sizes alternate. The channel fill deposits are
overlain by a 2-m-thick tempestitic deposit made of a basal,
massive, lenticular conglomeratic bed with an erosional
base which grades into fine-grained, cm-bedded sandstones
presenting HCS structures.
SCS structures are present 300 m southeast of the Punta
Zacate (Figs. 2, 6g). In one locality situated 390 m south-
east of the Punta Zacate (Fig. 2), wavy bedding structures
appear within a 40-cm-thick bed made of mm- to cm-sized
alternations of sandy and silty sediments (Fig. 6i).
Playa Rajada section Clast-supported, polymict con-
glomerates crop out in an 18-m-thick section located on
the northwestern Playa Rajada (Figs. 2, 4, 6k). These met-
ric beds are composed of a cobble-sized, moderately sorted
Fig. 8 Field photographs and thin-section photomicrographs of the
upper Oligocene Juanilla Formation. a Close-up view of the Lepi-
docyclina-encrusting coral bindstone facies (F7) (N 10°5859.0/W
085°4305.0). Scale bar 5 cm. b Thin-section photomicrograph of
the Lepidocyclina-encrusting coral bindstone facies (F7) shown in a.
Scale bar 5 mm. c Outcrop exhibiting the rhodolith-coral rubble rud-
stone facies (F5) overlain by the nodular coral framestone facies (F6)
(N 10°5848.9/W 085°4248.8). The contact is highlighted with a
dashed line. Hammer length 21 cm. d Thin-section photomicrograph
of the nodular coral framestone facies (F6). Scale bar 5 mm. eh
Close-up views of the rhodolith-coral rubble rudstone facies (F5).
Same outcrop as in c. e Coral rubble consisting of branching coral
debris. Scale bar 2 cm. f Coherent coral framework within the rubble.
Scale bar 5 cm. g, h Centimetric warty rhodoliths displaying sphe-
roidal (g) and ellipsoidal (h) shapes. Scale bars 1 cm. i Thin-section
photomicrograph of the rhodolith-rich part of the rudstone facies
(F5) shown in c. Scale bar 5 mm. j Sandy coral rudstone facies (F4)
exhibiting encrusting and domal corals embedded in a detrital wacke-
stone matrix. Same outcrop as in c. Scale bar 20 cm. kp Lepidocy-
clina packstone facies (F3) (N 10°5854.9/W 085°4249.9). k Mod-
erately packed accumulation of Lepidocyclina tests aligned parallel to
the bedding. GPS device length 14 cm. l Close-up equatorial view of
the discoidal Lepidocyclina tests shown in k. Scale bar 5 cm. m, n
Bivalve shells. m Scale bar 1 cm. n Note the double meter stick for
scale. o Centimetric horn-shaped, solitary corals. Scale bar 2 cm. p
Thin-section photomicrograph of the Lepidocyclina packstone facies
(F3) displaying lepidocyclinids, bivalve shell debris and solitary coral
fragments. Scale bar 5 mm
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matrix which contains randomly scattered boulders (diam-
eters: 0.3–0.8 m). The majority of the clasts originate from
various differentiated volcanic lithologies. Scarce sedimen-
tary clasts correspond to shallow-water limestones often
containing lepidocyclinids, asterocyclinids, Dicytioconus
sp., and few miliolids, such as Quinqueloculina sp. (CR14-
10; Fig. 9). The clast-supported, polymict conglomerates
alternate with arenites and pebble-sized, matrix-supported
Isla Juanilla The Junquillal Fm. is only observed as
boulders of arkosic-lithic arenites fallen from the northern
slope of the island and which contain a few fragments of
LBF, red algae and echinoderms (facies F2; Figs. 6j, 7). Its
thickness on the island is probably less than 10 m (Figs. 4,
7). The sedimentary contact with the thin-bedded turbidites
(facies F1) of the underlying Descartes Fm. has not been
directly observed.
Other sections HCS developed in the most fine-grained
deposits of the Bahia El Jicote and Bahia Junquillal out-
crops (Figs. 2, 6h). Large-scale, low-angle cross-stratifica-
tions are present in the outcrops of the Bahia Jobo shores
(Fig. 2). In both localities, these structures developed in
medium to coarse-grained arenites comprised in subma-
rine dune sets which present metric heights and pluri-met-
ric widths (Fig. 6l, m). Carbonate beds rich in LBF have
been encountered in the outcrops of the Bahia Junquillal
southwestern shore (1529, N 10°5749.9/W 085°4147.6;
Fig. 10).
Age of the Junquillal Formation
During our fieldwork, several LBF-bearing samples
were collected (Figs. 9, 10). The sample COC5 from the
Playa El Coco contains an individual open spiral form
as in Operculinoides Hanzawa (Fig. 9). In the sample
1529 (Bahia Junquillal, N 10°5749.9/W 085°4147.6;
Fig. 10), we determined Lepidocyclina (Nephrolepidina)
chaperi Lemoine and Douvillé, Lepidocyclina (Neph-
rolepidina) macdonaldi Cushman, Lepidocyclina ariana
Cole and Ponton, Lepidocyclina pustulosa H. Douvillé,
Fabiania A. Silvestri in fragments, Nummulites willcoxi
Heilprin, Nummulites striatoreticulatus L. Rutten, Astero-
cyclina asterisca (Guppy), Asterocyclina sp., Discocy-
clina sp. and fragments of Heterostegina sp. Even if some
of these species have their first occurrence in the middle
Eocene, the presence of Lepidocyclina chaperi indicates a
Priabonian age according to Frost and Langenheim (1974),
Butterlin (1981), and Robinson and Wright (1993). More
recently, Molina et al. (2016) concluded that some forms
may have survived until the early Oligocene (see remarks
under age of the Descartes Fm. above). On the other
hand, reworking of LBF from Priabonian strata cannot be
excluded. Therefore the Junquillal Fm. may reach into the
earliest Oligocene.
Depositional environment
The facies succession observed from the top of the
Descartes Fm. through the Junquillal Fm. (Figs. 4, 5, 6)
is characterized by shallowing-upward conditions that
recorded a rapid regression. The lithologies of the Bahia
Cuajiniquil northern shore and the roadside outcrops west
of the Cuajiniquil village (Fig. 2) clearly show the rapid
transition from Descartes Fm. distal facies to the shallow
shelf facies of the Junquillal Fm. This transition occurs
within a few meters and is accompanied by the input of rel-
atively coarser, platform bioclast-rich sands and platform-
derived pebbles preceding the onset of shelf, siliciclastic
The different sections of the Junquillal Fm. are charac-
terized by amalgamated storm deposits that take the form
of fining-upward tempestites. The tempestites comprise
three facies that correspond, from base to top, to poorly
stratified basal conglomerates, hummocky-stratified fine-
grained arenites and flat-bedded silts which are rarely pre-
served. This facies succession records one or several storm
events of which the peak energy is illustrated by the depo-
sition of the conglomeratic lag bed that may be the result
of multiple winnowing events. Lag deposits are overlain by
HCS structures that formed during the waning stage of the
last event. The overlying thin-bedded silts correspond to
suspension fallout during fair weather conditions occurring
between the storms. The HCS and SCS structures observed
in the Junquillal Fm. are indicative of storm-influenced,
shelf environments (Dott and Bourgeois 1982; Walker et al.
1983; Duke 1985; Cheel and Leckie 1993; Dumas and
Arnott 2006). Also, the development of large-scale, low-
angle to high-angle cross-stratifications suggests the depo-
sition under the influence of multi-directional currents and
Fig. 9 Larger Benthic Foraminifera of the Descartes (br) and Jun-
quillal Formations (a, sz). Figures are light microscopic images, and
optical cathodoluminescence images (CL), where indicated. Scale
bar is 2 mm unless indicated otherwise. ac Nummulites floridensis
Heilprin. dg Amphistegina parvula (Cushman). g CL. Scale bar
200 µm. h Nummulites trinitatensis (Nutall). il Lepidocyclina pus-
tulosa H. Douvillé. j, l CL. Scale bar 200 µm. m, n Lepidocyclina
(Pliolepidina) cf. pustulosa. n CL. Scale bar 200 µm. o. Lepidocy-
clina chaperi Lemoine and Douvillé (stellate form). p, q Asterocy-
clina cf. asterisca. q Detail under CL showing the equatorial layers
in a ribbed fragment. Scale bar 200 µm. r Lepidocyclina ariana Cole
and Ponton. su Miliolids. Scale bar 10 µm. s Quinqueloculina sp.
t Miliolid. u Triloculina sp. v Victiorella sp. w Nummulites striato-
reticulatus. x Operculinoides kugleri. y, z Dictyoconus sp. y apical
part of Dictyoconus sp. Scale bar 100 µm. a Sample COC5 (Playa El
Coco); b, c Sample MZO1 (Playa Manzanillo); dr Sample CR14-07
(Playa Manzanillo); su, y, z Sample CR14-10 (Playa Rajada); vx
Sample 1529 (Bahia Junquillal). See Figs. 2 and 4 for locations and
GPS coordinates
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waves in shoreface conditions (Ricci Lucchi 1995; Dumas
and Arnott 2006; Santra et al. 2013).
The metric, stacked conglomeratic deposits observed at the
Punta Zacate (Figs. 2, 4) are interpreted as the results of lateral
migration of channels which produced beds presenting planar
cross-stratifications with opposite bed dip angles (Fig. 6b).
The local occurrence of boulder-bearing, clast-supported
conglomerates (Playa Rajada, Punta Descartes; Figs. 2, 6k)
is interpreted as the result of submarine, rock avalanche
deposits in the vicinity of a river or delta mouth. Prior to
the avalanching process, the clasts were rounded in a fluvi-
atile and/or beach environment and subsequently deposited
in the nearshore area, close to a river mouth (small Gilbert
type delta?). Given the absence of matrix and the poor sort-
ing, the cobbles and the boulders were finally transported
and mixed in a rock avalanche by gravity, possibly trig-
gered by storm events or flood events in the river system, as
well as earthquakes.
The general absence of mud- to silt-sized particles in the
Junquillal Fm. is the consequence of continuous influence
of high-energy currents and/or wave action. Consequently,
the finest sediments remained suspended and were win-
nowed towards more distal, offshore settings. In the rare
cases where fine-grained sediments have been preserved,
it is possible to observe wavy bedding structures (Fig. 6i),
which are typical in environments where higher and lower
energy conditions alternate.
In conclusion, the studied tempestite facies suggest that
the Junquillal Fm. deposited at or above the storm-wave
base (SWB) in a proximal shelf (Myrow 1992; Einsele
2000). Indeed, it is not possible to ascertain the depths
reached by the upper Eocene storms. In modern settings,
the depth of the SWB can be very variable. However, it
does not exceed 200 m (Immenhauser 2009). Regarding the
modern eastern Pacific, Clifton (1988) estimated a depth of
150 m for the SWB on the central California shelf.
The proposed facies interpretation is in contradiction
with the previous studies of the Punta Descartes-Bahia Jun-
quillal area published by Astorga (1987, 1988), Winsemann
and Seyfried (1991) and Winsemann (1992). The men-
tioned authors interpreted these deposits as multicycle fan
complexes deposited in basinal environments. Their facies
description pointed out the presence of channelized lobes
interstratified with channel overbank deposits and basinal
plain turbidites. However, deep-water facies have not been
observed in the area coinciding with the Junquillal Fm.
Juanilla Formation: a new lithostratigraphic unit
The Juanilla Fm. corresponds to an ancient carbonate plat-
form exposure which has not been described or mentioned
in the literature. This exposure represents a geographi-
cally limited, formal lithostratigraphic unit, given that it
is restricted to the Isla Juanilla (~0.2 km2; Figs. 2, 7). The
Juanilla Fm. does not present a lateral continuity mappable
at a regional scale. Therefore, the description of the Juanilla
Fm. does not follow the same scheme as the previously
described formations. The type area, the type section and
the reference sections are comprised within the geographi-
cal extent of the Isla Juanilla (Figs. 2, 7). The lower bound-
ary of the formation is equivalent to the upper boundary of
the Junquillal Fm. (defined above).
On the Isla Juanilla, field investigations permitted to
distinguish the three formations discussed here compris-
ing seven different facies, with an estimated thickness of
approximately 50 m (Figs. 7, 8). The facies F1 is part of
the uppermost Descartes Fm., whereas the facies F2 is part
of the Junquillal Fm. These facies have been described in
the previous parts (Figs. 5, 6). The facies F3 to F7 compose
the Juanilla Fm. (upper Oligocene; Figs. 7, 8). The facies
F3-F4-F5 are considered as a pre-reef, platform facies asso-
ciation. The facies F6-F7 are defined as the Isla Juanilla
Coral Reef.
Stratigraphic architecture of the Isla Juanilla Coral Reef
The dominant facies (F6) of the Juanilla Fm. corresponds
to well-bedded, nodular, coral framestones (Figs. 7, 8c).
The facies F6, along with the facies F7, represent the Isla
Juanilla Coral Reef which deposited over a substratum
made of the facies F3 to F5. The Isla Juanilla Fm. uncon-
formably overlies the Junquillal and Descartes fms. (facies
F1 and F2; Fig. 7b). The reef facies F6 occurs in six dif-
ferent stratigraphic units (U1 to U6; Fig. 7). In addition,
Fig. 10 Larger Benthic Foraminifera of the Junquillal Formation. All
figures are light microscopic images. Scale bar is 2 mm for all fig-
ures, except for b, which is 1 mm. a Asterocyclina cf. marianensis
(Cushman). Vertical oblique section showing 18 lateral chambers on
each side of the median plane. b Fragments of the ogival equatorial
chambers probably of Lepidocyclina pustulosa (H. Douvillé). c Lepi-
docyclina pustulosa (H. Douvillé), central vertical section of megalo-
spheric form of a small specimen. d Discocyclina sp. Vertical oblique
section of megalospheric specimen with 12 lateral chambers arranged
in regular tiers fragment showing the ribbed form in the distal part.
e, f Lepidocyclina (Lepidocyclina) pustulosa (H. Douvillé). e Central
vertical section of megalospheric form with a pronounced umbo and
thin flange. f Central vertical section of megalospheric form showing
the embryonic chambers in an inflated specimen. g Discocyclina sp.
h Lepidocyclina ariana Cole and Ponton. Eroded vertical, oblique
section of a megalospheric form. The equatorial layers in the vertical
plane are observed near the protoconch side. The lateral chambers are
straight and thick. i Lepidocyclina cf. chaperi. j Lepidocyclina (Lepi-
docyclina) macdonaldi Cushman. Vertical section of megalospheric
form showing the embryonic chambers, the pillars and the thick open
roofs and floors of lateral chambers. k Lepidocyclina chaperi Lem-
oine and Douvillé. Oblique section of saddle-shaped specimen. a
Sample CR14-10 (Playa Rajada); bk Sample 1529 (Bahia Junquil-
lal). See Figs. 2 and 4 for locations and GPS coordinates
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the facies F7 is present only in the unit U1 (Fig. 7). The
sedimentary structure of the six units is well exposed in the
cliffs of the western-southwestern coast (Fig. 7a). There,
a 700-m-long outcrop gives an insight to the reef growth
(profile D–D’; Fig. 7b). In the central part of this sec-
tion, the reef growth occurred towards the S–SE, through
a series of steeply inclined, sigmoidal clinoforms. These
clinoforms are distributed in four distinct, prograding units
(U2 to U5) which onlapped an initial, slumped unit (U1,
reef core). The sedimentary structure of the oldest unit is
not easily observed on the northwestern shore as this zone
of the island is strongly folded and corresponds to a dip
slope. The first prograding unit (U2) is 6 m thick (topset
dip angle = 30°; foreset = 60°). The following prograd-
ing unit (U3) is 80 m long and 20 m thick (topset = 20°;
foreset = 40°). The last two prograding units (U4 and U5)
are ~5 m thick, each, and exhibit well-developed sigmoi-
dal clinoforms (foreset = 40°). The unit’s U4 foreset and
the unit’s U5 bottomset downlap the unit’s U3 bottomset.
Finally, the southern part of the coral reef section (230 m
long, 20 m thick) is included within an aggrading unit (U6)
which onlaps the unit U5. In summary, the oldest rocks
of the Isla Juanilla Coral Reef are located in the western-
northwestern part of the island. This area corresponds to
the initial unit (U1) from which the reef developed through
four prograding units (U2 to U5) and finally one aggrading
unit (U6).
Facies F3: Lepidocyclina packstone
A foraminiferal limestone (F3) is observed solely in a small
outcrop of ~25 m2 located on the eastern part of the Isla
Juanilla (Fig. 7a). The limestone presents a pale blue-green
fresh surface which derives from the mud-sized matrix and
progressively alters to a brown patina. The outcrop shows
a moderately packed accumulation of lepidocyclinids (35–
60 % of the rock volume; Fig. 8k) which displays undu-
lated, discoidal tests with mm- to cm-sized diameters (up
to 6 cm) and thicknesses smaller than 4 mm (Figs. 8l, 11).
The thickness/diameter ratio of the measured tests is com-
prised between 0.05 and 0.12. The foraminifers are gener-
ally accumulated with their major axis aligned parallel to
the bedding. The large lepidocyclinid tests, especially their
rims, are easily broken due to their small thickness, hence
they are not always entirely preserved. Apart from a large
proportion of LBF, the limestone contains well-preserved
centimetric tests of conical-shaped solitary corals and a few
Pectinid and other bivalve shells (Fig. 8m–o). The solitary
corals are randomly scattered in the sediment and do not
occur in growth positions (Fig. 8o). Rare hermatypic corals
and very flat, irregular echinoids (“sand dollar”) also occur.
The relation between the foraminiferal limestone and the
overlying facies cannot be observed because of a 1 m-gap
in the outcrop.
The microfacies analysis allows the following obser-
vations. Smaller benthic foraminifers are frequent in the
matrix (3–5 %, 0.2–0.6 mm). Mollusk shells and debris
are common constituents (5–9 %, 0.5–9 mm). Subangular,
poorly sorted calcitic debris are widespread (6–9 %, 0.1–
0.75 mm). They probably represent fragments of LBF and
mollusk tests. Echinoderm plates and spines are present as
fragments (3–4 %, 0.2–0.5 mm). Sub-millimetric, coral-
line red algae fragments occur (<5 %). Pale green glauco-
nite frequently fills in the different tests and appears as fine
clayey material which is associated to anhedral, opaque
minerals (pyrite, magnetite). Individual grains of glauconite
are absent from the matrix. A detrital component is notice-
able and corresponds to anhedral to subanhedral, often cal-
cified plagioclase crystals (2–8 %, 0.1–0.7 mm).
The different constituents are enclosed in a brown-
green micrite which makes up to 30 % of the foraminif-
eral limestone. Rare nannofossils could be extracted from
this matrix (see below). This LBF-rich facies is defined
as a Lepidocyclina packstone with a biolithoclastic matrix
(Fig. 8p).
Facies F4: sandy coral rudstone
Sandy coral limestones (F4) crop out on the eastern part
of the island and are approximately 3 m thick (Figs. 7, 8j).
This facies could not be observed in thin-section, due to its
highly brittle nature. Although it contains detrital minerals,
the brown, mud-sized matrix of the sandy coral rudstone
appears to be more carbonate-rich when compared to the
one described for the Lepidocyclina packstone. The detri-
tal fraction is composed of sand-sized feldspars and opaque
minerals. Millimeter-sized bivalve shells are common in
the matrix. The proximal part of the bed is progressively
enriched in mm- to cm-sized, hermatypic coral debris (rub-
ble), from the bottom to the top. In the distal part of the bed
(to the SE), the coral communities are preserved in origi-
nal growth position and appear as dm-sized, encrusting to
sometimes domal, moderately packed specimens enclosed
in a soft wackestone matrix.
Fig. 11 Larger Benthic Foraminifera of the Juanilla Formation. Fig-
ures are light microscopic images, and optical cathodoluminescence
images (CL), where indicated. Scale bar is 2 mm, unless indicated
otherwise. a Lepidocyclina (Nephrolepidina) vaughani Vaughan and
Cole. b Miolepidocyclina cf. panamensis. Scale bar 1 mm. c Mio-
gypsinoides sp. d, e Helicosteginoides sp. Scale bar 200 µm. e CL. f
Lepidocyclina foresti Vaughan. g Lepidocyclina (Eulepidina) favosa
Cushman. h Lepidocyclina canellei Lemoine and Douvillé. CL.
Scale bar 200 µm. i Lepidocyclina undosa Cushman. j Lepidocy-
clina (Eulepidina) favosa Cushman. af, h, i Sample IJ5 (facies F3;
Figs. 7, 8); g, j Sample IJ18 (facies F7; Figs. 7, 8)
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Facies F5: rhodolith-branched coral rubble rudstone
The rhodolith-branched coral rubble rudstone (F5) rep-
resents a 1.5-m-thick deposit, which is discontinuously
exposed below the main nodular coral facies and overlies
the sandy coral rudstone (Figs. 7, 8c, e–h). It is mainly
made of mm- to cm-sized, closely packed rhodoliths (40–
70 %). The forms are concentric, spheroidal to ellipsoidal
to sometimes columnar rhodoliths, sometimes covered by
encrusting Foraminifera and embedded in a muddy matrix
(Fig. 8g, h). Reworked debris of branched corals (rubble)
are present in the upper part of the rudstone in 0.5-m-thick
lenses (Fig. 8c). Coral debris appear like intermingled
tubes (60–70 % of the bed volume) with lengths up to 5 cm
and correspond to dismembered branches of an originally
arborescent framework (Fig. 8e). Coherent pieces of the
framework are also observed (Fig. 8f). Although the coral
debris are sometimes encrusted by red algae, a pervasive
algal binding as well as rhodoliths are absent from the rub-
ble. The latter is bound by diagenetic cements. The rhodo-
lith-coral rubble facies is considered as a rudstone, as the
rhodoliths and the rubble do not build a rigid framework
and were possibly locally transported/reworked.
The microscopic observation of the rhodolitic part of
the rudstone facies allows for a more detailed description
of the red algae. The mm- to cm-sized rhodoliths display
a thick concentric structure developed around a nucleus,
with crusts of Melobesioid crustose red algae (Fig. 8i). The
monospecific, millimetric algal crusts show the presence
of conceptacles, encrusting Foraminifera and sometimes
trapped matrix. Rhodoliths smaller than 1 cm in diameter
(elongated forms) adopt the shape of the nucleus, while
the ellipsoidal to spheroidal, larger forms display warty
crusts that do not usually outline the nucleus shape. For the
smaller forms, the crust thickness is roughly equivalent to
the nucleus radius. When it exists, the nucleus corresponds
mainly to mm-sized debris of corals and in minor propor-
tions to matrix intraclasts and unrecognizable calcitic
clasts. This rhodolith-rich facies shows in places a nodu-
lar fabric which is related to a pressure solution process.
Effects of this process are partly localized on the rims of
the rhodoliths, which display ferruginous dissolution seams
at the boundaries with a microspar matrix (sensu Folk
1965). The latter also develops dissolution seams high-
lighted by ferruginous zones.
Other bioclasts (15 %) are present in the rudstone
matrix. Millimeter-sized gastropod shells commonly occur
in longitudinal and transverse sections. Smaller benthic
Foraminifera are scarce (0.25–1 mm). Altered fragments
of echinoderm plates and spines display sizes from 0.2 to
2 mm. Finally, the least common skeletal grains correspond
to mm-sized fragments of Udoteacean green algae and sub-
millimetric fragments of bivalves. The detrital component
is almost absent and consists of anhedral to subanhedral
plagioclase crystals (0.25–0.75 mm).
Facies F6: nodular coral framestone
The coral framestone facies (F6) crops out all over the Isla
Juanilla in nodular beds which take the shape of prograding
clinoforms in the central area of the island (units U2 to U5;
Figs. 7, 8c). The maximum exposed thickness of this facies
does not exceed 20 m, but may be thicker in the central part
of the island (see transect D-D’, Fig. 7). The coral nodu-
lar framestones display a beige fresh surface and are pri-
marily made of massive corals associated to red algae. The
nodular aspect of the decimetric beds (0.2–0.5 m) is due to
the abundant presence of coral colonies (40–65 %) and is
particularly visible on the southwestern part of the island.
The bedding is not developed in the first 3 m of the facies,
which appear as a structureless, massive lithology. The cor-
als show recrystallized, sometimes well-preserved, sphe-
roid to domal shaped colonies with decimetric sizes (up to
0.8 m). We have distinguished a few species and genera in
the field (Fig. 12), which are listed below. Millimeter-sized
red algae clasts are commonly observed in the coral frame-
stone matrix.
Microscopically, the corals are observed as mm- to cm-
sized corralite frameworks, as well as reworked debris
(Fig. 8d). The corals are set in a dark grey, poorly sorted
matrix showing characteristics of a packstone, which also
fills in the coral septas. The original mineralogical compo-
sition of the coral skeletons disappeared since they display
drusy and coarse sparite in the studied sections. Melobe-
sioid and Sporolithacean coralline red algae are present in
the micritic matrix as well-rounded clasts (0.15–10 mm,
15 %; up to 40 % in some grainstones). Other bioclastic
elements (15 %) are present in the nodular coral frame-
stone. Millimeter-sized gastropod shells commonly occur
in longitudinal and transverse sections. Smaller benthic
Foraminifera (0.25–1 mm) and altered fragments of echino-
derm plates and spines (0.25–2 mm) are generally scarce.
The least common bioclasts correspond to mm-sized frag-
ments of Udoteacean green algae and sub-millimetric
bivalve fragments. The detrital component is almost unno-
ticeable and consists of anhedral to subanhedral crystals of
plagioclase (0.2–0.4 mm). The coral-rich facies can be con-
sidered as a coral framestone with a bioclastic packstone
matrix, as it appears as a three-dimensional in situ rigid
framework which supports the rock. This facies displays
the most diverse coral fauna of the Juanilla Fm.
Facies F7: Lepidocyclina-encrusting coral bindstone
This 5-m-thick nodular facies overlies the coral frame-
stone facies and crops out in the overturned limb of the
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recumbent fold (slump) located in the northwestern Isla
Juanilla (Fig. 7). The lithology consists mainly of cm-thick,
elongated, closely packed encrusting corals (Hydnophora
sp.) which can reach lengths up to 50 cm (Fig. 8a, b), and
a few massive, decimetric corals of the genus Caulastrea
(Fig. 12c). Some bindstone beds display cm-sized, curved
Lepidocyclina tests (25–30 %, IJ18; Figs. 8a, b, 11g, j).
The corals and the lepidocyclinid tests are commonly
encrusted by coralline red algae (0.75–1 mm thick) and
show evidence of bio-erosion by boring organisms. The
packstone matrix of the bindstone contains other bioclasts
(20–25 %, 0.15–1 mm) which correspond to smaller ben-
thic Foraminifera and poorly sorted fragments of crustose
red algae, echinoid spines and corals. The bioclasts are
embedded in a microspar matrix (crystals 10–40 μm in
size; Fig. 8b).
Age of the Juanilla Formation
Larger Benthic Foraminifera The beds rich in LBF (facies
F3, sample IJ5; Figs. 7, 8k, 11) yielded up to 90 % of Lepi-
docyclina sp. consisting commonly of very thin, up to 6 cm
in diameter, microspheric forms. Some of these have undu-
lations like the forms described by Vaughan (1919) from
the Oligocene of Trinidad, such as Lepidocyclina undu-
lata Cushman. Among the macrospheric forms, we dis-
tinguished many nephrolepidine and eulepidine embryon
types. The sample IJ5 contains Lepidocyclina (Nephrole-
pidina) vaughani Cushman, Lepidocyclina foresti Vaughan,
Lepidocyclina undosa Cushman, Lepidocyclina (Eule-
pidina) favosa Cushman, Lepidocyclina canellei Lemoine
and Douvillé and Miolepidocyclina cf. panamensis in few
oblique sections. We also determined Miogypsinoides sp.,
in which we could not count the number of periembryonic
chambers by lack of equatorial sections. Hence, we could
not distinguish between Miogypsinoides butterlinus and
Miogypsinoides complanatus. Several of these taxa indicate
a late Oligocene age. This sample also contains questiona-
ble Helicosteginoides sp., identified under cathodolumines-
cence by its nepionic stage, and considered to be restricted
to the Eocene. However, this form shows corrosion that
suggests reworking. Forms of the same family, such as Hel-
icosteginoides soldadensis, Helicocyclina paucispira, Heli-
costeginoides intermedius have been found in the Cipero
Formation of Trinidad known from the Oligocene (Kugler
2001). These forms were also considered as reworked by
Caudri (1975) because they are until now only known from
the Eocene of the Caribbean.
The sample IJ18 (facies F7; Figs. 7, 8a, 11) contains
Lepidocyclina (Eulepidina) favosa Cushman and Lepido-
cyclina canellei Lemoine and Douvillé. This association of
LBF indicates also a late Oligocene age (Cole 1952, 1961;
Butterlin 1962; Caudri 1996).
Hermatypic corals Several sites on the east side and at
the NW-tip of Juanilla Island have yielded well-preserved
hermatypic coral species (Fig. 12). Siderastrea conferta
Duncan (Fig. 12a, b), was described from the upper Oli-
gocene Antigua Fm. of Antigua and Barbuda (Lesser Antil-
les; Vaughan 1919). This species has also been described
from the lower Oligocene of Jamaica (Stemann 2004). This
seems to be the oldest occurrence. Antiguastrea cellulosa
Duncan (Fig. 12d, e) has been also described form the
upper Oligocene Antigua Fm. (Frost and Weiss 1979). It
is also present in the lower Oligocene of Jamaica and His-
paniola (Vaughan et al. 1921; Stemann 2004). We found at
least two species of Hydnophora (Fig. 12g–i). Today, this
genus is restricted to the Indian Ocean and Australia. It is
so far unknown from the American margin. It ranges, how-
ever, from the Mesozoic to Recent. In particular, it has been
described form the Eocene and Oligocene of the Caribbean.
The genera Caulastrea (Fig. 12c) and Diploastrea are typi-
cal of the modern Indopacific realm, but were present in the
Caribbean until the Pleistocene. In summary, the two iden-
tified coral species confirm a (possibly late) Oligocene age
of the Juanilla Fm.
Nannofossils We have extracted scarce calcareous nan-
nofossils from the mud-sized matrix of the Lepidocyclina
packstone (F3). Several species were determined (written
communication: Simonetta Monechi, University of Flor-
ence). Discoaster tanii and D. deflandrei range through
the Eocene–Oligocene, whereas D. lenticularis, D. sublo-
doensis and D. barbadiensis seem to be reworked from the
underlying Eocene formations. Unfortunately, no nanno-
fossils characteristic of the Oligocene could be found, per-
haps due to the paralic conditions at that time.
Reef growth
General features
Tectonic uplift that was responsible for the erosion of much
of the underlying Junquillal Fm. gave way to moderate
subsidence, creating accommodation space for reef growth
during a time of an overall constant eustatic sea-level (Haq
et al. 1987; Haq and Al-Qahtani 2005; Miller et al. 2005).
A 4th–5th order glacio-eustatic sea-level rise, coinciding
with the Milankovitch frequency band (Tucker 1993; Ein-
sele 2000), could also have triggered reef growth, but its
preservation implies at least moderate subsidence.
The unconformable onset of photozoan carbonate sedi-
mentation on the substantially eroded uplifted high (facies
F1–F2) could not be clearly observed due to the lack of
outcrops (Fig. 7). It occurred after a time of erosion and/or
non-deposition encompassing at least the early Oligocene.
However, the onset of reef building in the youngest coral
reef unit (U6) has been described with more detail and is
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noticeable in the shallowing-upward transition from the
facies F3–F4–F5 to the facies F6 (Figs. 7, 8). As suggested
in the U1 stratigraphic column (Fig. 7c), we admit that
the shift from the uplifted siliciclastic shelf deposits (F2,
Junquillal Fm.) to the reef facies (F6) possibly occurred
trough the deposition of the facies F3–F4–F5. It is not clear
whether the substrate underlying the coral reef facies F6 is
similar for the different units (U1–U6). The facies F3–F4
represent platform sediments that deposited prior to the
reef core unit (U1), in a rather low-energy, siliciclastic-
influenced environment. Subsequently, the coral reef pro-
graded and aggraded on these platform deposits, with the
deposition of the units U1 to U6. The unit U6 possibly rep-
resents the evidence for such a process (Figs. 7, 8c, d).
The period over which the Isla Juanilla Coral Reef grew
cannot be estimated with the dated fossils (low time reso-
lution). Although the exposure of the Juanilla Fm. may
be incomplete, the Isla Juanilla Coral Reef presents a size
which is comparable to some Holocene fringing reefs.
Holocene coral reefs of similar sizes commonly con-
structed most of their framework within periods shorter
than 8000 years (Kennedy and Woodroffe 2002; Montag-
gioni 2005). In the Caribbean realm, Holocene reefs grew
at mean rates of 6 m/1000 years, which is comparable to
growth rates of pre-Holocene reefs exposed around the
world (Dullo 2005).
Pre-reef stage 1: calm water, offshore conditions (facies
F3: Lepidocyclina packstone)
The first phase of carbonate platform development is
marked by the colonization of the uplifted substrate by
an oligophotic fauna (LBF and solitary corals). The facies
F3 is also characterized by the occurrence of solitary cor-
als and abundant presence of mud (Fig. 8o), which indi-
cates low light/low energy conditions. The accumulation of
well-preserved, bed-parallel Lepidocyclina tests (Fig. 8k)
is indicative of a low-energy environment of deposition.
Moreover, the well-preserved, delicate tests have appar-
ently not suffered much transport and have accumulated
in situ, forming a 7-m-thick bed. This is also the case for
the other large bioclasts, which have not experienced any
significant breakage. The poorly sorted fragments of mol-
lusks and lepidocyclinids probably originate from the
in situ destruction by predators.
This environment was under the influence of a slight
terrigenous input, highlighted by the presence of angu-
lar, sand-sized plagioclase grains and a clay-rich micritic
matrix. Coral reef-derived bioclasts are absent, implying
that there was no connection with a reefal environment.
Glauconite growth in the porosity of bioclasts in association
with pyrite is thought to have been favored by the presence
of organic matter, acting as a reducing micro-environment
(Huggett and Gale 1997; Kelly and Webb 1999; Chafetz
and Reid 2000; Baldermann et al. 2012). The pale blue-
green fresh surface of the rock suggests fine-grained
impregnation of the matrix by glauconite, although it is not
clearly visible in thin-section. The depositional paleodepth
of the Lepidocyclina facies is certainly restricted to the
photic zone (Frost and Langenheim 1974; Sartorio and Ven-
turini 1988; Hottinger 1997), as the lepidocyclinids have
probably hosted photosymbionts in their tests (Chapronière
1975; Brasier 1995). Large lepidocyclinids (very low thick-
ness/diameter ratios) could have proliferated in quiet, low-
light environments down to the lower photic zone (Hal-
lock and Glenn 1986; Hottinger 1997). Behforouzi and
Safari (2011) describe a clear morphologic change of the
Lepidocyclina tests, with increasing depth and decreasing
light intensity and hydrodynamic energy. Very thin tests are
characteristic of low light/low energy conditions. This is in
accordance with the presence of numerous solitary corals,
which can settle on soft substrates in turbid, reduced light
conditions (Schuster and Wielandt 1999). Whereas clayey
sedimentation and turbidity prevented the settlement of
larger, hermatypic corals (Sanders and Baron-Szabo 2005).
Such conditions also explain the occurrence of irregular,
infaunal echinoids. These different elements suggest that
the Lepidocyclina facies was deposited in a low-energy,
probably rather mesotrophic environment which was con-
nected to a source of suspended detrital material (Seyrafian
et al. 2011). The low-energy conditions allowed also the
deposition of planktonic organisms (coccoliths) brought
in by open-marine circulation. Nevertheless, it remains
difficult to estimate the water depth of deposition, as the
photic zone could have been reduced due to detrital input
and turbidity.
In the literature, there is a long debate on the origin of
massive LBF accumulations. Hypotheses of in situ growth
and accumulation are opposed to accumulation due to lat-
eral transport by bottom currents and wave action. A nice
overview of the early work is given by Matteucci and
Pignatti (1989) showing Arni’s (1963) nummulite bank
hypothesis as well as Aigner’s (1982, 1985) model of mas-
sive almost monospecific nummulite accumulations, con-
sidered as an interaction of biological and hydrodynamical
processes. Based on the relative abundances of A- versus
B-forms, Aigner (1985) distinguished: (1) thanatocoenoses
and in situ winnowing (A B); (2) selectively winnowed
assemblages enriched in B-forms (A > B); (3) residual
assemblages (B only); (4) allochthonous assemblages (A
only) formed by hydrodynamic transport and settling (e.g.,
Our facies F3 seems to be relatively enriched in B-forms
at the outcrop scale (Fig. 8), but A-forms are well repre-
sented in thin-sections. From this point of view the assem-
blages can be considered as para-autochthonous.
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More recently, Briguglio and Hohenegger (2009, 2011)
and Seddighi et al. (2015) combined laboratory settling
experiments, observations in modern coastal-offshore envi-
ronments of Okinawa (Japan) and modeled hydrodynamic
conditions to understand the relationship between LBF
shape and the hydrodynamic densities and conditions in the
specific habitat. Although these studies mainly dealt with
nummulitids, they may give some hints to the interpretation
of the Lepidocyclina beds (facies F3) of Juanilla Island.
The large, very flat lepidocyclinids have a relatively low
density, a low shape entropy (Hofmann 1994; Briguglio
and Hohenegger 2009), and hence a low settling velocity.
Once settled, they tend to be more easily resuspended by
wave action and transported by lateral currents than ellip-
soidal forms. The conclusion is that facies F3 was formed
in calm waters, essentially beneath the storm weather wave
base. This is confirmed by few broken or abraded forms
and the abundance of clays in the matrix. On the other
hand, the large Lepidocyclina tests, once settled on a cohe-
sive sediment, are less prone to resuspension (Hjulström
diagram; Hjulström 1935). Therefore, water depths derived
from modern observations in sandy, carbonate environ-
ments cannot be directly applied to the Lepidocyclina beds
of Juanilla.
Pre-reef stage 2: shallowing conditions (facies F4: sandy
coral rudstone)
Although rather mesotrophic conditions still prevailed at
this stage of platform sedimentation, the deposition of the
sandy coral rudstone (facies F4; Fig. 7) represents the first
occurrence of hermatypic coral communities. Although the
presence of mud was reduced in comparison with facies F3,
the corals probably grew on a pavement of LBF and other
bioclasts from the previous, turbid, poorly illuminated
environment. This facies change was the result of a shal-
lowing of the platform floor in response to a 4th–5th order
eustatic sea-level drop and the accumulation of LBF-rich
sediments. The shallowing resulted in an increase of water
energy and evacuation of mud towards offshore settings,
thus increasing light penetration.
The facies F4 proximal deposits consist of coral rubble,
whereas the distal ones display low-diversity coral assem-
blages in growth position (Fig. 8j). This facies distribu-
tion suggests that initial coral growth may have been pref-
erentially located in the deeper, less agitated parts of the
uplifted high. The shallowest areas were prone to constant
wave action and periodical storms. These conditions could
have prevented the permanent settlement of corals. The
laminar morphology of the corals in the distal deposits sug-
gests a rather low-energy environment with low turbidity
and limited sediment accumulation. In these reduced light
conditions, the corals produced thin, encrusting shapes to
maximize their light-capturing surface (Wilson and Lok-
ier 2002). However, the low light conditions hindered the
construction of a reef framework. The corals remained
as individual bioconstructions, separated by episodically
deposited, mud-rich sediments. It is not excluded that some
of the corals switched from autotrophic to heterotrophic
modes of nutrition during episodic turbid pulses (Anthony
and Fabricius 2000).
Pre-reef stage 3: shallow conditions (facies F5:
rhodolith-branched coral rubble rudstone)
The subsequent deposition of mud-poor, rubble- and rho-
dolith-rich sediments attest for more oligotrophic condi-
tions. The increasing wave energy permitted the circulation
of clear waters that lasted for most of the following reef
growth. Moderate- to high-energy, euphotic conditions are
recorded by spheroidal to ellipsoidal, concentric rhodoliths
that developed thick, homogeneous crusts (Braga and Mar-
tin 1988; Bosence 1991; Fig. 8g–i). Coral rubble was pro-
duced by the action of waves and storms in water depths
shallower than 20 m (Rasser and Riegl 2002). The rubble
and the rhodoliths originated in biodetrital crests formed
around the shallowest edge of the uplifted high.
Coral rubble (facies F5) is composed of small, almost
monospecific branching forms that colonized the platform
floor (=facies F4), shortly prior to the growth of the Isla
Juanilla Coral Reef (Figs. 7, 8). Initially, the relatively soft
platform floor favored the establishment of low-diversity,
branching forms due to their wide physiological tolerances,
including the ability to proliferate by fragmentation and
resilience to taphonomic destruction (Wood 2011). Subse-
quently, the rubble deposits acted as a hard, stable substrate
for the larval recruitment of other, more diverse coral spe-
cies (large coral heads, facies F6; Figs. 7, 8). These species
rapidly supplanted the delicate, pioneer forms, initiating
the building of the reef framework.
Reef stage 1 (=unit U1): catch-up phase (facies F6–F7:
coral framestone and bindstone)
The first phase of reef framework accretion (unit U1)
occurred vertically, as accommodation space was still
available between the platform floor and the sea surface.
The available space was filled with reef facies F6 and F7
(Figs. 7, 8).
The framestone facies (F6) consists of massive, domal
to tabular morphologies which typically proliferated in
moderate- to high-energy, oligotrophic conditions (James
1983). They colonized the shallowest and most agitated
parts of the carbonate platform, previously occupied by
rubble- and rhodolith-rich crests. The close co-occurrence
of photosynthetic corals in growth position and coralline
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algae, as observed in the nodular coral framestone (facies
F6), has been reported as a typical reef-building association
in Cenozoic, tropical to subtropical environments (Heckel
1974; James 1983; Fagerstrom 1988). These communities
generally colonized hard substrates in the agitated, shallow
photic zones, where the input of nutrients and terrigenous
sediments was low (Muscatine and Porter 1977; Hallock
and Schlager 1986; Brasier 1995; Wood 1999).
The deposition of the bindstone facies (F7) occurred on
a reef crest, forming a reef top. The resulting intense wave
energy is reflected in the coral morphology/size (mainly
cm-thick encrusting forms; see Montaggioni 2005 for
Holocene examples; see Fravega et al. 1994 for an Oligo-
cene example) and diversity (only 2 genera). The encrust-
ing, slow-growing coral habits evidence an adaptation
to frequent and intense wave energies. The decline of the
coral diversity and the disappearance of domal morpholo-
gies may have occurred in a very short period (<10 years;
Wood 1999). Moreover, the physical disturbances led to
the subsistence of only a few other resilient species. These
include crustose red algae, borer communities, and LBF
occurring in few niches. Except for red algae, these organ-
isms are absent from the reef facies F6, which accumulated
in less agitated conditions.
During the reef building initiation, the available accom-
modation space may have been at most 20 m. This is
deduced from the unit U1 thickness (~18 m), which upper-
most 5 m (facies F7) deposited once the reef reached the
sea surface. Similarly, in Holocene fringing reefs, the abil-
ity of corals to construct a vertically accreting framework is
Fig. 12 Hermatypic corals of the Juanilla Formation. Scale bar is
1 cm for all figures except for c 3 cm. a, b Siderastrea conferta Dun-
can. c Caulastrea sp. d, e Antiguastrea cellulosa Duncan, very simi-
lar to holotype from the upper Oligocene Antigua Fm. (Antigua and
Barbuda). f Diploastrea sp. g, h Hydnophora sp. A. i Hydnophora sp.
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> 1000 m
Initial eustatic sea-level
Deep-basin gravity flows Pelagic rain
< 200 m
To the basin depocenter
Nicaraguan Isthmus
No accommodation space
Coastal plain
To the shoreline
Shoreline migration
No accommodation space To the shoreline
To the high relief
To the high relief
To the high relief Bahia Junquillal anticline
No accommodation space To the shoreline
To the high relief
Carbonate platform development (Juanilla Fm.)
Shoreline migration
Ephemeral LBF-rich bank
~ 3:1 VE
b Middle Priabonian (~ 36 Ma)
a Earliest Priabonian (~ 38 Ma)
c Early Rupelian (~ 33 Ma)
d Late Rupelian (~ 30 Ma)
e Early Chattian (~ 27 Ma)
f Middle Chattian (~ 25 Ma)
Deep-basin turbidites
Juanilla Fm. (upper Oligocene)
Junquillal Fm. (upper Eocene)
Deep-shelf turbidites (deposited
between frames a and b)Descartes Fm.
upper Eocene)
Uplift (extensive to reduced)
Subsidence (extensive to reduced)
Subaerial unconformity
Lithostratigraphic units Symbols
Eustatic sea-level
(dashed line = previous frame s.-l.)
Planktonic Foraminifera
Fig. 13 Schematic model depicting the upper Eocene–Oligocene
tectono-sedimentary evolution of the southeastern Sandino Forearc
Basin (3:1 vertical exaggeration). See “Discussion” for more details.
a Interplay between turbiditic and pelagic sedimentation in a deep-
basin environment (Descartes Fm.). The basin depocenter is located
a few kilometers seaward. b Differential uplift of the forearc basin
leading to emergence of the Nicaraguan Isthmus. The deep-shelf tur-
bidites correspond to the facies F1 in Fig. 7. Seaward progradation
of shallow-shelf units (Junquillal Fm.) under a slightly falling sea-
level regime. c The conjunction of sea-level fall (55 m) and ongoing
uplift causes the subaerial erosion of the Junquillal Fm. The locus
of shelf sedimentation migrated seawards (not visible in the frame).
d Localized folding of the sedimentary pile, thus creating the Bahia
Junquillal anticline. e Deep erosion of the Junquillal Fm. due to the
mid-Oligocene sea-level fall (~55 m). Tectonic quiescence. f The sub-
sidence of the shelf provokes a relative sea-level rise (transgression).
The newly created accommodation space favorizes the establishment
of a carbonate platform (Juanilla Fm.) on the uplifted tectonic high
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limited to the uppermost 20 m of the water column, where
favorable light saturations occur (Bosscher and Schlager
1992; Montaggioni 2005).
Post-depositional deformation of the unit U1
The unit U1 is comprised in a recumbent fold overturned
towards the SW (profiles A–A’, B–B’; Fig. 7b). On the
northwestern coast, the unit U1 appears as a 300-m-long
dip slope with SW-directed dips varying from 65° to 45°
(profile D–D’; Fig. 7b). The recumbent fold affects the reef
framework with more than m-sized coral colonies. How-
ever, no brittle fracturation can be observed in the lime-
stone. We conclude that this deformation occurred prior to
lithification of the reefal matrix and may represent post-
depositional, gravitational slumping of the unit U1 (facies
F6–F7). The fact that the underlying facies (F1–F5) are less
consolidated and rich in clays may have favored slumping
in the seismically active forearc area. The units U2–U6
show no evidence of deformation, which indicates that
they deposited after the slumping event. Actually, a major
unconformity separates the unit U1 from the prograding
units U2–U3 (profile D–D’; Fig. 7b). The steep dips (~60°)
of the unit U2 reflect the topography inherited from the
slumped unit U1. Further to the SSE, the dips of the units
U3–U6 become flatter, as these units deposited farther from
the slumped unit.
Reef stage 2 (=units U2–U5): progradational phase (facies
F6: coral framestone)
Due to limited accommodation space, the reef started to
accrete laterally. This progradational phase is recorded in
the units U2–U5. These steeply inclined, prograding units
(profile D–D’; Fig. 7b) formed a reef front which progres-
sively advanced towards the SE, over a distance of 150 m.
The wave-influenced, open marine environment was
favorable to the further establishment of coral head com-
munities. The lateral accretion was facilitated by: (1) a rel-
atively flat topography which provided a constant accom-
modation space; (2) cemented rubble and rhodoliths-rich
deposits (facies F5; Fig. 7) which provided a hard substrate
for the advancing reef front.
Reef stage 3 (=unit U6): keep-up phase (facies F6: coral
The last phase of the reef development is recorded in the
unit U6 (Fig. 7). The unit U6 consists of flat-lying frame-
stone beds (facies F6). These beds aggraded over a fore-
reef facies (F5) made of rubble and rhodoliths (Fig. 7c) and
progressively onlapped the steeply inclined reef front of
the unit U5 (profile D–D’; Fig. 7b). The unit U6 evidenced
the end of lateral reef accretion, following an increase in
accommodation space. The coral communities tended to
vertical accumulation, in attempt to match the relative
sea-level rise. The latter possibly resulted from tectonic
subsidence or 4th–5th order eustatic sea-level rise. Owing
to optimal growing rates, the unit U6 coral communities
successfully tracked the relative sea-level rise, which is
deduced from: (1) the facies homogeneity of the unit U6,
indicating an immediate adaptation of coral communities
to the changing conditions; (2) the facies similarity (F6)
between the aggrading unit U6 and the prograding units
U2–U5, previously deposited at constant sea level.
The shallowing‑upward trend in the southern Sandino
Forearc Basin: the role of tectonic uplift and eustatic
sea‑level changes
Prior to the late Eocene shallowing-upward trend, the SFB
deposits accumulated in deep-sea settings during a period
of 30 m.y. (Figs. 3, 13a). During this time, the basin
development was primarily dependent on subsidence and
volcanic arc activity supplying turbidites. Around the
Paleocene–Eocene boundary (Buenavista Fm.), the tem-
porary interruption of arc-derived turbidite activity may
be related to a transgression (sea-level rise; Haq and Al-
Qahtani 2005; Miller et al. 2005) in combination with a
short-lived, cessation of arc activity, related to subduction
The late Eocene epoch is characterized by a shallowing-
upward succession of depositional units of the SFB as
a result of tectonic uplift of the nascent Nicaraguan Isth-
mus (Figs. 1, 2, 13b). This forearc segment underwent
a Priabonian uplift event that provoked the abrupt end of
deep-water facies deposition (Descartes Fm.) and onset
of coarse-grained, shelf detrital sedimentation (Junquillal
Fm.). Due to limited accommodation space, the Junquillal
Fm. deposits prograded seaward from the uplifted Nica-
raguan Isthmus towards the present-day Punta Descartes
area and downlapped over the Descartes Fm. (Fig. 13b).
The transition from basinal to shelf facies happened rap-
idly (<2 m.y.) and is easily recognizable in the field, even if
no angular unconformity accompanied this event (Fig. 5g).
Actually, a major upper Eocene unconformity exists in the
Nicaraguan part of the basin (treated in a separate paper).
The absence of a major angular unconformity in the Costa
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Rican part may be explained by a distant tilt axis resulting
in uplift without important rotation.
The detailed study of the tectonic events that caused the
upper Eocene–lower Oligocene shallowing-upward of the
SFB is the scope of a separate paper. However, here are
summarized the key points which led us to consider that the
shallowing-upward trend was primarily controlled by tec-
tonic uplift;
1. In the southeastern part of the SFB (Nicaragua), major
unconformities exist between Eocene turbidites (Brito
Fm.) and upper Eocene tempestites (new formation to
be defined);
2. The SFB depocenter exhibits impressive, along-basin
thickness variations. The southeasternmost part of the
SFB consists of a 5 to 6-km-thick sedimentary pile
(seismic profile NIC-20, Berhorst 2006; Funk et al.
2009; this study; Fig. 2b). The depocenter thickness
increases constantly towards the NW and reaches
16 km in the northwesternmost part of the depocenter
(250 km to the NW; seismic profile NIC-100; McIn-
tosh et al. 2007). Interestingly, the pre-upper Eocene
sequences present uniform facies successions and a
constant, along-basin thickness of 5–6 km. On the
other hand, the thickness of post-Eocene sequences
increases towards the NW (<0.1 km in SE-most SFB
to 10 km in the NW-most SFB), with changing deposi-
tional paleoenvironments.
We conclude here that these along-basin thickness and
facies variations reflect the effects of an upper Eocene,
differential uplift of the SFB. The southeastern SFB was
affected by the strongest uplift. This uplift possibly affected
the Tempisque Forearc Basin (SFB southern equivalent,
NW Costa Rica) and may explain the absence of post-
Eocene forearc deposits in this area (Denyer and Alvarado
2007). The uplift markedly limited further development of
the southeasternmost SFB. Here, sedimentation probably
stopped during the late Oligocene, after a period dominated
by erosion and/or non-deposition (see below). At the same
time, the central and northwestern SFB experienced much
slower uplift rates. In consequence, the shallowing-upward
trend is more progressive. In these parts, the Eocene Brito
Fm. turbidites are conformably overlain by km-thick, deep-
shelf carbonate-detrital sequences of the lower Masachapa
Fm. (lower Oligocene; Elming et al. 1998). Later, shore-
face conditions were reached during the late Oligocene,
which is ~5 to 7 m.y. later than in the southeasternmost
SFB. These depositional environments suggest a diachro-
nous, along-basin shallowing-upward trend supporting a
differential uplift of the SFB. In contrast with the south-
eastern SFB, shelf sedimentation continued in the central
and northwestern SFB until the middle Miocene.
An eustatic sea-level fall (<50 m for the Priabonian;
Haq and Al-Qahtani 2005; Miller et al. 2005) could have
contributed to the observed facies changes by a decrease in
accommodation space and filling-up of the basin. However,
the eustatic sea-level drop alone does not explain the Pria-
bonian, rapid shift from several hundreds of meters deep,
turbiditic to near-shore environments and the important
post-Eocene, along-basin facies and thickness variations.
The early Oligocene corresponds to a period of global
sea-level fall which was related to the formation of major
Southern Hemisphere ice sheets (Miller et al. 2005). A
sea-level fall of ~55 m occurred at the Eocene–Oligocene
boundary and enhanced forced regression in the southeast-
ern SFB. The sea-level fall led to the erosion of the upper
Eocene tempestites in a coastal setting (Junquillal Fm.;
Fig. 13c). The eroded proximal shelf sediments contributed
to the regressive-forced sedimentation which was possibly
localized in the distal shelf.
Compressional tectonics were still active and caused
the folding of the central and southeastern SFB. The SFB
exhibits folds that have been imaged by seismic profiles in
its southeastern and central segments, and include the Cor-
vina and Argonaut anticlines (Ranero et al. 2000; Berhorst
2006; Sallarès et al. 2013). The origin of these localized
structures remains debated. They formed either in reac-
tion to transpressional forces (arc-parallel strike-slip faults;
Ranero et al. 2000) or by a mechanism of out-of-syncline
thrusting (orthogonal compression in an extensional con-
text) which accompanied the uplift of the Nicaraguan Isth-
mus (Funk et al. 2009). Indeed, the co-occurrence of these
regimes is common in active margins (Bally et al. 2012). In
any case, the folds grew during the deposition of the lower
Masachapa Fm. (lower Oligocene; Ranero et al. 2000)
and caused an uplift of the forearc lithologies comprised
between 0.4 and 1.7 km. In the studied area, these compres-
sional and/or transpressional forces led to the local folding
and uplift of the forearc formations, thus forming the Bahia
Junquillal anticline (Figs. 2, 13d; formed prior to the Bahia
Junquillal synform). The folding/uplift event initiated the
local, subaerial erosion of the Junquillal Fm. which would
continue until the early Chattian. The conjunction of eus-
tatic sea-level fall and tectonic uplift caused a general ero-
sion/non-deposition of sediments in the former proximal
The global sea-level stabilized during the Chattian, after an
important fall at the Rupelian–Chattian boundary (160 m
after Haq et al. 1987; 100 m after Haq and Al-Qahtani
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2005; 55 m after Miller et al. 2005). The sea-level fall
caused further erosion of the Junquillal Fm. tempestites.
Due to the conjunction of the Rupelian uplift (see above)
and continuous sea-level fall, the tempestites comprised
in the Bahia Junquillal anticline were almost completely
eroded to <10 m on the Isla Juanilla (Figs. 7, 13e). In com-
parison, the tempestites located off the Bahia Junquillal
anticline show much thicker exposures (apparent thick-
ness up to 2000 m; Fig. 2a). This differential erosion of the
Junquillal Fm. indicates that the Bahia Junquillal anticline
was an uplifted tectonic high during the Oligocene and
became the substratum on which the Juanilla Fm. deposited
(Fig. 13f). The ephemeral development of the carbonate
platform in a siliciclastic-poor environment was facilitated