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Origin and significance of lamination in Early Cretaceous stromatolites and
proposal for a quantitative approach
Pablo Suarez-Gonzalez, I. Emma Quijada, M. Isabel Benito, Ram´on
Mas, Ra´ul Merinero Palomares, Robert Riding
PII: S0037-0738(13)00206-6
DOI: doi: 10.1016/j.sedgeo.2013.11.003
Reference: SEDGEO 4702
To appear in: Sedimentary Geology
Received date: 28 August 2013
Revised date: 5 November 2013
Accepted date: 8 November 2013
Please cite this article as: Suarez-Gonzalez, Pablo, Quijada, I. Emma, Benito, M. Isabel,
Mas, Ram´on, Palomares, Ra´ul Merinero, Riding, Robert, Origin and significance of
lamination in Early Cretaceous stromatolites and proposal for a quantitative approach,
Sedimentary Geology (2013), doi: 10.1016/j.sedgeo.2013.11.003
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Origin and significance of lamination in Early
Cretaceous stromatolites and proposal for a
quantitative approach
Pablo Suarez-Gonzaleza,b,*, I. Emma Quijadaa,b, M. Isabel Benitoa,b, Ramón Masa,b,
Raúl Merinero Palomaresc, and Robert Ridingd
a Departamento de Estratigrafía, Universidad Complutense de Madrid, C/ José Antonio
Nováis 12, 28040 Madrid, Spain.
b Instituto de Geociencias IGEO (CSIC, UCM), C/ José Antonio Novais 12, 28040
Madrid, Spain.
c Departamento de Cristalografía y Mineralogía, Universidad Complutense de Madrid,
C/ José Antonio Nováis 12, 28040 Madrid, Spain.
d Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN
37996, USA.
* Corresponding Author: Tel.: +34913944783. Fax: +34913944808. Email:
pablosuarez@geo.ucm.es
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ABSTRACT
Stromatolite lamination is typically defined as alternation of dark and light
laminae. Study of Lower Cretaceous stromatolites from the Leza Fm (N Spain) supports
this statement, but recognizes additional complexities in lamination that have
implications for interpreting accretion processes. These stromatolites are partial
analogues of present-day coarse-grained carbonate stromatolites in the Bahamas and
Shark Bay (Australia) that mainly form by trapping and binding carbonate sand. The
Leza examples contain both grain-rich and micrite-rich laminae with scarce particles,
suggesting that they accreted both by trapping and not trapping grains. Lamination in
modern and ancient coarse-grained stromatolites is commonly defined by thin micritic
crusts that formed during interruptions in accretion and separate Contiguous grainy
laminae (repetitive lamination). Leza examples also contain these thin hiatal crusts and
locally show repetitive lamination, but their conspicuous macroscopic lamination is
defined by thicker alternating grain-rich and micrite-rich laminae (alternating
lamination). This indicates that, although hiatuses in accretion occurred, change in
accretion process was the main cause of macroscopic lamination. These differences in
accretion processes and lamination styles between Leza examples and modern coarse-
grained stromatolites may reflect differences in their environmental settings. Modern
examples occur in shallow marine tidal environments, whereas Leza Fm coarse-grained
stromatolites developed in tide-influenced water-bodies in coastal-wetlands that
experienced fluctuations in water salinity and hydrochemistry. Analysis of lamina-
thickness in these Cretaceous stromatolites and similar published examples provides a
quantitative approach to the processes that underlie stromatolite lamination.
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KEYWORDS: Microbialites, agglutinated stromatolites, accretion processes,
lamination, coastal-wetlands.
1. INTRODUCTION
Lamination is a defining feature of the microbial sediments that Kalkowsky
(1908) named stromatolites. Clearly visible in examples as old as 3.5 billion years
(Hofmann et al., 1999; Allwood et al., 2006), lamination distinguishes stromatolites
from other microbial carbonates such as dendrolites, thrombolites and leiolites (Riding,
2011). The shape, continuity, and stacking of laminae are important in stromatolite
description and classification (Hofmann, 1969, 1973; Monty, 1976; Semikhatov et al.,
1979; Grey, 1989; Grotzinger and Knoll, 1999), including the definition of stromatolite
morphotypes (Maslov, 1960; Vologdin, 1962; Walter 1972; Semikhatov and Raaben,
2000). Stromatolite lamination has been examined for periodicity (e.g. Jones, 1981;
Takashima and Kano, 2008; Petryshyn et al., 2012) and pattern of arrangement (e.g.
Zhang et al., 1993; Batchelor et al., 2000; Dupraz et al., 2006; Wagstaff and Corsetti,
2010; Petryshyn and Corsetti, 2011; Mata et al., 2012), but quantitative analyses of
stromatolite lamina thickness are not common (e.g., Komar et al., 1965; Bertrand-
Sarfati, 1972; Petryshyn et al., 2012).
Microbial mat communities can develop well-layered distributions in response to
vertical physicochemical gradients (e.g., Schulz, 1936; Javor and Castenholz, 1981;
Nicholson et al., 1987), but this biological stratification does not appear to be the
principal precursor to the lamination that is preserved in lithified mats (Golubic, 1991).
Early studies of present-day stromatolites and other laminated microbial deposits, such
as oncoids, related layering to seasonal variations in growth and calcification (Roddy,
1915) and to the size of trapped grains and alternation of sediment-rich and organic-rich
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layers (Black, 1933). Subsequent work has supported and extended this view, and it is
now widely accepted that primary lamination reflects episodic, in some cases iterative,
changes in accretion variously related to variations in microbial growth and
calcification, inorganic precipitation, and grain trapping (e.g., Cloud, 1942; Ginsburg
and Lowenstam, 1958; Logan, 1961; Hofmann, 1973; 1977; Doemel and Brock, 1974;
Monty, 1976; Park, 1976; Semikhatov et al., 1979; Jones 1981; Braga et al., 1995;
Grotzinger and Knoll, 1999; Reid et al., 2000; Riding , 2000; 2011; Seong-Joo et al.,
2000; Storrie-Lombardi and Awramik, 2006; Planavsky and Grey, 2008; Dupraz, et al.,
2009; Wagstaff and Corsetti, 2010; Petryshyn and Corsetti, 2011; Mata et al., 2012;
Petryshyn et al., 2012).
A goal of stromatolite research is to be able to confidently discriminate between
these accretion processes in order to interpret the origin of lamination. In ancient
examples this effort is often hindered by poor preservation, but there is an additional
complication in that lamination can also be generated by hiatuses, as observed in
modern coarse-grained carbonate stromatolites (sensu Awramik and Riding, 1988) from
Shark Bay and the Bahamas (Monty, 1976; Reid and Browne, 1991; Reid et al., 1995;
2000; 2003). This key distinction was recognized by Monty (1976) in a wide-ranging
study of present-day stromatolites. He distinguished two main lamination styles:
alternating lamination, produced by superposition of laminae of differing texture, and
repetitive lamination, where hiatuses, marked by thin dark horizons, separate laminae of
similar texture (Monty, 1976).
In this study, we examine well preserved Cretaceous stromatolites from the Leza
Formation (Cameros Basin, N Spain) that exhibit both repetitive and alternating
lamination. These examples mainly consist of ooids, peloids and bioclasts, together
with micritic laminae. Their fabrics resemble those of well-known present-day coarse-
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grained carbonate stromatolites (Logan; 1961; Golubic and Hofmann, 1976; Monty,
1976; Dravis, 1983; Dill et al., 1986; Awramik and Riding, 1988; Reid and Browne,
1991; Riding et al., 1991a; Reid et al., 1995; 2000; 2003; Macintyre et al., 1996; 2000;
Feldmann and McKenzie, 1998; Planavsky and Ginsburg, 2009; Dupraz et al., 2011;
Jahnert and Collins, 2011; 2012; 2013). The Leza Fm contains some of the earliest
known examples of coarse-grained carbonate stromatolites, and these are unusual in
exhibiting both of the lamination styles defined by Monty (1976). Co-occurrence of
these contrasting lamination styles sheds light on their processes of formation. They
reveal how lamination can be produced by either hiatuses in accretion or by changes in
the process of accretion (i.e. trapping and binding of grains vs. in-situ calcification of
microbial mats without significant grains), and how these in turn reflect environmental
controls. It also draws attention to distinct differences in macroscopic appearance;
repetitive lamination is much less conspicuous, and the prominent lamination that
chacterizes Leza stromatolites in field occurrences and hand-specimens is dominantly
alternating lamination. We develop a metrical methodology to quantitatively describe
and distinguish these lamination styles, which could be applied in other studies of
ancient and present-day stromatolites.
2. GEOLOGIC SETTING
The stromatolites studied here belong to the Early Cretaceous Leza Formation in
the Cameros Basin, the northernmost basin of the Mesozoic Iberian Rift System (Mas et
al., 2002b) (Fig. 1). The Cameros Basin developed during Tithonian (latest Jurassic) to
Albian (late Early Cretaceous) times, and records up to 6000 m of continental and
transitional sediments (Alonso and Mas, 1993; Quijada et al., 2010; 2013a; 2013b;
Suarez-Gonzalez et al., 2010; 2013). The late Barremian-early Aptian Leza Fm was
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deposited in a series of small fault-bounded depressions on the northern margin of the
basin (Fig. 2) (Suarez-Gonzalez et al., 2013). It consists of up to 280 m of carbonates
with variable siliciclastic input (Suarez-Gonzalez et al., 2010; 2013; in press). The
depositional setting of the Leza Fm has been interpreted as a system of coastal-wetlands
formed by broad and relatively vegetated plains with shallow water-bodies, which had
influence of both freshwater and seawater (Suarez-Gonzalez et al., 2013; in press). Leza
coarse-grained stromatolites have only been observed in the eastern area of this system
(Figs. 1B, 2), where four facies associations can be distinguished (Fig. 3; Suarez-
Gonzalez et al., 2013): clastic facies, interpreted as alluvial sandstones and
conglomerates, laterally related to the water-bodies of the coastal-wetlands; black
limestones facies, deposited in water-bodies influenced by freshwater and/or seawater,
as shown by the presence of both continental (charophytes, terrestrial vertebrates) and
marine (dasyclad algae) fossils ; oolite-stromatolite facies, deposited in tide-influenced
water-bodies dominated by seawater (with ostracodes and miliolid foraminifers);
evaporite-dolomite facies, deposited in relatively restricted water-bodies dominated by
seawater, with high salinity and probably local tidal influence.
Leza coarse-grained stromatolites occur in the oolite-stromatolite facies
association, which mainly consists of cross-bedded grainstones (Fig. 4) with flaser,
wavy, and lenticular bedding (Suarez-Gonzalez et al., in press). Stromatolites pass
laterally into, and are interbedded with, the grainstones (Fig. 4). These grainstones are
medium-coarse grained carbonate sand (mean 0.5 mm) (Figs. 4B, D) composed of
ooids, peloids, intraclasts and bioclasts (ostracodes, miliolid foraminifers). These fossils
are abundant, but show very low diversity. Ooid nuclei are generally peloids, intraclasts
and quartz grains, but can also be bioclasts (ostracodes, dasyclads, charophytes). Flat
pebble breccias formed by micritic intraclasts and stromatolite fragments are common
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in the oolite-stromatolite facies association, closely associated with the stromatolites
(Fig. 4C). This facies association is interpreted as tide-influenced water-bodies seaward
of the Leza coastal-wetlands (Suarez-Gonzalez, et al., 2013, in press). Given the general
paralic setting, freshwater input is a likely cause of the low diversity of marine fauna
(Suarez-Gonzalez et al., in press). However, local pseudomorphs after evaporites (see
sections 4.2.5 and 5.1.4, below) indicate that hypersaline conditions also occurred.
3. METHODS
In the eastern outcrops of the Leza Fm (Figs. 1B, 2), 26 horizons of coarse-
grained stromatolites were observed along 6 measured stratigraphic sections (Fig. 2). A
total of 29 stromatolite samples were examined in polished hand specimens and in
corresponding thin-sections prepared perpendicular to the lamination, that were partially
stained with Alizarin Red S and potassium ferricyanide (Dickson, 1966), to distinguish
calcite and dolomite. Thin-sections were compared with their corresponding hand
specimens in order to relate the macroscopic lamination to accretion processes,
interpreted by petrographic study.
Quantitative analysis of lamination was based on lamina thickness, using 14
representative thin-sections of Leza coarse-grained stromatolites. Laminae were
characterized by microfabric under the microscope (see section 4.2, below), and 12 to
15 lamina-thickness values were measured in each thin-section using an ocular
micrometer. A total of 192 lamina measurements were obtained. This metric approach
was inspired by methods applied to Proterozoic stromatolites by Komar et al. (1965),
Bertrand-Sarfati (1972), Walter (1972), and Preiss (1973), in which the relative
thicknesses of dark and light laminae were used to characterize different stromatolite
taxa (see sections 4.4 and 5.3, below). We also applied this methodology used in Leza
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coarse-grained stromatolites to published examples of coarse-grained stromatolites.
Data management and analysis were performed using the free software R 2.15.2 (R
Development Core Team, 2012).
4. STROMATOLITE DESCRIPTION
4.1 Macroscopic features
Leza coarse-grained stromatolites are interbedded with and pass laterally into
ooid grainstones (Fig. 4), forming beds up to 40 cm thick that extend laterally for up to
100 m. Their morphologies range from laterally linked domes 70 cm across and 40 cm
high (Fig. 4A) to stratiform deposits with elevated areas (Figs. 4C, 4E). They show
distinct macroscopic lamination (Figs. 4C, 4D, 4E) formed by darker and lighter
laminae, 0.5-4 mm thick. The laminae have gradational to sharp contacts with generally
smooth surfaces and a high degree of inheritance (Fig. 5A).
4.2 Microscopic features
When observed under the microscope, laminae of Leza coarse-grained
stromatolites contain a variety of textural components, creating different microfabrics
(Fig. 5). The mineralogy is generally calcite, but partial dolomitization of ooids and
micritic matrix is observed. In addition to ooids and other sand-size carbonate grains
(peloids, intraclasts and bioclasts) which are common (Figs. 5-12), micrite is also
present, together with rare calcified filaments and pseudomorphs after evaporites.
Various combinations and proportions of these components produce the variety of
laminae in Leza coarse-grained stromatolites.
4.2.1 Grainy laminae
Grainy laminae are formed almost exclusively by medium-coarse grained
carbonate sand (Figs. 5B, 6): ooids (typically superficial ooids, sensu Carozzi, 1957),
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peloids, bioclasts (ostracodes, foraminifers), micritic intraclasts and rare quartz grains
(Figs. 5B, 6). Identical grain types occur in the oolitic grainstones surrounding the
stromatolites. Ooid nuclei are generally peloids and micritic intraclasts, and less
commonly quartz grains and fragments of ostracodes, dasyclads and charophytes.
Composite ooids occur occasionally (Fig. 6B). Grain-size in the stromatolites ranges
100-800 µm (mean 350 µm), and is finer than in the surrounding sediment (mean 500
µm). Intergranular space is filled by sparite cement and/or clotted-peloidal micrite (Figs.
5B, 6). Laminae are up to 3 mm thick and generally smooth, forming horizontal to
steeply dipping (up to 90º) layers on the tops and flanks of stromatolite domes,
respectively. Macroscopically, they appear generally continuous, but often pinch or
disappear laterally in thin section.
4.2.2 Micritic laminae
Micritic laminae are common in Leza coarse-grained stromatolites (Figs. 5C,
6B, 7). They are mainly formed by clotted and clotted-peloidal fabrics, and less
commonly dense fabric, and contain very few scattered carbonate grains (Figs. 5C, 6B,
7A). Clotted fabric is composed of irregular micrite clots, 30-
by irregularly shaped fenestrae filled with microsparite or sparite cement (Fig. 7A).
Clotted-peloidal fabric is composed of peloids, 20-
clots (Fig. 5C). Peloids are surrounded by microsparite cement, and typically
concentrated in areas 0.1-1.5 mm across, which may be separated by irregularly shaped
fenestrae (Figs. 5C, 6B). Dense micrite is less common (Fig. 7C). Micritic laminae of
Leza coarse-grained stromatolites are generally thinner (<2 mm) and more irregular
than grainy laminae, locally even developing micro-domes (Fig. 7A). Lateral pinching
is common. As in the case of grainy laminae, micritic laminae occur in both flat and
domal areas of the stromatolites, with varied dip angles.
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4.2.3 Mixed laminae
Grainy and micritic laminae form a series ranging from purely grainy to wholly
micritic, but with most laminae of the Leza coarse-grained stromatolites being mixtures
with different proportions of both carbonate grains and micrite, here termed mixed
laminae (Figs. 8, 9). When observed in detail, subtle and gradual changes between
micritic and grainy microfabrics can be observed in some mixed laminae (Fig. 9A).
Nonetheless, it remains useful to distinguish grain-dominated mixed laminae (with
abundant subordinate intergranular micrite, Figs. 7B, 8) and micrite-dominated mixed
laminae (micritic laminae with locally abundant grains, typically concentrated in
isolated pockets, Figs. 7C, 8). Very occasionally, mixed laminae also contain poorly
preserved calcified filaments (Figs. 7B, 7C). These lack a well-defined calcified sheath
and instead occur as elongated clusters of clotted-peloidal micrite (Fig. 7B) or thin
micritic rims perpendicular to lamination (Fig. 7C). Mixed laminae are smooth, laterally
quite continuous, and generally 1-4 mm thick; but laminae up to 6 mm are also present.
Mixed laminae exhibit varied dip angles, both in flat and flank areas of domes.
4.2.4 Thin micritic crusts
Dark thin micritic crusts, 25-500 µm in thickness (average 140 µm), overlie
laminae of differing microfabric composition. They typically overlie grainy laminae or
grain-dominated mixed laminae, and less commonly micritic laminae or micrite-
dominated mixed laminae (Figs. 9, 10, 11). They are composed of micrite that is either
dense, clotted or clotted-peloidal. They differ from micritic laminae in not forming
thick laminae, and in only occurring superposed on other, and thicker, laminae. They
characteristically show diffuse and irregular lower surfaces and sharp upper surfaces
(Figs. 9, 10, 11). Although thin micritic crusts are common in Leza coarse-grained
stromatolites, they are macroscopically inconspicuous and are only clearly noticeable
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under the microscope (compare Fig. 5A with Figs. 8, 9A). Thin micritic crusts are
varied and complex: less developed crusts are typically very thin (~100 µm) and form a
thin micritic film gradually passing downwards into clotted-peloidal micrite (Figs. 9D,
10A). More developed crusts are generally thicker (~500 µm) and involve the
micritization and fusion of carbonate grains underlying the crust (Figs. 9D, 10B, 11).
Small crystals of pyrite, <20 µm, can form thin levels, commonly near the upper surface
of the crust (Figs. 7C, 9E).
4.2.5 Evaporite laminae
These are very minor components of the Leza coarse-grained stromatolites and
have only been observed in four samples. They are formed by contiguous aggregates of
calcite, dolomite and quartz pseudomorphs after sulphates (Fig. 12). Aggregates are up
to 6 mm across, they deform the adjacent laminae (Fig. 12A), and incorporate primary
components such as ooids and micrite clots (Fig. 12B). Individual pseudomorphs are
0.2-1.5 mm long (Fig. 12) and display lenticular and tabular habits (Fig. 12B),
characteristic of gypsum and anhydrite, respectively (e.g., Warren, 2006 and references
therein).
4.3 Lamination
In outcrop and hand-specimens, Leza coarse-grained stromatolites are well-
laminated (Fig. 3). To relate this characteristic lamination to the stromatolite
microfabrics, we compared polished hand specimens with thin-sections of the same
samples (Fig. 5, 8, 9). In addition to the well-defined macroscopic lamination, a
microscopic lamination is also observed that is produced by the thin micritic crusts.
4.3.1 Macroscopic lamination
Conspicuous macroscopic lamination is produced by alternation of darker and lighter
layers typically 0.5-4 mm thick (Figs. 4E, 5A, 8A). When observed in detail, this
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shading is more complex than just dark and light. Under the microscope, the
conspicuous colour contrast between laminae that defines macroscopic lamination can
be related to changes on the microfabric of the laminae (i.e., changes in proportions of
grains, micrite and cement; see Fig. 8). These changes are generally an alternation
between grain-rich and micrite-rich laminae, even in samples dominated by successive
mixed laminae (Figs. 5, 8). This style of stromatolite lamination, formed by
superposition of laminae with contrasting microfabric composition, corresponds with
that defined as alternating lamination by Monty (1976). Under the microscope, the
contact between successive macroscopic laminae is generally abrupt and sharp. It can be
marked by a thin micritic crust (Fig. 8), although transitional contacts are also observed
(Fig. 6B). Erosive contacts occur rarely (Fig. 10B), and are typically associated with the
top surface of thin micritic crusts.
4.3.2 Microscopic lamination
When observed under the microscope, some areas of stromatolite thin-sections
show a fine-scale lamination that is not readily observed in hand specimen, and which is
formed by contiguous laminae of similar microfabric composition that are separated by
thin micritic crusts (Figs. 9A, 9C, 9D, 10A, 11). Thin micritic crusts can occur at the
contact between successive laminae of differing microfabric (Figs. 8B, 9B), but they are
most conspicuous when they separate similar laminae. In this case they create
microscopically distinct lamination. This is due to their dense and dark appearance,
which contrasts with the adjacent laminae (typically grainy laminae or grain-dominated
mixed laminae, but also micrite-dominated mixed laminae), that are more cement-rich
and therefore lighter in appearance (Figs. 9, 10, 11). This style of stromatolite
lamination, formed by superposition of laminae of similar microfabric composition
separated by thin dark horizons, corresponds to repetitive lamination defined by Monty
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(1976). He recognized repetitive lamination in present-day coarse-grained carbonate
stromatolites from Shark Bay as thin lithified micritic layers (~500 µm thick) that mark
discontinuities in loosely packed unlithified grainy laminae, up to 3 mm thick. The same
lamination-style was subsequently recognized and studied in Bahamian coarse-grained
stromatolites (Reid and Browne, 1991; Reid et al., 1995, 2000; Macintyre et al., 1996,
2000; Feldmann, 1997; Feldmann and McKenzie, 1998; Visscher et al., 1998, 2000).
Although lamination in Leza coarse-grained stromatolites is in general
dominated by alternating lamination (Figs. 4E, 5, 8), repetitive lamination dominates
some areas of the samples (see upper part of Fig. 5A). In addition, thin micritic crusts
also occur locally within some of the thicker laminae that define the macroscopic
alternating lamination, forming a subordinate smaller-scale lamination (Fig. 8B). This
is very similar to what Monty (1976) defined as composite alternating lamination.
4.4 Quantitative lamination data
These results show that Leza coarse-grained stromatolites contain the two main
stromatolite lamination styles defined by Monty (1976), alternating and repetitive
lamination. This offers an opportunity to further examine and analyze both of them in
detail. The original definitions and schematic representations of these lamination styles
(see Figs. 1A, 1D of Monty, 1976) suggest that they are essentially differentiated not
only by the microfabrics that form them, but also by the relative thicknesses of their
constituent laminae: alternating lamination is formed by consecutive laminae of
different microfabric with variable, but overall similar, thickness; and repetitive
lamination is formed by laminae of similar microfabric and similar thickness separated
by much thinner discontinuity horizons. This suggests that both these lamination styles
could be quantitatively distinguished by measuring the relative thicknesses of their
laminae.
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To test this hypothesis we conducted a metrical analysis of these lamination
styles in Leza coarse-grained stromatolites following previous quantitative analyses of
relative lamina-thickness (Komar et al., 1965; Bertrand-Sarfati, 1972; Walter, 1972;
Preiss, 1973), which compared thicknesses of dark (D) and light (L) laminae using a
variation in the laminae of Leza coarse-grained
stromatolites is more complex than simply dark and light (see section 4.3, above), we
used microfabric rather than colour for metrical analysis: we considered alternating
lamination as micrite-rich laminae (M) alternating with grain-rich laminae (G) (Figs. 5,
8), and repetitive lamination as thin micritic crusts (M) separating contiguous laminae
of the same, typically grain-rich (G), microfabric (Fig. 9). Evaporite laminae were not
measured because they are rare, and therefore not a characteristic constituent of Leza
coarse-grained stromatolites lamination.
For this analysis we used 14 thin-sections of Leza coarse-grained stromatolites,
and selected areas of the thin-sections which clearly displayed one of the two lamination
styles. In each selected area, we measured, under the microscope, 6-10 thickness values
of its constituent laminae (i.e., M, G, M or G). A mean value was obtained from all
measured laminae of the same lamina-type in each selected area (i.e., Mmean,and Gmean,
for areas with alternating lamination, and Mmean and Gmean for areas with repetitive
lamination). The relative thickness data for each area were summarized using
Mmean/Gmean or Mmean/Gmean values. These data show that areas dominated by
alternating lamination have Mmean/Gmean values in the range 0.71-2.26 (mean = 1.35),
and areas with repetitive lamination have Mmean/Gmean values in the range 0.04-0.32
(mean = 0.13). For visual comparison, these relative thickness data were plotted in
diagrams (Fig. 13), in which each line joins the maximum and minimum values of the
lamina-thicknesses from a measured area of a thin section, and thus, each line shows the
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full thickness range of each measured area of the thin-sections. In these diagrams we
recorded the minimum thickness as zero in the common situation of areas where
laminae thin and disappear laterally.
To further investigate our quantitative thickness data, we statistically examined
thickness-values of all the measured laminae (n=192), irrespective of their microfabric
and of whether they formed alternating or repetitive lamination. These values range
from 0-3250 µm, with small values being predominant, producing a right-skewed
histogram (Fig. 14A). To obtain a symmetric distribution, we transformed these data,
applying natural logarithms (Fig. 14B). The resulting histogram of transformed data
suggests a bimodal distribution produced by mixture of two, apparently normal,
distributions (Fig. 14B). A model-based clustering method is required to characterize
both distributions and to test their normality. Using software
(Fraley et al., 2012), we obtained a mixture model of two normal populations with their
characteristic mean, variance and mixing proportion (Fig. 14B): the first population
represents 41% of the data, and the second population represents 59%. This model also
calculated the probability of each datum of belonging to each population. With a
probability higher than 0.7, laminae with thicknesses <450 µm belong to the first
population, and those with thicknesses >600 µm belong to the second population.
Laminae with thicknesses ranging 450-600 µm have similar probabilities of belonging
to both populations and can therefore be regarded as the intersection area of both
populations (Fig. 14B).
5. DISCUSSION
5.1 Microfabric interpretation
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The variety of microfabric components in laminae of Leza coarse-grained
stromatolites implies that diverse processes were involved in their accretion,
lithification, and the origin of their lamination styles.
5.1.1 Grain-rich laminae
Sand-size carbonate grains are very common, forming grainy laminae and grain-
dominated mixed laminae (Figs. 5-11), in both horizontal and inclined (up to 90º)
portions of the stromatolite domes. They have the same composition as the grains in the
surrounding ooid grainstone facies, but overall are consistently finer. This difference in
grain-size has been noted in present-day marine coarse-grained carbonate stromatolites
formed by stabilization (trapping and binding) of previously mobile grains by microbial
mats (Logan, 1961; Monty, 1976; Dravis, 1983; Reid and Browne, 1991; Riding et al.
1991a; Reid et al., 1999). In these examples, grain-trapping is produced by erect
filaments, mat irregularities, and extracellular polymeric substances (EPS) secreted by
cyanobacteria and other mat microbes (Logan, 1961; Playford and Cockbain, 1976;
Dravis, 1983; Dill et al., 1986; Awramik and Riding, 1988; Riding et al. 1991a;
Visscher et al., 1998; Reid et al., 2000; Decho et al., 2005; Dupraz et al., 2009; Browne,
2011; Bowlin et al., 2012; Jahnert and Collins, 2012). Rare filaments in some mixed
laminae of the Leza coarse-grained stromatolites (Figs. 7B, 7C) may be relicts of
cyanobacteria. Calcified filaments, similarly preserved, also occur rarely in present-day
Bahamian examples (Dravis, 1983; Reid and Browne, 1991; Reid et al., 1995;
Macintyre et al., 1996; Feldmann and McKenzie, 1998; Planavsky et al., 2009), and as
filament molds at Shark Bay (Reid et al., 2003).
We infer that grain-rich laminae of Leza coarse-grained stromatolites formed in
a similar manner to those in Bahamian and Shark Bay examples. The Leza coarse-
grained stromatolite palaeoenvironment may have had regular currents (e.g. tides,
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waves, and/or storms, Suarez-Gonzalez et al., in press.) that continuously supplied
grains (preferentially the finer fraction) to the tops and flanks of the stromatolite domes.
5.1.2 Micrite-rich laminae
Microfabrics mainly composed of clotted micrite and clotted-peloidal micrite are
also common in Leza coarse-grained stromatolites (Figs. 5- 9). Both clotted and clotted-
peloidal microfabrics are common in ancient and modern microbial carbonates, and they
are widely attributed to calcification of microbial mats induced by heterotrophic
bacteria (Dalrymple, 1965; Chafetz, 1986; Chafetz and Buczynski, 1992; Reitner, 1993;
Dupraz et al., 2004; Riding and Tomás, 2006; Heindel et al. 2010; Spadafora et al.,
2010). Micritic fabrics similar to these in Leza coarse-grained stromatolites also occur
in present-day coarse-grained carbonate stromatolites and thrombolites, generally filling
the intergranular space of grain-rich microfabrics but not typically as relatively thick
micritic laminae (Reid and Browne, 1991; Reid et al., 1995; 2003; Feldmann, 1997;
Feldmann and McKenzie, 1998; Planavsky and Ginsburg, 2009; Planavsky et al., 2009;
Browne, 2011; Jahnert and Collins, 2011; 2012), which is the case in Leza coarse-
grained stromatolites, where micrite-rich laminae typically alternate with grain-rich
laminae and both display similar mm-scale thicknesses. Nonetheless, there are examples
of subtidal stromatolites at Shark Bay, which are dominantly micritic with very scarce
grains, and are composed of clotted and clotted-peloidal microfabrics similar to those of
Leza coarse-grained stromatolites (Reid et al., 2003; Jahnert and Collins, 2011; 2012).
Precipitation of clotted and clotted-peloidal micrite in all these present-day examples
has been interpreted to be induced, under anaerobic conditions, by heterotrophic
microbes (chiefly sulphate-reducing bacteria) which degrade the EPS mainly secreted
by the primary producers of the mat (Feldmann, 1997; Feldmann and McKenzie, 1998;
Visscher et al., 1998; 2000; Reid et al., 2000; Andres et al., 2006; Dupraz et al., 2009;
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Planavsky and Ginsburg, 2009; Planavsky et al., 2009; Jahnert and Collins, 2012).
Therefore, bioinduced precipitation of micrite in these examples can be considered a
subsurface process (Feldmann, 1997; Feldmann and McKenzie, 1998) in intergranular
spaces of the uppermost millimetres of the microbial mat (Visscher et al., 1998; 2000).
Based on comparisons with these and other present-day coarse-grained stromatolites,
we infer that micritic laminae formed when the surfaces of Leza coarse-grained
stromatolites accreted without significant trapping, and that calcification was primarily
achieved by subsequent precipitation of clotted-peloidal and/or clotted micrite,
bioinduced by heterotrophs within anaerobic areas of the mat. Micrite-dominated mixed
laminae are interpreted to have formed by similar processes, but under circumstances
where the surface mat did trap some grains, which were typically concentrated in
particular areas of the laminae. Intergranular clotted and clotted-peloidal micrite found
in grain-rich laminae is interpreted as similarly bioinduced, but in areas between trapped
grains. Laminae of mixed micritic-grainy composition occur in present-day coarse-
grained carbonate stromatolites in the Bahamas (Reid and Brown, 1991; Feldmann and
Mckenzie, 1998; Planavsky and Ginsburg, 2009) and Shark Bay (Monty, 1976, Reid et
al., 2003; Jahnert and Collins, 2001; 2012).
5.1.3 Thin micritic crusts
Thin micritic crusts very similar to those in Leza coarse-grained stromatolites
have been recognized in present-day coarse-grained carbonate stromatolites. Monty
(1976) described thin lithified micritic layers (~500 µm thick), rich in organic matter,
separating thicker (~3 mm) loosely packed unlithified grainy layers, from intertidal
Shark Bay stromatolites. He noted that the micrite appears to have been precipitated in
situ and is associated with bored and micritized grains. Similar lithified horizons were
subsequently described in coarse-grained Bahamian stromatolites from the Exuma Cays
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(Reid and Browne, 1991; Reid, et al., 1995) as thin micrite crusts (20-40 µm thick) that
generally overlie layers of micritized and truncated grains, 200-1000 µm thick. These
crusts are interpreted as formed by biogeochemical processes during hiatuses in
stromatolite accretion (Macintyre, et al., 1996; Feldmann, 1997; Feldmann and
McKenzie, 1998). Visscher et al., (1998; 2000) related formation of these micritic crusts
to carbonate precipitation-dissolution processes induced by sulphate-reducing bacteria,
a few mm below the surface of mats. In a detailed study, Reid et al. (2000) related thin
micritic crusts in Bahamian stromatolites to successive mat processes in which
Schizothrix mats promoted grain-trapping, heterotrophic bacteria induced precipitation
of thin micritic horizons when accretion ceased, and endolithic Solentia cyanobacteria
bored and micritized grains during longer hiatal periods (see also Macintyre et al.,
2000).
Thin micritic crusts in Leza coarse-grained stromatolites share many of these
features, and we infer similar origins for them. Small pyrite crystals observed in the thin
micritic crusts of Leza coarse-grained stromatolites (Fig. 7C, 9E) are consistent with an
origin related with sulphate-reduction, and resemble the small framboidal pyrite crystals
that are by-products of sulphate-reduction in present-day microbial mats (Jørgensen and
Cohen, 1977; Visscher et al., 1998; Popa et al., 2004; Jones et al., 2005, Spadafora et
al., 2010). Locally, thin micritic crusts show signs of erosion (Fig. 10B), suggesting that
significant early lithification of the thin micritic crusts occurred during prolonged hiatal
periods.
5.1.4 Evaporite laminae
Pseudomorphs after sulphates occur in aggregates that deform the adjacent
laminae and include fragments from them (ooids or micrite) (Fig. 12), which suggests
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that the original minerals grew displacively and replacively, as intrasediment sulphates,
once the overlying laminae where already deposited.
Stromatolites are very commonly found interbedded in evaporite-rich modern
environments and ancient units (e.g., Von der Borch et al., 1977; Pope et al., 2000;
Schreiber and El Tabakh, 2000). Stromatolites partially composed of evaporites
(typically sulphates) also occur, as products of syndepositional alteration referred to as
and Monty, 1981; Babel, 2007). Additionally, many examples
of stromatolites and other microbial carbonates contain displacive and replacive
sulphate laminae, similar to those of Leza coarse-grained stromatolites (Gunatilaka,
1975; Horodyski and Vonder Haar, 1975; Park, 1977; Aref, 1998; Gerdes et al., 2000;
Rouchy and Monty, 2000; Ortí, 2010). Similarly, we interpret evaporite laminae of the
Leza coarse-grained stromatolites as formed by very early diagenetic intrasedimentary
precipitation of evaporite minerals when interstitial waters reached oversaturation. The
facies association with Leza coarse-grained stromatolites does not typically contain
evaporites (see section 2 above), but it alternates with facies rich in pseudomorphs after
sulphates (Fig. 3). This indicates that the tidally-influenced coastal-wetlands that
contained Leza coarse-grained stromatolites (Suarez-Gonzalez et al., in press), locally
and temporarily became restricted, allowing precipitation of intrasedimentary sulphates
within the stromatolites. Similarly, in some areas of Shark Bay, gypsum crystals occur
within supratidal microbial mats (Hagan and Logan, 1974; Jahnert and Collins, 2013).
5.2 Accretion processes and lamination style: origins of lamination
Just as microfabrics and laminae can be related to process in Leza stromatolite
development, their lamination styles reflect the mechanisms by which the stromatolites
accreted.
5.2.1 Macroscopic lamination
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Alternating lamination (sensu Monty, 1976), the predominant macro-lamination
in Leza coarse-grained stromatolites (Figs. 4, 5, 8), is formed by alternation of laminae
with different microfabric composition, typically grain-rich and micrite-rich laminae
(Fig. 8). This reflects alternation of two main accretion processes: grain trapping and
binding, and bioinduced calcification. The mixed grainy-micritic composition of many
of the stromatolite laminae (Figs. 7, 8, 9) indicates that these two processes were not
mutually exclusive, although one was generally predominant. The typically sharp
transition between these alternating laminae (Fig. 8), suggests interruptions in accretion,
although gradual transitions are also observed (Fig. 6B), indicating that both accretion
processes could progressively grade into each other. In addition, thin micritic crusts
occur at lamina contacts (Figs. 8B), and were presumably produced by longer hiatuses
in accretion.
Predominance of alternating lamination distinguishes Leza coarse-grained
stromatolites from most recent examples of coarse-grained carbonate stromatolites,
which generally lack thick micrite-rich laminae (Logan, 1961; Dill et al., 1986; Reid et
al., 1995; 2000, Feldmann and McKenzie, 1998). Planavsky and Ginsburg (2009)
describe four successive processes responsible for the development of Bahamian
microbialites: 1) sediment trapping, binding and initial lithification; 2) disruption and
truncation of the initial fabric; 3) pervasive cementation and clot formation; and 4) late-
stage boring. Thus, the only accretionary process is trapping and binding of grains and
the other processes are very early diagenetic and are related either to alteration of the
original fabric or intrasedimentary precipitation. Dupraz et al. (2011) indicate that
microbial mats and their Figure 4 shows that the first type of mat is responsible for
accretionary grain-trapping, the second is involved in precipitation of a thin micritic
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crust during hiatuses, and the third causes micritization and fusion of grains below the
stromatolite surface. Bowlin et al. (2012) summarize previous studies of Bahamian
stromatolite mats and add three more mat types to those described by Dupraz et al.
(2011), all of which are involved in trapping and binding of sediment. In Shark Bay
stromatolites, Jahnert and Collins (2013) describe six mat types, and Jahnert and Collins
(2012) attribute stromatolite development to grain accretion plus four additional
constructional mechanisms: 1) superficial micrite generation within organic gel of the
mats that stabilizes sediment; 2) bioturbation, micritization and recrystallization; 3)
pervasive micrite generation, filling spaces and enveloping grains; and 4) fibrous
aragonite precipitation in void spaces. However, in Shark Bay stromatolites, micritic
microfabrics do also occur. Reid et al. (2003) interpreted them as calcified mats with
little or no trapped and bound sediment, but these are only observed in subtidal grain-
poor stromatolites and in the upper parts of some intertidal grainy specimens.
Leza coarse-grained stromatolites therefore differ in detail from these examples
of recent coarse-grained carbonate stromatolites in that their macroscopic lamination
reflects the alternation of two fabrics, and thus of two accretion processes: grain
trapping and calcification of mats that trapped few or no grains. We propose that the
development and alternation of these two distinct microfabrics in Leza coarse-grained
stromatolites reflects both extrinsic (environmental) and intrinsic (biotic) factors. The
Leza Fm was deposited in a system of coastal-wetlands, influenced by both freshwater
and seawater, and therefore experienced significant changes in salinity (Suarez-
Gonzalez et al., 2013). In addition, facies containing Leza coarse-grained stromatolites
were influenced by tides (Suarez-Gonzalez et al., in press) and the presence of evaporite
pseudomorphs within the stromatolites indicates that their sedimentary environment
underwent changes in salinity. These hydrochemical fluctuations together with the
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hydrodynamical changes typical of tidal environments could alternately promote grain
trapping and mat calcification in Leza coarse-grained stromatolites. However, biotic
changes (e.g., alternation of mats of differing microbial composition at the stromatolite
surface) cannot be ruled out, especially since mats of varied biotic compositions can
alternate at the surface of present-day coarse-grained carbonate examples (Bowlin et al.,
2012; Jahnert and Collins, 2013).
5.2.2 Microscopic lamination
Repetitive lamination (sensu Monty, 1976) is also common in Leza coarse-
grained stromatolites, but since it is formed by microscopic thin micritic crusts within
laminae of similar microfabric composition (generally grain-rich laminae) it is
macroscopically inconspicuous (Figs. 5, 8, 9). Thin micritic crusts of Leza coarse-
grained stromatolites are likely to have formed during hiatuses in accretion, as in
present-day examples (Macintyre, et al., 1996; Feldmann, 1997; Feldmann and
McKenzie, 1998; Visscher et al., 1998; 2000; Reid et al., 2000; 2003; Dupraz et al.,
2009; 2011). During these interruptions, precipitation of micrite occurs near the
stromatolite surface (Visscher et al., 1998; 2000) and, if the hiatus is long enough,
micritization and fusion of subsurface grains also occurs (Macintyre et al., 1996; Reid et
al., 2000) (Fig. 11). Following this interpretation, thin micritic crusts in coarse-grained
carbonate stromatolites can be considered as essentially hiatal products of alteration and
precipitation near the stromatolite surface, rather than as accretionary events (Feldmann,
1997; Feldmann and McKenzie, 1998; see Dupraz et al., 2011, Figure 4, for a graphical
explanation). Evidence of erosion on some previously lithified thin micritic crusts in
Leza coarse-grained stromatolites (Fig. 10B) suggests extended hiatuses in accretion.
Erosion of partially lithified stromatolites is also commonly described in present-day
coarse-grained examples, due to bioerosion (e.g., Dill et al., 1986), subaerial exposure
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in supratidal areas (e.g., Jahnert and Collins, 2011), burial and exhumation of the
stromatolites by migrating sand waves (Andres and Reid, 2006; Planavski and
Ginsburg, 2009), or physical stress by tidal currents during long periods when no grains
were supplied (Feldmann & McKenzie, 1998). In Leza coarse-grained stromatolites,
erosion was likely related to subaerial exposure, since stromatolites are laterally and
vertically associated with flat pebble breccias formed by micritic intraclasts and
stromatolite fragments (Fig. 4C). It is also possible that changes in hydrodynamic
conditions (i.e., abnormal tides or storms) were involved in erosion of the stromatolite
surface. Resumption of the accretion process occurring prior to the hiatus produced a
new lamina, similar in microfabric to the preceding one, creating repetitive lamination.
Repetitive lamination is the dominant lamination style in most present-day
examples of coarse-grained carbonate stromatolites (Monty, 1976; Reid and Browne,
1991; Reid et al., 1995; 2000; 2003; Macintyre et al., 1996; Feldmann and McKenzie,
1998; Dupraz et al., 2009; 2011). Since the thin micritic crusts that define this
lamination style are essentially microscopic and often discontinuous and laterally
impersistent, the predominance of repetitive lamination in these examples helps explain
the irregular and crude macrolamination commonly observed in parts of them (Logan,
1961; Dravis, 1983; Dill et al., 1986; Planavsky and Ginsburg, 2009), that is also
reflected in composite descriptive terminologies
Riding et al., 1991a), or -
.
5.2.3 Other ancient examples of coarse-grained carbonate stromatolites
Although present-day coarse-grained carbonate examples are well-known in the
Bahamas and Shark Bay, grainy carbonate laminae are rarely the main components of
ancient stromatolites (see Monty, 1977; Awramik & Riding, 1988; Fairchild, 1991;
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Altermann, 2008; Browne, 2011). Riding, et al. (1991b), Arenas and Pomar (2010) and
Bourillot et al. (2010) studied examples of coarse-grained carbonate stromatolites of
Messinian (late Miocene) age in Spain, and Immenhauser et al. (2005) described Aptian
(Early Cretaceous) microbial buildups with crudely-layered coarse-grained carbonate
microfabrics in Oman. Other Cretaceous stromatolites from Spain containing some
laminae composed of trapped carbonate grains (mainly peloids) have been described
(Turonian, Rodríguez-Martínez et al. 2012; Berriasian, Quijada et al. in press).
Oxfordian-Kimmeridgian (Upper Jurassic) stromatolites in Poland (Matyszkiewicz et al.
2006; 2012) contain coarse-grained carbonate microfabrics, and Triassic coarse-grained
carbonate stromatolites have been described from NE Spain (Mercedes-Martín et al.,
2013) and from the SW USA (Woods, 2013).We are not aware of any definite examples
of coarse-grained carbonate stromatolites older than Early Triassic.
As in modern examples, ancient coarse-grained stromatolites typically lack thick
micrite-rich laminae, and exhibit crude macrolamination mainly defined by thin micritic
crusts, similar to those of Leza coarse-grained, Bahamian and Shark-Bay examples
(Immenhauser et al., 2005; Matyszkyewitz et al., 2006; 2012; Arenas and Pomar, 2010;
Mercedes-Martín et al., 2013). Arenas and Pomar (2010) provide a detailed description
of thin micritic laminae c microbial
The predominant style of these scarce fossil examples is therefore repetitive
lamination. To our knowledge, the thin micritic crusts of Leza coarse-grained
stromatolites are the oldest (~125 M.y.) well-documented analogues of those in present-
day coarse-grained carbonate stromatolites.
5.2.4 Significance of Leza coarse-grained stromatolites lamination
Leza coarse-grained stromatolites differ from most published examples of
coarse-grained carbonate stromatolites in that they clearly contain both alternating and
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repetitive lamination, the two main styles defined by Monty (1976). Thin micritic crusts
create repetitive lamination but they can also be present at contacts between laminae of
differing microfabric in alternating lamination. This demonstrates that hiatuses
occurred throughout the development of Leza coarse-grained stromatolites, but they
only produced noticeable lamination when they repeatedly interrupted accretion by the
same process. When hiatuses occurred between periods of accretion by different
processes, macroscopic lamination largely reflects this difference in accretion
mechanism, not the hiatus itself. Nonetheless, Leza coarse-grained stromatolites do
show that interruptions in accretion can be an additional source of microscopic
lamination in stromatolites, although it is readily overshadowed by more conspicuous
alternating lamination. However, modern coarse-grained carbonate stromatolites
(Monty, 1976; Reid and Browne, 1991; Reid et al., 1995; 2000; 2003; Macintyre et al.,
1996; Feldmann and McKenzie, 1998), as well as some parts of Leza coarse-grained
stromatolites (Figs. 5A, 9A), show that if only one accretion process predominates and
is periodically interrupted, then repetitive lamination becomes the main lamination
style, and is typically relatively indistinct.
5.3 Quantitative analysis of lamination
The methodologies presented here show that differences in lamination can be
quantified and compared using a metric analysis of lamina thickness. Analysis of
transformed thickness data (Fig. 14B) reveals two different populations, and the
intersection between both populations (in the range 450 to 600 µm) is approximately the
upper limit of the thickness range of thin micritic crusts in Leza coarse-grained
stromatolites (500 µm). This suggests that the presence of a distinct lower population
(values <450 µm) among the lamina thickness data broadly reflects the abundance of
thin micritic crusts in Leza coarse-grained stromatolites. Therefore, graphical data
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analysis by histograms provides useful information that can assist process
interpretation; for example, abundant thin micritic crusts could reflect the importance of
hiatuses during stromatolite development.
Furthermore, analyses of the relative thicknesses of constituent laminae in both
alternating and repetitive lamination (section 4.4 and Fig. 13) indicate that stromatolite
lamination style can be quantitatively assessed. Micrite-rich and grain-rich laminae in
alternating lamination are similar in thickness (mean M/G = 1.35) with grain-rich
laminae being typically slightly thinner. In contrast, repetitive lamination is formed by
thin micritic crusts that are much thinner than the adjacent, typically grain-rich, laminae
(M/G = 0.13). These data suggest a systematic relationship between relative thickness
values and the lamination styles that were defined by Monty (1976).
To further examine this relationship, the same methodology has been applied to
published examples of present-day and ancient coarse-grained stromatolites, which
typically show repetitive lamination (Monty, 1976; Reid and Browne, 1991; Reid et al.,
1995; 2000; 2003; Feldmann and McKenzie, 1998; Matyszkyewitz et al., 2006; Arenas
and Pomar, 2010). From these, we selected 13 thin-section images with measurable
laminae. In each photomicrograph, maximum and minimum thicknesses of its
constituent laminae (grain-rich laminae and thin micritic crusts) were measured to show
the full thickness range of each example (Table 1). As in Leza coarse-grained
stromatolites, we recorded minimum thickness as zero where laminae thinned and
disappeared laterally. Our analysis assumes that the measured images were from
sections of the stromatolites cut essentially parallel to growth direction, as is usual in
stromatolite studies. The values we obtained should therefore reflect actual lamina
thickness. Nonetheless, our lamination analyses are based on the proportional
thicknesses of thin micritic crusts relative to grain-
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the lamination in these examples. Thus, these relative values should not be significantly
affected by oblique cutting of the samples, since the thicknesses of all laminae will be
similarly affected by the angle of the cut. Data from published examples are plotted in
the same way as for Leza coarse-grained stromatolites (Fig. 15). Their M/G values
range from 0.02-0.5 (mean = 0.16). The similarity between these thickness data (Fig.
15) and those measured in areas of Leza coarse-grained stromatolites with repetitive
lamination (Fig. 13) supports the view that most coarse-grained carbonate stromatolites
typically show repetitive lamination defined by thin micritic crusts that interrupt the
accretion of much thicker laminae, mainly formed by trapped carbonate grains.
Therefore, the quantitative approach presented here suggests a systematic
relationship between relative lamina-thickness values and lamination styles, with values
~1 characteristic of alternating lamination and much lower values (typically <0.3)
being characteristic of repetitive lamination Since lamination style can be directly
linked to fundamental accretion mechanism in stromatolites (i.e., periodic interruption
of a single accretion process vs. alternation of different accretion processes), the
characterization of lamination styles with this quantitative approach offers a valuable
additional tool that may be applied to studies of the origins and significance of
lamination in other examples from the wide variety of ages and settings in which
stromatolites have formed.
CONCLUSIONS
The Leza Fm contains one of the oldest known and best-preserved examples of
coarse-grained carbonate stromatolites. These partially resemble some present-day
Bahamian and Shark Bay examples that mainly form by trapping and binding of
carbonate grains. However, Leza coarse-grained examples show certain peculiarities
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which differentiate them from present-day examples, and which have important
implications in the interpretation of accretion processes:
- Laminae of Leza examples show a wider variety of compositions that include
grain-rich laminae formed by trapping and binding of particles, micrite-rich laminae
formed by microbially-induced precipitation of clotted and clotted-peloidal micrite, and
mixed grainy-micritic laminae formed by a combination of these processes. This
implies that surface microbial mats of Leza coarse-grained stromatolites could accrete
by grain trapping but also by in-situ calcification, trapping few or no grains. In addition,
thin micritic crusts developed at the tops of laminae during hiatuses in accretion due to
microbially-induced alteration and precipitation of carbonate. Scarce evaporite laminae
are relicts of intrasedimentary sulphates.
- Combinations of these processes created two distinct lamination styles in these
stromatolites: macroscopic alternating lamination formed by alternation of laminae of
contrasting microfabric (grain-rich, micrite-rich), and microscopic repetitive lamination
formed by successive laminae of similar microfabric (typically grain-rich laminae)
separated by very thin hiatal micritic crusts. Repetitive lamination appears to
predominate in most present-day and ancient examples of coarse-grained carbonate
stromatolites, but alternating lamination dominates Leza examples, showing that when
both lamination styles coexist within the same stromatolite, it is alternating lamination
that creates the conspicuous macroscopic lamination.
These differences in accretion process and lamination styles between the Leza
and other coarse-grained carbonate stromatolites are likely to be produced by differing
environmental conditions, since most other examples of coarse-grained carbonate
stromatolites are known from marine environments, whereas Leza examples formed in
tide-influenced coastal-wetlands in the varying presence of both seawater and
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freshwater. This sedimentary environment provided hydrodynamic and hydrochemical
fluctuations that help to account for the contrasting accretion processes reflected in the
lamination.
Numerical analysis of lamination in these Leza examples and in other coarse-
grained carbonate stromatolites shows that lamination style can be quantitatively
distinguished using the relative thickness of the constituent laminae. Stromatolite
lamination is often regarded as a relatively simple alternation of dark and light layers.
Our detailed petrographic and numerical study of Leza coarse-grained stromatolites
reveals additional complexity that is likely to help rather than hinder interpretation of
stromatolite accretion processes.
ACKNOWLEDGEMENTS
This study was funded by the Spanish DIGICYT Project CGL2011-22709, by
-CM 910429 of the
Complutense University of Madrid, and by a FPU scholarship from the Spanish
Department of Education. We are grateful to Concha Arenas and an anonymous
reviewer for detailed and helpful comments on the manuscript. We thank Beatriz Moral,
Gilberto Herrero and Juan Carlos Salamanca for thin-section preparation, to Modesto
Escudero for help with computing and scanning, and to Laura Donadeo for help with
bibliography.
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FIGURE CAPTIONS
Figure 1: A: Location of the Mesozoic Iberian Rift System (MIRS) and the
Cameros Basin in the Iberian Peninsula. B: Geological map of the Cameros Basin.
Outcrops of the Leza Fm are outlined in green and a rectangle shows the location of the
eastern outcrops of the Leza Fm, mapped in Figure 2. Modified after Mas et al. (2002a).
Figure 2: Geological map of the eastern outcrops of the Leza Fm and adjacent
units (see Fig. 1B for location). Asterisks mark localities where coarse-grained
carbonate stromatolites of the Leza Fm have been observed and sampled. CAN:
Canteras. ARN: Arnedillo. PÑ: Peñalmonte. PRW: West Préjano. PR: Préjano. PRE:
East Préjano.
Figure 3: A: Simplified log of the Arnedillo section, which is representative of
the facies and their vertical distribution in the eastern outcrops of the Leza Fm. B:
Facies sequence of the oolite-stromatolite facies association, which contains the
stromatolites studied here.
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Figure 4: Field photographs of the Leza Fm oolite-stromatolite facies
association. A: Vertical and lateral relationship between ooid grainstone beds and
relatively large domal coarse-grained stromatolites. Canteras section. B: Close-up view
of the ooid grainstones. Canteras section. C: Stromatolite horizon developed on top of a
rippled grainstone bed. Arrows point to flat-pebbles associated with the stromatolite.
Préjano section. D: Contact between coarse-grained stromatolite (below) and ooid
grainstone (above). Préjano section. E: Stromatolite horizon showing laterally-linked
domes and stratiform morphologies displaying the characteristic lamination of Leza
coarse-grained stromatolites. Préjano section.
Figure 5: Macroscopic lamination of Leza coarse-grained stromatolites and its
main microfabric components. A: Cut-slab of a sample from the Arnedillo section.
Macroscopic lamination is formed by alternation of dark and light laminae (see Fig. 8
for details). Additionally, microscopic lamination can be also observed in areas where
macroscopic lamination is inconspicuous (Fig. 9A). B: Microfabric mainly composed of
carbonate grains (grainy lamina). C: Microfabric mainly composed of clotted-peloidal
micrite (micritic lamina).
Figure 6: A: Grainy lamina with benthic forams (arrows). West Préjano section.
B: Transitional contact between a grainy lamina with some composite ooids (below)
and a micritic lamina (above). West Préjano section.
Figure 7: A: Micritic laminae composed of clotted micrite forming a millimetric
domal shape. Arnedillo section. B: Calcified microbial filaments preserved as clusters
of sinuous elongate clotted-peloidal micrite in a grain-dominated mixed lamina.
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Canteras section. C: Subvertical calcified microbial filaments (green arrow) preserved
as micritic rims in the micrite-dominated mixed lamina of the lower half of the image.
This lamina is topped by thin micritic crust which includes a level of small pyrite
crystals (yellow arrow). West Préjano section.
Figure 8: Alternating lamination. Mirror-image, same-scale comparison of a cut
slab of stromatolite from Figure 5A (A) and a photomicrograph of a thin section of the
same area (B). Blue solid lines mark approximate contacts between macroscopic
laminae. Differences in lamina colour broadly relate to changes in microfabric
composition and show alternating lamination. Yellow arrows indicate grain-dominated
mixed laminae and green arrows micrite-dominated mixed laminae. Red dotted lines
partially outline the thin micritic crusts that can be seen in the photomicrograph
(B).Note that these are generally macroscopically inconspicuous and only the thickest
ones can be seen in the magnified cut slab (A). Arnedillo section.
Figure 9: Repetitive lamination and thin micritic crusts. A: Detail of upper part
of Figure 5A showing an area of the sample dominated by microscopic repetitive
lamination. Lamination is inconspicuous in hand specimen (Fig. 5A) but is evident
under the microscope as thin micritic crusts (yellow arrows) over successive grain-
dominated mixed laminae. Note subtle differences in fabric in the middle mixed laminae
(red arrow): abundant clotted-peloidal micrite with scattered grains in the lower part,
changing gradually to a continuous thin level of grains in the middle part, which grades
upwards to grain-dominated with intergranular micrite. B: General view of various thin
micritic crusts (arrows) that mainly cap grain-rich, but also micrite-rich, laminae.
Peñalmonte section. C: Area of a sample (Préjano section) showing repetitive
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lamination. D: Detail of C showing repetitive lamination formed by thin micritic crusts
(arrows) overlying successive grainy laminae. Better developed thin micritic crusts
(yellow arrows) are thicker, more conspicuous, and typically involve grain
micritization, whereas less developed ones (green arrows) are thinner and less
conspicuous. E: Detail of Figure 7C showing small pyrite crystals (<20 µm) (arrow) at
the top of a thin micritic crust. West Préjano section.
Figure 10: A: Thin micritic crust; relatively poorly developed, consisting of a
thin micritic film gradually passing down into intergranular clotted micrite. Préjano
section. B: Erosively truncated thin micritic crust, more developed than in A, involving
micritization and grain fusion (arrowed). Additional accumulation of darker micrite
above the erosion surface might represent a superposed thin micritic crust. Arnedillo
section.
Figure 11: Micritization in thin micritic crust. A: Relatively well-developed thin
micritic crust. Grains below the crust surface have diffuse boundaries due to intense
micritization and grain fusion. Note a vertical trend from strongly micritized grains in
the upper part, to partially micritized grains in the middle part, and very little
micritization in the lower part. Arnedillo section. B and C: Less developed thin micritic
crusts, in which grains immediately below the crust surface are only partially micritized
(arrows). Préjano section.
Figure 12: Evaporite laminae from West Préjano section. A: General view of
laminae formed by aggregates of pseudomorphs after sulphates. Note how the
aggregates deform adjacent laminae. B: Detail of A showing that aggregates are
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composed of small pseudomorphs after sulphates (arrows) and include components of
other laminae (carbonate grains and micrite).
Figure 13: Plots of relative lamina-thickness from 14 thin-sections of Leza
coarse-grained stromatolites. Each line in both plots summarizes data from a particular
area of a thin-section clearly displaying one of the two lamination styles observed in
Leza coarse-grained stromatolites. Several laminae were measured in each area under
the microscope, but only maximum and minimum values are joined by each line, in
order to show the full thickness range of the measured area. In the common situation
where laminae thin and disappear laterally, minimum thickness is plotted as zero.
Photomicrographs show an example of each lamination style with bars marking the
thicknesses of laminae. Blue: grain-rich laminae; orange: micrite-rich laminae; yellow:
thin micritic crusts. A: Plot of areas displaying alternating lamination. B: Plot of areas
displaying repetitive lamination.
Figure 14: Histograms of 192 lamina-thickness values measured in 14 thin-
sections of Leza coarse-grained stromatolites. A: Histogram of lamina-thickness
showing a right-skewed distribution. B: Histogram of the natural logarithm of lamina-
thickness. Mixture of two normal populations was obtained for these
transformed data by model-based clustering (Fraley et al., 2012). First population: red;
second population: blue; mixture of populations: green. Shaded areas represent the
interval.
Figure 15: Plot of thirteen relative lamina-thickness data from eight published
studies of present-day and ancient coarse-grained carbonate stromatolites (Table 1).
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Each line represents one example in which both maximum and minimum thicknesses of
the constituent laminae were measured, using the same methodology as in Figure 13.
Table 1: Lamina thickness data from published photomicrographs of coarse-
grained carbona/ values were obtained using the midpoints of the
thickness ranges of thin micritic crusts and of grain-rich laminae: (min+(max-
min)/2)/ (min+(max-min)/2). These data are plotted in Fig. 15.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Figure 7
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Figure 8
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Figure 9
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Figure 10
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Figure 11
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Figure 12
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Figure 13
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Figure 14
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Figure 15
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Reference
Minimum
thickness of thin
micritic crusts –
M’min (µm)
Maximum
thickness thin
micritic crusts –
M’max (µm)
Minimum
thickness of
grain-rich
laminae – G’min
(µm)
Maximum
thickness of
grain-rich
laminae – G’max
(µm)
M’/G’
Arenas & Pomar (2010) p.
478 Fig. 10e
19
115
2308
3269
0.02
Arenas & Pomar (2010) p.
478 Fig. 10f
0
114
314
2571
0.04
Arenas & Pomar (2010) p.
478 Fig. 10g
0
70
330
609
0.07
Feldmann & McKenzie
(1998) p. 206 Fig. 11a
0
500
1250
3000
0.12
Feldmann & McKenzie
(1998) p. 206 Fig. 11b
200
550
750
2800
0.21
Matyszkiewicz et al.
(2006) p. 258 Fig. 5d
0
381
0
6190
0.06
Monty (1976) p. 214 Fig.
13c
0
667
0
3333
0.20
Reid & Browne (1991) p.
17 Fig. 5a
0
29
100
714
0.04
Reid et al. (1995) p. 17
Plate 7/1b
0
313
213
500
0.44
Reid et al. (2000) p. 991
Fig. 4b
0
938
0
7188
0.13
Reid et al. (2003) p. 307
Plate 46/1c
80
680
360
1160
0.5
Reid et al. (2003) p. 307
Plate 46/1g
60
200
800
1040
0.14
Reid et al. (2003) p. 309
Plate 47/1b
0
231
0
1923
0.12
Table 1: Lamina thickness data from published photomicrographs of coarse-grained
values were obtained using the midpoint of the thickness
range omin+(max-min)/2)/ (min+(max-min)/2).
These data are plotted in Fig. 15.
