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Pedogenic and early diagenetic processes in Palaeogene alluvial
fan and lacustrine deposits from the Sado Basin (S Portugal)
N.L.V. Pimentel *
Departamento e Centro de Geologia, Faculdade de Cieˆncias da Universidade de Lisboa, Campo Grande C-2, 5j, 1700 Lisbon, Portugal
Accepted 1 August 2001
Abstract
The Palaeogene deposits of the Sado Basin were deposited in a continental basin that shows a typical pattern with alluvial
fans system in the margins of the basin, passing towards distal areas of mudflat facies where, in some areas, the installation of
shallow water bodies favoured the development of palustrine conditions. The deposits of this basin vary form coarse
conglomerates and sandstones to palustrine carbonates. These sediments were affected by pedogenesis and early diagenetic
processes that promoted important modifications on their primary features. These modifications have been studied by the
analyses of four profiles, developed on proximal, middle and distal fan deposits and the fourth one on lacustrine deposits. The
overall analyses of the sedimentological, pedogenic, diagenetic features and their relationships indicate that three main
processes took place throughout the basin: soil formation, palygorskite neoformation and dolomitization. Soil formation
processes led to illuviation of clays and carbonate precipitation mostly around roots. Pedogenic carbonates increase towards
distal areas, whereas hydromorphic features are present throughout the basin. Palygorskite neoformation was partially
diagenetic, being maximum in proximal areas and palustrine deposits. This neoformation is attributed to the percolation of
alkaline Mg-rich soil and groundwaters through smectitic-rich sediments, promoting important clay transformation.
Dolomitization was an early diagenetic process that occurred mainly in carbonate-rich deposits of distal and lacustrine
environments, as a result of the increasing Mg/Ca ratio of the percolating groundwaters. In all these processes there has been a
close spatial and temporal interplay between pedogenesis and diagenesis, driven by the chemistry of soil particles and
groundwaters. D2002 Elsevier Science B.V. All rights reserved.
Keywords: Alluvial fans; Lacustrine carbonates; Palygorskite; Dolomite; Paleosols; Groundwater
1. Introduction
Closed terrestrial basins enclose a widespread
record of sedimentary deposits ranging from (i) the
‘‘classical’’ freshwater model in which alluvial fan
deposits grade to floodplain/mudflat deposits and to
freshwater lacustrine and palustrine carbonates to (ii)
the saline model in which alluvial fan deposits grade
towards the basin centre to ephemeral or perennial
saline lakes in which a variety of evaporite minerals
precipitated. Commonly, it has been shown that there
is an evaporitic trend towards the centre of the basin,
resulting in the precipitation of the more soluble salts
in these areas (Eugster and Hardie, 1978; Arakel,
1986; Arenas and Pardo, 1999), and also in an increase
in Mg content as calcium rich minerals tend to precip-
0037-0738/02/$ - see front matter D2002 Elsevier Science B.V. All rights reserved.
PII: S 0037-0738(01)00213-5
*
Fax: +351-21-7570064.
E-mail address: Pimentel@fc.ul.pt (N.L.V. Pimentel).
www.elsevier.com/locate/sedgeo
Sedimentary Geology 148 (2002) 123– 138
itate from more dilute solutions in less central areas
(Armenteros et al., 1995; Calvo et al., 1995). In these
closed basins, the precipitation of carbonate minerals
and the distribution of the clay mineral associations
not only are related to the sedimentary processes, but
also to the pedogenic and early diagenetic processes
that affected the primary deposits (Freytet and Plaziat,
1982; Armenteros et al., 1995; Colson and Cojan,
1996; Arenas et al., 1999). The result is a complex
assemblage of lithofacies in which it may be not easy
to know if precipitation of carbonates, evaporites and/
or silica, as well as neoformation of clays, took place
in the pedogenic, shallow lake or early diagenetic en-
vironment (Calvo et al., 1986; Spo¨tl and Wright, 1992;
Rodas et al., 1994; Pimentel et al., 1996; Pimentel,
1998a).
During the Palaeogene, the Sado Basin (in south-
western Portugal) was a closed basin in which the
mineralogy and texture of the primary facies (flood-
plain/paleosols and palustrine deposits) underwent
significant changes that resulted in the extensive
occurrence of palygorskite, calcretes and dolocretes.
The aim of this paper is the study of the pedogenic
and early diagenetic processes that operated on the
alluvial and lacustrine deposits of the Sado Basin. In
doing so, we try to provide some criteria to identify
strictly pedogenic, palustrine and early diagenetic
processes. Moreover, the distribution of these features
along the basin can provide a new example that con-
tributes to understand the hydrology, chemistry and
diagenesis of closed semi-arid basins.
2. Geological setting
The Sado Basin is a small Tertiary basin, located in
the southwestern area of the Iberian Peninsula, which
was in connection with the larger Lower Tagus basin
(Fig. 1). The formation and tectono-sedimentary evo-
lution of these basins are related to the relative move-
Fig. 1. Location and geological setting of the study area, at the Sado Basin. L—Lisbon, M—Madrid, GF—Gra
ˆndola Fault, MPF—Messejana –
Plase
ˆncia Fault, TF—Torra
˜o Fault.
N.L.V. Pimentel / Sedimentary Geology 148 (2002) 123–138124
ments between the African and the European litho-
spheric plates, during the alpine orogeny (Tapponier,
1977; Boillot et al., 1984; Uchupi, 1988; de Galdeano,
1996). The convergent movement of those plates
started in the Palaeocene, leading to intense deforma-
tion in the western Mediterranean, but only in the
Eocene the compression was transmitted further west
into the Iberian Massif. Here, the N –S compression
reactivated several originally late Hercynean faults,
most of them as NE –SW strike-slip sinistral faults.
Several tertiary iberian basins were then formed, such
as the Tagus– Sado Basin between the portuguese
Central System and the Messejana –Plase
ˆncia Fault
(MPF, Fig. 1).
The Sado Basin is bounded by the previously
mentioned Messejana–Plase
ˆncia Fault and by two
WNW–ESE late Hercynean reactivated faults, the
Torra
˜o and Gra
ˆndola Faults (TF and GF, Fig. 1). These
two accidents acted mainly as dip-slip faults, forming a
small graben between them, in relation to the first
Middle Eocene compressive phases. The sedimentary
response was the infilling by thick alluvial fan deposits
(Vale do Guizo Formation), described in this paper.
During the Oligocene and Early Miocene, with the
rotation of the main compression from NE –SW to-
wards E–W (de Galdeano, 1996), subsidence was at-
tenuated and active sedimentation stopped in this
basin, restricting the Neogene sedimentary record to
less than 40 m of Messinean fluvio-marine deposits
and Piacenzian fluvial sands (Pimentel, 1998b).
The Neogene deposits lie unconformably and cover
most of the Palaeogene, mainly at the SW half of the
basin, reducing the outcrops for study. However, the
sub-surface data (from drillings aiming the underlying
Palaeozoic rocks) brought some knowledge about the
Palaeogene deposits in other areas. The Palaeozoic
basement lies at altitudes between + 100 and 250 m,
showing a general dip towards the SW (with several
small horsts and grabens), accompanied by an increas-
ing thickness of the Palaeogene sequence, from about
20 m (near the Torra
˜o Fault) to a maximum of 250 m
(near the Gra
ˆndola Fault). There are strong evidences
for a syn-sedimentary origin of this dip, namely the
presence at the SW border of the Basin of porphyritic
basic clasts — with indubitable provenance from the
igneous areas forming the footwall of the Torra
˜o Fault
to the NE (Fig. 2).
The Palaeogene fill of the Sado basin is composed
by reddish coarse detrital sediments of the Vale do
Guizo Formation (Pimentel, 1998c,d). This formation
Fig. 2. Simplified paleogeographic reconstruction for the Palaeogene fill of the Sado Basin (Vale do Guizo Formation). Fan areas are represented
in the upper horizontal plan, members of the formation in the right-hand vertical plan, and carbonate accumulations in the front vertical plan.
The four studied profiles are represented by a square in the sedimentary sequence and by a circle in their geographic location: A—Vale do Gaio,
B—Sa
˜o Roma
˜o, C — Vale do Guizo, D — Porches. S — Synthetic litostratigraphic section of the Vale do Guizo Formation.
N.L.V. Pimentel / Sedimentary Geology 148 (2002) 123–138 125
has been divided into three members which are in part
laterally equivalents (Fig. 2). The lower member (up
to 25 m thick) is represented by coarse conglomeratic
deposits (with quartzic and porphyritic clasts), show-
ing an intense white/pinkish mottled cementation by
carbonates and clays. The intermediate member (gen-
erally about 40 m thick) includes alternating brick-
reddish sandstones and clays with abundant post-
depositional features (peds, mottling and carbonate
concretions). The upper member (up to 20 m thick) is
composed mainly of yellowish clays and sandstones
with important carbonate accumulations, including a
massive 5-m-thick carbonate layer capping the Palae-
ogene deposits in distal areas. The sedimentary
sequence clearly shows a fining-upwards trend, indi-
cating a retrogradation of the depositional system
(Fig. 2). This may be due to a single tectonic up-lift
of the Torra
˜o Fault and subsequent ageing of the
hanging-wall relief (Pimentel, 1998c).
The Palaeogene deposits mainly consist of polimic-
tic conglomerates, arkosic sandstones, clays and car-
bonates, corresponding to the following lithofacies
(modified from Miall, 1977): Gms (gravelly debris-
flow deposits), Sc (sandy debris-flow deposits), Sm
(sandy massive settling deposits), Fm (suspended-load
settling deposits) and P (carbonate accumulations).
The deposits are organized in tabular 2 –5-m-thick
fining-upwards cycles, representing repeated episodes
of mainly non-channelized transport and aggradation,
with almost no erosive scouring (Nemec and Steel,
1984; Blair, 1987; Blair and McPherson, 1994). Four
basic architectural elements were recognized (modi-
fied from Miall, 1985): SG (sediment gravity flows),
LS (laminated sand sheets), OF (overbank fines) and
PL (carbonate layers). SG corresponds to debris flows,
LS is interpreted as the product of flash floods and
ephemeral streams, OF includes waning flood sands or
clays and floodplain muds (Miall, 1977, 1985) and PL
corresponds mainly to palustrine carbonate precipita-
tion. This association seemingly resulted from high
energy flows inundating large areas of sheet-flood
dominated alluvial fans, representing the proximal
and middle fan facies (Loewe, 1982; Bluck, 1987;
Harvey, 1987, 1989; Galloway and Hobday, 1996).
Distal fan or floodplain deposits are also present,
consisting of fine sandstones and clays, with abundant
pedogenic features and important lenses of palustrine
carbonates.
3. Methods
Previous studies carried out in the Sado Basin
(Pimentel, 1998b,c) have clearly established the dis-
tribution of the alluvial sedimentary environments
during the Palaeogene, and a clear transition from
proximal alluvial fan to distal fan and palustrine
environments was recognized (Fig. 2). The alluvial
deposits formed in these environments underwent
different pedogenic and diagenetic modifications
whose origin and significance are the aim of this
paper. Taking in mind the previous studies, we have
selected four main profiles which are representative of
the four main sedimentary environments that pre-
vailed in the basin during the Palaeogene (see Fig. 2
for location): (A) proximal fan facies of the lower
coarser member, close to the basin’s northeastern
border (Torra
˜o Fault); (B) middle fan facies of the
intermediate sandy member, about 15 km from the
Torra
˜o fault; (C) distal fan facies of the intermediate
sandy member, about 30 km from the Torra
˜o Fault;
and (D) distal fan and palustrine facies of the upper
member, close to the previous profile.
For each profile, four to six samples representing
the main different horizons were obtained. Thin-sec-
tions were studied with Ana Alonso-Zarza (Madrid
University) under the transmitted light microscope,
most of them after alyzarine-red staining for carbo-
nates identification. The clay mineralogy was deter-
mined by Diamantino Pereira (Minho University) for
the < 2 Afraction (after treating the powdered samples
with HCl 10% for 24 h), using a Philips PW-1710
XRD system operating at 40 kV and 30 mA with a
Monochrome CuK
a
radiation. Scanning Electron
Microscopy was carried out at the Universidad Com-
plutense de Madrid with a JEOL 6400 at 20 kV, on
several fracture surfaces covered with gold.
4. Pedogenesis and diagenesis of the alluvial
deposits of the Sado Basin
4.1. Proximal fan areas
4.1.1. Description
The studied profile is located on the eastern side of
the Vale do Gaio dam (A in Fig. 2). It shows the
Palaeozoic basement and the first 3 m of the lower
N.L.V. Pimentel / Sedimentary Geology 148 (2002) 123–138126
member at this site (Fig. 3A). The low-grade meta-
morphic slates present an argilic alteration (1 – 2 m
thick) grading from brown-reddish into whitish col-
ours towards the top. The overlying deposits are
organized in 30–50 cm thick tabular layers of com-
pact and light coloured appearance, formed by mas-
sive conglomerates and clays, strongly indurated by a
dull whitish low-density material. This material
involves medium-sized (up to 10 cm) igneous por-
phyritic and quartzic clasts, along with large ones (up
to 30 cm) eroded from the local metamorphic base-
ment. There is an upwards vertical gradation in which
the layers become thinner and finer, the sediments are
less indurated and the carbonate content (almost
absent in the first 2 m) increases towards the top.
These are mainly chalky carbonates occurring in cm-
thick layers with thin irregular horizontal laminae.
The petrographic study shows that both the meta-
morphic substrate and the breccia have been highly
modified, showing an important corrosion of the
substrate as well as of the detrital grains. The sub-
strate presents an intense alteration, with patches of
iron oxides and clays penetrating the slate cleavage.
There are also abundant elongated sinuous stripes
with oriented clays crossing the altered slate. The in-
durated deposits show, from base to top of the profile,
a clear evolution trend. In the lower layers, the clayey
matrix is partially replaced by carbonates, which then
corrode and etch the grains by micritic strings, enter-
ing and breaking the grains or in other cases coating
them (Figs. 4A and 6A). Locally, the micritic strings
are included in elongated coarse calcite crystals (spar)
that also enter the grain or coat them. The upper
layers show an abundant microcrystalline matrix,
with some irregular patches of clay, containing cor-
roded and replaced grains. In some cases, carbonate
completely replaces the feldspar, mica or even quartz
grains.
The clays of the whitish weathered substrate are
mainly palygorskite (80%) and smectite. The Palae-
ogene sediments have a similar content, showing an
upward increase in palygorskite (from 20% up to
90%, av. 55%), while smectite decreases accordingly,
and illite presents values around 15% (Fig. 3A).
4.1.2. Interpretation
These sediments represent coarse debris-flows
deposited in proximal areas of alluvial fans (Fig. 2).
In the studied profile, the substrate was covered by
massive conglomerates with an abundant clayey
matrix (smectitic and illitic), inherited from the alter-
ation of the igneous rocks (including basic lithologies)
at the source area. The paleoalteration of the substrate
might have been previous to deposition, or partially
overprinted by the same post-depositional processes
that affected the whole profile. In outcrop, no clear
pedogenic horizon development is seen in the first
meters of the sequence, but the massive aspect may be
due to diagenetic modification: as seen in thin-section,
pedogenic processes were able to produce some car-
bonates that partially (and increasingly upwards) sub-
stituted the clayey matrix and etched the grains.
The most outstanding feature of this profile is the
widespread presence of palygorskite. The occurrence
of palygorskite in continental deposits is generally
ascribed to lacustrine settings, to soils of arid and
semi-arid regions, or to early diagenetic transforma-
tions (Singer and Gala
´n, 1984; Leguey et al., 1985;
Velde, 1985, 1995). Lacustrine deposits are totally
absent in this profile. Pedogenesis can be important in
proximal areas of alluvial fans (Wright and Alonso-
Zarza, 1990), but it seems insufficient to justify, for
itself, the large amounts of authigenic palygorskite
along the profile. Therefore, early diagenesis associ-
ated with phreatic circulation must have played an
important role in this massive and intense cementa-
tion. Highly modified waters, namely with an in-
creased Mg content, may have circulated through the
porous deposits, modifying them downwards and
eventually reaching the substrate. This process would
promote the partial dissolution of silicates by alkaline
waters (Velde, 1985; Singer, 1989; Wang et al., 1993)
as well as the neoformation of palygorskite from smec-
tite. This neoformation explains the antithetic behav-
iour of those two minerals along the profile, and might
have taken place as a simple replacement after dis-
solution (Vidales et al., 1987; Jones and Galan, 1988),
or as a result of repeated solution-precipitation events
(Velde, 1985).
In pedogenic calcretes, sepiolite and palygorskite
formation is a common process that can be explained
by the increase of Mg/Ca ratio due to the precipitation
of calcite within the soil profile (Watts, 1980). On the
contrary, in subsurface non-pedogenic environments,
the presence of these clays is less frequent. Some
examples come from Sanchez and Blanco (1986) and
N.L.V. Pimentel / Sedimentary Geology 148 (2002) 123–138 127
Fig. 3. Sketch of the profiles and clay mineralogy of the four studied sites (see Fig. 2 for location). A — Proximal fan areas profile (Vale do
Gaio); B — Middle fan areas profile (Sa
˜o Roma
˜o); C — Distal fan areas profile (Vale do Guizo); D — Palustrine areas profile (Porches).
* Position of the samples depicted in Fig. 4 and shown in Fig. 6. Scale bar = 30 cm.
N.L.V. Pimentel / Sedimentary Geology 148 (2002) 123–138128
Rodas et al. (1994), who describe the formation of
‘‘groundwater calcretes and palycretes’’ (sic) in Palae-
ogene deposits of the Tajo and Duero basins in Spain.
In these situations, the precipitation of calcite in
proximal areas would be responsible for the increase
of Mg/Ca ratio downwards in the groundwaters drain-
age system and consequent distal precipitation of
palygorskite. In the Sado Basin, the abundance of
this mineral in coarse proximal deposits is interpreted
as a result of (i) the high Mg content of groundwaters,
due to the alteration of igneous basic rocks in the
source area, and (ii) the irregularity and impermeabil-
ity of the Palaeozoic basement, confining groundwa-
ters in closed depressions subjected to intense chem-
ical modifications.
To explain the complex textural and genetic rela-
tionships among carbonates, detrital silicates and
clays of this profile, we propose that both pedogenic
processes (related to meteoric waters) and phreatic
processes (related to groundwaters) might have coex-
isted in space and alternate in time, as a result of
repeated small-scale variations. Factors such as sed-
imentation rate, rain abundance and infiltration, sea-
sonal temperatures, internal drainage and evaporation,
would promote frequent variations in pH, Si content
and Mg/Ca ratio. These variations are enough to allow
either the precipitation of pedogenic carbonates, dis-
solution of primary silicates or neoformation of paly-
gorskite (Wang et al., 1993).
4.2. Middle fan areas
4.2.1. Description
The study site is located on the left bank of the
Sado River at Sa
˜o Roma
˜o, where most of the Palae-
ogene basin-fill (here about 60 m thick) is exposed.
Fig. 4. Sketch of thin-sections from selected samples of the studied profiles (see Figs. 2 and 3 for location). A — Vale do Gaio; B — Sa
˜o Roma
˜o;
C — Vale do Guizo; D — Porches. Scale bar = 1 mm. See also Fig. 6.
N.L.V. Pimentel / Sedimentary Geology 148 (2002) 123–138 129
The selected profile is located at the middle part of the
Palaeogene sequence (B in Fig. 2, intermediate mem-
ber), where the deposits are composed mainly of
coarse sandstones with minor clays and conglomeratic
layers, organized in 2–3 m thick cycles. The profile
comprises a single cycle with a reddish sandy – con-
glomeratic lag, grading upwards into brick-red sand-
stones and mottled clays (Fig. 3B). Visual pedogenic
features are abundant and include hydromorphic mot-
tling, prismatic peds, slickensides, carbonate crossed
filaments and nodules, all of them increasing upwards.
The top of the profile is marked by an erosive surface
with coarse deposits of the following cycle, which
shows identical sedimentary and pedogenic character-
istics (Fig. 5A).
The petrographic study shows that these deposits
consist of a mixture of sand-size grains, clays and fine
crystalline carbonate (Figs. 4B and 6B). At the base of
the profile, the matrix composed of clay and iron
oxides is crossed by a dense network of micritic car-
bonates, while at the top it contains mainly carbo-
nates. The particle arrangement indicates clear
pedogenic micromorphologies such as clayey vosepic
and skelsepic fabrics in an overall aglomeroplasmic to
intertextic textures (Brewer, 1964). Other pedogenic
features commonly recognized are: alveolar struc-
tures, root tubes, thin micritic filaments and coatings,
increasing in abundance towards the top of the profile.
Most of the carbonate is dolomite that appears as fine
(0.015–0.05 mm) rhombohedral crystals or just as
dolomicrite forming the previously mentioned tex-
tures. SEM studies have shown that some of the
dolomite rhombs are coated by masses of palygorskite
fibres (Fig. 7).
The clay content is composed predominantly by
smectite (av. 50%) and palygorskite (av. 40%), with
Fig. 5. Outcrop photos of the Vale do Guizo Formation (see Fig. 2 for location). A—Middle fan deposits at Sa
˜o Roma
˜o: notice the two
depositional finning-upwards cycles (Fig. 3B represents the first cycle), the erosive surface covered with coarse sands of the second cycle and
the network of white carbonates increasing towards the top of each cycle; B—Palustrine deposits at Porches: notice the presence of large-scale
pseudomicrokarst (arrows) (vd. Fig. 3D).
N.L.V. Pimentel / Sedimentary Geology 148 (2002) 123–138130
minor illite, showing no significant variation through-
out the profile (Fig. 3B).
4.2.2. Interpretation
These deposits represent conglomeratic and sandy
sheets, as well as floodplain clays deposited in middle
fan areas (Fig. 2). The profile represents the develop-
ment of a paleosol on a fining-upwards single depo-
sitional cycle. This paleosol presents abundant
hydromorphic and vertic features, indicating seaso-
nal drought and short-lived water-table fluctuations
(Wright, 1989; Retallack, 1990). Petrographic fea-
Fig. 6. Photomicrographs of thin sections from the four studied profiles (see position of samples in Fig. 3), representing the different alluvial
fan to palustrine deposits. A — Proximal fan (Vale do Gaio): detrital grains presenting a thin micritic coating and oriented palygorskitic
argilanes (skelsepic fabric); B — Middle fan (Sa
˜o Roma
˜o): skelsepic fabric with rhizogenic voids partially covered by micrite and palygorskite;
C — Distal fan (Vale do Guizo): argilic glaebules, some fine detrital grains and circum-granular cracks; D — Palustrine (Porches): micritic
nodules with desiccation cracks partially filled by microsparite, containing some fine detrital grains; E — Palustrine (Porches): dolomitization
fabric with an homogeneous and crystalline texture of euhedral to sub-hedral rhombs presenting a notorious inclusion-rich darker centre. Scale
bar = 1 mm (except in E). Compare with sketches of Fig. 4, depicting other areas of the same samples.
N.L.V. Pimentel / Sedimentary Geology 148 (2002) 123–138 131
tures point to processes of clay illuviation throughout
the paleosol and precipitation of mainly rhizogenic
carbonates (Alonso-Zarza et al., 1998) at the top. The
profile exhibits evidence of rising phreatic table
periods, indicated by increasingly reducing mottling
and carbonate accumulation upwards, such as des-
cribed by Alonso-Zarza et al. (1992) in the Madrid
Basin. Internal drainage was therefore gradually
reduced as the water-table rose, promoting accu-
mulative hydromorphy (Bown and Kraus, 1987)
and pseudo-gley conditions (PiPujol and Buurman,
1994).
Fig. 7. SEM photos from samples representing paleosols developed on distal fan deposits (Vale do Guizo profile, Fig. 3C). A — Dolomitic
rhombs covered by a thin film and fibres of clay; B—Detail of an intensively corroded dolomite crystal, covered by palygorskite fibres;
C — Amalgamated palygorskite fibres.
N.L.V. Pimentel / Sedimentary Geology 148 (2002) 123–138132
Most of the pedogenic calcite was affected by
dolomitization, which overprinted the initial carbonate
vertical pattern. Dolomitization is generally found in
the proximity or around lacustrine environments with
saline waters subjected to strong evaporation, in which
gypsum precipitation is attained (Calvo et al., 1995;
Colson and Cojan, 1996; Arenas et al., 1998, 1999).
But in this case, there is no evidence of gypsum or
halite, and lacustrine-related dolomitization fabrics
were not identified (see below, interpretation of palus-
trine areas). Therefore, the increase of the Mg/Ca ratio
needed for dolomitization may be attributed mainly to
groundwater evolution. In this profile, palygorskite
might have resulted from neoformation by pedogenic
processes, commonly associated with calcrete forma-
tion as described in various situations (Sancho et al.,
1992; Rodas et al., 1994; Long et al., 1997; for
example), but the random variation of palygorskite
content along the profile does not show it clearly. SEM
observations indicate that part of it had to be formed
after dolomite (Fig. 7), and therefore it was also
associated with Mg enrichment of soil and/or ground-
waters, as described by Verrechia and Le Coustumer
(1996) in Pleistocene calcretes in Israel, or Pozo and
Vidales (1989) and Suarez et al. (1994) in Tertiary
lacustrine deposits in Spain.
4.3. Distal fan areas
4.3.1. Description
This study site is located along a road from Vale
do Guizo to Porches, with exposures of most of the
Palaeogene sedimentary fill (here 80 m thick) of
distal areas. The profile is approximately at 60 m of
the Palaeogene sequence (C in Fig. 2, upper mem-
ber), where interbedded sandstones and clays are
organized in 2–3 m thick cycles. The profile corre-
sponds to a single cycle with reddish sandstones
grading upwards into whitish clays (Fig. 3C). These
clays show well-developed pedogenic features, in-
cluding mottling, slickensides with a pinkish thin
film, and abundant carbonate accumulations increas-
ing upwards. Commonly, and increasingly towards
the top of the sequence, the paleosols are overlain by
lenticular carbonate beds, up to 50 cm thick. The
carbonates are relatively homogeneous dolomicrites
that show horizontal and circum-granular desiccation
cracks.
The petrographic study shows the presence of some
carbonates in the sandy base of the profile. The matrix
is mainly composed by consistent oriented clays and
some iron oxides, with restricted patches of micritic
calcite, involving altered feldspar, lithoclasts and
quartz grains (Figs. 4C and 6C). The upper layers
show an abundant carbonate matrix composed mainly
by dolomitic rhombs involving small etched detrital
grains. Some micritic patches with iron oxides give a
mottled appearance, and oxides are also present among
the larger rhombs with inclusion-rich dark centres.
Mineralogy of clays shows no significant variation
along the profile, always with palygorskite (av. 45%)
and smectite (av. 45%) predominating over illite (Fig.
3C).
4.3.2. Interpretation
These tabular sand-sheet deposits represent flood-
plain environments with interbedded mudflat clays,
deposited in distal areas of alluvial fans (Fig. 2). The
sub-aerial exposure of the mudflat promoted intense
pedogenesis that induced the precipitation of carbo-
nates within the soil profiles. Precipitation of calcite
was either biochemically and physicochemically in-
duced. Physicochemical processes include repeated
wet–dry cycles and the episodic rise of carbonate satu-
rated water-tables, which could favour the weathering
of the silicates and the increase of Ca and HCO
3
in the
profiles (Wang et al., 1993). The presence of both
smectite and palygorskite possibly reflects the result
of pedogenic replacement processes, such as described
by Rodas et al. (1994). Episodically saturated water-
table and vegetal colonisation might have contribu-
ted to the development of these carbonate paleosols
(Wright and Tucker, 1991). The occasional presence of
carbonate layers capping paleosols is due to the sub-
sequent installation of palustrine conditions, producing
alternating carbonate accumulation and exposure fea-
tures (slickensides and desiccation cracks). This suc-
cession is attributed to a rise of water, eventually
originating a pond, such as in some examples from
the Tertiary in Spain (Platt, 1989; Alonso-Zarza et al.,
1992; Sanz et al., 1995).
Calcite produced by pedogenic processes and in
palustrine conditions was partially substituted by dolo-
mite, with preservation of its primary fabric. Condi-
tions for dolomitization could have been produced by
an increase of Mg/Ca ratio in the soil profiles, caused
N.L.V. Pimentel / Sedimentary Geology 148 (2002) 123–138 133
by the previous precipitation of calcite. Dolomitization
is therefore considered as an early diagenetic process.
Coarse dolomitic crystals and xenotopic fabrics indi-
cative of more advanced diagenetic processes (Gregg
and Sibley, 1984) are not common in the study area.
4.4. Palustrine areas
4.4.1. Description
The study site is located near Porches, and corre-
sponds to the top part of the Palaeogene sequence (D in
Fig. 2, upper member). The general finning-upwards
trend of the sequence is accompanied by increasing
carbonate content, as palustrine deposits become
thicker towards the top. The profile corresponds to a
4-m-thick carbonate capping layer, which shows a
transitional contact with the underlying detrital depos-
its (Fig. 3D). It shows a lower nodule-rich term,
occasionally with prismatic structure, and an upper
more massive one, both showing typical palustrine
features (see Freytet and Plaziat, 1982; Alonso-Zarza
et al., 1992; Armenteros et al., 1995; Wright and Platt,
1995) such as nodules, desiccation cracks, mottling
and granular textures. Pseudomicrokarst cavities, filled
by fragments of the surrounding limestones, are
present mainly in the lower term and show an impor-
tant vertical development, with tubes of more than 50
cm in length and angular fragments up to 3 cm across
(Fig. 5B). Occasionally, some gravel lenses occur
eroding part or the lower term; the passage from the
coarse detrital gravels to the top of the sequence is
gradual as the gravels become finer-grained and are
progressively embedded in the micritic carbonate.
The microfabric of the carbonates in the profile
varies from the base to the top. At the base the
microstructure consists of a mosaic of microspar with
horizontal desiccation cracks, corroded detrital grains
and dense micritic carbonate nodules (Figs. 4D and
6D). The mineralogy of these carbonates is mostly
dolomite, with minor calcite. Upwards, there is an
increase in dolomite content, resulting in a relatively
homogeneous and crystalline texture with euhedral to
sub-hedral rhombs having a distinctive inclusion-rich
darker centre (Fig. 6E) (vd. Colson and Cojan, 1996;
Pimentel et al., 1996). The desiccation and root cracks
are filled by coarse spar calcitic cement (Fig. 4D).
In these palustrine deposits, the clay content de-
creases upwards and is dominated by palygorskite (av.
70%), with minor smectite, and illite (Fig. 3D). The
palygorskite increases upwards (from 40% to 80%),
apparently accompanying the dolomite trend.
4.4.2. Interpretation
The macro and microstructure of these deposits
reflect the gradual installation of a shallow lake in
distal parts of alluvial fans (Fig. 2), whose arrangement
is similar to the ones described for carbonate ponds in
the Palaeogene of Languedoc (Freytet, 1973; Freytet
and Plaziat, 1982) or in the Miocene of the Madrid
Basin (Sanz et al., 1995). The extensive occurrence of
pseudomicrokarst, mottling and desiccation cracks,
clearly indicates the existence of very shallow water
bodies with periods of desiccation, followed by a rise
of the water-table leading to renewal of lacustrine con-
ditions. These processes are typical of palustrine envi-
ronments (Freytet and Plaziat, 1982; Wright and Platt,
1995) that are very common in low gradient, low ener-
gy lake margins (Platt and Wright, 1992). The occur-
rence of gravel lenses, sometimes overlying pseudomi-
crokarst structures of large vertical development, may
indicate a lowering of the base level, which led chan-
nels to reach more distal areas of the basin. No gypsum
or other soluble salts have been recognized in these
carbonates, and some charophytes (Chara aff. dollfusi,
Psilochara sp.) are present in laterally equivalent beds
(Azere
ˆdo and Carvalho, 1986), altogether indicating a
predominance of shallow and fresh water conditions.
Dolomitization took place later on and very prob-
ably was not related to the same chemistry of lake
water. Very early dolomitization processes driven by
evaporation or by an increase in Mg/Ca rate of lake
waters tend to form micritic fabrics such as the ones
described by Calvo et al. (1995) in the Miocene of the
Madrid Basin, or Arenas et al. (1999) in the Tertiary
of the Ebro Basin. The fabrics described here indicate,
very probably, early dolomitization driven by the
circulation of Mg-rich fluids whose origin might be
in freshwater meteoric fluids that increased their Mg/
Ca ratio when passing trough the Mg-rich clays and
thus could dolomitize parts of the lacustrine sequen-
ces. Crystalline fabrics recognized in these carbonates
and the fact that the primary fabric is only preserved at
the base of the profiles suggest an early but not syn-
depositional origin for these dolomites, with progres-
sion of the dolomitization process from top to base of
the lacustrine limestones previously deposited.
N.L.V. Pimentel / Sedimentary Geology 148 (2002) 123–138134
5. Summary and conclusions
The study carried on the Palaeogene deposits of the
Sado Basin has shown that three main processes
modified the primary alluvial and lacustrine deposits.
These processes are: pedogenesis, palygorskite authi-
genesis and dolomitization.
Pedogenesis was undoubtedly an important process
affecting most of the deposits throughout the basin,
having an important geomorphic control. In proximal
areas of the alluvial fans, paleosols show intense clay
neoformation and illuviation. This can correspond to
areas somewhat apart from the main canyons and fan
outlets, eventually interfan areas originated by the
irregular topography of the basement. In these set-
tings, fluid circulation was favoured in the contact
between the basement rocks and the coarse detrital
sediments, allowing palygorskite authigenesis. In
middle and distal fan areas, the predominant episodic
sedimentation led to sub-aerial exposure periods and
controlled groundwater fluctuations, and so promoted
intense physical modifications (desiccation, hydro-
morphy, clay mobilization), as well as (bio)chemically
accumulated carbonates in the paleosols. In the distal
parts of the basin, freshwater lacustrine areas devel-
oped. These ephemeral and shallow lakes were sub-
jected to temporary exposure, generating frequent
palustrine conditions, as shown by the development
of large-scale pseudomicrokarst.
Authigenesis associated with paleosols affected the
clay mineralogy of the deposits. Part of the smectitic
content of the detrital sediments, possibly inherited
from the source-area regolith, has been transformed
into palygorskite short after deposition. This trans-
formation is believed to have continued even after the
initial sub-aerial exposure and single profile develop-
ment, long after burial by the next deposits. Both
pedogenesis and groundwater related processes (dur-
ing early diagenesis) are therefore considered as pro-
moters of palygorskite authigenesis.
The palygorskite content is maximum in proximal
areas, decreasing towards more distal parts of the
basin, which is not the trend in the ‘‘classical’’ models
(e.g. Arakel, 1986; Rodas et al., 1994; Armenteros et
al., 1995). This situation can be explained by the
abundance of Si and Mg introduced into the basin by
the solutions coming from the Palaeozoic basement,
including meteoric waters, soil solutions and shallow
groundwaters, gradually depleted by clay authigene-
sis. In more distal areas, a small decrease in palygor-
skite may be a consequence of a lower Si content,
partially consumed in proximal areas. Throughout the
basin, these solutions interacted in such a way that
small physico-chemical and mixing rate variations led
to the complex silicates –carbonates precipitation and
dissolution relationships.
Pedogenic processes also generated important car-
bonate accumulations. Due to the different geomor-
phologic and hydrologic characteristics of proximal
and distal areas, carbonate precipitation increased
towards distal areas, where palustrine conditions ori-
ginated thin irregular layers and massive beds of
carbonates. Most of the carbonates were affected by
dolomitization in early diagenetic stages, producing a
mosaic of dolomitic rhombs involving etched detrital
grains. The chemistry of those inter-stratal solutions
must have been alkaline and rich in Mg, in order to
partially dissolve the primary silicates and replace
calcite by dolomite. The same solutions would be able
to transform smectite into palygorskite, depending on
the Si, Al and Mg concentrations. Once again, an
interaction between meteoric, pedogenic and ground-
waters is believed to have originated both dolomite
and palygorskite neoformation. The increase in dolo-
mite towards distal areas may simply reflect the initial
carbonate contents of the deposits. But it may also
result from a distal increase in Mg/Ca ratio, due to the
initial calcite precipitation in lacustrine areas, and to an
inhibited palygorskite authigenesis resulting from
lower Si availability.
Pedogenesis and diagenesis were highly active
processes in the modification of the primary features
of the deposits, especially of their mineralogy. The
most striking aspect in this case study, however, is that
both processes interacted in space and time, in a
continuum since the moment of deposition until very
early sub-surface diagenesis. Palygorskite neoforma-
tion was developed all along that time-interval, as a
result of meteoric, soil and groundwaters circulation
and mixing. Dolomitization started later but was par-
tially contemporary with the clay transformations,
probably interacting with them.
The inferred general conditions controlling these
modification processes correspond to a semi-arid hot
climate. Alternating dry and wet cycles, associated to
seasonal variations, originated the abundant hydro-
N.L.V. Pimentel / Sedimentary Geology 148 (2002) 123–138 135
morphic features, including iron and carbonates mobi-
lization. Predominant high temperatures promoted
intense fluid evaporation and concentration, as well
as the precipitation of authigenic palygorskite, calcite
and dolomite.
In short, the study carried on the Palaeogene
deposits of the Sado Basin shows that pedogenesis
and early diagenesis in terrestrial basins notably con-
tribute to the modification of both the primary fabric
and the mineralogy of the deposits. These processes
may have close spatial and temporal relations due to
the continual percolation of alkaline fluids, since
deposition until shallow burial. The presence of shal-
low water bodies and ephemeral lacustrine conditions
also contributes to the geochemistry of these fluids.
The detailed study of those modifications is important
as it may give important data on some environmental
controls that operated in the basin such as: climate,
sedimentation rates, pedogenesis, groundwater com-
positions and their evolution.
Acknowledgements
Ana Alonso-Zarza (Madrid University) is most
gratefully indebted for accompanying this work since
the field observation of the selected profiles and de-
tailed description of thin-sections, until the proposal of
interpretations at several scales and first reviews of the
manuscript. Diamantino Pereira (Minho University) is
thanked for the laboratorial determination of clay
mineralogy. Many thanks are due to C. Arenas, M. A.
Bustillo and P. Freytet for their careful and construc-
tive reviews, which greatly improved the manuscript,
as well as the editorial work of J.J. Tiercelin. This
study has been supported by the ‘‘Reitoria da
Universidade de Lisboa’’ and the ‘‘Rectorado de la
Universidade Complutense de Madrid’’.
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