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Jan van Eden Geology
Pyrenees Eocene
Original publication in:
GEOLOGIE EN MIJNBOUW VOLUME
49 (2), p. 145-157
1970
A RECONNAISSANCE OF DELTAIC
ENVIRONMENT
IN THE MIDDLE EOCENE
OF THE SOUTH-CENTRAL PYRENEES, SPAIN
J.G. VAN EDEN
ABSTRACT
A regressive sequence of Eocene sediments is exposed in an area West of Tremp
(South-Central Pyrenees, Spain). The sequence forms part of deposits formed in
the Upper Cretaceous-Eocene marginal basin south of the Pyrenees. Insignificant
amounts of continental and littoral deposits are preserved on the north coast of
this marginal basin. In the studied areas, however, on the eastern margin of the
basin, a variety of continental environments is found.
Three formations are distinguished within the Eocene deposits. At the base is
the Roda Formation with a regressive marine series of limestone, marl, and
sandstone. Partly overlying this formation and partly laterally transitional to
it is the Montañana Formation, with continental and littoral deposits. The Santa
Liestra Formation, formed after a major regressive phase, is the youngest. The
distribution of these formations on the geological map and their main
sedimentary facies are presented in figure 1.
The Montañana Formation is regarded as a deltaic association. Two major
sedimentary environments are distinguished: (1) a flood-plain environment with
fluvial sandstones, conglomerates and finer sediments of the inter-distributary
lagoons and swamps, and (2) a transitional environment with channel mouth, bay,
tidal flat, and deltafront deposits, containing marine fauna.
Two types of large-scale cross-bedding, with different origins are compared. One
is interpreted as river sub-deltaic formations in lagoons, while the other has
been formed by lateral deposition in a migrating river channel. Several small
sedimentary structures occur, of which “current crescent marks” and
“longitudinal furrows-and-ridges” are discussed in some detail.
Excellent exposure of the Montañana Formation provided the opportunity for an
almost complete paleogeographicinterpretation. The paleogeographic pattern of
the floodplain is not that of the ideal delta, in which one major stream forms a
fan-shaped deposit. Instead there is a concentration of supply from the north
and east by several small rivers, caused by the configuration of the upland
area.
INTRODUCTION
Eocene formations west of the “Basin of Tremp” display marine and
continental deposits. Because of the interesting foraminifera and algae fauna of
the marine units, previous geologic research has been done mainly by
paleontologists. Where fossils are abundant, stratigraphic division is complete
and detailed, but the continental deposits have never been described in any
detail. The continental and littoral deposits have the highly differentiated
lateral pattern that is characteristic of deltaic sediments.
The deposits are part of a sequence formed in Upper
Cretaceous-Eocene marginal basins, the position of which is shown on the
inset-map of Figure 1. The development of this marginal basin on both sides of
the Pyrenees began after the first period of Alpine folding during Lower
Cretaceous (d e S i t t e r, 1965). During a simultaneous subsidence of the
basin and rising of the axial zone of the Pyrenees, the western part of the
southern marginal basin was filled with marine flysch deposits. Generally along
the northern coast of the southern marginal basin no differentiated coastal
sediments are preserved. Owing to the subsidence of the basin and subsequent
uplift of the borders, sediments were rapidly buried in deeper parts, while most
of the deposits along the coast were removed by erosion in later stages.
However, the Eocene sea gradually decreased in depth towards
the East and formed a relatively stable and wide-spread shallow area towards the
eastern margin of the basin, just west of the small town of Tremp. The general
coastline of the early-Eocene sea is indicated by a broken line on the inset-map
of Figure 1. In this shallow and relatively stble part of the basin, a variety
of littoral and continental environments were developed. The gentle folding and
excellent exposures permit the same stratigraphic unit to be followed over an
extensive area. This provides an opportunity to study the lateral change in
facies from continental to marine. The geologic map of Figure 1 shows the
situation of particular sedimentary environments within the stratigraphic units.
This paper is based on the author’s M. Sc. thesis, which
forms part of a larger regional study of the sedimentary geology carried out by
the students and staff of the geology department of Leiden State University. The
fieldwork for the present study was executed during the summer months of 1964,
1965, 1966.
Figure 1
Geologic map. Santa Liestra Formation, (1) clastic wedge and fluvial environment
conglomerates, (2) flood-plain environment, (3) transitional environment, (4)
marine environment. Montañana Formation, (U) Upper, (M) Middle. (L) Lower, (5)
flood-plain environment, (6) transitional environment. Roda Formation, (7)
marine environment. Paleocene, (8) mostly red continental sediments.
STRATIGRAPHY
Detailed work on the stratigraphy has been
done and some age determinations have been made in several locations near the
area studied, in the lower Eocene marine deposits, such as those near Figols (H
o t t i n g e r, 1960). However, in the present study only relative age
relations between the units will be mentioned.
Red coloured, continental sediments of Paleocene age occur in
the eastern part at the base of the Eocene. The overlying Eocene unit, part of
the Roda Formation, is composed of Alveolina-limestone and marls that were
deposited after a transgressive phase. The time-stratigraphic boundary between
these two formations is easy to recognize.
The Roda and Montañana Formations have a complex relation,
which cannot be defined by simple time-stratigraphic boundaries. As shown in the
stratigraphic section of Figure 2, young Eocene sediments soon lose their marine
characteristics in the eastern part of the studied area, where continental
environ-ments predominated. In the western part, however, marine sedimentation
continued. The marine deposits are called the Roda Formation, while the
transitional and continental deposits are included in the Montañana Formation.
The abundance of foraminifera in the marine sediments and their absence in the
transitional and continental sediments has been used to differentiate between
the Roda and Montañana Formations. Division of the Montañana Formation into
Upper, Middle and Lower is mainly based on a remarkably continuous white
sandstone which will be discussed later. This sandstone, called White Sandstone,
has been of major importance in correlating the highly variable sedimentary
environments.
Overlying the Montañana Formation is the Santa Liestra
Formation, which was formed during a regressive phase and is easy to recognize
in the northern part of the basin, where it consists of thick red-coloured
conglomerates.
 Figure
2
Generalized cross section, showing facies units within the Roda and Montañana
Formations. (1) Continental deposits: sandstone, conglomerate, light coloured
mudstone. (2) Littoral deposits: sandstone, dark coloured mudstone, marine shell
fauna. (3) Pro-deltaic marine deposits: sandstone, marl, limestone, foraminifera
fauna. (4) and (5) marine deposits: respectively, marl and bioclastic limestone.
RODA FORMATION
General
At the base of the Roda Formation, blue-grey,
mostly nodular limestones are found (Alveolinalimestone, - M i s c h, 1934 -).
The limestone layers alternate with, and are progressively replaced by marls.
The limestone and marl were formed in a shallow sea where the foraminifera and
algae fauna can give us a detailed picture of the environment (Hottinger, 1960;
Hillebrandt, 1962). In the continuous regressive sequence, the sant content
increases towards the top. The sandstones consist of coarse shell debris,
limestone fragments, quartz and occasionally some mica, embedded in calcilutite.
Quartz percentages vary between 10 and 45 percent. The more quartzose sandstones
are probably derived from the mature sand deposits that werc formed in the
high-energy zone along the coast. Layering has a remarkable lateral continuity,
which is in contrast to the irregular bedding in the littoral deposits of the
Montañana Formation. This sandy marine deposit at the top of the Roda Formation
merges towards the east into the transitional and continental deposits of the
Montañana Formation, as is illustrated in Figure 2.
 Figure
3
Distribution of the mature sandstone of the Middle Montañana
Formation.
The dotted area shows the presence of the Montañana
Sandstone; the dashed area indicates its absence. The eastern boundary line is
approximately parallel to the ancient coastline. The total thickness and number
of the layers of mature sandstone for some locations are given in columns.
Beach-barrier complex
Several kilometers south of La Puebla de
Roda, the river Isabena breaks through a relatively hard sandstone complex
fornung a narrow cleft. This sandstone overlies the marls and forms the base of
the sandy top-part of the Roda Formation. The sandstone unit is 20 meters thick.
The stratification is very regular, has a constant primary dip of 6°, and can be
followed over several tens of meters in the direction of dip. A high-angle
cross-stratification can be found in the bottom part of the sandstone unit. Here
the primary dip direction is contrary to that of the low-angle stratification.
The sandstone is a calcarenite with a somewhat higher quartz content than normal
sediments in this area. The sub-horizontal beds have a typical laminated
structure with good sorting within the laminae, but a great difference in
composition. Quartz percentages can vary from 10 to 40 percent in adjacent
laminae.
The extensive sub-horizontal layering and laminated structure
are common in fore-beach and similar deposits (T h o m p s o n, 1937; M c K e e,
1957). The high-angle cross-bedding with a contrary dip direction may also
indicate an origin as a beachbarrier deposit (Werner, 1963). Lateral transition
of the pure sandstone in sandy marls with an abundant marine fauna on both
sides, indicates an off-shore barrier.
M a n g l i n (1959) remarks tht the Cuisien ends in a
detritic period that has a different development from place to place as a result
of tectonic activity. Slight folding caused broad ridges (anticlines) in a
direction parallel to the northern coast. The development of the deposits near
La Puebla de Roda is parallel to the axial zone. The pure, well-sorted,
sandstone of the barrier complex could have been formed on a locally shallow
part of the sea, above such a tectonic ridge.
MONTAÑANA FORMATION
General
The transitional and continental deposits of the Montañana
Formation are the top-part of a regressive series that started at the base of
the Roda Formation. The stratigraphic relation of the Montañana Formation to
other formations is shown in Figure 2. Two major sedimentary environments may be
distinguished within the Montañana Formation: (1) a floodplain environment with
fluvial sandstones, conglomerates and light coloured mudstones, and (2) a
transitional environment with littoral sediments, dark coloured mudstones and a
shell fauna. Both environments are indicated on the geologic map of Figure 1 and
will be described separately.
Flood-plain environments
L i t h o l o g y - Almost all sediments in the area are
clastic, and mainly consist of limestone fragments. The limestone fragments were
derived from Cretaceous rocks that must have been exposed towards the
north-east. Other rock fragments and quartz grains may make up as much as 30
percent of the rock, but most sediments of the flood-plain environment contain
much less. In a few locations sandstones with a higher quartz content are
present; they will be discussed separately. Minor occurrences of micritic
limestone in thin lenses and very fine layers of gypsum are of local importance.
Thin layers of carbonaceous material may be found within the fluvial sandstones.
Very fine sandstone and siltstone, which are subsequently
referred to as mudstone, have a high clay content, while the coarser sandstones
are relatively clean. Original lamination in the mudstone has disappeared by
homogenization caused by burrowing animals and probably plant roots. Randomly
oriented worm tracks are preserved in a few places, and irregular vertical
structures occur. The concretionary and sometimes red coloured mudstone is
indicative of pedogenesis, but no organic remains are found in situ.
D i s t r i b u t i o n and g e o m e t r y -
Conglomerate, sandstone, and siltstone give a broad picture of decreasing
grain-size in a southwesterly direction. No conglomerates occur near the margins
of the flood-plain. The sandstone has the typical characteristics of fluvial
channel deposits, such as (1) longitudinal bodies of clean, cross-bedded,
sandstone several hundred meters wide and 5 to 20 meters thick, (2) an erosional
base and numerous conglomerate lenses in the bottom-part, and (3) towards the
top of the sandstone a gradual transition to fine sand and mudstone. Fine
mudstone alternates with thin layers of somewhat coarser mudstone, at distances
of several meters. This indistinct layering somethnes has a pronounced
regularity over a distance of a few kilometers, which suggests a deposition in
lagoons. The coarser mudstone layers are laterally transitional to the coarse
channel-sands; this indicates an origin as natural levees.
Large scale cross-bedding - Thick crossbedded units are
developed in many of the fluvial deposits. Two different types can be
distinguished: a) cross-bedding produced by lateral sedimentation in a migrating
channel, and b) cross-bedding of a deltaic origin.
a) Photograph 1 shows an example of irregular cross-bedded
layers, which are formed by a migrating channel. The sediment often varies from
fine sand to conglomerate in different layers of the same channel deposit.
Directions as well as the thickness of the foresets are highly variable, some
deposits having a rather chaotic appearance. Internal erosive surfaces indicate
alternating periods of sedimentation and erosion, due to variable water supply
and shifting of the main flow in the channel. Slump structures occur and they
probably originated on the steep eroded side of a channel. The lower part of
Photograph 1 shows steeply inclined layers, due to slumping. Several
cross-bedded units occurring on top of each other were probably formed by the
sane migrating channel.
b) Photographs 2 and 3 show unilateral large-scale
cross-bedding. This type of structure is often found in the flood-plain deposits
around Montañana. The thickness of the sedimentation units is 2 to 5 meters. The
foresets have angles of dip less than 20° and are tanaeritial to the underlying
horizontal beds. Layers Which can be followed in exposure over about 100 meters
in the direction of dip, show a homogeneous composition of medium or fine
sandstone and have an even thickness. The cross-bedded units have an isolated
position within the horizontally bedded mudstones, and consist of one
cross-bedded unit only. They do not contain any (marine) fossils and belong to
the fluviatile deposits.
The homogeneous sediment, the less chaotic appearance, the
constant direction of the foresets, the even thickness of the units and their
isolated position are remarkably different from the characteristics of the
migrating channel deposits, and provide enough evidence to assume a “deltaic”
mechanism of deposition.
Russell (1939) and Hill & Webb (1958) have described recent
subdeltas, which were constructed by rivers in coastal lagoons. The conditions
are probably similar to those under which these deposits have formed.
 Photograph
1
Cross-bedded units, formed by a migrating channel. Just below these units occur
steeply inclined layers, due to slumping.
 Photograph
2
Unilateral, large-scale cross-bedding (subdelta), in the flood-plain deposits.
 Photograph
3
Detail of Photograph 2. The thickness of the cross-bedded unit is
approximately 4 metres.
E r o s i o n a l s t r u c t u r e s - A great
variety of erosional structures can be seen on the base of sandstone beds. These
include channels with a depth of several meters and a width of tens of meters,
as well as small-scale dragmark and flutecast structures. Two types of lesser
known structures, “current crescents” and “longitudinal furrows-and-ridges”, are
discussed in more detail.
“Current crescents” are semicircular depressions, which are commonly excavated
on the up-current side of an obstacle by the flow of water, particularly in
water of very shallow depth (P o t t e r and P e t t i j o h n, 1963, p. 121).
The crescent-shaped depression is preserved on the underside of the sandbed as a
cast, forming a so-called “current crescent cast”. In these deposits one may
find small structures only a few cm. wide, or larger ones up to 50 cm. wide and
6 cm. deep (Photograph 4). The size of the larger structures indicates rather
big obstacles, but these are never present. Later transport of such large
obstacles would probably have destroyed the cast. If the obstacles are still in
place they must have had a similar composition as the underlying mudflat. Plant
roots may locally have indurated the soil, and the so-formed mudlumps may have
acted as obstacles to the running water. The presence of several crescent casts
on the same exposed bed also suggests rather a local characteristic than an
occasional object from outside the environment.
“Longitudinal furrows-and-ridges” is the second type of erosional structure
which is preserved on the base of fluvial sandstone beds. Many parallel gullies,
with a sharp V-form cross-section, must have been eroded in the mud and
preserved as casts on the base of overlying channel sands. Photograph 5 shows
the underside of a sandstone bed with a pattern of furrows-and-ridges.
The casts are not more than 6 cm deep and their mutual distance is about 20 cm.
Over several square meters of a bedding plane the gullies have a consistent
direction, whereas in somewhat larger exposures considerable changes in
direction are noticable. The individual casts can be followed over several
meters and at their up-current end they often join in crescent-like casts. As
with the “current crescent casts” the kind of obstacles and conditions which
have initiated this erosional pattern can only be guessed at. Vegetation may
have prevented larger scour phenomena and caused the refined pattern of erosion.
Thin layers of running water are probably involved, which could be separated
into many current lines to form the regular erosion pattern of parallel gullies.
Such conditions are easily realized on a natural levee during overflow of the
river. Erosional structures of multi-directional eddies and currents, created by
plants and levee-relief are known from the natural levees in recent sediments (M
o o r e, 1966)
Transitional environment
The sediments of this area are distinguished from those of the
flood plain by their dark-coloured mudstone, the absence of conglomerates and
the presence of the marine fauna, which consist mainly of pelecypods and
gastropods. The light, and sometimes reddish, colours of the flood-plain
sediments suggest an exposure to air for longer periods, during which oxidation
could take place. The dark-coloured sediments in the transitional area have most
likely been covered by water most of the time. Three main types of environment
can be distinguished: (1) distributary channels near the delta front, (2)
inter-distributary bays, tidal flats, small tidal channels, and (3) the high
energy delta front.
D i s t r i b u t a r y - c h a n n e l d e p o s i t s
n e a r t h e d e l t a f r o n t - Longitudinal
bodies of coarse, rather clean sandstone, alternating with darkcoloured sandy
mudstone are exposed along the Rio Isabena. Photograph 6 shows the channel
deposits in section. The composition of the sandstone here is similar to that of
the sandstone in the fluvial channels, but some layers have a high percentage of
shell debris. Coarse shell fragments (often Ostrea) are mostly found in the
central bottom part of the channels. The sandstone deposits are fairly
homogeneous in composition and have fewer internal erosion planes than the
fluvial channel deposits of the flood-plain area. The bodies are several meters
thick and about 100 meters wide. Most of their crossbedding is of the
trough-type indicating a current direction parallel with the long dimension of
the channel deposits, towards the southwest or west.
Large-scale unilateral cross-bedded units with a thickness of
several meters occur. The low-angle foresets mostly dip towards the ancient
area. These structures are interpreted as tidal deltas.
 Photograph
4
Current crescent casts. The semicircular casts are 40 cm wide.
 Photograph
5
Longitudinal “furrows-and-ridges” on the underside of a sandstone bed.
 Photograph
6 Distributary channel deposits in the transitional environment of the Montañana
Formation, near the delta front.
B a y a n d t i d a l - f l a t d
e p o s i t s - Sediments of these environments are very poorly
sorted, most of the deposits give the impression of being thoroughly mixed by
burrowing organisms. The sandstones consist of coarse shell detritus, limestone
fragments and quartz grains in a calcilutite matrix, sometimes making up more
than 50% of the rock. The layering is very irregular. In the vertical sequence
this is reflected by a strongly varying bed thickness, and in the horizontal
direction by rapid wedging-out layers and many wash-outs and channels.
Alternation of coarseand fine-grained layers is often indistinct. Finely
laminated clay and siltstone with very thin carbonaceous intercalations are
sporadically found, but in most deposits all laminations and minor structures
have disappeared. The organic content of this deposit is higher than that in the
flood-plain sediments.
In an area within a radius of 2 or 3 km from Castigaleu the
fauna consists almost entirely of Ostreas. This abundance of Ostreas indicates a
brackish-water environment probably derived in a part from the isolated sea bay.
Minor channels have a typical sequence of several types of
laminated beds. In the bottom part, the coarse sandstone contains mud-pebbles
and large shell fragments, which are derived from the underlying eroded
mudstone. This cross-bedded sandstone in the bottom part of the channel is 1 or
2 meters thick. It is often overlain by massive sand layers with parallel
lamination. This type of lamination originated in the higher flow regime, where
the supply of sand was abundant. Towards the top of the sequence the sand supply
diminished and more quiet conditions returned. This sandy layers and lenses show
poorly developed cross-bedding. The transition from the channel sequence into
the deposits of the bay facies with their abundant shell fauna is gradual.
D e l t a - f r o n t d e p o s i t s - A remarkable white
sandstone, with a relative high quartz content of up to 60% was probably formed
by reworking of the calcareous sediments along the agitated coastal areas. The
sandstone is cross-bedded and has a sparse fauna of marine character. The
position of these clean sandstones on the seaward margins of the sub-aerial
delta environment and their alignment parallel to the coast defines them as
delta front sand sheets or beach deposits.
White Sandstone
This sandstone body has a special site within
the deposits of the Montañana Formation. It is composed of the same mature
sandstones that have been mentioned above in the delta-front environment and it
forms an exceptionally extensive sheet-like body in the eastern part of the area
studied. It is an important marker-bed between the discontinuous continental and
deltaic deposits and it is the stratigraphic boundary between the lower and
upper part of the Montañana Formation.
Its elongate shape and its position parallel to the presumed
coast is shown in Figure 3. On the eastern boundary it changes to a normal
fluvial sediment. The White Sandstone lies almost entirely between the
continental flood-plain deposits. Towards the southwest, it splits up into
several clean sand layers which are enclosed by sediments of the
transitional-coastal environments. Occasionally the sand layers have sparse
marine faunae.
The White Sandstone (10 to 15 metres thick) often has an
erosional base that shows longitudinal channels in parallel directions similar
to the channels in nearby fluvial deposits. Large-scale cross-bedded units that
are 0.5 to 1 metre thick have irregular wavy boundaries and steep-dipping (to 30
°) foresets that often have remarkably constant direction. They resemble eolian
cross-bedding, and the paleocurrent pattern and the channels at the base give
the impression of blown-out river sands. But regular small lenses of
conglomerate, aqueous current ripples and small wash-outs point to a
predominantly aqueous deposition, and the regional distribution of the bodies,
and their relation to the deposits of coastal environment suggest an origin as
sandy coastal plains.
It is possible that during the Middle Montañana phase erosion
of the sea could dominate over the prograding activity of the delta. In this
destructional phase the coastal plains might be covered with mature sands of the
delta front. The wide extension of the deposits towards the east over the
flood-plain deposits, could be the result of occasional floods or wind action.
 Figure
4
Paleocurrent pattern of the Montañana Formation. Directions measured in the
estuarine and coastal environments have a bcomponental distribution. The
northwestern direction is parallel to the assumed coast.
 Figure
5
Paleocurrent directions in the area around Montañana. Directions of large-scale
cross-bedding (subdeltas) and fluviatile channels. Note the convergent pattern
with directions towards the south and west in the areas west and east of
Montañana, respectively.
SANTA LIESTRA FORMATION
This formation overlies the Montañana and
Roda Formations. Its most characteristic facies is a thick red-coloured
limestone conglomerate. The conglomerates are exposed over an extensive area
north of the place Santa Liestra and can locally be found on topographic highs
towards the east; their distribution is shown on the geologic map of Figure 1.
In the eastern part of the area the conglomerates are
gradually transitional into sandy and silty fluvial deposits and still further
in the direction of transport there is a change to the transitional and marine
environment. The facies of this part of the formation is very much the same as
that of the Montañana and Roda Formations.
North of Santa Liestra there is an abrupt change from the
unsorted, red-coloured conglomerates into marls and sandstones, which contain
abundant foraminifera. The conglomerates must have been transported over a short
distance and deposited along a relative steep coast, where no transitional
environments could be developed.
Between the villages Luzas and Montañana, clastic sediments
have intensive red colours. Micritic limestone lenses have locally a total
thickness of 6 m. This, together with the occurrence of gypsum and plant
remains, suggests aerial exposure of the sediments and a relative warm and dry
climate.
PALEOGEOGRAPHY AND CONCLUSIONS
L o w e r M o n t a ñ a n a F o r m a t i o n
- The pattern of environments that can be recognized in the Montañana
Formation is shown on Figure 6. The general paleocurrent pattern of the
floodplain does not have the characteristics of an ideal delta, where one major
stream will split up into several branches near the shore. Instead, the
configuration of the basin caused a certain concentration of supply from the
north and east by several small rivers. The convergence of paleocurrent
directions is most clear near Montañana; west of this village current directions
towards the south prevail, while eastwards a pre-dominant current direction to
the west can be observed (Figure 5).
The limestone composition of the pebbles points to a nearby
source and to a rapid uplift with fast erosion of the source area. The size of
the pebbles decreases rapidly towards the southwest, in the direction of
transport. In the area around Montañana coarse sand was deposited in
distributary channels, finer sand on natural levees and mudstone in the
inter-levee swamps and lakes. The previously discussed longitudinal
furrows-and-ridges indicate multidirectional currents on levees that probably
were caused by local relief and vegetation. Subdeltas were constructed by the
river channels in lakes and lagoons.
A brackish-water bay with abundant Ostreas extended south of
Cajigar. This bay was partly isolated from the sea by sandy barriers on the
delta front. Channel mouths, bays, lagoons and tidal fiats are transient
environments, and their preserved deposits alternate and intermingle.
 Figure
6
Paleogeographic interpretation of the Lower Montañana Formation.
 Figure
7
Paleogeographic interppetation of the Santa Liestra Formation.
M i d d l e M o n t a ñ a n a F o r m a t i o n
- The reduced sediment supply caused a domination of erosion over the
constructing activity of the delta. A coastal plain of mature sand was formed
and its extension is indicated as a dotted area in Figure 3. The sandstone has
covered the topographic lower parts of the floodplain and can be traced to the
west in a small zone along the coast, overlying the sediments of the
transitional environment of the Lower Montañana Formation.
U p p e r M o n t a ñ a n a F o r m a t
i o n - Renewed uplift and extension of the tectonically active
areas southwards from the axial zone caused a strong sediment supply and a rapid
progression of the continental deposits over the transitional deposits of the
Middle and Lower Montañana Formation. Deltaic deposits similar to those of the
Lower Montañana Formation were thus developed further to the southwest.
S a n t a L i e s t r a F o r m a t i o n -
The major units of the Santa Liestra Formation are shown in Figure 7. The
formation was deposited during a rapid regression of the sea, during which thick
conglomerates have advanced over the northern part of the continental and marine
deposits of the Montañana Formation. These conglomerates, which merge into the
marine deposits without the development of any transitional environments, do not
belong to the deltaic complex, but can be described as a “elastic wedge” deposit
(K r u m b e i n and S l o s s, 1963, p. 542).
In the conglomerates north of Cajigar,
Alveolinalimestone pebbles of the Lower Eocene age have been found. These
pebbles indicate that a strong uplift occurred at the northern border of the
basin, making the erosion of the Lower Eocene formations possible.
The paleogeographic picture can be completed by mentioning
the remains of the flora and fauna. Fragments of tortoises, crocodiles and
mammals have been found, mostly in the flood-plain deposits above the White
Sandstone. C r u s a f o n t (1964) describes specimens from nearby locations
east of the studied area. Large wood and leaf fragments have been found in all
flood-plain deposits and a few impressions of palmleaves have been observed in
the Upper Montañana and Santa Liestra Formations.
ACKNOWLEDGMENTS
The author is indebted to Dr. J.D. de Jong and Dr. P.J.C.
Nagtegaal, for the interest they have shown and the valuable suggestions they
have made throughout the course of this study and for their criticism of this
manuscript.
Geological Institute, State
University of Leiden, Leiden, The Netherlands.
Present address [1970]: Geologic Research Unit, RST Technical Services Ltd.,
Kalulushi, Zambia.
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Some more (previously unpublished) photographs from this
area:
 Foto
7
Imprint of a palmleave.
 Foto
8
Variety of current indicators
 Foto
9
Mudcracks
 Foto
10
Grazing trails of bottom dwellers
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