Darwin's great patagonian tertiary formation at the mouth of the río Santa Cruz: a reappraisal
Ana Parras and Miguel Griffin
Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa y CONICET, Santa Rosa, La Pampa. Emails: aparras@exactas.unlpam.edu.ar, miguelgriffin@aol.com
ABSTRACT
The classical section described by Darwin at the mouth of the Río Santa Cruz was a landmark in the later stratigraphy of the marine Cenozoic of Patagonia. Sedimentological and paleontological studies of rocks from the Monte León Formation at the Darwin and Mount Entrance sections - mouth of the Río Santa Cruz - suggest that they were deposited in an early Miocene inner shelf to subtidal - or even intertidal at the top - environment. The base of the sections is covered and the top is unconformably overlain by Quaternary deposits. The invertebrate fauna (mainly mollusks) mentioned herein - the earliest of which were collected by Darwin and described by Sowerby - were collected from the two sections with a detailed stratigraphic control. It includes brachiopods, echinoderms, scaphopods, bivalves, and gastropods. The similarity of this fauna with that one from the sections at Restinga Norte, Cabeza de León, Las Cuevas, Yegua Quemada and Monte Observación, all south of the study area, allow correlation with them. In addition to the equivalence of the faunas, sedimentological features at all these localities are also the same. Therefore, all the shell beds involved should be referred to the Punta Entrada Member of the Monte León Formation. The Monte Observación Member should be restricted to its original use and included as a member at the base of the overlying Santa Cruz Formation, outcrops at Cerro Monte León and Cerro Observación.
Keywords: Darwin Section; Mount Entrance Section; Mouth of Santa Cruz River; Paleoenvironments; Invertebrates; Patagonia.
RESUMEN: La Gran Formación Terciaria Patagónica de Darwin en la desembocadura del río Santa Cruz: una revaluación. La sección clásica descripta por Darwin en la desembocadura del río Santa Cruz fue un hito en el posterior desarrollo de la estratigrafía del Cenozoico marino de Patagonia. Los estudios sedimentológicos y paleontológicos realizados en sedimentitas de la Formación Monte León, en las secciones Darwin y Monte Entrada, en el área de la desembocadura del río Santa Cruz, sugieren que las mismas fueron depositadas en un ambiente de plataforma interna a submareal en la base, hasta intermareal en el techo, durante el Mioceno temprano. La base de las secciones se encuentra cubierta y su techo es cortado en forma discordante por depósitos del Cuaternario. La fauna de invertebrados (mayormente moluscos) mencionada en este trabajo, cuyos primeros representantes fueran coleccionados por Darwin y descriptos por Sowerby, fue colectada con un detallado control estratigráfico a lo largo de las dos secciones. La misma incluye braquiópodos, equinodermos, escafópodos, bivalvos y gasterópodos. La similitud en la composición taxonómica, como así también en las características sedimentológicas, entre estas secciones y aquellas ubicadas más al sur como son Restinga Norte, Cabeza de León, Las Cuevas, Yegua Quemada y Monte Observación, sugiere que las mismas se correlacionan lateralmente. Por lo tanto estos niveles deben ser referidos al Miembro Punta Entrada de la formación Monte León, restringiéndose el Miembro Monte Observación a los niveles basales de la suprayacente Formación Santa Cruz, aflorantes en Cerro Monte León y Cerro Observación.
Palabras clave: Sección Darwin; Sección Monte Entrada; Desembocadura del río Santa Cruz; Paleoambientes; Invertebrados; Patagonia.
INTRODUCTION
On April 13th 1834, HMS Beagle anchored
within the mouth of the Río Santa
Cruz, a few miles upstream from the
Atlantic Ocean. During his numerous excursions
ashore, Darwin made geological
observations on the cliffs that rise along
the southern bank of the river. He also
assembled a collection of invertebrate
marine fossils later described by Sowerby
(1846). Thus, the locality from where this
material was collected became the type
locality of most of the taxa described by
Sowerby. The main purposes of this
work are to pinpoint and re-describe as
accurately as possible the locality at the
mouth of the Río Santa Cruz where
Charles Darwin (1846, p. 112-113) described
the geological section of marine
Tertiary rocks, and discuss the possible
paleoenvironmental conditions in which
it was deposited. This should aid in the
formulation of more precise correlations
of the beds mentioned by Darwin with
others exposed in the area, and thus clarify
the meaning of the formal names proposed by later authors for the rocks
involved. Darwin (1839, 1846) himself
included these rocks in his "great Patagonian
tertiary formation" or "Patagonian Tertiary
Formation", a unit subject to very different
interpretations ever since. According
to his description of the beds exposed
and after a survey of the area to try
to identify the exact location of his section,
we were able to identify it on the
southern bank of the Río Santa Cruz, a
short distance away from its mouth in
the Atlantic Ocean.
The rocks exposed in this area are now
included in the lithostratigraphic unit
known as Monte León Formation (Bertels
1970) and deemed to be of late
Oligocene-early Miocene age (Malumián
1999, 2002, Barreda and Palalmarczuk
2000). This unit is exposed along the
coast of Patagonia between Puerto San
Julián and Coy Inlet, at the mouth of the
Río Coyle (or Coig) (Fig. 1). It comprises
mainly sandstones and siltstones with a
high content of pyroclastic material, and
carries numerous shell beds.
Figure 1: Location map of the Darwin Section and the Mount Entrance Section at the mouth of the Río Santa Cruz.
In order to clarify the exact location of Darwin's section, we surveyed the coast along the southern bank of the river. Based on his writings (1846, p. 112-113) it became clear the section is the one lying just east of the present port of Punta Quilla (S 50° 07' 39.9"; W 68° 24' 10.04"), a point where the marine Tertiary rocks are best exposed (Fig. 2), and where they best meet the description and measurements he offers in his "Geological Observations…".
Figure 2: Monte León
Formation at the mouth of
the Río Santa Cruz, Darwin
Section, showing Darwin's
strata and facies associations
described herein.
As there has been some confusion as to
the exact location of Darwin's locality,
we also measured and described the section
lying further east, approximately one
kilometer away from Mount Entrance, at
the very mouth of the river (S 50° 08'
32.2"; W 68° 22' 23.6"). The two localities
lie within the present Department of
Corpen Aike, in the province of Santa
Cruz (Fig. 1). They both carry a rich
fauna of exceptionally well preserved
mollusks that can be counted among the
best in southern South America. They
have been mentioned in the literature
(Becker 1964, Bertels 1970, 1980, Di
Paola and Marchese 1973, Olivera 1990,
del Río and Camacho 1998, Barreda and
Palamarczuk 2000), but detailed descriptions
that could enable correlation and
paleoenvironmental reconstructions are
still lacking.
Study of these two stratigraphic sections
allowed recognition of sedimentary facies,
and hence the dominant deposition
mechanisms could be interpreted. These
facies were grouped into three facies
associations and a depositional model
thus emerged. Detailed analysis of the
taxonomic composition of the fossil
assemblages revealed the presence of at
least 38 species of bivalves and 70 species
of gastropods, in addition to scaphopods,
echinoderms, and brachiopods.
Diffe-rences among the different shell
beds suggest a complex history. The different
fossil concentrations were originated
in different settings, ranging from
inner shelf to nearshore subtidal environments
- and even intertidal, representing
deposition during the early Miocene transgression in Patagonia.
GEOLOGICAL FRAMEWORK
The surface of Patagonia is covered
mainly by marine rocks deposited during
the Atlantic transgressions that occurred
during the late Cretaceous, Paleogene
and Neogene. These are intercalated with
non marine rocks and non deposition
and erosion intervals (Malumián 1999).
One of these transgressions is commonly
referred to as "Patagoniano" and
occurred during the high sea level period
that began in the latest Oligocene and
ended in the early middle Miocene. The
final phases of this transgression are
coincident with the Neogene climatic
optimum. It covered the largest extent of
land among the various Cenozoic transgressions
that occurred in Patagonia
(Malumián 2002). This transgression
deposited rocks containing the Paleogene/Neogene boundary in Patagonia and
in the Cordillera sector of southern
Chile. They are presently included in the
San Julián and Monte León formations
along the coast of Santa Cruz and in the
Centinela Formation in the Andean region
of this province. Exposures of these
units lie within the Austral Basin.
Further north, the rocks deposited during
the Patagonian transgression are included
in the Chenque and Gaiman formations,
in the San Jorge Basin. In southern
Chile, these rocks are known as
Guadal Formation, and are exposed
south of Lago General Carrera.
The basement rocks underlying the Cenozoic
deposits are a Jurassic lava-pyroclastic
complex known as Bahía Laura
Group, including the Chon Aike Formation
(ignimbrites and scarce rhyolitic lavas
and tuffs) and the La Matilde Formation
(tuffs, claystones, siltstones and
coal beds). These two units are interdigitated
and unconformably overlain, in the
San Julián area, by the San Julián Formation.
This late Oligocene unit includes
predominantly marine clastic -sandstone
to siltstone - rocks (Parras and Casadío
2005, Parras et al. 2008).
Deposition of the probably unconformably
overlying Monte León Formation
occurred during the late Oligocene and
early Miocene. This unit includes siltstones,
sandstones and tuffs (Bertels 1970,
1980), and is overlain by the early to middle
Miocene continental rocks of the
Santa Cruz Formation. Unconformably
overlying these are variably thick Quaternary
deposits of the Shingle Formation
described by Darwin too (Darwin
1839, 1846).
The earliest record of marine rocks in
Patagonia was by d'Orbigny (1842 a, b),
who explored this part of South America
during the years 1826-1833. D'Orbigny
himself never went ashore further south
than the coast of northern Patagonia.
However, he named all the deposits that
constitute the cliffs along the coast between
Carmen de Patagones (Río Negro)
and the Strait of Magellan as "Tertiaire
Patagonien". Although he did not inspect
the cliffs further south, he did have available
the large oysters from San Julián,
which he believed were the same ones as
he had collected in Río Negro and further
north in Entre Ríos. Darwin was
aware of this, and therefore used an equivalent
name for these rocks. However, he
also noticed that there were differences
between the faunas from the southern
and northern areas in which his "Patagonian
Tertiary Formation" was exposed.
After these initial surveys the area remained
largely unexplored from a geological
point of view. A Chilean settlement near
the present town of Puerto Santa Cruz was headed by Vidal Gormaz, who
collected a few fossils from the cliffs
surrounding the settlement and sent
them to R.A. Philippi, then at the Museo
Nacional de Historia Natural in Santiago
de Chile. These fossils were described by
Philippi (1887), but hardly any comments
were made on the geology of the area,
except to note that the specimens came
from the Tertiary marine beds there
exposed. It was not until the final years
of the 19th Century that this region was
again surveyed and its geology described
in more detail. This was due mainly to
three early explorers who visited the
mouth of the Río Santa Cruz and assembled
the data that were then - and for
many years - used to formulate the first
stratigraphic divisions and the earliest
paleogeographic and paleobiogeographic
interpretations about the rocks in the
area.
In the first place should be mentioned
Carlos Ameghino, brother of Florentino
Ameghino-the famed Argentine paleontologist.
C. Ameghino surveyed the area
and collected fossils that were subsequently
studied by his brother. Although
he did not publish his results extensively,
his keen and detailed geological observations
were later used by Florentino to
formulate the earliest biostratigraphic
scheme of the Tertiaire Patagonien or the
Great Patagonian Formation. Thus, based on
the fossil content (mainly mollusks) of
these marine beds at the mouth of the
Río Santa Cruz and in the surroundings
of Puerto San Julián, F. Ameghino (1898)
subdivided the marine beds he had
named (Ameghino 1896) "Formación Patagónica" into two chronostratigraphic
units he called "Juliense" and "Leonense".
Overlying these (Ameghino 1900-1902)
he recognized the continental "Formación
Santacruceña", at the base of which he
could distinguish a marine stage he called "Superpatagónico". The faunas of marine
invertebrates that were the basis of
Ameghino's subdivisions were collected
by Carlos and described by Maurice Cossmann
(1899) and Hermann von Ihering
(1897, 1899, 1902, 1907, 1914).
During the same years, the area was also
visited by J. B. Hatcher, geologist in charge
of the Princeton Expedition to Southern
Patagonia. Hatcher's observations
were published in the Reports and Geology
of this expedition (Hatcher 1903)
and in a series of papers in the American
Journal of Science (Hatcher 1897, 1900).
The large collection of Tertiary marine
fossils that he assembled was described
by Ortmann (1902). Hatcher's conclusions
on the geology of the area were
different from those of Ameghino.
Hatcher did not recognize the existence
of the "Superpatagónico", as the fauna described
by Ortmann was essentially the
same one as the one in the underlying "Formación Patagónica". This initial controversy
pervaded all further work on these
units and has never been satisfactorily
resolved. The reasons for this are varied
and include imperfect knowledge on the
sedimentology and stratigraphy at the
localities involved, inaccurate location of
some of these localities, outdated taxonomic
placement of some of the fossil
material on which the biostratigraphy
was based, among other.
At about the same time, C. Bicego was
entrusted by H. von Ihering with the task
of assembling further collections at the
localities initially visited by C. Ameghino.
His large collection of mollusks was described
by von Ihering, but Bicego himself
did not provide further geological
data beyond that mentioned by Ameghino.
Wichmann (1922), Windhausen (1931)
and Feruglio (1949) published detailed
accounts on the geological features of
Patagonia, including the marine Cenozoic
rocks. Their detailed sections and
geological observations are generally easy
to corroborate in the field. However,
interpretations largely reflect the ideas of
Ameghino and von Ihering.
After these initial studies, the use of "Patagonian
Formation" for the Cenozoic
marine rocks in southern South America
was widespread, although the exact chronostratigraphic
and lithostratigraphic
meaning of this formational name remained
somewhat unstable. Accounts on the
uses and controversies attached to such
usage can be found in Camacho (1974,
1979), Zinsmeister (1981), Legarreta and
Uliana (1994), and Malumián (1999,
2002).
At the mouth of the Río Santa Cruz,
these rocks were formally named Monte
León Formation by Bertels (1970). The
unit includes mainly yellowish-grey siltstones
and fine sandstones with a high
pyroclastic content and intercalated shell
beds. These rocks record a relative sea
level increase characterized by the greatest
extent of the Patagonian Cenozoic
sea, thus allowing input of corrosive
Antarctic water onto the Argentine continental
shelf (Malumián 2002). Most
authors agree that in the study area, i.e.
the mouth of the Río Santa Cruz, these
rocks were deposited in a shallow marine
environment, under littoral and neritic
conditions (Panza et al. 1995, Barreda and
Palamarczuk 2000). Bertels (1980) suggested
outer shelf conditions for the
lower part of this formation. Panza et al. (1995) stated that towards the top of the
unit conditions became progressively
shallower, ending in a marshy environment
just before the onset of the continental
environment in which the overlying
Santa Cruz Formation was deposited.
The abundant tuff beds and cineritic
material in these rocks led Bertels (1970)
and Di Paola and Marchese (1973) to
suggest they originated in coetaneous
volcanic events in the Cordillera. The ash
was probably transported towards the
coast by the western winds and deposited
in an environment that was protected
from the action of marine currents.
Although discrepancies still exist concerning
the age of the Monte León Formation,
most authors refer it to the late Oligocene (Bertels 1970) or late Oligocene
- early Miocene (Malumián 1999,
2002). Yet, Náñez (1988) restricted it to
the Oligocene (middle section of P19
Zone and top of P21 Zone) while suggesting
that the top may reach into the
early Miocene. This was later confirmed
by 40Ar/39Ar ages published by Fleagle et al. (1995). Their sample came from the
top of the unit at Cerro Monte León and
it yielded an age of 19.33 Ma (Burdigalian,
early Miocene). More recently Barreda
and Palamarczuk (2000) considered
that in the study area the Monte León
Formation should be restricted to the
early Miocene on the basis of its palynomorph
content, an age we agree with
herein.
METHODOLOGY
Just east of Punta Quilla (Darwin section; S50°07'39.9"; W68°24'10.04") and at Mount Entrance (S50°08'32.2"; W68° 22'23.6") we measured detailed (1:10) stratigraphic-sedimentologic sections (Fig. 3). We recorded bed geometry, lithology, grain size, sedimentary structures, and color. Each bed was sampled and fossils and trace fossils were photographed and collected. Taphonomic observations were recorded too. Close-packing and size-sorting were estimated following the semi-quantitative categories for coarse bioclastic fabrics of Kidwell and Holland (1991). Terminology for geometry of concentrations and orientation of bioclasts follows Kidwell et al. (1986). Sedimentary facies were distinguished and grouped in facies associations for paleoenvironmental analysis. The invertebrate fauna collected was identified taxonomically. The different beds mentioned by Darwin (1846, p. 112-113) were identified as was the exact provenance of the type material of the species described by Sowerby (1846).
Figure 3: Studied stratigraphic
sections showing identified
facies associations.
SEDIMENTOLOGY
Sections
The type section of the Monte León
Formation lies on the right bank of the
Río Santa Cruz, one kilometer upstream
of its mouth into the Atlantic Ocean
(Bertels 1970, p. 499). However, Bertels
(1970) did not describe or illustrate her
section until a decade later (Bertels 1980,
p. 214-217), when she also defined the
Punta Entrada Member of the Monte
León Formation. This locality is precisely
where our Mount Entrance section lies.
According to her description (Bertels
1980) the section measures there 103.8
m, but this could not be confirmed in the
field by us, as the bottom of the section
is completely covered by modern foothill
deposits and only 60 m are exposed.
Therefore, the base of Bertels' section -
as inferred from her description - must
have been measured at Darwin's section,
where the exposure is complete, although
the top part is here extremely difficult
to reach. Riggi (1978, p. 166) measured
89 m but it remains unclear exactly
where his section is (his Mount Entrance
section lies "on the right bank of the Río
Santa Cruz and its mouth in the ocean").
Barreda and Palamarczuk (2000) measured
80 m south of the mouth of the
river, but most other published sections
are compounds based on partial sections
measured at various points ranging between
south of the mouth of the river to
30 km upstream along the right bank
(Olivera 1990, del Río and Camacho
1998). Other authors based their stratigraphic
framework on unpublished data
(Becker 1964) or previous work by others
(Di Paola and Marchese 1973).
According to Darwin's description, the
section mentioned by him is the one just
east of Punta Quilla (our Darwin Section, Figs. 2 and 3). This is where the cliff
is best exposed and where the description
and measurements provided by
Darwin himself best agree with field
observations. In order to refer the beds
described by Darwin to the type section,
we provide herein a description of both,
even if lithology is very similar - as is fossil
content. The Darwin Section is more
complete because the lower beds lie exposed
at low tide. However, access to the
topmost beds is extremely difficult. This
is probably the reason why the section at
Mount Entrance (and nearby) has been
better studied.
a) Darwin Section (S50º07'39.9"; W68º 24'10.04")
0-1.2 m: 1.2 m Grey tuffaceous clayish
siltstones with abundant fossils and
numerous Ophiomorpha isp. Dominant
species is "Turritella" patagonica. It appears
in clump-geometry packets. Also recorded
are Nucula sp., Limopsis insolita,
Cucullaea alta, Crassostrea? hatcheri, Fasciculicardia
patagonica, Pleuromeris cruzensis, Dosinia
laeviuscula, Ameghinomya darwini. Close-packing within these clumps is dense/loose (25%), size-sorting is poor,
orientation chaotic, disarticulation is total,
fragmentation low in some clumps
and high in others, abrasion is nil. Encrusting
and bioerosion is observed on
the valves of C.? hatcheri. The taxa recorded
were en-crusting bryozoans, sponge
borings (En-tobia isp.), polychaetes (Maeandropolydora isp.), and bivalve borings
such as Gastro-chaenolites isp.
1.2-6 m: 4.8 m Grey tuffaceous clayish
siltstones similar to the previous ones.
This bed carries three lenses, each ca. 0.2
m thick and dominated by "Turritella" patagonica. These lenses are separated by
siltstones with very few clumps that carry
almost exclusively "T". patagonica. Also
present are, among many other taxa,
Nucula (Lamellinucula) reticularis, Iheringinucula
crassirugata, Scaeoleda? ortmanni, Neilo
ornata, Arca patagonica, Cucullaea alta, Limopsis
insolita, Glycymeris cuevensis, Atrina
magellanica, Neopanis quadrisulcata, Swiftopecten
nodosoplicatus, Zygochlamys geminata,
Zygochlamys quemadensis, Reticulochlamys proxima,
Jorgechlamys centralis, Crassostrea? hatcheri,
Pteromyrtea crucialis, Pleuromeris cruzensis,
Fasciculicardia patagonica, Spissatella lyelli,
Hedecardium? ameghinoi, Hedecardium? puelchum,
Trachycardium pisum, Cardium patagonicum,
Lahillia patagonica, Maorimactra indistincta,
Macoma? santacruzensis, Retrotapes
striatolamellata, Dosinia laeviuscula, Ameghinomya
darwini, Ameghinomya meridionalis,
Caryocorbula hatcheri, Panopea quemadensis,
Liotia scotti, Gibbula (Phorcus) margaritoides,
Valdesia dalli, Solariella dautzenbergi, Calliostoma
santacruzense, Calliostoma peraratum,
Calliostoma cossmanni, Calliostoma garretti, "Turritella" ambulacrum, Struthiochenopus
santacruzensis, Perissodonta ornata, Perissodonta
patagoniensis, Crepidula gregaria, Cerithioderma patagonica, Polinices santacruzensis,
Polinices ortmanni, Glossaulax vidali,
Lunatia consimilis, Sassia bicegoi, Cerithiopsis
juliana, Cirsotrema rugulosum, Cirsotrema quemandense,
Trophon santacruzensis, Xymenella dautzenbergi, Xymene cossmanni, Enthacanthus
monoceros, Crassilabrum hatcheri, cf. Ocenebra
iheringi, Urosalpinx archipatagonica, Peonza
torquata, "Cominella" annae, Penion subrectus,
Miomelon petersoni, Miomelon gracilior,
Adelomelon pilsbryi, Neoimbricaria patagonica,
Dentimargo deuterolivella, Austroimbricaria
quemadensis, Zeadmete ameghinoi, Zeadmete
cruzialis, Antimelatoma quemadensis, Splendrillia
santacruzensis, Austrotoma cuevensis,
Fusiguraleus iheringi, Terebra quemadensis,
Terebra santacruzensis, Odostomia synarthrota,
Turbonilla cuevensis, Turbonilla observatonis,
Semiacteon argentinus, Cylichna juliana, Kaitoa
patagonica, "Odontostomia" euryope, Dentalium sp. Within the lens close-packing is dense (55%), size-sorting poor - with
juvenile and adult specimens alike - ,
orientation is chaotic, disarticulation is
total, fragmentation very low, and abrasion
nil. Encrusting and bioerosion occur
on the oysters.
6-27 m: 21 m Yellowish-brown tuffaceous
siltstones with at least two beds
dominated by "Turritella" patagonica. They
measure 1 and 2.5 m thick respectively.
The top bed is the most conspicuous and
shows sectors in which it is hardened
into concretions that outstand in the section.
Faunal composition almost identical
to that of the underlying bed. Contacts
are wavy, close-packing is dense/loose
(25%) at some places and dense (55%) at
others, size-sorting is poor (full size range
of "T." patagonica), orientation is chaotic,
disarticulation high with very few
specimens keeping conjoined valves,
fragmentation is high in some sectors
and low in others, abrasion nil. Between
15 and 18 m above the base of this bed
there are thin intercalated lenses of 0.05-0.3 m thickness dominated also by "T." patagonica. At the very top there is
Dentalium in life position and Limopsis
insolita.
27-30 m: 3 m Grey tuffaceous siltstones.
Lithology similar to previous ones, but
the color change from yellowish to grey
is notable.
30-34 m: 4 m Grey tuffs. The first 0.4 m
carry large Ophiomorpha isp. The rest of
the bed is massive, with only a few sparse
fragments of shells.
34-56 m: 22 m Yellow tuffaceous siltstone
highly bioturbated, with numerous very
large Thalassinoides isp. (up to 0.08 m
diameter). Sparse body fossils include
crabs, very small Lucinoidea in life position
and larger fragments of maybe the
same taxon. Preservation is gypsum replacement.
The top 8 m carry very sparse
valves of Cucullaea alta and Dosinia laeviuscula,
in all cases disarticulate and convex-up, some of them broken. Numerous
concretions (as recorded by Darwin)
of 0.8 to 1.4 m diameter and conjoined
in a "necklace" manner. An especially
conspicuous bed of concretions is
at 6 m from the bottom of this bed.
56-56.5 m: 0.2-0.5 m Very fine sandstone
with abundant specimens of Dosinia laeviuscula.
This is a monospecific bed with
a wavy bioturbated contact at the bottom
and a wavy one at the top. Close-packing
is dense (55%), size-sorting is well sorted,
orientation chaotic, disarticulation
high, fragmentation is moderate to low,
abrasion nil. At the base there are numerous
tubes that reach down up to 0.7 m
into the underlying bed. This shell bed is
replaced laterally by light grey to white
tuff of approximately the same thickness.
56.5-75.5 m: 19 m Interstratified siltstone,
claystone and fine sandstone. At the bottom
it carries right valves of Crassostrea?
hatcheri, in all cases convex-down. At 11
m from the bottom the color changes
from brown to yellowish-green.
b) Mount Entrance Section (S50º08'32.2";
W68º22'23.6")
0-17 m: 17 m Grey and greenish tuffaceous
clayish siltstones with a few specimens
of "Turritella" patagonica that may be
isolated or forming clumps. At 7 m from
the base there is a sandy lens dominated
by this species but also containing a few
brachiopods and bivalves. At 14 m there
are large "necklace" concretions of about
0.3 m diameter. Between 16 and 17 m the
bed is strongly bioturbated (Ophiomorpha isp. and other small tubes in all directions).
17-27 m: 10 m Very bioturbated tuffaceous
sandstones.
27-27.8m: 0.8 m Fine tuffaceous sandstones,
dark grey and very hard, with crossbedding.
At the base it carries load structures.
These sandstones are very bioturbated,
with many tubes of about 0.01 m
diameter placed obliquely to the stratification
planes.
27.8-38.8 m: 11 m Tuffaceous bioturbated
sandstones similar to the underlying
bed.
38.8-39.4 m: 0.6 m Very fine sandstones
with abundant fossils. This is a shell bed
with a wavy contact at the base and a gradual
transition at the top. Close-packing
is dense (55%) at the base and loose
(15%) at the top, size-sorting is well sorted,
as complete specimens and fragments
show approximately the same size,
a few large whole bivalves are also observed,
orientation is chaotic, disarticulation complete, fragmentation high, and many
naticid and muricid boreholes are evident
on the shells. This bed carries a very rich
fauna of well preserved mollusks, including
among others Nucula (Lamellinucula)
reticularis, Iheringinucula crassirugata, Scaeoleda?
ortmanni, Neilo ornata, Arca patagonica,
Cucullaria darwini, Cucullaea alta, Limopsis
insolita, Glycymeris cuevensis, Atrina magellanica,
Neopanis quadrisulcata, Swiftopecten nodosoplicatus,
Zygochlamys geminata, Zygochlamys
quemadensis, Reticulochlamys proxima,
Jorgechlamys centralis, Crassostrea? hatcheri,
Pteromyrtea crucialis, Pleuromeris cruzensis,
Fasciculicardia patagonica, Spissatella lyelli,
Spissatella kokeni, Hedecardium? ameghinoi,
Hedecardium? pauciradiatum, Hedecardium?
puelchum, Trachycardium pisum, Cardium patagonicum,
Lahillia patagonica, Maorimactra
indistincta, Serratina jeguaenis, Macoma? santacruzensis,
Retrotapes ortmanni, Retrotapes
striatolamellata, Dosinia laeviuscula, Ameghinomya
darwini, Ameghinomya meridionalis,
Caryocorbula hatcheri, Panopea quemadensis,
Liotia scotti, Homalpoma philippii, Gibbula
(Phorcus) margaritoides, Valdesia dalli, Solariella
dautzenbergi, Calliostoma santacruzense,
Calliostoma peraratum, Calliostoma cossmanni,
Calliostoma garretti, "Turritella" ambulacrum, "Turritella" patagonica, Struthiochenopus
santacruzensis, Perissodonta ornata,
Perissodonta ameghinoi, Perissodonta patagoniensis,
Calyptraea eleata, Sigapatella americana,
Crepidula gregaria, Cerithioderma patagonica,
Vermetus incertus, Polinices santacruzensis,
Polinices ortmanni, Glossaulax vidali, Lunatia
consimilis, Sconsia ovulum, Sassia bicegoi,
Sassia morgani, Cerithiopsis juliana, Cirsotrema
rugulosum, Cirsotrema qemandense, Trophon
santacruzensis, Xymenella dautzenbergi,
Xymene cossmanni, Enthacanthus monoceros,
Crassilabrum hatcheri, cf. Ocenebra iheringi,
Urosalpinx archipatagonica, Peonza torquata, "Cominella" annae, Penion subrectus, Austrocominella
cosmanni, Retizafra improvisa, Miomelon
petersoni, Miomelon gracilior, Adelome lon pilsbryi, Neoimbricaria patagonica, Dentimargo
deuterolivella, Austroimbricaria quemadensis,
Zeadmete ameghinoi, Zeadmete cruzialis,
Sveltia maior, Scalptia vidali, Antimelatoma
quemadensis, Splendrillia santacruzensis,
Austrotoma cuevensis, Fusiguraleus iheringi,
Eoturris santacruzensis, Borsonia patagonica,
Terebra quemadensis, Terebra santacruzensis,
Odostomia synarthrota, Odostomia suturalis,
Turbonilla cuevensis, Turbonilla observatonis,
Turbonilla iheringi, Semiacteon argentinus,
Cylichna juliana, Kaitoa patagonica, "Odontostomia" euryope, Dentalium sp.
39.4-59.4 m: 20 m Interstratified fine
sandstone, siltstone and claystone. It
carries a few interstratified concretions
with bivalves and echinoderms. At 12 m
from the base of this bed there is a color
change from brown to yellowish green.
Facies Analysis
Vertical and lateral facies distribution at the two studied sections allows recognition of three facies associations (Figs. 2 and 3). Facies Association 1 (FA1): Siltstones and fine sandstones rich in pyroclastic material and clumps, lens and beds of skeletal fossils concentrations. Facies Association 2 (FA2): Highly bioturbated siltstones and tuffs with concretions. Facies Association 3 (FA3): Interstratified siltstones, fine sandstones and claystones, thinly laminated in some sectors.
Facies Association 1 (FA1)
This facies association is roughly equivalent to Darwin's (Darwin 1846, p. 112) lowermost stratum (i.e., "… to a thickness of fifty or sixty feet, consists of a more or less hardened, darkish, muddy, or argillaceous sandstone…"). It includes yellowish grey siltstones and fine muddy sandstones with abundant pyroclastic material and skeletal concentrations forming clumps, lens or beds, dominated by "Turritella" patagonica (Fig. 4a). Also intercalated are specimens of Crassostrea? hatcheri in life position or slightly rotated but still with both valves articulated. This facies association also carries most of the rich and diverse fauna of the Monte León Formation (see description of Sections for a list of the most conspicuous taxa and their stratigraphic distribution). Also present are abundant bioturbations as small tubes running in all directions, and galleries assigned to Ophiomorpha isp. Concretions of variable size and shape are abundant, sometimes joined together forming "necklaces". This facies association measures 30 m thick at Darwin's Section and 17 m thick at Mount Entrance Section.
Figure 4: a) Clumps of "Turritella" patagonica (FA1); b) Ophiomorpha isp. (FA2); c) Concretions (FA2); d) Skeletal concentration dominated by Dosinia laeviscula at the top of FA2.
Some of the skeletal concentrations occur
in laterally continuous beds, with
wavy contacts and hardened concretionary
sectors that form ledges in the section.
There are a least three beds 1 - 2.5
m thick. The shell beds are densely to
loosely packed (25%) in some of the sectors,
and densely packed (55%) in others.
Size-sorting is poor; although by far the
numerically most abundant taxon is "T." patagonica; it is represented by specimens
of all sizes. Orientation is chaotic, disarticulation
is very high and only a few
shells are conjoined. Fragmentation is
low in some sectors and high in others,
while abrasion is nil. Encrusting and
bioerosion are evident in valves of C.? hatcheri, which carry encrusting bryozoans,
sponge borings (Entobia isp.), polychaetes
(Maeandropolydora isp.) and bivalve
borings (Gastrochaenolites isp.).
In addition to the aforementioned shell
beds also common are lenses of between
0.05 and 0.3 m thickness. The fossil content
of these lenses is the same as the
beds, as are all taphonomic attributes too.
The uppermost part of this facies association
shows a sharp decrease in fossil
content, as the beds and lenses become
much less frequent. However, a few disperse
fossils such as a Dentalium in life
position and Limopsis insolita can be
observed.
Facies Association 1 represents a transition
environment between the inner shelf
and the subtidal part of a tidal plain, below
fair weather wave base but above
storm wave base. The mollusks contained
in these beds are commonly considered
to be indicative of shallow, fully marine
conditions with normal salinity. Well
over half the taxa present in this fauna
are either very shallow infaunal (Turritella,
Cucullaea, Cardium, etc) or epifaunal (Zygochlamys,
Reticulochlamys, Swiftopecten, Trophon,
Austrotoma, etc.). Most trophic levels
are represented, including suspension feeders,
deposit feeders and carnivores.
Taxonomic composition of the assemblage
and the beds with articulate and
highly bored oyster clusters suggest that
the background sedimentation rate was
low.
The presence of thin consolidated lenses
with fragments of shells or specimens
lying chaotically is interpreted as of deposits
formed by sporadic events recording
a sharp increase in current velocity.
The lenticular structure of some of these
skeletal concentrations and their taphonomic
features suggests they belong to
the infilling of small scale tidal channels
in a subtidal to inner shelf environment.
Facies Association 2 (FA2)
This facies association overlies FA1 and
is equivalent to Darwin's lower two strata
of the upper part of the cliff. It comprises
massive and bioturbated tuffaceous
siltstones and very fine sandstones,
and tuffs. These are grey at the base and
light yellow towards the top. The thickness
is approximately 27 m at Darwin's
section and 22 m at the Mount Entrance
Section. The base carries large galleries
assigned to Ophiomorpha isp. (Fig. 4b) and
also few isolated shell fragments.
Towards the top there are frequent galleries
assigned to Thalassinoides isp. (up to 8
cm diameter). It also carries very sparse
remains of crabs and small monospecific
clumps of a small bivalve possibly referable
to Pteromyrtea, and very few fragments
of larger bivalve shells. The uppermost 8
m carry a few isolated valves of Dosinia
laeviuscula. These are disarticulate, convex-
up, and mostly unbroken. Throughout
this facies association are common
concretions of 0.8 - 1.4 m diameter that
may be isolated or joined forming "necklaces" (Fig. 4c). Darwin had already noticed them as a conspicuous feature in
this part of the section: "…Of the three
beds, the central one is the most compact, and
more like ordinary sandstone: it includes numerous
flattened spherical concretions, often united
like a necklace, composed of hard calcareous
sandstone, containing a few shells: some of these
concretions were four feet in diameter, and in a
horizontal line nine feet apart…"(Darwin
1846, p. 113).
At the Mount Entrance Section there is a
hard ledge of about 0.8 m thickness with
cross-bedding. It is highly bioturbated
with numerous tubes of about one cm
diameter placed obliquely to stratification.
At the Darwin Section this facies association
ends with a monospecific shell concentration
of 0.2 - 0.5 m thickness, with
a very fine sandstone matrix, and containing
adult specimens of Dosinia laeviscula (Fig. 4d). Contacts of this bed are wavy
and bioturbated, with many tubes reaching
up to 0.7 m down into the underlying
bed. Laterally it gives way to a white
tuff of equal thickness. Close-packing is
dense (55%), size-sorting good, orientation
chaotic, disarticulation high, fragmentation
moderate to low, abrasion nil.
At the Mount Entrance Section this
facies association ends with a skeletal
concentration of about 0.6 m thickness,
also with a fine sandstone matrix, but
with a polyspecific taxonomic composition,
carrying an abundant fauna of gastropods
and bivalves (see description of
Sections for a list of the most conspicuous
taxa and their stratigraphic distribution).
The lower contact is wavy and
the upper one transitional, as close-packing
is dense at the base and dispersed at
the top. Size-sorting is good, as the
whole specimens and the shell fragments
are of the same size with only a few
interspersed large bivalves and gastropods.
Orientation is chaotic, disarticulation
is complete, and fragmentation of
large specimens is high but with a few
whole specimens, while small specimens
are largely unbroken. Many specimens
show muricid and naticid boreholes.
Facies association 2 represents a subtidal
environment at the lowest part of a tidal
plain. The low diversity in the fauna
throughout the facies association (Pteromyrtea
and Dosinia), suggests a more marginal
shallow environment than FA1.
The high degree of bioturbation and dominance
of trace fossils assigned to Thalassinoides isp. and Ophiomorpha isp. suggests low sedimentation rate and an environment
with moderate to relatively low
energy conditions. Taphonomic features
of the specimens in the skeletal concentrations
at the top, together with the presence
of apparently unselected material
suggests that these concentrations were
formed during a high energy event, perhaps
representing a storm event.
Facies Association 3 (FA3)
This facies association comprises the uppermost bed of Darwin's "upper part of the cliff" (Darwin 1846, p. 113). It includes 19 - 20 m of interstratified siltstones, fine sandstones and claystones, sometimes with heterolithic lamination. At the base there are convex-down right valves of Crassostrea? hatcheri. It carries a few concretions with bivalves and echinoderms. At 11 m above the base color changes abruptly from brown to yellowish green. Fine grain-size and sedimentary structures produced by variations between calm and turbulent water indicates deposition by combined traction and decantation processes. Such a process suggests that this facies association developed in a subtidal to intertidal environment.
Paleonvironments
Facies associations together with the
invertebrate fauna recorded in each bed
allow a detailed paleoenvironmental reconstruction
of the rocks of the Monte
León Formation in the Río Santa Cruz
area. These rocks can be included in the
regressive stage of the high sea-level
period that began at the end of the
Oligocene. The proposed deposition
model suggests that it took place in a
shallow sea, with variations that range
from the inner shelf to subtidal or even
intertidal environments.
At the base of the section (FA1) - i.e., the
inner shelf to subtidal environment -
there was a continuous deposition of
siltstone with volcanic material (bioturbated)
as result of decantation, with a
few intercalated episodes recording higher
energy events. These were recorded
as beds and lenses of skeletal concentrations
interpreted as having been deposited
by channeled high energy unidirectional
currents, perhaps tidal channels.
Towards the top of the sections (FA2)
these lenses become sparser, while the
bioturbation and pyroclastic content of
the deposit increase. The top carries
finely laminated siltstones and sandstones
(FA3) that suggest a subtidal - or
even intertidal - environment.
PALEONTOLOGY
One of the most conspicuous features of the Monte León Formation is the rich and excellently preserved mollusk fauna contained in the beds and lenses within the Punta Entrada Member; it occurs mainly in FA1 and the most conspicuous and common elements are mentioned above. This fauna has been known ever since Darwin collected the first specimens at the mouth of the Río Santa Cruz, and has been used over the years in the formulation of diverse hypothesis on the biostratigraphic and paleobiogeographic distributions of the taxa involved (del Río 2004 and references therein). Darwin's initial collection of 16 species of mollusks has been increased to over 110 recognized from that locality nowadays. Of these, virtually all are also known from the shell beds that were described later by Ameghino and Hatcher. The type locality of the species described by Sowerby (1846) is the section described here as Darwin Section. Thus, all his taxa come from beds lying within the Punta Entrada Member. It is important to note that taxa later described by von Ihering (1897, 1907, 1914) and Ortmann (1902) came from further south. The beds bearing these taxa were then included in the Superpatagonian, although Hatcher and Ortmann could see no difference between the Patagonian and Superpatagonian fauna. This is not surprising, as the Superpatagonian included two suites of taxa from entirely different sources. In the first place it included the species that is unique to the lowermost beds of the Santa Cruz Formation, i.e., Crasssotrea orbignyi. This oyster is quite different from Crassostrea? hatcheri, and is widespread in Patagonia and always appears associated to the marginal marine rocks at the base of the Santa Cruz Formation or its equivalent units. Secondly, the Superpatagonian included many taxa common to the "Leonense" chronostratigrapic unit of Ameghino, together with a suite of exclusive taxa not known from the underlying strata. However, these were based on material coming from two different sources. In the first place, it included material collected from the shell beds at the top of the Punta Entrada member, at localities where this unit is exposed along the coast between the mouth of the Río Santa Cruz and Monte Observación, just off limits to the south of Monte León National Park. It also includes material collected from beds overlying the former and containing Crassostrea orbignyi, exposed a few kilometers further inland and clearly at the base of the Santa Cruz Formation. The taphonomic attributes of the material other than the autochtonous accumulations of C. orbignyi suggest that it is in all cases reworked material from the underlying shell beds. In most cases, taxonomic differences can be attributed to the differential preservation of the two suites of material.
MEANING OF THE GREAT PATAGONIAN FORMATION AT THE MOUTH OF THE RÍO SANTA CRUZ
Upstream the Río Santa Cruz, the rocks
described by Darwin at the mouth of the
river are gradually replaced by continental
beds nowadays included in the Santa
Cruz Formation (Darwin 1846, p. 113-115). Although he described a section at
about 160 km west of Santa Cruz, Darwin
was doubtful that the rocks could be
included in "the great Patagonian tertiary formation",
as the only fossils he could find
were very worn pieces of Ostrea patagonica (i.e. Crassostrea? hatcheri or more likely
C. orbignyi). As Darwin never went ashore
further south than Mount Entrance,
he was unable to observe neither the
beds at Las Cuevas nor those within the
Monte Observación Member. Therefore,
his concept of "great Patagonian tertiary formation" or "Patagonian Tertiary Formation" only included those rocks exposed at
Santa Cruz (and other places further
north).
At Mount Entrance Bertels (1980, p.
214) defined a lower member of the
Monte León Formation that she named
Punta Entrada Member and included in
it all the marine beds exposed there.
Darwin's Section is only 2.6 km away and
no major structural features alter the
stratigraphic relations between the beds
exposed at the two localities. Consequently,
we can confidently assume that
(as confirmed by descriptions above) the
two sections are equivalent. About 40 km
further south along the coast and within
the present Monte León National Park,
these rocks continue to form part of the
coastal cliff and lithological features
remain overall constant. However, a
short distance inland a second higher
ledge is composed mainly by the continental
Santa Cruz Formation. The contact
between the two units has been subject
of heated debates over the years,
ever since the first subdivisions of the
Patagonian Tertiary were made by Ameghino.
The Patagonian Tertiary Formation
in Darwin´s sense was restricted by
Ameghino (1894) to the marine beds
exposed along the coast between Chubut
and Monte Observación in Santa Cruz.
His subdivision of this unit (Ameghino
1898) into "Juliense" and "Leonense" was
followed shortly afterwards (Ameghino
1900-1902) by his recognition that the
overlying "Formación Santacruceña" included
a lower marine bed he called "Superpatagoniano".
In this he included the shell
beds at the top of the cliff exposed at
Las Cuevas, Yegua Quemada, Jack Harvey
and Monte Observación, and also
the oyster beds at the base of the Santa
Cruz Formation exposed further inland
at Cerro Monte León and Cerro Observación.
He based such a subdivision on
the fossil content, a fact that was later
disputed by other authors (Wilckens
1905, Hatcher 1900, Ortmann 1902) who
could see no difference in taxonomic
composition between the Superpatagonian
beds and those appearing in the
typical Patagonian Formation.
Di Paola and Marchese (1973) did not
recognize Bertels' introduction of a formational
status for Ameghino's Juliense and Leonense (i.e., her San Julián and
Monte León Formations). Instead they
subdivided the Patagonia Formation
(sensu Ameghino 1894) into three members
based on lithological differences
among them. The uppermost one, overlying
the Monte León Member, they
named Monte Observación Member.
They defined this unit as the rocks appearing
below the Santa Cruz Formation at
Cerro Observación and that showed features
common to the Monte León Member
at the base and to the Santa Cruz
Formation at the top. The bottom of this
member is - according to them - an oyster
bed of Crassostrea orbignyi, and the top
is the last bed of this same oyster before
giving way completely to the continental
mammal-bearing rocks of the Santa Cruz
Formation (the contact here is transitional).
The total thickness measured by
them between the two oyster beds was 27
m. When introducing her Punta Entrada
Member, Bertels (1980, p. 214) also acknowledged
the difference between this
unit and the topmost beds of the Monte
León Formation. Therefore she admitted
the formal status given by Di Paola and
Marchese (1973, p. 214) to the Monte
Observación Member. However, she stated
that a) the Monte Observación
Member included all the shell beds at the
top of the sections at Monte León (Cabeza
de León), Las Cuevas, and Monte
Observación (i.e., Cerro Observación,
just west of Las Cuevas), and b) it replaced
Ameghino's chronostratigraphic
term "Piso Superpatagónico". However, these
assumptions appear to be unwarranted.
The lithological features described
by Di Paola and Marchese (1973) for the
Monte Observación Member are quite
clearly distinct from those observed (as
noted by Bertels herself; Bertels 1980, p.
216) in the shell beds at the top of the
sections at Monte León (Cabeza de
León) and Las Cuevas. The lithology in
these shell beds is clearly the same as that
in the underlying beds attributed by her
to her Punta Entrada Member. The
Superpatagonian of Ameghino originally
included not only the shell beds at Monte
León and Las Cuevas, but also those at
Yegua Quemada, Jack Harvey, and Monte
Observación all of them south of Las
Cuevas. He also included the shell beds
with Crassostrea orbignyi at Cañadón de los
Artilleros, inland from San Julián, also
underlying the continental Santa Cruz
Formation. Neither Ameghino nor any
author after him considered the fact that
the shell beds at these places are thicker,
with a far more abundant and better preserved
fauna than those at the mouth of
the Río Santa Cruz, the most likely reason
for them supporting the difference
between the Patagonian and Superpatagonian
beds. Such a difference was
minimized by Ortmann (1902) and Hatcher
(1900) based on the very abundant
fauna from Santa Cruz (possibly near
Mount Entrance) that they had available.
In fact, these authors were right in stating
that the different composition of the
fauna was due to local facies variations.
Moreover, the far more abundant fauna
at Las Cuevas is also surely the cause of
bias in this sense, as the chances of
collecting uncommon taxa are naturally
much higher there.
Di Paola and Marchese (1973), while correctly
defining their Monte Observación
Member, never mentioned the fact that
between the base of this unit (i.e., the
lowest oyster bed at Cerro Observación)
and the top of the shell beds at Las
Cuevas there are ca. 20 m of rocks that
are not exposed anywhere between Santa
Cruz and the last exposures with marine
shells at Puerto Coyle. As the lithology
(and sedimentary structures as well) of
the Monte Observación Member (other than the in situ beds with oysters in life
position) is identical with that of the
Santa Cruz Formation and the beds with
other marine shells included in this unit
are clearly reworked material with taxa
that appear at Las Cuevas too, we suggest
that the Monte Observación Member
should be restricted to the rocks described
by Di Paola and Marchese (1973, p.
214) and included in the Santa Cruz
Formation. Likewise, the shell beds at the
top of the sections at Monte León
(Cabeza de León) and Las Cueva, Yegua
Quemada, Jack Harvey and Monte Observación
should be definitely referred to
the Monte León Formation as they are
equivalent to the shell beds lying within
the Punta Entrada Member at Mount
Entrance and Darwin's Section. Therefore,
the use of the term Superpatagonian
becomes superfluos.
CONCLUSIONS
As a result of this study we were able to
pinpoint the most probable location
where Charles Darwin made his observations
in the Río Santa Cruz mouth. At
this locality, i.e., our Darwin Section, we
also identified the beds mentioned by
him, including the location of the beds
from where he collected the fossil mollusks
that were described by Sowerby.
The strata recognized by him are equivalent
to those described by us and of
which a detailed lithological description
is provided.
Rocks belonging to the Monte León
Formation exposed at the mouth of the
Río Santa Cruz can be included in the
regressive stage of the Atlantic transgressive-regressive cycle that flooded Patagonia
during the late Oligocene-early Miocene.
The deposition model suggests that
it took place in a shallow sea, with variations
that range from the inner shelf to
subtidal or even intertidal environment.
Analysis of sedimentological features of
these rocks, together with the mollusk
fauna contained in them, and comparison
with those from nearby exposures
further south along the coast and traditionally
referred to the "Superpatagonian",
suggest that the Monte Observación
Member of the Monte León Formation
should be restricted to its original
concept. As a consequence, the term "Superpatagonian" becomes meaningless,
at least as a chronostratigraphic
unit in the area studied.
ACKNOWLEDGEMENTS
We would like to thank Sergio Martínez (Montevideo, Uruguay) and Jeff Stilwell (Melbourne, Australia) for their thoughtful comments that improved the original version of this manuscript. Help in the field by Juan Griffin is duly acknowledged too. The Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa supported part of the fieldwork (Project PI188). CONICET financed field and lab work (Project PIP1442 /05).
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Recibido: 1 de octubre de 2008
Aceptado: 20 de noviembre de 2008