Sedimentology and paleontology of a miocene marine succession first noticed by Darwin at Puerto Deseado (Port Desire)
Silvio Casadío and Miguel Griffin
Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa, Santa Rosa, La Pampa. Email: miguelgriffin@aol.com
ABSTRACT
Rocks exposed just south of Puerto Deseado (Port Desire), Santa Cruz Province, were surveyed by Darwin during his journey on board HMS Beagle. The fossil mollusks collected there were studied later by Sowerby, who described four species based on the material from "Port Desire". Sedimentological and stratigraphical observations suggest that the marine rocks cropping out at Darwin's locality should be assigned to the Monte León Formation (early Miocene). The rocks were deposited in subtidal environments, most precisely in an offshore environment near fair weather wave base at the bottom to a lower shoreface at the top of the section.
Keywords: Darwin; Early Miocene; Monte León Formation; Sedimentology; Paleontology.
RESUMEN: Sedimentología y paleontología de una sucesión marina miocena citada por primera vez por Darwin en Puerto Deseado. Las rocas expuestas al sur de Puerto Deseado, provincia de Santa Cruz, fueron relevadas por Darwin durante su viaje a bordo del HMS Beagle. Los moluscos fósiles que coleccionó fueron estudiados por Sowerby quien describió cuatro especies basadas en ejemplares procedentes de Puerto Deseado. Las observaciones sedimentológicas y estratigráficas sugieren que estos depósitos marinos, estudiados originalmente por Darwin, son asignables a la Formación Monte León (Mioceno temprano). En general, las rocas representan ambientes submareales que en el tramo inferior de la sucesión corresponden a un costa afuera, próximo al nivel de base de olas de buen tiempo, mientras que hacia el techo pasan a un shoreface inferior.
Palabras clave: Darwin; Mioceno temprano; Formación Monte León; Sedimentología, Paleontología.
INTRODUCTION
The Cenozoic marine deposits of Patagonia
have been known since the early 19th century and have been studied ever
since d'Orbigny (1842) first described his
Tertiaire Patagonien. This author also described
the fossils that he collected during
his explorations in southern South
America. All of the specimens he described
came from the marine rocks exposed
in northern Patagonia - in the vicinity of
the mouth of the Río Negro, and in the
area surrounding the present city of
Paraná, in northeastern Argentina. Likewise,
he did mention the presence of a
large oyster similar to Crassostrea patagonica (d'Orbigny 1842) in Puerto San Julián,
which had been given to him by a sailor.
However, he never actually visited the
coast further south of Río Negro. During
his journey to Chile, Bolivia and
Peru around Cape Horn, he must have
seen from afar the cliffs rising along the
coast, as he included all these exposed
rocks in his Tertiare Patagonien.
A few years later, HMS Beagle sailed
along the coast of Patagonia, and on this
occasion the geology of the region was
studied by Charles Darwin, who published
his observations (Darwin 1846) together
with descriptions of the fossil taxa
he collected during this trip (Sowerby
1846). In addition to the localities already
mentioned by d'Orbigny, Darwin visited
four other places that became classic fossiliferous
localities over the years, and
that were crucial in establishing the basis
of Patagonian Cenozoic stratigraphy.
These four localities are San José, Puerto
Deseado, Puerto San Julián, and Santa
Cruz. San José lies on the northern coast
of Península Valdés in Chubut (northern
Patagonia). The other three localities lie
much further south along the coast of
the present province of Santa Cruz.
Darwin included all the rocks with marine
fossils from these localities, and from
Paraná and Uruguay, in what he called
the Great Patagonian Formation. Although
this appears to be roughly equivalent
to d'Orbigny's Tertiaire Patagonien,
Darwin was aware that this succession
included two distinct units, the lower one
marine, and the second one, continental
with land vertebrate remains. Nowadays,
the four localities visited by Darwin
include different formal units, deposited
in different basins and environments. For
example, the marine deposits at San José locality belong to the Puerto Madryn
Formation (late Miocene). The marine
rocks and fossils mentioned by Darwin at
San Julián belong either to the late
Oligocene San Julián Formation (Bertels
1970), or to the early Miocene Monte
León Formation (Bertels 1970). At the mouth of the Río Santa Cruz, the rocks
described by Darwin belong to the
Monte León Formation.
Darwin's observations and fossil collections
drew the interest of later explorers, geologists and paleontologists alike, especially
during the second half of the
19th century and early decades of the 20th
century. During this period, Ameghino
(1896, 1898, 1900-1902) established the
earliest stratigraphic subdivision of the
Cenozoic rocks of Patagonia. This subdivision
was mostly based on the fossil
content of these richly fossiliferous beds.
However, since Ameghino's subdivision,
there has been a lot of controversies, not
only because of the age of the rocks, but
also because of the stratigraphic relationships
and correlations of several outcrops
that lie spread over a huge area in
Patagonia, and that were attributed to the "Patagonian beds". Most of the rocks
exposed there are now included in different
formal units, although the exact relationships
among them are still partially
unresolved (Parras et al. 2008). Detailed
history of these controversies can be
found in Camacho (1974, 1979), Zinsmeister
(1981), Legarreta and Uliana
(1994), and Malumián (1999, 2002). Many
of the rich faunas contained in the
marine and continental Cenozoic rocks
are crucial in the reconstruction of the
evolutionary history of the South American
biota. Therefore, a clear understanding
of the stratigraphic relationships -
especially for those type localities of taxa
that are used with biostratigraphic and/or paleobiogeographic purposes - is prerequisite
to any further inference involving
these rocks and the fossils that they
contained.
During its voyage around the world, the
HMS Beagle anchored in the Río Deseado
mouth. There, Darwin had the opportunity
to explore the riverbanks upstream
and to provide detailed descriptions
of the rocks he encountered. The
northern bank of the river - where the
modern town of Puerto Deseado (Fig. 1)
lies and along where the road and railroad
run - is mainly composed of Jurassic
volcanic rocks for at least 30 km upstream.
These rocks were well recorded by
Darwin, who was aware of their lack of
fossils. However, a few kilometers further
upstream of "Darwin's Rock", the
furthest upstream point reached by him,
marine Cenozoic rocks rich in fossils become
exposed.
Figure 1: Location map, showing Darwin's locality at Port Desire
These rocks remained unknown to Darwin.
Nevertheless, among the fossils collected
by him and described by Sowerby
(1846), there are several that come from "Port Desire". Darwin mentioned as the
fossiliferous locality, the cliffs lying a few
miles south of the river mouth. This locality
comprises two very small outcrops
that ever since remained almost unexplored
by geologists or paleontologists, as
they are in a relatively inaccessible area
and are not visible from the town. Four
of the species of mollusks described by
Sowerby come from this locality. Despite
the fact that it is a type locality of taxa
later used in biostratigraphic and paleobiogeograpic
reconstructions, it has remained
obscure and little known ever
since Darwin discovered it.
The late Oligocene - early Miocene marine
beds exposed along the Atlantic coast
of Patagonia are presently included in
the San Julián (late Oligocene - early
Miocene) and Monte León (early Miocene)
formations (in the Austral Basin),
and in the Chenque Formation (in the
San Jorge Basin). These rocks were deposited
during Atlantic transgressions that
covered large portions of Patagonia.
The Monte León Formation was formally
described by Bertels (1970), and it
includes those deposits at the mouth of
the Río Santa Cruz, which are characterized
by their abundant and well preserved
fauna. These rocks and faunas were extensively
studied throughout the years
(Ihering 1897, 1899, 1907, 1914, Ortmann
1902, Feruglio 1949, Di Paola and
Marchese 1973, Bertels 1980, Náñez
1988, del Río 2004, del Río and Camacho
1998, Crawford et al. 2008, and references
therein), but most of these studies
were carried out on the exposures found
around the mouth of the Río Santa Cruz
and in the San Julián area. In contrast,
outcrops with similar rocks in the Puerto
Deseado area received far less attention
(Frenguelli 1931, Feruglio 1949, del Río
2004, Giacosa et al. 1998). In this area,
the Neogene marine rocks overlie Jurassic
volcanic rocks of the Bahía Laura
Group, and are reduced to patchy thin
exposures isolated one another. Giacosa et al. (1998) used the informal name "Patagoniano" to designate these outcrops
and suggested that these were equivalent
to the San Julián and Monte León formations.
However, del Río (2004) included
them in the Chenque Formation, a unit
formally described for rocks exposed
about 150 km to the northwest, in the
vicinity of Comodoro Rivadavia city.
Sedimentological and stratigraphical observations
suggest that the Neogene
marine rocks at Darwin's locality near
Puerto Deseado should be assigned to
the Monte León Formation. In this study
we present a detailed description of these
rocks and comments on its paleontological
content.
Institutional abbreviations used in this
work are as follows: NHM-L (The Natural
History Museum, London, Department
of Palaeontology); GHUNLPam
(Colección Paleontológica del Departamento
de Ciencias Naturales de la Universidad
Nacional de La Pampa).
GEOLOGICAL SETTING
South of Puerto Deseado, the Monte León Formation (early Miocene) is exposed between Punta Guanacos and Punta Norte. The lowermost portion of the section is exposed during low tide along the coast, with the remainder of the formation outcropping along the base of cliffs. At the studied locality (47º48'37''S; 65º52'32''W, see Fig. 1), the Monte León Formation is 10 m thick (Fig. 2) and unconformably overlies Jurassic volcanic rocks of the Bahía Laura Group. The contact between the two units is only visible during low tide. The Monte León Formation is unconformably overlain by Quaternary conglomerates (Fig. 3a).
Figure 2: Stratigraphic section at
Darwin's locality.
Figure 3: a) Puerto Deseado section. The Monte León Formation is unconformably overlain by Quaternary conglomerates; b) Lenticular shell concentrations
dominated by "Turritella" patagonica;
c) Specimens - mostly articulated - of Crassostrea? hatcheri; d) Thalassinoides isp. Filled with alternating sandy and
muddy laminae.
Facies associations (FA)
FA1: This association includes fine bioturbated sandstone and mudstone, intercalated with lenticular shell concentrations (less than one meter wide and 0.4 m thick) with trough-cross stratification, dominated by "Turritella" patagonica (Fig. 3b). At the top of these concentrations there are specimens - mainly articulated - of Crassostrea? hatcheri (Fig. 3c). Specimens of Crassostrea? hatcheri in life position are also abundant. They are mainly obvious du-ring low tide on the wide abrasion platform, where some specimens are found forming clumps. Commonly, the valves of this oyster are bioeroded by sponge (Entobia isp.), polychaetes (Maeandropoly-dora isp.) and Pholadidea patagonica (Gastro-chaenolites isp.). Large burrow systems, with horizontal and vertical components are frequent in this facies association. These excavations are 5 cm in average diameter, and are assigned to Thalassinoi-des isp. The fill of these structures consists of alternating snady and muddy laminae (Fig. 3d).
Interpretation
FA1 represents an environment lying
above storm wave base and below normal wave base. The alternating sandy and
muddy laminae filling the Thalassinoides burrows probably indicate tidal cyclicity.
The abundance of Thalassinoides isp. suggests
that the benthos was well-oxygenated
during deposition (Bottjer et al. 1986,
Bromley 1990) and the substrate was
probably firm. However, the abundance
of "Turritella" patagonica - member of a
very shallow infaunal group - suggests
that the surface sediments were most
likely poorly consolidated. The concentrations
of "Turritella" patagonica may represent
the infilling of small tidal channels.
The beds with articulated and highly
bored oyster clusters point towards
periods of lower sedimentation rates.
FA2: comprises mainly fine- to mediumgrained
massive tuffaceous sandstones.
Bioturbation completely obliterated the
sedimentary structures. There are large
burrow systems mainly with vertical
components, with an average diameter of
5 cm, and more than 1.5 m long, branching
points not observed, precluding
ichnoespecific identification and therefore
referred to Thalassinoides isp. The fill of these burrows is mainly composed by
sand and abundant fragments of "Turritella" patagonica. Most of the fossils are
disarticulate and the degree of fragmentation
is low. In addition to semi-infaunal
and epifaunal taxa such as Atrina magellanica,
Neopanis quadrisulcata, Swiftopecten nodosoplicatus,
Zygochlamys geminata, Zygochlamys
quemadensis, Reticulochlamys proxima,
Jorgechlamys centralis among others, specimens
of the deep burrowing bivalve Panopea
quemadensis and shallow infaunal Cucullaea
alta and Lahillia patagonica were
found articulated and in life position.
Interpretation
FA2 represents a lower shoreface. Bioturbation
was reported for mixed sediments
to a depth of as much as 1 m in
shallow marine environments (Smith and
Nelson 2003) resulting in a homogenized,
poorly-sorted layer (Fürsich and
Pandey 2003). Mollusks suggest that
background sedimentation rate was low.
Most of the taxa found in FA2 represent
either very shallow infaunal (e.g. arcoids,
cardiids) or epifaunal forms (e.g. pectinids,
muricids), indicating that the substrate
was stable for extended periods.
In general, the rocks exposed at Darwin's
locality represent subtidal facies from an
offshore environment near the fair weather
wave base at the bottom, to a lower
shoreface at the top of the section.
PALEONTOLOGY
The most conspicuous feature of the
rocks described here is the rich and well
preserved fauna of mollusks contained
in them. This fauna lies almost entirely
within FA 2 and amongst the most abundant
elements are Nucula (Lamellinucula)
reticularis, Neilo ornata, Cucullaea alta, Limopsis
insolita, Glycymeris cuevensis, 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?
puelchum, Cardium patagonicum, Lahillia
patagonica, Dosinia laeviuscula, Ameghinomya
darwini, Panopea quemadensis, Valdesia
dalli, Solariella dautzenbergi, Calliostoma santacruzense, "Turritella" ambulacrum, "Turritella"
patagonica, Polinices santacruzensis,
Glossaulax vidali, Cirsotrema rugulosum, Trophon
santacruzensis, Xymenella dautzenbergi,
Xymene cossmanni, Enthacanthus monoceros,
Crassilabrum hatcheri, cf. Ocenebra iheringi,
Urosalpinx archipatagonica, Neoimbricaria patagonica,
Antimelatoma quemadensis, and
Dentalium sp. All these taxa are also found
in the Punta Entrada Member of the
Monte León Formation, at the mouth of
the Río Santa Cruz, the only difference is
the relative abundance of taxa in the two
areas. They are far less numerous at
Puerto Deseado, but this can be due to
the fact that the exposures are also very
much reduced there. Darwin's specimens
were described by Sowerby and four of
the species described by him were based
on material from "Port Desire". Three of
these were bivalves and one is a gastropod.
The latter is the most conspicuous
fossil at both localities:
Neilo ornata (Sowerby, 1846) (Fig. 4a). It
is not strange that Sowerby had only "fragments" (see Griffin and Nielsen
2008), as most of the specimens of this
species, although complete in the rock,
are very easily broken when collected,
possibly as a result of the shell structure
in this group. The species is quite common
in Puerto Deseado, but it also occurs
in the Santa Cruz area (sensu Darwin,
i.e., just upstream from the mouth of the
river in the Atlantic Ocean), where it is
generally found at the base of the section.
Other species of Neilo are known
from Paleogene and Neogene rocks exposed in different areas of Patagonia and
Tierra del Fuego, but the relationships
among them remain rather obscure, as in
most cases the material consists of fragmentary
molds.
Figure 4: a) Neilo ornata (Sowerby, 1846). Lectotype, NHM-L 27974; b) Cucuallaea alta Sowerby, 1846, GHUNLPam 16859, external view; c) Jorgechalmys centralis (Sowerby, 1846), GHUNLPam 16860, left valve; d) "Turritella" patagonica Sowerby, 1846, GHUNLPam 16861.
Cucullaea alta Sowerby, 1846 (Fig. 4b). Also common at Monte León, this species
appears sparsely in FA2, and is less
abundant than at the localities further
south. Darwin's original material - i.e.,
Sowerby´s syntypes - included three specimens,
one of them a juvenile from "Santa Cruz", and the other two (possibly
the two separate valves of the same
specimen) are from "Port Desire". One
of the latter is a large well preserved specimen,
similar to those found by us at
this locality. One of Darwin`s specimens
from Puerto Deseado is an adult, but the
two are fairly well preserved. This species
also appears at other Patagonian localities
in which rocks of similar age are
exposed, although it is never quite as
abundant as it is at the exposures at the
mouth of the Río Santa Cruz and environs.
Jorgechlamys centralis (Sowerby, 1846) (Fig. 4c). This large pectinid is known
from rocks belonging to the Monte León
Formation, or to its equivalents such as
the Centinela and Chenque Formations.
The type material comes from "Santa
Cruz" and "Port Desire", but lectotype
designation by Griffin and Nielsen
(2008) has restricted the type locality to
Puerto Deseado. It is an important species
as it has been used in biotratigraphic
zonations of Patagonian Cenozoic units
(del Río 2004).
"Turritella" patagonica Sowerby, 1846 (Fig. 4d). This species is the most common
mollusk in the Monte León
Formation. It appears in great numbers
and constitutes a significant part of the
deposits in which it occurs. The exact
generic placement of this species remains
as yet uncertain, as pointed out by
Griffin and Nielsen (2008). This taxon
occurs in many other localities where
marine rocks of this age are exposed. It
is very common in the Chenque, Centinela,
and Carmen Silva formations, albeit
in none of these it reaches the abundance
showed in the Monte León Formation.
CONCLUSIONS
South of Puerto Deseado the rocks exposed along the cliffs show a lithological composition that is not distinguishable from those of the Monte León Formation at its type locality further south. The succession exposed in the Puerto Deseado area contains subtidal facies from an offshore environment near fair weather wave base at the bottom to a lower shoreface at the top of the section. The type specimens of Neilo ornata, Cucullaea alta, Jorgechlamys centralis and "Turritella" patagonica described by Sowerby (1846) come - in all cases - from the cliffs south of Puerto Deseado. The mollusks collected by us from FA2 are also found in the Punta Entrada Member of the Monte León Formation at the mouth of the Río Santa Cruz suggesting an early Miocene age for Darwin's Port Desire locality.
ACKNOWLEDGMENTS
Prefectura Naval Argentina (Puerto Deseado) was instrumental in carrying out field work at Darwin's locality. The help of Miguel Santillán is also acknowledged. J. Todd (The Natural History Museum, London) provided photographs of Sowerby's type material. The manuscript benefited from thorough reviews by Noelia Carmona, Sven Nielsen and Guido Pastorino. Financial support of CONICET and Universidad Nacional de La Pampa are also duly acknowledged.
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Recibido: 14 de octubre de 2008
Aceptado: 27 de noviembre de 2008