On the formations of the Pampas in the footsteps of Darwin: south of the Salado
Marcelo Zárate1 and Alicia Folguera2
1 CONICET- Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa, Santa Rosa, La Pampa.
E-mail: marcelozarate55@yahoo.com.ar
2 Servicio Geológico Minero Argentino (SEGEMAR), Buenos Aires. Email: alifolguera@yahoo.com
ABSTRACT
In 1833 during his journey across the Buenos Aires Pampas, Charles Darwin made observations that reflected his thoughts on two major landscape units, Pampa interserrana and Pampa deprimida, later identified by other authors. Darwin grouped the Pampean sediments into a single unit, the Pampean Formation, based upon the lithological homogeneity and the large extension of the deposits; the unit was thought to be of estuarine-marine origin and attributed to the Recent Epoch considering the paleontological content (vertebrates and mollusks). At present, the Pampean sedimentary succession, which accumulated approximately during the last 11-12 Ma, is interpreted as a pedosedimentary sequence due to the ubiquity of pedogenetic features throughout the deposits. Four main subcycles of sedimentation are identified related to reactivations of the Pampean landscape. At a regional scale, the outcrop distribution of Pampean sediments of different ages suggests the dominance of more stable conditions since the late Miocene-Pliocene in a vast area of Pampa interserrana, documented by the formation of calcretes. However, sedimentation during the late Pliocene-Pleistocene was active within the domain of the Salado tectonic basin and Sierras de Tandil. The regional disparity shown by the Pampean stratigraphic record reveals the major morphostructural differences of its basement.
Keywords: Charles Darwin; Pampean sediments; Calcrete; Geomorphology; Buenos Aires.
RESUMEN: Sobre las formaciones de las Pampas en los pasos de Darwin: al sur del Salado. Durante su viaje por la Pampa bonaerense en 1833 Charles Darwin efectuó observaciones que reflejaban las dos grandes unidades de paisaje posteriormente reconocidas en la región, la Pampa interserrana y la Pampa deprimida. La homogeneidad litológica y la vasta extensión de los depósitos fueron los criterios básicos empleados para agruparlos en una única unidad, la Formación Pampeano; basado en criterios paleontológicos le atribuyó origen estuárico-marino y la asignó a la época Reciente. Si bien han existido otras propuestas estratigráficas, Formación Pampeano o simplemente Pampeano, son denominaciones empleadas informalmente y con un significado algo vago. Actualmente, sobre la base de la ubicuidad de los rasgos pedológicos de la sucesión sedimentaria se la interpreta como una secuencia pedosedimentaria acumulada durante los últimos 11-12 millones de años, se han reconocido 4 ciclos de sedimentación principales relacionados con reactivaciones del paisaje pampeano. Regionalmente, la distribución areal de niveles aflorantes de distintas edades sugiere una mayor estabilidad relativa en el sur, marcada por la formación de calcretos en la Pampa interserrana a partir del Mioceno tardío-Plioceno y mayor continuidad del proceso sedimentario en el ámbito de la cuenca tectónica del Salado y de las sierras de Tandil. Las variaciones regionales exhibidas por el registro estratigráfico del Cenozoico tardío constituyen una manifestación del comportamiento diferencial de las unidades morfoestructurales que integran el basamento de la sucesión sedimentaria.
Palabras clave: Charles Darwin; Sedimentos pampeanos; Tosca; Geomorfología; Buenos Aires.
INTRODUCTION
In the course of the winter of 1833,
Charles Darwin travelled across the Buenos
Aires Pampas. His observations, as
well as those by Alcide d´Orbigny (1842),
the French naturalist who explored the
region some years before Darwin, were
part of the initial expansion stage of
geology as a science. The explorations of
both naturalists are framed within the
creation of a universal scheme of the
Earth history in which all the regions of
the world would be incorporated (Podgorny
et al. 2008).
In early August of 1833, Darwin disembarked
from the Beagle at Patagones, a
town by the Rio Negro, in northern Patagonia
and began his journey heading
north towards Bahía Blanca. From there
he travelled across the southern Pampas
to the Rio Salado area and Buenos Aires,
continuing later to Santa Fe. Logistics for
the trip was provided by Juan Manuel de
Rosas - the Argentine administrator at
the time - in the form of a passport to
travel, horses, and a group of soldiers
and gauchos to escort him. In his diary,
Darwin took notes on the Pampean scenery,
referring to the dominant vegetation,
the available water sources, the topographic
features and the sediments
and rocks he found along the journey. He also provided a historical picture of Argentina
with descriptions and his own
viewpoints on the people and the ongoing
war with the aboriginal groups to
expand the frontier of the southern territories
westward. The information gathered
during the field trip was the raw material
to write On the Formations of the Pampas,
the fourth chapter of his Geological Observations on South America (1846).
The vast extent, the disputed origin and
the abundance of fossil mammal remains
were the three main aspects underlined
by Charles Darwin when he summarized
the characteristics of the Pampean sediments.
What was the meaning of the
Pampean Formation as used by Darwin?;
what is the meaning of this term at present?;
what is our present understanding
of the late Cenozoic Pampean geology?.
This paper is particularly focused on the
Buenos Aires Pampas, south of the Rio
Salado, the region that Darwin explored
during the winter of 1833; it includes
comments on specific observations he
made elsewhere to understand better his
interpretations and inferences. Consequently,
the main goal is to analyze and
discuss his observations on the landscape
and the sediments within the framework
of our present understanding of the late
Cenozoic geology of the southern Pampean
plain. With this purpose in mind,
the present contribution brings in Darwin ´s comments and interpretations on
the landscape along with the nature,
composition and areal distribution of the
Pampean sediments (terms, descriptions,
and special comments are quoted in italics
and referred to his diary and the
chapter on the Formations of the Pampas).
These particular issues are then analyzed
from our present geological perspective,
providing an updated geological
review.
DARWIN´S ITINERARY AND HIS PERCEPTION OF THE PAMPEAN LANDSCAPE
Charles Darwin travelled around 650 km (400 geographical miles) from Rio Colorado to Buenos Aires following the road that connected the military outposts (Postas) set up along the western frontier in 1833 during the ongoing war with the aboriginal groups (Fig. 1). On the way to Bahía Blanca, once he crossed the Río Colorado, Darwin noticed a major change between the Patagonian and the Pampean environments "...The country had a different appearance from that South of the Colorado: there were many different plants and grasses and not nearly so many spiny bushes and these gradually became less frequent; until a little to the North there is not a bush..."(Darwin's Beagle Diary 1831-1836). In southern Buenos Aires province he examined the neighboring areas of Bahia Blanca and Punta Alta where he spent around a week. On August 27th Darwin continued to the next military outpost, "Sauce Posta", situated by the Rio Sauce Grande, "…the distance was about 6 leagues from Sierra de la Ventana…" (Darwin's Beagle Diary 1831-1836). It is likely that this Posta was situated in the vicinity of the present village of Saldungaray.
Figure 1: Darwin´s itinerary across the Pampean plain of Buenos Aires province. S: Sauce Posta, T:
Tapalqué, G: Guardia del Monte; 1833 frontier adapted from Rolleri et al. (2005)
From the Sauce Posta, Darwin headed to Sierra de la Ventana (Ventania range). There he stayed for around 3 days exploring the ranges which were described as "…a desert mountain of pure quartz rock.Its height is between 3 and 4000 feet …" (Darwin's Beagle Diary (1831-1836). From the notes reported in his diary he probably examined the rock outcrops and the landscape of the upper basin of Rio Sauce Grande and its surroundings, reaching the area of the highest peaks (Cerro Tres Picos). His observations include remarks on the vast extent of the plain surrounding Sierra de la Ventana "…the plain was like the ocean…", (Darwin's Beagle Diary 1831-1836); "...the high plain round this range sinks quite insensibly to the eye on all sides..."(Darwin 1846, p. 79). After surveying this sector of Sierra de la Ventana, he returned to the Sauce Posta. Several days later he continued northward to Sierras de Tandil (Tandilia range). Proceeding to the 3rd Posta, the landscape was perceived as "…a dry grassy plain and on our left hand at a greater or less distance were low hills.." (Darwin's Beagle Diary 1831-1836); this description seems to depict the plain sector northward of Ventania (Fig. 2). From there he travelled around 160 km between the two mountain systems. Heading to the 4th Posta the landscape is reported as "…a low swampy country which extends for nearly 80 miles to the Sierra Tapalken…" (Darwin's Beagle Diary 1831-1836). After crossing this large swampy area Darwin arrived to the 7th Posta in the proximity of Sierra Tapalken (spelled Tapalqué nowadays) depicted as "…a low broken ridge of quartz rock, 2 or 300 feet high extending to the east to Cape Corrientes…" (Darwin's Beagle Diary 1831-1836).
Figure 2: Geographical location of the Laprida depression, fluvial streams and localities mentioned
in the text. 1: Río Quequén Grande, 2: Río Quequén Salado, 3: Río Sauce Grande, T: Tapalqué, S:
San Miguel del Monte.
From this range Darwin headed to Buenos Aires along the basin of Rio Tapalken (at present arroyo Tapalqué); the 8th and 9th Postas were situated along this stream. On the way to the Rio Salado he described the plain as "…partly swamp and partly good to the east of rio Tapalguen…" (Darwin's Beagle Diary, 1831-1836). He arrived at Guardia del Monte (the present town of San Miguel del Monte) on September 19th, 34 days after disembarking in Patagones, Rio Negro.
THE SOUTHERN PAMPEAN LANDSCAPE
Swampy or dry areas, scarcity or more frequency of tosca outcrops and rounded or flat topped hills were some of the fundamental geographical attributes used by Darwin to describe the landscape of the Buenos Aires Pampas. Southward of Tandilia, he perceived the region as a dominantly dry environment; northward, in the Rio Salado basin, Darwin referred to more fertile lands, the dominance of swampy environments and a general paucity of tosca exposures. The attributes of the landscape and the environmental features reported by Darwin essentially reflect the major geomorphological differences between the so-called Pampa deprimida (depressed Pampa) and Pampa interserrana (intermontane Pampa) (Frenguelli 1950), a still current subdivision of the Buenos Aires Pampean plain (Fig.1). The Tandilia and Ventania mountain systems together with the Pampa interserrana, situated in between the two ranges, constitute the geomorphological expression of a geologically complex structural block named Positivo Bonaerense by Yrigoyen (1975). This positive structural unit separates the Salado and the Colorado tectonic basins (Fig. 3).
Figure 3: Morphostructural units of Buenos Aires province, modified after Yrigoyen (1975).
The Pampa interserrana, characterized by Darwin as a dry plain of tosca higher than the country rock (Darwin 1846, p. 79) is a large-scale plain extending across most of southern Buenos Aires province and continuing southwestward into La Pampa Province, where it has been interpreted as a structural plain (Calmels 1996). The plain is dissected by fluvial systems that drain the central part of Pampa interserrana and the neighboring ranges of Tandilia and Ventania. Fluvial terraces are present along the main river valleys generating a stepped topography. In the upper basins of some of the main rivers (Quequén Grande, Quequén Salado, Sauce Grande), as well as other streams draining the Ventania range, the erosional retreat of the plain surface during the excavation of river valleys gave rise to a hilly and moderately undulating landscape composed of flat-topped and smooth-shaped isolated hills (Fig.4a).
Figure 4: Plain capped by the tosca rock (calcrete). a) Isolated hill formed by fluvial erosion of the
Río Sauce Grande, south of Saldungaray; Sierra de la Ventana at the background;
b) Calcrete crust
and resulting tableland aspect of the plain by the Río Sauce Chico valley, west of Bahia Blanca.
The plain is capped by a calcareous duricrust
(Fig. 4b) or calcrete (Darwin´s tosca
rock, see below), in turn covered by a large
apron of eolian sandy silts and silty
sands, around 1.5- 2 meters thick, making
up the parent material of the presently
cultivated Pampean soils. North-northwestward
the landscape grades into a low
-gradient area characterized by very poor
drainage conditions with numerous shallow
lakes and flooded environments,
known as Laprida depression (Fig. 2), an
endorheic sector (Dangavs 2005) still poorly
known. The environmental features
of this area fit Darwin´s description of a
large swampy environment before he
reached Sierras de Guitru Gueyú (the present
group of low ranges at the northwestern
part of Tandilia).
Since Darwin´s observations on some
general morphological characteristics of
the southern ranges of Buenos Aires, the
geomorphology of Tandilia and Ventania
has been the focus of only a few studies,
although these have documented
significant stages of landscape development,
some not yet fully understood. In
Sierra de la Ventana, Keidel (1916) pointed
out the presence of old erosion surfaces
at different altitudes in the area of
the upper basin of Rio Sauce Grande. In
turn, Teruggi and Kilmurray (1980) mentioned
the presence of monadnocks and
old peneplain surfaces in the Sierras de
Tandil. Recently, a stepped topography
composed of erosion surfaces at different
elevations was recognized and geomorphologically
correlated at both
mountain ranges (Demoulin et al. 2005).
A model of landscape evolution has
been proposed hypothesizing that the
highest erosion surfaces of both ranges
formed prior to the break-up of Gondwana
and the rifting process which led to
the opening of the South Atlantic
Ocean. This long-lasting history of
denudation included intervals of landscape
stability and reactivation which are
thought to be related to the evolution of
the Colorado and Salado tectonic basins
(Demoulin et al. 2005). In this context,
one of Darwin´s observations in Sierra
de la Ventana, becomes particularly revealing.
He reported a rock outcrop, "…a
few small patches of conglomerate and breccia
firmly cemented by ferruginous matter at a height
of 300 to 400 m…" (Darwin 1846, p. 79).
This description is in agreement with the
geomorphic setting and the general lithological
characteristics of Brecha Cerro Colorado (Andreis et al. 1971) or Conglomerado
Rojo (Harrington 1936), a deposit associated
with one of the regional erosion
surfaces identified in Sierra de la Ventana
(Keidel, 1916, Demoulin et al. 2005).
Northward of Tandilia range, in the domain
of the Salado tectonic basin, the
landscape can be subdivided into two
main geomorphic settings. Along the Sierras
de Tandil, Tricart (1973) pointed out
the occurrence of extensive areas of fluvial
discharge interpreted as alluvial fans
that reached the Río Salado. Later, Fidalgo
(1983) described a 20-km wide
piedmont region and also identified alluvial
fan-like landforms exhibiting a distributary
drainage pattern. The area of
arroyo Tapalqué visited by Darwin is located
in this piedmont setting, characterized
by a progressive altitudinal decrease
northward, reflecting a gradual change to
imperfect drainage conditions. A very
flat topography with gradients ranging
from 0.1% to 0.01% typifies the central
part of the Salado tectonic basin; numerous
shallow lakes (lagunas) and swampy
environments are present. The Río Salado
floodplain, several kilometers wide
in some areas, and including shallow lakes
interconnected during flooding events, was crossed by Darwin who pointed
out: "…12th to 13th to 14th posta: we had
to ride for a long distance in water above the
horse knees..." (Darwin's Beagle Diary,
1831-1836). After passing the Río Salado
and before arriving to Guardia del Monte,
he stayed at a large estancia owned by
Juan Manuel de Rosas. Several isolated
hills (Los Cerrillos), occur in this sector.
These features are around 10 meters above
the surrounding plain and are located
on the eastern side of shallow lakes (Fig.
6). The hills represent lunettes generated
by eolian deflation of the topographic
depressions, presently occupied by shallow
water bodies, under drier conditions
(Tricart 1973, Dangavs 2005).
Figure 6: The Río Salado Valley, south of San Miguel del Monte (Guardia del Monte). a) Hill on the
northeastern-eastern side of the Río Salado floodplain;
b) Quarry excavated at the hill showing a 3
m thick loess section; present soil developed on top.
Stratigraphy and lithology of Pampean sediments: the meaning of the Pampean Formation
Darwin typified the sediments of the
Pampean Formation as "...a dull reddish,
slightly indurated argillaceous earth or mud,
often but not always, including in horizontal
lines concretions of marl, and frequently passing
into a compact marly rock..." (Darwin 1846,
p. 89). The reddish argillaceous earth was
named "Pampean mud". Darwin emphasized
the great extent of the Pampean Formation
as one of its most striking features.
According to his own observations
and the reports from d´Orbigny and
other travelers he considered that the
unit extended from the vicinity of Rio
Colorado in the south to beyond Santa
Fe in the north "…M. d´Orbigny traced it
for 250 miles further north…"(Darwin 1846,
p. 97); and from Banda Oriental (Uruguay)
in the east to Cordoba to perhaps
Mendoza in the west. Finally he stated "… the area of the Pampean Formation, as remarked
by d´Orbigny is at least equal to that of
France and perhaps twice or three as great…" (Darwin 1846, p. 97). Thus Darwin correctly
perceived what we now know to
be the great geographic extent of the
Pampean Formation.
Darwin also stressed the lithological homogeneity
of the sediments that he
grouped into a single unit, the Pampean Formation. The concept of Formation
(gebirgsformation), as a geological unit, was
coined by Abraham Gottlob Werner
(1749-1817), a German geologist from
the School of Mines at Freiberg, with the
purpose of defining a historical entity
that included rock bodies formed during
the same time interval; at a higher hierarchical
rank, the term terrain grouped several
formations. Both terms, formation
and terrain, although representing units
of different stratigraphic hierarchy, were
later used as synonyms by other authors.
Nevertheless, during the decade of the
1840s, the term formation took root in the
Anglo-Saxon community, while terrain,
as a synonym of formation, was adopted
in the French geology literature (Podgorny
et al. 2008).
Since Darwin´s visit, several other researchers
have stressed the apparent lithological
homogeneity of the Pampean sediments.
At a regional scale of analysis the
deposits look rather alike, dominantly composed of sandy silt and silts of light
brown to reddish brown colors, common
presence of calcareous accumulations of
various morphologies and a general massive
appearance. This perception of lithological
homogeneity was accentuated
by the nature of the exposures, scattered
and discontinuous outcrops of relatively
reduced thickness and mostly situated
along river banks, which constrain the
detailed analysis of the spatial and temporal
relationships of the sediments. Exceptions
were the exposures along the
Río Paraná and the Monte Hermoso seacliff,
where Darwin was able to examine
the deposits and their paleontological
content in better exposures. To examine
the Pampean subsurface he described the
lithology of some wells in the vicinity of
Buenos Aires and south of Bahia Blanca:"… in a well at the depth of seventy feet, according
to Ignatio Nunez, much tosca-rock was
met with, and at several points, at 100 feet deep,
beds of sand have been found..." (Darwin
1846, p. 77). Also, 50 km SE Buenos Aires,
along the coast of Rio de la Plata at
Ensenada (Fig. 2), Darwin examined the
exposures of a quarry "…specimens from
near Ensenada, given me by Sir W. Parish,
where the rock is quarried just beneath the surface
of the plain..." (Darwin 1846, p. 78).
Since the mid-XIX century, many contributions
have been focused on the stratigraphy
and several different classifications
were proposed. The purpose herein
is not to make a review of the different
stratigraphic classifications, but to stress
the concepts and ideas on which the subdivision
of the Pampean sediments is
based. Up to the present, however, Pampean
stratigraphy has remained a controversial
and debatable issue; various different
schemes are still used simultaneously
(Table 1).
TABLE 1: Some of the best known stratigraphic schemes of the Pampean sediments.
Those by Fidalgo et al. (1973) and Riggi et al. (1986) are currently used.
The homogeneity of the deposits together
with the discontinuity of the exposures
forced the search of a criteria other
than the lithology to separate the Pampean
sediments. Eventually this led to the
utilization of the fossil vertebrate content
to separate the deposits into different
stratigraphic units. Therefore, those
localities with exposures laterally continuous
for several km as the Mar del Plata
sea-cliffs, the Río Paraná bank exposures
and the sea-cliff at Monte Hermoso
were subdivided on the basis of their paleontological
content (among others
Ameghino 1908, Frenguelli 1957, Kraglievich
1952). At the end of the 19th century,
the stratigraphic analysis of the
Pampean deposits was benefited significantly
by deep excavations during the
construction of the harbours at Buenos
Aires and La Plata. These temporary exposures
provided sections several meters
thick that became type localities, not later
available, to characterize the uppermost
interval of the Pampean deposits (Ameghino
1908). The resulting schemes became
the groundwork that guided subsequent
stratigraphic studies.
Much later, the identification of units on
the basis of the vertebrate fossil content
evolved into the concept of land-mammal
ages (Pascual et al. 1965), a fundamental
stratigraphic tool still widely
accepted and used. In recent years, these
biostratigraphic units have been redefined
and updated (Cione et al. 2007 and
references therein). The relative degree
of evolution inferred from the fossil assemblages
was used in the stratigraphic
arrangement of the Pampean deposits.
Accordingly, and considering their paleontological
content, several units were
identified, and classified as lithostratigraphic
units (formations) receiving their
names after the locality where they were
described (among others, Epecuén Formation,
Saldungaray Formation, Irene
Formation, Arroyo Chasicó Formation;
Fidalgo et al. 1975 and references therein).
As time passed, in spite of the numerous
schemes proposed, Pampean Formation or
simply Pampeano developed into terms
that became deeply rooted in the Argentine
geological literature; both became
synonyms to refer to the Pampean deposits.
At present, the term Pampeano is still
used informally with a rather loose significance.
However, general agreement
exists to separate the surficial Late
Pleistocene-Holocene sedimentary cover
from the underlying Pampean deposits.
Following this line of reasoning, the prelate
Pleistocene deposits exposed in the Pampa interserrana and the Salado Basin
were grouped again into a single unit, the Formación Pampeano (Pampean Formation) identified exclusively on the basis of lithological
attributes (Fidalgo et al. 1973).
This proposal is somewhat close to the
original meaning given by Darwin to the
term Pampean Formation. Regardless of its
vertebrate fossil content, the unit defined
is nevertheless useful for mapping purposes
and well descriptions. More recent
schemes tend to divide the upper part of
the Pampean deposits into two units
(Ensenada and Buenos Aires Formations)
distinguished on the basis of their
paleontological content (Table 1). In turn, the uppermost sedimentary mantle
was subdivided into several lithostratigraphic
units that represent lithofacies of
different geomorphic processes and environmental
settings such as eolian, fluvial
and littoral deposits (among others
Fidalgo et al. 1973 a,b).
The Pampean sediments today: current ideas and interpretations
At present the Pampean sediments are
lithologically characterized as light brown
to reddish brown sandy silts or silty
sands, moderately indurated with a general
massive appearance. Darwin´s interpretation
was that the "...deposits were accumulated
in the former estuary of Rio de la Plata
and in the sea adjoining it…" (Darwin 1846,
p. 99); the source area of the Pampean
mud was thought to come from the weathering
of igneous and metamorphic
rocks of Brazil and Uruguay (Darwin
1846, p. 100).
The mineral composition of the Pampean
sediments reflects their derivation
from volcaniclastic material in turn mostly
derived from Andean rocks, with minor
contributions (Precambrian granitic
and metamorphic rocks and Paleozoic
quartzites) from the Tandilia and Ventania
ranges in the southern part. Northward,
the Pampean ranges of Córdoba
and San Luis and the Brazilian shield were
secondary source areas (Teruggi 1957,
González Bonorino 1965, Zárate 2003
and references therein). A detailed analysis
of the deposits reveals grain size
changes and variations in the density, color,
and mineralogical composition according
to the sediment age and the geographic
setting considered within the
Pampean plain.
The massive appearance and the rather
uniform lithology, among other attributes,
were usually taken as evidence of an
eolian origin and the sediments have
been interpreted as loess. Nervertheless,
Teruggi (1957) pointed out that the distribution
of primary loess was quite limited,
a statement reconfirmed by more recent
studies (Zárate 2003 and references
therein). Across the Pampean environment,
primary loess facies have a low
preservation potential. The deposits are
locally reworked either by aqueous transport
agents (loess-like or loessoid deposits)
or modified by pedogenesis (weathered
loess). Loess-like deposits exhibit a
weak and poorly expressed sedimentary
structure consisting either of horizontal
bedding or cross bedding. Along the
piedmont areas, channelized lithofacies
representing fluvial paleoenvironments
include rock fragments from bedrock of
the Tandilia and Ventania ranges. At distal
settings from the ranges, the coarse
fluvial facies are made up of bones and
tosca fragments embedded in a sandy
matrix.
Paleosols are a very common feature
with pedogenic traits (pedogenic structure,
textural changes resulting from eluviation,
carbonate leaching, evidences of
bioturbation features from soil microfauna
and vegetation) distributed throughout
the Pampean sediments. Welded paleosols
resulting from the superposition
of pedogenetic process have been reported
in both Pliocene and Pleistocene stratigraphic
sections of the Pampean succession.
Micromorphological studies also
point to the occurrence of intervals
when sediments accumulated progressively
on the surface while soil formation
was active, resulting in accretionary soil
surfaces. From a sedimentological perspective,
the occurrence of numerous
paleosol levels throughout the deposits
documents an episodic sedimentation
process. Due to the ubiquity of pedogenic
features, the Pampean succession is
interpreted as a pedosedimentary sequence
(i.e. Kemp and Zárate 2000,
Zárate et al. 2002). The presence of welded
paleosols and intervals of accretionary
pedogenesis suggest a variable
balance of pedogenesis and sedimentation
through time. At some sections, the
occurrence of discrete paleosol levels,
stratigraphically superposed, allow inferring
some sort of cyclicity in soil formation
that has been related to climatic
changes (among others Nabel et al. 2000,
Kemp and Zárate 2000).
Yet not assessed, bioturbation structures
are an outstanding feature of the Pampean
sediments. Not only are there traces
of invertebrates in the sediments,
particularly those beds more intensely
modified by pedogenesis, but also burrows,
caves and galleries (krotovinas) excavated
by small rodents and larger mammals,
mainly xenarthrans (Vizcaino et al. 2001). The resulting structures are filled
in by diamicton-like deposits, including
fragments of tosca, paleosol aggregates
and bones redeposited by water. Also,
some burrows and caves are filled with
eolian and/or fine grained laminated sediments.
In the Pampean sediments, minor erosive
episodes are recorded by both the partial
truncation of paleosol surfaces and the
local incision of fluvial paleochannels
(Ruocco 1989, Zárate and Fasano 1989).
Three major episodes of landscape reactivation,
inferred from stratigraphic
analysis and gemorphological evidence,
are documented by cycles of fluvial incision.
These generated erosional unconformities
in the sedimentary record as
well as terrace formations and different
aggradational surfaces in the Sierra de la
Ventana piedmont (Zárate 2005).
Carbonate accumulations of the Pampean sediments: the Tosca-rock
Darwin gave special attention to the calcareous
accumulations in his general characterization
of the Pampean Formation.
The duricrust (calcrete) that caps the sediments
in the Pampa interserrana area, and
constitutes massive accumulations in some
other locations, was described as a
marly rock and named tosca-rock, after the
popular term given to the carbonate deposits
by the local Pampean inhabitants
(Darwin 1846). He reported the morphology
(nodular and stalactiform concretions,
compact stratified tosca rock), the relation
with the Pampean mud, as well as the
geographical distribution and relative occurrence
(more frequent outcrops of
tosca in the area of the ranges and the Pampa interserrana, general scarcity of tosca exposures in the Rio Salado basin, almost
no tosca-rock in northeastern Buenos
Aires province:"… At Arroyo del Medio
the bright red Pampean mud contains scarcely
any tosca-rock…", "… At Rosario there is
but little tosca-rock..." (Darwin 1846, p. 87).
He also made comparisons of tosca samples
collected throughout the Pampas,
referring to their similar characteristics
(the absolute identity in his own words) of
specimens collected in the northern
Pampas, Tandilia (Tapalqué) and south of
Bahia Blanca. In addition, thin sections
of tosca were petrologically analyzed by a
colleague "...Dr Carpenter has kindly examined
under the microscope, sliced and polished
specimens of these concretions and of the solid
marl-rock collected in various places between the
Colorado and Sta Fe Bajada…." (Darwin
1846, p. 77).
Despite the economic importance of calcareous
crusts as a source of road building
material and in agricultural development
across most of the southern Buenos
Aires Pampean plain, no detailed and
systematic studies of these materials have
been carried out. In the Positivo Bonaerense area, the morphology and thickness
of calcretes vary according to the relative
age of the geomorphic surfaces that they
cover. The oldest crust capping late Miocene-Pliocene deposits is up to 1.5-2 m
thick. The general morphology of calcretes
suggests a high degree of relative development
close to morphological stages
5 and 6 as defined by Machette (1995).
Thus, a typical calcrete section is usually
composed of three main parts from top
to bottom. The uppermost part is made
up of platy calcrete consisting of 5-10
cm thick indurated layers, showing an
irregular horizontal fracturing (Fig 5B).
Lamination, brecciation, and dissolution
are very common features suggesting a
very complex history of multiple episodes
of carbonate precipitation and weathering
on the surface. The middle part
consists of powdery and massive carbonate
grading downward into the host
Pampean sediments or showing a very
sharp lower contact (erosional surface)
on those deposits. Carbonate nodules,
concretions and massive accumulation of
tosca are also very frequently found. Studies
on specific accumulations suggest
different mechanisms of formation, such
as precipitation from phreatic waters,
capillary rise, and pedogenic carbonate
leaching (Bk horizons) (Tricart 1973,
Imbellone and Teruggi 1986).
Figure 5: Plain edge in the
area of Bahía Blanca. a)
Plain surface with a thin
eolian cover and fragments
of calcrete, Atlantic Ocean
in the background; b)
Calcrete section at a quarry
showing typical platy structure
in upper part, grading
downward into Pliocene
sediments. Note hammer
for scale.
Age and origin of the Pampean sediments
On the basis of its lithological homogeneity (similar composition) and "...the apparent
absolute specific identity of some of its
mammiferous remains..." (Darwin 1846, p.
100), Darwin inferred that all over its
large area the Pampean Formation belonged
to the same geological epoch.
This age interpretation followed Charles
Lyell´s ideas, where the percentages of
living and extinct mollusks was a fundamental
tool for assigning relative ages to
the Tertiary strata. He collected some of
the mollusk shells from late Pleistocene-Holocene marine deposits exposed along
the Pampean coastal fringe. At Punta
Alta Darwin considered that the proportion
and specimens of shells found in the
sediments was comparable to those species
living today in the bay. Therefore,
taking into account the paleontological
content including living species of mollusks
that he considered contemporaneous
with the mammiferous remains found in
the sediments, Darwin attributed the
Pampean Formation to the Recent Period
or what is now called the Holocene.
From a stratigraphic viewpoint, Darwin
recognized stages in the Pampean
Formation on the basis of both the different
altitudes shown by the outcrops
of Monte Hermoso, Punta Alta and
Sierra de la Ventana, and an observation
now regarded as a taphonomic attribute
of a fossil remnant: "...From the rolled fragment
of black bone and from the plain of Punta
Alta being lower than that of Monte Hermoso,
I conclude that the course sub-littoral deposits of
Punta Alta are of subsequent origin to the
Pampean mud of Monte Hermoso; and the beds
at this latter place… are probably of subsequent
origin to the high tosca-plain round the Sierra
Ventana..." (Darwin 1846, p. 87).
The Pampean succession is now interpreted
as the late Cenozoic (approximately
the last 12 Ma) continental record of
central Argentina. Sediments of the Pampean
succession accumulated after a
Miocene regression of the Paranense sea
(Folguera and Zárate, submitted). Major
differences are evident with regard to the
regional distribution of the deposits. Fidalgo
et al. (1975) mapped the exposures
bearing Late Miocene and Pliocene vertebrate
fossil assemblages, showing a distribution
restricted to the Positivo Bonaerense
area. Darwin examined an outcrop
on the bank of the Río Sauce Grande
noting that "...there is an imperfect section
about 200 feet high displaying in the upper part
tosca rock and in the lower part red pampean
mud...". This likely corresponds to late
Miocene?-early Pliocene sandy siltstones.
Several other outcrops are discontinuously
exposed along other neighboring
river systems as well as some railroad
cuts. Northward, in the Río Salado valley,
the relatively oldest exposures of Pampean
sediments are much younger (Pleistocene
sensu lato, see below). These deposits
consist of compacted clayish silts
and sandy silts, showing evidence of hydromorphic
conditions (mottles, greenish
colors, manganese nodules, massive
carbonate accumulations).
Considering the stratigraphic relations,
numerical ages and relative ages inferred
from the vertebrate fossil assemblages,
the Pampean succession of sedimentation
has been recently subdivided into
four stratigraphic intervals, interpreted as
subcycles of sedimentation (Zárate 2005)
(Table 2).
TABLE 2: Age, geographic distribution and fossil mammal remains of the subcycles of
sedimentation identified in the Pampean deposits (after Zárate 2005).
The oldest sedimentary subcycle, is exposed
in the Positivo Bonaerense and the
northern part of the Colorado tectonic
basin. It is composed of Late Miocene
deposits (11-12 Ma to circa ?7-6 Ma) bearing
Chasicoan and Huayquerian fossil
remains. These deposits have an average
thickness of 150-200 m, and overlie the
Precambrian and Paleozoic bedrock of Pampa interserrana. The geographical distribution
of this unit is broad, extending
southwestward into La Pampa province
(Folguera and Zárate, submitted). Geomorphologically,
these late Miocene
deposits, capped by the thickest calcrete crust form the very extensive plain within
which the present river valleys were
excavated.
The second sedimentary subcycle, represented
by late Miocene?-early to late Pliocene
deposits (?7-6-3.2 Ma) is discontinuously
exposed along the river banks of
the drainage system. The deposits include
Huayquerian, Montehermosan and
Chapadmalalan fossil remains. The outcrops
are located in the Positivo Bonaerense and distributed throughout the Sierra de
la Ventana piedmont, the valleys of Quequén
Salado, Quequén Grande as well as
of minor streams of the southern part of Pampa interserrana, and along the lower
section of the Mar del Plata-Chapadmalal
sea-cliffs. The Monte Hermoso
stratigraphic section (Quattrocchio et al. 2009) is included in this cycle of sedimentation.
Farther southward, deposits of
comparable age crop out in the Colorado
tectonic basin. Exposures up to 15 meter
thick and 200-300 m long can be
found at various quarries in the surroundings
of Bahia Blanca. The sediments
consist of fluvial facies, including rock
fragments from Sierra de la Ventana, capped
by a 1-1-5 meter thick calcrete (Folguera
and Zárate, submitted). Darwin
explored the very top part of the exposures
(Fig. 5a) "…At the settlement of Bahia
Blanca, the uppermost plain is composed of very
compact, stratified tosca rock containing rounded
grains of quartz distinguishable by the naked
eye...". Further eastward, deposits of this
cycle still poorly known, crop out along
the lower reaches of the Quequén Salado
and the Quequén Grande rivers; Huayquerian,
Montehermosan and Chapadmalalan
fossil remains have been exhumed
(among others Verzi et al. 2008).
Along the Mar del Plata-Chapadmalal sea
cliffs, this stratigraphic interval is composed
of early to late Pliocene (3.2 to 4.5-5
Ma), massive siltstones with well developed
paleosols bearing Chapadmalalan
fossil remains.
The third subcycle of sedimentation (late
Pliocene-late Pleistocene circa 3.2 Ma-
0.040/0.030 Ma) includes deposits bearing
Marplatan, Ensenadan and Bonaerian
fossil remains. They are distributed
in the Salado tectonic basin, forming the
bedrock in which fluvial valleys are excavated.
The lower sections of deep quarries
sometimes penetrate levels containing
Ensenadan fossil mammal remains.
Rabassa (1973) reported sediments bearing
Ensenadan fossil remains in the central
part of Tandilia range. These deposits
are also discontinuously exposed
along the SW piedmont of Tandilia,
along the Atlantic coast between Mar del
Plata and Miramar. The late Pliocene-late
Pleistocene subcycle unconformably overlies
Pliocene siltstones along the Mar del
Plata sea cliff. Included in this subcycle
are the Sauce Grande fluvial terrace sediments
with Bonaerian fossil remains reported
by Deschamps (2005).
Along the Atlantic coast, between Miramar
and west of Necochea, the sea cliffs
are composed of fine sandy facies including
diamicton layers with a high content
of calcareous nodules and capped by a
platy calcrete. Inland, at a distance of several
kilometers from the coastline, the
uppermost 2 meters of these deposits
are exposed in numerous quarries and
roadcuts, suggesting that they form an
extensive plain. The stratigraphic and
geomorphologic relationship with older
deposits is still unknown. The lithofacies
are similar to the Pleistocene exposures
of Mar del Plata and might be part of the
late Pliocene-late Pleistocene subcycle of
sedimentation which should be confirmed
in future studies.
The most recent subcycle of sedimentation
(40-30 ka-to the present) is composed
of a large eolian apron along with
fluvial deposits accumulated during the
last glacial cycle. Recent numerical ages
obtained in fluvial deposits of northern
Buenos Aires point to an older age for
the beginning of this cycle of sedimentation
(Blasi et al. 2008). The eolian lithofacies
constituting the parent material of
the present cultivated soils are composed
of sandy deposits towards the west, grading
into sandy silts and silt east and northeastward.
In the Pampa interserrana area
the eolian mantle consists of loessial
sands, sand mantles, and sandy loess (Zárate
and Blasi 1993, Bidart 1996), representing
proximal eolian facies derived by
deflation from the Río Colorado flood-plain. In the area of the Río Salado valley,
the eolian facies are made up of silty deposits
(loess) named La Postrera Formation
by Fidalgo et al. (1973) (Fig. 6b).
FINAL REMARKS
Darwin as well as many other researchers
after him emphasized the relatively uniform
lithological composition, the vast
extent and the rich vertebrate fossil content
of the Pampean sediments. His perception
of the Pampean environment
illustrated not only the major landscape
differences but also some key lithological
features of the deposits. To date, the geographical distribution of exposures
of different ages suggests significant variations
of the sedimentary dynamics
throughout the region, and four main
subcycles of sedimentation have been
identified. The outcrops of the first two
cycles recording the late Miocene and
most of the Pliocene sedimentation are
restricted to the Positivo Bonaerense (Sierra
de la Ventana piedmont, part of Pampa
interserrana and Sierras de Tandil). A remarkable
geomorphological characteristic
of this southern area is the occurrence
of several intervals of landscape stability
documented by the development of
calcrete crusts (tosca-rock) on topographic
surfaces of different ages. During
the very late Pliocene-Pleistocene, sedimentation
continued in the piedmont of
Sierras de Tandil and the Salado tectonic
basin. In the meantime, the Positivo Bonaerense was geomorphologically much
more stable with episodes of fluvial incision
along the main river systems. Yet, no
information is available on the Laprida
depression, the swampy environment
crossed by Darwin, and the Atlantic fringe
where possible Pleistocene deposits
are exposed. In both the Positivo Bonaerense and the Salado tectonic basin, the last
subcycle of sedimentation is related to
incision of the present fluvial valleys
sometime circa 40 ka and the accumulation
of an eolian mantle during the last
glacial cycle.
What was the primary control of sediment
accumulation at a regional scale?.
Why did sedimentation cease in the late
Miocene over a vast area of Positivo Bonaerense?.
Which factor triggered the subcycles
of sedimentation now identified?. At
present, research in progress is addressing
these main questions. The studies
under way are focused on the potential
role that might have been played by both
Andean tectonism and tectonic dynamics
of the Colorado and Salado basins. With
this purpose, future stages of current research
will be mainly directed towards
establishing improved chronological control
of the deposits along with detailed
mapping and sedimentological analysis of the areas, still poorly known (the northern
area of Pampa interserrana, the Laprida
depression and the Atlantic sea cliff
west of Mar del Plata.
Last but not least, the Geological Observations
on South America was read by one of
us (MZ) in the early 1980s. From the present
perspective, after almost three decades
of exploring the geology of this vast
environment, Darwin´s careful and detailed
descriptions and his interpretations,
capture the attention of those of us who
have devoted time and patience to
understanding the nature of the Pampas.
ACKNOWLEDGEMENTS
We thank Beatriz Aguirre-Urreta for inviting us to participate in this special volume. The critical and helpful comments and suggestions on the text by the reviewers Daniel Muhs, Edward Derbyshire and Cecilia Deschamps, improved and smoothed the manuscript. We are grateful to Alejandra Pupio and Irina Podgorny for helping us with literature and information used in this paper. Vivi Martinez helped with the illustrations. This paper was financially supported by Universidad Nacional de la Pampa (Research P.186-06), and is a contribution to PIPCONICET 5627.
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Recibido: 29 de agosto de 2008
Aceptado: 24 de octubre de 2008