Adriana Blasi

Late Pleistocene-Holocene eolian deposits of the southern Buenos Aires province, Argentina: A preliminary model

Abstract The aim is to reconstruct the sedimentological history of the Late Pleistocene-Holocene eolian deposits of southern Buenos Aires province. Hence, a preliminary model is proposed in order to analyze the different stages of the sedimentological cycle. The deposits include sandy loess, fine sand sheets and dunefields. The southern Buenos Aires province represents the depositional area, whereas the source and transport environments are located in the northern Patagonian Andes and northern extra-andean Patagonia. According to the mineralogical characteristics of the material, explosive volcanism is considered the predominant process of particle formation, particularly as generator of pyroclastic deposits which were reworked by different erosive processes (glacial, fluvial, eolian). During the Late Glacial Maximum and the Holocene, alluvial deposits which were mainly generated by fluvial erosion of volcaniclastic sedimentary units, were deposited along the floodplains of the Colorado and Negro rivers, including those distal segments located in the emerged offshore platform. The fluvial regime depended upon the regime of the glaciers, hence it reflected the major glacial fluctuations which took place in the Andes. Floodplains were deflated by southwest winds, transporting the material towards the southern Buenos Aires province (sandy loess and loessial sands) and to La Pampa province (dunefields).

Late pleistocene and holocene loess deposits of the Southeastern Buenos Aires province, Argentina

The Late Pleistocene and Holocene loess deposits of the SE Buenos Aires province are composed of four allostratigraphic units that represent four episodes of loess deposition. The first and the second episodes occurred in Late Pleistocene times. The second episode was followed by a soil forming interval (Early Holocene to Mid-Holocene times). The third episode took place at about 5000 yr BP, after the Holocene sea-level maximum when marine regression began. The fourth episode constitutes a historical event of only local significance.Loess shows a fairly constant granulometric and mineralogical composition. The modal fraction consists of very fine sand and coarse silt (3 to 5 phi). They are classified as sandy silts or silty sands. Three grain-size subpopulations are differentiated: coarse, medium and fine. The medium-size subpopulation, which is the most important, consists of most of the very fine sand and coarse silt. It is thought to be transported by modified saltation and short-term suspension during local dust-storms.The mineralogical composition of loess consists of a volcaniclastic assemblage derived mainly from reworked pyroclastic deposits, primary tephras and volcaniclastic sediments. The source area of these materials was located in the lower Colorado river valley about 400 km SW of the studied area. There was also a direct supply by volcanic ash falls.

Identification and differentiation of Pleistocene paleosols in the northern Pampas of Buenos Aires, Argentina

Abstract A combined macromorphological and micromorphological approach was used to identify and differentiate paleosols at a representative exposure of the Pleistocene sedimentary succession in the northern Pampas of Buenos Aires (La Plata area). Five pedological units (Pu), which apparently represent four discrete paleosols plus the surface soil, were initially differentiated on the basis of field-scale morphological properties. The succession was divided into B and C horizon with an A horizon only clearly identified in the surface soil and a weak A horizon at depth. Micromorphology suggests a complex pedosedimentary history of welding, with some degree of water reworking indicated by fragments of sorted layers and the significant grain-size heterogeneity of the parent material. The micromorphological data do not support the field differentiation of Pu4, Pu3, and Pu2 into discrete paleosols. Pedological features (i.e. excrements, secondary carbonate coatings, illuvial clay coatings) occur throughout without any obvious breaks or patterns. Pu4, Pu3, and Pu2 are therefore interpreted as an accretionary and/or welded pedocomplex. The Gorina section does not conform to the simple classical model proposed by other authors in the region of alternating loess and paleosol units associated with arid (loess deposition) and wet (soil formation) intervals, respectively.

The Arrival of Homo sapiens into the Southern Cone at 14,000 Years Ago

The Arroyo Seco 2 site contains a rich archaeological record, exceptional for South America, to explain the expansion of Homo sapiens into the Americas and their interaction with extinct Pleistocene mammals. The following paper provides a detailed overview of material remains found in the earliest cultural episodes at this multi-component site, dated between ca. 12,170 14C yrs B.P. (ca. 14,064 cal yrs B.P.) and 11,180 14C yrs B.P. (ca. 13,068 cal yrs B.P.). Evidence of early occupations includes the presence of lithic tools, a concentration of Pleistocene species remains, human-induced fractured animal bones, and a selection of skeletal parts of extinct fauna. The occurrence of hunter-gatherers in the Southern Cone at ca. 14,000 cal yrs B.P. is added to the growing list of American sites that indicate a human occupation earlier than the Clovis dispersal episode, but posterior to the onset of the deglaciation of the Last Glacial Maximum (LGM) in the North America.

The origin and construction of pre-Hispanic mounds in the Upper Delta of the Paraná River (Argentina)

Pre-Hispanic mounds, known as “cerritos,” “cerritos de indios,” or “aterros” across southeastern South America, are one of the most conspicuous and well-studied cultural manifestations in lowlands archaeology. Nevertheless, in the Upper Delta of the Paraná River, mounds are rarely studied, and even their anthropic origin is under debate. This could be related to the fact that anthropogenic mounds are located on a floodplain where other “mound-like” natural geoforms (generated by fluvial processes) are also present. In addition to this, the natural geoforms also contain evidence of Holocene human occupation (sherds, bones, charcoal, humans burials, etc.), which can lead to interpretive errors of their origin and formation. Thus, this project set out to determine the genesis and evolution of these mounds and also to identify the cultural occupation and transformation of natural landforms found in the area. In this article, natural and anthropogenic systems and processes were identified and characterized through the application of proxy record analysis (i.e., sediment composition, stratigraphy, micromorphology, silica bodies and chronological analysis) at the Los Tres Cerros archaeological locality in the Upper Delta of the Paraná River of Victoria County, Entre Ríos Province, Argentina. This analysis allowed for the recognition of natural anthropogenic interfaces, such as the “pre-mound” occupation as well as evidence of cultural activities such as mound construction, between 1,000 and 500 14C years bp. These findings were integrated into current research on the variability of mound construction during the Late Holocene in the lowlands of South America.

The Colorado River of Argentina : source, climate, and transport as controlling factors on sand composition

Abstract Detrital modes of the Colorado River (Argentina) sands provide a clear example of contradictory plate tectonic interpretations of depositional paleoenvironments. The sands were derived from an arc adjacent to the leading edge of the South American Plate and deposited in areas progressively closer to a trailing edge downstream. These sediments are rich in unstable volcanic rock fragments and, in lesser proportions, feldspars (mainly plagioclase). Two kinds of deposits are present within the Colorado River: proximal sands and gravels that characterize a back-arc basin and are derived mainly from a volcanic arc; and distal sands with the same provenance. The average detrital mode for proximal sands is QM-F-L 8-39-53. The average for intermediate sands is 13-30-57, and the distal average mode is 18-22-60. These data are coherent with the average composition of Argentinian coastal sands between the Rio de La Plata and Tierra del Fuego. The data show that sand composition is controlled by the relief, drainage systems, climate, and the tectonic environment of the source area, which is not necessarily the tectonic environment of deposition.

Geoarqueología en Pali Aike: Cueva Orejas de Burro 1(Patagonia, Argentina)

We present stratigraphic and geoarchaeological evidences from the Orejas de Burro 1 archaeological site, located in the southern part of the Pali Aike Lava Field (Santa Cruz province, Argentina). We develop discussions about formation processes of the sedimentary deposits, the chronology of the archaeological assemblages found in these deposits, and the regional palaeoclimatic implications of certain sedimentological features observed in the stratigraphy. This is a first step towards a geoarchaeological approach that aims to integrate the archaeological record of stratified rockshelters and caves within a broader biogeographical framework for southern Patagonian archaeology.

Pre-Hispanic settlements in hydrometeorologically susceptible areas during the late Holocene: The Upper Delta of the Paraná River Case

In this paper, we present the results of the geoarchaeological studies carried out in two archaeological localities of the Upper Delta of the Paraná River (Argentina). The main objective of these studies is to depict the pre-Hispanic strategies involved in the colonization and settlement of southern South America wetlands. Paraná Delta is one of the most conspicuous areas of these lowlands and comprises a large wetland macrosystem. Its current geomorphological configuration was established after the last transgressive mid-Holocene event c. 6000 14C yr BP. In this environment, a high ecological heterogeneity, with diverse and abundant tropical and temperate biota, was developed. These features were important factors to the human colonization and utilization of these wetlands. However, this environment has the highest hydrometeorological susceptibility of La Plata basin. This susceptibility had an impact on settlement systems and resource exploitation strategies established in the area since at least 2000 14C yr BP. These strategies involved at least two settlement types: semi-permanent residential camps and transitory camps oriented to exploit particular resources. The semi-permanent settlements were located in anthropogenic elevated mounds, locally known as ‘cerritos’, and were not subjected to seasonal inundations. Conversely, the transitory camps are found in levees exposed to recurrent flooding.

Quaternary ontogeny of a pampean ‘laguna’

Our sedimentological study of Lobos Lake sediments has shown that, below modern deposits, there is a clastic and biological record of three different paleoenvironments: two aquatic (freshwater and mixohaline respectively) and one subaereal. This record documents the climatic and geomorphologic changes that affected the basin during its evolution up to its present water level, from the upper Pleistocene to the Holocene.