Paleosol development in response to extrinsic and intrinsic factors: The Mata Amarilla Formation (lower Upper Cretaceous), an example of southern Patagonia

Abstract

Soils are the product of the interrelationship between the lithosphere and the biosphere. The study of the fossil soils is a valuable tool for the interpretation of ancient sequences. The aim of this contribution is to study the extrinsic (tectonic, climate and eustacy) and intrinsic (paleotopography, palaeodrainage and relative sedimentation rates) controls over the paleosols development. The Mata Amarilla Formation is framed within the Austral foreland basin, which is located in the southwestern of Argentina. Its shape is elongated N-S; the eastern border is parallel to the Río Chico. The western tectonic limit are the Patagonian Andes, and the southern boundary is the Scotia plate. The study area is situated around the town of Tres Lagos, southwest Santa Cruz province. Detailed sedimentological logs, facies analysis, pedofeatures and soil horizons description were useful to interpret palaeoenvironment and paleosols development. The Mata Amarilla paleosols have hydromorphic characteristics and are grouped within the gleysoils group. This designation refers to gray, greenish-gray to olive green colors, which reflect the reduced state of iron oxides. These paleosols have features consistent with seasonal humid tropical climates, in concordance with the beginning of the greenhouse period during Cenomanian times, which continued throughout the Upper Cretaceous to the Oligocene. The Cenomanian is an important period in the evolution of vegetation, as there was a change from a mesophytic flora to floras dominated by angiosperms. These changes are also recorded in the paleosols, since the Cenomanian pedogenic processes are very similar to recent processes. Previous studies in the Mata Amarilla Formation allowed the deffinition of three sections that represent different accommodation/sediment supply condition. These changes are inferred to be promoted by relative sea-level oscillations in response to the tectonic evolution of the Austral fold and thrust belt. The lower and upper sections of the Mata Amarilla Formation are characterized by histosols associated with low gradient coastal environments (lagoons, estuaries and distal fluvial systems). A special type of histosol with acid sulfate properties, such as jarosites rhizoliths, motts and nodules was developed in the coastal plain. In the lower portion of the middle section, a thick paleosols associated to the Maria Elena Petrified Forest is developed. This level is composed by vertic alfisols and vertisols. In the eastern part of the study areas there are fossil trees in life position associated with anastomosing rivers, while to the west trunks are pseudotransported related to meandering rivers. The rest of the middle section is characterized by vertisols and inceptisols developed on distal and proximal fluvial floodplains, respectively. Summarizing, the type of paleosols and drainage conditions developed in the three sections responded to extrinsic factors, while the variations in the middle section might respond to intrinsic factors within the fluvial system. Finally, a sequential stratigraphic model for paleosols succession for the Mata Amarilla Formation was developed. This model contrasts with the classic model developed by Wright and Marriott (1993), in that the Mata Amarilla model does not have incised valleys, and transgressive system are composed of very poorly drained soils. Models are similar in that during the first stage of forced regression a thick paleosol level develops. In contrast, the rest of the regressive stage of the Mata Amarilla Formation is dominated by low to moderate maturity paleosols. The classic model works well during major eustatic changes occurred during icehouse period, while the Mata Amarilla model could be extrapolated to other sequences deposited during climatic optimum conditions.