Ichnological and paleoclimatic analysis of weakly developed paleosols: Punta San Andrés Alloformation (Plio-Pleistocene, Buenos Aires province, Argentina) as a case study.

BEILINSON, ELISA

Mendoza, Argentina

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists (IAS)

The Plio-Pleistocene continental deposits of the Punta San Andrés Alloformation that crop-out in the marine cliffs of Mar del Plata City (Buenos Aires Province, Argentina) show the development of different kinds of paleosols (Protosols, Vertisols and Calcisols) and a variety of trace fossils related to them. The presence of structures related to biological activity in these deposits has been pointed out by several authors, but they have never been properly described. Amongst the variety of traces related to invertebrate activity, burrows are the most abundant and three different morphologies can be recognized. The first group consists of un-lined meniscated burrows. They are slightly sinuous and their orientation is predominantly vertical to oblique. The burrows are 10 mm in diameter and 10 to 15 cm long. Secondary successive branching may occur, but true branching is absent. The absence of lining makes this ichnofossil assignable to Taeniduim isp. and they have been interpreted as produced by earthworm activity. These burrows are developed in Protosols with an index of bioturbation estimated in 4 or 5. The second group consists of variably orientated burrows with also a characteristic backfill menisci but this time, lining is present. This burrows, interpreted as Beaconites isp., are found mainly in Protosols, more rarely in Vertisols, and their estimated index of bioturbation is between 3 or 4. Finally, the last burrow morphology is that of vertical to oblique orientated, un-lined, essentially cylindrical, straight to slightly sinuous burrows with active filling. They are 6 to 10 mm in diameter and 10 to 15 cm long and are found in calcic Protosols. The index of bioturbation for strata containing this kind of burrows is between 4 and 5 and they tend to be the only ichnofossil present in this silty to sandy strata. Another kind of trace fossil that can be found in these paleosols are multi-layered, spherical chambers 15 to 20 mm in diameter assigned to Castrichnus incolumnis. The chamber walls are 1 mm thick and they are built with imbricated pellets. Some of these chambers are connected to meniscated burrows assigned to Taenidium isp. Abundance of this ichnofossil is very low, less than 5%, and they have been interpreted as earthworm aestivation chambers. They are found in Protosols. The presence of chambers and burrows associated with earthworm activity provides valuable paleoclimatic and paleoecological information. Most earthworms can´t survive in water logged soils and need water to be confined to films on soil aggregates surfaces or in pore spaces. In those soils affected by seasonally strong dry periods, earthworms built aestivation chambers. Protosols are immature paleosols that characterize levees, crevasse splays and proximal floodplain deposits and they show, for the studied deposits, micromorphological evidence of a well drained area. Calcic Protosols in the study area show micromorphological evidence of seasonally dry periods and Vertisols show evidence of water logging. The fact that the burrows and chambers described here were found mostly in Protosols and calcic Protosols, and that merely none of them were found in Vertisols confirms the prior paleoclimatic interpretations made on the basis of the micromorphological and sedimentological work.