Taphofacies from fluvial and alluvial sequences dominated by silt: the Plio-Pleistocene cliffs of Mar del Plata.

ISLA, F; TAGLIORETTI MATIAS

Mendoza

Simposio; 18 International sedimentological Congress; 2010

Conceptual models are good guides for the interpretation of fluvial and alluvial environments. Primary and biogenic structures are the best descriptors of the different facies. At the Pampian cliffs of Mar del Plata, Plio-Pleistocene sequences, either representing braided or meandering systems, are dominated by silt. Although some tectonics has been accepted most of the paleosurfaces resemble the original landscape, with high preservation of soils and groundwater levels. Based on fossil assemblages, climatic changes have been proposed. However, at the upper formations semiarid conditions with episodic rains would have prevailed. Within this context, several facies could be discriminated. Bottom-channel deposits are characterised by lag gravel and disarticulated bones with evidences of bedload transport. Point-bar facies are less evident than in sandy rivers, but they can be differentiated when primary structures represent variations in the flux. Small channels are differentiated by mesoscale crossbedding, fining-upwards graded bedding and heavy-minerals layers. Levees and crevasse-splay facies are denoted by laminated layers and convolute bedding. Ox-bow lakes are recognised as massive fine-grained strata of limited extension, and characterised by small burrows produced by invertebrates. These environments were subject to rapid sedimentation induced by flood events. At alluvial areas, large biogenic structures (about 2 m diameter and several meters long) dug by large mammals (milodontids) are the more conspicuous evidences of stability in areas not subject to floods or groundwater-level fluctuations. Mesoscale caves, dug by rodents or dasypodids, have been assigned to distal portions of the alluvial plain. Paleosoils are also present in stable areas, with burrows made by worms when the preservation was maximised by ashfalls. Massive layers composed of well-sorted silt are assigned to loess accumulations or consolidated tuffs. A 4 m-depth shallow lake was preserved, spanning from the euxinic bottom to the shore facies subject to floods and droughts. All these facies are subject to pedogenetic transformations and reworking processes. Time averaging varies from 101 to 105 years increasing at transgressive paleocaves that silted slowly by sediment supported by eolian transport or weathering of the top of the cave. To both sides of the Tandilia Range, the accommodation space was different during time. Structural levels (calcrete crusts) controlled the erosive action of channels and the digging capacity of some mammals.