PEDOGENIC MUD AGGREGATES OF AN ALLUVIAL-LACUSTRINE SYSTEM (CUYANA BASIN, ARGENTINA)

Pedogenic mud aggregates are silt- to sand-sized particles derived from the aggregates of Vertisols. They are useful paleoindicators of bedload transport processes in sheetflood alluvial systems in rift basins. The crucial factors needed for their formation are: (1) the presence of smectitic clays from alkaline rocks or ash, (2) seasonal climate (arid and humid periods), and (3) soil aggregate formation. Pedogenic mud aggregates are removed from soils and deposited by traction load processes, not by suspension settle-out.

Pedogenic mud aggregates are identified in sheetflood sandstones from the Cerro Puntudo Formation (Anisian, Triassic) of the Cuyana Basin of central Argentina. This basin formed during the early rifting of the South Atlantic Ocean and filled with 6000 m of continental sediments. The Cerro Puntudo Formation is a 65 m thick sequence characterized by alternating stromatolitic limestones, mudrocks, shales, fanglomerates, sandstones, and tuffs, representing alluvial, fluvial, palustrine, and lacustrine systems. The pedogenic mud aggregates occur in ripple cross-laminated to trough cross-bedded, fine quartz sandstone and flaser bedded units that have a sheet morphology. Thickness of these units is on a decimeter scale and they can contain oncolitic clasts. These sandstone and flaser bedded units are interbedded with oncolitic to palustrine limestone beds. Interpreted as sheetflood deposits, the sandstones contain very thin mud lamina within traction-load sedimentary structures. Pedogenic mud aggregates were identified within these laminae in thin sections under a microscope using blue light. The distribution of aggregates within these traction load deposits signifies the mixing of pedogenic mud aggregates and siliciclastic grains as bedload during deposition during sheetflood events.