ALPINE GLACIATION IN WESTERN GONDWANA: THE CARBONIFEROUS GLACIGENIC DEPOSITS OF THE PROTOPRECORDILLERA OF WEST-CENTRAL ARGENTINA

Earliest Pennsylvanian (Bashkirian) glacigenic strata associated with the Protoprecordillera were deposited in the Calingasta-Uspallata (CU) and Rio Blanco (RB) backarc basins and the Paganzo foreland basin (PFB) in western Argentina. These basins formed due to convergence along the western margin of Gondwana during the Chañic and the Río Blanco tectonic events. Uplift of the Protoprecordillera fold and thrust belt during the latest Visean-Serpukhovian resulted in the development of a regional unconformity that formed the pre-glacial basin floors. During the Bashkirian, alpine glaciers carved deep valleys into the upland with ice draining radially away from the Protoprecordillera. Valley glaciers or ice caps also occupied basement uplifts in the Sierras Pampeanas to the East. Ice grounded below sea level occurred in the CU and RB Basins where thick glacimarine successions were deposited. In the PFB, a thin glacial succession was deposited in both terrestrial and glacimarine settings. Throughout the region, deposition occurred subglacially (rare), in morainal banks, settling from suspension out of meltwater plumes, rain-out from melting icebergs, and/or from subaqueous sediment gravity flows. A rapid transition from diamictites to marl-bearing, dropstone-free shales marks glacial retreat and establishment of sediment starved marine conditions. Coarsening-upward successions and truncation surfaces (>200 m of relief) signal either deltaic progradation during a forced regression (PFB) or fluvial incision (CU and RB) associated with a base-level fall. The base-level fall was previously attributed to glacial rebound. Such an interpretation is problematic for several reasons. These reasons include: 1) loading by alpine glaciers and ice caps are of small magnitude, 2) the base-level fall occurred several million years after glaciation ended, and 3) base-level fall was greatest in areas outboard of glacial centers. Therefore, sequence stratigraphy in this region was likely driven by tectonism or tectonic unloading (erosion) in the fold and thrust belt. This glacial event lasted less than 10 Ma with non glacial conditions prevailing in this region throughout the rest of the Paleozoic. During the Bashkirian only small glacial centers occurred in South America.