Deglaciation in a Pennsylvanian paleovalley of western Argentina: the Agua de Jagüel Formation

The Pennsylvanian Agua de Jagüel Formation of west central Argentina was deposited adjacent to the Protoprecordillera, a Paleozoic fold-thrust belt, in the Calingasta-Uspallata Basin. Upon examination of the geometry of the formation and the angular unconformity that separates the formation from Mississippian and older bedrock, it was determined that the formation was deposited in a paleovalley, likely from alpine glaciers draining the Protoprecordillera. The formation is composed of three depositional sequences separated by erosional unconformities: Sequence I contains a deglaciation succession, and Sequences II and III contain fluvial and shallow marine deposits. The paleovalley contains the lower two depositional sequences and is estimated to have been at least 700 m deep, and at least 5 km wide. The identification of this paleovalley aligns with studies of other paleovalleys of the Protoprecordillera and the conclusion that the Protoprecordillera was glaciated by alpine glaciers that were not large enough to have caused a major eustatic rise during deglaciation.

The deglaciation succession of Sequence I is composed of massive and stratified diamictites passing upward to mudrocks with striated dropstones, which then grade upwards into marl-bearing mudrocks. Geochemical and mineralogical analysis provides information on the sediment source and the redox conditions during deposition of the marl-bearing mudrocks. X-ray diffraction reveals the presence of albitic plagioclase, quartz, calcite, chlorite, and illite. Albite, chlorite, and illite are characteristic of continental weathering and may have been supplied to the basin by meltwater streams emanating from the base of the glaciers as they retreated up the paleovalley. X-ray fluorescence analysis provided V/Cr ratios above 2, indicating anoxia due to the presence of H2S. Anoxia may have occurred as a result of a sill at the end of the paleovalley that inhibited circulation with waters in the adjacent seaway.