REVISING THE LATE CARBONIFEROUS SEQUENCE STRATIGRAPHY OF WEST-CENTRAL ARGENTINA: DISPARATE DEGLACIATION SUCCESSIONS IN THE CALINGASTA-USPALLATA BASIN

The Hoyada Verde and Tramojo Formations of western Argentina contain upper Namurian-lowermost Westphalian glacigenic strata. The two formations occur within the Calingasta-Uspallata Basin, which was bounded to the east by the Protoprecordillera, a Paleozoic fold-thrust belt. Facies analysis indicates that the strata were deposited in fjords (Tramojo Formation) and on the basin floor (Hoyada Verde Formation) by temperate (wet-based) glaciers as indicated by the abundance of strata deposited by meltwater/outwash and iceberg rafting.

Two large-scale glacial advance and retreat cycles are preserved in the strata; however, variations in the two formations suggest dissimilar depositional environments following glacial retreat. In the Hoyada Verde Formation, the uppermost diamictites are overlain by dropstone-free mudrocks containing marl beds, indicative of sediment starvation. Conversely, in the Tramojo Formation, diamictite units grade into interbedded conglomerates and sandstones, indicative of regression and high rates of sedimentation. The absence of transgressive facies following glacial retreat in the Tramojo Formation is significant, as the traditional view is that a glacioeustatic rise occurred in the Protoprecordillera region. The post-glacial stratigraphy of the Tramojo Formation and the creation of alluvial accommodation space can be explained by glacial retreat into a terrestrial environment with concomitant lengthening of a proglacial alluvial system. Thus, the Tramojo Formation requires revision of the current sequence stratigraphy of western Argentina. In lieu of a transgression, the apparent sea-level rise recorded in the Hoyada Verde Formation may simply reflect retreat of a marine glacier (grounded below sea-level). This reinterpretation of the sequence stratigraphy aligns with other studies concluding that the Protoprecordillera was glaciated by alpine glaciers that were not large enough to have caused a major eustatic rise during deglaciation.