LITHOSTRATIGRAPHY AND CHRONOSTRATIGRAPHY: A MASSIVE ACCUMULATION OF LARGELY SUBAERIAL CARBONIFEROUS SILICICLASTICS IN BOLIVIA THAT CHALLENGE STANDARD STRATIGRAPHIC CORRELATION

Stratigraphic nomenclature can be difficult to assign and apply in thick nonmarine successions where lateral variability in lithology and thickness of units is significant. The Carboniferous Macharetí Group, a glacially influenced siliciclastic unit consisting largely of interbedded diamictites and sandstones, is deposited within the Tarija-Chaco Basin in southern Bolivia, northern Argentina, and western Paraguay. Within this depositional area, the Macharetí Group has been divided into 2, 4, and 5 formations with each country dividing the group based upon the dominant lithologies in their region, neglecting to account for lithostratigraphic changes across political boundaries. Sandstone interbeds within the Macharetí Group increase to the north where it becomes impossible to establish the location of formation contacts using the same methodology used in southern sections (first occurrence of sandstone). Significant lateral variability in thickness of the Macharetí Group due to deposition within paleovalleys and syndepositional tectonics further complicates the establishment of lithostratigraphic units, especially in the lower formations which may be only locally derived. Additionally, the establishment of a chronostratigraphic framework for these units has been challenging due to a lack of geographically wide ranging macrofossils within limited marine units, poorly productive palynological samples, and contrasting ages of micro- and macro-fossil data. For these reasons, numerous age schemes have been suggested for the Macharetí Group and individual formations with ages ranging from the Famennian (late Devonian) to the Moscovian (mid Pennsylvannian). Given the differing ages and lithologies of the formations across the depositional area and the difficulty in applying formation names, a revision of the current stratigraphic nomenclature is necessary.