FOSSIL-BEARING RESISTATES: A NEGLECTED SOURCE OF KEY TAPHONOMIC, PALEONTOLOGICAL AND PALEOGEOGRAPHIC DATA

Resistates (residual sediments consisting primarily of minerals chemically resistant to weathering processes) lying as lag gravel deposits or in stone lines of soil profiles in the Present-day landscape seem unpromising as fossil sources. These relict sedimentary particles have long been neglected as potential sources of key taphonomic and paleontological/geological data. Notably, fossil-bearing resistates can be abundant in some regions given the post-Cretaceous tectonic/climatic and geomorphologic evolution of the Brazilian territory and its denudational history tied to the elaboration of distinct planation surfaces (e.g., South American Planation Surface). In highly dissected reliefs where morphogenetic processes are significantly associated to the origin of plateaus, mesas and terraces, developed in distinct topographic levels, fossil-rich resistates can offer extremely relevant and surprising paleontological/geological information. In this contribution, we present data from Caldas do Jorro county, Bahia, Tucano Basin, Brazil, where fossil-bearing resistates have abundant bivalve/gastropod shells. Limestones (including shell-beds) were the original source rocks of these fossil-bearing resistates. Pronounced silicification ensured their persistence in the surface landscape or in the weathering profile. In the study area, fossil-bearing resistates can be found/tracked for tens of kilometers. Despite their long history and rock resistance to desegregation and erosion caused by Earth's surface processes, in some cases well-preserved fossils can be extracted from these relict materials. Notably, in the studied example, the detailed examination of taphonomic style and the precise identification of the remaining fossils indicates no direct correlation with any known lithostratigraphic unit cropping out in the region. Hence, an immediate question arises: from which sedimentary succession such fossils come from? Undoubtably, the identified bivalves/gastropods, and the taphonomic style of the former shell beds are similar to the Lower Cretaceous of the Santana Group of the Araripe Basin. Two possible explanations are considered: a- complete erosion of correlatable source rocks deposited in the Tucano Basin, and b- the long-distance provenance/transport of these resistates from the Araripe Basin, what would be incompatible with the Quaternary drainage system. Possible erosion of the Santana Group succession in the southern Tucano Basin also reinforces that the Lower Cretaceous marine transgression recorded in the Araripe Basin probably came from the south.