FINGERPRINTS OF INCREASED TERRESTRIAL RUNOFF OR UPWELLING? A NEW LOOK ON EARLY PERMIAN MIXED-CARBONATE SILICICLASTIC SEDIMENTS ALONG THE WESTERN MARGIN OF GONDWANA (BOLIVIA AND CHILE)

Mixed-carbonate siliciclastic sediments exposed in the region of Lake Titicaca (Bolivia: Copacabana Formation) and in the Atacama Desert of Northern Chile were deposited during the Early Permian in an equatorial setting. The presence of scattered fusulinids and rugose corals indicates close faunal relationships to the North American continent and has been traditionally regarded as an indicator of clear and warm water in the tropical realm. Based on our field data, there is evidence for a reinterpretation of the depositional environment, because prevailing metazoans are brachiopods, crinoids, bryozoans, and siliceous sponges (represented by their spicules), which all are members of the heterozoan (formerly bryomol) association. Fusulinids and rugose corals, which are predominantly members of the photozoan (formerly chlorozoan) association, are restricted to a few horizons. Further, the metazoans are not equally distributed throughout the rock sequence. Beds rich in bioclasts are intercalated with monotonous, finely laminated mud/siltstones and green tuffs. Based on these field data, increasing evidence exists that most carbonates do not have a tropical signature, although they occur within the equatorial realm. Using our field-derived sedimentological and biological data, we speculate that tropical carbonate production was hampered by periodic terrestrial runoff as indicated by abundant siliciclastic sediments and/or by oceanic upwelling as indicated by finely laminated sediments without bioturbation. Thus, carbonate sediments of the heterozoan association prevail in a tropical setting. The presence of intervals rich in fusulinids can be explained with the temporal presence of a thermocline. Our hypothesis of the depositional environment will be tested using a combined approach of petrography, stable isotope analysis, and measurements of Corg- and CaC03-contents. By comparison with Carboniferous and modern counterparts, we will discuss a more realistic depositional model under the consideration of feedback loops between terrestrial and oceanic ecosystems.