Geochemical evidence for lacustrine microbial blooms in the vast Permian Main Karoo, Parana, Falkland Islands and Huab basins of southwestern Gondwana

Early Permian black pyritic shale units, i.e, the Whitehill, Irati and Huab formations and Black Rock Member, in southwestern Gondwana basins, have elevated delta(13)C values of total organic carbon up to -17 parts per thousand (PDB), hydrogen indices between 300 and 1000 mg CO(2)/g C and oxygen indices <30 mg CO(2)/g C, These data suggest a manifestation of a microbial bloom event. Values for delta(13)C (between -26 and -22 parts per thousand, PDB), hydrogen (<100 mg CO(2)/g C) and oxygen indices of total organic carbon above and below these rock units are typical for Palaeozoic terrestrial (Type III) organic matter. Carbon-13 isotope stratigraphy concur with palaeontological evidence, that these formations are stratigraphical equivalents, and therefore, that the microbial blooms covered a vase area (similar to 5 million km(2)), possibly Earth's most expansive microbial bloom. The palaeoenvironment of the rock units preceding and including the black shales was most likely lacustrine with fresh to brackish water, as shown by stable isotope (C, O, and S) evidence from organic matter, carbonates and pyrite. The implication is that a broad tectonic belt, which formed as a result of compressional tectonics during the Late Carboniferous to Early Permian from the palaeo-west, must have been sufficiently well developed to cut-off any contact with seawater, earlier than predicted (Late Permian) from previous stratigraphic and sedimentological studies. Stable isotope data, supported by mineralogical evidence, indicate that conditions, particularly in the upper portions of the lacustrine oil shales, were brackish and locally may have been evaporitic. We envisage that the microbial blooms (possibly Botryococcus) peaked in the upper half of the black shale units. Conditions rapidly became unfavourable for microbial blooms across the entire basin in the overlying formations, manifested by a change from oil shales (Type I organic matter) to organic-poor (Type III) shales, This may possibly have been a consequence of tectonic activity and interrelated causes. Stable isotope data of carbonates in the overlying sediments indicate that in the palaeo-north the basin became evaporitic (drying-up), and in the palaeo-southern part of the basin, high latitude/altitude water flooded the basin, and so brought to a close the microbial blooms. (C) 1999 Elsevier Science B.V. All rights reserved.