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 el evated delta(13)C values of total organic carbon up to -17 parts per thousa nd (PDB), hydrogen indices between 300 and 1000 mg CO2/g C and oxygen indic es <30 mg CO2/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 CO2/g C) and oxygen indices of total organic carbon abov e and below these rock units are typical for Palaeozoic terrestrial (Type I II) organic matter. Carbon-13 isotope stratigraphy concur with palaeontolog ical evidence, that these formations are stratigraphical equivalents, and t herefore, that the microbial blooms covered a vase area (similar to 5 milli on km(2)), possibly Earth's most expansive microbial bloom. The palaeoenvir onment 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 impl ication is that a broad tectonic belt, which formed as a result of compress ional tectonics during the Late Carboniferous to Early Permian from the pal aeo-west, must have been sufficiently well developed to cut-off any contact with seawater, earlier than predicted (Late Permian) from previous stratig raphic and sedimentological studies. Stable isotope data, supported by mine ralogical evidence, indicate that conditions, particularly in the upper por tions 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 becam e unfavourable for microbial blooms across the entire basin in the overlyin g formations, manifested by a change from oil shales (Type I organic matter ) to organic-poor (Type III) shales, This may possibly have been a conseque nce of tectonic activity and interrelated causes. Stable isotope data of ca rbonates in the overlying sediments indicate that in the palaeo-north the b asin 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 reser ved.