A COMPOSITE REFERENCE SECTION FOR TERMINAL PROTEROZOIC STRATA OF SOUTHERN NAMIBIA

Integrated sequence stratigraphic and chemostratigraphic data yield a framework for correlations of stratigraphic units in the terminal Prot erozoic to Cambrian Witvlei and Nama Groups of Namibia. Coupled with p recise U-Pb zircon age constraints, these correlations make it possibl e to construct a composite reference section for use in calibrating te rminal Proterozoic chronostratigraphy, The Namibian reference section starts with two distinct glacial horizons and extends up to within 1 m illion years of the Proterozoic-Cambrian boundary. The two glacial hor izons may represent each of two distinct Varanger-age glaciations bett er known from the North Atlantic region. From the higher of the two gl acial horizons up, the composite stratigraphy preserves one of the thi ckest and most complete available records of carbon-isotope variabilit y in post-Varanger terminal Proterozoic seawater. Four carbon-isotope chemostratigraphic intervals are recognized: (1) a postglacial negativ e delta(13)C excursion (Npg interval); (2) a rising interval (Pr inter val) of increasing positive delta(13)C values; (3) a falling interval (Pf interval) characterized by decreasing positive delta(13)C and culm inating in near zero or negative values; and (4) an interval of modera tely positive, relatively invariant delta(13)C values (I interval) tha t extends up to the unconformity that contains the Proterozoic-Cambria n boundary. Each of these chemostratigraphic intervals can be recogniz ed in widely separated correlative sections around the world. By compa ring sediment accumulation rate in the radiometrically calibrated Nami bian stratigraphy with sediment accumulation rates in correlative sect ions in Arctic Canada and Oman, a maximum age of 564 Ma is estimated f or the end of the younger Varanger glaciation, 25 m.y. younger than pr evious estimates.