GEOCHEMISTRY OF LATE ORDOVICIAN GLACIAL DEPOSITS AND ASSOCIATED SEDIMENTARY ROCKS, SOUTH AFRICA: RELEVANCE TO THE OCCURRENCE OF GLACIATION AT LOW PALEOLATITUDES

The Pakhuis Formation of the Table Mountain Group in South Africa includes two thin diamictites succeeded by a coarsening upward shale-siltstone unit known as the Cedarberg Formation. These glaciogenic rocks are peculiar in that they occur within one of the world’s greatest accumulations of mineralogically and chemically mature sandstones. Chemical investigation of shales and sandstones below and above the diamictites reveals that they have undergone extensive alteration. Some diamictite samples have low values for a Chemical Index of Alteration, in keeping with their inferred glacial origin. Black shales, comprising the basal Soom Member of the Cedarberg Formation immediately above the diamictites are enriched, particularly at the base, in Mo, U and other trace elements. The unusual chemical composition of these shales is attributed to starved basin conditions and a reducing environment during a rapid sea level rise that accompanied destruction of the Late Ordovician glaciers. The glacial deposits of western South Africa have been regarded as the distal deposits of a widespread continental ice sheet centred on North Africa but they may have been partly derived from piedmont glaciers descending to near sea level from highlands formed as a result of Late Ordovician orogeny. Descent of glaciers to sea level in low latitudes (snowball Earth scenario?) suggests extreme, if short-lived global cooling. The occurrence of what are interpreted as fossil ice-wedge structures is enigmatic because of implied strong seasonality at supposedly low paleolatitudes. Glaciation may have occurred as a result of short-lived co-incidence between Late Ordovician orogenic activity (Taconic, Famatinian, Shackleton) in low paleolatitudes, and drift of the Gondwana supercontinent into a sub-polar position.