CLAST PROVENANCE AND GEOCHEMISTRY OF THE GHAUB FORMATION, NORTHERN NAMIBIA

Rocks of the Ghaub Fm. exposed on the Fransfontein ridge in Namibia are composed of carbonate diamictite and interbedded, stratified, detrital carbonate sands and mudstones (siliciclastic intervals are rare). U-Pb ages of volcanic zircons and composite carbon-isotope profiles of the Otavi group sediments constrain the age of the Ghaub Fm. to ~635-Ma, which correlates globally to the younger Cryogenian (Marinoan) glaciation of the Neoproterozoic. Based on this correlation and the relevant sedimentary stratigraphy some interpret the Ghaub Fm. as a glacial deposit. This interpretation is challenged by arguments that the Ghaub diamictites were deposited entirely as a package of sub-aqueous mass flows in response to syntectonic rifting of the Otavi platform.

We counted and sampled distinct clast types both within beds of structureless diamictite and within the stratified upper member of the Ghaub formation (the Bethanis member). We also sampled for clast types along strike, particularly within a massive sequence of diamictites that are found near the Duurwater farm. Our data indicates that most of the Ghuab diamictites are not mono-lithologic (intraclast) dominated units but rather demonstrate a diversity of clast types; a characteristic not typically associated with carbonate bank debris flows. Clasts found in diamictites near the town of Fransfontein are the most diverse and include: dark calcitic limestone, oolitic limestone, tan dolostone, and various silicified types. The overlying dropstone-bearing Bethanis member also contains these clasts but locally is dominated by large stromatolitic stones and boulders, not commonly found in the underlying diamictites. Massive diamictites on Duurwater Farm are also dominated by stromatolitic dolostones, identical in character to those in the Bethanis member. We speculate that the Duurwater diamictites were deposited as a morainal bank (or medial ridge) during the final stages of deglaciation and are equivalent stratigraphically to the Bethanis member, which is largely an ice rafted and sediment gravity flow sequence which passes transitionally into an overlying cap carbonate.

Samples of each clast type within the Ghaub Fm. have been subjected to stable carbon isotope analysis in order to determine the stratigraphic level from which each different clast type originated.