The sedimentary history of the major Cryogenian glacial unit of central Africa : evidence against the Snowball Earth theory [abstract]

The Cryogenian Grand Conglomerat Formation (<765 & >735 Ma) is an association of interbedded glaciogenic, clastic periglacial and non-glacial deposits, within the Katanga Supergroup of central Africa. Correlation of regional unconformities and facies distribution suggest that the Grand Conglomerat strata were deposited (during and after eruption of flood basalts) in an asymmetrical rift, with a strongly uplifted southern shoulder, and a graded shelf defining the northern margin. Glaciomarine sediments along the southern margin of the Katangan rift are preserved within fandelta conglomerates supplied from an elevated rift shoulder. By contrast, the northern margin of the rift was the site of continental glaciation with cross-bedded glaciofluvial and marginal marine sandstones and conglomerates, associated with massive tills (diamictites) that pass laterally towards the south into glaciomarine mixtite interlayered with wedges of dolomitic sandstone. A cap carbonate (Kakontwe Limestone) is present only in the distal parts of the basin. Its absence in proximal regions is considered to reflect very high rates of sedimentation of fine-grained glaciogenic debris derived from deglaciated source areas. Palaeomagnetic data indicates that the Grand Conglomerat glaciogenic sediments were deposited close to the Equator during the Cryogenian. This low-latitude setting, coupled with the absence of a topographical trigger would suggest that glaciation was related to global atmospheric cooling. However, the presence of water-borne glaciogenic on-shore sediments and offshore sediments derived from floating glaciers suggests that the ocean during this part of the Cryogenian was not completely frozen. Associations of glaciogenic facies with non-glaciogenic sediments imply glaciation with interglacial periods and gradual deglaciation, instead of severe conditions of permanent sea ice cover and rapid change to the greenhouse environment.