A CRYOGENIAN-EARLY EDIACARAN CARBONATE SHELF BREAK DOMINATED BY GLACIAL PALEOTOPOGRAPHY, FRANSFONTEIN RIDGE, KUNENE REGION, NAMIBIA

We present a new fence diagram composed of 64 measured sections from a continuously outcropping homocline that exposes a Neoproterozoic carbonate shelf break in oblique cross-section. The Otavi Group is a 2-2.4-km-thick carbonate platform sequence, which was deposited between 770 and 590 Ma on the southwest promontory of the Congo craton. It is exposed in an arcuate 600-km-long fold belt in northern Namibia. The carbonate platform was glaciated twice during the Cryogenian, and regionally extensive erosion surfaces beneath the respective glacial deposits divide the carbonate succession into three subgroups (S1 to S3). Subsidence of the platform occurred in two stages, the first in response to north-south crustal stretching and the second to post-rift cooling. The ‘rift-drift’ transition occurs in the middle of S2, between glaciations. After the transition, a south-facing shelf break quickly developed, dividing a shallow-water platform in the north from a distally tapered foreslope and basin in the south. The shelf break was presumed to have developed in response to a north-south change in the degree of crustal stretching and subsequent thermal subsidence, but this is conjectural. Fransfontein Ridge, a continuously outcropping homocline of the Otavi Group, obliquely transects the shelf break and contiguous foreslope and platform strata. We examined the stratigraphic architecture of shelf break development in a 60 km strike-length of Fransfontein Ridge with closely-spaced measured sections and high-resolution δ¹³C data. To our surprise, the location and development of the shelf break in this transect are not structurally controlled—no significant syndepositional fault is intersected. Instead, the dominant control on S2 stratigraphy and sedimentation was <1.8 km of local paleotopography on the older glacial surface, which is underlain by crystalline basement in this area. The exact location of the shelf break is dictated by >0.5 km of relief on the younger glacial surface, which ramps across the 0.3-km-thick upper S2 carbonate platform (Ombaatjie Fm) on farm Kranspoort 475 with a cutoff angle of 8 degrees in the line of section. Preservation of <1.8 km paleotopography on the older glacial surface, inferred from stratigraphic onlap, is consistent with a nonglacial medial Cryogenian epoch (S2) of short duration.