GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 44-2
Presentation Time: 8:20 AM


QUINN, Daven P., University of Wisconsin – Madison Department Of Geoscience, 1215 W Dayton St., Mail Code 170-23, University of Wisconsin – Madison Department Of Geoscience, Madison, WI 53706, GROTZINGER, John P., Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125 and PRESENT, Theodore M., Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125

The Zebra Nappe is the youngest allochthonous tectonic unit of the NaukluftNappe Complex (NNC), an outlier of the 650-540 Ma Damara orogen that consistsof carbonate and clastic strata thrust south- and southwestward over the Namaforeland basin. The tectonic units of the NNC are Neoproterozoic indepositional age, while Damara orogenesis and foreland-basin deposition spanthe Precambrian-Cambrian boundary.

Building on a revised geologic map and tectonic framework for the southern Naukluft Mountains, we present a new facies model and stratigraphic framework for the Zebra Nappe. The ~600 m of carbonate and clastic sediments in the Zebra Nappe are divided into units along the lines of Hartnady (1978), and here we define three depositional sequences. The lower sequence, containing peritidal, upper-shoreface, and shallow subtidal facies, records a shallow-water environment across the study area. The second sequence is a deepening facies system that consists of west-thickening shale wedges interspersed with compressed regressive intervals of shallower facies belts, culminating in reworking surfaces. The final sequence is a grainy limestone shoal varying between ribbon and intraclast limestone, and cross-stratified grainstone.

The Zebra Nappe does not contain facies indicative of “cap-carbonate” strata, and δ13C and δ18O chemostratigraphy does not capture known terminal Proterozoic isotope excursions (e.g. the Shuram excursion), but consistent values of -2 near the top of the section potentially match early Ediacaran deposition. Detrital zircons within several key sandstone packages are sourced entirely from the Kalahari Craton. The lack of sediment input from the Damara orogenic belt contrasts strongly with the allochthonous Nama group. Thus, the Zebra Nappe is not correlated with any known stratigraphic package exposed in southern Namibia; it likely represents a preserved fragment of a missing pre-Damara passive margin of the Kalahari craton.