SEDIMENT ACCUMULATION ON A BASALT FLOW OF THE JURASSIC KALKRAND FORMATION (NAMIBIA)

The Kalkrand Formation in central Namibia (Africa) comprises a part of the African Etendeka flood basalts deposited simultaneously with the breakup of Gondwana. Within the Kalkrand Formation, two Jurassic sedimentary interlayers were deposited in depressions between basalt flows. These sedimentary interlayers are located within Hardap Recreational Resort, northwest of Mariental. Studying these interlayers will aid in understanding the environment and evolution of plant and animal life associated with aqueous conditions after volcanic devastation.

Sedimentary interlayer I (the lowermost interlayer and the target of this study) is a 4-5 meter-thick succession composed of sandstone, siltstone and mudstone, located along the northern shoreline of the Hardap Dam Reservoir, west of the resort swimming pool and restaurant building. The lowest rock unit is a red-colored, massive to laminated, medium- to fine-grained sandstone infilling the cracks in the underlying flood basalt. This basalt is highly weathered, indicating a long exposure time before sediment deposition. The sandstone transitions via a gray/brown, wavy but continuous, 1-2 mm layer of mudstone into a gray-red siltstone that is poorly laminated and randomly strewn with possible volcanic clasts (less than 1-2 mm wide) with a thickness of about 7-10 cm. Above this a 1-2 cm greenish siltstone layer with wave ripple cross-lamination underlies a greenish-gray siltstone layer (10-15 cm) that is poorly laminated throughout and contains conchostracans within its top 5 cm. Overlying the conchostracan layer is undifferentiated weathered siltstone to silty mudrock approximately two meters thick that is very poorly exposed. A red-colored, massive, medium-grained sandstone unit, about 25-30 cm thick, caps the stratigraphic section of sedimentary interlayer I. These deposits are interpreted to be mostly subaqueous, accumulating via traction load and suspension settle out. The conchostracans signify a shallow water environment. No deeper subaqueous conditions are indicated in this faulted depression, probably due to the fractured and weathered nature of the underlying porous basalt.