SEDIMENTATION BETWEEN BASALT FLOWS (JURASSIC KALKRAND FORMATION, NAMIBIA)

The basalt flows of the Kalkrand Formation located in central Namibia (southwestern Africa) are the western edge of the extensive Karoo flood basalts, representing the rifting of Africa from Antarctica and India. Two Jurassic sedimentary interlayers within Hardap Recreational Resort near Mariental were deposited between these basalt flows within down-faulted blocks during a time of volcanic quiescence. These interlayers were initially interpreted as lake deposits. However, further research has revealed that these Jurassic sediments may in fact be alluvial fill cemented by groundwater fluids.

The two sedimentary interlayers are six to eight meters in thickness and are sandwiched between porphyritic theolitic basalts. The lithology of these interlayers is dominated by sandstone, siltstone, and minor mudrock, with calcite as the dominant cement. Ten facies were documented: (1) massive fine-grained sandstone, (2) coarse-grained sandstone with or without volcanic clasts, (3) laminated siltstone and mudrock, (4) flaser bedding, (5) fine-grained sandstone with large interfingering, calcite crystal fibers, (6) ripple cross-laminated sandstone to siltstone with conchostracans, (7) brown mudrock with rhizoliths and possible pine cones, (8) ripple cross-laminated siltstone to mudrock, (9) disrupted silty mudrock with rhizoliths, and (10) trough cross-bedded sandstone. Interpretation is as follows: (1) grain flow deposition, (2) traction deposition from stream flow either eroding basalt or from outside the basaltic area (quartz present), (3) high regime flow during sheetflood deposition, as evidenced by the presence of pedogenic mud aggregates, (4) sheetflood deposition with irregular flow, (5) alluvial sandstone permeated with diagenetic (hydrothermal?) fluids, (6) traction alluvial infill or sheetfloods, (7) marshy deposits, (8) traction alluvial infill or sheetfloods with subsequent temporary ponding, (9) paleosols, and (10) fluvial deposition under a higher flow regime.

There is no evidence of deposition within playa lakes or deep fresh lakes. Coarse-grained sandstone fill large cracks in the basalt below indicating that regional groundwater within this porous basaltic area remained beneath the sedimentary infill. No permanent standing water for the genesis of lakes was possible.