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Short-term sediment accumulation directly on basalt flows without the damming of drainage is not well studied. Can lakes form on basalt surfaces during times of volcanic quiescence between outpourings? Large faulted downthrown blocks of the lower Jurassic Kalkrand basalt within the Hardap Recreational Resort in central Namibia provided depressions for sediment accumulation during two periods of volcanic quiescence. The sedimentary infill was preserved as two sedimentary interlayers from 6 to 8 m in thickness between lava flow units. These sedimentary rocks, formerly interpreted as dominantly lacustrine deposits, now can be recognized as alluvial fill because of the presence of pedogenic mud aggregates. Sedimentation processes filling these fault-bound depressions included sheetflooding and grain flow, archived in successions containing sandstone, siltstone, and mudrock. Large cracks and fractures on the basalt surfaces, filled with quartz sandstone, showed that the regional water table was below the basalt flow surface, making the formation of lakes impossible, especially because of the permeability and porosity of such flows in general. No pattern or cyclicity during sediment deposition was found and paleocurrent measurements showed random sedimentary influx directions, probably related to the lava field surface topography. Autochthonous weathering of basalts added to the allochthonous drainage input of quartz-rich sand and silt that produced sandstone and siltstone containing smectitic clays. Most of these clay-rich siltstones and sandstones contained mud laminae within traction-load sedimentary structures, providing evidence for the presence of pedogenic mud aggregates within these sheetflood deposits. Geothermal groundwaters circulating through the basalt and the sediments, mixing with meteoric waters, produced Ca-rich diagenetic waters with an elevated temperature to precipitate calcite cement and dendritic crystals. This work demonstrates that characteristics of mud deposition can provide important clues in determining the true depositional paleoenvironment of these Jurassic sedimentary interlayers between basalt flows.

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