Sedimentation and palaeoenvironments of Late Cretaceous crater‐lake deposits in Bushmanland, South Africa

Abstract

ABSTRACTA large diameter borehole core from an epiclastic kimberlite remnant on the farm Stompoor in the Prieska district, Cape Province, contains a continuous 76 m section of fossiliferous sediments interpreted as having accumulated within a crater‐lake during the Late Cretaceous. Three distinct facies associations reflect depositional processes that prevailed in offshore areas of the original lake. Facies Association A: matrix‐supported pebble conglomerates comprising a chaotic assemblage of pyroclastic, basement and country rocks set in a fine‐grained matrix. Flat, non‐erosional basal surfaces with ‘frozen’ rip‐up clasts, the protrusion of matrix‐supported clasts above the upper surfaces and a direct relationship between maximum clast size and bed thickness suggest deposition from debris flows that originated subaerially on pyroclastic talus cones surrounding the crater. Facies Association B: alternating thin beds of matrix‐supported granule conglomerate, structureless fine‐grained sandstone and parallel laminated mudrock. Small fining‐upward sequences within these beds are comparable to turbidite Bouma Tade, Tde. Numerous partings display petrified fish and frog skeletons, as well as bivalve, gastropod and ostracode shells, leaf impressions, insect wings and a possible bird bone. These beds were deposited by thin debris‐flows and turbidity underflows interspersed with periods of ‘pelagic’ sedimentation. Facies Association C: microlaminated mudstone beds containing scattered ‘dropstone lapilli’. The lamination is imparted by alternating Ca‐rich/Ca‐poor layers which may reflect climatic seasonality. They are interpreted as the result of seasonally influenced suspension settling through a thermally stratified water column.Short‐term periodicities in conglomerate bed thicknesses are interpreted as the result of successive block caving of a slump scar giving rise to several debris flows from the same source area. Seismic shock from nearby volcanism may have simultaneously triggered slumps on both subaerial and subaqueous slopes. Dropstone lapilli in Type C beds and the preponderance of load casting in Type B beds support this interpretation. An estimate of the time span involved in accumulating 76 m of crater lake sediments based on the possible seasonal imprint of Type C beds gives a figure of some 220,000 yr.