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Lithology and depositional environments of the Lower Jurassic Clarens Formation in the eastern Cape, South Africa

Institut für Geologie, Universität Würzburg, Pleicherwall 1, D-97070 Würzburg, Germany e-mail: f.holzfoerster{at}geologie.uni-wuerzburg.de

The Lower Jurassic Clarens Formation of the main Karoo Basin, South Africa, is regarded a climatically controlled fluvioaeolian to aeolian lithostratigraphic interval. It reveals a tectonically quiet period of orogenic unloading of the Cape Fold Belt and initiation of the rifting along the Indian Ocean rift axis. In the Clarens Formation outcrop area west of 27°E its lithofacies spectrum differs significantly from that known from the major part of the basin. Abundant pyroclastic and lacustrine facies elements characterise relatively small-scale local depocenters reaching below the regional base of the Clarens Formation. Such depositories have subcircular outlines and are surrounded by massive structureless sandstones being the product of phreatomagmatic crater formation. They separate the basin-wide aeolian facies succession from one illustrating the shallowing-upward crater lakes infilled due to gravitational and aeolian sediment supply. The existence of the crater lakes evidence shallow groundwater levels and reflect increased precipitation towards the end of Clarens Formation deposition. Low-frequency severe storms are suggested by hummocky cross-stratified beds in the lake-fill sequences. The strong impact of phreatomagmatic volcanic activity on the sediment supply and facies development of parts of the Clarens Formation illustrate a new facet of increasing rifting activity prior to the Lower Jurassic Karoo flood volcanism.