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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Petrology
Vol. 43 (1973)No. 1. (March), Pages 147-159

Open-Cast Slump Sheets and their Relationship to Sandstone Beds in an Upper Cambrian Flysch Sequence, Tasmania

Keith D. Corbett


Slump sheets up to three meters thick are abundant in the Upper Cambrian Singing Creek Formation, a proximal flysch sequence on the Denison Range, southwest Tasmania. The slumps range from coherent types, in which bedding is well preserved, through semi-coherent, in which there has been partial disaggregation, to incoherent, in which scattered remnants of original beds "float" in a reconstituted matrix. The latter type include "pebbly mudstones," and may show a weak slump cleavage. In a measured 96-m section there are some 48 slump sheets, many of them separated only by a single graded sandstone layer which truncates and smoothes off the top of the underlying slump and forms a flat decollement surface beneath the overlying one. The truncation indicates that the slumps occurred at the eafloor prior to burial, i.e., they are open-cast.

Fold-axis orientation has been measured in five of the more coherent slump sheets in this section, and confirms what in outcrop appears to be a strong alignment. This alignment, together with other evidence of movement, suggests a direction of transport at right angles to that of the turbidity currents which deposited the intercalated sandstone beds. The slumps were not triggered by current activity in the manner proposed by Crowell (1957) for some "pebbly mudstones," but by some mechanism which repeatedly formed a local slope of constant orientation and affected only the upper metre or so of sediment. Slumping from the wall of a fan channel is a possible explanation, but there is little to suggest that the beds are channel deposits. A mechanism involving contemporaneous seafloor faul ing seems more likely, and a cross-fault which could have been responsible for the slumping occurs adjacent to the measured section.

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