A sandy, bed-load dominated fluvial system deposited by lateral-accretion: Permian Karoo Sequence, South Africa

Abstract

A Lower Permian fluvial channel sandstone has been studied from the Karoo Sequence, South Africa. Drill hole core data supplemented by studies of the vertical and lateral facies associations obtained from outcrop have delineated the channel geometry and channel-fill associations.The channel has an average width of between 2 and 5 km, a width to depth ratio of ca. 420 and can be traced in the subsurface for 60 km. Vertical profiles of channel-fill associations were defined and include: (1) large-scale fining-upward sequences, (2) large-scale coarsening-upward sequences, and (3) a fining then coarsening-upward sequence. The dominant sedimentary structure of these sequences is tabular cross-stratification. The analysis of photomosaics of the channel sandstone revealed the existence of channel-fill sequences comprising a second-order scour surface overlain by large-scale tabular cross-stratification and compound cross-stratification. The upper parts of the channel-fill sequences are structured by small-scale tabular cross-stratified sets.The channel-fill sequences represent the deposition of a sandy, bed-load dominated braided fluvial system. Palaeocurrent data derived from the sedimentary structures show a transport direction to the southwest which is oblique to the south-southwest trend of channel geometry. A conceptual model of channel-fill sedimentation and progressive lateral migration of the fluvial system in a westerly direction is presented. The migration of dunes and sand waves over braid and compound bars in an oblique downstream direction permits formation of large-scale tabular and compound cross-stratification on the side and downstream ends of these bars. The lateral migration of first-order channels in a westerly direction causes the development of new compound bars lateral to, and downstream of, earlier formed compound bars. With continued lateral migration of the first-order channel, the trailing second-order channel and initially formed compound bar is abandoned. In this model, lateral accretion deposits, formed from the oblique downstream migration of compound bars, are preferentially preserved in the rock record.