AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Significance of the Tide-Influenced Facies Distribution Within the Shelf-Margin Deposits, Kookfontein Formation, South Africa
(1) Geology, Colorado School of Mines, Golden, CO.
Most shelf-edge successions are expected to be dominated by wave-generated, deltaic, or soft-sediment deformed sedimentary packages, however in the Tanqua shelf-edge depocenter of the Permian Karoo Basin in South Africa, tidal features are surprisingly common. The emphasis of this paper is to understand how the tidal facies change both laterally and downdip within this shelf-edge succession. Three clinothems located in the middle Kookfontein Formation were studied by measuring 18 detailed stratigraphic sections, measuring paleocurrents directions, and walking-out key surfaces on Pienaarsfontein Se Berge, Grasberg, Watervleihoek Se Berg, and Kookfonteinberg. The facies of this prograding system indicate a seaward increase in tidal structures and a landward increase in fluvial structures. The lowest, upper-slope interval of each clinothem, consists of 5 to 20 m of thinly bedded siltstone and mudstone that contain a combination of the following features: 1) mud drapes, 2) bidirectional paleocurrents, 3) flaser, lenticular, and wavy bedding, 4) bioturbation, 5) climbing ripples, and 6) planar laminations. The bases of these tide-dominated upper slope pro-delta packages typically have very thin apparently structureless beds that gradually grade into laminated thin beds and slightly thicker ripple laminated beds. These beds are overlain by 3 to 7 m thick turbidite packages that consist of mud-draped laminated sandy beds alternating with in-situ deformed or un-deformed mudstone, siltstone, and very fine-grained sandstone. These tidal packages are superimposed by homogeneous or heterolithic slump deposits ranging from 1 to 5 m thick. Finally, each clinothem is capped by outer-shelf inner-delta front fluvial dominated very fine to fine-grained distributary mouth bars and distributary channels. Faint low-angle and planar cross-stratification commonly occur within these clean sandbodies, but occasionally mud-drapes are observed in these channels and wave ripples within the bars. Results from this study emphasize the significance of tidal processes in the construction of shelf margins.