Stratigraphic Prediction of Deepwater Clastic Systems Along the Exhumed 900 km Karoo/Falklands Basin Margin

Abstract

A major exploration challenge is understanding along margin variability of broadly coeval deepwater systems in terms of geometry, architecture and net: gross. Many years of outcrop studies integrated with 20 fully cored research wells permits construction of a time-stratigraphic evolution of the NW-SE trending Permo-Triassic Karoo basin margin. Similar stratigraphy along the margin belies important differences in thicknesses, volumes and partitioning of sand between basin floor and submarine slope. A consistent hierarchy is applied based on depositional sequences that stack into composite sequences and composite sequence sets (CSS). The Tanqua Karoo depocentre includes 800 m of mudstone overlain by 4 basin floor fans, each 50 m thick with dip lengths of 40 km+, showing a progradational stacking pattern. The submarine slope section is 120 m thick, above which are shelf edge clinoforms and mixed influence deltas. Some 80 km along margin the 1200 m thick Laingsburg succession comprises distal basin plain turbidites punctuated by 3 mass transport complexes (MTCs). A 300 m+ basin floor fan composite sequence set is overlain by 3 further CCSs marking progradation of an 800 m thick muddy slope succession to the east with slope valleys feeding fan system 80–100 km long and up to 60 km³. Shelf edge clinoforms mark eastward progradation of a 500 m thick mixed influence delta system. 100 km east the margin is characterized by MTCs derived from a southern lateral slope, which alternate with undeformed 20–40 m thick lobe complexes. Overlying shelf deposits are sand-poor. 400 km east, the 1700 m Ecca Pass section includes a 300 m thick basin floor fan complex with no basal MTCs, overlain by a 100 m thick siltstone and slope channels. The shelf section is sand poor, suggesting a switch in sand delivery location between deepwater and shelf. The Falkland Islands lay 250 km east of Ecca Pass in the Permian and the 3 km+ succession is similar in terms of a basin floor fan complex with no MTCs and a 100 m siltstone cap, above which is 300 m of slope channel levee complexes with no slope valleys and a wave-dominated shelf succession. The exhumed Karoo-Falklands margin provides a well exposed example of lateral variability in basin margin physiography convolved with late icehouse to greenhouse transition glacio-eustatic sea level and long wavelength subsidence to control spatial and temporal variability in deepwater systems architecture and overlying shelf delta style.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015