Deepwater Clastic Systems Along the 1000 km Exhumed Gondwana Margin: Lessons from Outcrop Studies

Abstract

Abstract

Over 15 years of outcrop studies integrated with 20 cored research wells and radiometric dating has constrained the time-stratigraphic evolution of the Permo-Triassic Karoo-Falklands Gondwana margin. The stratigraphy is remarkably similar along the margin but with important difference in thicknesses, volumes and partitioning of sand between basin floor and submarine slope. A consistent hierarchy is applied based on depositional sequences (recognised in outcrop and well data) that stack into composite sequences (mappable at seismic scale) and composite sequence sets (CSS). The Tanqua Karoo depocentre includes 800 m of silty mudstone overlain by 4 basin floor fans, each up to 50 m thick with dip lengths of 40 km+, showing a progradational stacking pattern. Grain size never exceeds fine sand. The submarine slope section is only 120 m thick, above which are shelf edge clinoforms and a 500 m of mixed influence shelf edge to shelf deltas. Some 80 km along margin the 1200 m thick Laingsburg succession comprises distal basin plain turbidites punctuated by 3 MTCs. A 300 m+ basin floor fan composite sequence set is overlain by an 800 m thick muddy slope succession 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. The Prince Albert area 100 km east is characterized by large MTCs likely derived from a southern lateral non-depositional section of margin, which alternate with undeformed 20-40 m thick lobe complexes. Overlying shelf deposits are sand-poor. Some 400 km further east, the 1700 m Ecca Pass section includes a 300 m thick basin floor fan complex overlain by a 100 m thick siltstone and slope channel deposits. Grain size is coarse sand. 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 includes a similar basin floor fan complex and a 100 m siltstone cap, then 500 m of slope channel levee complexes with no slope valleys. The shelf succession is wave-dominated. 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 #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016