Basin floor-slope-shelf edge stratigraphy and architecture, Laingsburg Karoo basin, South Africa.

The Laingsburg depocentre of the SW Karoo basin, South Africa, preserves a well-exposed 1.3 km thick succession of late Permian siliciclastic deposits that record the early filling of a high latitude back arc to proto-foreland basin during an icehouse climate. Uniformly fine-grained sandstones were derived from far-field granitic sources, possibly Patagonia. The coeval staging and delivery systems (fluvial and shelf) are not preserved. The deepwater systems are overlain by mixed influence shelf edge deltas. Initial mud-prone basin floor turbidites of the Vischkuil Formation contain three regionally developed zones of soft sediment deformation related to emplacement of major debris-flows that mark the initiation of a major sand delivery system. The overlying 300 m thick sandy basin floor fan system (Fan A) and 150 m thick base-of-slope channel/ levee system (Unit B) are divisible into four composite sequences that show long term forward stepping, aggradation and backstepping stacking patterns, followed by basinwide pelagic mud deposition. The superjacent 800 m thick mud-dominated submarine slope succession is characterized by 60 – 120 m thick sand-prone to heterolithic packages that show abrupt lateral and down-dip changes in thickness and facies, which are separated by extensive 30 – 70 m thick mudstone packages. On the largest scale the slope stratigraphy is defined by two major cycles, each comprising three composite sequences of repeating architectural style. Each composite sequence comprises three sequences. The lower cycle comprises lithostratigraphic Units B/C, C and D while the upper cycle includes Units E/F, E and F. In each case a sandy basal sequence is dominated by intraslope lobe deposits (Units B/C and E/F). The second composite sequence in each cycle (Unit C and Unit E) is characterized by slope channel-levee complexes that feed lobes down dip. The uppermost composite sequence in each cycle (Units D and F) comprises deeply entrenched slope valley/canyon systems. Architectural style is similar at individual sequence scale in comparable positions in each major cycle, but along-strike changes in sequence expression as a function of shelf edge delivery system and variable substrate compaction complicate the resultant stratigraphy. Although complicated in detail, the deepwater succession shows a consistent hierarchical order, enabling prediction of temporal changes in architectural style within each composite sequence. This stratigraphic organisation is interpreted as due to high magnitude/high frequency glacial-eustatic sea level changes during a tectonically quiescent pre-foreland basin period.