AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

New Time Constraints on the Karoo Deepwater Physical Sequence Hierarchy

Abstract

The Karoo Basin outcrop provides system-scale exposures of deep-water deposits. Mapping has resulted in multiple examples of slope valleys that pass down dip through channel-levee systems to genetically-related basin-floor lobe complexes. The depositional architecture of these systems has been widely used as analogues to subsurface deep-water systems for exploration and production. The Karoo succession exhibits a physical sequence hierarchy in which individual depositional sequences comprise a 10-30 m thick sandy lowstand systems tract, overlain by a 1-5 m thick regional mudstone drape, interpreted as a combined transgressive and highstand systems tract. These sequences typically stack in groups of three, forming a lowstand sequence set (LSS), 50-100 m thick and typically mappable on seismic data. The LSS is overlain by a 20-40 m thick regional mudstone (combined transgressive and highstand sequence set). Absolute age dating has been a challenge and the duration of the systems has been poorly constrained, limiting their full analogue potential. U/Pb dating of zircons in volcanic ash beds has involved issues such as lead loss and possible changes in zircon source, leading to a range of interpretative scenarios. However, recent TIMMS ages integrated with magnetostratigraphy showing that the whole deep-water through shelf/deltaic section lies within the Kaiman reversed magnetic superchron provides compelling age control at last. The 2 km thick succession was deposited in 6 My, based on multiple dates of 270 Ma for the distal basin plain deposits and 264 Ma at the base of the overlying fluvial Beaufort Group, where the magnetostratigraphy shows the first switch back to normal polarity (top Kaiman superchron). The data support an average 300Ky duration for each composite sequence and 100Ky (Milankovitch eccentricity) for each sequence. Detailed correlation to the Tanqua depocentre, 80 km along margin, has allowed comparison of the sequence stratigraphy in a lower subsidence setting with later onset of sand supply. These sequence durations scale with the Quaternary Golo fan system, east Corsica, where lobe complex sets have durations of 50-100Ky and support the notion that deep-water sand delivery occurs over far shorter time periods than commonly assumed, thus challenging biostratigraphic resolution.