CORRELATION IN FLUVIAL DEPOSITS: INTEGRATION OF PHYSICAL STRATIGRAPHY, CHEMOSTRATIGRAPHY AND MAGNETOSTRATIGRAPHY, KAROO BASIN, SOUTH AFRICA

Fluvial successions are notoriously difficult to correlate, typically due to the absence of regionally unambiguous markers such as dated marine shales or limestones, and a characteristically high degree of lateral facies variability.

The Permo-Triassic Beaufort Group is continuously exposed over hundreds of kilometres and has served as a field laboratory for a multidisciplinary approach to fluvial correlation. Sedimentological and sequence stratigraphic studies of 1 km of the Beaufort Group strata reveal an aggradational succession, rare valley-fills and no mature regional palaeosols. Fluvial style includes lateral and downstream accretion of bars, mud plugs, crevasse splays and variably drained overbank environments.

Sedimentary logs served as a template for chemostratigraphic & magnetostratigraphic sampling, samples being tied to sedimentary facies. Correlation involved i) physical correlation of two 77 m sections near Ouberg Pass, exposed continuously between logged sections 7 km apart. This was achieved by a walking out beds and analysis of a virtual outcrop model from airborne LiDAR, ii) correlation of chemostratigraphic profiles from the two sections without a priori knowledge of the physical correlation. The chemostratigraphic correlation agreed well with the physical correlation, based upon changes in chemistry of the sandstones and mudstones that reflect changing sediment provenance, climate and facies.

The full 549 m Ouberg Pass section contains a characteristic magnetic polarity profile with several normal and reversed magnetozones, indicating a post-Kiaman superchron depositional age. This is confirmed by U/Pb dating of zircons in interbedded volcanic ash deposits, which provide direct correlation to the latest Guadalupian stage of the Permian Global Polarity Timescale.

Magnetostratigraphic correlation to this reference section is observed in a second, 169 m thick section located 85 km away across depositional strike. The chemostratigraphic profile of this section can also be correlated to the Ouberg Pass section and demonstrates the utility of this integrated approach over both short and long distances. Ongoing work is addressing the physical and chemical origin of the chemostratigraphic profiles and integrating the dataset into a regional sequence stratigraphic model.