Reinterpreting vintage geophysical data from the Algoa and Gamtoos Basins, South Africa: an integrated sequence stratigraphic framework since the middle Mesozoic
Sequence stratigraphy is a branch of stratigraphy that is concerned with how genetically related geological successions are deposited in time and space. This requires the integration of diverse types of datasets (drill core, outcrop, wireline, reflection seismic surveys, etc.) to build robust depositional models, the cornerstones of stratigraphic frameworks. Although the application of sequence stratigraphy has been a successful tool to predict the lithology of geobodies in the petroleum industry, terminology is inconsistently used by the different schools of thought to define stratigraphic surfaces. This has resulted in multiple sequence stratigraphic models that interpret the same data differently. The limited exploration, to-date, and poor dataset quality have impeded the understanding of the geological evolution of the offshore Algoa and Gamtoos Basins in the southern Cape region of South Africa. To reconstruct the main geological events in the area since the late Early Jurassic, we integrated vintage borehole and seismic data as well as key outcrop observations, generated contemporary gross depositional environment models for the basin fill, and tested the applicability of different sequence stratigraphic models. The studied stratigraphic interval formed since the inception of Gondwana break-up, in syn- and post-rift systems that were increasingly dominated by marine processes, especially in the distal hanging walls. Marine incursions are detected in the Upper and Middle Jurassic in the Algoa and Gamtoos Basins, respectively. However, the severely eroded Algoa Basin syn-rift succession, exacerbated by poor data quality, makes it challenging to understand the timing of the marine incursion in this compartmentalized half-graben. Sedimentation within these half-grabens primarily occurred above the hanging walls, whilst the footwalls (i.e., basement highs) formed the dominant sediment source areas. The geological characteristics of the studied synrift succession prevents the application of the depositional sequence stratigraphic or the tectonic system tracts models. Because subaerial unconformities (SUs) in the distal syn-rift sequence are not detectable, a diachronous, northward advancement of the shoreline until the late Valanginian can be postulated. The observations in the syn-rift sequence, which is bound by a basal SU, followed by third and fourth-order transgressive and regressive cycles and a second-order maximum flooding surface at the top, can be explained with a modified genetic sequence stratigraphic model. In the transitional to drift phase interval, from Hauterivian to Holocene, the successions are bound by third-order SUs and their correlative conformities. In the successions without evidence for subaerial exposure in the drift successions, flooding surfaces are used as sequence-bounding stratigraphic contacts, validating the applicability of the genetic and transgressive-regressive sequence stratigraphic models for this upper part of the studied stratigraphic interval. This study reaffirms the notion that while the sequence stratigraphic concept is model independent, sequence models are sensitive to depositional scale and data resolution. Moreover, it also reiterates that sequence boundaries should not be limited to subaerial unconformities, but rather to correlative surfaces that bound genetically related sedimentary successions.
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