Abstract
The main Karoo Basin of southern Africa contains the continental record of the end-Triassic, end-Permian, and end-Capitanian mass extinction events. Of these, the environmental drivers of the end-Capitanian are least known. Integrating quantitative stratigraphic architecture analysis from abundant outcrop profiles, paleocurrent measurements, and petrography, this study investigates the stratigraphic interval that records the end-Capitanian extinction event in the southwestern and southern main Karoo Basin and demonstrates that this biotic change coincided with a subtle variation in the stratigraphic architectural style ∼260 Ma ago. Our multi-proxy sedimentological work not only defines the depositional setting of the succession as a megafan system that drained the foothills of the Cape Fold Belt, but also attempts to differentiate the tectonic and climatic controls on the fluvial architecture of this paleontologically important Permian succession. Our results reveal limited changes in sediment sources, paleocurrents, sandstone body geometries, and possibly a constant hot, semi-arid paleoclimate during the deposition of the studied interval; however, the stratigraphic trends show upward increase in 1) laterally accreted, sandy architectural elements and 2) architectural elements that build a portion of the floodplain deposits. We consider this to reflect a long-term retrogradational stacking pattern of facies composition that can be linked to changes on the medial parts of southward draining megafans, where channel sinuosity increased, and depositional energy decreased at the end-Capitanian. The shift in the fluvial architecture was likely triggered by basin-wide allogenic controls rather than local autogenic processes because this trend is observed in the coeval stratigraphic intervals from geographically disparate areas in the southwestern and southern main Karoo Basin. Consequently, we propose that this regional backstepping most likely resulted from tectonic events in the adjacent Cape Fold Belt.
Highlights
Tectonics and climate, the two main allogenic forcing mechanisms of continental deposition (Shanley and McCabe, 1994; Allen et al, 2002; Catuneanu, 2006), are fundamentally associated with the history and evolution of the Lower Beaufort Group in the main Karoo Basin (MKB) of South Africa (Figure 1)
Channel deposits in the studied interval of the Lower Beaufort Group are volumetrically dominated by sandstone facies that are organized in classical upward-fining successions, in which the facies are often found in a vertical succession that comprises volumetrically subordinate but common gravel facies (e.g., Gmm1) and/or facies Sl/St at the base, followed by Sp, Sh, Sm, and Sr at the top
We suggest that, through the same time interval, the amount of the paleoslope inclination was more pronounced in the SW-MKB than in the questions: Why did regional slope become gentler with time? How do the different trends in architecture type fit in the larger evolutionary history of the main Karoo Basin? What were the processes controlling the architecture trends? Why do the changes in architecture trends coincide with the mass extinction event reported by Day et al (2015)? Who investigated the same stratigraphic interval? While the data presented in this work may not be robust enough to fully address all of these questions, some considerations and conclusions can be drawn from the information available
Summary
The two main allogenic forcing mechanisms of continental deposition (Shanley and McCabe, 1994; Allen et al, 2002; Catuneanu, 2006), are fundamentally associated with the history and evolution of the Lower Beaufort Group in the main Karoo Basin (MKB) of South Africa (Figure 1). Using quantitative stratigraphic architecture analysis, including paleocurrent and petrographical data collected from a regional outcrop network, this study aims to untangle the climatic, tectonic, and other, more intrinsic syn-sedimentary signals of a ∼260 Ma old rock record in the southwestern and southern MKB (Figure 1) The significance of this stratigraphic interval, which is conformable, and span ∼2.0–2.5 Myr in total duration and ∼600 m in thickness (Figure 1C and references in there), is that it contains evidence for fluvial architectural changes, and for a rich and diverse paleontological record associated with the end-Capitanian mass extinction event (Day et al, 2015), known as the “dinocephalian extinction event” of Lucas (2017). In this quantitative facies analysis study, we aim to 1) demonstrate that the depositional setting for this ∼260 Ma old paleontologically important Permian succession was a spatiotemporally dynamic megafan system that drained the foothills of the adjacent Cape Fold Belt (CFB) and 2) differentiate the tectonic and climatic controls on the changing fluvial architecture, while emphasizing the limitations of the fluvial sedimentary record for capturing megaevents, which are sudden, high magnitude environmental perturbations that may have caused mass extinction events
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
