Clinoform architecture and along-strike variability through an exhumed erosional to accretionary basin margin transition

Exhumed basin margin-scale clinothems provide important archives for understanding process interactions and reconstructing the physiography of sedimentary basins. However, studies of coeval shelf through slope to basin-floor deposits are rarely documented, mainly due to outcrop or subsurface dataset limitations. Unit G from the Laingsburg depocentre (Karoo Basin, South Africa) is a rare example of a complete basin margin scale clinothem (>60 km long, 200 m-high), with >10 km of depositional strike control, which allows a quasi-3D study of a preserved shelf-slope-basin floor transition over a ca. 1200 km2 area. Sand-prone, wave-influenced topset deposits close to the shelf-edge rollover zone can be physically mapped down dip for ca. 10 km as they thicken and transition into heterolithic foreset/slope deposits. These deposits progressively fine and thin over 10s of km farther down dip into sand-starved bottomset/basin floor deposits. Only a few km along strike, the coeval foreset/slope deposits are bypass-dominated with incisional features interpreted as minor slope conduits/gullies. The margin here is steeper, more channelized, and records a stepped profile with evidence of sand-filled intraslope topography, a preserved base-of-slope transition zone and sand-rich bottomset/basin-floor deposits. Unit G is interpreted as part of a composite depositional sequence that records a change in basin margin style from an underlying incised slope with large sand-rich basin-floor fans to an overlying accretion-dominated shelf with limited sand supply to slope and basin-floor. The change in margin style is accompanied with decreased clinoform height/slope and increased shelf width. This is interpreted to reflect a transition in subsidence style from regional sag, driven by dynamic topography/inherited basement configuration, to early foreland basin flexural loading. Results of this study caution against reconstructing basin margins.