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

Abstract

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. 1,200 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 tens 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 the 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 margin successions from partial datasets without accounting for temporal and spatial physiographic changes, with potential implications on predictive basin evolution models.

Inherited Depositional Topography Control on Shelf-Margin Oversteepening, Readjustment, and Coarse-Grained Sediment Delivery to Deep Water, Magallanes Basin, Chile

“…Like other outcrops of large-scale shelf-margin systems, the Tres Pasos-Dorotea deposits at Cerro Cazador provide primarily a two-dimensional (2-D) perspective of the ancient basin margin, limiting interpretations of stratigraphic evolution in 3-D. Considerable along-strike variability of continental margins on the modern seafloor (e.g., Olariu and Steel, 2009;Ryan W. B. et al, 2009) suggests that the same variation ought to be expected in the stratigraphic record of the Magallanes foreland basin margin (e.g., Martinsen and Helland-Hansen, 1995;Driscoll and Karner, 1999;Jones et al, 2015;Madof et al, 2016;Paumard et al, 2018;Poyatos-Moré et al, 2019).…”

“…However, the detailed sedimentology and stratigraphy of major shelf-margin readjustments, like those described by Ross et al (1994) across a large-scale shelf-margin transect, have not been well documented. Furthermore, comparably little work has been focused on the sedimentologic record along the entirety of the transition from shallow-marine to deep-water depocenters along basin margins of high relief (>500 m) (Dixon et al, 2012a;Poyatos-Moré et al, 2019). Deciphering the details of siliciclastic basin margins is challenging as a result of their large scale (several hundreds of meters to kilometers of relief; McMillen, 1991;Helland-Hansen, 1992;Hubbard et al, 2010;Patruno et al, 2015).…”

Sedimentological and stratigraphic criteria to distinguish between basin-floor and slope mudstones: Implications for the delivery of mud to deep-water environments

“…The architecture and depositional environments of the Laingsburg and Fort Brown formations are constrained by a robust three-dimensional stratigraphic framework, established through extensive mapping over 2500 km 2 ( Figure 1B, D) (Grecula et al, 2003a(Grecula et al, , 2003bSixsmith et al, 2004;Van der Merwe et al, 2009, 2014Figueiredo et al, 2010;Di Celma et al, 2011;Flint et al, 2011;Hodgson et al, 2011;Brunt et al, 2013aBrunt et al, , 2013bMorris et al, 2014aMorris et al, , 2014bMorris et al, , 2016Spychala et al, 2015Spychala et al, , 2017Brooks et al, 2018a;Poyatos-Moré et al, 2019). Palaeoflow was dominantly to the northeast and east, and the main sediment entry point was located to the southwest (Van der Merwe et al, 2014).…”

“…The stratigraphic framework of the Laingsburg (Units A, A/B, B) and Fort Brown (Units B/C, C, D, D/E, E, F, G) formations is based upon the recognition and mapping of continuously exposed regional mudstone units intercalated between sandstone-prone units. Mudstone units drape the underlying depositional topography, and thin gradually basinward (to the east) forming a wedge-shape geometry ( Figure 1D) (Brunt et al, 2013b;Brooks et al, 2018a;Poyatos-Moré et al, 2019). The relative thicknesses of the mudstone units have been used as a tool to propose a hierarchical subdivision of the deep-water succession (Flint et al, 2011).…”

Clinoforms and clinothems: Fundamental elements of basin infill

“…the relative power and direction of waves, tides and currents) and other environmental factors (e.g. Helland‐Hansen & Gjelberg, 1994; Patruno & Helland‐Hansen, 2018; Poyatos‐Moré, Jones, Brunt, Tek, et al, 2019). This gives rise to different architectural styles on the margin and determines the long‐term regression, aggradation or retrogradation of depositional systems.…”