Abstract
AbstractSalt tectonics is an important part of the geological evolution of many continental margins, yet the four‐dimensional evolution of the minibasins, the fundamental building block of these and many other salt basins, remains poorly understood. Using high‐quality 3D seismic data from the Lower Congo Basin, offshore Angola we document the long‐term (>70 Myr) dynamics of minibasin subsidence. We show that, during the Albian, a broadly tabular layer of carbonate was deposited prior to substantial salt flow, diapirism, and minibasin formation. We identify four subsequent stages of salt‐tectonics and related minibasin evolution: (i) thin‐skinned extension (Cenomanian to Coniacian) driven by basinward tilting of the salt layer, resulting in the formation of low‐displacement normal faults and related salt rollers. During this stage, local salt welding led to the along‐strike migration of fault‐bound depocentres; (ii) salt welding below the eastern part of the minibasin (Santonian to Paleocene), causing a westward shift in depocentre location; (iii) welding below the minibasin centre (Eocene to Oligocene), resulting in the formation of a turtle and an abrupt shift of depocentres towards the flanks of the bounding salt walls; and (iv) an eastward shift in depocentre location due to regional tilting, contraction, and diapir squeezing (Miocene to Holocene). Our study shows that salt welding and subsequent contraction are key controls on minibasin geometry, subsidence and stratigraphic patterns. In particular, we show how salt welding is a protracted process, spanning > 70 Myr of the salt‐tectonic history of this, and likely other salt‐rich basins. The progressive migration of minibasin depocentres, and the associated stratigraphic architecture, record weld dynamics. Our study has implications for the tectono‐stratigraphic evolution of minibasins.
Highlights
A minibasin is a syn‐kinematic succession of sediment that subsides into a body of salt (e.g. Jackson & Talbot, 1991; Peel, 2014a)
Minibasins are commonly found in passive margin salt basins, such as the Gulf of Mexico (e.g. Hudec, Jackson, Vendeville, Schultz‐Ela, & Dooley, 2011; Lamb, Toniolo, & Parker, 2006; Prather, Booth, Steffens, & Craig, 1998), the West African margin (e.g. Hudec & Jackson, 2004; Marton, Tari, & Lehmann, 2000) and the Brazil margin (e.g. Quirk et al, 2012), as well as cratonic salt basins such as the North and South Permian basins (e.g. Hodgson, Farnsworth, & Fraser, 1992), and the Precaspian Basin (e.g. Barde et al, 2002; Duffy et al, 2017; Jackson et al, 2019)
As minibasin subsidence and subsequent salt welding have a direct impact on the formation of related salt structures, a better understanding of minibasin evolution and growth improves our knowledge of the development of genetically related salt structures
Summary
A minibasin is a syn‐kinematic succession of sediment that subsides into a body of salt (e.g. Jackson & Talbot, 1991; Peel, 2014a). Based on stratal geometry and the relative locations of the depocentres, we divide subsequent minibasin development into four stages: (i) Cenomanian to Coniacian: depocentres initiation and lateral migration to the north and south (Figure 7b‐d); (ii) Santonian to Paleocene: across‐strike shift of minibasin depocentres to the west (Figure 7e and f); (iii) Eocene. Subsidence shifted along strike towards the south into a location where salt was still relatively thick and accommodation generation, driven by salt expulsion, was still ongoing (Figure 8e) This is evident by the Turonian strata laterally onlapping over the Cenomanian strata near Depocentre D1 (Figure 6). By the late Miocene, the minibasin is defined by a single, 4–9 km wide, NE‐trending depocentre, the axis of which lies midway between the flanking salt walls (D9; Figure 7j) Overall, this succession thins towards flanking salt walls, suggesting that the latter were rising at this time (Figure 5). Further contraction in Pleistocene was accommodated by thrusting and secondary weld formation (Figures 5a and 9k)
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