The bathymetric datum with respect to global sea level for Aptian salt deposition on the deep-water Angolan rifted margin and the composition of underlying basement are hotly debated. Quantitative analysis of deep seismic reflection and gravity anomaly data together with reverse post-breakup subsidence modelling has been used to investigate ocean-continent transition structure, continent-ocean boundary location, crustal type and the palaeo-bathymetry of Aptian salt deposition. The analysis has been applied to the ION-GXT CS1-2400 deep long-offset seismic reflection profile and the P3 and P7+11 seismic cross sections of Moulin (2005) and Contrucci et al. (2004) offshore northern Angola. The palaeo-bathymetry of base Aptian salt deposition has been determined using reverse post-breakup subsidence modelling consisting of the sequential flexural isostatic backstripping of the post-breakup sedimentary sequences, decompaction of remaining sedimentary units and reverse modelling of post-breakup lithosphere thermal subsidence. We predict that Aptian salt was deposited between approximately 0.2km and 0.6km below global sea level, and that the inner proximal salt subsided by post-rift (post-tectonic) thermal subsidence alone, while the outer distal salt formed during syn-rift, prior to breakup, resulting in additional tectonic subsidence. Our analysis argues against Aptian salt deposition on the Angolan margin in a 2-3km deep isolated ocean basin, and supports salt deposition on hyper-extended continental crust formed by diachronous rifting migrating from east to west, culminating in the late Aptian. Gravity inversion to give Moho depth and crustal thickness, RDA analysis to identify departures from oceanic bathymetry and subsidence analysis shows that the distal Aptian salt is underlain by hyper-extended continental crust rather than exhumed mantle or oceanic crust.
The Demerara Rise is a prominent bathymetric feature that has been considered as a broad expression of shallow continental basement and used in conjunction with the Guinea Plateau as a pinning point for circum-Atlantic plate reconstructions. Previously, shallow-penetration, poorly imaged seismic data over the Demerara Rise were modeled with the lower sequences interpreted as continental crust at relatively shallow depths. However, new long-offset, deeply penetrating seismic data provide evidence that basement nearly or entirely comprises excessively thick volcanic strata (approximately 21 km). Seismic character and geometry, 2D gravity modeling, and volcanic margin analogs were used to identify unfaulted, convex-upward seaward dipping reflector (SDR) packages. These steeply dipping (approximately 20°) igneous successions are westwardly divergent, and occur as offlapping reflector sets in trains as long as 250 km. This rift-related volcanism now recognized at the Demerara Rise was probably conjugate to syn-rift volcanism in South Florida/Great Bahama Bank, and from this we have predicted a volcanic element for the Guinea Plateau. This volcanism could be linked to a Bahamas hot spot at the initial opening of the Central Atlantic. Six SDR packages have been interpreted below the Late Jurassic-Early Cretaceous carbonate section of the rise, indicating that the early volcanism produced a marine substrate upon which the subsequent carbonate bank section developed. We have inferred that this Early Cretaceous volcanic/carbonate margin continued into the Guinea Plateau of West Africa. The pre-Aptian section was inverted and peneplained with a strong angular unconformity prior to the Early Cretaceous opening of the Equatorial Atlantic seaway. The newly identified Central Atlantic volcanic margin of the Demerara Rise holds implications of a volcanic origin for its conjugate margins. We have confirmed a voluminous magma-rich opening of the southeastern Central Atlantic.
Abstract Outer marginal collapse (OMC), a recently proposed process by which top-rift and base-salt unconformities formed near sea level may subside rapidly to 2.5–3 km at continental margins as mantle exhumation or seafloor spreading begins, needs further examination. We examine salt deposition at three margins and find that the differing positions and volumes of salt can be related to different durations of salt deposition as OMC and subsequent mantle exhumation proceed. Along NW Florida, salt is thin but deep and is interpreted as having formed at the start of OMC, before drowning further to abyssal depths. In the Campos Basin, salt is thick and extends across tens of kilometres of interpreted exhumed mantle, interpreted as having formed during the entire period of OMC before spreading onto mantle during exhumation. In the Santos Basin, salt is thick and extends across c. 100 km of interpreted exhumed mantle and/or oceanic crust, arguably requiring ‘lateral tectonic accommodation’, whereby salt deposition persists near global sea level across the conjugated salt basin during mantle exhumation beneath mobile salt. The supposition that OMC can account for salt deposition in three different basins without invoking problematic 1.5–2 km-deep subaerial depressions provides further support for the process.
Sparse data have hindered efforts to characterize the general geology and petroleum systems in the Siberian Arctic in and east of the Laptev Sea, a region whose potential has often been discounted. Recent acquisition and interpretation of 13,000+ line-km of new long offset, long record reflection data in the North Chukchi, East Siberian, and Laptev Seas has clarified the geometry and inter-relationships of several basins in this enormous 3 × 106 km2 area devoid of wells. The 16 sec (PSTM) and 40 km (PSDM) data image a number of attractive late Mesozoic and Cenozoic extensional basins superimposed on older Phanerozoic fold belts that lie below acoustic basement. These basins all relate in various ways to the opening of the Arctic Ocean, and many contain 7.5 to 10 km of sedimentary fill and, up to 20 km in the case of the North Chukchi Basin. A variety of stratigraphic fill styles related to their underlying tectonics can be observed. For example, late-stage (postrift) architecture in the North Chukchi Basin shows Tertiary deltaic sequences traversing over 400 km northward overlying Late Cretaceous rift-fill sediments which contain potential source rocks. In contrast, the Laptev Sea exhibits successions related to a passive margin subsidence history, with low-angle sedimentary systems tracts including well-developed ancient shelf margins and lowstand systems, all cut by intra-continental extensional structures on trend with the active Gakkel Ridge spreading center. Slightly older sediment fill occupies rifts under the East Siberian Sea. The observed potential petroleum systems in this region offer source, reservoir and seal lithologies and hydrocarbon migration geometries to access shelf margin, lowstand depositional systems in addition to the potential within the Neogene rifts.
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