Shelf-to-Slope Subsurface Petroleum Systems
2013. "Shelf-to-Slope Subsurface Petroleum Systems", Shelf Margin Deltas and Linked Down Slope Petroleum Systems–Global Significance and Future Exploration Potential, Harry H. Roberts, Norman C. Rosen, Richard H. Fillon, John B. Anderson
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Unravelling end-Cretaceous paleobathymetric dip-profiles along strike in the northern Gulf of Mexico continental margin (nGoM) is important because it provides us with the initial framework in which to assess the evolution of Cenozoic clastic systems and burial history. Light can be shed on the problem by integrating seismic and potential field data with analytical basin modelling techniques and palinspastic-kinematic paleo-geographic analysis. Progressive palinspastic reconstruction of the Gulf is critical to setting up the appropriate lithospheric models to assess subsidence history. Kinematically, the opening of the GoM involved an early rift stage of northwest–southeast stretching (with minor counterclockwise [CCW] rotation) between North America and Yucatán (Triassic–Early Oxfordian), followed by a drift stage of seafloor spreading between the opposing rifted margins that entailed significant CCW rotation of Yucatán Block. This Stage 2 rotation was accommodated by transform motion of Yucatan/Chiapas Massif along the foot of the very narrow eastern Mexican margin, but transform motion stopped once the Central Gulf Spreading Ridge passed any point along this margin. Thus, the crustal boundary along eastern Mexico is ultimately an igneous, constructional contact that is overlain by the entire stratigraphy visible on seismic, and no transform faults are to be expected (Pindell, 1985).
The rift stage was asymmetric (Pindell et al., 1986; Marton and Buffler, 1994) such that Yucatán collapsed off the mainly northwest-vergent Alleghenian Orogen of the southern USA. The nGoM margin (foot-wall) underwent large and rapid tectonic subsidence during the rift stage (because the crust was highly stretched), but little thermal subsidence during the drift stage and thereafter (because the lithosphere was NOT stretched much). In contrast, the Yucatán hanging wall underwent little syn-rift subsidence (because the crust was not stretched much), followed by considerable postrift (Late Jurassic and younger) thermal subsidence (because the lithosphere was stretched). During the rift stage, thick red beds, possibly with lacustrine or even marine intervals, effectively buried basement in most nGoM areas and, toward the end of the rift stage (Callovian–Early Oxfordian), gave way to salt deposition across much of the half-open Gulf basin. Original salt thickness is generally considered to exceed what could have been achieved by thermal subsidence alone (~2km) during Callovian–Early Oxfordian time (presumed agespan of salt deposition); thus, salt accumulation was coeval with Stage 1 tectonic subsidence (syn-rift) and/or involved the marine inundation of pre-existing, isolated, sub-sea level accommodation space, which, by Oxfordian time, was almost definitely filled to sea level with salt. Together, red beds and salt probably are 5–10km thick beneath much of the nGoM rifted margin.
Oxfordian onset of CCW rotational seafloor spreading in the Gulf split the pre-existing red-bed/salt basin into the Louann and Campeche halves. Backstripping shows that ocean crust was generated near its typical 2.6km depth below sea level, and not at an Icelandic-setting near sea level. The continent-ocean boundary typically is marked by a large step up from continental to oceanic crust (i.e., the rifted continental crust was already buried by red beds and salt far thicker than 2.6km ocean-generation depth, so the basement step to ocean crust is UP). As spreading ensued, a central, widening “chasm” was produced that was floored by oceanic crust and that, once Smackover open marine conditions were established, received no new depositional salt. To our knowledge, truly autochthonous salt cannot be shown to overlie definite oceanic crust; thus, initial spreading was effectively coeval with the onset of Smackover open marine conditions, and there may have been a causal relationship between the onset of spreading and the breaking of the evaporitic sill, wherever that was (Florida Straits or Veracruz Basin are equally viable guesses).
As seen south of the Middle Grounds margin, the immediately adjacent shoulders of these salt walls halo-kinetically collapsed into the widening chasm, but, given the enormous width of the nGoM rifted margin, an important question is to assess how far north into the salt basin such early collapse occurred. The Red Sea analogue shows that the salt could have supported shelf platforms at least into the Cretaceous; we typically observe minor (<20km) extrusion of salt across the step up onto oceanic crust, but locally, such as at Sigsbee Escarpment, salt may have collapsed much farther (100km) onto the ocean crust, possibly as early as Late Jurassic–Cretaceous times. In such places, the term “parautochthonous salt” applies, because the salt still underlies most stratigraphy although it acquires a tapered-wedge cross-sectional geometry as it collapses.
Because the nGoM margin was the footwall during Jurassic asymmetric rifting, thermal subsidence had far less influence on paleobathymetry than is commonly believed. Thus, determination of paleobathymetry can be roughly gauged by structural analysis of halokinesis. Thus, for large areas of the nGoM margin, we propose (1) that a relatively shallow, “supra-salt platform” persisted until the Late Paleocene onset of the well-known Wilcox growth faulting, and (2) that the Upper Jurassic–Cretaceous supra-platform section remained shallow, and was never deeply buried until halokinetic collapse began. This contrasts sharply with the Campeche Salt margin of Mexico, which was drowned to truly basinal depths in the Late Jurassic-Early Cretaceous due to far higher rates of post-rift thermal subsidence and weak clastic sediment supply. Thus, in the north but not in the south, we envision a very broad, relatively shallow supra-salt platform with a thinner-than-often-assumed Upper Jurassic-Cretaceous section that extended well beyond much of today's coastline. In this case, the true continental slope and rise would have been located much farther out than the Stuart City carbonate trend (which is often inferred as the paleo-shelf edge). This platform may have been stepped due to early halokinesis, particularly at Sigsbee, and along its outer reaches probably sloped or ramped down to the area of oceanic crust.
Given this scenario for the paleobathymetry, it should not be surprising to find early Paleogene sands at the foot of that platform slope (e.g., Perdido area). The sands could have been transported there from (1) the north or northwest by shelf bypass across the suprasalt platform, or (2) the west, out of a proto-Rio Grande river system, or both. By the end of the Paleocene, salt collapse in updip areas of the supra-salt platform began due to differential burial by prograding clastics, producing syn-depositional counter-regional faults and basin-facing half-grabens at the Wilcox and younger fault trends, which controlled the [new, syntectonic] position of the paleo-shelf edge. Such collapse fed downslope shortening behind (landward of) the Paleogene sands at the foot of the true continental slope. We infer detachment on salt, such that the Mesozoic marine supra-salt section was cut both updip and downdip by at least some of the faults. Apparent rafting of the Mesozoic shelf section at the landward limit of the Wilcox trend (e.g., Anderson and Fiduk, 2003, and also observed in NE Mexico on seismic by the authors) demonstrates that the salt (and inferred Upper Jurassic–Cretaceous shallow shelf) was mobile during end-K/early T time. The concept of the Mesozoic supra-salt platform in the nGoM: (1) requires significant changes to commonly-accepted Late Jurassic through Paleocene paleobathymetric and paleogeographic maps of nGoM, and therefore of reservoir and source rock distribution; (2) indicates the need to develop maturation models for the inner shelf areas that do not assume a pre-existing deep-water setting outboard of the Sligo/Stuart City “reef trends” prior to Tertiary clastic deposition; and (3) provides a new paleogeographic context for assessments and models of Cenozoic deltaic and progradational depositional systems along the northern Gulf of Mexico.