Crustal distribution in the central Gulf of Mexico from an integrated geophysical analysis

This study addresses the question of the crustal composition in the central part of the northern Gulf of Mexico (GOM) – the region of the major disagreement between published tectonic models. The location of the Ocean-Continental Boundary (OCB) for different tectonic models varies within 140 km (87 mi) in the study area. I have developed a 2D model integrating the seismic reflection and refraction data with potential fields (gravity and magnetics) along the profile through the debated region. Two alternative OCB locations were tested. The preferred model suggests the OCB position near the Sigsbee Escarpment, which is in agreement with the result of Eddy, 2014 and with the findings of the LithoSPAN experiment (Makris et al, 2015). However, the model with an alternative OCB location (further to the north of the Sigsbee Escarpment) may also satisfy the observed gravity and magnetic fields, although the crust in the oceanic domain is thicker than normal. Since the potential fields do not offer the unique answer, the other geophysical data should be examined, such as the Vp/Vs ratio. This parameter was analyzed for the LithoSPAN (Makris et al., 2015) and allowed distinguishing between continental and oceanic domains; it was also examined for GUMBO 3 and 4 (Duncan, 2013). However, the values of Vs derived during retraction experiment for GUMBO 2 are not publically available at this time

Plio-Pleistocene evolution of the upper continental slope, Garden Banks and East Breaks areas, northwestern Gulf of Mexico

textOver 7000 sq. km of salt and six Plio-Pleistocene biostratigraphic horizons were mapped in the East Breaks and Garden Banks areas using a 12,000 km grid of seismic data and all obtainable well data. Structure mapping of allochthonous Jurassic salt and the six horizons (Globoquadrina altispira, Lenticulina 1, Angulogerina B, Hyalinea B, Trimosina A, and Sangamon Fauna) and isopachs of the intervals between these horizons revealed notable lateral variations in the area underlain by salt, in the degree of salt deformation, and in the size and thickness of associated intraslope basins. East of 94.5° W salt structures occupy 40% of the area and exhibit complex shapes that suggest a high degree of salt deformation. West of 94.5° W salt structures occupy 11% of the area and consist mostly of structurally simple salt stocks. A zone of high-offset north-south trending faults mark the transition between these two areas. Isopach maps of the six Plio-Pleistocene intervals (from 2.9 Ma to the present) reveal major shifts in the rates and locations of sediment accumulation. From 2.9 to 1.0 Ma. sediment-accumulation rates averaged only 0.8-1.3 mm/y with a maximum rate of 2.7 mm/y. From 1.0 to 0.69 Ma. sediment-accumulation rates averaged 5.8 mm/y with a maximum rate of 11.6 mm/y. This interval correlates to sediments deposited between the extinctions of Hyalinea balthica and Trimosina denticulata and recorded a major period of sediment loading/salt withdrawal between 1.0-0.69 Ma. From the end of this time to the present, sediment -accumulation rates averaged 1.7-2.1 mm/y with a maximum rate measured at 6.2 mm/y. Increased sediment influx during 1.0-0.69 Ma coincides with a major third order sea level lowstand and was focused in central Garden Banks. The restriction of such dramatically increased accumulation rates to this area suggests that sediment influx was accompanied by large-scale salt withdrawal. The increase in accommodation space created by salt withdrawal appears to be the most important factor affecting accumulation rates. Salt structural styles found on the upper continental slope are transitional between those found on the lower slope and those on the shelf. The shelf is dominated by isolated, individual salt stocks (km²) surrounded by kilometer thick sedimentary sections. The lower slope is dominated by broad, laterally continuous, allochthonous salt sheets (10³ km²) with moderate to thin sediment cover. The upper slope contains both of these structural styles plus intermediate size (10-10² km²) salt ridges and massifs. Observations made during this study suggest that differential sediment loading is the mechanism causing the changes in structural style. A Loading/Dissection model is presented to explain the formation of the three primary salt structural styles, their genetic relationship, and their observed distribution. Differential loading has dissected large salt sheets into numerous smaller and irregularly shaped ridges and stocks (like those found on the upper slope). Salt found on the upper slope originated in the Jurassic Louann Formation, but is now surrounded by Pleistocene age sediments. To achieve this relationship, it appears that some Jurassic salt has undergone at least two cycles of sediment loading and consequent diapirism. Salt/sediment relationships suggest that virtually all of the mapped salt is allochthonous. Repetitive sediment loading and salt structural development has not been previously documented and represents a step beyond the limits of current salt structural models.Geological Science

Origin of the Neoproterozoic Rim Dolomite as Lateral Carbonate Caprock, Patawarta Salt Sheet, Flinders Ranges, South Australia

The ‘rim dolomite’ of South Australia’s Central Flinders Ranges is a prominent ridge-forming, layered dolomitic and siliceous unit. The rim dolomite present at the salt-sediment interface between Patawarta diapir and the Ediacaran-aged Bunyeroo formation. The rim dolomite is classified as a lateral dolomite caprock based on the following field observations: 1) the rugose dolomicrite base that parallels the contact of the diapiric matrix and the bedding in the overlying stratigraphy, 2) the exclusive presence of dolomite at the salt-sediment interface, 3) the lack of sedimentary structures or fossils (cyanobacterial laminites and stromatolites), 4) the lack of interbedded Bunyeroo lithofacies, and 5) the inability to trace the rim dolomite capstone away from the diapir margin into the outboard stratigraphy. The rim dolomite displays the following capstone types: 1) massive – microcrystalline dolomite, 2) porphyritic – two distinct crystal sizes, one forming microcrystalline dolomite groundmass and the other forming rosettes of silica, 3) banded – microcrystalline dolomite forming pressure-dissolution layers of silica and authigenic hematite, and 4) brecciated – mosaic to disorganized, forming a microcrystalline dolomite groundmass, which locally contains remnant clasts of Callanna non-evaporite lithologies, such as quartz arenite to arkosic sandstones and basalts, surrounded by an anastomosing cement-filled vein network. All capstone types contain various amounts of anhydrite, quartz, feldspar, and non-evaporite grains that represent the insoluble residue during halite dissolution and caprock accretion. The rim dolomite is a caprock formed in the crestal position and rotated to the diapir flank by halokinetic drape-folding, which matches the field relationships and capstones of other lateral caprocks in salt basins, such as the Paradox Basin and Gulf Coast, USA