Cenozoic Structural Evolution and Tectono-Stratigraphic Framework of the Northern Gulf Coast Continental Margin
Published:January 01, 1995
F. A. Diegel, J. F. Karlo, D. C. Schuster, R. C. Shoup, P. R. Tauvers, 1995. "Cenozoic Structural Evolution and Tectono-Stratigraphic Framework of the Northern Gulf Coast Continental Margin", Salt Tectonics: A Global Perspective, M.P.A. Jackson, D.G. Roberts, S. Snelson
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The Cenozoic structural evolution of the northern Gulf of Mexico Basin is controlled by progradation over deforming, largely allochthonous salt structures derived from an underlying autochthonous Jurassic salt. The wide variety of structural styles is due to a combination of (1) original distribution of Jurassic and Mesozoic salt structures, (2) different slope depositional environments during the Cenozoic, and (3) varying degrees of salt withdrawal from allochthonous salt sheets. Tectono-stratigraphic provinces describe regions of contrasting structural styles and ages. Provinces include (1) a contractional foldbelt province, (2) a tabular salt-minibasin province, (3) a Pliocene-Pleistocene detachment province, (4) a salt dome-minibasin province, (5) an Oligocene-Miocene detachment province, (6) a lower Oligocene Vicksburg detachment province, (7) an upper Eocene detachment province, and (8) the Wilcox growth fault province of Paleocene-Eocene age.
Within several tectono-stratigraphic provinces, shale-based detachment systems, dominated by lateral extension, and allochthonous salt-based detachment systems, dominated by subsidence, can be distinguished by geometry, palinspastic reconstructions, and subsidence analysis. Many shale-based detachments are linked downdip to deeper salt-based detachments. Large extensions above detachments are typically balanced by salt withdrawal.
Salt-withdrawal minibasins with flanking salt bodies occur as both isolated structural systems and components of salt-based detachment systems. During progradation, progressive salt withdrawal from tabular salt bodies on the slope formed salt-bounded minibasins which, on the shelf, evolved into minibasins bounded by arcuate growth faults and remnant salt bodies. Associated secondary salt bodies above allochthonous salt evolved from pillows, ridges, and massifs to leaning domes and steep-sided stocks.
Allochthonous salt tongues spread from inclined salt bodies that appear as feeder faults when collapsed. Coalesced salt tongues from multiple feeders formed canopies, which provided subsidence potential for further cycles of salt withdrawal. The Sigsbee escarpment is the bathymetric expression of salt flows that have overridden the abyssal plain tens of kilometers since the Paleogene. The distribution and palinspastic reconstruction of Oligocene-Miocene salt-based detachments and minibasins suggest that a Paleogene salt canopy covering large areas of the present onshore and shelf, may have extended as far as the Sigsbee salt mass.
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Salt Tectonics: A Global Perspective
The conceptual breakthroughs in understanding salt tectonics can be recognized by reviewing the history of salt tectonics, which divides naturally into three parts: the pioneering era, the fluid era, and the brittle era.
The pioneering era (1856-1933) featured the search for a general hypothesis of salt diapirism, initially dominated by bizarre, erroneous notions of igneous activity, residual islands, in situ crystallization, osmotic pressures, and expansive crystallization. Gradually data from oil exploration constrained speculation. The effects of buoyancy versus orogeny were debated, contact relations were characterized, salt glaciers were discovered, and the concepts of downbuilding and differential loading were proposed as diapiric mechanisms.
The fluid era (1933–1989) was dominated by the view that salt tectonics resulted from Rayleigh-Taylor instabilities in which a dense fluid overburden having negligible yield strength sinks into a less dense fluid salt layer, displacing it upward. Density contrasts, viscosity contrasts, and dominant wavelengths were emphasized, whereas strength and faulting of the overburden were ignored. During this era, palinspastic reconstructions were attempted; salt upwelling below thin overburdens was recognized; internal structures of mined diapirs were discovered; peripheral sinks, turtle structures, and diapir families were comprehended; flow laws for dry salt were formulated; and contractional belts on divergent margins and allochthonous salt sheets were recognized. The 1970s revealed the basic driving force of salt allochthons, intrasalt minibasins, finite strains in diapirs, the possibility of thermal convection in salt, direct measurement of salt glacial flow stimulated by rainfall, and the internal structure of convecting evaporites and salt glaciers. The 1980s revealed salt rollers, subtle traps, flow laws for damp salt, salt canopies, and mushroom diapirs. Modeling explored effects of regional stresses on domal faults, spoke circulation, and combined Rayleigh-Taylor instability and thermal convection. By this time, the awesome implications of increased reservoirs below allochthonous salt sheets had stimulated a renaissance in salt tectonic research.
Blossoming about 1989, the brittle era is actually rooted in the 1947 discovery that a diapir stops rising if its roof becomes too thick. Such a notion was heretical in the fluid era. Stimulated by sandbox experiments and computerized reconstructions of Gulf Coast diapirs and surrounding faults, the onset of the brittle era yielded regional detachments and evacuation surfaces (salt welds and fault welds) along vanished salt allochthons, raft tectonics, shallow spreading, and segmentation of salt sheets. The early 1990s revealed rules of section balancing for salt tectonics, salt flats and salt ramps, reactive piercement as a diapiric initiator resulting from tectonic differential loading, cryptic thin-skinned extension, influence of sedimentation rate on the geometry of passive diapirs and extrusions, the importance of critical overburden thickness to the viability of active diapirs, fault-segmented sheets, counter-regional fault systems, subsiding diapirs, extensional turtle structure anticlines, and mock turtle structures.