Tectono-Sedimentary Models for Rift-Basin Carbonate Systems
Published:January 01, 2008
Nigel E. Cross, Dan W. J. Bosence, 2008. "Tectono-Sedimentary Models for Rift-Basin Carbonate Systems", Controls on Carbonate Platform and Reef Development, Jeff Lukasik, J.A. (Toni) Simo
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Detailed outcrop study of Miocene platform carbonates along the southwestern margin ofthe Gulf of Suez rift, together with an extensive review of comparable systems elsewhere, has generated new tectono–sedimentary models for marine rift–basin carbonate systems. Both syntectonic and post–tectonic platforms are described for various extensional settings in rift basins. Syntectonic platforms are defined as systems deposited during periods associated with active faulting in rift basins. Post–tectonic platforms are defined as carbonate systems deposited after periods of active extensional faulting that develop over preexisting, fault–generated rift topography. Carbonate systems that occupy transfer zones are also defined, which display the effects of active tectonism and intervening phases of tectonic quiescence. These platform types produce recurring tectono–sedimentological signatures recognizable throughout the strati– graphic record.
The footwall margins to syntectonic platforms generally develop through the tectonic modification of existing platforms and display features associated with relative sea–level fall such as emergence and, depending upon the prevailing climate, meteoric diagenesis. In addition, fault activity truncates platforms that have prograded across the fault line during preceding phases of tectonic quiescence. Hanging–wall dip–slope margins can be complex, inasmuch as they display the effects of hanging–wall subsidence and up–dip footwall uplift. Down–dip of the fault–block fulcrum, depositional sequences are more likely to be bounded by flooding surfaces and display retrogradational to backstepping depositional sequences, while up–dip of the fault–block fulcrum, sequences may form as a result of forced regression driven by relative sea–level fall. In rift–margin areas, clastic influx is likely to play a more important part in tectonically active hanging–wall dip–slope stratigraphies because of the rejuvenation of adjacent footwall highs.
Post–tectonic footwall platforms develop on the uplifted highs of fault blocks during periods of relative tectonic quiescence. Such areas reflect favorable shallow–water sites for carbonate production, because commonly they are isolated from significant clastic drainage fairways. The geometry of a footwall margin is highly variable, because it is inherited from the morphology of the faulted margin. Low– relief footwall margins produce progradational platforms that can bridge the block–bounding fault zone. High–relief blocks produce smaller, aggradational platforms in which the platform margin corresponds to the position of the previous fault scarp. Hanging–wall dip– slope margins commonly have large progradational geometries and typically evolve from ramp–type to rimmed–shelf morphologies through time. Platforms are more likely to form on rotated fault blocks that dip toward the rift margin or upon intrabasinal blocks that are set well away from rift–margin clastic input.
In areas of low clastic flux, transfer–zone platforms occur with either faulted (hard linkage) or low–relief and progradational (soft linkage) margins. Such platforms commonly pass laterally into rift–parallel footwall margins to produce complex platform–margin geometries.
Hydrocarbon reservoirs in platforms from these settings have a range of tectonic, sedimentary, and diagenetic controls, but the 3D models presented form templates that can be used as analogues for exploration and/or production from fault–block carbonate platforms.
The active (syntectonic) and passive (post–tectonic) platform models presented can be used at scales varying from individual depositional sequences to large–scale rift–basin stratigraphies characterized by episodes of tectonic activity and intervening periods of tectonic quiescence.
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Controls on Carbonate Platform and Reef Development
Carbonate platforms and reefs emerge, grow and die in response to intrinsic and extrinsic mechanisms forced primarily by tectonics, oceanography, climate, ecology and eustasy. These mechanisms, or controls, create the physical, biological and chemical signals accountable for the myriad of carbonate depositional responses that, together, form the complex depositional systems present in the modern and ancient settings. If we are to fully comprehend these systems, it is critical to ascertain which controls ultimately govern the “life cycle” of carbonate platforms and reefs and understand how these signals are recorded and preserved. Deciphering which signals produce a dominant sedimentological response from the plethora of physical and biological information generated from superimposed regional to global-scale controls is critical to achieving this goal. With this understanding, it may be possible to extract common time- and space-independent depositional responses to specific mechanisms that may, ultimately, be used in a productive sense. Extensive research on a wide variety of carbonate platform and reefal systems in the past few decades has provided the foundation and understanding necessary to take carbonate research to a new level. With assistance from rapidly advancing computer software and an increasing use of cross-disciplinary integration, carbonate research is shifting from description and morphological analysis towards a science that is more focused on the assessment of process and genetic relationships. The aim of this special publication is to present a cross section of recent research that shows this evolution from a variety of perspectives and scales using examples distributed throughout the Phanerozoic.