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Abstract:

Carbonate platforms and reefs are more common in foreland basins than is generally appreciated and may provide a better record of basin evolution and relative sea-level change than siliciclastic strata. Carbonate platform and reefal facies may develop in the proximal foredeep on a variety of topographic highs, in the distal foreland area far from terrigenous influx, or across the entire foreland basin during tectonically quiescent stages of basin development. Basin geometry, dispersal of siliciclastic sediment, subsidence patterns, and deformation structures across the foreland affect carbonate facies distribution and platform morphology during synorogenic stages of foreland basin evolution.

Synorogenic foreland carbonate platforms typically have ramp profiles that mimic the flexural profile produced by tectonic loading. During active convergence, the flexural profile is driven toward the foreland by the advancing orogenic wedge. Synorogenic carbonate ramps are forced to onlap and/or backstep cratonward. Basinward parts of some foreland carbonate platforms may be drowned (sensu stricto) during active convergence, especially when: (1) the underlying lithosphere has low rigidity; (2) the orogenic wedge advances rapidly; (3) a eustatic sea-level rise occurs at the same time as migration of the flexural profile; or (4) some other environmental stress affects carbonate-producing benthos. Two-dimensional forward models show that flexural drowning of some Phanerozoic carbonate platforms, even in the absence of a coeval eustatic sea-level rise or other environmental stress, is possible in less than 250,000 yr.

In some foreland areas, complex patterns of synorogenic differential subsidence and foreland deformation can affect carbonate facies tracts many hundreds of kilometers cratonward of the proximal foredeep. These patterns of differential subsidence and deformation probably reflect the response of preexisting basement structures or rheological anisotropies in the foreland area to tectonic loading along the plate margin or the response to sublithospheric processes. Quantitative subsidence analyses from some foreland areas suggest that differential subsidence in the distal foreland is related temporally to tectonic loading along the continental margin, but cratonward limits of the differential subsidence are beyond reasonable limits of flexurally produced subsidence. In addition, patterns of differential subsidence in the distal foreland do not have "normal" flexural wavelengths, amplitudes, or orientations with respect to the orogenic wedge. Therefore, while subsidence in the distal foreland is temporally related to convergence along the plate margin, alternative models for lithospheric deformation are necessary to explain the differential subsidence in the distal foreland.

Siliciclastic sediment dispersal is another first-order control on carbonate sedimentation in foreland basins. Coarse-grained, siliciclastic sediment may have less affect on suspension-feeding, carbonate-producing benthic organisms than clay and silt. Hence, bedrock geology, paleoclimate, and depositional gradients in the hinterland and foreland sides of the basin indirectly affect carbonate sedimentation. Effects of siliciclastic sedimentation on foreland carbonates also depend on the evolutionary stage of a foredeep. During "underfilled" stages of basin evolution, siliciclastic sediment is trapped in the proximal foredeep and will not affect carbonate-producing benthos on the distal side of the basin. The distal parts of clastic wedges may fill accommodation during "intermediate" stages of basin evolution and provide a substrate for carbonate platforms that prograde from the peripheral bulge. Progradation of siliciclastics during later "overfilled" stages of basin evolution may terminate carbonate platforms even in the distal foreland.

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