Abstract

A fundamental control on the evolution of carbonate platforms is accommodation, the space available for sediment deposition. Here we adopt a sensitivity-analysis approach, using the process-based forward model CARB3D+, to quantitatively investigate the sedimentary response of isolated carbonate platforms to individual accommodation drivers of eustasy, subsidence, and surface dissolution. Sediment production, the mechanism invoked to fill accommodation, is also investigated.

Fourth-order Milankovich-driven sea-level cycles dominate icehouse carbonate sedimentology, with 400 ky cycles modulating the effect of 100 ky cycles of comparable magnitude when both cycles are present. Both overwhelm higher-frequency, lower-amplitude fifth-order cycles, which are rarely preserved on the platform top. Subsidence is a fundamental control on sedimentary stacking patterns, sequence-boundary frequency (SBF), and platform geometry, with rapid subsidence increasing the preservation of platform-top sequences. Most icehouse platforms are subject to extended periods of platform-top exposure. "Missed beats," when the platform top remains emergent at a sea-level maximum, are common and create non-Milankovich SBFs. Furthermore, during platform emergence, surface lowering via surface dissolution also creates accommodation for the subsequent platform flooding event, with surface dissolution rates in humid climates comparable to moderate rates of subsidence. However, accommodation generation by surface dissolution is self-limiting, in as much as accommodation generated by dissolution is rapidly refilled on flooding.

Sediment production is a key control on unfilled accommodation, which our simulations suggest is considerable on icehouse platforms. Increased shallow-water (< 3 m depth) sediment production enhances intra-platform bathymetric relief between the margin and interior; this is driven in large part by a higher sedimentation rate at the margin than in the interior. Platforms can be flooded to depths up to 45 m during marine transgressions, and accommodation often remains unfilled. There are two distinct phases of platform-top sedimentation. During the transgression and early highstand, platform margins accumulate at a greater rate than interior sediments, generating differential intra-platform bathymetry. Subsequently, accommodation in the interior fills by forced regression and progradation from the margins into the platform interior. Numerical forward models can augment our understanding of icehouse carbonate-platform evolution by identifying the nature of the sedimentary response to individual controls. They can thus contribute to improving characterization of carbonate reservoirs in icehouse platforms, such as the large hydrocarbon fields in the North Caspian and S.E. Asia.

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