--> ABSTRACT: Termination of Carbonate Platforms: Eustatic Fluctuations in Base Level, by Paul Crevello; #91003 (1990).

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ABSTRACT: Termination of Carbonate Platforms: Eustatic Fluctuations in Base Level

Paul Crevello

Various processes can lead to the termination of carbonate platforms: tectonic foundering, eustatic drowning or exposure, local environmental stress, or changes in sedimentary regimes. In Liassic carbonate platforms of the High Atlas rift, Morocco, both tectonic foundering and eustatic base-level shifts are recorded in the platform stratigraphy. The stratigraphic signature of the eustatic base-level fluctuations reported here recorded subaerial exposure, transgressive sequences, and drowning of platforms.

Regressive carbonate sequences record base-level lowering and exposure in the late Domerian. The sequences consist of progressive seaward shifts in facies belts, upward thinning in cycles and cycle bundles, cycle skipping (i.e., a decrease in number of cycles per bundle), and an increase in intensity of exposure features.

Transgressive carbonate sequences mark the renewal of marine deposition across the platforms in early to middle Toarcian. These carbonates are comprised of amalgamated, noncyclic, top-truncated (i.e., erosional upper surfaces), subtidal, open-marine, oolitic skeletal-rich (corals and megaladonts) lithofacies. Exposure surfaces separating subtidal lithofacies are evidence of fluctuating eustatic base level.

Drowning of the platform along the Sahara craton is marked by the change to low-energy deposits containing the bivalve Gryphaea sp., which passes abruptly upward into downlapping(?) ammonite-bearing (middle Toarcian, Bifrons zone, to Aalenian) marine shales. Over 100 m of marine shales were deposited over the Liassic platform before carbonate platform deposition resumed.

Drowning of an isolated axial-rift platform, Jebel Bou Dahar, is represented by only a thin (2-3 m) condensed sequence [middle Toarcian(?) to early Aalenian] of glauconitic ammonite-brachiopod floatstones. A consequence of the drowning and sediment starvation of the Bou Dahar platform was the development of an angular onlap unconformity, which was formed by the steeply dipping Domerian slope facies being progressively onlapped by middle Toarcian to lower Aalenian basin-filling shales and distal carbonate turbidites sourced from the Saharan platform to the south. The condensed sequence on Bou Dahar and the onlap drowning unconformity are evidence of base-level rise and termination of platform growth by sediment starvation in the rift axis.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990