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ABSTRACT: Tectonic and Eustatic Controls on Carbonate Platforms of the Jurassic High Atlas Rift of Morocco

Paul Crevello

Stratigraphic studies of Lower and Middle Jurassic carbonates along the southern margin of the High Atlas rift document five major stages of platform development, a response to rift tectonics and eustasy. The five stages are as follows.

(1) Early Sinemurian marine transgression over Triassic-Liassic continental red beds and basalts lead to the development of regionally extensive, cyclic carbonate platforms.

(2) Middle to late Sinemurian marked the initiation of Liassic rifting, with synrift platforms restricted to the rift margin and to localized horsts within the rift axis. Synrift platforms developed diverse depositional systems, with a marked change to rimmed margins flanked by steep slopes and deep (400-500 m) marine basins. Two orders of cyclicity shallowing-upward cycles and bundles of cycles dominated the platform tops.

(3) Late Pliensbachian subaerial exposure, resulting in termination of Liassic platform development, was recorded by regressive seaward shifts in facies belts, microkarstification, and deposition of continental red beds across the platforms.

(4) Early to middle Toarcian transgression, yielding landward shifts in facies belts on platform tops, was signified by deposition of noncyclic, skeletal carbonate sequences.

(5) Middle Toarcian platform drowning was followed by deposition of Toarcian-Aalenian amonite-bearing marine shales (100 m thick) blanketing (downlapping, width source from the south) the southern platform, whereas only a condensed sequence (2-3 m thick) of glauconitic, ammonite-skeletal carbonate strata covered the submerged, isolated, axial rift platform of Jebel Bou Dahar. Prolonged sediment starvation on Bou Dahar combined with depositional onlap of its slope by basin-filling shales (Toarcian to Bajocian) and distal carbonate turbidites (Aalenian), also sourced from the southern margin of the rift, produced a drowning onlap unconformity. Eventually, Bajocian marine shales blanketed the drowned Bou Dahar platform.

Thus, synrift tectonics controlled platform and basin configuration, depositional facies trends, and, in part, platform margin morphology. In contrast, termination of the Liassic platforms and the internal stratigraphic development of small-scale platform cyclicity was strongly influenced by eustatic base-level fluctuations.

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