Late Triassic to Middle Jurassic History of the North-Central High Atlas, Morocco

Dieter Karl Letsch

The Lower and Middle Jurassic (Liassic and Dogger) rocks in the north-central High Atlas and on the adjacent Oran Meseta, Morocco, were deposited on the subsiding margin of the Triassic/Jurassic High Atlas trough. This and the Middle Atlas trough formed as a result of rifting of the Moroccan Meseta and Oran Meseta from the Saharan craton during initial stages of the opening of the modern Atlantic. The Tethys seaway flooded these troughs in the early Liassic, resulting in deposition of several thousand meters of Liassic and Dogger limestone and marlstone.

Four major Liassic stratigraphic units were identified and termed Units 1 through 4 from oldest to youngest. Unit 1 consists of intertidal and supratidal skeletal pelletal wackestones, algal boundstones, and claystones located on the Oran Meseta, and shelf-margin skeletal pelletal oolitic packstones and grainstones along the High Atlas margin. Unit 2 intertongues with and overlies shelf-margin Unit 1 and consists of slope and basin-floor mudstones, wackestones, and turbidite packstones, cyclically interbedded with marlstones. Unit 2 is punctuated by base-of-slope slumps and debris beds near the margin of the basin. At the top of Unit 2 is a condensed zone overlain by basin-floor Unit 3 marlstones, which are cyclically interbedded with mudstones, wackestones, and turbidite packstones. oth Units 2 and 3 thicken basinward, off the ancient shelf margin. Unit 4 lies within Unit 3 and consists of a displaced block (3 km by 30 km) of the upper Liassic shelf margin and associated coarse-grained debris, which is thickest on the marginward side of the block.

The deepening-upward Liassic section in the north-central High Atlas reflects the rapid development of the short-lived High Atlas trough, which formed in the Late Triassic-Early Liassic flooding by the Tethys established carbonate tidal flats on the Oran Meseta, a shelf margin at the basin's edge, and slope and basin-floor deposition within the trough. Rapid subsidence of the margin brought slope and basin-floor sediments on top of the platform margin as the trough developed. Subsidence slowed toward the end of the Lias, resulting in progradation of the shelf-margin environments. At the end of the Lias, a portion of the margin slid into the basin, followed by debris shed off the slide scar. Continued marlstone and limestone deposition filled the basin during the Dogger, marking the en of rift-related sedimentation in the High Atlas trough.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.