ABSTRACT: Rift Tectonics and Eustatic Overprint, High Atlas of Morocco

John E. Warme, Paul D. Crevello, Manfred Hauptmann

The central and eastern High Atlas of Morocco coincide with an east-west aulacogen filled with 5-10 km of Liassic and Dogger limestones and marls, and structurally inverted in the Tertiary.

A mosaic of carbonate environments evolved and migrated within first-order rift subsidence, lasting for 25 m.y., and over individual fault blocks foundering and rotating. Deposition was complicated by synsedimentary transtensional wrenching, but exhibits a pattern of fault blocks representing medial platforms flanked outward by troughs, shelves, and shorelines.

Tests for eustatic vs. tectonic control on deposition include (1) evidence for longer term sea level changes that affect all rift blocks, overriding their independent motions, and (2) evidence for shorter term cyclic deposition within the Milankovitch scale. Because carbonate deposition is sensitive to sea level, stacked facies represent a hierarchy of relative sea level changes.

In the High Atlas, limestone deposition is everywhere overwhelmed by Toarcian marls that overstep the basin margins. These marls are related to a significant second-order circum-Mediterranean or global sea level rise. These marls also separate major lower and upper shallowing-upward sequences of approximately 10 m.y. duration.

Within these major sequences, lower orders of deposition typically are cyclic over all tectono-stratigraphic blocks, and are regarded as controlled by Milankovitch-scale sea level and/or climatic oscillations: shelfal shallowing-upward cycles, turbidite limestone-marl cycles, and midbasin pelagic limestone-marl cycles. These lower order cycles are each up to a few meters thick and commonly are arranged into cycle bundles that internally wax and wane by thickening and thinning, deepening and shallowing, and varying in proportions of limestone and marl.

With the exception of the Toarcian drowning, biostratigraphic control on the High Atlas is not yet refined enough to confidently correlate depositional cycles of any scale with published eustatic sea level histories for the Jurassic.

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