Jurassic Midbasin Platform, Central and Eastern High Atlas, Morocco

John E. Warme

The central and eastern High Atlas ranges of southern Morocco coincide with a Mesozoic 100 × 500-km east-west rift. Upper Triassic to lower Liassic continental red beds, evaporites, and basalts, unconformably overlying Paleozoic metasediments, comprise basin-wide initial synrift deposition. These are abruptly overlain by up to 40 m of Hettangian or lower Sinemurian black, laminated, unfossiliferous limestones deposited in a euxinic hypersaline and/or stratified sea. Continued syndepositional transtension differentiated the embryonic seaway into a mosaic of fault bocks comprising an axial midbasin platform symmetrically flanked to the north and south by turbidite troughs, basin-margin shelves, and shores.

The midbasin platform serves as an exposed model for structurally analogous platforms in oil-producing basins such as west Texas. It has a unique Liassic upward-shallowing limestone sequence approximately 450 m thick, now widely exposed by en echelon ridges upthrusted over a minimal axial area of 20 × 150 km. The basal laminated euxinic facies (potential source rock) becomes interbedded upward with bioturbated limestones, indicating cyclic presence of normal marine bottom waters. Overlying strata are completely bioturbated nodular limestones with rare diminuitive shelled benthos and chert nodules resembling sponge morphologies. The upper 250 m of the sequence exhibit increasing bed thickness, abundance and diversity of shelly benthos, and frequency and size of sponge-algal mud mo nds (potential reservoirs), indicating overall shallowing but generally below wave base. The sequence culminates in giant sponge-algal buildups with up to 200 m of relief, abruptly capped by a starved surface rich with ammonites related to a late Liassic (Toarcian) eustatic sea level rise. This surface is overlain by 1,000 + m of medium-bedded, deep-water limestone-marl cycles (potential seals) representing Toarcian drowning of the platform and another 2-4 km of limestones and marls deposited until the mid-Dogger when the rift filled and became terrestrial.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.