Rift Tectonics and Limestone Sedimentation: Jurassic of the Central and Eastern High Atlas, Morocco

John E. Warme, Beverly H. Hazlett, Paul D. Crevello, Dieter K. Letsch, Randolph B. Burke

The central and eastern High Atlas ranges of southern Morocco represent deposition in an Early to Middle Jurassic rift which first collected continental basalts, red beds, and evaporites. Carbonate deposition was initiated by a euxinic phase, followed by a mosaic of normal marine limestones and marls controlled by regional subsidence and local differential fault-block movements and overprinted by global sea level changes.

The High Atlas is now an en echelon series of high-angle reverse faults, creating abrupt and discontinuous fault-bounded ridges separated by broad synclines. Facies relationships of the Jurassic carbonates show that the faults were originally synrift normal faults, probably transtensional, now structurally reversed.

A north-south geotraverse demonstrates the symmetry of parts of the Jurassic rift: a linear central basin platform was flanked north and south by turbidite-filled deeps and outward in turn by slope-shelf-shore paleoenvironments. In the lower and middle Lias, the central basin platform exhibits an upward-shallowing sequence capped by upper Pliensbachian giant sponge-algal buildups. Turbidite-filled half-grabens, dipping away from the central platform, contain immense olistostromes broken from the shelf edges. Shoreline and shelf environments include coralgal reefs and cyclic lagoonal beds. Subsequently all paleogeographic elements were drowned by a Toarcian sea level rise, producing rhythmically bedded deeper-water limestones and marls across the basin.

Overlying Aalenian to Bajocian shelf limestones prograded into an axial seaway filled with thick marls, punctuated in the Bajocian by horizons of spectacularly exposed coralgal reefs which appear structurally isolated on separate fault blocks. Sedimentation eventually outpaced subsidence, culminating in continental deposits as the sea finally retreated in the mid-Dogger.

Although abundant potential source rocks of this rift are thermally overmature, the basin serves as a well-exposed model for comparison with carbonate-filled rifts elsewhere.

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