ABSTRACT: Controls on Inception, Growth, and Termination of Lower Devonian (Emsian) Carbonate Mudstone Pinnacles, Northern Tafilalet Basin, Morocco

AITKEN, S. A., J. C. HOPKINS, and C. M. HENDERSON, University of Calgary, Calgary, Alberta, Canada

More than 30 carbonate mudstone pinnacles developed on the outer margin of a carbonate ramp in northern Tafilalet basin during the Emsian (as determined by conodonts). The pinnacles consist dominantly of carbonate mudstone cores flanked by nodular mudstone to crinoid packstone beds and are circular, steep-sided, conical structures up to 60 m high and 100 m in diameter or occur as elongate complexes up to 100 m high and 1000 m in length.

Pinnacles were initiated at different stratigraphic horizons on low-amplitude mounds of carbonate mudstone colonized by branching corals and pelmatozoans rather than at one horizon on hardgrounds as suggested in other studies. Some pinnacles were smothered shortly after inception; others aggraded and attained significant relief above the adjacent ramp. Pinnacle growth was characterized by syndepositional submarine cementation and lithification that allowed mudstone to packstone flank beds to accumulate on depositional slopes up to 55% as indicated by present morphology and brachiopod geopetals. Southern slopes are steep, whereas northern slopes are more gently dipping, producing an asymmetrical outline.

Pinnacle growth was terminated by drowning and influx of fine-grained siliciclastics represented by shales, which skirt and onlap pinnacles forming an angular drowning unconformity. Significant changes in conodont biofacies and faunas from pinnacles into overlying shales support the drowning nature of this cover and suggest that siliciclastic deposition occurred for an extended period of time. Four shallowing-upward carbonate parasequences cap the pinnacles. Sediments of the youngest parasequence locally filled karst cavities and vugs formed at the tops of pinnacles during exposure and erosion.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)