Condensed Griotte Facies and Cephalopod Accumulations in the Upper Devonian of the Eastern Anti-Atlas, Morocco

Abstract

The Upper Devonian Griotte in the Tafilalt includes several types of thin variegated cephalopod limestones, ranging from a condensation facies to crinoidal limestones and nodular limestones. Excellent outcrops and the high fossil content facilitates reconstruction of depositional history and palaeogeography. Stratification patterns and facies indicate that condensed Griotte and crinoidal limestones were deposited in shallow water, temporarily under turbulent conditions, while nodular limestones may have been formed in a slightly deeper neritic environment.

While the Upper Devonian of the Tafilalt represents a particularly shallow shelf, palaeobathymetric values of the same order of magnitude may be applied to other Palaeozoic Griotte facies and to equivalent Mesozoic facies types: Condensation facies and crinoidal limestones are neritic sediments, deposited on swells or on large (mostly carbonate) platforms in water depths of a few up to about 200 m. The more marly nodular limestones can partly be attributed to the same setting, but may also occur at greater depths (e.g. reef or platform slopes), as indicated by slumpings, resedimented breccias, turbiditic intercalations and by vertical transitions into bathyal sequences (radiolarites, flysch).

Variegated cephalopod limestones are a characteristic member of a geodynamic facies succession documenting slow subsidence of shallow neritic areas (generally carbonate platforms, rarely sandy shelf deposits or volcanic swells) into the bathyal. Insignificant lateral change of facies and an extension over several thousands of km² indicate that the environmental conditions were uniform over large areas.