Mid-Triassic to Early Liassic clastic/evaporitic deposits over the Maghreb Platform

Abstract

The development of Triassic to Lower Liassic clastic/evaporitic series over the epicratonic Maghreb Platform is closely associated with the eastern opening of a Tethyan marine domain between Africa and Europe. West of the platform, Morocco became separated from North America in Late Triassic times by rifting along the axis of the Proto-Atlantic Ocean. In addition, NE–SW and ENE–WSW trending Atlasic half-grabens formed, essentially in Morocco, as part of a Late Triassic/Early Liassic Atlas rifting episode. This is the tectonic context in which the red bed to evaporite sequences were deposited. A first depositional pattern is illustrated by the areally extensive onlapping of Upper Triassic clastic/evaporitic series over the Algerian Saharan Platform. From the Carnian to the Norian, distal mudflats and evaporites graded westward (i.e. landward) into proximal fluvial facies. Seven stratigraphic cycles formed part of a general base-level rise of a major retrogradational clastic-to-evaporitic cycle, indicative of general tectonic subsidence of the Algerian Platform as a whole. The evaporites are thus interpreted as markers of persistent onlapping occurring at the edge of the Basin as clastic influxes gradually diminished. A second depositional pattern is provided by Ladinian to Rhaetian clastic–evaporitic–carbonate transgressive sequences of the Atlasic domain in Tunisia. These terminate characteristically in marine facies. In this domain, intense tectonic activity was consistently offset by a high rate of evaporitic or clastic sedimentation in shallow-water environments. A third depositional pattern is illustrated in the Moroccan High Atlas. In the central part of this range, Carnian red conglomerates and sheet-like sandstones of a distal braided fluvial plain were deposited in tectonic troughs that linked together as part of a rift system. Episodic connections occurred with the Tethyan domain to the northeast. In the western High Atlas, an Upper Carnian/Norian sequence several hundred meters thick displays a general trend from clastics to evaporites, reflecting the development of arid paleoclimatic conditions. The Moroccan Meseta provides examples of clastic/evaporitic depositional patterns within intracratonic troughs. Clastic filling of rift-induced troughs in the Oujda Mountains exhibits significant facies and thickness variations, ranging from local proximal alluvial and distal anastomosed channel systems through to dominant mudflat systems. In the >1000-m-thick series of the 60-km-long Khemisset Basin, facies range from red beds, located mainly along the basin margins, to salt-dominated deposits in the basin center. Increasing accommodation controlled the deposition of evaporites and the maximum flooding surface is to be found in typical halite deposits with interbedded bands of clays. At that time, greater tectonic subsidence rates allowed more marked, episodic, marine incursions. Clastic/evaporitic series from the passive margin of the Moroccan Atlantic coast are known only from bore-holes and onshore and offshore seismic data. Recent papers have reported interesting sections across strongly subsiding grabens or tilted blocks related to predominantly SSW–NNE faults.