Sedimentation-calcrete cycles in the Mesozoic Red formations from the central High atlas (Telouet area), Morocco

Abstract

In the Telouet-Ighrem area (southern side of the central High Atlas, Morocco) the Mesozoic “Couches Rouges” or red formations are composed of conglomerates, sandstones and siltstones organized into upward-fining sequences locally capped by swamp deposits. Epigenetic calcretes developed within these formations, subsequent to indeindependent of the initial sedimentation. Several types occur, controlled mainly by the permeability and structures of the original sediment: nodules, prismatic pillars, small columns, etc. These carbonate bodies appear in field sections as metric to plurimetric, commonly overhanging, lenses or beds. Siliceous micro-features are locally associated with the carbonate accumulations. These calcretes are the result of polyphase epigenetic processes among which surface weathering in a semi-arid context is predominant. During the wet season, carbonates in the upper part of the profile dissolve and migrate downwards. By the end of the wet season and the beginning of the dry season solute concentration occurs which induces hydrolysis of quartz and silicates (including clay minerals). During the dry season palygorskite develops and carbonates are precipitated. These pedogenic processes may have succeeded localized and ephemeral palustrine processes related to observed lateral water circulation or to an exposed water-table, the latter being particularly sensitive to exceptional events (e.g. heavy rainfall) in this type of climate. The accumulation of detrital deposits followed by the epigenetic wrocesses which occur according to the sometimes partial sequence of calcitization-palygorskitization-dolomitization-silicification, characterize “sedimentation-calcretization cycles”. The thickness of the carbonate bodies depends on the stability of the environment, which is probably controlled by climate and tectonics (Michard, 1976; Jenny et al., 1981; Jacobshagen, 1988). A new insight is thus provided on the Mesozoic palaeogeography of the central High Atlas.