Graded limestones and limestone-quartzite couplets: Possible storm-deposits from the Moroccan Carboniferous |
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| Authors: | Gilbert Kelling Peter R Mullin |
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| Institution: | University of Wales, Swansea Great Britain |
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| Abstract: | The Lower Carboniferous (Visean) successions of the Meseta region of central Morocco are dominated by mixed marine carbonate and terrigenous facies bearing the biological and sedimentological hallmarks of deposition in a neritic environment. This study concerns two inter-related types of sedimentary unit, relatively uncommon but displaying features not readily compatible with the depositional mechanisms conventionally envisaged for such an environment. They are described with reference to three sections in which they are well represented.Graded limestones are texturally immature, broadly lenticular or channel-fill units of calcarenite and calcirudite of variable thickness, exhibiting a general upwards diminution in modal or maximum grain size and a somewhat irregular sequence of internal structures, often including large-scale cross-stratification. The compound units, here termed limestone-quartzite couplets, generally comprise a lower calcarenite member, frequently graded, succeeded by more quartzose arenite, often but not invariably finer in grain size. Six main types of couplet are distinguished primarily on the nature of the contact between the carbonate and terrigenous members.From a review of published data on modern and ancient shelf deposits it is concluded that periodic storm-generated mechanisms of transportation and deposition are of considerable geological importance. Two main categories of storm-induced modification of “normal” sedimentation processes are recognised: (1) storm-stirring and erosion; and (2) lateral transfer of storm-mobilised sediment. A process-response model is developed which involves the introduction of coarse sediment by storms on to a substrate of finer indigenous sediment. Comparison of the theoretical sequences produced with the types of couplets and graded limestones commonly encountered in the field strongly indicates that these anomalous associations result from rapid, storm-induced addition of coarse carbonate detritus (from local elevations such as reefs and shell-banks) to slightly deeper environments in which muds and quartzose sands were being deposited by more protracted processes. |
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