Blomeier, D. and Reijmer, John (2002) Facies architecture of a Lower Jurassic carbonate platform slope (Jbel Bou Dahar, High Atlas, Morocco). Journal of Sedimentary Research, 72 (4). pp. 463-476. DOI 10.1306/111501720462.

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Abstract

The plateau of Jbel Bou Dahar (High Atlas, Morocco) represents an outstanding example of a well-preserved early Mesozoic carbonate platform. Field observations of the large-scale sedimentary structures and the arrangement of lithofacies associations within the mid- and lower slope were combined with detailed microfacies analyses and gamma-ray measurements. The investigations were carried out in order to (1) resolve the sequence stratigraphic architecture of the slope, and (2) reconstruct the sediment export patterns and depositional processes related to high-frequency sea-level variations.

The slope strata are arranged into two different types of lithofacies associations that are bounded by characteristic discontinuities and show a specific occurrence along the investigated slope section. The steep (8-18°) mid-slope is dominated by thick- to medium-bedded limestone successions that were able to maintain a high angle of repose due to their detrital composition. The sediment packages are bounded by basal low-angle unconformities and wedge out within the lower slope or thin into the adjacent basin as single carbonate layers. The sediments are characterized by coarse-grained and poorly sorted floatstones to rudstones, which show low gamma-ray values and contain components derived mainly from the platform edge. The facies associations were interpreted as debris-flow deposits that were exported during sea-level lowstands when the flattened platform top and extended parts of the margin were exposed and sediment production was restricted mainly to the outer margin and upper slope section.
In contrast, the gently dipping lower slope (4-18°) is characterized by rhythmic limestone-marl alternations that show increasing thicknesses with increasing distance from the platform. From the lower part of the mid-slope down, the successions onlap the debris-flow deposits, forming a characteristic interfingering pattern of different lithofacies associations. The medium- to thin-bedded carbonates of the limestone-marl alternations comprise two different microfacies types: (1) mudstones to wackestones with high gamma-ray values and a mixed shallow-marine to deep-marine microfacies, overlain by (2) well-sorted, arenitic packstones to grainstones with low gamma-ray values and a shallow-marine component composition. The latter points to the platform interior and margin as primary source areas. The shallow-marine material was exported during sea-level highstands when the carbonate factory included the entire platform margin and the flooded lagoons of the platform top. During these periods, the exported fine-grained sediments could maintain only low angles of repose. Thus they bypassed the steep upper and mid-slope sections, developed into turbidites during downslope transport, and were finally redeposited on the gently dipping lower slope and in the adjacent basin.

The architecture of the investigated slope area shows a characteristic interfingering of different facies associations that can be interpreted as genetic highstand and lowstand stratigraphic sequences. The arrangement of these sequences along the slope, as well as their composition, were controlled by high-frequency sea-level fluctuations, affecting the primary sediment production (source area), export patterns, and sedimentary processes, resulting in a shift of the depositional centers during different sea-level stands.

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