EVOLUTION OF A HIGH-RELIEF CARBONATE PLATFORM SLOPE USING 3D DIGITAL OUTCROP MODELS: LOWER JURASSIC DJEBEL BOU DAHAR, HIGH ATLAS, MOROCCO

摘要

Seismic-scale continuous exposures of an Early Jurassic carbonate platform located in the southern High Atlas of Morocco provide detailed quantitative information about the lithofacies and stratal geometries of an aggrading, retrograding, and prograding steeply inclined slope system that evolved in an active intracratonic rift basin. Aggradational, backstepping, and progradational outer-platform to slope transects are each characterized by distinct lithological features and stratal patterns. A digital outcrop model was constructed using real-time kinematic global positioning system and lidar, storing information on recorded stratal surfaces including lithofacies information. From these data 3D models of the slope system could be built. In the study area, four stages of slope development are recognized. Stage 1 corresponds to a low-relief carbonate platform characterized by widespread sub-wave-base depositional conditions. Stage 2 developed as an aggrading to retrograding platform, during which the platform built considerable relief. In addition, a massive organically bound upper slope fringe formed, dominated by coral-sponge-microbial boundstone. The in situ boundstone consisted of irregular wedges that accreted on the upper slope down to similar to 140 in below the platform break. After a period of sediment starvation Stage 3 involved a major backstepping of sediment deposition. Seventy-meter-high clinoforms prograded from the center of the platform across the platform top until reaching the relict platform break (of Stage 2). Subsequently, progradation occurred basinward, as indicated by the spilling of lobes down the existing slope deposits of Stage 2. Limited progradation, along with the reestablishment of the in situ coral-sponge-microbial boundstone on the upper slope, led to the progressive steepening of the slope profile to dips of 23 degrees. Clinoforms are planar in the upper part of the slope, and concave upwards in the lower part; with a total relief of over 460 in. Stage 4 is observed only in the lower slope and adjacent basin. Coarse detrital carbonate deposits form lens-shaped aprons that grade basinward into kilometer-scale lobes that alternate with thick wedges of basinal fallout sediments. Syndepositional extensional faults operating in the subsurface affected sedimentation along the lower slope profile throughout the four stages and are expressed in the stratal patterns through growth strata in monoclinal folds. Three-dimensional digital outcrop models reconstructing the architecture of the slope system allowed quantification of slope geometries and stratal relationships and spatial distribution of sedimentary bodies. The digital model enhanced the understanding of the depositional changes during slope evolution and also led to observations on, for example, angular relationships that would have been difficult to detect without the digital approach. The evolution from aggrading to retrograding, prograding, and retrograding was strongly influenced by synsedimentary tectonics within an intracratonic rift basin. The tectonics generated irregular distribution of accommodation space, variably interfering with eustatic sea-level movement. This in combination with the character of lithofacies types and style of sedimentary processes along the slope, such as in situ boundstone growth and gravity-driven resedimentation processes, directly controlled carbonate slope architecture and stratigraphy of the Djebel Bou Dahar and influenced stratal anatomy and lithofacies distribution.