Growth faults affecting depositional geometry, facies and sequence stratigraphy record on a carbonate platform edge (South Vercors Urgonian platform, SE France)

The first two calcarenite units at the base of the Urgonian limestones on the southern edge of the platform bear different depositional geometries depending on place (Cirque d’Archiane to Montagnette and Rocher de Combau). The lower calcarenite unit (Bi5 of Arnaud H. 1981. De la plate-forme urgonienne au bassin vocontien. Le Barrémo-Bédoulien des Alpes occidentales entre Isère et Buëch (Vercors méridional, Diois oriental et Dévoluy). Géologie Alpine, Grenoble, Mémoire 12: 3. Disponible sur https://tel.archives-ouvertes.fr/tel-00662966/document), is up to 200 m thick and shows three different patterns, in terms of accommodation space, from the western Archiane Cirque to the Montagnette to the east. On the western side of the Cirque, the unit begins on slope fine-grained limestone with thin sigmoïdal offlap geometry, suggesting little available space after a relative sea level fall. It is overlain by thick progradational/aggradational, then purely aggradational calcarenite capped by a coral and rudist-bearing bed. This bed is, therefore, interpreted as a maximum (although moderate) flooding facies. The depositional geometry is different on the eastern side of the Cirque, where a progradational pattern in the lower part of the unit is interrupted by a rotational movement affecting the depositional profile. The deformation promoted aggradation updip and retrogradation downdip as a result of starvation. The inferred growth fault updip (thought to be responsible for the change) began to function earlier at the Montagnette, explaining the huge calcarenite clinoforms found there, filling a deeper saddle created in the depositional profile. The same fault probably was reactivated later during the deposition of the overlying, thinner Bi6-1 unit, which appears at Rocher de Combau with an uncommon tidal facies at the base. A rotational bulge, created by the inferred growth fault, would have protected a small area behind it to spare the local calcarenite deposition from the waves for a while. These two examples show that sequence stratigraphic interpretation may differ from one place to the other, and even show opposite trends due to this kind of disturbance.

The Lower Cretaceous carbonate slope-to-platform-margin succession near Khatt, United Arab Emirates: sedimentary facies and depositional geometries

ABSTRACT The Jurassic and Lower Cretaceous carbonate succession exposed near Khatt provides exceptional conditions for the investigation of sedimentary facies and depositional geometries in a carbonate slope-to-platform-margin setting. A coarsening-upward sequence in Lower Cretaceous limestones indicates decreasing depth of deposition and platform progradation. A pronounced shedding of sediments containing reefal fragments occurs in a slope environment with a well exposed basin-to-platform transect. The carbonate succession consists of mudstone, wackestone, grainstone, coarse rudstone with conglomerate/breccia interbeds, and framestone at the top. The depositional architecture is characterized by the abundance of massive sheet- or channel-like limestone bodies within thinly bedded and generally uniform strata. Quantitative analysis of many carbonate channel deposits and their geometries measured in outcrop led to the distinction of two major types. Type I channel deposits are thin (0.3 to 5 m) but massive, and are commonly irregularly shaped in cross-section. They are as much as 200 m wide. Type I channel deposits are characterized by a wide size range of skeletal and non-skeletal carbonate components. Type II channel deposits, by contrast, are more regularly bedded and have much larger thickness-to-width ratios, in general close to 1:10. Furthermore, they are composed of packstone to grainstone calciturbidite sediments. As with some sheet deposits, they can be correlated through most of the 5.5-km-long Khatt outcrop. Stratigraphically, however, their occurrence is very much restricted, indicating significant alternation of depositional styles as a consequence of changing carbonate platform production and changing sedimentary environments. The data presented here can serve as input for 3-D geological modeling of equivalent depositional environments in the subsurface. They can also be applied to object-based deterministic and stochastic facies modeling.