ABSTRACT

Lower–Middle Ordovician platform carbonates are important oil and gas reservoir rocks in the Ordos Basin of the North China craton (NCC). In recent years, numerous cores have been recovered from these units, but the depositional environments and facies architecture of this large carbonate platform system have not yet been reported in detail. This study integrates sedimentological and magnetic fabric data collected from outcrops and cores to document variation in microfacies and facies associations within the Shanganning carbonate platform (SCP), infer paleowind directions and the paleogeographic location of the SCP, and evaluate tectonic and paleogeographic controls on platform development. This analysis documented 25 microfacies belonging to 7 microfacies fabric and sediment types: carbonate mudstone, wackestone, packstone–grainstone, evaporite, bindstone–framestone, siliciclastics, and dolostone. Microfacies can be grouped into facies associations that represent tidal-flat, open-platform, restricted-platform, evaporite-platform, platform-margin, slope, and deep-basinal depositional environments. These environments are mainly distributed zonally around the L-shaped Qingyang paleo-uplift on the western and southern margins of the SCP. Anisotropy of magnetic susceptibility measurements at five platform-margin locales yielded paleocurrent directions of (all approximately) 318°–348°, 51°–81°, 42°–72°, 80°–110°, and 54°–84° from paleo-north after correction for a 120°–150° clockwise rotation of the NCC since the Ordovician. The depositional architecture of the SCP depended on a combination of tectonic and paleogeographic (wind) influences. Local tectonic movements controlled the spatial distribution of facies associations around the Qingyang paleo-uplift, which was characterized by open, restricted, and evaporite-platform facies in the platform interior and mainly slope and basinal facies on the outer-platform margin (to the west and south of the Qingyang paleo-uplift). Vertical tectonic movements resulted in cyclic submergence and emergence of the platform over its approximately 20-m.y.-long formation history. Paleo–trade winds influenced aspects of facies zonation across the platform, including growth of metazoan reefs on the windward platform margin and formation of oolitic grainstone and microbial reefs on the leeward platform margin. The facies model for the SCP developed in this study should provide a useful reference framework for future carbonate oil and gas exploration in Lower–Middle Ordovician units of the Ordos Basin. Furthermore, this study makes innovative use of magnetic anisotropy data, in combination with standard petrographic, core, and field data, to discern facies patterns and controls on a paleo-carbonate platform, which is an integrated methodology that may serve as a model for future studies of other carbonate platforms.

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