Abstract
Three stratigraphic sections of the Callovian Tuwaiq Mountain Formation at Shaib El-Hisyan, Khashm Al-Giddayah and Dirab, near Riyadh City, central Saudi Arabia have been studied to distinguish their microfacies features and the equivalent probable depositional environments. Lithostratigraphically, the Tuwaiq Mountain Formation is subdivided into two lower and upper Tuwaiq members. The part that informally is known as upper Tuwaiq Member is subdivided from base to top into T1, T2, and T3 members, respectively. The microfacies analysis of the Tuwaiq Mountain Formation led to the recognition of 12 microfacies types (FT1-FT12) that are grouped into three associations: (1) open platform facies association (FT1-FT5); (2) high-energy shoals of ooids and patch reefs facies association (FT6-FT9); and (3) restricted carbonate platform facies association (FT10, FT12). The lower part of the Tuwaiq Mountain Formation is characterized by relative abundance of wackestone and wacke/packstone facies that are capped by thin peloidal molluscan floatstone facies. The former indicates a quiet open platform environment with periodic agitation, while the later indicates deposition in a sand shoal environment. The facies of basal unit of the upper Tuwaiq Mountain Formation, T1, are enriched with dense micrite matrix and shell fragments of epifaunal and deep infaunal organisms (e.g., Pholadomya spp.). The microfacies pattern of Baladiyah Member (T1) indicates a quiet open platform with low and sometimes high water circulation. The Maysiah Member (T2) is composed of hard limestone with abundance of coralline microfacies in framestone, grainstone, and packstone textures suggesting a high-energy shallow reefal environment with periods of quiet open platform conditions. This shallow reefal environment becomes more restricted southward, where shales and algal dolomitic grainstones were formed. The Daddiyah Member (T3) that made up of chalky limestone with chert intercalations is predominated by matrix-support lime mudstone, wackestone, and packstone facies. These microfacies reflect open platform with slightly low circulation below fair-weather wave base with period of restricted marine conditions forming rudstone microfacies. Southward, these environments became shallower and subjected to considerable quartz influx in an agitated sand shoal.