Genesis of thick, fine-grained calcite hardgrounds in the upper part of the Early to Late Cretaceous Natih Formation (Wadi Muaydin, Oman Mountains)

We studied two fine-grained calcite hardgrounds (Hg1, Hg2) from the upper part of the Early to Late Cretaceous Natih Formation. Both hardgrounds occur in a 60-m-thick succession of limestones, comprising the Natih members C, B and the basal part of A. The hardgrounds of interest are positioned in the members C (Hg1) and B (Hg2) and typified by abundant borings of homogenous distribution, filled with dolomite. They occupy the upper part of a mudstone to wackestone bed (Hg1) and a wackestone bed (Hg2), respectively, above which the grain size increases. Field work, petrographic, microfacies and cathodoluminescence analyses, allowed us to shed light on the their unusual great thickness (75 and 100-120 cm, respectively) as the thicknesses of such fine-grained horizons may generally represent barriers for calcite-precipitating fluids. The logged section represents a protected/lagoonal environment with a relatively low sedimentation rate as indicated by the hardgrounds, peloidal limestones and bioturbation-related nodular bedding. The hardgrounds contain sponge spicules, which we did not encounter elsewhere. The two hardgrounds formed by minor sediment aggradation of locally produced mud-rich sediment, in which near-surface cementation was able to keep pace with aggradation. These conditions were met due to the Cretaceous calcite sea water composition, tropical climate, relatively low relative sedimentation rate and relative sea-level rise shifting the depocenter landward. Episodic sea-level rise was caused by regional plate convergence. Slab-pull and pulsed thrust-loading caused down-bending of the platform twice and formation of the two hardgrounds. Temporary down-bending events were followed by isostatic rebound.