Abstract

The upper Peechee Member of the Ancient Wall reef complex that is well exposed on the southeast margin of Mount Haultain represents the end of the first main depositional cycle of this complex. The uppermost part forms a 15–30 m thick carbonate sequence that extends basinward over deeper water fore-reef detritus and is divisible into three distinct layers: the lower two consist mainly of stromatoropoid and coral bioherms and biostromes and associated calcarenites and calcilutites; the uppermost consists of five small micrite and wackestone bioherms 7.5–22 m long and 4–6 m high; three of these grew around and on top of a 30 m wide stromatoporoid–coral biostrome. Frame-building organisms include laminar and hemispherical colonies of Phillip-sastrea, renalcid algae (often associated with small fenestral cavities), encrusting calcareous algae (Sphaerocodium), and laminar stromatoporoids. Renalcid encrustations of micrites and wackestones on the vertical sides and undersides of bioherms indicate that early submarine cementation was also significant in forming these rigid structures. The bioherms formed during deepening water conditions with agitation and bioerosion too gentle to apron them with detritus but sufficient to fragment and disorient fragile skeletal elements. The sharp contacts and the lack of interfingering with basin strata indicate that the bioherms were drowned before burial by basin calcareous shales.

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