Sedimentological and petrological analysis of a section of the Thumb Mountain formation carbonates, Cornwallis Island, Nunavut, Canada

Abstract

The study section of Thumb Formation carbonates (Mid-Upper Ordovician) exposed on Cornwallis Island Nunavut, Canadian Arctic Archipelago is characterized by medium- to thickly- bedded units consisting of lime mudstone intercalated with wackestone. The sedimentology and petrology of a fifteen metre section was examined using field data, thin sections, polished sections and hand samples. Allochems include an abundance of disarticulated and minimally fragmented ostracods together with rarer brachiopods, corals, gastropods, bryozoans, algae, trilobites, and crinoids. The study section displays cyclic shallowing upwards sequences, interrupted by periods of nondeposition in the form of hardgrounds. Fenestral pores occur in the top intervals of the cyclic sequence. Examination of the carbonate matrix, cements and diagenetic properties indicate a movement from a marine to freshwater phreatic environment before deep burial. Ichnological data and microfacies determination agree with this proposal. The section is interpreted as a shallow water marine succession of diagenetically altered rocks deposited in a protected tidal to intertidal environment during a phase of periodic platform flooding and marine transgression. The presence of both open and restricted platform facies in the study area further constrains the extent of the shallow-water depositional environment that persisted on the Arctic carbonate platform during the Mid - Late Ordovician, allowing more precise articulation of the palaeogeography, tectonic and paragenetic history of the area.