The Codroy Group (Upper Mississippian) on the Port au Port Peninsula, western Newfoundland: stratigraphy, palaeontology, sedimentology and diagenesis

Abstract

Upper Mississippian sediments of the Codroy Group on the Port au Port Peninsula, western Newfoundland, are remnants of a once extensive cover that mantled a karsted palaeoridge of Cambro-Ordovician strata. Today, the sediment's are patchily preserved within karst depressions, and adjacent low-lying basins. The present topography of the peninsula is, in part, an exhumed Early Mississippian karst landscape featuring a variety of karren, karst valleys and caves. The Mississippian relic karst has been locally rejuvenated. -- Codroy strata constitute three coeval lithofacies: (1) marine carbonates, mainly bioherms, and interbedded fluvial sandstones, (2) marine evaporites, primarily gypsum/anhydrite, with minor, laminated limestone and fluvial elastics, and (3) alluvial-fan conglomerates and braided-stream sandstones. Prograding fluvial and alluvial sediments subsequently buried the marine basins. -- Fauna and sedimentology indicate that the marine sediments were deposited in a schizohaline environment adjacent to a, well-drained landmass. Preserved miospores and plant debris in the limestones, elastics, and gypsum indicate that plants, though not abundant, grew on the slopes of the ridge. Late Mississippian seas along (the northern and western margins of the Port au Port ridge supported a prolific community of brachiopods, bryozoans, blue-green algae, molluscs, and ostracodes. Cephalopods, foraminifers, and conodonts are low both in species and individuals, whereas true corals, and crinoids/echinoids are absent. Fauna traditionally used to differentiate between Upper and Lower Codroy strata are found to occur together within the marine carbonates, but, in two distinct facies. This indicates that the marine macrofauna, and probably microfauna, are facies-controlled prohibiting their use in detailed biostratigraphlc zonation. -- Marine carbonates infilling palaeokarst valleys may constitute either bryozoan/algal biolithites plastered against the valley walls and/or carbonate mounds of a similar lithology, with associated intermound sediment. The buildups were lithified early as evident by the abundant synsedimentary cement (interpreted to have been magnesium, calcite and aragonite). -- The complex diagenetic history of the marine carbonates records progression from a marine environment with synsedimentary cementation to a phreatic zone within which occurred fracturing, stylolitization, dissolution and cement precipitation. Mineralization by sulphides and sulphates spans the phreatic diagenetic history. Late stage phreatic cement is pervasive in all other sediments of the carbonate lithofacies as well as throughout the carbonate and clastic sediments of the other lithofacies.