Late Mississippian bryozoan/microbial build‐ups on a drowned karst terrain: Port au Port Peninsula, western Newfoundland

Abstract

ABSTRACTBryozoan and microbial carbonate build‐ups occur within the Late Mississippian (mid‐Viséan) Codroy Group on the Port au Port Peninsula, western Newfoundland. Build‐ups formed only adjacent to a rocky, cliffed, shoreline filling narrow submerged palaeovalleys of a well‐developed Late Devonian‐Early Mississippian karst terrain. The nearshore setting was a stressed environment as indicated by (1) the absence of normal marine biota, such as corals, echinoderms, and calcareous algae, (2) high numbers and low species diversity of dominant taxa (bryozoans, brachiopods, and microbial communities), and (3) abundant plant‐bearing siliciclastics deposited by the episodic influx of fresh‐water from adjacent uplands. Build‐up development was terminated by Meramec time, due to falling sea‐level and seaward progradation of terrestrial sediments.Preserved structural and constructional fabrics within the build‐ups include (1) thickets of bryozoans and microbial microthrombolites, structurally enhanced by probable marine Mg‐calcite and aragonite cements, (2) multigeneration internal sediment, and (3) small colonies of serpulid worms and terebratulid brachiopods. Submarine cementation within the build‐ups appears to have been abundant during arrested sedimentation, whereas intermound sediments (carbonates and siliciclastics) were lithified only during burial diagenesis. Shallow‐burial fracturing, stylolitization, sulphide and sulphate mineralization, and precipitation of phreatic, iron‐poor, calcite cement occurred during burial diagenesis.Codroy build‐ups are distinct from the more common Mississippian mud mounds. Their internal characteristics and the geologic setting are more like other Late Palaeozoic and Holocene bryozoan‐rich build‐ups.