Origin and Significance of Middle Ordovician and Mississippian Calcarenites

Abstract

Calcarenites are common throughout geologic history, but the range of depositional settings and conditions under which they form is poorly constrained. This study compares physically structured calcarenites in the Mississippian of the Western Canada Sedimentary Basin, and the Ordovician of the northern Appalachian Basin. Calcarenites in these successions are used to reconstruct paleo-environments and postulate sequence-stratigraphic subdivisions. Mississippian sediments in the Western Canada Sedimentary Basin were deposited on a sub-thermocline mid to outer ramp where wave action predominated, producing oscillatory and combined-flow sedimentary structures. It is interpreted that occasional storm-generated currents produced offshore-migrating dunes. Cyclonic storms appear to have been rare due to the low latitude and west-coast location of the study area. Swell waves were instead more common, reworking sediment across the mid ramp. Comparatively, carbonate sands in the Ordovician Chazy Group are interpreted to have been deposited as a tropical transgressive system with contemporaneous siliciclastic and carbonate sedimentation. A mixed-lithology barrier-island system is interpreted to have migrated landward as relative sea level rose. Low-energy siliciclastic and carbonate sediments accumulated in the back-barrier lagoon and carbonate dune fields formed seaward of the barrier islands, covering lagoonal deposits along a wave ravinement surface. It is interpreted that the subaqueous dunes developed between storm and fair-weather wave bases due to residual tidal currents that were likely amplified by a regional funnel-shaped embayment and local bathymetric constrictions. As flooding continued, mixed wave and tidal, open-marine carbonates buried the subaqueous dune fields. Broader-scale implications can be drawn from both successions. Mississippian calcarenites suggest that swaley cross-stratification can form down to storm (or swell) wave base on carbonate platforms, and is not restricted to the shoreface as it is in siliciclastics. Furthermore, the deposits suggest that swell-wave deposits are also more likely to be produced in carbonate deposits because of in situ grain production. The Ordovician calcarenites are strikingly similar to those along transgressive, wave-dominated, siliciclastic shorelines, suggesting that barrier-island complexes and associated transgressive sand sheets could be an under-appreciated origin for other carbonate sands.