Models for circulation patterns in epicontinental seas applied to Paleozoic facies of North America craton


  • Brian J. Witzke


Circulation in epicontinental seas develops in response to thermohaline, wind-driven, and tidal motions in various climatic, geographic, and bathymetric settings. Mass balance relations in a seaway are established by the relative rates of outflow, inflow (freshwater and ocean water), and evaporation, which constrain two contrasting styles of epicontinental circulation. Quasietuarine circulation (QEC) is established where rates of freshwater influx exceeds evaporation. Surface outflow from a seaway with QEC typically is balanced by inflow of oceanic water at depth. A density stratification can be established below the limits of vertical mixing. A bottom layer of oxygen poor or anoxic water may develop in a seaway with or without an entrance sill if the bottom waters are derived from the oxygen minimum zone of the open ocean or if organic productivity exceeds the limits of aerobic respiration. Upwelling may be promoted in specific epicontinental settings with QEC. Antiestuarine circulation (AEC) is developed in arid climates where evaporation exceeds freshwater influx. Evaporative concentration in the surface layer causes oxygenated water to sink. Bottom outflow of denser, more saline water is balanced by surface inflow from the open ocean. A sill or barrier may inhibit bottom circulation in seas with AEC, permitting development of a basal brine layer. Stagnation in the brine layer may lead to oxygen depletion. Seas with AEC commonly are characterized by extensive carbonate-evaporite facies. Geographic and stratigraphic syntheses of lithofacies and biofacies provide the empirical basis for interpretations of seaway dynamics in ancient epicontinental settings. Examples from the Upper Ordovician (Maquoketa formation) and Upper Devonian (Lime Creek-Sweetland Creek formations) of cratonic North America are interpreted in the context of stratified QEC patterns. In each case, oxygen poor to anoxic waters appear to have impinged along the basin slope below the margin of an oxygenated carbonate shelf. Upwelling along the shelf margin in the Maquoketa example helps explain the distribution of phosphatic sediments. A stratified AEC pattern is interpreted for the Gower formation (Silurian) of eastern Iowa. A barrier to bottom circulation at the mouth of an embayment in the epicontinental sea resulted in a basal layer of hypersaline water devoid of shelly benthos. Surface water salinities increased progresively shoreward within the embayment, but benthic organisms flourished where carbonate mounds built upward into the oxygenated layer.