SWELL-DOMINATED CARBONATES ON A MISSISSIPPIAN RAMP IN THE CANADIAN ROCKY MOUNTAIN FRONT RANGES

摘要

The Livingstone Formation and the middle Opal Member of the Mount Head Formation of western Canada are Mississippian carbonate deposits, tens to hundreds of meters thick, that are interpreted to have accumulated in a mid-ramp, wave-dominated, sub-thermocline environment, at water depths up to 100 m or more. The rocks are predominantly pelmatozoan grainstones and packstones that contain abundant physical sedimentary structures. The most common sedimentary structures are planar lamination, swaly and hummocky cross-stratification, and normally graded event beds that are interpreted as having been formed by oscillatory (wave) currents or oscillatory-dominated combined flows. The abundant oscillatory currents are interpreted to have been generated by swell waves and to a lesser extent by local storms. High-angle, planar-tabular cross bedding is also present in rare, discrete beds produced by unidirectional offshore-directed currents. The unidirectional currents that produced the dunes are interpreted to have been generated by coastal set-up that produced downwelling storm currents during the rare storms that occurred along the Mississippian coastline. It is hypothesized that local storms were not common due to the west-coast location and the tropical latitude of the study area. Instead it is postulated that swell waves, approaching from the broad Panthalassa Ocean to the west, were responsible for most sediment remobilization and deposition across the mid and outer ramp. Swell waves are believed to have resuspended sediment to greater depths than short-period, less-organized storm waves. These Mississippian deposits suggest that in situ grain production on the ramp, without need for the offshore transport of sediment from the shoreface, resulted in two major differences from siliciclastic wave-dominated settings: 1) swaly cross-stratification was not restricted to environments shallower than hummocky cross-stratification on the upper shoreface, because sand-size sediment was generated across a large portion of the ramp; and 2) aggradation by means other than sediment supply from shallower water caused preservation of deep-water swell-dominated deposits.