ORIGIN OF LOWER MISSISSIPPIAN SEDIMENTARY SEQUENCES IN NORTH AMERICA

Mississippian formations and members were examined in Alberta (Banff and Pekisko), Montana (Madison), Arizona (Redwall), Iowa (Burlington and Keokuk), and West Virginia (Greenbrier). These units are correlative and show the same shapes and marker beds. The carbonate types change laterally with the facies.

Quartz sand was left by the Tamoroan transgression created by erosion of the Acadian Orogen. Diachronous limestone beds occur above. The first shelf with isochronous sequences (Bighorn) developed in the Bighorn Mountains, just south of carbonate mounds at the shelf margin in the Big Snowy Range. The inner shelf in Alberta and Iowa is made of diachronous oolitic limestone beds.

The overlying beds are rhythmic bands of silty or fine grained limestone, a response to orbital climatic change as sealevel rose due to shelf sedimentation and accretion pf extraterrestrial material.

The second shelf consists of the lower 3 cycles in the lower Woodhurst as proposed by J.L. Wilson. Thick crinoidal or oolitic beds of a high stand interglacial stage alternate with silty beds with millimetere lamination and mudcracks of the low stand glacial stage. A shelf margin of carbonate mounds extends along the western and southern craton margin, a response to subsidence of the continent margin as the plate drifted from the oceanic rise. The margin thins from 100 to 2 meters from Crown Mountain, MT to Crowsnest Pass, Alberta.

The Massive crinoidal Pekisko beds mark the end of the silt supply as the sea covered the craton source.