Development of cyclic ramp-to-basin carbonate deposits, lower Mississippian, Wyoming and Montana

Abstract

The Lower Mississippian Lodgepole/lower Madison Formations (20-225 m thick) developed along a broad (>700 km) stormdominated cratonic ramp. Three types of shallowing-upward cycles (5th order) are recognized across the ramp-to-basin transition. Peritidal cycles consist of very shallow subtidal facies overlain by algal-laminated tidal flat deposits, which are rarely capped by paleosol/breccia layers. Shallow subtidal cycles consist of stacked ooid grainstone shoal deposits or deeper subtidal facies overlain by ooid-skeletal grainstone caps. Deep subtidal cycles occur along the outer ramp and ramp-slope and consist of sub-storm wave base limestone-argillite, overlain by graded limestone, and are capped by storm-deposited skeletal-ooid grainstone. They pass downslope into rhythmically interbedded limestone and argillite with local deepwater mud mounds; no shallowing-upward cycles occur within the ramp-slope facies. Average cycle periods calculated along the outer ramp range from 30-110 k.y. The cycles likely formed in response to 5th order (20-100 k.y.) sea level oscillations.

The cycles are stacked to form three 3rd to 4th order depositional sequences which are defined by regional transgressive-regressive facies trends. The ramp margin wedge (RMW) developed during long-term sea level fall lowstand conditions and consists of cyclic crinoidal bank and oolitic shoal facies which pass downdip into deep subtidal cycles. The transgressive systems tract (TST),which onlapped the ramp during long-term sea level rise, includes thick deep and shallow subtidal cycles; peritidal cycles are restricted to the inner ramp. The highstand systems tract (HST) developed during long-term sea level highstand and fall, and along the ramp is composed of early HST shallow subtidal cycles which are overlain by late HST peritidal cycles; shallow through deep subtidal cycles composed the HST along the ramp-slope.