Cyclic Ramp-to-Basin Carbonate Deposits, Lower Mississippian, Wyoming and Montana: A Combined Field and Computer Modeling Study

Abstract

ABSTRACT The Lower Mississippian Lodgepole/lower Madison Formations (20-225 m thick) developed along a broad (> 700 km), storm-dominated pericratonic ramp. Three types of fifth-order upward-shallowing cycles 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 located along the outer ramp consist of basal sub-storm wave base limestone-argillite, overlain by storm-deposited limestone, which are capped by hummocky stratified to massive skeletal-ooid grainstone. Deep subtidal c cles pass downslope into ramp-slope facies composed to rhythmically interbedded limestone and argillite, with local deep-water mud mounds; no upward-shallowing cycles occur within the ramp-slope facies. Average cycle durations calculated along the outer ramp are between 30-110 ky. The fifth-order cycles are stacked to form three third- to fourth-order depositional sequences, which are recognized by regional transgressive-regressive facies trends and cycles stacking patterns. Ramp-margin wedges (RMW) developed during long-term sea-level fall and lowstand and consist of cyclic crinoidal bank and oolitic shoal facies, which pass downdip into deep subtidal cycles. Transgressive systems tracts (TST), which onlapped the ramp during long-term sea-level rise, include thick shallow and deep subtidal cycles; peritidal cycles are restricted to the inner ramp. Highstand systems tracts (HST) developed during long-term sea-level highstand and fall, and along the ramp are composed of early HST shallow subtidal cycles overlain by late HST peritidal cycles; shallow through deep subtidal cycles characterize the HST along the ramp-slope. Two-dimensional computer modeling of the cyclic sequences suggests that for the assumed water depths of facies, fifth-order sea-level oscillations of at least 20-25 m were required to generate deep subtidal cycles along the ramp-slope. Synthetic sequences run with fifth-order sea-level amplitudes End_Page 1194----------------------- and TST deposition, a feature not observed in the actual sequences. Other factors, in addition to fifth-order sea-level oscillations, likely played a role in generating synchronous peritidal and deep subtidal cycles during HST deposition. These factors may include short-term climatic changes, which influenced the depths to storm-wave reworking. The moderate-amplitude sea-level oscillations suggested by the cyclic sequences may reflect the initial effects of Carboniferous glaciation that was occurring in Gondwana.