--> ABSTRACT: Rift Tectonics, Sea Level Rise, Carbonate Sedimentation and Sequence Stratigraphy: Keys to Understanding the Mississippian Lodgepole Limestone in Central Montana, by William F. Precht, Warren Shepard; #91002 (1990).

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ABSTRACT: Rift Tectonics, Sea Level Rise, Carbonate Sedimentation and Sequence Stratigraphy: Keys to Understanding the Mississippian Lodgepole Limestone in Central Montana

William F. Precht, Warren Shepard

A major sedimentary and tectonic feature, the Central Montana Trough stretches east-west across Montana. This trough is an aulacogen that had its inception in mid-Proterozoic time. Tectonic elements of this trough have been periodically reactivated and strongly influenced mid-Paleozoic sedimentation; major stratigraphic units thicken or thin within the rift, often in harmony with the Williston basin.

Throughout Siluro-Devonian time, the site of the aulacogen was a positive feature and minor amounts of sediment were locally deposited. In the latest Devonian, the central part of the aulacogen was uplifted and much of the preexisting Devonian strata were eroded. In the Early Mississippian subsidence resumed. This subsidence, related to reactivation of the rift margin, coupled with eustatic sea level rise, led to the progressive development of the Kinderhookian Lodgepole Limestone.

Sequence stratigraphic response of the Lodgepole shows a strong interrelationship between differential subsidence, carbonate production, depositional setting (shallow vs. distal ramp), and the cumulative changes in relative sea level (accommodation space). These changes formed three distinct members of the Lodgepole, including (1) a transgressive surface marked by the Cottonwood Canyon Member, (2) a transgressive systems tract of the Paine Member that is comprised of deeper-water, bedded carbonate mud-to-wackestones and Waulsortian-type carbonate buildups encased within these rhythmically bedded deposits, and (3) a high-stand systems tract characterized by high-energy, cyclic, shoaling-upward crinoidal grainstones and oolites of the Woodhurst Member. Numerous shallowing-upward, parase uences observed within the context of the larger third-order Vail-type stratigraphic framework are interpreted as allocyclic carbonate sedimentation responses to Milankovich-scale sea-level perturbations.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990