Deposition and diagenesis of the Mississippian Lodgepole Formation, central Montana
Jenks, Susan Elizabeth
Wilson, James Lee
Master of Arts
The lower Mississippian Lodgepole Formation is exposed in central Montana in the anticlines which form the Big Snowy and Little Belt Mountains. Four sections averaging 130 feet in length were measured at the base of the Woodhurst Limestone, the uppermost member of the Lodgepole. Three of the sections were located in the western end of the Big Snowy Mountains. These were composed of two major bioclastic and ooid grainstone units, and a succession of mudstones, wackestones, packstones and argillaceous dolomites and pellet grainstones and pelleted mudstones. Field, faunal, and petrographic evidence indicate these rocks were deposited in very shallow water, the grainstones in the form of carbonate sand shoals, the remaining rock types in a broad lagoon behind the shoals. One section was measured 70 miles to the west in the Little Belt mountains. Rocks here consist of crinoid grainstones and packstones, skeletal and ooid grainstones, mudstones, bryozoan packstones and wackestones, and calcareous shales. Evidence suggests these rocks formed down paleoslope from those in the Big Snowys, some of the sediments being deposited in deeper water in a normal marine shelf environment. A number of diagenetic processes affected the sediments after deposition. Morphology and distribution of cements and evidence of timing relative to other diagenetic events indicate cementation of the carbonate sands took place in the intertidal or shallow subtidal environment soon after deposition. Dolomitization of crinoid debris with magnesium derived from the high magnesian calclte of the crinoid skeletons themselves also took place very early in the history of the sediment. This was followed by silicification and pyritization of skeletal debris and ooids. Another, more extensive period of dolomitization occurred which affected micrite and, to a lesser extent, ooids and tended to avoid skeletal material and spar. The last diagenetic event to affect these rocks was compaction and stylolitization filled remaining pore space as coarse blocky calcite spar.