Depositional Cycles of Lodgepole Formation (Mississippian) in Central Montana: ABSTRACT

Abstract

Detailed stratigraphic and petrographic investigations of lower Madison Group outcrops reveal that the Lodgepole Formation is composed of 5 two-part depositional cycles. The stratigraphically lowest cycle includes the entire Paine Member; the remaining cycles comprise all but the lower part of the Woodhurst Member. Each cycle is characterized by a fine-grained lower unit and a coarser upper unit. The lower unit is dominated by horizontally laminated carbonate mudstones, pellet carbonate grainstones, and finely crystalline dolomites. These lithologies are interpreted to be the deposits of calm, nonturbulent lithotopes. The upper unit of each cycle is characterized by cross-laminated, medium- to coarse-grained, bioclastic and oolitic carbonate grainstones, interpreted to have been deposited in shallow, turbulent environments. From the lithology, sedimentary structures, lateral petrographic and stratigraphic continuity, and modern analogues, these oolitic and bioclastic beds are interpreted to be generally synchronous within individual outcrop belts in central Montana. Regionally, however, cycle-ca ping intervals are probably diachronous stratigraphic units. Facies interpretations of Lodgepole depositional cycles suggest that rocks of the fine-grained lower unit are deposits of deeper water transgressive phases of the Lodgepole sea; lithologies of the upper coarse-grained unit are accumulations of the shallower water regressive phases. End_of_Article - Last_Page 654------------