Abstract: Recent and Cretaceous Methane "Vent" Communities as Analogs for Dickinson Field's Mississippian Lodgepole Mound (Stark County, North Dakota)

LONGMAN, MARK W.

The Lower Mississippian Lodgepole carbonate buildup reservoir at Dickinson Field is about one mile in diameter and 300 ft thick. It has been described as being a Waulsortian mound and the flank facies around the buildup are typical of such mounds, consisting of skeletal wackestones to grainstones in debris beds containing common fragments of fenestrate bryozoans, crinoids, and sponge spicules. The mound "framework," however, consists of marine-cement, microbial (bacterial) boundstones, and stromatactis cavities. Ostracods are common in the mound framework, but interbedded limestones composed of crinoid and bryozoan debris, also occur. The microbial limestones and odd ostracod-rich layers are interpreted as "vent" communities that formed around methane and CO[2] seeps on the Lodgepole sea floor. These gases had a biogenic origin and probably escaped along fracture trends during compaction of underlying black shale in the Bakken Formation, which is unusually thick beneath the Dickinson mound. When the gas vents periodically became inactive, crinoids and bryozoans colonized the microbial boundstones at Dickinson Field.

Possible modern analogs for the Lodgepole "vent" communities occur in the Gulf of Mexico where methane seeps form over salt domes in more than 1000 ft of water. These modem methane-based communities consist of bacteria, mussels, and tube worms. Gas hydrate forms nodules in and under bacterial limestone crusts and may contribute to forming stromatactis structures.

Upper Cretaceous methane vent communities in the Pierre Shale of the southern Denver Basin form mounds tens of feet high consisting mainly of marine cement and microbial limestones. These mounds, aligned along regional lineament (fracture) trends, are known as "tepee buttes." Molluscs are common around the Cretaceous methane vents, but are absent in the Lodgepole vent communities. Ostracods may have filled the niche of the molluscs.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado