Dolomitized crinoidal grainstones locally form the upper part of the Mississippian Madison Group that unconformably underlies Jurassic strata in the Disturbed belt, northwestern Montana. Surface exposures of these grainstones exhibit a significant vuggy and intercrystalline porosity (4 to 12%) and are permeable (6 to 12 md). Many of the pores are filled with dead oil.

The porous crinoidal grainstone unit at the top of the Madison has undergone eogenetic secondary dolomitization, probably in Late Mississippian time, and hence prior to any significant erosional events. Porosity most likely resulted from solution effects during erosion and was fully developed before deposition of the Jurassic strata. Phreatic calcite cement partly occludes some of the pore space and developed after migration of liquid hydrocarbons into the grainstone unit.

Variations in thickness of the grainstone unit are mainly the result of pre-Jurassic erosion. In places the grainstones are completely eroded beneath the Jurassic rocks but, where present, they thicken to more than 100 m as observed in a north-south direction along the strike of imbricate thrust slices of the pre-Tertiary section. These thickness changes resulted either from broad warping of the Mississippian strata followed by planar erosional truncation, from erosional relief on the Jurassic erosion surface carved into unfolded Mississippian strata, or from some combination of these two effects which may have different geographic trends. Location of thickness maxima of the grainstone unit in each of several thrust plates enables correlation of the thickness patterns in an east-west irection from thrust sheet to thrust sheet. Projections of such patterns into the subsurface will be a valuable guide for exploration in the Disturbed belt.

End_of_Article - Last_Page 757------------