VITRINITE REFLECTANCE AND STABLE ISOTOPE COMPARISON OF AUTOCHTONOUS AND ALLOCHTONOUS COAL DEPOSITS IN THE MORROWAN (PENNSYLVANIAN) OF NORTHWEST ARKANSAS

Coal stringers and seams occur within the marine Brentwood Member and the terrestrial Woolsey Member of the Bloyd Formation (Pennsylvanian-Morrowan) of northwest Arkansas. Both coals are high-volatile A bituminous in rank and are closely associated stratigraphically. A coal stringer in the marine Brentwood Member probably represents a saturated log that was transported into the shale environment from a terrestrial source farther north. The Baldwin coal seam of the overlying terrestrial Woolsey Member is autochthonous, deposited as terrestrial environments supplanted marine environments in northwest Arkansas. The type Brentwood Member represents maximum flooding and high stand conditions associated with the initial transgression of Morrowan seas across the southern Ozark region. Maximum flooding in the Brentwood is represented by a thick basal black shale. The upper Brentwood is a combination of black shale and dark, high energy, quartz-bearing carbonates that thin up depositional dip (shoreward). The coal stringer discovered in the basal shale is the first, high rank occurrence of coal reported from the Brentwood Member. The Woolsey Member represents regression of the Morrowan seas and is marked by coastal plain environments. Organic matter settled out and was buried in this fine grained, silt-interbedded shale, forming the Baldwin coal, present as two or more seams within the member. The uppermost Baldwin coal defines the top of the Woolsey Formation. The stable isotopic composition of carbon and oxygen in the coal was characterized at the University of Arkansas Stable Isotopes Laboratory. Results accrued provided information on the sources of plant material constituting the coal as well as on the depositional and burial history. Isotope data enabled documentation of whether the transported coal of the Brentwood Member was derived from terrestrial environments similar to those that developed later during deposition of the Woolsey Member.