Stratigraphic Relationships of the Brentwood and Woolsey Members, Bloyd Formation (Type Morrowan), Northwest Arkansas
The Brentwood Member of the Bloyd Formation conformably overlies the Prairie Grove Member, Hale Formation in the type Morrowan succession of northwestern Arkansas. At its type locality, the Brentwood is separated from the underlying Prairie Grove Member by nearly 6 m of dark shale. Away from this area, the shale thins rapidly and the Hale-Bloyd boundary may be placed with difficulty. At some localities east of type section, the boundary is thought to be erosional rather than the more typical gradational contact. The Brentwood consists of discrete carbonate bodies separated by dark shales. The carbonates consist principally of open shelf deposits, such as crinozoan biosparites and oolites. All carbonate lithologies contain varying amounts of fine to medium grained, rounded, quartz sand. Regionally, the Brentwood Member becomes more shaly to the west of its type locality and loses the quartz sand content in its carbonate lithologies. To the east, the Brentwood shales become less prominent and quartz sandstone intervals characterize the succession. The Woolsey Member overlies the Brentwood Member and consists of light to dark, argillaceous shales with occasional sandstones, carbonates, and a thin coal. The coal is confined essentially to Washington and northern Crawford counties and never exceeds 45.7 cm. The shales are thought to be of terrestrial origin, but marine fossils and thin limestones in the lower part of typical Woolsey strata indicate a transitional change within the member rather than at its base. In contrast, the detrital fraction of the upper Brentwood carbonates seem to decrease rather than increase towards the Woolsey contact. To the west of the type area, the Woolsey gives rise to marine shales and carbonates. To the east, the Woolsey is equivalent to a thick, quartz pebble-bearing sandstone of fluvial origin. The top of the Woolsey Member is a regional unconformity with the overlying Dye Shale Member, Bloyd Formation.
McGilvery, Thomas A. and Berlau, Charles E.
"Stratigraphic Relationships of the Brentwood and Woolsey Members, Bloyd Formation (Type Morrowan), Northwest Arkansas,"
Journal of the Arkansas Academy of Science: Vol. 34, Article 24.
Available at: https://scholarworks.uark.edu/jaas/vol34/iss1/24