TIDAL INFLUENCE NEAR THE TERMINUS OF A BRAIDED FLUVIAL SYSTEM, MORROWAN (PENNSYLVANIAN) OF NORTHWEST ARKANSAS

The middle Bloyd sandstone is an informally named, terrestrial unit within the dominantly marine Bloyd Formation (Morrowan, Pennsylvanian) of northwest Arkansas. The unit accumulated on a broad, gently sloping ramp that extended southward into central Arkansas prior to development of the Arkoma foreland basin during middle Pennsylvanian time. Normal faults, active during basin subsidence, displaced the middle Bloyd into the subsurface of the Arkoma basin and restricted the outcrop belt to the structural shelf north of the basin. Sedimentological studies on the northern part of the structural shelf concluded that the middle Bloyd was deposited on a broad braid plain by south-flowing stream systems. Sedimentary patterns recorded on the northern part of the structural platform and the transition to the northern Arkoma basin differ in character.

In the southern zone, the middle Bloyd ranges in thickness from 14 to over 34m. The unit is divided into genetic and repetitive intervals that begin with an erosion surface overlain directly by tabular, cross-stratified sets up to 2m in thickness. They are succeeded by thinner sets of trough cross-strata that grade upwards into intervals of ripple-laminated sand with thin shale interbeds. Trough cross-stratified sets in numerous occurrences display bidirectional foresets. These genetic intervals contrast sharply with intervals on the northern part of the structural platform that are dominated by numerous erosion surfaces, thick tabular cross bed sets and abundant quartz granule and pebble conglomerates. Intervals of ripple laminated sand are rare. Shale is absent and bidirectional cross-bed sets have not been observed. Middle Bloyd strata on the northern part of the platform were deposited in broad braided stream valleys without tidally influenced processes. Braided stream sedimentation continued across the southern part of the platform, but alternated between pulses of stream-dominated sediment supply characterized by coarse tabular sets at the base of genetic sequences and tidally dominated intervals characterized by small scale trough cross-stratified sets, bidirectional current indicators and ripple-laminated intervals interbedded with shale.