Abstract

The Bloyd Formation (Morrowan) of northwest Arkansas is a heterogeneous succession of sandstone, limestone, shale, and siltstone units that accumulated in shallow-marine and non-marine environments during Early Pennsylvanian time. Sedimentation occurred on a gentle paleoslope inclined to the south. The basal Brentwood Limestone Member is overlain in extreme western Arkansas by coal-bearing shale and siltstone strata of the non-marine Woolsey Member. Eastward, Woolsey strata pass into a thick and laterally extensive sandstone body characterized by abundant sets of tabular cross-strata, medium coarse grain size, and beds of quartz pebble conglomerate. The unit is bounded by a marked unconformity that displays up to 6 ft (2 m) of erosional relief on the underlying Brentwood Member.

Erosion surfaces occur throughout the unit and bound genetically significant intervals characterized by a consistent succession of sedimentary structures. Quartz-pebble conglomerates resting directly on erosion surfaces are overlain by sets of large-scale tabular cross-strata. Thin intervals of ripple-laminated sandstone containing well-developed ripple bed forms with occasional clay drapes overlie the tabular sets and are bounded above by erosion surfaces. These genetic intervals are repetitive throughout the unit and reflect sedimentation by initially competent but rapidly waning current systems. Paleocurrent measurements derived from tabular cross-strata indicate that the sediment was emplaced by south-following unidirectional currents characterized by low dispersion.

The succession of sedimentary structures contained within genetic sequences and paleocurrent data indicates that the middle Boyd Sandstone body accumulated in several south-flowing braided stream systems in northwest Arkansas.