ALLUVIAL ARCHITECTURE OF THE UPPER POTTSVILLE FORMATION, LACKAWANNA COUNTY, PENNSYLVANIA

A roadcut exposing the upper part of the Pennsylvanian Pottsville Formation within the Lackawanna syncline, west of Scranton, PA, was examined to determine shifts in fluvial architecture by documenting changes in lithology, sedimentary structures and fluvial stacking patterns. The upper Pottsville succession grades vertically into the coal-bearing measures of the Llewellyn Formation and records the transition from a channel-dominated system into finer-grained coal swamp deposits.

Approximately 80 m of section from the upper Pottsville Formation was measured up to the Dunmore #4 coal bed which is just below the contact with the overlying Llewellyn Formation. In addition, photomosaics were created from roadcut photographs to trace erosion surfaces and to record lateral changes in sandbody style. The succession consists of blue-gray, quartz-pebble conglomeratic sandstone facies ranging from 1.5 m to 9 m thick. The sandstones are medium-grained, planar-tabular and trough cross-bedded. The sandstones often contain conglomeratic lenses at their base with well-rounded, spherical pebbles up to 2 cm in diameter. A brown mudstone facies is primarily in the lower parts of the section and is 1 – 2 m thick. Vertically, the finer-grained units grade into gray, finely-laminated shales and culminate in black carbonaceous shales and anthracite coal. The geometry of these sandstone and mudstone facies is an alternation of couplets consisting of stacked sheet sandstones with overlying channel-shaped bodies that pinch out laterally. Within the roadcut there are three of these couplets that vary in thickness from 5 m to 60 m.

The couplets are interpreted in terms of shifting rates of aggradation within the fluvial system. The lower sheet sandstones represent channel fill under conditions of lower aggradation and higher rates of channel migration. The transition to the overlying channel-shaped bodies would indicate a higher rate of aggradation. Whether these changes are attributable to autocyclic or allocyclic processes cannot be determined. However, the overall shift of the facies from coarser-grained within the Pottsville Formation to finer-grained in the Llewellyn Formation may reflect basinal changes in the accommodation to sediment supply ratio.