Cross-bedding and scour in sandstone of the Logan Formation (Lower Carboniferous) of Jackson County, Ohio
Sandstone & pebbly sandstone in the Pennsylvanian of Ohio, USA. (Synthesized from info. provided by several geologists during the 2003 Annual Field Conference of the Great Lakes Section, Society of Economic Paleontologists and Mineralogists): The Lower Pennsylvanian Sharon Formation is a 10-15 meter thick, ledge-forming, erosion-resistant unit. The Sharon is paleovalley-filling in places, so it is thicker than 10-15 meters in some spots. The jointing patterns of the Sharon Formation allow for 3-D examination around large blocks of outcrop - can see the 3-D architecture of sedimentary structures. The Pottsville Group lies over a major unconformity, which was formed by eustatic sealevel fall & erosion. The Sharon Formation is the basal unit of the Pottsville sediments over this unconformity. In terms of the tectonic setting, this is in the Appalachian Foreland Basin. What influenced sedimentation and sediment supply of the Sharon Formation during the Early Pennsylvanian? Probably a migrating forebulge and Early Pennsylvanian climatic changes. The Sharon is correlatable with the Olean Conglomerate in Pennsylvania. Both the Sharon and the Olean are time-equivalent to the Tumbling Hill Member & the Huylkill Member of the lower Pottsville Formation of central Pennsylvania (both of those members are below the major unconformity in Pennsylvania, unlike in northeastern Ohio). The Sharon Conglomerate/Formation & the Olean Conglomerate were deposited under strong north-to-south paleoflow conditions. About twelve lithofacies can be seen in the Sharon Formation in the Akron, Ohio area. The Sharon Formation is dominantly conglomerate and sandstone, with lots of sedimentary structures. It is light on fine-grained materials. The Sharon has horizontally bedded gravels, cross-bedded gravels (including trough and tabular cross bedding), deformed/overturned cross-bed sets, basal scours up to 2 meters deep (but typically 0.5 to 1 meter deep; scours are backfilled by dune/bar back migration), whole channel fills, chute fills, and gravel bar platform deposits (usually 1-2 meters thick in the Sharon; these include bar head deposits, bar core deposits, bar tail deposits, and bar margin deposits - can usually use the presence of imbricated clasts to ID bar-head & bar-core portions of gravel bar platforms, but in the Sharon, clasts are mostly spheroidal, so it is difficult to tell specific portions of gravel platforms here). In the gravel-rich Sharon deposits, get calculated average bankfull depths of 2.1 meters, 19.9 meter average paleochannel widths, and 34.3 meter maximum paleochannel widths. Get different numbers for the sandy Sharon deposits. The Sharon is typically more conglomeratic at the base & more sandy near the top. The Sharon’s interpreted depositional environment is gravel & sand bedload streams. Paleovalleys underneath the Sharon Formation were formed when the subsidence rate was greater than the sediment supply. Paleovalley backfilling (i.e., Sharon deposits) occurred when the subsidence rate was less than the sediment supply. The change in fluvial style seen in Sharon deposits is probably due to filling & overtopping of paleovalleys. Beds of the Sharon Formation are usually cliff-forming. The Sharon in the Akron area consists of quartz-pebble conglomerate & quartzose sandstone & pebbly quartzose sandstone & sandy quartz-pebble conglomerate & some lenses or thin intervals of granulestone. The basal Sharon is conglomeratic - the “lower conglomerate”. An “upper conglomerate” can be seen in places - it is usually quite thin (1-2 pebbles thick in places), and in some places, it splits into two horizons; in some places it’s not there at all. Pebbles are almost entirely white vein quartz, with an uncertain source from the north. Detrital muscovite in the Sharon has been dated to about 370 and 406 Ma (Devonian), so the source area includes Acadian Orogeny materials. The Sharon has relatively common cross-bedding, with a few overturned cross-beds visible in areas. Abundant iron oxide staining is present in the Sharon sandstones, with a variety of morphologies - this can weather out as resistant ridges or as 3-D surfaces. Many vugs have thick goethite linings. Many goethite-stained quartz pebbles are present. Seeps & springs occur sporadically along the sandstones of the lower Sharon Formation in places. These spring waters have widely variable pH and TDS (total dissolved solids). Some dry springs are present - conduits without water emerging. A few places in basal Sharon strata have obvious rip-up shale clasts, derived from uppermost Meadville Shale beds (below the Mississippian-Pennsylvanian unconformity). One outcrop is known with many Meadville Shale clasts mixed in with Sharon quartz pebbles - this appears to represent paleobank failure of Meadville material during near-earliest Sharon deposition. The outcrop shown above is at Virginia Kendall Ledges in Cuyahoga Valley National Park. Virginia Kendall Ledges is an isolated platform of Sharon Formation, surrounded by a lower land surface of Lower Mississippian Cuyahoga Formation shales & siltstones & sandstones. The lower Sharon Formation at this site is quite pebbly - many pebble-filled channelform features are present. Upon 3-D examination of their architecture, these are not channels or chutes, but are interpreted by Professor Neil Wells as bar confluence scours with subsequent pebble fills. The edges of the Virginia Kendall Ledges platform have large Sharon blocks separating from the rest of the platform. Abundant overturned recumbent cross beds are present - some of the world's best developed and best exposed examples. The mechanism by which crossbeds get overturned seems straightforward (unidirectional shear by fluvial currents), but the cause is not clearly understood - some cohesive agent may be required? Someone suggested biomats. Some of the scour pits in this area seem to have fairly steep margins - perhaps whatever cohesive agent was responsible for simple deformation of crossbeds was also responsible for overly steep, stable margins of depressions/chutes/channels/scours. Shown above is "Ice Box Cave", a natural crevice extending far back into a sandstone cliff face. The opening is gated off to prevent vandalism by hominids. Stratigraphy: Sharon Formation (also known as Sharon Sandstone or Sharon Conglomerate or Sharon Member), lower Pottsville Group, upper Lower Pennsylvanian Locality: Ice Box Cave, Virginia Kendall Ledges, Cuyahoga Valley National Park, north of Akron, northern Summit County, northeastern Ohio, USA (~~vicinity of 41° 13' 44.76" North latitude, 81° 30' 37.76" West longitude)