DESCRIPTION OF FIELD STOPS

The Silurian and Lower Devonian Calm Between The Taconic and Acadian Orogenies

As described in the introduction, the geologic record largely consists of alternations between carbonate domination and the influx of clastic wedges. On stratigraphic evidence alone this is the way orogenic episodes are recognized. Carbonates represent periods of tectonic stability, and the large influxes of clastics indicate a mountain source being eroded. The carbonate rocks at this next stop (8) are sandwiched between clastic wedges of the Taconic and Acadian orogenies and represent a period of tectonic calm.

Stop 8-Oak Flat Section

Stop 8 is 6.7 mi (11.2 km) east of the U.S. 220 and 33 junction in Franklin, W.Va. (1.1 miles [1.8 km] west of Oak flat) on the south side of U.S. 33, across the highway from a small roadside park. The exposure is a large but discontinuous 0.3 mi (0.5 km) long road cut.
Seven steeply dipping Upper Silurian and Lower Devonian formations from the Tonoloway to the Oriskany are exposed. This stop is an excellent example of almost the full Helderberg Group and the overlying Oriskany (and down the road a little, the Needmore).
Tonoloway carbonate tidal flat. At the west end of the cut the Tonoloway Formation is exposed in a broad, gentle anticline extending a couple of hundred yards along the highway. The (algal) laminated micrites appear uniformly monotonous at first, but on closer study many other features can be found, including: Ostracods (Leperditia) and intraformational conglomerates. Near the formation top, clay content increases and the rocks weather out in thin fissile plates with occasional mud cracks and salt casts. All of these features indicate a tidal flat environment with occasional restricted circulation and high evaporation. (The Salina salt beds to the west are a facies of the Tonoloway).
Keyser Carbonate Barrier or shallow subtidal. The Keyser is divided into a lower and upper member separated by the Big Mountain Shale Member. The Lower and Upper Keyser are largely structureless, coarse-grained, fossiliferous calcarenites. The abundant well-washed fossils imply a shallow, clear-water, high energy environment such as a carbonate barrier or large areas of fossil debris in a shallow subtidal region. With the Keyser Foundation a major transgression began to advance northeastward into the Silurian Central Appalachian Basin easing the restricted circulation that prevailed during the Tonoloway.
Big Mountain shallow subtidal. The Big Mountain Shale, although exposed, may be hard to see, especially with summer vegetation. The shale is brown, fissile, and locally calcareous and fossiliferous. The Big Mountain is interpreted as a clastic influx transported north and northeastward by longshore currents during a minor regressive phase of the basin. The source was the roots of the now eroded. Low, and tectonically stable Taconic mountain region in the Clifton Forge, Virginia area.
Coeymans deep subtidal. The Coeymans is well exposed above the Upper Keyser in the large, prominent road cut. The base of the formation can be recognized by a bed with numerous, large bryozoan colonies. Although the Coeymans is very fossiliferous, it contains more calcisiltite and is darker than the Upper Keyser, and is interpreted as a deeper subtidal environment than the Keyser. It represents a continuation of the major transgression that began with the Lower Keyser.
New Scotland, Mandata, and Licking Creek deep subtidal. The New Scotland is a thick-bedded fossiliferous limestone with numerous chert beds that are easily visible from the road. The Mandata and Licking Creek (Shriver Chert) are poorly exposed, badly weathered, and hidden in the trees. The New Scotland, Mandata and Licking Creek, like the Coeymans, are deeper subtidal, closer to the basinal axis, and were deposited during the maximum stages of the transgression.
Oriskany quartz arenite beach. The quartz arenitic Oriskany contains, in places, abundant molds of brachiopods. Near the top of the formation is a very thick bed with laminations parallel to bedding. Above that are large-scale planar cross beds, usually seen because brachiopod molds are aligned along the foresets. Systematic environmental studies of the depositional environments of the Oriskany have not been done, but it is generally interpreted as a beach and near shore sand.
Unconformities have been recognized both below and above (the Wallbridge discontinuity) the Oriskany at various places in the Central Appalachians, but no direct evidence for their presence exists at this stop. Overall, however, the Oriskany represents a time of shallowing and even complete withdrawal of the Devonian sea from the craton at the boundary between the Tippecanoe and Kaskaskia sequences, just prior to the Acadian Orogeny.