DESCRIPTION OF FIELD STOPS
The Taconic Clastic Wedge Of the Western Cratonic Basin
|This series of stops shows the transition from the Middle Ordovician carbonates of the passive margin to the Middle to Upper Ordovician formations deposited in the clastic wedge of the western cratonic basin (Figure 6). This entire section contains facies that range from intertidal to subtidal shelf, but no deeper. This is in sharp contrast to the deep basinal clastics of the Martinsburg basin. This western belt was exterior to the flexural moat (Figure 7 and 8).
Stop 4-Germany Valley Overlook
At the overlook you can observe Germany Valley (Ordovician limestones at the culmination of the Wills Mountain anticline); the ridge crest of North Fork Mountain (Tuscarora Sandstone on the east limb of Wills Mountain anticline) to the east; and River Knobs (Tuscarora Sandstone on the west limb of Wills Mountain anticline), which also form the first set of "razorback" hills or ridges to the west. At the northeast end of Germany Valley the Tuscarora ridge forms a spectacular example of closure at the nose of the lunging anticline. Also to the west, beyond the River Knobs, are the Fore Knobs that are developed along the Upper Devonian Greenland Gap Group/Hampshire Formation contact east of the Allegheny Front. West of the Fore Knobs is Spruce Knob, formed on a resistant bed of Pennsylvanian Pottsville Sandstone.
Stop 5-Germany Valley Section: Trentonian (Dolly Ridge Formation)
Stop 5 is about 2.5 mi (4.1 km) east of Judy Gap on U.S. 33. (Judy Gap is several miles west of the Germany Valley overlook, and is easily recognized by the spectacular cut through the nearly vertical Tuscarora sandstone). Turn left on to Bland Hills Road. The Dolly Ridge Formation (upper Trenton Group) is most accessible along Bland Hills road on Dolly Ridge, about 1.6 mi (2.7 km) north of U.S. 33.
The Dolly Ridge Formation is a dark gray, dense, fine grained, medium to thinly bedded limestone that weathers yellowish brown and contains olive gray shale beds and bentonites. These limestones represent the waning stages of an expansive carbonate bank that was situated on the eastern margin of North America during the Cambrian and Ordovician prior to Taconian flexure folding and clastic sedimentation. The onset of these clastic sediments is evident here as shale beds in the Dolly Ridge Formation. The bentonites are not identifiable here.
Stop 6-Western facies of the Ordovician clastic wedge
Stop 6a (lower Reedsville). The lower, calcareous part of the Reedsville Formation is observed along U.S. 33 where it begins to climb the western slope of North Fork Mountain, about 0.8 mi (1.1 km) east of the junction with Bland Hills Road.
At this locality the Reedsville consists of medium gray to grayish olive, calcareous shale that weathers to a light olive gray limestone-like surface. It is interbedded with laminae of very thin beds of medium gray, bioclastic calcarenite that weathers moderate yellow brown and rare thin interbeds of medium gray, calcareous siltstone that weathers light olive gray. Fossils in this part of the section include the brachiopods Rafinesguina, Sowerbyella, and Zygospira; the bryozans Prasopora and Hallopora; the gastropod Sinuites; the trilobite Cryptolithus; the cephalopod Orthoceras; crinoid stalks and columnals; and the graptolites Diplograptus and Climacograptus. The vioclastic layers are probably storm rip-up deposits as described by Kreisa. This outcrop represents the transition from the carbonate-dominated regime below, to the clastic-dominated lower part of the Taconian clastic sequence.
Stop 6b (middle Reedsville) is at the Germany Valley overlook along U.S. 33, about 0.7 mi (1.1 km) east of stop 6a.
Exposed at this stop is the shaley middle part of the Reedsville Formation that is composed of light olive-gray shale, with thin interbeds of medium gray, calcareous siltstone that weathers grayish orange (storm shelf hummocky bedding), rare thin interbeds of medium gray, bioclastic calcarenite (often megarippled) that weathers grayish orange, and rare medium to thin interbeds of yellow brown, fine-grained sandstone that weather light olive gray. The percentage of sandstone increases up-section, and the percentage of calcarenite decreases upward, typical of a clastic wedge.
Stop 6c (upper Reedsville, Oswego) is the next outcrop uphill (east) along U.S. 33, about 0.2 mi (0.3 km) from Stop 6b.
Exposed here is the upper Reedsville Formation and the transition to the Oswego Sandstone. The Reedsville is a bioturbated, medium gray, fossiliferous mudstone that weathers light olive gray. The uper Reedsville contains a completely different fauna (Orthorhynchula assemblage biozone) than in its lower beds, including the brachiopods Orthorhynchula and Lingula; the bivalves Ambonychia, Ischyrodonta, Modiolopsis, and Tancredopsis, the trilobite Isotelus; and phosphatized remains of the gastropod Plectonotus. The Orthorhynchula zone is a shallow water fauna. Small phosphate nodules and Lingula are common in the uppermost part of the Reedsville. The abrupt litologic and faunal change that occurred in the upper part of the Reedsville (Orthorhynchula Zone) is thought to represent a eustatic sea-level drop associated with the Late Ordovician glaciation. The phosphate and Lingula zone probably represents continued shallowing to shoaling or brackish water conditions.
The Oswego Sandstone, which overlies the Reedsville Formation in a gradational contact, is a light brownish gray, medium fine-grained, medium bedded, cross-bedded sublitharenite, with interbedded silt-shale. The lower contact of the Oswego is arbitrarily chosen at the base of the lowest cross-bedded sandstone above the Orthorhynchula zone. The Oswego represents the culmination of the upward-coarsening cycle that started at the base of the Reedsville. The immature composition of the Oswego reflect its provenance in an orogenic sourceland to the east.
Stop 6d (Oswego-Juniata) is the next exposure uphill along U.S. 33, about 0.3 mi (0.5 km) east of stop 6c, and across from the picnic area.
This outcrop contains the contact between the Oswego and the Juniata Formations. This contact is chosen at the base of the lowest red mudstone, but is uncertain here because of the large covered interval that occurs between the top of the outcrop at 6c and the base of the outcrop at 6d. The Oswego makes up about the lower 10 ft (3 m) of the exposure at Stop 6d. The overlying red beds of Juniata formation clearly exhibit their cyclical nature at this exposure. A typical cycle consists of a lower red, cross-bedded, fine-grained sandstone (sub-litharenite); a middle red, sub-lithic wacke with vertical burrows; and an upper red bioturbated mudstone. The Juniata represents deposition in nonmarine to marginal marine conditions, probably a delta plain that existed during the glacio-eustatic low stand of sea level.
Stop 7-Junitata and Tuscarora Formations
The Juniata Formation and its contact with the overlying Tuscarora Sandstone are well exposed along U.S. 33 on the east slope of North Fork Mountain, about 1.5 mi (2.5 km) east of Stop 6d.
Rt. 33 cuts through the Tuscarora three times, and this stop is the middle of these three exposures. The top of the Juniata is very similar to its base, implying that the same conditions existed until the end of the Ordovician. The contact with the overlying Tuscarora Sandstone is gradational. This contact is arbitrarily regarded as the Ordovician-Silurian boundary.
The Tuscarora Sandstone is a very fine-grained to very coarse-grained quartzrenite. Vertical Skolithos burrows occur near the base, overlain by a thinly bedded to massive cross-bedded internal that is in turn overlain by a burrowed interval. This probably represents nearshore marine conditions, implying that sea level rose at the beginning of the Silurian. About two-thirds of the way up through the Tuscarora section is an interval of thinly bedded sandstone interbeded with dark gray shale, containing Arthrophycus (annulated bedding plane trails) at the base of the sandstone beds. This interval may represent lagoonal, or possible marine conditions. The upper part of the Tuscarora, like the lower part, contains cross-bedded sandstone and Skolithos, probably indicating a return to nearshore marine conditions. The Tuscarora Sandstone, on the eastern limb of the Wills Mountain anticline, is about 170 ft (52 m) thick and dips about 25 degrees to the east. The Tuscarora Sandstone is overlain by the Rose Hill Formation, a sequence of interbedded hematitic sandstone and olive-gray marine shale. Because a covered interval occurs at the top of the Tuscarora exposure, the contact with the Rose Hill is not observable. In fact, much of the remainder of the Silurian is poorly exposed in the field area.