DESCRIPTION OF FIELD STOPS

Formations of the Divergent (Passive) Continental Margin and Deposited in the Eastern Flysch Basin During the Taconic Orogeny

These three stops illustrate the transition from the Lower to Middle Ordovician carbonate bank of the passive margin, to the eastern facies of the Middle to Upper Ordovician deep orogenic basin and clastic wedge (Figure 6). The regional paleogeography and plate tectonic relationships responsible for the orogeny are shown in Figure 7. The eastern flysch basin is interpreted as a flexural moat (Figures 7 and 8) into which the clastics were deposited.
The regional unconformity at the top of the Beekmantown has been interpreted as due to a global sea-level drop. Alternately, this unconformity is interpreted as due to uplift of a peripheral bulge that would have included most of the Appalachian basin. The carbonate-clastic transition is associated with the development of a deep basin (Martinsburg basin) that was present in the area now known as the Shenandoah Valley. Volcanic bentonites are also found within this transition.

Stop 1-Shenandoah Valley Section: Beekmantown, New Market, Lincolnshire, and Edinburg Formations.

The field guide begins at the bridge where Rt 33 (Market Street) crosses I-81 heading east past a shopping strip and the Valley Mall. From the bridge, travel 8.5 miles to Rt 991. As you travel east you will occasionally glimpse the Blue Ridge Mountains in front of you. As you approach Stop 1, Massanutten Mountain will appear as a long ridge on your left. At route 991, make a U-turn and return east about 2 miles. The outcrop is a road cut along the south side (east lane) of Rt. 33. The outcrops are sparse, patchy and not well exposed. These rocks are on the east flank of the Massanutten syncline. They are nearly vertical and strike nearly parallel to the roadcut.
These formations at other places have distinct contacts, or are gradational over short distances. At this roadcut facies typical of each formation tend to alternate in the first half of the outcrop. The section may be repeated by faulting, or facies relationships at this locality may be more complex than other places. In any event, our purpose is to compare the facies upsection and their environmental interpretations, rather than delineate formation contacts. Be alert for facies changes.
The Beekmantown formation is interpreted as upper intertidal to supertidal. It consists if interbedded light and dark gray dolomites and micrites (ribbon rock). It weathers to a very fine sandy texture (feels slightly gritty). On a fresh surface the rock looks very clean, and light gray.
Beekmantown-New Market contact. Regionally this contact is a well-developed unconformity (Knox). But because the unconformity lessens to the east it may not be present at this stop. In any event, the contact between the Beekmantown and New Market formations is not exposed here, although it can be narrowed down to within a few feet.
The New Market formation is a change from Beekmantown supertidal dolomites to inter-to supertidal New Market limestones. The change represents a relative rise in sea-level during the Early Middle Ordovician. The New Market is a very pure, light gray (fresh and weathered) birdseye limestone (micrite). Birdseyes are calcite crystals deposited in vugs in the original sediment. They look like dark gray flecks or blebs scattered randomly through the micrite. On weathered surfaces birdseyes are more resistant and stand out in high relief forming a rough textured surface.
The Lincolnshire formation is interpreted as a carbonate shelf environment, probably deepening up section. The micrites are dark gray to black, and are interbedded with medium to coarse calcarenites (fossil fragmental sands). Bedding planes are often very fossilferous. Black chert nodules are a characteristic feature, and become more pronounced up section.
The Edinburg formation is a transition from the passive continental margin to an orogenic foreland basin. At this stop it consists of black (weathered tan) graptolitic shales.

Stop 2-Shenandoah Valley Section: Martinsburg Formation

Stop 2 is 1.6 miles west of Route 991 (the U-turn) about 100 yards east of the entrance road to Spotswood School. The outcrop is low but well exposed at the east end of the right turn lane. These rocks strike diagonally out of the cut, and are nearly vertical. Up is to the left. First impressions are of interbedded gray sands and shales. Sands are a foot or more thick, shales about half a foot. On close inspection graded bedding is seen in the sand. Flutes and scours are visible on the bottoms of some sands. Rare laminations are at the top of some sands.
These are Bouma sequences produces by turbidity currents flowing into a deep water basin. Of the complete Bouma sequence TABCDE, these represent primarily TABD. These sands tend to be fledspathic-lithic wackes, a very immature sediment typical of those eroding from volcanic arcs (see QFL ternary diagram on Figure 2).

Stop 3-Shenandoah Valley Section: Middle Ordovician bentonite in the Edinburg Formation

From Stop 2 return to I-81 at Harrisonburg, and get on I-81 heading north toward Winchester. At the next exit (Mauzy/Broadway) get off. When the exit ramp joins Rt. 11 turn right, and then about 100 yards down Rt. 11 turn right again onto Rt. 608. The stop is .2 miles down Rt. 608, on the north side of the road.
This is a low, obscure outcrop covered with vegetation. This is the Edinburg formation, but is different in make up from the Edinburg at stop 1. There the Edinburg was a black, graptolitic shale. Here the Edinburg is mostly black micrites with interbedded shaley micrites producing a cobbly weathering. The difference in Edinburg at these two stops reflect the two sources feeding the now tectonically subsiding basin. The micrites are coming off the carbonate platform to the west (Dolly Ridge=Trenton equivalent) and sliding down the western slope of the basin. The shales are the first clastics coming from the rising Taconic mountains to the south. The shales are followed by the Martinsburg turbidites.
The significance of this stop is, however, the bentonite layers, the weathered remains of ash falls. We will not visit the Piedmont, but the mountain building, and roots of the volcanic arc are there. The ash falls seen here are direct evidence the arc came in and caused the Taconic orogeny.

After Stop 3 we travel westward a little over an hour to reach Stop 4 at Germany Valley. We then turn around and work our way back toward Harrisonburg, making Stops 4-13 on the way. The stops are conveniently arranged in sequence upsection from older to younger formations.