Middle Ordovician limestones in the valley of the North Fork of the Roanoke River, Montgomery County, Virginia

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Virginia Polytechnic Institute


This study was threefold: first, to determine the facies relationships of the limestones between the top of the Knox group and the base of the Liberty Hall shale; second, to study the faunas of the limestones; and third, to determine the relief of the erosion surface developed on the surface of the Knox group prior to the deposition of the overlying sediments.

Study of the area in the North Fork of the Roanoke River was begun in the fall of 1950 and the work was continued in the summer of 1951. The lithologic divisions of the Middle Ordovician limestones was mapped. The 15-minute Blacksburg quadrangle was used es a base map with a modified scale of approximately 1:30,000. Sections were measured with a Brunton compass and a steel tape at several localities, and faunas were collected from several stratigraphic horizons.

The facies relationships of the limestones were determined by detailed mapping, measurement and comparison of sections, and by tracing key beds.

The thickness of the middle Ordovician limestones was determined by measuring in the direction of dip. Minor warps and possible collapse in places complicated the measurement, The variation of the strike and dip is probably related to the structural position of the beds in the Paris-Catawba Mountain syncline.

Faunas were collected from the Middle Ordovician limestones. The lowest of these faunas came from a ledge about 10 feet above the Knox group, whereas the youngest of the faunas came from a horizon about 40 feet below the base of the Botetourt limestone. Species of these faunas have been identified and correlated with faunas from other Middle Ordovician limestones elsewhere.

The following conclusions seem to be warranted by evidence presented in this paper:

  1. The relief on the Knox group is about 225 feet in the area discussed in this report.

  2. The limestone units discussed mw be correlated in the following way:

a. The Ellett siltstone member of the New Market limestone is equivalent to the red shales reported in the Blackford formation in Tazewell County. However, the Blackford there included overlying impure limestones and dolomites that overlie the Ellett in the Valley of the North Fork of the Roanoke River.

b. The impure magnesian limestones that overlie the Ellett are a lithofacies of the New Market limestone.

c. The entire New Market limestone of the Valley of the North Fork of the Roanoke River is the equivalent of the New Market limestone of the Shenandoah Valley area in northern Virginia.

d. The Whistle Greek limestone of the Valley of the North Fork of the Roanoke River is the equivalent of the Whistle week limestone of the areas in northern Virginia and Maryland. Fossils indicate a correlation with the New York Chazy.

e. The beds herein called "Lincolnshire limestone" occupy the same position ae the Lincolnshire limestone of the northern areas of Virginia.

f. The Botetourt limestone occupies the same stratigraphic position in the Valley of the North Fork of the Roanoke River as the unit named Botetourt by Cooper and Cooper in the Shenandoah Valley area.