50 Stratigraphy (Posters)

Monday, 21 March 2011
1:30 PM - 5:30 PM
Grand Ballroom

STRATIGRAPHY, FACIES CHANGES, AND PETROLOGY OF SANDSTONES IN THE SILURIAN SEQUENCE OF THE VALLEY AND RIDGE PROVINCE IN HIGHLAND AND BATH COUNTIES, VIRGINIA

Paper number: 50-3
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

The Silurian of western Virginia includes three medium- to thick-bedded quartz arenites that are discrete, 5-20 m thick units in northern sections but which coalesce farther south into the massive 100-120 m “Eagle Rock Sandstone.” Facies changes in associated shales, siltstones, and limestones contribute to this southward thickening. Recent bedrock mapping in the Williamsville 7½ min. quadrangle and nearby areas has clarified relationships among these Silurian sandstones in this region. Three newly measured and described sections in Highland and Bath Counties are in the Bullpasture River Gorge, at Lower Gap, and at Trimble; these sections are in stratigraphically strategic positions by being between two sections (Muddy Run and Fork of Waters) that have previously been measured and described. Measurements from these three exposures allows better constraint of (1) the areal extent of the unnamed quartz arenite in the McKenzie Formation, (2) the lateral continuity of the Williamsport Sandstone, (3) what is true Keefer Sandstone vs. the amalgamated “Keefer” Sandstone of some previous workers that included the McKenzie and Williamsport horizons, and (4) the lateral extent of hematitic and oolitic “Clinton” ores in the Keefer of this region. The unnamed McKenzie sandstone persists 20-25 km farther north than previously known, making prominent ledges in the Bullpasture River. The Williamsport had not even been recognized in earlier reports, but it is mappable in this area. In some earlier reports the Keefer was mapped as being overlain by Tonoloway, but the true Keefer is overlain by McKenzie, so the Keefer of those reports is more accurately characterized as “Keefer” or even “Eagle Rock.” The Keefer oolitic beds are ferroan dolomites with berthierine and hematite ooids; they extend from the Fork of Waters section south at least to the Bullpasture River.

Calcarenaceous sandstones in the Upper Silurian Tonoloway Formation are also now better delimited as a result of our bedrock mapping efforts. A fossiliferous quartz arenite 1-3.5 m thick separates the lower and middle members of the Tonoloway, and a calcareous quartz wacke 1-1.3 m thick occurs consistently 25 m downsection. Other Tonoloway sandstones are thinner and more dolomitic, but overall the Tonoloway of this region is more quartzose than previously reported.