A NEW BEDROCK GEOLOGIC MAP OF THE RILEYVILLE QUADRANGLE IN PAGE AND SHENANDOAH COUNTIES, VIRGINIA

A 1:24,000 scale geologic map of the Rileyville 7.5 minute quadrangle, northwestern Virginia, is being created with support from the EDMAP and STATEMAP programs of the US Geological Survey. The geologic map is digitized in ArcGIS 10 from data collected in the field using iGIS on iPad tablets.

The Rileyville 7.5 minute quadrangle extends from the South Fork of the Shenandoah River in Page Valley, across the Massanutten Mountains and Fort Valley, to the North Fork of the Shenandoah River in Shenandoah Valley. Cambrian and Ordovician carbonates are concentrated in the southeastern region of the map around the South Fork of the Shenandoah River. To the west, Ordovician carbonates transition to Silurian clastic rocks associated with the Massanutten Mountain ridges. Devonian shales are found in Fort Valley in the center of the Massanutten syncline. Ordovician clastic rocks transition back to Cambrian carbonates on the western margin of the mapping area.

Page Valley, Fort Valley, and Shenandoah Valley are dominated by northeast-southwest trending structures. These structures include west-vergent folds, west-directed thrust faults, and associated joints and veins. East-directed back thrusts are locally apparent, as are occasional northwest-striking normal or strike-slip faults. In Page Valley, the South Fork of the Shenandoah River is often constrained by jointing or faulting as exhibited by the parallel nature of river meanders. The Shenandoah Valley displays similar structures in areas around the North Fork of the Shenandoah River.

The central part of the Rileyville quadrangle is dominated by the Massautten synclinorium, where Silurian sandstones core ridges with 1000+ feet of relief over central Devonian shales. The Massanutten synclinorium is a first order syncline that reaches a width of more that 6 kilometers, and within it contains parasitic anticlines and synclines. The southernmost Massanutten Sandstone ridge is fault duplicated across the full extant of the Rileyville quadrangle, extending from the Hamburg quadrangle in the southwest, to the Bentonville quadrangle in the northeast. Deformation features throughout the Rileyville quadrangle are interpreted to have resulted from the northwest-directed collision of Gondwana with eastern Laurentia during the Alleghenian orogeny.