Stratigraphy, structure, and geomorphology of the central Appalachians across the North Mountain fault zone near Harrisonburg, Virginia, USA

By: , and 

Links

Abstract

This field trip focuses on the geology of the central Appalachian Valley and Ridge province near Harrisonburg, Virginia, USA. Recent geologic mapping utilizing 1-m resolution lidar data has revealed new insights into the Paleozoic stratigraphy, structural geology, and Neogene landscape evolution of the region. The detailed mapping reveals the presence of the Big Spring Station Member and multiple thrombolite zones in the Cambrian Conococheague Formation extending as far south as the Briery Branch 7.5 min quadrangle, providing insights into Late Cambrian sea-level fluctuations. Multiple outcrop exposures in the study area of this guidebook confirm recent work in Pennsylvania, USA, showing that the Ordovician Reedsville Shale overlies the Martinsburg Formation and that the two are distinct and mappable as separate formations rather than laterally equivalent units as previously interpreted. Our work extends the Silurian Williamsport Sandstone into Shenandoah County, Virginia, and describes its facies relationships with the Bloomsburg Formation along strike and across the Adams Run anticline. Mapping within the thick Devonian siliciclastic sequence reveals the presence of the Mahantango Formation on the western limb of Supin Lick syncline and illustrates its complex facies relationship with the Millboro Shale. In addition, we highlight new mapping criteria for the Brallier and Foreknobs Formations and demonstrate how the specific changes to the placement of the contact between them addresses previous challenges in their differentiation. We present cosmogenic burial ages of broad alluvial fan sediments in the Shenandoah Valley near Timberville and Briery Branch, Virginia, and erosion rates estimated for the Briery Branch stream basin. Both analyses provide new constraints on the timing of landscape evolution and karst development since the middle Pliocene. This field guide also highlights some significant structural features within the North Mountain fault zone, such as evidence of imbricated thrust sheets cut by cross-strike faults that have been exploited by Eocene igneous intrusions. Map-scale horses of Silurian and Ordovician rocks hold up ridges that are oblique to the regional strike. Deformation internal to one of these horse blocks is shown to be non-coaxial with respect to the main regional northwest directed transport.

Study Area

Publication type Book chapter
Publication Subtype Book Chapter
Title Stratigraphy, structure, and geomorphology of the central Appalachians across the North Mountain fault zone near Harrisonburg, Virginia, USA
DOI 10.1130/2025.0072(06)
Volume 72
Year Published 2025
Language English
Publisher Geological Society of America
Contributing office(s) Florence Bascom Geoscience Center
Description 50 p.
Larger Work Type Book
Larger Work Subtype Monograph
Larger Work Title From the Ozark Plateaus and Arkansas River Valley to the Shenandoah Valley: Field guides for the 2025 GSA south-central and southeastern section meetings
First page 93
Last page 142
Country United States
State Virginia
City Harrisonburg
Other Geospatial North Mountain fault zone
Additional publication details