Stratigraphy, Structure, and Tectonics: an East-To-West Transect of the Blue Ridge and Valley and Ridge Provinces of Northern Virginia and West Virginia
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FLD016-05 2nd pgs page 103 The Geological Society of America Field Guide 16 2010 Stratigraphy, structure, and tectonics: An east-to-west transect of the Blue Ridge and Valley and Ridge provinces of northern Virginia and West Virginia Lynn S. Fichter Steven J. Whitmeyer Department of Geology and Environmental Science, James Madison University, 800 S. Main Street, Harrisonburg, Virginia 22807, USA Christopher M. Bailey Department of Geology, College of William & Mary, Williamsburg, Virginia, USA William Burton U.S. Geological Survey, Reston, Virginia 22092, USA ABSTRACT This fi eld guide covers a two-day east-to-west transect of the Blue Ridge and Valley and Ridge provinces of northwestern Virginia and eastern West Virginia, in the context of an integrated approach to teaching stratigraphy, structural analysis, and regional tectonics. Holistic, systems-based approaches to these topics incorpo- rate both deductive (stratigraphic, structural, and tectonic theoretical models) and inductive (fi eld observations and data collection) perspectives. Discussions of these pedagogic approaches are integral to this fi eld trip. Day 1 of the fi eld trip focuses on Mesoproterozoic granitoid basement (associated with the Grenville orogeny) and overlying Neoproterozoic to Early Cambrian cover rocks (Iapetan rifting) of the greater Blue Ridge province. These units collectively form a basement-cored anticlinorium that was thrust over Paleozoic strata of the Val- ley and Ridge province during Alleghanian contractional tectonics. Day 2 traverses a foreland thrust belt that consists of Cambrian to Ordovician carbonates (Iapetan divergent continental margin), Middle to Upper Ordovician immature clastics (asso- ciated with the Taconic orogeny), Silurian to Lower Devonian quartz arenites and car- bonates (inter-orogenic tectonic calm), and Upper Devonian to Lower Mississippian clastic rocks (associated with the Acadian orogeny). Alleghanian structural features include the Little North Mountain thrust, Cacapon Mountain anticlinorium, Broad Top synclinorium, and Wills Mountain anticlinorium. Within the road log of this fi eld guide we include both planned and optional stops, so that readers can explore the pedagogic concepts discussed herein in more detail, if desired. Fichter, L.S., Whitmeyer, S.J., Bailey, C.M., and Burton, W., 2010, Stratigraphy, structure, and tectonics: An east-to-west transect of the Blue Ridge and Valley and Ridge provinces of northern Virginia and West Virginia, in Fleeger, G.M., and Whitmeyer, S.J., eds., The Mid-Atlantic Shore to the Appalachian Highlands: Field Trip Guidebook for the 2010 Joint Meeting of the Northeastern and Southeastern GSA Sections: Geological Society of America Field Guide 16, p. 103–125, doi: 10.1130/2010.0016(05). For permission to copy, contact [email protected]. ©2010 The Geological Society of America. All rights reserved. 103 FLD016-05 2nd pgs page 104 104 Fichter et al. INTRODUCTION The Geological Society of America (GSA) fi eld trip described in this guidebook is based on the SST semester-long This fi eld guide focuses on an east-to-west, cross-orogen project described above. We hope that this fi eld guide provides transect of the Blue Ridge and Valley and Ridge provinces of a provocative way of thinking about student fi eld trips and fi eld northwestern Virginia and eastern West Virginia, generally fol- education in general, as well as serving as a useful resource for lowing Route 33 from Ruckersville, Virginia, to the Allegheny educators leading their own fi eld trips through this region. Front near Judy Gap, West Virginia. Over the past few decades several fi eld trip guides have documented the stratigraphy and GEOLOGIC SETTING depositional environments of this region (e.g., Diecchio, 1986; Fichter, 1986; Fichter and Diecchio, 1993), as well as the struc- Blue Ridge tural geometry and tectonic history of this part of the Appala- chian orogen (e.g., Perry and DeWitt, 1977; Spencer et al., 1989; The fi rst day of this fi eld trip begins south of Madison, Vir- Bailey et al., 2006). It is not the purpose of this fi eld guide to reit- ginia, and proceeds west to Route 33, crosses the Blue Ridge erate all of this prior work, except to briefl y summarize pertinent at Swift Run Gap, and ends in Harrisonburg, Virginia. In north- models and interpretations in the context of fi eld locations that central Virginia, the Blue Ridge province forms a basement- are highlighted. Instead, this fi eld guide focuses on the pedagogy cored anticlinorium that was thrust over Paleozoic strata of the of an integrated approach to teaching stratigraphy and structural Valley and Ridge province during late Paleozoic (Alleghanian) analysis within the overarching framework of tectonics. In the contractional tectonics. Mesoproterozoic granitoid basement and text that follows we present a holistic, systems approach to teach- a cover sequence of Neoproterozoic to Early Cambrian rocks ing these topics from both inductive and deductive perspectives underlie the Blue Ridge. The basement complex formed at mid- that are rooted in fi eld observations and data collection. dle to lower crustal depths during and after the assembly of the In 2007, the James Madison University Geology and Envi- Rodinian supercontinent, while the cover sequence records rift- ronmental Science Department initiated a new course for earth ing, Rodinian breakup, and the subsequent transgression of the science majors, entitled “Stratigraphy, Structure, Tectonics,” or Iapetus Ocean at the onset of the Phanerozoic. “SST.” The course builds on a central theme of the earth sci- The basement suite formed between 1.2 and 1.0 Ga and ence degree, which is to simultaneously think about, evaluate, includes three temporally distinct groups of granitoids (Fig. 1). and integrate the relative importance of many variables that The oldest group crystallized between 1190 and 1150 Ma, the infl uenced the geology and natural history of a region. In gen- middle group between 1120 and 1110 Ma, and the youngest eral, we want students to learn to evaluate how different sys- group between 1090 and 1020 Ma (Aleinikoff et al., 2000; Tollo tems infl uence each other, rather than think about each system et al., 2004; Southworth et al., 2009). The older group is gneissic individually. Thus, components of traditional stratigraphic and and characterized by a high-grade compositional layering. Both structural analyses techniques are incorporated within the broad the older and younger groups are chemically diverse and range umbrella of tectonics, in a course co-taught by specialists in from low-silica charnockites to alkali feldspar leucogranitoids. these respective fi elds. The volumetrically less signifi cant middle group is restricted to A major component of the SST course is a semester-long orthopyroxene-bearing granites. project based on several fi eld trips that collectively transect the The felsic 730–700 Ma Robertson River plutonic and vol- Blue Ridge and Valley and Ridge geologic provinces of north- canic complex cuts Mesoproterozoic basement and a thick ern Virginia and West Virginia. During the fi eld trips, structural sequence of Neoproterozoic metasedimentary rocks (Lynchburg, and stratigraphic data are collected at each outcrop, with students Fauquier, and Mechum River units) overlies older rocks (Tollo and instructors evaluating the data to assemble a regional tectonic and Aleinikoff, 1996; Bailey et al., 2007). This sequence of plu- history. Stratigraphic data include detailed facies analysis and tonic, volcanic, and sedimentary rocks formed during an episode environmental interpretations; structural data include orientation of rifting that created signifi cant accommodation space but did measurements and geometric relationships. Students incorporate not generate ocean crust. fi eld observations, data, and interpretations into a fi nal report that A later Neoproterozoic cover sequence of metasedimen- consists of two principal parts: (1) a cross section from the Blue tary and metavolcanic rocks unconformably overlies the base- Ridge to the Allegheny Front that incorporates multiple scales ment complex in the western Blue Ridge and includes the clastic of stratigraphic and structural fi eld evidence, and (2) a com- Swift Run Formation and predominantly basaltic greenstones prehensive tectonic and geologic history of the region that uses of the Catoctin Formation (Fig. 1). The Swift Run Formation stratigraphic and structural fi eld observations and is informed is heterogeneous with a highly variable thickness. The Cato- by theoretical models. For this report it is not enough to simply ctin Formation forms a thick unit extruded over a large area recognize and identify specifi c outcrop features. Observations (>4000 km2) and was generated from mantle-derived tholeiitic and conclusions must be both inductively and deductively valid, magmas (Badger and Sinha, 2004). Metadiabase dikes of similar and to the degree possible, internally consistent and rooted in the composition to Catoctin metabasalts intrude the basement complex overarching mechanisms and driving forces of tectonics. (and older Neoproterozoic rocks) and are likely feeder dikes for the FLD016-05 2nd pgs page 105 Blue Ridge and Valley and Ridge Provinces of northern Virginia and West Virginia 105 overlying Catoctin lava fl ows. Badger and Sinha (1988) report between the underlying Catoctin metabasalts and the overlying a Rb-Sr isochron age of 570 ± 36 Ma, and zircons from metar- Chilhowee Group has traditionally been interpreted as an uncon- hyolite