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CAROLINA GEOLOGICAL SOCIETY OFFICERS 1959-1960 President: Owen Kingman Tennessee Copper Company Mine Office Ducktown, Tennessee Vice President: John McCauley Department of Geology University of South Carolina Columbia, South Carolina Secretary: E. Willard Berry Department of Geology Duke University Durham North Carolina Chairman of the Membership Committee: John St. Jean Department of Geology University of North Carolina Chapel Hill, North Carolina Chairman of Program Committee: Stephen G. Conrad N.C. Division of Mineral Resources Raleigh, North Carolina Field Trip Leaders: Bruce Bryant U.S. Geological Survey Denver, Colorado John C. Reed, Jr. U.S. Geological Survey Denver, Colorado LIST OF ILLUSTRATIONS Figure 1. Generalized geologic map of western North Carolina and northeastern Tennessee show- ing location of the Grandfather Mountain area and major tectonic features. Modified from King (1955) and Geologic Map of North Carolina (1958). Quadrangles: 1, Linville; 2, Table Rock; 3, Blowing Rock; 4, Lenoir: Figure 2. Generalized preliminary geologic map of the Grandfather Mountain area, N.C. - Tenn. CAROLINA GEOLOGICAL SOCIETY Guidebook for 1960 Annual Meeting Pages 1-16 ROAD LOG OF THE GRANDFATHER MOUNTAIN AREA, N.C.1 Bruce Bryant and John C. Reed, Jr. U.S. Geological Survey Denver, Colorado 1.Publication authorized by Director, U.S. Geological Survey. Retyped and formatted December 1999. INTRODUCTION In the Inner Piedmont southeast of the Grandfather Mountain window, the Predominant rocks are gneisses, More than a century ago Elisha Mitchell recognized that schists, and granitic rocks of Precambrian and (or) Paleozoic the rocks of the Grandfather Mountain area in western North age. These rocks were thrust over those of the Grandfather Carolina are unusual for the eastern Blue Ridge (Mitchell, Mountain window, (STOP 12). The structural relations of the 1905. The rocks of the area were briefly described by Kerr Piedmont rocks to the allochthonous rocks of the Blue Ridge (1875, p. 135-136), but the first systematic geologic mapping are unknown, but reconnaissance mapping to the southwest in the area was that by Keith and his associates in 1894 to of the window, especially in the Pisgah quadrangle (Keith, 1907 (Keith, 1903, 1905; Keith and Sterrett, 1954). In the 1907), suggests that they may also be in fault contact with southern part of the Cranberry quadrangle, and in parts of the the blue Ridge rocks (Reed and Bryant, 1960). Morganton and Mt. Mitchell quadrangles, Keith mapped Rocks of the Blue Ridge thrust sheet Rocks west and upper Precambrian and lower Cambrian sedimentary and north of the Grandfather Mountain window and structurally igneous rocks in the midst of a complex terrane of schists, overlying it comprise a complex terrane of mica schist and gneisses, and granites which comprises the bulk of the Blue gneiss, amphibolites, migmatites and generally layered gra- Ridge in northwestern North Carolina and Northeastern Ten- nitic gneiss of early Precambrian age intruded by plutons of nessee. Keith believed that the younger rocks occupy a com- granite and quartz monzonite of early Precambrian age plex syncline overridden by thrust sheets of lower (STOP 4), stocks and dikes of gabbro of late Precambrian Precambrian rocks from three sides. The structure was inter- age (Bakersville gabbro, Keith, 1903) and by stocks, sills, preted on the geologic map of the United States (Stose and and small lenticular bodies of granodiorite, pegmatite, and Ljuntgstedt, 1932) as a window, since called the Grandfather ultramafic rock of early or middle Paleozoic age. This slices Mountain window (Stose and Stose, 1944, p. 383). of Cambrian (?) clastic rocks intercalated in the gneisses In 1956 the writers began geologic mapping in four mark thrust faults in the crystalline terrane. quadrangles which include the Grandfather Mountain win- The mica schist and gneiss and amphibolite southwest dow and adjacent portions of the Blue Ridge and Inner Pied- of Newland pass northward through a transition zone in mont. Mapping in the Linville and Table Rock quadrangles which layers and lenses of granitic material become increas- has been completed; mapping in the Blowing Rock and ingly abundant into layered migmatitic gneiss in which gra- Lenoir quadrangles is in progress. nitic layers predominate, and finally into rudely layered and During the field trip there will be time to examine only nonlayered granitic gneiss. (fig. 2). This transition zone some of the more important rock types and structural fea- between schist and gneiss is wide west of Newland, but to tures. the south, layered gneiss in which granitic layers predomi- nate is in almost direct contact with amphibolite and mica SUMMARY OF GEOLOGY IN THE GRANDFA schist lacking granitic layers. (STOP 3). THER MOUNTAIN AREA IN NORTHWESTERN The granitic rocks and layered gneiss were sheared and NORTH CAROLINA partly recrystallized so that they are now cataclastic gneiss, The Blue Ridge is composed largely of schists, gneisses, porphyroclastic gneiss, phyllonitic gneiss, phyllonite and and granitic rocks of early Precambrian age which were blastomylonite (STOP 4). The mica schist and amphibolite involved in large-scale thrusting and subjected to one or have a crystalloblastic texture, except in a few places near more episodes of thermal and dynamic metamorphism dur- the layered gneiss ing Paleozoic time. These rocks have been thrust northwest- The schists, gneisses, and amphibolites are intruded by ward over upper Precambrian and Cambrian rocks of the light colored granodiorite (alaskite of Hunter and Mattocks, Unaka belt (fig. 1). The upper Precambrian and lower Cam- 1936, and subsequent workers) and mica pegmatites in the brian rocks in the Grandfather Mountain window show that the allochthonous1 rocks traveled relatively at least 30 miles northwestward during the Paleozoic thrusting. 1.*See glossary on last page for definitions of some of the more specialized terms used in this road log. 1 BRUCE BRYANT AND JOHN C. REED, JR. 2 ROAD LOG OF THE GRANFATHER MOUNTAIN AREA, N.C. Spruce Pine district. In many places the granodiorite and matrix (STOP 14). The younger (STOP 15) is massive to pegmatite look cataclastic, but actually, as the microscope well foliated and light colored, and may be correlative with shows, they are coarsely recrystallized like the wall rock. the Toluca quartz monzonite of the Shelby area described by The dikes and stocks of Bakersville gabbro cut the gra- Griffiths and Overstreet (1952). nitic gneisses and migmatites and are clearly younger than The rocks of the Inner Piedmont differ from the rocks of the plutonic metamorphism; during subsequent retrogressive the Blue Ridge thrust sheet in having less mica schist, metamorphism they have been converted to amphibolite and amphibolite, and migmatite, different structural and petro- metagabbro. Near the northern edge of the Spruce Pine dis- graphic types of granitic rocks. The pegmatites in the Inner trict, Wilcox and Poldervaart (1958) inferred that they are Piedmont are mica-poor pegmatites and typically they con- older than the pegmatites and granodiorite. tain accessory xenotime, monazite, and zircon. Those of Minerals from the pegmatites have been dated by several Spruce Pine district are mica rich and typically contain gar- methods, and the results are in fair agreement at about 350 net, but lack xenotime, monazite, and zircon. million years (Ecklemann and Kulp, 1957; Long, Kulp and Adjacent to the boundary of the Grandfather Mountain Ecklemann, 1959; Tilton and others, 1959). It is not entirely window, rocks of the Piedmont were sheared to porphyro- certain whether these results give the age of emplacement of clastic schist and gneiss and blastomylonite (STOP 12). the pegmatites or the age of their metamorphism, but it seems likely that they were emplaced immediately before or ROAD LOG FOR FIELD TRIP OF THE CARO during an early stage of the metamorphism in which they LINA GEOLOGICAL SOCIETY SATURDAY, were deformed. OCTOBER 8, 1960 ROCKS OF THE GRANDFATHER MOUNTAIN Topographic maps: The field trip will pass through the following published quadrangles: Ashford, Linville Falls, WINDO Chestnut Mountain, Oak Hill, Morganton North and Col- In the Grandfather Mountain window granitic rocks of lettsville 7 -minute quadrangles and Linville 15-minute early Precambrian age (STOPS 8, 9, 11) are unconformably quadrangle. General Instructions: Eat a good breakfast and overlain by a thick sequence of interlensing arkose, siltstone, carry a snack because the lunch stop will be late. shale, graywacke, and volcanic rocks, STOPS 5, 6, 7, 8, 10), Mileage all of which probably are correlative with the Ocoee series of 0.0 Leave center of Morganton (elev. 1,200 ft.), *:00 A.M.; late Precambrian age. The basement rocks were pervasively go west on U.S. 70. sheared and locally phyllonitized at the same time that the 1.0 Mimosatel. 1.5 Jct. U.S.70 bypass overlying sequence was progressively metamorphosed (STOP 8). Both the basement rocks and the overlying 1.6 Silver Creek. To the north and northwest are (left to sequence are apparently autochthonous. In the southeastern right); Table Rock (elev. 3,020 ft.) Hawksbill (elev. part of the window is an isolated outcrop strip of predomi- 4,020 ft.), and Grandfather Mountain (elev. 5,939 ft.). nantly felsic flows and pyroclastics with minor interbedded 3.1 Boxwood Court sedimentary rocks (STOPS 10, 12); this sequence is proba- 3.5 White alluvial clay in gully fill passes up to red clay, typ- bly correlative with the main sedimentary and volcanic ical of the Piedmont. sequence mentioned above. In the southwestern part of the window arkosic quartzite, quartzite, phyllite and dolomite 5.7 Town of Glen Alpine occur in a thrust sheet between the autochthonous basement 7.2 Typical medium-colored, fine grained biotite gneiss with and sedimentary rocks of the window and the rocks of the 1- to 6-inch layers of medium -grained schist contain- Blue Ridge thrust sheet to the west. They are correlative with ing pods of pegmatite.