Geologic map of the northern portion of the Pittsboro 7.5-minute quadrangle, Chatham County, North Carolina

DEPARTMENT OF ENVIRONMENT AND NATURAL RESOURCES DIVISION OF ENERGY, MINERAL, AND LAND RESOURCES TRACY E. DAVIS, DIRECTOR NORTH CAROLINA GEOLOGICAL SURVEY KENNETH B. TAYLOR, STATE GEOLOGIST This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program OPEN FILE REPORT 2012-03 MAP UNITS £ < 2 Ш 5 э о Qal alluvium (Qal) О О N о и ш 7 • Jd / и diabase о О О Ё О а: Q- о ш Metamorphic Rocks Нусо Formation metamorphosed volcaniclastic sedimentary and pyroclastic rocks associated with Hyco Formation: upper portion (stratigraphic relations uncertain) ca. 613 - 616 Ma (Wortman et a!., 2000; Bowman, 2010; and Bradley and Miller, 2011) Zq Zhat (u) Zhdsi (u) Zhe/pl Zhdlt (u) Zhadlt (u) Zhablt quartz bodies (Zq) altered tuffs (Zhat(u)) mixed epiclastic-pyroclastic rocks with interlayered dacitic lavas (Zhe/pl) dacitic lavas and tuffs (Zhdlt (u)) dacitic shallow intrusive (Zhdsi (u)) andesitic to dacitic lavas and tuffs (Zhadlt (u)) andesitic to basaltic lavas and tuffs (Zhablt) Equal Area Schmidt Net Projection of Contoured Poles to Primary Layering, Bedding and Welding/Compaction Foliation. Contour Interval = 1 N = 29 N Equal Area Schmidt Net Projection of Contoured Poles to Foliation and Cleavage Contour Interval = 1 N = 288 § Unidirectional Rose Diagram of Joints N = 314 Outer Circle = 8% Mean direction = 313 Zhablt Zhablt Pittsboro Zhe/pl Zhe/pl i - % 1 1 Art'. КГ) ,Jbp lain Webster Poultry Company Pond Dam Zhe/pl YELLOW В „uce'b'ukns RD HpL'LY.RipQE RD JOE WOM В IE RP VIEW CHURCH Kf> ★ 2010 Magnetic North Declination at Center of Sheet = 8 degrees 29 minutes West SCALE 1 :24 000 1 0.5 0 1 I I I I I Ч I - 1 III I Miles 1.000 500 0 1.000 2,000 3,000 4,000 5,000 6,000 7,000 FH I I I I I I I^^^Feet 1 0.5 0 1 I — I I — I I — I I — I I — I I ~l Kilometers CONTOUR INTERVAL - 10 FEET Geology mapped between August 2011 through July 2012. INTRODUCTION Pre-Mesozoic crystalline rocks in the Pittsboro Quadrangle are part of the Virgilina sequence of the Neoproterozoic to Cambrian Carolina terrane of the Carolina Zone (Harris and Glover, 1988; Hibbard et al., 2002; and Hibbard et ah. 2006). In the region of the map area, the Virgilina sequence can be separated into two lithotectonic units: I ) the Hyco Formation and 2) the Aaron Formation. The Hyco Formation consists of ca. 615 to 633 Ma (Wortman et ah. 2000; Bowman, 2010; Bradley and Miller. 201 1) metamorphosed layered volcaniclastic rocks and plutonic rocks. Available age dates (Wortman et ah. 2000; Bradley and Miller, 2011) indicate the Hyco Formation may be divided into lower (ca. 630 Ma) and upper (ca. 615 Ma) members (informal) with an apparent intervening hiatus of magmatism. In northeastern Chatham County, Hyco Formation units are intruded by the ca. 579 Ma (Tadlock and Loewy, 2006) East Farrington pluton and associated West Farrington pluton. The Aaron Formation (not present in the map area) consists of metamorphosed layered volcaniclastic rocks with youngest detrital zircons of ca. 578 and 588 Ma (Samson et ah, 2001 and Pollock. 2007, respectively). The Virgilina sequence was folded and subjected to low grade metamorphism during the ca. 578 to 554 Ma (Pollock, 2007) Virgilina deformation (Glover and Sinha, 1 973; Hams and Glover, 1985; Harris and Glover, 1988; and Hibbard and Samson, 1995). In the map area, original layering ofVirgilina sequence lithologies are interpreted to range from shallowly to steeply dipping due to open to isoclinal folds that are locally overturned to the southeast. Jurassic-aged diabase dikes intrude the crystalline rocks of the map area. Quaternary-aged alluvium is present in most major drainages. Map units of meta- volcaniclastic rocks include various lithologies that when grouped together are interpreted to indicate general environments of deposition (or lithofacies). The dacitic lavas and tuffs unit is interpreted to represent dacitic domes and proximal pyroclastics. The andesitic to basaltic lavas and tuffs unit is interpreted to represent eruption of intermediate to mafic lava flows and associated pyroclastic deposits. The epiclastic/pyroclastic units are interpreted to represent deposition from the erosion of dormant and active volcanic highlands. Meta-volcaniclastic units within the map area display lithologic relationships similar to units present in northern Orange and Durham Counties. Due to these similarities, the meta-volcaniclastic units have been tentatively assigned to the upper portions of the Hyco Formation; geochronologic data is needed to confirm this interpretation. All pre-Mesozoic rocks in the map area have been metamorphosed to at least the chlorite zone of the greenschist metamorphic facies. Many of the rocks display a weak or strong metamorphic foliation. Although subjected to metamorphism, the rocks retain relict igneous, pyroclastic, and sedimentary textures and structures that allow for the identification of protolith rocks. As such, the prefix “meta” is not included in the nomenclature of the pre-Mesozoic rocks described in the quadrangle. Jurassic diabase dikes are unmetamorphosed. The nomenclature of the International Union of Geological Sciences subcommission on igneous and volcanic rocks (IUGS) after Le Maitre (2002) is used in classification and naming of the units. The classification and naming of the rocks is based on relict igneous textures, modal mineral assemblages, or nonnalized mineral assemblages when whole-rock geochemical data is available. Past workers within the adjacent areas (Eligman, 1987 and Wagener. 1964 and 1965) have used various nomenclature systems for the igneous rocks. The raw data, when available, of these earlier workers was recalculated and plotted on ternary diagrams and classified based on IUGS nomenclature. Pyroclastic rock terminology follows that of Fisher and Schminke (1984). DESCRIPTION OF MAP UNITS SEDIMENTARY UNIT Qal - Alluvium: Unconsolidated poorly sorted and stratified deposits of angular to subrounded clay, silt, sand and gravel- to cobble-sized clasts, in stream drainages. May include point bars, terraces and natural levees along larger stream floodplains. Structural measurements depicted on the map within Qal represent outcrops of crystalline rock inliers surrounded by alluvium. INTRUSIVE AND METAINTRUSIVE UNIT Jd - Diabase: Black to greenish-black, fine- to medium-grained, dense, consists primarily of plagioclase, augite and may contain olivine. Occurs as dikes up to 100 ft wide. Diabase typically occurs as spheriodally weathered boulders with a grayish-brown weathering rind. Red station location indicates outcrop or boulders of diabase. METAVOLCANIC UNITS HYCO FORMATION - UPPER PORTION (STRATIGRAPHIC ORDER UNCERTAIN Zq - Quartz body: White, beige, red. and tan; sugary to porcelaneous; very fine- to medium-grained massive quartz rock to quartzite-like rock. Outcrops are usually massive. May contain vugs with crystal shaped terminations. Map areas contain boulders (up to several feet in diameter) and/or outcrops of white colored massive quartz. Zhat (u) - Altered tuffs: Very light gray to light greenish gray (whitish in areas) with red and yellow mottling, altered volcaniclastic rocks. Alteration consists of silicified. sericitized and pyrophyllitized rock. Sericite phyllite. pods of pyrophyllite, and quartz + phyrophyllite rock all with less than 1 mm to 2 mm diameter weathered sulfides are common. Relict lithic clasts and kaolinitized feldspar crystal shards are visible in some exposures. Relict structures are obliterated in heavily altered rocks. Map area contains boulders (up to several feet in diameter) and outcrop of massive milky quartz and quartz + sericite rock. Zhe/pl - Mixed epiclastic-pyroclastic rocks with interlayered dacitic lavas: Grayish-green to greenish-gray, locally with distinctive reddish-gray or maroon to lavender coloration; metamorphosed: conglomerate, conglomeratic sandstone, sandstone, siltstone and mudstone. Lithologies are locally bedded; locally tuffaceous with a cryptocrystalline-like groundmass. Siltstones are locally phyllitic. Locally contain interbedded dacitic lavas identical to Zhdlt unit. Contains lesser amounts of fine- to coarse tuff and lapilli tuff with a cryptocrystalline-like groundmass. Minor andesitic to basaltic lavas and tuffs present. Silicified and/or sericitized altered rock similar to Zhat unit are locally present. Conglomerates and conglomeratic sandstones typically contain subrounded to angular clasts of dacite in a clastic matrix. Portions of the Zhe/pl unit are interpreted to have been deposited proximal to active volcanic centers represented by the Zhdlt unit but are also interpreted to record the erosion of proximal volcanic centers after cessation of active volcanism. Zhdlt (u) - Dacitic lavas and tuffs of the upper portion of the Hyco Formation: Greenish-gray to dark gray, siliceous, aphanitic dacite. porphyritic dacite with plagioclase phenocrysts. and flow banded dacite. Dacite with hyaloclastic textures are common. Welded and non-welded luffs associated with the lavas include: greenish-gray to grayish-green, fine tuff, coarse plagioclase crystal tuff and lapilli tuff. Locally, interlayers of immature conglomerate and conglomeratic sandstone with abundant dacite clasts are present. The dacites are interpreted to have been coherent extrusives or very shallow intrusions associated with dome formation. The tuffs are interpreted as episodic pyroclastic flow deposits, air fall tuffs or reworked tuffs generated during formation of dacite domes. The unit occurs as map scale pods surrounded by clastic rocks of Zhe/pl unit. Wortman et al. (2000) reports an age of 615.7+3.7/-1.9 Ma U-Pb zircon date for a dacitic tuft' from the unit in the Rougemont quadrangle. Zhdsi (u) - Dacitic shallow intrusive of the upper portion of the Hyco Formation: Gray-green, light green to green, greenish-gray to light gray; dacite, plagioclase poiphyritic dacite with a granular-textured groundmass to micro-granodiorite (intrusive texture visible with 7x hand lens). Locally fine- to medium grained granodiorite present. Plagioclase phenocrysts, when present, range from less than 1 mm to 4 mm. Black colored amphibole, when visible, occurs as phenocrysts (less than 1 mm to 1 mm) and as intergrowths with plagioclase. Amphibole intergrowths distinguish rock from fine-grained tuffs. Interpreted as shallow-ly emplaced dacite probably co-magmatic with Zdlt (u) unit. Zhadlt (u)- Andesitic to dacitic lavas and tuffs of the upper portion of the Hyco Formation: Black to dark gray, gray-green to green; aphanitic andesite to dacite and porphyritic andesite to dacite with plagioclase phenocrysts. Hyaloclastic textures are common. Interlayed with the lavas are gray to black; welded and non-welded; coarse tuff, lapilli tuff, and tuff breccia. Rocks interpreted as andesites have distinct interior weathering rind of light brown to gray and fresh surfaces exhibit non-vitric like textures in contrast to dacites. Zhablt - Andesitic to basaltic lavas and tuffs: Green, gray-green, gray, dark gray and black; typically unfoliated, amygdaloidal. plagioclase porphyritic. amphibole/pyroxene porphyritic and aphanitic; andesitic to basaltic lavas and shallow intrusions. Hyaloclastic texture is common and imparts a fragmental texture similar to a lithic tuff on some outcrops. Locally interlayered with meta-sediments identical to the Zhe/pl unit. Bowman. J.D., 2010.. The Aaron Formation: Evidence for a New Lithotectonic Unit in Carolina, North Central North Carolina, unpublished masters thesis. North Carolina State University, Raleigh. North Carolina, 1 16 p. Bradley, P.J., Gay, N.K.. Bechtel. R. and Clark. T.W.. 2007. Geologic map of the Farrington 7.5-minute quadrangle, Chatham, Orange and Durham Counties, North Carolina: North Carolina Geological Survey Open-file Report 2007-03, scale 1:24,000. in color. Bradley, P.J. and Miller, B.V., 2011. New geologic mapping and age constraints in the Hyco Arc of the Carolina terrane in Orange County, North Carolina: Geological Society of America Abstracts with Programs. Vol. 43, No. 2. Eligman. D.. 1987. Volcanic stratigraphy in the Carolina slate belt near Chapel Hill. North Carolina, unpublished masters thesis. University of North Carolina at Chapel Hill. 51 p. Fisher, R.V. and Schmincke H.-U.. 1984, Pyroclastic rocks. Berlin. West Germany. Springer- Verlag. 472 p. Glover. L.. and Sinha. A.. 1973. The Virgilina deformation, a late Precambrian to Early Cambrian (?) orogenic event in the central Piedmont of Virginia and North Carolina. American Journal of Science, Cooper v. 273- A. pp. 234-251. Harris, C., and Glover, L., 1985, The Virgilina deformation: implications of stratigraphic correlation in the Carolina slate belt. Carolina Geological Society field trip guidebook. 36 p. Harris. C., and Glover. 1988, The regional extent of the ca. 600 Ma Virgilina deformation: implications of stratigraphic correlation in the Carolina terrane. Geological Society of America Bulletin, v. 1 00. pp. 200-217. Hibbard. J.. Samson, S.. 1995. Orogenesis exotic to the Iapetan cycle in the southern Appalachians, In. Hibbard. J., van Staal. C., Cawood. P. editors. Current Perspectives in the Appalachian- Caledonian Orogen. Geological Association of Canada Special Paper, v. 41. pp. 191-205. Hibbard. J.. Stoddard. E.F.. Secor, D., Jr., and Dennis. A., 2002. The Carolina Zone; Overview of Neoproterozoic to early Paleozoic peri-Gondwanan terranes along the eastern flank of the southern Appalachians: Earth Science Reviews, v. 57, n. 3/4, p. 299-339. Hibbard. J. P. van Staal. C. R.. Rankin. D. W.. and Williams, H.. 2006. Lithotectonic map of the Appalachian Orogen, Canada-United States of America. Geological Survey of Canada. Map-2096A 1: 1.500.000-scale. Le Maitre. R.W.. Ed., 2002. Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Igneous Rocks: Cambridge, Cambridge University Press, 252 p. Pollock. J. C.. 2007. The Neoproterozoic-Early Paleozoic tectonic evolution of the peri-Gondwanan margin of the Appalachian orogen: an integrated geochronological, geochemical and isotopic study from North Carolina and Newfoundland. Unpublished PhD dissertation. North Carolina State University. 194 p. Samson, S.D. and Secor, D., 2001, Wandering Carolina: Tracking exotic terranes with detrital Zircons, GSA Abstracts with Programs Vol. 33, No. 6. p. A-263 Tadlock, K.A. and Loewy, S.L.. 2006, Isotopic characterization of the Farrington pluton: constraining the Virgilina orogeny, in Bradley, P.J., and Clark, T.W.. editors. The Geology of the Chapel Hill. Hillsborough and Efland 7.5-minute Quadrangles, Orange and Durham Counties, Carolina Terrane. North Carolina. Carolina Geological Society Field Trip Guidebook for the 2006 annual meeting, pp. 17-21. Wagener, H.D.. 1964, Areal modal variation in the Farrington igneous complex, Chatham and Orange counties. North Carolina, unpublished M.S. thesis. University of North Carolina at Chapel Hill. 51 p. Wagener, H.D.. 1965. Areal modal variation in the Farrington igneous complex, Chatham and Orange Counties, North Carolina, Southeastern Geology, v. 6, no. 2. p. 49-77. Wortman. G.L.. Samson. S.D.. and Hibbard. J.P.. 2000. Precise U-Pb zircon constraints on the earliest magmatic history of the Carolina terrane. Journal of Geology, v. 108. pp. 321-338. Qal - T Jd / L Zq Zhat (u) Zhe/pl Zhdlt (u) Zhdsi (u) Zhadlt (u) Zhablt REFERENCES: CONTACTS, FOLDS AND OTHER FEATURES Lithologic contacts - Distribution and concentration of structural symbols indicates degree of reliability. contact - location known contact - location inferred contact - location concealed Qal contact brittle fault - inferred fold axis - overturned anticline - inferred fold axis - overturned syncline - concealed diabase contact - dashed where inferred, dotted where concealed in cross section, fold form lines in cross section, inferred axial trace of large-scale fold A A' cross section line 15 , strike and dip of inclined primary ' bedding and layering Strike and dip of inclined mylonitic foliation strike and dip of inclined primary bedding and layering (multiple observations at one locality) Strike and dip of inclined mylonitic foliation (multiple observations at one locality) 74 У strike and dip of inclined overturned primary bedding Fault plane - normal strike and dip of primary flow banding V strike and dip of inclined joint surface 74 * strike and dip of primary volcanic r compaction and/or welding foliation strike and dip of inclined joint surface (multiple observations at one locality) strike and dip of inclined regional foliation У strike of vertical joint surface strike and dip of inclined regional foliation (multiple observations at one locality) У strike of vertical regional foliation (multiple observations at one locality) У strike of vertical joint surface (multiple observations at one locality) У bearing and plunge of aligned clast lineation У bearing and plunge of crenulation lineation by foot by car Roberson Town Lake cross section scale - 1:24 000 no vertical exaggeration strike and dip of cleavage strike and dip of cleavage (multiple observations at one locality) diabase station location У У © bearing and plunge of mineral lineation bearing and plunge of slickenline station location active quarry - crushed stone (3M pit) Base map is from USGS 2010 GeoPDF of the Pittsboro 7.5-minute quadrangle. Aerial photo, map collar and select features removed. Bounds of GeoPDF based on 7.5-minute grid projection in UTM 17S; North American Datum of 1983 (NAD83). GEOLOGIC MAP OF THE NORTHERN PORTION OF THE PITTSBORO 7.5-MINUTE QUADRANGLE, CHATHAM COUNTY, NORTH CAROLINA By Philip J. Bradley and Heather D. Hanna Digital representation by Michael A. Medina and Philip J. Bradley 2012 This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program, award number G1 1 AC20296. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government. Disclaimer: This Open-File report is preliminary and has been reviewed for conformity with the North Carolina Geological Survey editorial standards or with the North American Stratigraphic Code. Further revisions or corrections to this preliminary map may occur prior to its release as a North Carolina Geological Survey map. GEOLOGIC MAP OF THE NORTHERN PORTION OF THE PITTSBORO 7.5-MINUTE QUADRANGLE, CHATHAM COUNTY, NORTH CAROLINA NCGS OPEN FILE REPORT 2012-03