Geologic map of the Chatham County portion of the Silk Hope 7.5-minute quadrangle, Chatham and Alamance Counties, North Carolina

го с 1- ш +■* га з О и о N О <л ш о о N О V. ш о к. о. о (1) alluvium ' Г Jd \ L diabase Metamorphic Rocks Нусо Formation - upper portion metamorphosed plutonic rocks ca. 613 and 614 Ma (Wortman et al., 2000) zgr Zdi Zdi-porphyrilic metamorphosed volcaniclastic sedimentary and pyroclastic rocks (stratigraphic relations uncertain) ca. 612 - 616 Ma (Wortman et al., 2000; Bowman, 2010; Bradley and Miller, 2011) Equal Area Schmidt Net Projections and Rose Diagram Plots and calculations created using Stereonet v. 8.6.0 based on Allmendinger et al. (2013) and Cardozo and Allmendinger (2013) Equal Area Schmidt Net Projection of Contoured Poles Primary Bedding and Layering Contour Interval = 2 sigma N = 52 Equal Area Schmidt Net Projection of Contoured Poles to Foliation and Cleavage Contour Interval = 2 sigma N = 277 Unidirectional Rose Diagram of Joints N = 259 Outer Circle = 6% Mean vector = 205 degrees DEPARTMENT OF ENVIRONMENT AND NATURAL RESOURCES DIVISION OF ENERGY, MINERAL, AND LAND RESOURCES TRACY E. DAVIS, DIRECTOR KENNETH B. TAYLOR, STATE GEOLOGIST This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program NORTH CAROLINA GEOLOGICAL SURVEY OPEN FILE REPORT 2014-02 79 22' 30" at Center of Sheet = 3 degrees 26 minutes \ Ф a О Ф c <0 О -£ £ I £ ■S I щ c £ £ Ф a CO 2 CD 1 >. ■q ~o СИ CL ■£ £ го £ w О h- in Q> Ф о in В 6 с 6 'jz b о 3 z S Q СО cross section scale - 1:24 000 no vertical exaggeration by car MAJOR ELEMENTS IN WEIGHT PERCENT OXIDE SELECTED TRACE ELEMENTS IN PPM or PPB** SAMPLE MAP UNIT Si02 TiO, ai2o3 Fc203* MnO MgO CaO Na,<> k2o p2o< 0203 LOI TOTAL Co Ni Cu Zn As Rb Sr Y Zr Mo Ba La Ce Nd Sm Au** SH-437 Zhdlt (u) 73.33 0.33 13.72 2.48 0.11 0.33 1.11 5.75 2.14 0.04 <0.002 0.5 99.85 0.7 0.4 1.6 113 2.4 33.7 300 32.1 181.3 0.4 609 18.7 39.9 23.6 5.33 0.6 SI 1-2221 Zhasi 46.86 3.07 14.23 14.55 0.26 4.58 9.61 2.76 0.6 0.95 0.002 2.3 99.75 29.1 7.7 18.4 74 1.8 14 440.2 31.1 91.6 0.2 196 14.9 38.6 29.4 7.33 1.1 SI 1-2661 Zhadlt (u) 51.52 1.51 15.65 11.57 0.19 4.37 8.88 2.84 0.26 0.39 <0.002 2.6 99.76 26.9 6.5 64.5 101 1.4 3 558.4 20.1 54.7 0.2 174 6.9 17.3 13 3.5 <0.5 SI 1-2662 Zhadlt (u) 53.01 1.09 15.73 1 1.99 0.19 3.8 8.08 2.99 0.33 0.18 0.005 2.4 99.8 27.3 5.1 7 83 0.9 3.7 406.1 20.8 59.4 0.1 186 6.4 16.3 12.1 3.16 1.2 Major and trace element geochemical analyses completed by Acme Labs, Vancover, Canada. ICP-ES and ICP-MS whole rock analyses using method code LF202. Total iron measured as Fe203* **AII trace elements are in PPM (parts per million) except Au which is in PPB (parts per billion) LOI = loss on ignition DESCRIPTION OF MAP UNITS Pre-Mesozoic crystalline rocks in the Silk Hope Quadrangle are part of the redefined Hyco Arc (Hibbard et al.. 2013) within the Neoproterozoic to Cambrian Carolina terrane of the Carolina Zone (Hibbard et al., 2002; Hibbard et al., 2006). In the region of the map area, the Carolina terrane can be separated into two lithotectonic units: 1 ) the Hyco Arc and 2) the Aaron Formation of the redefined Virgilina sequence (Hibbard et al.. 2013). The Hyco Arc consists of the Hyco Formation which include ca. 612 to 633 Ma (Wortman et al., 2000; Bowman. 2010; Bradley and Miller, 201 1) metamorphosed layered volcaniclastic rocks and plutonic rocks. Available age dates (Wortman et al., 2000; Bradley and Miller, 201 1) 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 detrita! zircons of ca. 578 and 588 Ma (Samson et al., 2001 ; Pollock. 2007. respectively). The Hyco Arc and Virgilina sequence lithologies were folded and subjected to low grade metamorphism during the ca. 578 to 554 Ma (Pollock, 2007) Virgilina deformation (Glover and Sinha, 1973; Harris and Glover. 1985; Harris and Glover, 1988; Hibbard and Samson. 1995). In the map area, original layering of Hyco Formation lithologies are interpreted to range from shallowly to steeply dipping due to open to isoclinal folds that can be 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. The dacitic lavas and tuffs unit is interpreted to represent dacitic domes and proximal pyroclastics. The epiclastic/pyroclastic units are interpreted to represent deposition from the erosion of dormant and active volcanic highlands. Some of the meta-volcaniclastic units within the map area display lithologic relationships similar to dated units present in northern Orange and Durham Counties. Due to these similarities, the meta-volcaniclastic units have been tentatively separated into upper and lower 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 metamoiphic 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. A preliminary review of the area geology is provided in Bradley (2013). Unit descriptions common to the Bynum quadrangle (Bradley et al., 2013) and nearby detailed geologic maps (see Bradley. 2013) were used for conformity with on strike units in neighboring quadrangles. 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 normalized mineral assemblages when whole-rock geochemical data is available. Past workers in the Silk Hope quadrangle and adjacent areas (Hauck, 1977; Wilkinson, 1 978; Schmidt et al., 2006) have used various nomenclature systems for the igneous rocks. To assist in naming of units, select raw data of earlier workers, when available from neighboring quadrangles, were recalculated and plotted on ternary diagrams and classified based on IUGS nomenclature. Pyroclastic rock terminology follows that of Fisher and Schminke ( 1984). SEDIMENTARY UNIT Qal Jd. / Zgr Zdi Zdi-porphyritic Zhel Zhe/pl Zhdlt (u) Zhdsi (u) Zhadlt (u) Zhasi Zhablt-dcp 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 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. META1NTRUS1VE UNITS Hyco Formation - Upper Portion Zgr - Granite: Leucocratic, light brownish to beige or creamy, and locally pale pink or green; medium- to coarse-grained, equigranular metamorphosed leucocratic granodiorite and granite; locally weakly porphyritic with beta-quartz forms; grades to quartz porphyry in zones of cleavage development; quartz may be bluish; locally reddish weathering; locally contains epidote and/or chlorite clots possibly pseudomorphic after a hornblende; feldspar and quartz grains resist weathering and produce a bumpy surface; plagioclase and quartz phenocrysts sit in a granophyric matrix of alkali feldspar and quartz. Correlative to the Chatham granite of Hauck ( 1977) as identified by Wilkinson (1978). Zdi - Diorite: Mesocratic (CI-50). greenish-gray to grayish-green, fine- to coarse-grained, hypidiomorphic granular diorite to quartz monzodiorite. Major minerals include plagioclase and amphibole. Plagioclase crystals are typically sericitized and saussuritized. Amphiboles are typically altered to chlorite and actinolite masses. May be gabbroic locally. Quartz monzodiorite of unit identified northeast of Sapling Ridge Church. Zdi-porphyritic: Diorite porphyry: Mesocratic to almost melanocratic, greenish-gray to gray diorite porphyry with fine- to medium-grained groundmass and euhedral phenocrysts (up to 18 mm) of light gray to white plagioclase. Plagioclase crystals may be saussuritized. Unit locally includes equigranular. diorite to monzodiorite like Zdi unit. METAVOLCANIC UNITS Hyco Formation - Upper Portion Zhcl - Epiclastic rocks and lavas: Conglomerate, conglomeratic sandstone, sandstone, siltstone and mudstone. Siltstones and mudstones typically display bedding ranging from mm-scale up to 10 cm, bedding layers traceable for several feet locally, may exhibit soft sediment deformation. Locally tuffaceous with a relict vitric texture. Locally contain interbedded dacitic to basaltic lavas. Conglomerates and conglomeratic sandstones typically contain subrounded to angular clasts of dacite in a clastic matrix. Deposition interpreted as distal from volcanic center, in deep water(?). and via turbidite flows. Correlative in part to Haw River sequence of Hauck ( 1977). Zhe/pl - Mixed epiclastic-pvrociastic 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 relict vitric texture. Siltstones are locally phyllitic. Locally contain interbedded dacitic lavas identical to Zhdlt unit. Contains lesser amounts of fine- to coarse tuff and lapilli tuft' with a vitric groundmass. Minor andesitic to basaltic lavas and tuffs present. Silicified and/or sericitized altered rock 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. Hyaloclastic textures are common. Welded and non-welded tuffs associated with the lavas include: greenish-gray to grayish-green, fine tuff, coarse plagioclase crystal luff 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 and/or Zhel units. Wortman et al. (2000) reports an age of 61 5.7+3. 7/-1 .9 Ma U-Pb zircon date for a dacitic tuff 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 porphyritic 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 I 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 shallowly 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. Locally include andesites identical to Zhasi unit - especially on prominent ridge in southern portion of quadrangle, lnterlayed with the lavas are gray to black; welded and non-welded; coarse tuff, lapilli tuff, and tuff breccia. Locally interlayered with meta-sediments identical to adjacent Zhe/pl and/or Zhel units. Rocks interpreted as andesites have distinct interior weathering rind of light brown to gray and fresh surfaces exhibit weakly vitric like textures in contrast to dacites. Zhasi - Andesitic shallow intrusive: Grayish-green to light green, plagioclase porphyritic andesite with a granular-textured groundmass to very fine-grained diorite (with intrusive texture visible with 7x hand lens). Contains lesser amounts of fine- to medium grained diorite. Plagioclase phenocrysts typically range from 1 mm to 4 mm. Dark green to black colored amphibole, when present, occurs as phenocrysts (less than 1 mm to 1 mm) and as intergrowths with plagioclase. Zhablt-dcp - Andesite to basalt porphyry of the Dry Creek area: Distinctive, green to grayish-green, andesite porphyry with aphanitic groundmass and euhedral phenocrysts (up to 20 mm) of greenish- white plagioclase; phenocrysts typically constitute 20 to 50% of the rock; lesser pyroxene/amphibole phenocrysts. Correlative to Dry Creek Porphyry complex of Hauck (1977). REFERENCES: Allmendinger, R. W., Cardozo, N. C., and Fisher. D., 2012, Structural Geology Algorithms: Vectors & Tensors: Cambridge, England, Cambridge University Press, 289 pp. Bowman. J.D., 2010.. The Aaron Formation: Evidence for a New Lithotectonic Unit in Carolinia, North Central North Carolina, unpublished Master's thesis. North Carolina State University. Raleigh. North Carolina. 1 16 p. Bradley, P.J.. and Miller, B.V., 201 1. 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. Bradley, P.J., 2013, The Carolina terrane on the west flank ofthe Deep River Triassic basin in the northern Piedmont of North Carolina - a status report, in Hibbard. J.P. and Pollock, J. editors. One arc, two arcs, old arc. new arc - The Carolina terrane in central North Carolina. 2013 Carolina Geological Society field trip guidebook. Salisbury. North Carolina, p. 139-15 1. Bradley. P.J.. Hanna. H.D.. Stoddard. E.F.. and Bechtel, R. 2013, Geologic map of the Bynum 7.5-minute quadrangle. Orange. Chatham and Alamance Counties, North Carolina: North Carolina Geological Survey Open-file Report 2013-03, scale 1:24,000. in color. Cardozo. N.. and Allmendinger. R. W., 2013. Spherical projections with OSXStereonet: Computers & Geosciences, v. 51. no. 0. p. 193 - 205. doi: 10.1 01 6/j.cageo.2012. 07.021 . 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. 100. pp. 200-217. Hauck. S.A.. 1977, Geology and petrology of the northwest quarter of the Bynum quadrangle, Carolina slate belt. North Carolina, unpublished M.S. thesis. University of North Carolina at Chapel Hill. 146 p. Hibbard. J., and 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. p. 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. Hibbard, J.P.. Pollock. J.C.. and Bradley. P.J., 2013. One arc. two arcs, old arc, new arc: An overview of the Carolina terrane in central North Carolina. Carolina Geological Society field trip guidebook. 265 p. 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., Secor, D.. and Hamilton, M.A., 2001. Wandering Carolina: Tracking exotic terranes with detrital Zircons. GSA Abstracts with Programs Vol. 33, No. 6. p. A-263. Schmidt, R.G., Gumiel, P.. and Payas, A.. 2006. Geology and mineral deposits of the Snow Camp-Saxapahaw area. Central North Carolina: United States Geological Survey Open-file Report 2006-1259 (http://pubs.usgs.gov/of/2006/1259/index.html). Steponaitis, Vinca P.. Jeffrey D. Irwin, Theresa E. McReynolds. Christopher Moore (eds.), 2006. Stone Quarries and Sourcing in the Carolina Slate Belt. Research Report No.25, Research Laboratories of Archaeology. University of North Carolina, Chapel Hill (www.ria.unc.edu/Publications/Res_reports.html): 5 1, 57. 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.. eds.. 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. Wilkinson, S.E., 1978, The geology of the northeast quarter of the Silk Hope quadrangle, Carolina Slate belt. North Carolina, unpublished M.S. thesis. University of North Carolina at Chapel Hill. 56 p. 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. CONTACTS, FOLDS AND OTHER FEATURES Lithologic contacts - Distribution and concentration of structural symbols indicates degree of reliability. lithologic contact-inferred lithologic contact-concealed gradational contact-inferred brittle fault-inferred Qal contact diabase dike - inferred interpreted fold hinge of large scale anticline; £ — ?-► question mark where existence is questionable; dotted where concealed; arrow indicates direction of plunge inteipreted fold hinge of large scale syncline; - i — -?->• question mark where existence is questionable; dotted where concealed; arrow indicates direction of plunge — xi - interpreted fold hinge of large scale overturned anticline — У - inteipreted fold hinge of large scale overturned syncline A A' cross section line in cross section, fold form lines in cross section, inferred axial trace of large-scale fold 22 S/ 34 80 У V 60y 50 strike and dip of inclined primary bedding and/or layering strike and dip of inclined primary bedding and/or layering (multiple observations at one locality) strike and dip of inclined overturned primary bedding strike and dip of primary volcanic compaction and/or welding foliation strike and dip of inclined regional foliation strike and dip of inclined regional foliation (multiple observations at one locality) X strike of vertical regional foliation strike of vertical regional foliation /* (multiple observations at one locality) strike and dip of cleavage 65^/ strike and dip of cleavage / 77 (multiple observ ations at one locality) 5 у strike and dip of inclined joint surface X strike of vertical joint surface # strike of vertical joint surface / (multiple observations at one locality) © A diabase station location station location 50 3*/. strike and dip of inclined joint surface / 70 (multiple observations at one locality) SH-437, station location with conspicuous amounts of quartz cobbles and/or boulders FBL lithic samples of Steponaitis et al. (2006) geochemical sample location (NCGS) prospect pit quarry - abandoned GEOLOGIC MAP OF THE CHATHAM COUNTY PORTION OF THE SILK HOPE 7.5-MINUTE QUADRANGLE, CHATHAM AND ALAMANCE COUNTIES, NORTH CAROLINA By Heather D. Hanna and Philip J. Bradley Digital representation by Michael A. Medina and Heather D. Hanna 2014 1. quarry (flagstone) - abandoned 2. quarry (crushed stone) - abandoned 3. prospect pit (commodity unknown) - abandoned 4. prospect pit (copper) - abandoned Base map is from USGS 2010 GeoPDF of the Silk Hope 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). This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program, award number G13AC00204. The views and conclusions contained in this document are those ofthe authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, ofthe U.S. Government. ACKNOWLEDGEMENT: Randy Bechtel assisted in data collection. 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. This map and explanatory information is submitted for publication with the understanding that the United States Government is authorized to reproduce and distribute reprints for governmental use. Scan with smartphone for link to GeoPDF of map. Third party App required. GEOLOGIC MAP OF THE SILK HOPE 7.5-MINUTE QUADRANGLE, CHATHAM AND ALAMANCE COUNTIES, NORTH CAROLINA NCGS OPEN FILE REPORT 2014-02