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
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2
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5
э
о
Qal
alluvium (Qal)
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О
N
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и
ш
7
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Jd
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diabase
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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
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lain
Webster Poultry
Company
Pond Dam
Zhe/pl
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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