DurhamSub-basin of the TriassicDeepRiver Basin Alluvial Fan Deposits
The Falls Lake-Wake Forest map area extends westward into sedimentary rocks of the Chatham Group in the Durham sub-basin of the TriassicDeepRiver basin. A separate article in this guidebook by C.W. Hoffman summarizes the stratigraphic framework in this part of the basin. Alluvial fan deposits of conglomerate (Trcc), sandstone containing interbedded conglomerate (Trcs/c), pebbly sandstone (Trcsc), and sandstone (Trcs) along the eastern margin of the basin are collectively equivalent to Hoffman and Gallagher’s (1989) Lithofacies Association III. An exposure of alluvial fan conglomerate south of the Falls Lake-Wake Forest map area will be examined on the 1994 Carolina Geological Society field trip.
Fluvial Deposits
In the central area of the rift basin (at the western edge
of Figure 2), a unit of sandstone and interbedded siltstone
(Trcs/si) in the Chatham Group is part of Hoffman and Gallagher’s
Lithofacies Association II and is interpreted as fluvial
in origin. This sandstone and interbedded siltstone unit
consists mainly of fining-upward sequences, 2-5 meters
thick, of grayish-pink to pale red, medium- to coarse-grained
arkose grading upward into fine-grained arkose and reddishbrown,
bioturbated siltstone. Pink K-feldspar is abundant,
and the presence of detrital muscovite distinguishes these
sandstones from those of the alluvial fans. Hoffman and Gallagher
(1989) interpret this fluvial unit as meandering stream
deposits.
Jonesboro Fault
The Jonesboro fault forms the eastern boundary of the
TriassicDeepRiver basin (Figure 1). In the Falls Lake-Wake
Forest area, this steeply northwest-dipping normal fault separates
moderately northwest-dipping foliated metamorphic
rocks of the Beaverdam diorite-gabbro complex and Cary
formation from gently southeast-dipping units of Upper Triassic
conglomerate, sandstone, and siltstone of the Chatham
Group in the Durham sub-basin of the DeepRiver basin on
the western side of the map (Figure 2). No evidence for pre-
Mesozoic movement on the Jonesboro fault has been found
in this area (Horton and others, 1992). Horton and others’
(1992) geologic map includes the Jonesboro fault segment
(just west of Figure 2) described by M.J. Bartholomew and
others in a separate article in this guidebook. An exposure of
the Jonesboro fault south of the area shown in Figure 2 will
be visited on the 1994 Carolina Geological Society field trip.
Diabase Dikes
Early Mesozoic diabase dikes (not shown on Figure 2)
cut across the Jonesboro fault and Triassic sedimentary strata
of the Durham sub-basin, as well as older rocks throughout
this region of the Piedmont (Horton and others, 1992). Most
of the dikes on the 1:24,000-scale geologic map by Horton
and others (1992) have steep dip angles and strike north to
northwest. The diabase in this area is typically dark gray to
black, fine- to medium-grained olivine diabase, and it is
probably Early Jurassic in age (Sutter, 1985, 1988). A comprehensive
synthesis of diabase dikes in North Carolina and
South Carolina is provided by Ragland (1991).
Carolina terrane
The Cary formation (informal name of Farrar, 1985a) in the Falls Lake-Wake Forest area is approximately equivalent to the informal Cary sequence of Parker (1979), but extends east of the arbitrary metamorphic boundary shown on the geologic map of WakeCounty (Parker, 1979, Plate 1). In thisarea, the Cary formation consists of phyllitic to almost massive,felsic metavolcanic rocks (CZcf) and interlayeredmetasedimentary units. On the 1994 Carolina GeologicalSociety field trip, metavolcanic rocks of the Cary formationwill be examined in a quarry at HollySprings, southwest ofthe map area. The age of the Cary formation is interpreted tobe Late Proterozoic based on a preliminary U-Pb zircon ageof 574±12 Ma (see article by S.A. Goldberg in this guidebook)as well as similarity and proximity to volcanogenicrocks near Durham, North Carolina, which contain Late Proterozoicmetazoan fossils and have a U-Pb zircon age of620±20 Ma (Glover and Sinha, 1973; Cloud and others,1976; Harris and Glover, 1988).The Cary formation is bounded on the west by theJonesboro fault, which is the eastern border fault of the earlyMesozoicDeepRiver basin (discussed below). The contactbetween the Cary formation and the FallsLake melange isinferred by Horton and others (1992) to be a west-dippingthrust fault as discussed above. The nature of the easterncontact of the Cary formation south of the melange is uncertainand is at least partly obscured by intrusive contactswhere the Cary formation is intruded by the Crabtree Creekpluton (Blake and Stoddard, 1993).
Beaverdam Diorite-Gabbro Complex
The Beaverdam diorite-gabbro complex (informal nameof Parker, 1979) consists of metamorphosed plutonic rocksthat range in composition from hornblende gabbro (CZgb) tobiotite quartz diorite (CZdi) and tonalite. Hornblendite(metamorphosed pyroxenite?) occurs as cumulate(?) layersin metagabbro near the northern end of the complex. Gabbroicdikes are locally conspicuous in diorite. The metamorphic
foliation generally dips northwest. The age of theBeaverdam complex, although undetermined, is inferred tobe Late Proterozoic or Cambrian based on similarity to datedplutons in nearby parts of the Carolina terrane as summarizedby McSween and others (1991, Table 7-1) and referencestherein. A road cut in metamorphosed quartz diorite ofthe Beaverdam diorite-gabbro complex (Locality 5 on Figure2) is an optional stop for the 1994 Carolina Geological Societyfield trip
Falls LakeSchist and related ultramafic pods
The melange matrix is predominantly biotite-muscoviteschist (CZfs) composed mainly of quartz, sodic plagioclase,biotite, and muscovite. Minor amounts of chlorite, epidote,and garnet are common, but kyanite and staurolite occuronly locally. This biotite-muscovite schist, which we interpretas a metamorphosed mudstone, contains lesser amountsof biotite-muscovite-plagioclase-quartz gneiss which weinterpret as metagraywacke. The schist matrix containsblocks and pods of amphibolite (CZfa) and several types ofultramafic rocks ranging from pebble-size to mappabledimensions. A saprolite exposure of the melange in the eastcentralpart of the Bayleaf quadrangle (Locality 7 on Figure2) will be visited on the 1994 Carolina Geological Societyfield trip.Metamorphosed ultramafic rocks, which constituteabout 15%-20% of the melange, include serpentinite, chlorite-actinolite schist, soapstone or talc schist, and hornblendite(Horton and others, 1986, 1992). All of these ultramaficrock types can be observed at Stop B of Stoddard and others(1986) (Locality 6 on Figure 2) which is being visited on the1994 Carolina Geological Society field trip. Protolithsinferred from geochemical and mineralogical studies areconsistent with an interpretation of the metamorphosed ultramaficrocks as ophiolite fragments (Moye, 1981; Stoddardand others, 1982). Many ultramafic bodies have chloritic orleucocratic metasomatic rims, but none have thermal contactaureoles (Horton and others, 1986). Some ultramafic bodiesin the melange contain pods of chromitite. Metamorphicminerals observed along fractures in the chromitite includechlorite, margarite, fuchsite, kyanite, tourmaline, corundum,and rutile (Stoddard and others, 1989).Horton and others (1986) interpreted the lenticularshapes, concordant schistosity, and pinch-and-swell structure
of most ultramafic and mafic blocks in the melange as boudinageresulting from the ductility contrast between blocks andmatrix during regional deformation. However, the earliestschistosity recognized in the matrix, and parallel stringers ofgranite and pegmatite, cut across some of the smaller blockswithout deflection (Figure 2 of Horton and others, 1986),indicating that some of the fragment-in-matrix texture predatesthis schistosity. Locally, the schist matrix containsellipsoidal lumps, interpreted to be rounded pebbles up to afew centimeters across, of granite and pegmatite. Evidencethat some small fragments of ultramafic and mafic rock originatedas sedimentary clasts is equivocal, although the possibilityhas been suggested on the basis of their round toangular shapes and embayed margins (Figure 3 of Hortonand others, 1986). Horton and others (1986) and Blake(1986) suggested that the FallsLake melange, like manyother melanges, may have formed by a combination of sedimentaryand tectonic processes. Whether or not this is thecase, the fabrics observable now are predominantly tectonicin origin
Crabtree terrane
Felsic Gneiss of Crabtree Creek and Crabtree
Creek Pluton
The felsic gneiss of Crabtree Creek (cf) is very lightgray to pinkish-gray, weakly layered and well foliated. It iscomposed of quartz (50-80%), oligoclase (10-35%), microcline(0-10%), and muscovite (0-15%), with accessorybiotite, chlorite, and epidote. The unit includes the “quartzdiskgneiss” of Parker (1979). High quartz content, localquartz disks interpreted by some observers as flattened pebbles,relict plagioclase phenocrysts, and interlayered aluminousand graphite schists (cgk and cgs) have been used asevidence in support of a metasedimentary or metavolcanicorigin for the unit (Parker, 1979; Farrar, 1985a, 1985b; Stoddardand others, 1991). Parts of the felsic gneiss of CrabtreeCreek (cf) appear to be plutonic in origin as proposed byKish and Campbell (1986) on the basis of chemical and isotopicdata, and by Blake and Stoddard (1993) on the basis ofgeologic mapping in the Raleigh West quadrangle. Subhorizontallineation associated with the Nutbush Creek faultzone is prominent on the eastern side of the felsic gneissunit.The felsic gneiss of Crabtree Creek (cf) as mapped byHorton and others (1992) encompassed but did not distinguishthe northern end of the Crabtree Creek granitic pluton.This leucogranitic to granitic pluton, as described south ofthe map area by Blake and Stoddard (1993), is now recognizedas a separate unit (CZg) in the area of Figure 2, whereit lies in the southern part of the map west of the graphiteschists.
Falls Leucogneiss
The segment of the Nutbush Creek fault zone in theFalls Lake-Wake Forest area contains a highly elongate massof subhorizontally lineated, granitic orthogneiss (SOgf)informally known as the Falls leucogneiss (Farrar, 1985a) orFalls lineated gneiss (Mims and others, 1990), which containsabundant accessory magnetite. Geologic mapping byHorton and others (1992) supersedes earlier reconnaissanceby Farrar (1985b) which located the Nutbush Creek faultzone several kilometers farther west. However, evidence ofsubhorizontal lineation and dextral simple shear associatedwith the Nutbush Creek fault zone is intermittently presentfor several kilometers east and west of the Falls leucogneiss(Mims and others, 1990; Blake and Stoddard, 1993; D.E.Blake, this guidebook).