carolina geological society 2000 field trip ... - my articles/dennis...d. e. wyatt and m. k. harris...

WSRC-MS-2000-00606

CAROLINA GEOLOGICAL SOCIETY2000 FIELD TRIP GUIDEBOOK

Savannah River SiteEnvironmental Remediation Systems In Unconsolidated

Upper Coastal Plain Sediments - Stratigraphic andStructural Considerations

Edited by:D. E. Wyatt and M. K. Harris

Carolina Geological Society, 2000 Annual Field Trip Guidebook WSRC-MS-2000-00606

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Outline of the Geology of Appalachian Basement Rocks Underlying theSavannah River Site, Aiken, SC

Allen J. Dennis, Biology and Geology, University of South Carolina Aiken, Aiken SC 29801-6309John W. Shervais, Department of Geology, Utah State University, Logan, UT 84322 andHarmon D. Maher, Jr., Geology and Geography, University of Nebraska Omaha, Omaha, NE 68182-0199

INTRODUCTION

Basement core collected over a forty yearcampaign at the Savannah River Site andsurrounding environs affords an unparalleledopportunity to observe Appalachian basementrock and its alteration during Mesozoic riftingand subsequent Tertiary tectonism. Thestructures that are preserved would surely notsurvive long at the earth�s surface, and havethe potential to change how we think about thehistory of Piedmont rock exposures we use fordetailed bedrock geologic mapping.Remarkably no less than 5000m of coresample fault zones that have beenintermittently active throughout thePhanerozoic. These fault zones acted asconduits for fluids throughout the Mesozoicand Cenozoic. Additionally, evidencepresented here and elsewhere in this volumeindicates that syn- and post-depositionalfaulting of the Tertiary section isfundamentally controlled by long-livedAppalachian structures.

The following short paper draws on our inpress or submitted manuscripts for GeologicalSociety of America Bulletin, AmericanJournal of Science, and Geological Society ofLondon treating the geochemistry andpetrology of metamorphosed Neoproterozoicarc rocks underlying SRS, the Paleozoicstructure of those rocks, and evidence forMesozoic and younger fluid flow andpseudotachylyte generation. This work wassupported by South Carolina UniversitiesResearch and Education Foundation contract170.

We are grateful to Randy Cumbest, SharonLewis, Van Price, Doug Wyatt (all at SiteGeotechnical Services for all or some fractionof the time this work was done), Tom Temples

(at DOE during the time this work was done)for their assistance and support during thestudy. Josh Mauldin, Hampton Uzzelle,Lynda Bolton, John Harper, and Tom Creechassisted in this work.

Lithology

Greenschist to granulite faciesmetamorphosed volcanic arc rocks thatunderlie the Savannah River Site are divided,from northwest to southeast, by metamorphicgrade, into the Crackerneck MetavolcanicComplex, Deep Rock MetaigneousComplex, and Pen Branch MetaigneousComplex (Fig. 1; Dennis and others, 2000,Shervais and others, 2000). The CrackerneckMetavolcanic Complex are the lowest graderocks and comprise very weaklymetamorphosed mafic and felsic tuffs, withrelict lapilli. These are best observed in wellsP30 and P6R (Fig. 2). Deep RockMetaigneous Complex is made up of the DeepRock Metavolcanic Suite and the DRBMetaplutonic Suite. Rocks of the Deep RockMetavolcanic Suite are generally dark grey-green mafic metavolcanic rocks, �well-exposed� in DRB wells 2-7 at depths from ca.1100� to ca. 1900�. These metavolcanic rocksare locally garnet-bearing, indicatingamphibolite facies conditions. These rocksare cut by metamorphosed mafic and felsicdikes that locally preserve good primarytextures. The DRB Metaplutonic Suitecomprises metadiorite and (locally mylonitic)quartz diorite gneiss. In the DRB-1 where thisrock is observed, a serrated contact betweenthe diorite and well-foliated maficmetavolcanic rock is observed. Furthermore,deformation in the diorite is veryheterogeneous and may be best described asthin, late stage anastomosing shear zones.The Pen Branch Metaignous Complex is


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likewise made up of The Pen BranchMetavolcanic Suite and PBF MetaplutonicSuite. The Pen Branch Metavolcanic Suite ismade up of distinctive layered (transposed)intermediate to mafic orthogneisses. Theserocks are observed in PBF-7 between 2500-3500 where they are screens within the PBFMetaplutonic Suite, and between 3600-3648�(TD) in that well. Pen Branch MetavolcanicSuite is also observed in the SeismicAttenuation (SA) Core in spot cores between1959� and 4002�. The PBF MetaplutonicSuite is best observed in PBF-7 and is agarnet-bearing granodiorite to granodioritegneiss, locally with hornblende megacrysts.In that well it is heterogeneously deformed,with a locally intense K-metasomaticoverprint. The variety of deformation andmetamorphic textures in this unit, as well asthe variability of the metasomatic overprint isdisplayed in PBF-7 2550�-3600�.

A schematic interpretation of the temporal orstratigraphic relations between these units ispresented in Figure 3.

Dennis and others (1997) report U-Pb zirconages and Sr and Nd isotopic data for the DRBMetaplutonic Suite and PBF MetaplutonicSuite. Dennis and others (1997) report U-Pbzircon crystallization ages of 625.1± 1.4 Ma(weighted mean 207Pb/206Pb dates of foursize fractions) and 619.2 ± 3.4 Ma (average of207Pb/206Pb dates from only two sizefractions analyzed) for the PBF-7 granodioriteand DRB-1 diorite respectively. Neithersample showed evidence of inheritance. �Ndvalues for PBF-7 and DRB-1 rocks are 2.0 and3.5 respectively (Dennis and others, 1997).DRB-1 tonalite yields a 87Sr/86Sr ratio of0.701939. Sr isotopic systematics of PBF-7granodiorite are significantly complicated bythe addition of Rb during the pinking eventdiscussed below, but a primary initial ratio of0.703 may be estimated.

Based on the differences between Nd isotopesin PBF Metaplutonic Suite and DRBmetaplutonic suite and similarities in REEchemistry (unpublished data, Dennis andothers, 2000, Shervais and others, 2000)

between Pen Branch Metaigneous Complex,Deep Rock Metavolcanic Suite and differencesin REE patterns between those two “units”and DRB Metaplutonic Suite we interpret thatThe DRB Metaplutonic Suite may be faultedinto a Pen Branch Metaigneous Complex-Deep Rock Metavolcanic Suite intrusive-extrusive complex. We interpret that thisfaulting may have occurred in Neoproterozoictime, and may have served to initially localizethe Tinker Creek nappe.

Alleghanian (Pennsylvanian) Structure

A body of evidence supports a faultedoverturned nappe limb as the likely contactbetween the Pen Branch MetaigneousComplex and Deep Rock MetaigneousComplex. The overturned nappe limb is calledthe Tinker Creek nappe, and the shear zonethat �decapitates� the Tinker Creek nappe iscalled the Four Mile Branch fault. The nameFour Mile Branch fault has been usedpreviously in the SRS region to describeoffsets of Tertiary strata by Wyatt and others(1996) and Harris and others (1997). Weinterpret the Tinker Creek nappe and FourMile Branch fault to be Carboniferous in agebased on the similarity of structural style withthat observed in the exposed eastern Piedmontof South Carolina (e.g., Maher , 1987, Secorand others, 1986), supplemented by a single40Ar/39Ar age (305±5 Ma) of a hornblendeneoblast from these rocks reported by Rodenand others (1996).

The evidence supporting the interpretationthat the structure underlying the DRB (1-7)well array is an overturned nappe limbincludes parasitic folds, chloritic slip surfacesand shear bands, and evidence of extensivetransposition (Fig. 4). These data have beensupplemented by P-T calculations based ongarnet-biotite thermometry that supportinferences based on map-scale petrology, andthe seismic reflection processing ofDomoracki (1995). These structures (TinkerCreek nappe - Four Mile Branch fault)controlled the location of the TriassicDunbarton basin border fault. An


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Andersonian (1951) analysis supports thisinterpretation. Tinker Creek nappemesoscopic structures such as the chloriticslip surfaces were later reactivated as conduitsfor fluid flow (pinking event), gouge zones,and ultimately pseudotachylyte.

Mesoscopic structures that indicate nappeemplacement from southeast to northwest areabundant in DRB formation and includeparasitic folds, chloritic slip surfaces andshear bands (Fig. 5). These structures are bestrepresented in DRB well series, particularly inDRB-6. Mesoscopic folds interpreted to beparasitic have a down dip vergence thatindicates an overturned limb geometry. Thetrailing limb of these mesoscopic folds has adip of 50-60° typically (assuming that the coreis vertical), and an overturned limb that isnearly horizontal. Interlimb angles are usuallyin the range of 30-60°. It can be shown thatthe axes of these folds are nearly horizontal.Most foliation dips are in the range of 40-60°in DRB formation; rarely are dipssubhorizontal. There are also �floating�isoclinal fold hinges, intact isoclinal folds, andevidence for transposition within the DRBformation. Probably most of this isoclinal-transposed fabric records nappe deformationbut some may record a Late Precambrianorogenic event.

Chloritic slip surfaces cross-cut parasitic,mesoscopic folds and transposed layering, andshow hanging wall up offset. Chloritic slipsurfaces have the same strike as foliation andalmost always dip in the direction of foliation,but with a shallower dip, ca. 30-50° (Fig. 5g).A small fraction have the opposite dip. Thesurfaces range in thickness from less than 1mm thick to 4 mm thick. Sense of offsetacross these features is consistentlyhangingwall up. The amount of offset can beup to several cm. Chloritic slip surfaces areseparated by 2-10 cm. In DRB-6 arespectacular examples of down-dip vergent(overturned limb) parasitic folds cross cut byhangingwall up chloritic slip surfaces. Theprecise origins of the geometry of thesesurfaces and the chlorite along them areunclear, but are of considerable interest.

Mesocopic shear bands are present in bothvertical and horizontal sections of core, andindicate hangingwall up, and dextral motionrespectively. Horizontal sections of a fewsamples showed sinistral shear sense. It wasnot possible to discern overprinting relationsof dip-slip and strike-slip shear bands. It maybe simplest to intepret these structures ashaving formed during dextral tranpression thathas not been partitioned spatially as it seemsto be the case of the Irmo shear zone andAugusta fault (Secor and others, 1986; Maherand others, 1991).

Norian (Late Triassic) Metasomatism –“The Big Pink”

The Dunbarton Triassic basin (Marine, 1974;Marine and Siple, 1974) is one of a wellstudied array of basins described by Olsen andothers (1991) related to opening of the presentday Atlantic. The border fault rocks andfanglomerates of the northwestern margin ofthe basin are well exposed in coreholes PBF-7and 8. Mesozoic and younger veining andalteration paragenesis are generally bestexpressed in these wells.

Pinking Event

The effects of a hydrothermal alteration(�pinking�) event are widespread andheterogeneous in Pen Branch MetaigneousComplex and Deep Rock MetaigneousComplex. Hot waters with abundantdissolved silica, K and Al were responsiblefor metasomatic alteration along fracturesurfaces and foliation planes. Hydrolysis ofgroundmass biotite in the PBF-7 granodiorite

is interpreted to be the source of K+; thisphase is consistently altered to chlorite in theunpinked granodiorite. In DRB rocks, pinkingis observed as zones typically less than 1 cmthick that reactivate chloritic fractures orquartz veining.

Pinking of the PBF granodiorite occurs aslimited alteration proceeding along foliationplanes with grain size reduction andnucleation of quartz and potassium feldspar


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grains along the faces of potassium feldsparmegacrysts parallel and adjacent to foliationplanes. It is also observed as veins ofalteration cross-cutting foliation. In pinkedrocks, sericitization of plagioclase feldspars isobserved, and these crystals appear to have adusty coating in thin section. Microprobeanalysis indicates that the An content ofplagioclase is ≤An5.

Pinking may occur along fractures in as aband less than 5mm thick, or as broad zonesthat affect ten�s of meters of core (Fig. 6).The most spectacular location that this effectmay be observed covers the range 3496� (box167) - 3602� (box 173) in PBF-7, over whichthe rocks are completely pinked with less than5% mafic minerals. The presence of garnetand altered hornblende megacrysts (altered tomagnetite and pyroxene) in a felsic gneissaffirms that the protolith of these rocks wasthe PBF-7 granodiorite. These rocks are veryhard and recrystallized as befits thesilicification accompanying their alteration.Few later fractures crosscut this zone. Belowthis zone, there is no evidence of pinking, andthe rock is a garnet amphibolite. Biotite andamphibole have not been chloritized.

An isocon diagram (Fig. 7; O�Hara, 1988,1990; O'Hara and Blackburn, 1989; Grant,1986, after Gresens, 1964) was prepared forthe PBF-7 granodiorite. The construction ofthis diagram requires the ratio of major andtrace element analyses for altered rock(pinked) and protolith (unpinked) becalculated, and scaled such that all ratios maybe observed on a single plot. If ametamorphic or hydrothermal event isisochemical the ratios of analysed elements oroxides in the altered rock and protolith willequal 1 and lie along a line with a slope of 1.Deviations from a slope of 1 may suggestrelative enrichment (>1) or leaching (<1).However if certain trace elements areimmobile (e.g., Ni, Sc, Zr, Cr, Ti, V), then theline that passes through those elements� ratioson the isocon diagram may demark theboundary between leaching and enrichment inother elements. Using this logic, it isconcluded that MnO, MgO, and to a lesser

extent CaO are relatively depleted in thepinked rocks and K2O, Na2O, SiO2, Rb, andto a lesser extent Al2O3 are enriched in thepinked rocks.

Pinking is observed to cross-cut differentdeformation-metamorphic facies of the PBFMetaplutonic Suite granodiorite and thus mustpostdate the event responsible for fabricdifferences. Kish (1992) reports a 2 point Rb-Sr isochron of 220±5 Ma (Norian) for agranite recovered from the C-10 borehole.Petrography and chemistry of this sampleconfirm that it has been pinked, and that it issimilar to the PBF-7 granodiorite. Theisochron is interpreted to date the pinkingevent in this area. That the pinking is mostintensive in the PBF-7 well, and that the

biotite (interpreted to be source of the K+) isubiquitously altered to chlorite in the PBF-7granodiorite suggests that the DunbartonBasin border fault was the conduit for thepinking event hydrothermal fluids. It isestimated that the temperature of the waterswas low enough to allow chlorite andoligoclase to form.

Greening Event

A "greening" event postdates the pinkingevent. In crystalline rocks (particularly of thePBF-7 core) this event is recognized byepidote (and rare pyrite) veining that maycause local brecciation as the vein is injected.Epidote veins are generally 1 mm- 1 cm thickand fine-grained. The veins typically have thesame strike as foliation, and dip steeply in thesame direction (concordant), or in the oppositedirection (discordant). The effects of thegreening are, however, most spectacularlydeveloped in Triassic fanglomerates of PBF-7(Fig. 8; boxes 10-21, footage 1150-1330).The matrix of these conglomerates is nolonger brick red but instead is a pale green,and an irregular white and grey rind (5mm to1 cm thick) appears on oxidized gneiss clastsgreater than 2-3 cm in diameter (Fig. 8). Thealternating white and grey bands correspondto gneissosity in the oxidized clast, andinterpreted to represent partially resorbededges and the penetration of reducing fluids


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along gneissosity. In box 10 it is apparent thatshallowly dipping fractures (≤ 25° dip) controlgreening, and greening extends 1-10 cm awayfrom these fractures. In boxes 13-18 thisgreening of conglomerate matrix is complete.Microprobe analyses of fibrous intergranularmaterial (cement?) in the (PBF-7-16) matrixindicate that this is green muscovite. Locallythis material contains fine disseminated pyrite.

Similar relations are observed in GCB-2-648'(Fig. 8). Here saprolitized Triassicfanglomerate is observed. The matrixcomprises coarser grained white to creamygreen material, while the clasts are finergrained (clayey) and brick red. The marginsof these clasts are discernible as reducedghosts in the matrix, while the clasts' reddishinteriors have very highly irregular shapedthat are again interpreted as the delimiting theextent of penetration of reducing fluids alongfoliation. In poorly sorted Cretaceous (?)sediments 30' above (GCB-2-615') the reducedfanglomerate, a clot of euhedral chalcopyritegrains 2.5 cm x 5 cm (long axis) parallel tobedding plane is recognized. It is unlikely that

this (≥ 10 cm3) euhedral sulfide clot is adetrital clast (in sand), and it is interpreted torecord the greening event in Coastal Plainrocks.

The "greening" event indicates a flushing ofreducing waters along Triassic basin borderfaults, that may be related to migration ofhydrocarbons derived from Triassicsedimentary rocks (e.g., D. Richers, pers.comm.) The presence of chalcopyrite in rockstentatively identified as Cape Fear,approximately 10 m above reduced clasts andmatrix of a thin zone of preserved Triassicfanglomerate in GCB-2-648, suggests that thisevent postdates Cape Fear deposition(Coniacian). Differences in oxidation statebetween Cape Fear rocks and overlying redand yellow Middendorf Formation (?) sandsindicate that the greening event occurred priorto Middendorf deposition in the Santonian(86.6-83.0 Ma). Furthermore the presence ofreduced fanglomerate in GCB-2 suggests thata border fault of another Triassic sub-basinmay lie in the the vicinity of the GCB-2 well.

Quartz Veining and Vuggy Quartz

Quartz veining, locally quite vuggy, cutsepidote veins in PBF-7 core. These veins aresubparallel to epidote veins when crosscuttingrelations are observed. The veins generallyhave the same strike as foliation, and usuallydip parallel to foliation or less commonly inthe opposite direction. The veins can beirregular and not simple planar features. Theyare typically not more than 2 cm thick, butthick veins may present open vugs up to 7 mmlined with well terminated crystals. Thesevuggy veins are most notable below PBF-7-(box) 111-2662, and occur with irregularincreasing frequency through box 163-3452',and then less frequently in completely pinkedmylonite through 173-3602'. Thick veins mayalso contain cm-scale clasts of the variablypinked granodiorite gneiss they cut. Boththese observations strongly suggest high fluidpressures in the vicinity of the DunbartonBasin border fault during Late Mesozoicthrough Paleogene time.

Zeolite ± calcite

Zeolite ± calcite, locally euhedral, veins cutand locally fill vuggy quartz veins throughoutDRB and PBF formations. These veins aresubparallel to foliation as the epidote andquartz veining events, but may crosscutfoliation. The zeolite is typically pinkishorange or less commonly white. Verycommonly it forms radiating sprays, less than2 cm in diameter, of euhedral crystals, lessthan 1 cm in length, on the fracture surface.Calcite is typically colorless and rarely formseuhedral blocky crystals.

Strike normal zeolite

Nearly vertical (75-90°), nearly "strike-normal" zeolite-filled fractures are verycommon in DRB and PBF formation rocks.Map view of individual core segmentsindicates that often the strike of veins is 15-20° clockwise of foliation dip direction ratherthan precisely strike-normal. The veins aretypically less than 1 cm thick, and are filledwith pinkish orange to white crystals. Some


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veins show evidence of (considerable)indeterminate amounts of offset, but thisoffset must predate filling of fractures by largewell-formed crystals.

Approximately strike-normal joint sets havebeen recognized in the eastern Piedmont ofSouth Carolina during the course of detailedbedrock mapping by the authors (unpublisheddata; Bartholomew and others, 1997, Brodieand Bartholomew, 1997, Heath andBartholomew, 1997). Assuming a regionalstrike of 065 it is estimated that the orientationof the "strike-normal" zeolite filled fractureset is 350-355°, and probably indicates anepisode of approximately E-W extension,accompanied by minor strike-slip motion.

Understanding the timing of this motion isimportant for an understanding of Teritaryregional kinematics; Dennis and others (2000)report an attempt was made to date thesemineralized veins. These efforts wereencouraged by the report of Secor et al. (1982,p. 6952) of the successful K-Ar dating of astrike-parallel (060) set of zeolites in theCarboniferous Winnsboro (SC) granite nearthe Virgil Summer nuclear station. The agereported is 45±5 Ma (South Carolina Electric& Gas Company 1977). This set is post-datedby a second zeolite array oriented 332(approximately "strike normal" to regionalstrike, Secor et al. 1982, their fig 5c.) thatshows several cm of oblique slip. Both setsare reported to be steeply dipping. Based onmicroprobe analysis, PBF-7 strike-normalzeolite crystals are laumontite-chabazite andhave K in the range of 0.2 weight percent. Asingle 3 gram sample (PBF-7-139-3089) ofhandpicked (2mmx7mm) zeolite grainsyielded a conventional K-Ar age of 22.9±1.5Ma. This is considered to be a minimum age,because radiogenic Ar may have been lostfrom the system continuously or episodicallysince the crystals formed. Other strike normalzeolites from the SRS basement should bedated in an effort to push this minimum ageback.

Pseudotachylyte

Over two dozen pseudotachylytes have beenrecognized in core (Fig. 9). The commoncharacterstics of these features are 1)formation in a fine-grained intermediate tomafic protolith; 2) nucleation along pre-existing, thin chloritic slip surfaces; 3)evidence that suggests K-metasomatism alongchloritic slip surfaces influencedpseudotachlyte generation, perhaps byreducing the melting temperature of chloriticgouge; 4) evidence for multiple events withina single pseudotachylyte vein; different eventsare recognized based on discrete banding ofhydrated or devitrified vs. glassy or simplyaphanitic matrix; 5) preservation of angularclasts at all scales; 6) mm scale teepee orinjection structures that may penetrate theadjacent rock from 3 mm to 2 cm. It hasgenerally not been possible to recognize themagnitude or sense of offset across thepseudotachylytes. Locally it has beenrecognized that presence of felsic layeringwithin several cm of the chloritic slip surfacesmay influence pseudotachylyte development;perhaps through a very local increase indifferential shear stress.

Pen Branch fault

The Pen Branch fault has been described bySnipes and others (1993) and Stieve andStephenson (1995, based on Domoracki,1995). These authors recognized the PenBranch fault as a surface across which thebasement-coastal plain unconformity has beenoffset as much as 50 m (southeast side up)with offset of middle Eocene units decreasingto as little as 3-4 meters. The extent of controlof the Pen Branch fault on outcrop patterns ofthe Upland unit, or surface drainages and therelative importance of strike slip motion alongthis fault system is a matter of somecontroversy (R.J. Cumbest, T.J. Temples,pers. comms.). Domoracki (his fig. 23, 1995)plots two recent hypocenters (6/1985 and8/1988) on SRS line 1 on the basement faultabove the Tinker Creek nappe, in the vicinityof the Pen Branch fault, at depths of .96 and2.86 km respectively.


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In our observations the Pen Branch fault cutscrystalline basement in PBF-7 between boxes25 (1375') and 78 (2183'). Over this intervalat least two intrusive units of granodioriticcomposition are hydrated and retrogressed sothat they appear as chlorite schists.Deformation is highly heterogeneous, andseveral less completely retrogressed slices orlenses are preserved. The largest slice occursbetween boxes 38-48, with a much smallerexample at about 2078'. None of thepreviously described vein sets appear tocrosscut the sheared, hydrated schists, while atleast some are observed in the protolith.Evidence of the orthogneiss protolith isdemonstrated by preservation of pink graniticvein arrays that maintain orientation in theretrogressed rock, and the presence of relictfeldspar grains (porphyroclasts), up to 1 cmlong. within the chloritic matrix. Gouge,breccia zones and chloritic fractures typicallyoccur with 15-20° dips, parallel to foliation(ca. 45°) and steeply dipping (ca. 75°).Breccia zones, up to 10's of cm thick, andgouge zones, up to 2 cm thick, appear tonucleate on chloritic fractures Within 15 m ofthe fault zone, chloritic fracture surfaces areoxidized to a black hematite. At somelocations these oxidized coatings are striated.

Pseudotachylyte and the Pen Branch fault areboth interpreted to largely postdate strike-normal zeolite though both may extend backthrough the Mesozoic. That pseudotachylytepostdates the pinking event (220±4 Ma, Kish,1990; Norian) is evinced by the observation ofpseudotachylyte crosscutting rocks that havebeen visibly pinked. Microprobe analyses ofpseudotachylyte glass and devitrifiedglass/chlorite grains show that in DRBformation mafic metavolcanic rocks with abulk composition that is typically well below4 weight % K2O (Shervais and others, 2000),glass and devitrified glass compositions aretypically in the range 8-12 weight % K2O.This indicates the likelihood of pinking fluidsaltering chloritic vein material and adjacentmafic metavolcanic and subsequentreactivation of these chloritic slip surfaces assites of pseudotachylyte generation.

In over two dozen observations there are onlya couple instances of any crosscuttingmineralized veins cutting pseudotachylyte. Inone very thin (<1 mm) calcite vein cutspseudotachylyte at an oblique angle (DRB-7-9-1419). In the second, it is clear thatpseudotachylyte predates at least one zeolitemineralized fracture event. Thepseudotachlyte (Fig. 9; PBF-7-89-3089) is anirregular brick red, oxidized, devitrified zone,greater than 1 cm thick that is subparallel toand crosscuts epidote and calcite + zeoliteveins and includes clasts of mineralized veinswithin it. These clasts are less than 1 cm andand are visible at all microscope scales. Thebrick red, cherty appearance of the featurestrongly suggests oxidizing fluids passedthrough a cryptocrystalline or glassy material.The relative timing of this event can only besaid to be post-zeolite + calcite veining. Theoxidizing fluids appear to predate the cross-cutting zeolite-filled fracture that issubparallel to the margin of thepseudotachylyte and "decapitates" (oxidized)teepees.

In the case of basement expression of the PenBranch fault (250 m (800') over PBF-7- 25through 78) it can be shown that this featurecross-cuts all mineralized veins includingstrike-normal zeolite. Intact mineralizedfractures are not preserved in the intervalPBF-7-25 through 78. Given the relativedecrease in offset of Tertiary units in theAtlantic Coastal Plain cover (Snipes andothers, 1993) above the expression of PenBranch fault motion in the crystalline rocks,the retrogression of fault rock protolith anddisruption of mineralized fractures and veinsin bedrock is interpreted to have occurredfrom the Late Cretaceous through theNeogene.

CONCLUSIONS

Repeated episodic Phanerozoic reactivationsof basement fault zones are documented inmore than 5000m of basement core recoveredfrom beneath the updip coastal underlying theUS Department of Energy Savannah River


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Site, near Aiken, South Carolina.Neoproterozoic (ca. 619 and 624 Ma)metadiorite and metagranodiorite rockscontain a heterogeneously developed,anastomosing mylonitic fabric and intrudefoliated mafic metavolcanic rocks. Atapproximately 305 Ma, granulite faciesorthogneisses were thrust over amphibolitefacies metaigneous rocks on (northwest-vergent? and dextral) structures called theTinker Creek nappe and Four Mile Branchfault. The overturned limb of the TinkerCreek nappe and Four Mile Branch faultcontrol the location the border fault of theDunbarton Triassic basin. The Dunbartonbasin border fault region acted as a conduit forfluids in Mesozoic and Cenozoic time. Ca.220±5 Ma, a potash and silica metasomaticevent ("pinking" event) affected crystallinerocks throughout the SRS basement region. A"greening" event flushed highly reducingfluids through crystalline rocks, Triassicfanglomerates and affected rocks interpretedto be as young as Santonian (Cape Fear Fm.).Based on the occurrence of saprolite offanglomerates affected by greening in core,the remains of a subbasin of the Dunbartonbasin were identified in the northwest part ofthe site (GCB-2 well). A consistent veinparagenesis interpreted to to be Cretaceousand younger overprints the previous eventsand includes 1) locally vuggy quartz veinswith the same strike as foliation, 2) zeolite ±calcite with the same strike as foliation, and 3)a near vertical strike-normal zeolite set. Aconventional K-Ar age of a separate ofeuhedral, 2mm x 7mm zeolite grains from thelast set yields an age of 22.9 ± 1.5 Ma,interpreted to be a minimium age. More than30 pseudotachylytes are found throughout themetavolcanic terrane and are preferentiallylocalized along Alleghanian age chloriticfractures dipping less steeply than foliationthat have been �pinked.� In virtually everycase, pseudotachylyte can be shown to post-date mineralized fractures. Thus,pseudotachylytes beneath SRS are likely noolder than Mesozoic, and many are probablyTertiary in age. The Pen Branch fault is"exposed" in approximately 250 m ofbasement core between PBF-7-(box) 25-

(footage) 1375 and PBF-7-78-2168. Mostrecent motion on this fault must postdatestrike-normal zeolites, because the zone cross-cuts all mineralized fractures.

REFERENCES

ANDERSON, E.M. 1951. The dynamics offaulting. Oliver & Boyd, Edinburgh, 206p.BARTHOLOMEW, M.J., HEATH, R.D., BRODIE,

B.M., & LEWIS, S.E. 1997. Post-Alleghaniandeformation of Alleghanian granites(Appalachian Piedmont) and the AtlanticCoastal Plain: Geologic Society of AmericaAbstracts with Programs, 29/3, 4.

BRODIE, B.M. & BARTHOLOMEW, M.J. 1997.Late Cretaceous-Paleogene phase ofdeformation in the Upper Atlantic CoastalPlain. Geologic Society of America Abstractswith Programs, 29/3, 7.

CUMBEST, R.J., PRICE, V. & ANDERSON, E.E.1992. Gravity and magnetic modelling of theDunbarton Triassic basin, South Carolina.Southeastern Geology, 33, 37-51.

DANIELS, D.L. 1974. Geologic interpretation ofgeophysical maps, central Savannah Riverarea, South Carolina and Georgia. USGeological Survey, Geophysical InvestigationsMap, GP-893.

DENNIS, A.J. , WRIGHT, J.E. , MAHER, H.D.,MAULDIN, J.C. & SHERVAIS, J.W. 1997.Repeated Phanerozoic reactivation of aSouthern Appalachian fault zone beneath theup-dip coastal plain of South Carolina.Geological Society of America Abstracts withPrograms, 29/6, 223.

DENNIS, A.J., MAHER, H.D, SHERVAIS,J.W.,MAULDIN, J.C, & UZZELLE, G.H.2000a. The role of fluid flow in basin-bounding faults and metasomatictransformation of basement: An example fromthe Mesozoic and Cenozoic deformation of aNeoproterozoic volcanic arc beneath theAtlantic Coastal Plain, Savannah River Site,South Carolina, USA, In: HOLDSWORTH,R.E. (ed) Nature and tectonic significance offault zone weakening, Geological Society ofLondon Special Volume, in press.

DENNIS, A.J. , WRIGHT, J.E. , MAHER, H.D., &SHERVAIS, J.W. 2000b. Neoproterozoic andLate Paleozoic deformation and metamorphismof a volcanic arc beneath the Atlantic CoastalPlain, Savannah River Site, South Carolina.ms.


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DOMORACKI, W. J. 1995. GeophysicalInvestigation of geologic structure andregional tectonic setting at the Savannah RiverSite. Ph.D. thesis, Virginia Polytechnic andState University.

GRANT, J.A. 1986. The isocon diagram - A simplesolution to Gresens' equation for metasomaticalteration. Economic Geology, 81, 1976-1982.

GRESENS, R.L. 1967. Compostion-volumerelationships of metasomatism. ChemicalGeology, 2, 47-55.

HARRIS, M.K., THAYER, P.A., & AMIDON, M.B.,1997, Sedimentology and depositionalenvironments of middle Eocene terrigenous-carbonate strata, southeastern Atlantic CoastalPlain, USA. Sedimentary Geology, 108: 141-161.

HEATH, R.D. & BARTHOLOMEW, M.J. 1997.Mesozoic phase of post-Alleghaniandeformation in the Appalachian Piedmont.Geologic Society of America Abstracts withPrograms, 29/3, 23.

KISH, S.A. 1992. An initial geochemical andisotopic study of granite from core C-10. In:FALLAW, W.C. & PRICE, V. (eds) GeologicalInvestigations of the central Savannah riverArea, South Carolina and Georgia: CarolinaGeological Society Guidebook. South CarolinaGeological Survey, Columbia, B-IV-1 - B-IV-4.

MAHER, H.D. 1987a. Kinematic history ofmylonitic rocks from the Augusta fault zone,South Carolina and Georgia. AmericanJournal of Science, 287, 795-816.

MARINE, I.W. 1974. Geohydrology of buriedTriassic basin at Savannah River Plant, SouthCarolina. American Association of PetroleumGeologists Bulletin, 58, 1825-1837.

MARINE, I.W. & SIPLE, G.E. 1974. BuriedTriassic basin in the Central Savannah RiverArea, South Carolina and Georgia. GeologicalSociety of America Bulletin, 85, 311-320.

O'HARA, K. 1988. Fluid flow and volume lossduring mylonitization: An origin for phyllonitein an overthrust setting, North Carolina,U.S.A.. Tectonophysics, 156, 21-36.

O'HARA, K. 1990. State of strain in mylonites formthe western Blue Ridge Province, southernAppalachians: The role of volume loss.Journal of Structural Geology, 12, 419-430.

O'HARA, K. & BLACKBURN, W.H. 1989.Volume-loss for trace-element enrichments inmylonites. Geology, 17, 524-527.

OLSEN, P.E., FROELICH, A.J., DANIELS, D.L.,

SMOOT, J.P. & GORE, P.J.W. 1991. Riftbasins of early Mesozoic age. In: HORTON,J.W., & ZULLO, V.A. (eds) The Geology ofthe Carolinas. University of Tennessee Press,Knoxville, 142-170.

PETTY, A.J., PETRAFESO, F.A. & MOORE, F.C.1965. Aeromagnetic map of Savannah RiverPlant area, South Carolina and Georgia. USGeological Survey, Geophysical InvestigationsMap, GP-489.

RODEN, M.F., LATOUR, T.E., CAPPS, C., &WHITNEY, J.A. 1997. Incompletemetamorphic reequilibration in a metadioritefrom the crystalline basement of SavannahRiver Site. Geological Society of AmericaAbstracts with Programs, 29/3, 65.

SECOR, D.T., JR. , PECK, L.S., PITCHER, D.M.,PROWELL, D.C., SIMPSON, D.H., SMITH,W.A. & SNOKE, A.W. 1982. Geology of thearea of induced seismic activity at MonticelloReservoir, South Carolina. Journal ofGeophysical Research, 87, 6945-6957.

SECOR, D.T., JR., SNOKE, A.W., BRAMLETT,K.W., COSTELLO, O.P. & KIMBRELL, O.P.1986. Character of the Alleghanian orogeny inthe southern Appalachians. Part I. Alleghaniandeformation in the eastern Piedmont of SouthCarolina . Geological Society of AmericaBulletin, 97, 1314-1328.

SHERVAIS, J.W., DENNIS, A.J. & MAULDIN, J.C.2000. Petrology and geochemistry of avolcanic arc beneath the Atlantic Coastal Plain,Savannah River Site, South Carolina. ms.

SHERVAIS, J.W., SHELLEY, S.A., & SECOR, D.T.1996. Geochemistry of volcanic rocks of theCarolina and Augusta terranes in central SouthCarolina. In: NANCE, R.D. & THOMPSON,M.D. (eds) Avalonian and related peri-Gondwanan terranes of the circum-NorthAtlantic. Geological Society of America,Boulder, Special Paper, 304, 219-236.

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SPEER, J.A. 1982. Description of granitoid rocksassociated with two gravity minima in Aikenand Barnwell counties, South Carolina. SouthCarolina Geology, 26, 15-24.


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STIEVE, A. & STEPHENSON, D. 1995.Geophysical evidence for post-Late Cretaceousreactivation of basement structures in thecentral Savannah River area. SoutheasternGeology, 35, 1-20.

WYATT, D.E., WADDELL, M.G., and SEXTON,G.B. 1996. Geophysics and shallow faults inunconsolidated sediments. Groundwater, 34,236-334.


E-11

84

83

82

81

34

33

32

A t l a n t i c

C o a s t a l P l a i n

Columbia

Atlanta

Augusta

GEORGIA

SOUTH CAROLINA

F

C-GS

H

B

CPS

GHMZ��

��

��

��

��

�

��

��

N

��

aeromag - E P F S

M O D O CZ O N E

Clark Hill Lake

AFZ

Belair fault zone

subsurface Belair belt

��

�

��

��

*

*

Tr-J mafic igneouscomplex from aeromag

33¡00'

33¡15'

81¡15'33¡30'

81¡15'33¡15'

81¡45'

33¡00'

81¡30'

82¡00'

33¡30'

82¡00'

Martin f.: southern border of Dunbarton basin

��

Carolina slate belt

aeromag lineament: Tinker Cr. nappe limb & EPFS dextral shear zone

Graniteville pluton

Aiken

Kiokee belt

Springfield pluton

C-7

C-5

SAL-1

C-2

DRB wellcluster

33¡15'

81¡45'

C-1

edge of associatedgravity anomaly

33¡30'

81¡45'

10 km

��33¡30'

81¡30'C-3

* subsurface Tr border faultreactivated as Pen Branch Fault

C-10

��

��

��

��

C-1

no s

trat

ord

erim

plie

d

Pen Branch MIC

Deep Rock MIC

Crackerneck MIC

Post meta plutons

Tr sedimentary basin

Tr mafic igneous rocks

fault offsetting Tertiary strata

aeromag linerosional window

through ACP

offsite welllocation

��Carb.Dev.

Four Mile Branch fault

Figure 1. Eastern Piedmont of Georg-olina. Compiled from our work and interpreted fromPetty et al. 1965, Daniels 1974, Speer 1982, Cumbest et al. 1992, and Snipes, pers. comm. Off-site C-wells and SAL-10 located on this map.


E-12

C-7

C-5C-1

C-3

C-2

DRB-1DRB-3

DRB-4

GCB-1

SSW-3

SSW-1

SSW-2

DRB-2

GCB-5.1

8

51

LINE 7

LINE 3

21

623

8

LINE 4

LINE 4

LINE 7

GCB-2

P6R

MMP-4P30

GCB-3

DRB-5

DRB-7DRB-6

GCB-4PBF-7PBF-8

6

N

0 6 km

ab

GCB-8

Figure 2. Index map showing location of cores used in this study, as well as location ofDomoracki's (1995) seismic lines 3, 4, 7, and epicenters (a, 6/9/1985; b, 8/4/1988) plotted byDomoracki (his p. 23). Locations where other seismic lines cross Lines 3, 4, 7 are indicated: 1,21, 8 cross line 7; 5 crosses line 4; and 6 and 8 cross line 3).


E-13

��

��

��

��

��

��

Deep Rock Metaigneous Complex

Pen Branch Metaigneous Complex

?

619±3.4 Ma (U-Pb z)

626.1 ± 4.4 Ma (U-Pb z)

PBF-7 extrusive equivalent?

Post-620 ? Persimmon Fork-Uwharrie equivalent?

Deep Rock Metavolcanic Suite

Crackerneck Metavolcanic Complex

DRB-1 metadiorite

Pen Branch Metavolcanic Suite

PBF-7 Metaplutonic Suite


pre 626 Ma volcanic pile

original nature of this contact unclear:fault or unconformity?

Figure 3. Schematic column of Appalachian litho-“stratigraphic” units beneath the US DOESavannah River Site.


E-14

Figure 4. Overturned limb structure of the Tinker Creek nappe from structural analysis ofover 3000m of basement core in the DRB well cluster.


E-15

Dunbarton basin border fault

Pen B

ranch fault

Deep Rock Metaigneous Complex


Pen Branch Metaigneous Complex

TR

lower grade (?)

Deep RockMetaigneousComplex ?

T i n k e rC r e e k n a p p e

Kiokee belt - Savannah River terrane of Maher and others (1991)interpreted to be more extensively Alleghanian remobilized

ca. 620 Ma basement

ca. 8 km

land surfacePiedmont-Coastal Plain unconf.

Figure 5. Schematic cross-section of Appalachian structure underlying the middle third of theSavannah River Site.


E-16

Figure 6. Comparison of the relative amounts of pinking or K-metasomatic alteration of PBFMetaplutonic Suite observed in PBF-7. Second and third numbers refer to box # and footage(depth from surface). XRF analysis of whole rock powders from this set was used to constructthe isocon diagram (Fig 8).


E-17

Pin

ked

Rb

Y

K O2

SiO2

Na O2

MnO

ZrV Sr

Ni Cu

Ba Fe O2 3

TiO2

Al O2 3

P O2 5

isochemical ?Sc

elements/ major element oxides enriched during pinking

elements/oxidesleached/removed during pinking

Unpinked (protolith)MgO

most enriched in K during pinking

"isochemical"slope of 1

CaO

Cr S

Figure 7. Isocon diagram showing changes in ratios of major and trace elements as a result ofpinking in PBF-7 gneisses of PBF Metaplutonic Suite. Discussed in text. Standard deviationsare shown. Slope of “1” represents isochemical conditions in theory; ratio of elements/majoroxides before/after pinking equals 1. Perhaps a line that drives through Ni, Zr, Sc, V, TiO2

supposedly immobile elements really represents isochemical conditions – ratios that have notbeen changed by metasomatism. Elements/major oxides that appear below this line have beenleached or depleted during the pinking event, elements/major oxides that appear above the linerepresent increasing additions of certain elements during pinking, in particular K2O, Y, Rb,SiO2, Na2O and (to a somewhat lesser extent ) Al2O3. 15 unpinked protolith (boxes 103, 107,108, 139, 143, 145, 149, 157, 159, 161, 165, 171, 173, 175, 177) and 8 pinked (101, 115, 115t, 167,171, 171-3568, 175, 176-3541) samples. Compare Figure 6.


E-18

Figure 8. PBF-7-16-1249' (and PBF-7-13) and GCB-2-648' illustrate the “greening” event. Abrick-red arkose (DRB-11-2741’) from within the Dunbarton basin is included to comparecolor. The matrix of the PBF-7 sample on a wet surface is grayish yellow green (5GY 7/2) onthe Munsell rock color chart. Microprobe analyses of a fibrous mineral that defines the matrixindicates that it is muscovite. Irregular margins of oxidized fanglomerate clasts have beenreduced and now appear cream colored. Reducing fluids penetrated along gneissosity. TheGCB-2 sample is saprolite, but shows a very similar texture. Coarser grained matrix iscompletely reduced. The ragged irregular margins of (brick-red) fine-grained clayey clastsshow incomplete penetration of reducing fluids parallel to foliation.


E-19

Figure 9. Pseudotachylyte hand specimens: a) DRB 3-13-1038, b) DRB 7-6-1394, c) DRB-7-1419’. Inspection of pseudotachylyte samples commonly shows that these features reactivatethin chlorite shear surfaces that dip less steeply than foliation, and show hangingwall up senseof displacement. It can be shown that most pseudotachylytes must post-date Alleghanian timebecause the rock they cut has been pinked, and pinking is a Triassic event. Somepseudotachylytes seem to form near felsic mafic contacts, and may reflect local shear stressaccumulations at these sites. PBF-7-89-2344: pseudotachylyte clearly postdates pinking, the“greening” as observed in crystalline rocks , and zeolite ±calcite veining (pale pinkish orange).

carolina geological society 2000 field trip ... - my articles/dennis...d. e. wyatt and m. k. harris...

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