Implications of Thermal Events on Thrust Emplacement Sequence in the Appalachian Fold and Thrust Belt: Some New Vitrinite Reflectance Data Sharon E

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11-1990 Implications of Thermal Events on Thrust Emplacement Sequence in the Appalachian Fold and Thrust Belt: Some New Vitrinite Reflectance Data Sharon E. Lewis Montana Tech

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Repository Citation Lewis, Sharon E. and Hower, James C., "Implications of Thermal Events on Thrust Emplacement Sequence in the Appalachian Fold and Thrust Belt: Some New Vitrinite Reflectance Data" (1990). Center for Applied Energy Research Faculty Publications. 11. https://uknowledge.uky.edu/caer_facpub/11

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Notes/Citation Information Published in The Journal of Geology, v. 98, no. 6, p. 927-942.

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Digital Object Identifier (DOI) https://doi.org/10.1086/629462

This article is available at UKnowledge: https://uknowledge.uky.edu/caer_facpub/11 IMPLICATIONS OF THERMAL EVENTS ON THRUST EMPLACEMENT SEQUENCE IN THE APPALACHIAN FOLD AND THRUST BELT: SOME NEW VITRINITE REFLECTANCE DATAl

SHARON E. LEWIS AND JAMES C. HOWER2 Montana Bureau of Mines and Geology, Montana Tech, Butte, Montana 59701

ABSTRACT

Interpretation of existing geothermometry data combined with new vitrinite reflectance data, within the framework of a detailed composite tectonic setting, elucidates the evolution of structural sequencing of thrust sheets during the Alleghanian event in the Valley and Ridge Province in Virginia. That the Pulaski thrust sheet preceded the Saltville thrust sheet in the emplacement sequence, and that both reached thermal maxima prior to, or during, respective emplacement may be inferred from vitrinite and other geothermometry data. In contrast, the Narrows and St. Clair thrust sheets probably each attained their thermal maximum after emplacement. New vitrinite reflectance data are consistent with CAI and other temperature-sensitive information heretofore ascertained in the Valley and Ridge Province and support previously established maximum temperatures of ca. 200°C for strata of the Saltville thrust sheet as young as Mississippian. Rmax values from Mississippian coals in the Price Formation of the Saltville sheet, beneath but near the Pulaski thrust, range from 1.61% to 2.60%. At the structural front of the fold and thrust belt, a single Mississippian coal sample from the Bluefield Formation yields an Rmax value of 1.35%. Those coals showing highest Rmax values are more intensely fractured with secondary minerals filling the fractures. Warm fluids introduced during tectonic events may have played at least as important a role as that of combined stratigraphic and tectonic burial.

INTRODUCTION (CAI) (Epstein et al. 1977), "illite crystallin- Although fold and thrust belts typically are ity" and clay mineralogy (Zentmeyer 1985), generated during a singular orogenic event, and the level of organic maturation (Stanley internally they are a composite of discrete and Schultz 1983) contribute independent in- structural sequences. Within crystalline por- dicators of conditions of subgreenschist fa- tions of orogens, timing of metamorphism cies "metamorphism" in a portion of the Vir- and plutonism is crucial for determining ginia Valley and Ridge Province of the thrust sequences and docking order of ter- Southern Appalachians (figs. 1 and 2). This ranes, (e.g., Rast 1989; Dallmeyer et al. "metamorphism" constrains the time of em- 1986). For fold and thrust belts, deciphering placement of individual thrust sheets and the thermal history of sedimentary strata pro- thus relates to the overall thrust emplacement vides similar, equally important data consequence as well. straining timing and the emplacement se- TECTONICS AND DISTRIBUTION OF STRATIGRAPHIC quence of thrust systems. SEQUENCES The Appalachian Valley and Ridge Prov- Four major thrust systems (figs. 2 and 3), ince is an example wherein geothermometry named for their lower bounding thrusts, jux- and related data, combined with an evolving tapose Paleozoic strata between the meta- understanding of the structural relationships morphic rocks of the Blue Ridge Province of component thrust sheets, provide keys to (fig. 2) on the southeast and the Appalachian Appalachian thrust belt evolution. Data from Plateau on the northwest, where the struc- vitrinite reflectance (this study; Ingram and Rimstidt 1984), conodont alteration index tural front of the fold and thrust belt is marked by the axis of the Glen Lyn syncline (McDowell 1982) beneath the St. Clair thrust. ' Manuscript received September 8, 1989; ac- The structural style and internal complexity cepted July 31, 1990. of the Pulaski system, as depicted by Bartho- 2 University of Kentucky, Kentucky Center for lomew (1987) and Schultz (1988), occupies an Applied Energy Research, 3572 Iron Works Pike, intermediate relationship between the struc- Lexington, Kentucky 40511. turally superjacent Blue Ridge thrust system [JOURNAL OF GEOLOGY, 1990, vol. 98, p. 927-942] and the more typical decollement thrust sys- © 1990 by The University of Chicago. All rights tems of the Valley and Ridge farther west. reserved. 0022-1376/90/9806-0011$1.00 Thrust sheets structurally below the Pulaski

927

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the Saltville sheet below the Pulaski thrust (figs. 2 and 3). These coals occur within the lower 40 m of the upper Price Formation (Mississippian) (fig. 4), just above the marine to nonmarine transition; environments of deposition of the Price Formation are discussed by Kreisa and Bambach (1973) and White- head (1979). The lower and upper of the two principal recognized coal units are the "Langhorne," and the "Merrimac," respec- tively. Environments of deposition of these Price Formation coals are addressed by Brown (1983) and Zentmeyer (1985). Approx- imately 0.5 km of Mississippian rocks remain preserved stratigraphically above the coal- bearing strata (fig. 4), but structurally beneath the Pulaski thrust in the Price Mountain window (fig. 3). Tournaisian (Early Mississip- pian) rocks are the youngest overridden strata exposed on the Saltville thrust sheet. FIG. 1.-Map showing location of sample area Middle Ordovician limestones (CAI obtained relative to state and county boundaries. by Epstein et al. 1977) on the Saltville thrust sheet are stratigraphically overlain by ap- sheet neither possess the internal complexity proximately 3 km of strata, including coal- exhibited by the Pulaski sheet (Lewis and bearing Mississippian strata similar to that of Bartholomew 1989), nor have as much hori- the Price Mountain window (fig. 4). zontal displacement as the Pulaski thrust Mississippian coal also crops out along the sheet (Bartholomew 1987). southeastern flank of the structural front Each major Valley and Ridge thrust sheet where, in the Glen Lyn syncline (fig. 4) north in southwestern Virginia has an endemic stra- of Bluefield, West Virginia, preserved Penn- tigraphy (fig. 4). Composite stratigraphic sec- sylvanian and Mississippian stratigraphic tions compiled by Bartholomew (1987) for overburden was at least 1.5 km (Cooper each thrust sheet permit reconstruction of 1961). Where limestones of the Mississippian stratigraphy intrinsic to that thrust sheet (fig. Greenbrier Formation (Visean) are exposed 4). Reconstruction of the particular strati- close to the structural front, they lie stratigraphic columns from which our samples, graphically beneath (fig. 4) approximately 1 and other data pertinent to this discussion, km of strata that includes the Bluefield For- were collected, is essential for evaluating mation (0.2 km), which contains the coal, possible effects of stratigraphic burial. plus part of the overlying Hinton Formation Mississippian coals crop out adjacent to (0.8 km). No strata older than Devonian are the present day leading edge of, and in win- exposed in this part of the Glen Lyn syncline, dows through the Pulaski sheet. Between the but the pre-Devonian stratigraphic section is outcrop belts, drill hole data (Bartholomew probably similar to that above the structur- and Lowry 1979; Stanley and Schultz 1983) ally superjacent St. Clair thrust (fig. 4). On provide limited information on coals within the St.Clair thrust sheet, approximately 2.5

FIG. 2.-Generalized geologic map (modified after Bartholomew 1987 and Schultz et al. 1986) showing distribution of Mississippian-age strata and major thrust faults. Numbers in parentheses are Rmax values for surface sample locations and range of Rmax values for drill hole locations from table 1. YSF = Yellow Sulphur fault; 7.5 minute quadrangle abbreviations: NAR = Narrows, NEW = Newport, BLA = Blacks- burg, RN = Radford North, STA = Staffordsville, WF = White Gate, PUL = Pulaski. SE-NW line is the line of cross section in figure 3. Dark line within the shaded Mississippian outcrop belt on the Saltville thrust sheet follows the Price Formation coals. G-Greenbrier Formation in the Glen Lyn syncline. CO-Cambro-Ordovician strata; SD-Siluro-Devonian strata.

This content downloaded from 128.163.2.206 on Thu, 21 Dec 2017 15:53:16 UTC All use subject to http://about.jstor.org/terms Samplelocation Thrustfault Mississippianrocks MiddleOrdovicianlimestone Metamorphicrocks EXPLANATION

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km of Ordovician to Devonian strata stratigraphically overlie middle Ordovician limestones from which CAI was obtained by Ep- stein et al. (1977).

VALLEY AND RIDGE THERMAL HISTORY

Previous Work.-Campbell and others (1925) delineated the various Valley Fields on the basis of coal rank as defined by the "fuel ratio" (fixed carbon/volatile matter). By their definition, the Price Mountain (Montgomery County) and Pulaski (Pulaski County) fields fall into the semi-anthracite rank range. Coals from the Brush and Little Walker Mountain areas (both sampled by us) are described by them as including both semi-anthracite and low-volatile bituminous rank coals. Fields, not sampled for this study but described by Campbell and others (1925), southwest of the Price-Brush-Little Walker Mountains area include semi-anthracites from the Max Mead- ows field (Wythe County) and low-volatile bituminous coals from Reed Creek (Wythe County), Lick Creek, and Bland (Bland County) fields. Their samples from the Lick Creek and Bland County fields are the only ones in the Narrows thrust sheet (below the Saltville fault). The data of Campbell and Others (1925) suggest that this field has the lowest rank coals from any of the Valley Fields. Farther west, out of the Valley Fields, Hower and Rimmer (1990) report that low- volatile bituminous coals from the Pocahon- tas basin in southern West Virginia-Virginia decrease in rank 15-20 km eastward from central Buchanan County, Virginia, toward the structural front and the St. Clair fault. Stanley and Schultz (1983) provided chemi- cal analyses for the major coal beds in W- 6534 (Sunnyside test; our samples 8085-8087) and W-6535 (Merrimac test; our samples

FIG. 4.-Stratigraphic columns showing relative stratigraphy on Pulaski (western Salem synclor- ium), Saltville, Narrows, and St. Clair thrust sheets and in Glen Lyn synclinorium (modified after Bar- tholomew 1987). P-Mc-Pennsylvanian-Mississip- pian clastic sequence; Mbf-Bluefield Fm; Mg-Greenbrier Fm; Mmc-Maccrady Fm; Mpr (u;l;c)-Price Fm (upper, lower, and Cloyd mem- FIG.3.-GeneralizedSE-NWbalancedcrosssectionwithprobablerestoredAlleghanianconfigurationprojectedabovepresent-daytopography. bers); Dc-Chemung Fm; Ss-Silurian sand- PC-PriceFmcoals;BC-BluefieldFormationcoal;PMW-PriceMountainwindow;CW-ChristiansburgP-Pennsylvania;M-Mississip- pian;G-GreenbrierFm;D-Devonian;OS-Ordovician-Silurian;OL-MiddleOrdovicianlimestones;CO-Cambrian-Ordoviciancarbonates; Cr-RomeFm.Horizontal=verticalscale. stones; Oc-Ordovician clastic sequence; Ols-Middle Ordovician limestones; O-Ccb-Or- dovician-Cambrian carbonates; Cr-Rome Fm.

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8081-8084) cores. Volatile matter (daf) The presence of probable epigenetic miner- ranges from 12.6-19.2% for the Sunnyside als was determined by optical examination of coals and 12.3-16.7% for the Merrimac coals. the coals (comments column of table 2), sup- If "coals" with greater than 20% ash (dry) plemented by reconnaissance X-ray diffrac- are excluded, the range for coals from both tion analysis of low-temperature ashes (K. A. cores is reduced to 12.6-14.9% volatile Frankie and F. L. Fiene 1983 pers. comm.). matter. Aside from quartz and kaolinite, potential Sample Locations.-Vitrinite reflectance epigenetic minerals include sulfides: pyrite, and anisotropy values (table 1) were deter- marcasite, and sphalerite (fig. 5); and carbon- mined for 29 samples from 11 separate loca- ates: calcite, magnesite, dolomite, ankerite, tions in Montgomery and Pulaski counties in and siderite. Carbonates occur as both veins Virginia and for one sample from northwest- and cements between brecciated coal frag- ern Giles County, Virginia (figs. 1 and 2). ments (fig. 6). Chlorite (composition un- Price Formation coals were sampled from known) occurs from trace amounts up to 20% both the outcrop belt and drill core holes just in several ashes. Illite "crystallinity" (M. R. northwest of the trace of the Pulaski thrust Bayan 1989 pers. comm.), determined for a and from the Price Mountain window. All variety of lithologies from the "Merrimac" samples, except one, are from the upper seam (from a core that included our samples (nonmarine) member of the Early Mississip- KCER 8081-8084), generally falls near the pian (Tournaisian) Price Formation. The ex- diagenesis/anchizone transition, as would be ception (8063) is from the Middle Mississip- expected based on the vitrinite maximum re- pian (Visean) nonmarine Bluefield Formation flectances of the associated coals in that core on the overturned southeastern limb of the (2.13-2.60% Rmax). Illite composition, partic- Glen Lyn syncline near the Virginia-West ularly the presence of NHI = illite common Virginia border in Giles County, Virginia in higher rank coals, can complicate the inter- (fig. 2). pretations of "crystallinity" (Juster et al. Coal Characteristics and Rank.-Vitrinite 1987; Daniels and Altaner 1990; Daniels et al. maximum-reflectance and anisotropy (Rmax - 1990). Rmean/Rmax) values of Mississippian-age coal Geothermometry: Depth Indicators.- samples for this study are given in table 1. Conodont alteration index values (CAI) (Ep- These coals generally are within the range for stein et al. 1977; Harris 1979) provide the semi-anthracite rank (see Davis 1978, for de- most extensive geothermometry data avail- lineation of rank by reflectance) but exhibit able for the entire Valley and Ridge Province. variations both vertically and geographically. CAI values are highest on the southeastern- The top coal from Coal Bank Hollow #1 most thrust sheets and decrease successively (8065) and the three coals from Lick Run #5 northwestward (fig. 6). Middle Ordovician (8071-8073) are within the low volatile bitu- rocks on the Pulaski sheet yield CAI values minous range; two of the lower coals from up to 5 and generally are between 4 and 5. Coal Bank Hollow #1 (8066; 8068) are transi- Regionally, on the Saltville thrust sheet, CAI tional from low volatile bituminous to semi- values range from 3 to 4 in Middle Ordovician anthracite in rank. The upper coal from the limestones, whereas, in Middle Ordovician State Highway 100 (8061) outcrop shows a limestones in the St. Clair thrust sheet, CAI medium volatile bituminous reflectance value values range from 2 to 3 (Epstein et al. 1977; (1.30% Rmax), which is likely the consequence Harris 1979). In Mississippian limestones of surface weathering of that coal (Ingram near the structural front beneath the St.Clair and Rimstidt 1984) and not an indication of thrust, CAI values (Epstein et al. 1977; Har- actual rank. The sample (8063, table 1) from ris 1979) are 2-2.5 except locally near Glen the Bluefield Formation also has a medium Lyn, Virginia where they reach 3 or higher. volatile bituminous reflectance (1.35% Rmax). Within part of our sampling area, Zentmeyer Surficial processes may have reduced the (1985) used CAI values of Epstein et al. true rank here as well, because the sample (1977) plus her study of clay mineralogy was obtained within only 30 cm of the surface (dickite/kaolinite) and "illite crystallinity" to of the exposure. suggest a temperature range of 150°C to

This content downloaded from 128.163.2.206 on Thu, 21 Dec 2017 15:53:16 UTC All use subject to http://about.jstor.org/terms wth wth,lower wth,upper

3.3 4.1 7.7 7.4 9.0

10.7 12.3 10.0 19.7 26.4 27.2 33.9 33.6 34.1

363.9 364.9 369.7 367.9 380.5 427.1 435.6 surface surface surface surface surface surface surface surface

103 068 075 590 073 078 078 069 064 084 109 080 061 080 062 048 061 066 092 047 071 073 051 063 063 061 082 067 096

Rmax-Rmean/DepthofBase

14 19 11 14 14 20 22 24 16 23 19 09 14 16 18 17 16 25 24 22 18 17 18 19 17 19 20 24

1.20 1.81 1.92 1.84 1.99 1.96 1.51 1.53 2.05 2.21 2.40 2.16 2.01 2.02 1.88 1.97 1.92 2.13 2.16 2.10 2.10 2.27 2.08 2.25 2.00 2.01 2.35 2.35

TABLE1

121.2114 12 11 17 13 13 12 10 16 16 13 13 13 15 10 15 09 94 25 07 10 16 13 14 16 13 16 13 09

VitriniteReflectanceAnistropy

1.35 1.30 1.92 1.61 1.61 1.98 2.20 2.39 2.56 2.33 2.18 2.03 2.20 2.00 2.12 2.13 2.28 2.38 2.21 2.28 2.26 2.07 2.39 2.19 2.40 2.13 2.19 2.52 2.60

LOCATIONANDVITRINITEREFLECTANCESOFVALLEYFIELDCOALSAMPLES

3lenLyn FosterMiningCo. EmpireMine Route11 Weldon Route100 Route100 CoalBankHollow#1 CoalBankHollow#1 CoalBankHollow#1 CoalBankHollow#1 CoalBankHollow#1 3t.ValleyMineDump LickRun#5 LickRun#5 LickRun#5 /DMRW-6534 /DMRW-6534 /DMRW-6534 PriceMountain#6 PriceMountain#6 PriceMountain#6 ?riceMountain#6 ?riceMountain#6 'riceMountain#6 VDMR#3 IDMR#3 VDMR#3 IDMR#3

8063 8057 8060 8058 8059 8061 8062 8065 8066 8067 8068 8069 8064 8071I 8070I 8072I 8085 8086 8087 80731 80751 80741 80761 80771 80781 8081 8082 8083 8084 NOTE.-wth=weathered.

SampleLocationRmaxs.d.Rmean(m)Comments BluefieldFm. PriceFm.

This content downloaded from 128.163.2.206 on Thu, 21 Dec 2017 15:53:16 UTC All use subject to http://about.jstor.org/terms 1 Ta Ta 2

C T T CTa,b T T T T T T T T1,2 T1,2 T2

A A AC AC A AT A A A ACT AT A AT AC AC AC ACT A ACT AC AT A AC A AC AC ACT C

TAC T T T T T A T T C T T T

CT T CT T C AC T T CT AT

TABLE2

T T T

T T T C T T T T T T T

MINERALOGYOFVALLEYFIELDCOALSAMPLES

T T T T T A T

T T

T T T T T T T T T T T T T T T T T TC T T T T T T T

Gypsum. Zircon.

8063 8057 8060 8058 8059 8061 8062 8065 8066 8067 8068 8069 8064 8071 8070 8072 8085 8086 8087 8073 8075 8074 8076 8077 8078 8081 8082 8083 8084 NOTE.-T=<1%;C1-5%;A>5%.1sphaleriteobservedmicroscopically;2marcasitemicroscopically.

SampleRutilePyriteMarcasiteCalciteMg-CalciteDolomiteAnkeriteSideriteQuartzKaoliniteIlliteChloriteFeldsparOthers BluefieldFm. PriceFm.

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FIG. 5.-Photomicrograph of carbonate cement in corpocollinite breccia.

FIG. 6.-Diagram depicting predeformational strata along line of cross section shown in figure 2, with inferred isothermal lines based on temperature ranges from CAI values and vitrinite reflectance values. CAI values along line of cross section supplied by A. G. Harris. PC-Price Mountain coals; BC-Bluefield coal; SC-St. Clair thrust; N-Narrows thrust; S-Saltville thrust; P-Pulaski thrust. Dashed temperature-range bars projected into line of section. Approximate 2° regional dip on Middle Ordovician limestone (solid lines) from regional decollement dip of Harris et al. (1981).

200°C for heating of the Price Formation generated fractures probably formed in asso- coals. ciation with Alleghanian tectonism, probably Using cathodoluminescence, Grover and at higher temperatures and greater depths Read (1983) described a complex cementa- (200°C to 300°C and 5 to 7 km, from Epstein tion history with different generations of ce- et al. 1977). They found burial cements to be ments in Middle Ordovician ramp carbonates best-developed in the southeastern versus the from across the Valley and Ridge Province northwestern portion of the Valley and Ridge in Virginia. Earlier-formed, dull calcite and Province. Dorobek (1987, p. 510) discusses ferroan dolomite cements reflect burial dia- the hinterlandward increase in the propor- genesis conditions of subtidal carbonate beds tion of late dolomite cement, plus formation during the Late Devonian to early Mississip- of fluorite, in Siluro-Devonian Helderberg pian in the foreland basin and are associated Group carbonate rocks of the central Appala- with hydrocarbons and temperatures from chians. He concluded that the formation of 75°C to 135°C. They concluded that the youn- late dolomite cement plus fluorite took place gest, clear, dull cements that fill tectonically ". . just prior to and during late Paleozoic

This content downloaded from 128.163.2.206 on Thu, 21 Dec 2017 15:53:16 UTC All use subject to http://about.jstor.org/terms 936 S. E. LEWIS AND J. C. HOWER deformation" at temperatures in the 200°C less than that of semi-anthracites from the range. Elliot and Aronson (1987) found that Bernice Field (one of the North-Central the extent of illitization of mixed-layer illite/ fields) in Sullivan County, Pennsylvania smectite in Ordovician-age K (potassium)- (Hower 1978). The Bernice semi-anthracites bentonites near, and west of, the structural occupy an apparent stable tectonic setting on front was similar, regardless of inferred depth the Allegheny Plateau and represent the high- of burial of the K-bentonites. They attributed est rank found among coals involved only in the illitization to exposure to hot brines ex- plateau tectonics. Hower (1978) and Hower pelled during Alleghanian tectonism farther and Paxton (1979) interpreted the depth of southeast. burial for the Bernice Field to be about 3.5-4 Burial.-Regional reconstruction of pre- km, similar to burial of nearby fields of lower erosion stratigraphy would restore at least 4 rank, and the rank to be a consequence of a km of Cambrian through Mississippian (lower higher paleogeothermal gradient than in fields Price Formation) strata, a section similar to to the northwest. Upon further consideration that preserved in the western Salem synclino- of sandstone porosity, Paxton (1983) pro- rium portion of the Pulaski thrust sheet (Bar- posed that the depth of burial of Pennsylva- tholomew 1987), structurally above the Price nian coals increased progressively from the Mountain window (figs. 3 and 4). Replacing North-Central fields to the Northern Anthra- an additional 2-3 km of late Paleozoic strata cite Field with little change in paleogeother- (removed by erosion) would suggest ca. 7 km mal gradient through the region. The Bernice of rock atop the Price Mountain coals on the semi-anthracites would have been metamor- Saltville thrust sheet. The 7 km of strata phosed at a depth of about 5.7-6 km accord- would include: 2-3 km of eroded late Paleo- ing to his revised model. Our range of 4.5-7 zoic strata; the preserved (4 km) Cambrian km of restored depositional strata plus struc- through (lower) Mississippian column above tural overburden above the Mississippian the Pulaski thrust; plus the thickness of the coals of the Saltville thrust sheet approxi- upper Price and Maccrady formations (0.5 mates the depth of burial for Bernice Field km) below the Pulaski thrust (figs. 3 and 4). coals determined by Hower (1978) and Indicated burial depths (this study; Epstein Hower and Paxton (1979). [The difference in et al. 1977; Zentmeyer 1985) are consistent anisotropy for the Valley Field coals com- with the total known stratigraphic thickness pared to those of the Bernice Field could be above Mississippian coal-bearing strata (on due to either greater burial depth (as sug- their respective thrust plates) plus known gested by Paxton 1983) for Bernice Field stratigraphic thickness from structurally su- coals or to shorter burial/heating time for perjacent thrust sheets. Thus Middle Ordovi- Valley Field coals.] cian limestones on the Saltville sheet were DISCUSSION beneath ca. 9-10 km of strata. The Mississippian Greenbrier Formation is Available data suggest that the Valley and only 0.4 km below the coal in the Bluefield Ridge Province, as well as adjacent areas, ex- Formation (fig. 4), but these rocks were likely perienced maximum thermal effects during buried to a greater depth beneath the struc- the Alleghanian orogeny (cf. Epstein et al. turally higher St. Clair thrust sheet originally 1977; Grover and Read 1983). Vitrinite re- containing about 3.5-4 km of Cambrian-Mis- flectance data of Mississippian coal, CAI sissippian strata above the overturned limb values (of Ordovician-Mississippian lime- of the Glen Lyn syncline in the Glen Lyn area stones), and cathodoluminescence patterns (fig. 3). of carbonate cements (in Ordovician and Comparison with Other Areas.-The di- Siluro-Devonian strata) are all consistent vergence between maximum and mean vitrin- with a heating maxima increasing hinterland- ite reflectances (anistropy) is considered a ward across the Valley and Ridge Province. signature of the tectonic history of coal meta- However, the signature of this thermal event morphism (Hower and Davis 1981; Levine varies as a function of intensity, character, and Davis 1989a, 1989b). Reflectance aniso- and timing across the Valley and Ridge in tropy of the Mississippian-age Valley Field Virginia. Relationships among the thermal semi-anthracites studied here is significantly event, mineral precipitation in carbonates

This content downloaded from 128.163.2.206 on Thu, 21 Dec 2017 15:53:16 UTC All use subject to http://about.jstor.org/terms THERMAL EVENTS ON THRUST EMPLACEMENT SEQUENCE 937 and coal, and regional structural features 190° to 230°C (Perry et al. 1979), whereas the serve to delineate some components of the former values are indicative of 150° to 175°C relative emplacement sequence of the thrust (Epstein et al. 1977). Locally, near Glen Lyn, sheets. Virginia, anomalously high CAI values (3.5 Thermal Event Relative to Thrust Sheet in Mississippian Greenbrier Formation and 4 Emplacement.-The Pulaski thrust sheet in Upper Ordovician Martinsburg Formation) consistently shows higher-rank CAI values are from an area where the St. Clair thrust (4-5: Epstein et al. 1977; Harris 1979; Orn- sheet is structurally very thin. dorff et al. 1988) than those thrust sheets to Thermocline and Its Cause.-The hinter- the west. Disruption of CAI values (Epstein landward-increasing thermocline could result et al. 1977; Harris 1979) indicates that the from:(1) stratigraphic thickening to the south- Pulaski sheet was emplaced after its peak east; (2) progressive hinterlandward struc- thermal event as Harris (1979, p. 9) noted: tural burial by stacking of multiple thrust ". . . in the southeastern part of the basin, sheets as proposed by Levine (1986) for the isograds are disrupted and irregular, because Pennsylvania anthracites and also discussed late Paleozoic thrusting has severed and tele- by Epstein et al. (1977); and/or (3) an undis- scoped original metamorphic isograd pat- cerned (fluid-laden) heat source to the south- terns." Our data suggest its emplacement east, similar to the proposed heat source for was just prior to, or during, the event affect- Pennsylvania anthracites (Daniels et al. 1990; ing the Saltville thrust sheet. Because of the Daniels and Altaner 1990). large transport distance (ca. 100 km, Bar- Stratigraphic burial certainly influenced tholomew 1987), the thermal event affecting geothermometry signatures within the Valley the Pulaski thrust sheet could be, but is not and Ridge thrust sheets. Total preserved required to be, different from those that af- stratigraphic thicknesses above the Middle fected the forelandward part of the Valley Ordovician limestones from successive thrust and Ridge Province. sheets show a hinterlandward decrease (figs. On the Saltville thrust sheet, geother- 3 and 4) from about 4.5 km near the Glen Lyn mometry indicators (CAI and vitrinite re- above the Middle Ordovician limestones near flectance) imply relatively high temperatures Glen Lyn to about 3 km near Price Mountain. well up into the Mississippian section (fig. 6). But more likely estimates of total thickness Although rocks of the Saltville thrust sheet determined using a predeformational dip of were subjected to a thermal maximum after ca. 2° (Bartholomew 1987; Harris et al. 1981) emplacement of the Pulaski thrust sheet, the are ca. 5.5-6 km of strata above the Middle rather steep thermal gradient from the Salt- Ordovician limestone near Price Mountain ville thrust sheet to the structurally lower compared with ca. 5 km at Glen Lyn (fig. Narrows thrust sheet, suggests significant 6). Regardless, reconstructed thicknesses are offset of isotherms by the Saltville thrust (fig. similar enough that if stratigraphic burial 6). The existence of this thermal break is also were the sole or dominant control, geother- implied by the data of Campbell and Others mometry values should be relatively uniform (1925), who found significantly lower rank across the area. coals on the Narrows as compared to the In some instances, structural burial influ- Saltville thrust sheet. The Saltville thrust enced geothermometry signatures. Along the sheet was thus emplaced during or slightly structural front of the fold and thrust belt (150 after its own thermal peak. km west of our area), O'Hara et al. (1990) Both the Narrows and St. Clair thrust found lower Rmax values above the Pine sheets show lower CAI values (3.5) in Middle Mountain thrust, as opposed to slightly Ordovician-Devonian strata than either the higher Rmax values below the thrust sheet in superjacent Saltville or Pulaski thrust sheets samples from equivalent coal units, indicat- (Epstein et al. 1977). CAI values are some- ing emplacement of the Pine Mountain thrust what higher (4-4.5) from the Lower Cam- sheet prior to the thermal peak. At Glen Lyn brian Knox Group both within the Narrows a similar relationship is inferred along the sheet and from structurally slightly below the structural front, although the front differs Narrows thrust sheet in the Bane dome. The structurally from the Pine Mountain thrust. latter values are indicative of temperatures of Here, the weathered, and hence minimum

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(see Ingram and Rimstidt 1984), 1.35% Rmax attributed the differences in rank to differ- value (1.21% Rmean) for Bluefield coal is ences in resistance to thrusting between the somewhat low compared with a nearby fractured Merrimac and the hard, resistant anomalously high CAI index of 3.0 (Epstein Langhorne. Bustin (1983), in a study of coals et al. 1977) from the Mississippian Greenbrier involved in thrusting in the Cordilleran fold Formation (400 m below the Bluefield). Both and thrust belt in Montana, Alberta, and Brit- are high for simple stratigraphic burial be- ish Columbia, found no evidence for any sig- neath <2 km of Mississippian and Pennsylva- nificant fault influence on metamorphism. nian strata. They are, however, compatible However, O'Hara et al. (1990) did document with the added burial depth beneath the some evidence for localized shear-influenced structurally higher St. Clair thrust sheet con- heating in coals on the Pine Mountain thrust taining about 3.5-4 km of Cambrian-Missis- sheet in southeastern Kentucky. Rmax values sippian strata above the overturned limb of (this study) from a different locality but from the Glen Lyn syncline in this area. the same seams Campbell and Others (1925) Thermal Event and Coal Metamor- examined in the Empire mine (samples phism.-In addition to the overburden added 8061-8062, Route 100; table 1) show an in- by the Pulsaki sheet to coals of the Saltville verse relationship in rank where the fractured sheet, the Pulaski thrust may have exerted Langhorne is the locus of a small footwall some control on the movment of hydrother- thrust associated with the Pulaski fault and mal fluids. Daniels and Altaner (1990) and has significantly higher rank than the rela- Daniels et al. (1990) considered similar mech- tively undeformed Merrimac seam. Inasmuch anisms in the Pennsylvania anthracite fields. as most of the fractured samples show evi- Progressive foreland-directed movements of dence of fluid-borne post-fracture mineraliza- thrust sheets, combined with fluid expulsions tion, we believe the observed rank differ- associated with orogenic uplift to the east, ences are more likely due to differences in may be mechanisms contributing to the accessibility to fluid flow at the time of meta- movement of thermal fluids. Such hydrother- morphism. Considering the effects of thermal mal waters may originate in overpressured perturbations induced by hydrothermal flu- zones and can contain high concentrations of ids, conventional arguments for estimation of metals such as lead and zinc (Jones 1977). depth of metamorphism using rank gradients Cathles and Smith (1983) and Oliver (1986) and inferred temperature gradients must be attributed Mississippian Valley-type ore de- used with deliberate caution in areas of fluid- posits such as in the Illinois-Kentucky Fluor- borne post-fracture mineralization. spar District, to periodic expulsion of brines Late-stage fracture-filling carbonate ce- from geopressured zones. Among our un- ments, plus fluorite or sulphides, in nearby weathered samples, carbonate fracture-filling Valley and Ridge carbonates (Grover and mineralization is present only in coals with Read 1983; Dorobek 1987), along with the Rmax >1.95%, suggesting that mineralizing epigenetic minerals associated with breccias fluids may have played a role in the metamor- and fracturing of Valley Field coals, appear phism. Intense fracturing of some coals, to be part of an extensive (maximum temper- noted in this study as well as by Campbell ature ca. 200°C) fluid movement event (such and Others (1925) and Stanley and Schultz as depicted by Morton 1985; Garven and (1983), may have provided conduits for the Freeze 1984; Oliver 1986) during Alleghanian fluids. deformation. These fluids likely had a major Campbell and others (1925) noted signifi- effect on coal metamorphism and diagenesis cant rank differences between the Langhorne as a whole, especially in the southeastern- (12.0-12.9% volatile matter, daf) (see re- most Valley and Ridge Province. Temporal/ flectance analysis for KCER 8060) and the spatial association between probable Alle- Merrimac (21.6% volatile matter, daf) coal ghanian fracturing and mineral-laden fluids beds in the Empire Mine (where they are only might be comparable with models similar to about 4 m apart), Little Walker Mountain earthquake-generated, locally dilational, min- Field, Pulaski County. Campbell and Others eralized zones (Sibson 1987), although orien- (1925), a proponent of thrust pressure as a tation of the stress field here would be en- prime contributor to organic metamorphism, tirely different from that associated with

This content downloaded from 128.163.2.206 on Thu, 21 Dec 2017 15:53:16 UTC All use subject to http://about.jstor.org/terms THERMAL EVENTS ON THRUST EMPLACEMENT SEQUENCE 939 transform fault zones such as the San An- beneath both it and the Blue Ridge thrust dreas. Here, the creation of localized subhor- sheet. Structural limitations are also im- izontal dilational zones with preferred fluid posed on the Glen Lyn syncline, for if the movement essentially parallel to bedding Narrows thrust sheet had extended this far planes would have been favored. Both Clen- forelandward overtop the St. Clair thrust denin and Duane (1990) and Leach and sheet for any significant length of time, the Rowan (1986) discuss the role of tectonically structurally doubled thickness would have re- derived fluids in the formation of Mississippi sulted in significantly higher Rmax values. Valley-type ore deposits. Clendenin and The distribution of low volatile bituminous Duane (1990) ascribe a major role to "seismic Rmax values (fig. 2; sample numbers 8065 and pumping" of hot fluids during faulting. Simi- 8070-8072) suggests that vitrinite reflectance lar mechanisms would foster pervasive redis- may decrease slightly just northwest of the tribution [and perhaps "boiling" conditions, present day Pulaski thrust trace. This small such as described by Sibson (1987)], of inter- difference, if significant, could have resulted stitial fluids. from (1) especially rapid erosion of the over- Age of Thermal Event.-K-Ar ages of illiti- lying Pulaski sheet from that region, perhaps zation determined by Elliot and Aronson because of significant tilting of the southeast (1987) suggest a minimum 30 m.y. duration limb of the Sinking Creek anticline during (303-273 Ma) for heating time. The 303 Ma Saltville thrust propagation (fig. 3); (2) struc- K-Ar date, which Elliot and Aronson (1987) tural thinning of the Pulaski sheet because indicate is their oldest mean age of illitiza- the Pulaski fault ramped from the Cambrian tion, comes from significantly west of the Pu- Rome Formation upward to a stratigraphic- laski sheet; hence, movement on the Pulaski ally higher glide horizon (fig. 3); and/or (3) thrust may have begun prior to the 300 Ma differential access for warm fluids escaping oldest age of illitization. Thus the age span from tectonic highlands to the east. Mecha- for the western Valley and Ridge thermal nisms (1) or (2) may have been operative with event is compatible with Alleghanian as de- limitations (cf. Edman and Furlong 1987; fined by the Valley and Ridge thrusting event. Warner and Royse 1987). They discussed Moreover, it largely overlaps other radiomet- thermal effects from emplacement of superja- rically constrained Alleghanian deformation cent thrust sheets but did not consider the in the Appalachians such as in the Kiokee role of introduced fluids, which may not have belt (315-295 Ma) and in the South Carolina been pertinent to their examples. We favor slate belt as determined by 40Ar/39Ar data differential fluid access to be at least as im- (Dallmeyer et al. 1986). Also, Sinha et al. portant, if not the dominant, reason for the (1989) place the Permo-Carboniferous (Al- observed variations because, in our samples, leghanian) igneous activity of the Piedmont fractures allowing fluid access produce con- at 330-270 Ma. trasting effects over several meters. This in- Thrusting and Metamorphism of Virginia terpretation is compatible with Grover and Valley and Ridge.-If the total duration of Read (1983) and Dorobek (1987). the heating event was pre- to syn-thrusting, The forelandward sequencing of thrusts the Blue Ridge thrust sheet could not have (Pulaski followed by Saltville and more west- extended as far as the Price Mountain win- erly thrusts) supports Perry's (1978) interpre- dow, which is 10-12 km forelandward of its tation that Appalachian thrust development present-day trace (see above-surface projec- was from the hinterland toward the foreland. tion, fig. 3), for any significant period of time; CAI isograds north of the Roanoke recess are or if it did, it did not affect geothermometry not offset (Harris 1979); hence (1) two sepa- signatures, because added overburden should rate thermal events affected the central and have resulted in both higher vitrinite reflect- southern Appalachians, or (2) the heating ance values (Mississippian) and higher CAI event in the central Appalachians north of the values (Ordovician), as well as a difference recess continued throughout thrusting, or (3) in other temperature indicators. Thus, at the the thrust movement there occurred before time of peak "metamorphism," rocks of the the thrusting west of the Pulaski-Staunton Price Mountain window lay (as shown in fig. thrust system in the southern Appalachians 3) beneath the Pulaski thrust sheet, but not where CAI isogrades are offset (Harris 1979).

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The last alternative is in contradistinction to and St. Clair thrust sheets. The steep temper- the conclusion of Bick (1986) that foreland- ature gradient across the Saltville thrust sug- ward of the Pulaski-Staunton fault, northern gests both significant displacement on the Appalachian thrusting post-dates southern Saltville thrust and that the respective ther- Appalachian thrusting. Bick's interpretation mal maxima were attained on both the Pu- is possible if (1) either two separate thermal laski and Saltville thrust sheets prior to, or events occurred, or if (2) the thermal event during, their emplacement above the more affected the central Appalachians for a sig- westerly Valley and Ridge thrust sheets. nificantly longer period of time. Thermal maxima on the Narrows and St. Clair thrust sheets were probably reached CONCLUSIONS post-emplacement. 1) Rmax values from Mississippian-age Price 3) Local variation in Rmax values within the Formation coals range from 2.07% to 2.60% Saltville thrust sheet and decrease of Rmax in the Price Mountain window, and from from southeast to northwest may, locally at 1.61% to 2.56% in Saltville thrust sheet rocks least, be influenced as much by the tempera- beneath the present day leading edge of the ture of, and access to, fluids expelled during Pulaski thrust. These Rmax values are compat- Alleghanian tectonism as by either strati- ible with CAI values and other geothermome- graphic or tectonic burial. try indicators (Epstein et al. 1977; Ingram and Rimstidt 1984; Zentmeyer 1985), which suggest temperatures of Alleghanian "metamor- ACKNOWLEDGMENTS.-The balanced cross phism" of 200 ±25°C on the Saltville thrust section in figure 4 is generalized from the geo- sheet. Depth-of-burial for the coals was at logic map of the Radford 1:100,000 quad- least 4.5 km, and more likely 6-7 km, as rangle (in preparation for the U.S. Geol. reconstructed from both stratigraphic and Survey misc. map series) by M. J. Bar- structural data. The Pulaski thrust sheet was tholomew, A. P. Schultz, S. E. Lewis, and emplaced before the thermal maximum was R. C. McDowell. A. P. Schultz, S. P. attained in the Saltville thrust sheet. Altaner, E. J. Daniels, W. J. Perry, Jr., M. J. 2) Temperatures of ca. 200°C were main- Bartholomew, and one anonymous reviewer tained higher in the stratigraphic section helped significantly to improve the manu- (Mississippian strata) on the Saltville thrust script. A. G. Harris generously supplied sam- sheet than on the both the subjacent Narrows ple locations for CAI data in the region.

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