Deep Gas Well Encounters Ultramafic Kimberlite-Like Material in the Sauk Sequence of Southeastern Ohio

Deep gas well encounters ultramaﬁ c kimberlite-like material in the Sauk Sequence of southeastern Ohio, USA

Mark T. Baranoski Ohio Division of Geological Survey, Columbus, Ohio 43224, USA V.M. Brown Department of Environmental Sciences, University of Toledo, Toledo, Ohio 43606, USA Doyle Watts Department of Geological Sciences, Wright State University, Dayton, Ohio 45435, USA

ABSTRACT tectonic fault slice, or eroded igneous depos- 20-m-thick zone of what appeared to be bio- its. Subsurface mapping of units above the tite-rich arkose. A limited volume of the total The fi rst indication of a potential deep- Sauk Sequence (Ordovician Black River drill cuttings was available for microscopic subsurface kimberlite-like material in the Group–Trenton Limestone) shows no signifi - study, ~31 g from the 18-cm-diameter bore- Appalachian Basin, without surface expres- cant variation in structural contours above hole of this 20 m interval. Other available cut- sion, has been discovered in oil and gas well the feature. Although the igneous material tings, examined from other deep wells in the drilling samples from a deep well drilled near has not been isotopically dated, the strati- area, did not contain anomalous lithologies. Zanesville, Ohio, USA. The well was drilled graphic position suggests that it was intruded Grain mounts were made to petrographically into the Cambrian Sauk Sequence within prior to Middle Ordovician time. analyze samples from this anomalous zone. a localized fault-bounded graben, where a Reconnaissance scanning electron microscopy zone of ultramafi c rocks was encountered. Keywords: Appalachian Basin, ultramafi c, (SEM) with energy-dispersive spectrometry The zone of ultramafi c rocks is within Sauk phlogopite, SEM, Sauk Sequence. was performed on selected points within pol- carbonates at 1720 m below sea level and is ished grain mounts. Subsurface mapping with not highly anomalous on geophysical wire- INTRODUCTION available deep geophysical wire-line logs was line well logs. Petrographic analyses of done using computerized hand-contouring grain-mounted samples show a 20 m zone of We report our preliminary fi ndings on the verifi ed coincidence with a reported seismic ultramafi c rock material near the top of the newly discovered Murray ultramafi c zone near refl ection anomaly described in unpublished Conasauga Group. Well cuttings from the Zanesville, Ohio, based on limited subsurface industry data (Peter McKenzie, 1998, personal ultramafi c zone include relatively fresh phe- data. We use the rock term “kimberlite(?)” commun.). As part of a graduate student proj- nocrysts of phlogopite with calcite, apatite, informally, until further geochemical analy- ect, gravity and magnetic data were acquired at and titaniferous magnetite, in a secondary ses can be performed. The discovery well was the surface above the seismic anomaly. matrix of amphibole, chlorite, Fe-oxides, and drilled in 1994 by Clinton Oil Company (per- Kimberlites are rare, small ultramafi c intru- possibly serpentine. The rocks have under- mit 278921) in search of natural gas, based on sive bodies that were fi rst named for diamond- gone deuteric alteration and probably later a seismic refl ection anomaly beneath the Mur- bearing peridotite rocks of Kimberly, South hydrothermal alteration. ray lease in Muskingum County, Falls Town- Africa (Lewis, 1887). According to Dawson A 20-m-thick, localized, natural gas–bear- ship (see Figs. 1 and 2). The well reached a (1984), kimberlite is a hybrid rock, consisting of ing sandstone unconformably overlies (Knox total depth of 2065 m in the Middle Cambrian a complex assemblage of altered high-tempera- unconformity) the Sauk Sequence above the Conasauga Group and was completed as a ture minerals and megacrysts, including a wide ultramafi c zone. Structural mapping indi- gas well in a 20-m-thick Middle Ordovician range of wall-rock material. Field studies indicates that the localized sandstone is confi ned sandstone overlying the Knox unconformity. cate that a wide range of mineralogy and rock to a small seismic-defi ned graben originating Examination of mud-logging well cuttings types is typical with rocks reported as kimber- in the faulted Precambrian Grenville base- from the Murray well using a 30× binocular lites in the Appalachian Basin. These extremely ment complex. Mapping of newly acquired microscope indicated abundant limonite stain- rare rocks, thought to be mantle derived, have gravity and magnetic data does not indicate ing of Cambrian Knox and Conasauga Group been reported from surface exposures in the the presence of an anomaly coincident with dolomite within a 200 m interval. Microscopic Appalachian Basin states of Kentucky, Mary- the graben, suggesting that the ultramafi c examination also revealed a very unusual land, New York, Pennsylvania, Tennessee, Vir- material must be a very small, localized ginia, and West Virginia (Bolivar, 1982; Parrish intrusive body (pipe or diatreme). Alterna- 1Permit number assigned by the Ohio Department and Lavin, 1982; Southworth et al., 1993; Watts tive interpretations include a thin intrusive of Natural Resources Division of Mineral Resources et al., 1992) (Fig. 1). Kimberlites of Elliott sill, extrusive tuff deposits, Grenville-aged Management. County, Kentucky, are associated with calcite of

Geosphere; June 2007; v. 3; no. 3; p. 177–183; doi: 10.1130/GES00063.1; 6 ﬁ gures.

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Figure 1. Map showing known central and northern Appala- chian Basin kimberlite loca- tions, Rome Trough fault system, Ohio Precambrian Provinces, the location of the Murray deep well near Zanes- ville, Ohio, and nearest deep structures from Parrish and Lavin (1982), Baranoski (2002), and Watts et al. (1992). Appa- lachian Basin is located east of the Grenville front. Inset map shows location of Falls Town- ship and Zanesville in Musk- ingum County, Ohio.

igneous origin and occur in vertical pipes (dia- (Fig. 1). Bass (1960), and subsequently many geochronology from deep wells to determine the tremes) as well as dikes, which weather rapidly others, interpreted a Precambrian magnetic timing of the Ohio Precambrian Provinces, i.e., at the surface (Bolivar, 1982). Parrish and Lavin boundary in Ohio, referred to as the Grenville the Eastern Granite-Rhyolite, the East Conti- (1982) noted the diffi culty in locating kimber- front or Grenville front tectonic zone, separat- nent Rift Basin, and the Grenville Province. The lite intrusions by magnetic methods because of ing the Grenville Province on the east from East Appalachian Basin architecture was controlled weathering, low magnetic susceptibility, and Continent Rift Basin and older Granite-Rhyolite largely by subsidence of Grenville Province small size. Province on the west. The Grenville front is thus basement rocks east of the Grenville front during a magnetic lineament or boundary. the Phanerozoic. Preexisting structural weakness REGIONAL STRUCTURAL SETTING The Precambrian crystalline rocks of Ohio zones in the Precambrian are hypothesized as AND GEOCHRONOLOGY have not been U/Pb age dated. Calculated an important factor in controlling Phanerozoic whole-rock Rb/Sr dates for Ohio range from geology (Beardsley and Cable, 1983; Riley et al., The Precambrian unconformity surface in ca. 0.9 to 1.3 Ga (Lucius and von Frese, 1988). 1993). Kimberlites, considered to be emplaced eastern Ohio is defi ned as the top of Grenville Consequently, Ohio Rb/Sr dates only indicate along deep-seated faults and fractures (Bolivar, Province metamorphic and igneous rocks, and the presence of Precambrian rocks and do not 1982; Parrish and Lavin, 1982; Shultz, 1999), in western Ohio by the top of East Continent show relationships to currently accepted prov- are well documented in the Appalachian Basin Rift Basin sedimentary and volcanic rocks inces or boundaries. Drahovzal et al. (1992) used along a regional fairway east of Ohio and above and Granite-Rhyolite Province igneous rocks seismic refl ection data and limited petrology and the Cambrian Rome Trough rift system (Parrish

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SW NE

Figure 2. Color amplitude (red positive; black negative) seismic refl ection profi le near the Murray well (permit 27892, Muskingum County, Ohio) with major refl ectors indicated. We interpret the disruption of the Knox to Precambrian refl ectors as a faulted extensional structure extending into the Precam- brian basement. The maximum extent of the structure on this section is about 2.8 km. The anticlinal structure shown in the middle of the section, most prominently shown in the Knox refl ector, just to the right of the Murray well, is not confi rmed by well log data, and is likely a processing artifact.

and Lavin, 1982). Shultz (1999, p. 217) specu- above the Ordovician Wells Creek Formation. overlying the Knox unconformity and indi- lated, on the basis of proprietary seismic refl ec- The structure map on the Wells Creek shows cates the expected north-south strike and east- tion data, that there might be subsurface kimber- east-dipping contours at ~20 ft/mi (~3.5 m/ ward dip of ~3.5 m/km. Structure mapping of lites in eastern Ohio and western Pennsylvania. km) (Fig. 4A). the Cambrian Copper Ridge dolomite in the The Murray kimberlite(?) is on the western Two anomalous lithologies were encountered area, however, indicates a localized structural fl ank of the Appalachian Basin, 80 km north- beneath the Black River Group: a localized 20- low dipping eastward at ~19 m/km. The posi- west of the Rome Trough. The Appalachian m-thick sandstone in the lower portion of the tion of the structural low coincides with both Basin strata in this area dip <1º east-southeast Wells Creek Formation at ~1500 m below sea the occurrence of the thick sandstone (Fig. 4B) on mapped Ordovician and Cambrian surfaces. level and a 21-m-thick arkosic interval in the overlying the Knox unconformity and a small The Murray kimberlite(?) is not near known Conasauga Group at ~1720 m below sea level fault-bounded graben. The graben originates local or regional Precambrian structures. The (Fig. 3). Sample cuttings from available wells in the faulted Precambrian Grenville basement nearest known structures are the Cambridge in the area were examined, and anomalous or complex and was interpreted from industry cross-strike structural discontinuity and the Starr exotic lithologies were not identifi ed. The local- seismic data near the Murray well (Fig. 2). The fault system (Baranoski, 2002) (Fig. 1). ized sandstone is gas bearing in the Murray Murray well reached total depth in the Middle well and overlies the Sauk Sequence or Knox Cambrian Conasauga Group at 2070 m. Struc- LOCAL ORDOVICIAN AND CAMBRIAN unconformity. In this area, Rose Run sandstone ture mapping of the Conasauga is the logical STRATIGRAPHY AND STRUCTURE of the Knox Dolomite is typically ~30 m thick next step; however, there have not been enough and consists of interbedded dolomitic subar- drill sites to the Conasauga Group near the The total thickness of Paleozoic sedimen- kose, quartz arenite, and dolomite (beneath the Murray well. The nearest wells drilled are tary rocks at the Murray well is estimated to Knox unconformity). The sandstone directly more than 16 km away, and are thus too dis- be 2100 m. Only the Cambrian and Ordovi- overlying the unconformity is white, very fi ne tant to use as control points for contouring this cian rock units are discussed in this paper. grained, moderately to well rounded, moder- localized graben. As a result, mapping beneath The Murray well penetrated 57 m of Trenton ately to well sorted, pyritic, and very friable. the Copper Ridge dolomite is not possible. Limestone, 152 m of Black River Group (lime- The unit is characterized on geophysical logs stone), 43 m of Wells Creek Formation (shale, by relatively low photoelectric and gamma-ray ULTRAMAFIC (KIMBERLITE LIKE) limestone, dolomite, and sandstone), 173 m of response compared with the underlying Rose ROCK FRAGMENTS Knox Dolomite (Rose Run sandstone and Cop- Run. It correlates to stratigraphically similar per Ridge dolomite), and 152 m of Conasauga sandstones encountered in wire-line–logged Kimberlites are mineralogically complex Group (dolomite and sandstone) (Fig. 3). The wells within ~1.6 km (Fig. 4B) of the Murray rocks that are derived from the mantle and vio- well did not penetrate Precambrian rocks. well. This thick sandstone was not observed lently emplaced within stable, usually ancient, Detailed stratigraphic correlation and structural during well drilling in adjacent areas. cratons. They are volatile-rich, potassic, ultra- analyses with wells in the area did not reveal A structure map of the Murray well mafi c alkaline rocks that occur as dikes, sills, signifi cant structural or thickness variations area (Fig. 4A) depicts the Ordovician units and diatremes (Winter, 2001). They can vary

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in this preliminary study, they may be present in the groundmass or in other sample cuttings not mounted on the grain mounts. In one rock fragment, the geometric arrangement of opaque granules suggests alteration from another maﬁ c phase (Fig. 5A). The phlogopite is conspicuous and abundant in some of the rock fragments, varying from 0.01 to 1.0 mm (Fig. 5B). It occurs primarily as relatively fresh, subhedral to nearly euhedral phenocrysts, and to a lesser extent as smaller grains in the matrix. Optically, phenocrysts are biaxial (–) with a 2V angle of 2º–5º. They exhibit parallel extinction, poor (nearly absent) to good cleavage, and are reddish-brown. A few grains exhibit bending or kinking. Reconnais- sance SEM analyses of the phlogopite indicate an Mg:Fe ratio of ~3:1 with a relatively high titanium content (Fig. 6).

DISCUSSION AND CONCLUSIONS

Overall, the mineralogy of the ultramafi c zone in the Murray well is similar to that of other kimberlites described in the region (Boli- var, 1982; Alibert and Albarede, 1988; Shultz, 1999; Watts et al., 1992). However, we have not identifi ed mantle-derived xenoliths contain- ing olivine, garnet, and other mafi c minerals in the well samples. Such an identifi cation is also conditional because of the low volume of recovered borehole samples. Nevertheless, we interpret the phlogopite, apatite, and magnetite, and the secondary alteration minerals amphibole, chlorite, Fe-oxides, and possibly serpentine, as representative common constituents found in kimberlite-like suites. Unequivocal comparison of Murray well ultramafi c material cannot be made to other well-studied regional kimberlites without core samples. Nevertheless, it is signifi - cant that the presence of ultramafi c material has been described in the Conasauga Group, >90 m above Ohio’s Grenville basement rocks. Figure 3. Geophysical wire-line logs and stratigraphic correlation of the pertinent Ordovi- Deep-seated faulting in the Precambrian cian and Cambrian units and ultramafi c zone for the Murray well (permit 27892, Musk- Grenville basement complex is evident, based ingum County, Ohio). Gamma ray increases from natural radiation at points A and B. upon industry seismic refl ection data collected Photo-electric effect increases toward calcite at point C. directly over the Murray well (P. MacKenzie, 1998, personal commun.; G. Mason, 1999, personal commun.). The seismic refl ection widely mineralogically, but usually contain small rock fragments ranging from 0.5 to 4 mm data indicate a highly faulted structural sag at some olivine, phlogopite, ilmenite, magnetite, that were handpicked from ~31 g total drill cut- depth, suggesting localized extension of the garnet, diopside, enstatite, and chromite (Mitch- tings for the 20 m interval. Epoxy grain-mount crust (Fig. 2). The basement faulting is roughly ell, 1995). Deuteric calcite, serpentine, and glass slides were ground to 3 µm and polished coincident with two features on regional con- chlorite are often present. Kimberlite suites also for optical analyses and SEM. Phlogopite, Na- tour maps by Hildenbrand and Kucks (1984a, characteristically contain xenocrysts and xeno- feldspathoids, titaniferous magnetite, ilmenite, 1984b), i.e., a north-south–trending total mag- liths of other mantle-derived alkaline rocks as apatite, and calcite were positively identifi ed. In netic intensity, and a gravity high. Structure and well as other wall-rock material picked up on addition, Ca-Mg pyroxenes (probably diopside) isopach mapping of deep gas-well data indi- their rise through the mantle and crust. and secondary amphiboles were identifi ed in the cates no signifi cant anomalies above the Ordo- The ultramafi c kimberlite(?) material groundmass by SEM. Although olivine, serpen- vician Wells Creek Formation. A thick sand- described in this study consists of a few very tine, and garnet were not positively identifi ed stone unit overlying the Knox unconformity

180 Geosphere, June 2007

A C

Figure 4. Well data from available electric logs were used to manually contour selected geologic subsurface units in Falls Township area of Muskingum County, Ohio. (A) Structure on the Wells Creek Formation. Tight area of contours indicates local monoclinal ﬂ exure. (B) Structure on the Copper Ridge dolomite. The structurally low area correlates with a localized graben, based up on proprietary seismic data (Fig. 2). (C) Thickness of the Wells Creek Formation (including the anomalous sandstone overlying the Knox unconformity). Area of increased thickness indicates incised valley ﬁ ll that overlies localized graben. Areas of zero thickness and 50 feet (15 m) are stratigraphically thin, and typical of nondeposition on the Knox unconformity over paleotopographic monadnocks. Contour interval is 20 ft (~6 m).

surface coincides with a localized structural units above the Wells Creek shows only the porosity, or lithology zones. In the absence of low area mapped on the Copper Ridge dolo- expected regional strike and dip trends. core samples, mineralogical verifi cation of mite and faulting in Grenville basement. The The Murray ultramafi c zone is recognized spiky wire-line log response is tenuous; how- deep-seated extensional faulting of the Gren- only in sample cuttings. The geophysical well ever, Murray well wire-line logs suggest pos- ville basement associated with the graben may log (Fig. 3) shows poor clay and porosity devel- sible correlation where the gamma ray increases have provided suffi cient conduits for transfer opment, which indicates that the zone is very slightly to the right at points A and B, with pho- of mantle-derived ultramafi c material into the thin and may not be weathered. The wire-line toelectric increase at point C (Fig. 3). Conasauga Group dolomite. Subsurface map- logs for the Murray well are not highly anoma- Because kimberlites are typically enriched ping with deep well control indicates that the lous in this ultramafi c zone. Gamma-ray and with mantle-derived carbonates, we specu- faulting in the basement occurred prior to depo- density log response in this zone are typical late that the Murray ultramafi c rock may sition of the Middle Ordovician Wells Creek of Cambrian carbonate rocks in eastern Ohio have passed through faulted Grenville-aged Formation. Structure and isopach mapping of and do not indicate washout zones, anomalous marble prior to being intruded into Cambrian

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A B

Figure 5. (A) Photomicrograph (0.65–1.0 mm) of Murray well cutting (6450–6980 ft [1966–2127 m] interval) exhibiting a reaction rim of alteration minerals from a maﬁ c phase (Op with arrows point to rim of opaques; P—phlogopite grain). (B) Photomicrograph (1.75–2.6 mm) of Murray well cuttings (1966–2127 m interval). Three well cuttings show abundant phlogopite.

localized strike-slip compressional stresses, versus what appears in the seismic refl ection data to be dominantly tensional stress. Planar deformation features, one of the diagnostic criteria for impact sites, were not observed in well cuttings. Thus a bolide impact interpreta- tion for the seismic anomaly, as presented by Mason et al. (2004), is not supported. The Murray ultramafi c zone has not been isotopically dated. Based on subsurface mapping, we speculate timing of original emplacement prior to Middle Ordovician time, which is much older than the well-defi ned eastern North America Mesozoic kimberlite magmatism ages (Heaman and Kjarsgaard, 2000). The Murray ultramafi c zone is signifi cant structurally because it is located on the stable craton on the Appalachian Basin and 80 km northwest of Figure 6. Backscattered scanning electron microscope–energy-dispersive spectrometer the fairway of shallow kimberlites in the Rome image of probable ultramafi c rock fragment and table of elemental analyses at cross hair of Trough (Parrish and Lavin, 1982). It follows phlogopite at 1960 m. that other kimberlites in southeastern Ohio may have been penetrated by drilling, but have not been identifi ed because they are not easily distinguished from country rock on the wire- carbonates. Two deep wells drilled in central phlogopite in the sample, and the correlation line logs, and may have been overlooked in and eastern Ohio recovered marble from the of this phlogopite-rich zone to a spiky gamma- well cuttings. We will not know if the Murray Precambrian Grenville rocks (Bass, 1960). In ray log response (at points A and B; see Fig. 3), ultramafi c zone is a kimberlite or another exotic this scenario, the additional carbonate country could support either thin vertical dikes or rock until further mineralogical and geochemi- wall rock further enriches kimberlites within horizontal sill scenarios. The fresh-appearing cal work is completed. The presence of this carbonate; thus, distinguishing small intrusive phlogopite recovered in the well cuttings sup- ultramafi c zone on the stable craton is excit- bodies on wire-line logs is diffi cult. Alterna- ports an intrusive or faulted structural scenario. ing, and leads us to question when it occurred, tive scenarios include horizontal tabular lay- Erosional or explosive sediments scenarios are and what is its mineralogy and geometry. It is ers or sills; a faulted tectonic slice of Grenville not likely, because phlogopite-rich sediments possible that there are more exotic zones in the metasediments; erosional deposits of materials probably would have weathered rapidly prior deep subsurface lacking surface expression; if shed from a nearby exposed igneous body; to burial on this portion of the stable craton. so, other oil and gas and mineral deposits may explosive tuffaceous sediments from a pipe; A slice of Grenville basement thrusted upward be associated with these zones, and locating or an impact site. The dominance of K-rich into the Conasauga could only be explained by them is a further challenge.

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Beardsley, R.W., and Cable, M.S., 1983, Overview of the structure; impact crater or tectonic feature [abs.]: ACKNOWLEDGMENTS evolution of the Appalachian Basin: Northeastern America Association of Petroleum Geologists Bulle- Geology, v. 5, p. 137–145. tin, v. 88, no. 13. We thank Jim Templin of Columbia Natu- Bolivar, S.L., 1982, Kimberlite of Elliott County, Kentucky: Mitchell, R.H., 1995, Kimberlites, orangeites, and related Kentucky Geological Survey Series XI, 36 p. rocks: New York, Plenum Press, 410 p. ral Resources, Charleston, West Virginia, for Dawson, J.B., 1984, Petrogenesis of kimberlite, in Glover, Parrish, J.B., and Lavin, P.M., 1982, Tectonic model for kim- making well cuttings available; Greg Mason of J.E., and Harris, P.G., eds., Kimberlite occurrence berlite emplacement in the Appalachian Plateau of Penn- The Energy Cooperative, Granville, Ohio, for and origin: University of Western Australia Geology sylvania: Geology, v. 10, p. 344–347, doi: 10.1130/0091- Department Publication 8, p. 103–111. 7613(1982)10<344:TMFKEI>2.0.CO;2. sharing seismic data; Robert Smith and John Drahovzal, J.A., Harris, D.C., Wickstrom, L.H., Walker, D., Riley, R.A., Harper, J.A., Baranoski, M.T., Laughrey, C.D., Barnes of the Pennsylvania Geological Survey Baranoski, M.T., Keith, B.D., and Furer, L.C., 1992, and Carlton, R.W., 1993, Measuring and predicting The East Continent Rift Basin—A new discovery: reservoir heterogeneity in complex deposystems: The for scanning electron microscope analyses; Ohio Division of Geological Survey Information Cir- Doug Pride of Ohio State University for ana- Late Cambrian Rose Run sandstone of eastern Ohio cular 57, 25 p. and western Pennsylvania: U.S. Department of Energy Heaman, L.M., and Kjarsgaard, B.A., 2000, Timing of east- lytical discussions; Lisa Van Doren for digital contract DE-AC22–90BC14657, 257 p. ern North American kimberlite magmatism: Continen- Shultz, C.H., 1999, Jurassic kimberlite dikes, in Shultz, artwork; Larry Wickstrom, Dennis Hull, and tal extension of the Great Meteor hotspot track?: Earth C.H., ed., Harrisburg—Pittsburgh: Geological Survey Chris Tickle of the Ohio Department of Natu- and Planetary Science Letters, v. 178, p. 253–268, doi: of Pennsylvania, Pittsburgh Geological Society, Spe- ral Resources, Division of Geological Survey, 10.1016/S0012-821X(00)00079-0. Hildenbrand, T.G., and Kucks, R.P., 1984a, Residual total cial Publication, p. 210–217. Chris Laughrey of the Pennsylvania Geologi- intensity magnetic map of Ohio: U.S. Geological Sur- Southworth, C.S., Gray, K.J., and Sutter, J.F., 1993, Middle cal Survey; an anonymous reviewer for critical vey Geophysical Investigations Map GP-961, scale Eocene intrusive igneous rocks of the central Appala- chian Valley and Ridge Province—Setting, chemistry, reviews of the manuscript. 1:500,000. Hildenbrand, T.G., and Kucks, R.P., 1984b, Complete Bou- and implications for crustal structure: U.S. Geological guer gravity anomaly map of Ohio: U.S. Geological Survey Bulletin 1839-J, p. 1–24. REFERENCES CITED Survey Geophysical Investigations Map GP-962, scale Watts, K.C., Jr., Grifﬁ tts, W.R., and Hinkle, M.E., 1992, 1:500,000. Evidence for a titaniferous diatreme of alkalic igne- Alibert, C., and Albarede, F., 1988, Relationships between Lewis, H.C., 1887, On diamondiferous peridotite and ous rock in the subsurface, Harrison, Taylor, and Dod- mineralogical, chemical, and isotopic properties of the genesis of diamond: Geological Magazine, v. 4, dridge counties, West Virginia: U.S. Geological Survey some North American kimberlites: Journal of Geo- p. 310–314. Open-File Report OF 92–0366, 39 p. physical Research, v. 93, no. 7, p. 7643–7671. Lucius, J.E., and von Frese, R.R.B., 1988, Aeromagnetic and Winter, J.D., 2001, An introduction to igneous and meta- Baranoski, M.T., 2002, Structure-contour map on the Pre- gravity anomaly constraints on the crustal geology of morphic petrology: Upper Saddle River, New Jersey, cambrian unconformity surface in Ohio and related Ohio: Geological Society of America Bulletin, v. 100, Prentice Hall, 695 p. basement features: Ohio Division of Geological Sur- p. 104–116, doi: 10.1130/0016-7606(1988)100<0104: vey Map PG-23, scale 1:500,000, 18 p. AAGACO>2.3.CO;2. MANUSCRIPT RECEIVED 27 JULY 2006 Bass, M.N., 1960, Grenville boundary in Ohio: Journal of Mason, G., Watts, D.R., Bean, D., Baranoski, M.T., and REVISED MANUSCRIPT RECEIVED 14 MARCH 2007 Geology, v. 68, p. 673–677. Brown, V.M., 2004, The Ohio Dillon Falls subsurface MANUSCRIPT ACCEPTED 22 MARCH 2007

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