Textures of Paleozoic Chert and Novaculite in the Ouachita Mountains of Arkansas and Oklahoma and Their Geological Significance

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Textures of Paleozoic chert and novaculite in the Ouachita Mountains of Arkansas and Oklahoma and their geological significance

WALTER D. KELLER Department of Geology, University of Missouri-Columbia, Columbia, Missouri 65211 CHARLES G. STONE Arkansas Geological Commission, 3815 West Roosevelt Road, Little Rock, Arkansas 72204 ALICE L. HOERSCH Department of Geology, LaSalle University, Philadelphia, Pennsylvania 19141

ABSTRACT logic imprints from plate tectonism. Moreover, a widespread thermally altered sedimentary formation may indicate elevated temperatures, not Scanning electron micrographs of cherts and novaculites from only in a local area, but over a region as large as a mountain belt, and as a the Ouachita Mountain fold belt of Arkansas and Oklahoma show a long-term definitive thermal event. sequential range in textures from cryptocrystalline, anhedral quartz Evidence that elevated temperature has been secondarily imposed < 1 /¿m in diameter in the nonmetamorphosed chert and novaculite, to upon sedimentary rocks is the presence of polygonal, triple-point texture as coarse, polygonal triple-point, euhedral quartz 100 fim or more in shown in Figure 1, a scanning electron micrograph (SEM) of the Arkansas diameter where the cherts have been thermally metamorphosed, the Novaculite from the Hot Springs, Arkansas, region. This texture is charac- coarsest being in xenoliths. The textures have been correlated with teristic of thermally metamorphosed silica rock, for example, chert- similar texture and crystal sizes in the chert from a contact metamor- novaculite, as discussed by Spry (1969) and Kretz (1966). In contrast to phic aureole on the Isle of Skye, Scotland, where classic metamorphic mineral assemblages from talc through tremolite, diopside, and forsterite grades have been identified. Texture of chert thus can be used as an indicator of elevated temperature. Mean apparent crystal diameters measured of the quartz in the Arkansas Novaculite and associated cherts were plotted on a map of the Ouachita Mountain fold belt extending from Little Rock, Arkan- sas, to near Broken Bow, Oklahoma. The regional trend in texture parallels the structural core trend and, in addition, is strongly overprinted by localized areas of coarser crystallinity near Little Rock, Magnet Cove, and Potash Sulphur Springs, Arkansas, where igneous intrusions have occurred, and near Broken Bow, Oklahoma. Temperature estimates from studies of fluid inclusions, stable isotope ratios, mineral-chemical phase relationships in associated rocks, and novaculite as xenoliths suggest that maximum temperatures as high as 760 °C may have been reached by portions of the novaculite. Other examples of triple-point texture in chert collected from distant localities, which range in age from Precambrian to Tertiary, show that polygonal triple-point texture in chert is world-wide in occurrence. It follows that small samples of chert-novaculite can yield evidence of a history of elevated temperature. Such evidence may be used to estimate maturation or degradation of hydrocarbons in the rocks and to furnish clues during exploration for thermally related metallic and nonmetallic minerals.

INTRODUCTION

Sedimentary rocks that show evidence of having been heated to Figure 1. Scanning electron micrograph (SEM) of Arkansas higher temperatures than normal for sedimentary environment may indi- Novaculite showing polygonal, triple-point texture. Collected from cate the possibility of thermal maturation in source beds of hydrocarbons, a honestone quarry a few miles east of Hot Springs, Arkansas, a locus for deposition of hydrothermal mineral deposits, a buried and shown as locality no. 1 in Figure 15. The scale bar represents 5 jum; otherwise hidden pluton, an episode of thermal metamorphism, or petro- original magnification (OM) is 2,000*.

Geological Society of America Bulletin, v. 96, p. 1353-1363, 25 figs., November

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Figure 2. SEM of nonmetamorphosed Arkansas Novaculite at Atoka, Oklahoma. Shows fine-grained, anhedral, mosaic quartz Figure 3. SEM of nonmetamorphosed chert in the Durness typical of sedimentary chert, OM 6,000x. dolomite, Isle of Skye, Scotland. A portion of a vug lined with quartz crystals secondarily deposited from solution, not triple- point, is shown in the upper-left corner, OM 4,400x. triple-point texture, the Arkansas Novaculite, where it is not metamorphosed, shows the texture of typical sedimentary chert, that is, anhedral, cryptocrystalline quartz (Fig. 2), collected from vertical novaculite beds at film of gold to carry away excess electrical charge from the electron Atoka, Oklahoma, far outside the thermal area and effects within the core microscope beam, they were micrographed at magnifications ranging from of the Ouachita Mountains. lOOx to 40,000* but 200x to 6,000x were those most commonly used. The purposes of this report are to illustrate with SEMs the full range After thorough visual examination of the entire areas of a specimen mount in sizes of triple-point crystals present in the chert-novaculite in Arkansas under the microscope, several micrographs judged to be representative of and Oklahoma, to match these textures with morphological counterparts the texture were taken for record and publication. Apparent diameters for collected from classic metamorphic-grade zones, to show on a map the the crystals were measured by counting the lengths of crystal intercepts distribution of the triple-point textures in the Ouachita Mountain fold belt, along lines plotted across the SEM photos and by using metric-circle to interpret in broad terms the thermal and geological significance of these charts. Although apparent diameters of crystals in rocks are smaller than textures, and to point out their possible application as guides in geologic their true diameters, apparent diameters provide reproducibly measurable exploration. data that are statistically comparable between diverse samples from a single region or from widely separated localities. These data on size are EXPERIMENTAL PROCEDURES reported for individual reference-texture specimens and to prepare the map of crystal sizes in the Ouachita fold belt. More than 2:50 samples were systematically collected in the field Scanning electron micrography adds an extra vertical dimension to from the major outcrops of the Arkansas Novaculite and of other cherts in the views seen by thin-section microscopy as used by Goldstein (1959), the Ouachita Mountain Belt from Little Rock, Arkansas, to Broken Bow, Goldstein and Reno (1962), and Goldstein and Hendricks (1963) in their Oklahoma, so as to represent the exposed distribution of those rocks. extensive petrographical studies of the Arkansas Novaculite. SEM also Ordinarily, several specimens were collected from each area of outcrop, extends the range in magnification beyond 20x to l,000x, which is the and/or from diffe rent levels in the stratigraphic section. At places of spe- typical range and optical limit of light microscopy, to as much as 4C,000x, cial interest, 10 or 20 specimens were taken at selected spacings, as along a or higher if needed, yielding excellent resolution. traverse outward from the intrusion at Magnet Cove, or at the Hot Springs and Little Rock areas. REFERENCE METAMORPHIC TEXTURES For micrography, small chips were broken from hand specimens, thereby yielding clean, fresh, natural-fracture surfaces. These were not A set of textures presented as standard reference examples for distin- ground or etched, thus avoiding the possibility of otherwise introducing guishing metamorphic grades was micrographed from the Cambrian artifacts during processing. After the chips were sputter-coated with a thin cherty Durness dolomite that was progressively metamorphosed in the

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Figure 4. SEM of talc, elemental composition confirmed by Figure 5. SEM of portion of Figure 4 enlarged to OM 5,000x. energy dispersive analysis and chert in the Durness dolomite, talc- Shows early development of triple-point texture representative of grade metamorphic zone, Isle of Skye, Scotland, OM 1,000*. talc-grade metamorphism in the Durness dolomite.

contact aureole where intruded by the Tertiary Beinn an Dubhaich granite, Isle of Skye, Scotland (Hoersch, 1981). Cherts were collected from the zones containing metamorphic mineral assemblages that characterize classic metamorphic grades, such as talc, tremolite, diopside, and forsterite; they are therefore indicative of the metamorphic grades constituting that standard metamorphic sequence. Unmetamorphosed Durness chert from outside the border of the metamorphic aureole is shown in SEM in Figure 3. A micrograph was selected that includes not only the sedimentary quartz, that here is slightly coarser than the Arkansas Novaculite at Atoka (Fig. 2), but also a vug containing coarser crystals of quartz that were secondarily deposited from solution, that is, texture other than the triple-point variety. In Figure 4, metamorphosed Durness chert and talc are shown, independently identified by energy dispersive analysis from the talc-grade zone on Skye. A part of the chert is shown at higher magnification in Figure 5; the mean apparent diameter (MAD) in this specimen is 0.9 ¡im. A textural counterpart to it from an outcrop of Arkansas Novaculite on Highway 375, in the SE'/4NE1/4, sec. 11, T. 3 S., R. 30 W., Polk County, is shown in Figure 6, at the same magnification as in Figure 5.

Figure 6. SEM of morphological counterpart to talc-grade metamorphic chert in the Arkansas Novaculite, collected along Highway 375 in the SE%SW%, sec. II, T. 3 S., R. 30 W., Polk County, Arkan- sas, locality no. 6 in Figure 15, OM 4,800*.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/11/1353/3444841/i0016-7606-96-11-1353.pdf by guest on 02 October 2021 Figure 7. SEM! of tremolite-grade metamorphic chert from the Figure 8. SEM of textural counterpart to tremolite grade from Durness dolomite. Polygonal, triple-point texture is clearly devel- the Arkansas Novaculite collected south of Shady Lake, in the oped, OM 5,000*. NW'/4SE'/4, sec. 31, T. 4 S., R. 2 W., Polk County, Arkansas, locality no. 8 in Figure 15, OM 4,800*.

Figure 10. SEM of morphological counterpart to diopside- grade metamorphic chert from a quarry in the NWViSW/4, sec. 9, T. Figure 9. SEM of diopside-grade metamorphic chert in the 3 S., R. 16 W., Hot Springs County, Arkansas, locality no. Id1 in Durness dolomite, OM 3,000*. Figure 15, OM 3,000*.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/11/1353/3444841/i0016-7606-96-11-1353.pdf by guest on 02 October 2021 Figure 11. SEM of forsterite-grade metamorphic chert in the Figure 12. SEM of morphological counterpart to forsterite- Durness dolomite, OM 2,000*. grade metamorphic chert collected at Reyburn Creek Gap in the center of the SEW, sec. 13, T. 3 S., R. 14 N., Hot Spring County, Arkansas, locality no. 12 in Figure 15, OM 2,000*.

Figure 14. SEM of textural counterpart to periclase-grade metamorphic chert occurring as inclusions in the intrusion at the Figure 13. SEM of periclase-grade metamorphic chert, Crest- Union Carbide Vanadium Mine, Potash Sulphur Springs, Arkan- more, California, OM 200*. sas, locality no. 14 in Figure 15, OM 200*.

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Figure 15. Map of the Ouachita Mountain fold belt in Arkansas and Oklahoma in which the main outcrop areas of Arkansas Novaculite are shown in dark shading. The generalized distribution of mean apparent diameters of the quartz crystals in the Arkansas Novaculite and chert in other formations is shown by symbols at 34 representative data points (of more than 200 measured) and by isopleths (dashed lines). The "0" nm" zone is transitional to polygonal triple-point texture, and it includes some crystals less than 1 /um in diameter. Coarsest crystallinities near Little Rock, Arkansas, and Broken Bow, Oklahoma, indicate areas of higher temperature history. Locations of collecting sites for specimens shown in SEMs are indicated by the same numbers as refer to their SEMs.

The texture of chert collected from the Durness tremolite-grade zone crystal size of the quartz occur in different samples from the areas of is shown in Figure 7. Triple-point texture, 1.35 ^m MAD, is clearly diopside and forsterite grades of metamorphism. The SEMs shown for developed in this grade. An Arkansas textural counterpart shown in Figure illustration are, however, typical mid-range crystallinities observed in nu- 8, at approximately the same magnification, was collected just south of merous specimen mounts chipped from reaction zones of the respective Shady Lake, in the NW'ASE'A, sec. 31, T. 4 S., R. 28 W„ Polk County, metamorphic grades. Arkansas. Periclase-grade metamorphosed chert, shown in Figure 13, wes col- Durness chert of diopside grade, 5.5 /urn MAD, is shown in Figure 9. lected by Carpenter at the Crestmore, California, metamorphic locality An Arkansas textural counterpart, in Figure 10, at the same magnification, (Carpenter, 1967). It is notably coarse grained, 57 |um MAD, and, like was collected from a quarry in the NW'ASW1/«, sec. 9, T. 3 S., R. 16 W., quartzite, breaks through the grains. A textural counterpart at the same Hot Spring County. magnification from Arkansas (Fig. 14) is in boulders of Arkansas Novacu- Forsterite-graiie Durness chert, 8.4 /im MAD, is shown in Figure 11. lite that occur as inclusions in the intrusive rock at the Union Carbide An Arkansas textural counterpart (Fig. 12), at the same magnification, was Vanadium Mine near Potash Sulphur Springs, Garland County, Arkansas. collected at Reyburn Creek Gap in the center of the SEW, sec. 13, T. 3 S., In hand specimen, the quartzitic character of the novaculite in the mine, as R. 17 W., Hot Spring County, Arkansas. also in the hottest parts of the Magnet Cove region, is similar to that at At the Skye locality, some variation and slight overlap in mean Crestmore.

VALLEY

ARKANSAS GEOLOGICAL COMMISSION

Figure 15. (Continued).

DISTRIBUTION OF THE APPARENT MEAN DIAMETERS fossils that usually are siliceous, and some vugs and patches that contain OF CRYSTALS IN THE CHERT AND NOVACULITE very small crystals of euhedral quartz and other minerals. UNITS IN THE OUACHITAS The main belt of triple-point texture lies within the most complexly deformed portions of the central "core" or "anticlinorial axis" of the The distribution of the textural varieties and crystal sizes in the Ouachita Mountains, as shown by Miser (1954), Flawn and others (1961), Devonian-Mississippian Arkansas Novaculite, the Ordovician Bigfork and Haley and others (1976). The two coarser crystal localities (Broken Chert, and other Paleozoic siliceous rocks is shown by symbols, and by Bow and Little Rock) within this trend were described by Honess (1923), isopleths of apparent mean diameters of their crystals, on a map of the Miser (1959), Viele (1973), Goldstein (1975), and Denison and others Ouachita Mountain belt, Figure 15. The belt containing triple-point tex- (1977) as containing the most intensely sheared and regionally metamor- ture is -250 km (155 mi) long, extending from Little Rock, Arkansas, to phosed rocks (low-temperature, lower greenschist facies) in the Ouachita near Broken Bow, Oklahoma. It is ~65 km (40 mi) wide in the east, near Mountains. The triple-point thermal belt also is closely congruent with the Little Rock, where crystals >35 |um in apparent diameter occur in part of zone of milky quartz veins, which were interpreted by Engel (1952), Miser the novaculite. The size of the triple-point texture gradually decreases (1959), Bass and Ferrara (1969), Bence (1969), and Stone and others westward to the Arkansas-Oklahoma state line near Hatton, Arkansas, (1981) to have been formed by hydrothermal fluids generated primarily where the mean crystallinity is only slightly more than 1 fim and the width from the enclosing sedimentary rocks during the closing phases of the late of the belt is -24 km (15 mi). Farther southwest, near Broken Bow, Paleozoic Ouachita orogeny. Oklahoma, crystallinity of the quartz increases to -20 /im, and the textu- The precise structural modeling of the Ouachita Mountains, includ- ral belt widens to -40 km (25 mi). Most of the chert and novaculite ing placement of subduction zones and accretion of >50,000 ft of Paleo- outside of the 0 tim isopleth boundary exhibits the typical cryptocrystal- zoic sedimentary rock, has been highly debated, for example, Viele (1973), line character of anhedral mosaic quartz, also an abundance of micro- Morris (1974), King (1975), Wickham and others (1976), Thomas

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(1977), Arbenz (1980), Haley and Stone (1981), and Lillie and others (1983). There is general agreement, however, that the rocks were dis- placed long distances by transport of mainly troughal sequences northward over foreland facies and other Paleozoic rocks. Arbenz (1980) considered that a later crustal imbrication and underthrusting were probably respon- sible for the arching, the southward vergence, and much of the thermal activity recorded in the rocks in the central "core" area. The timing of heating events is not always sharply defined, because there is polyphase structural deformation in the region. Goldstein (1975) and Denison and others (1977) described some possible heating events as being as early as Devonian time in some rocks present in the Ouachita Mountain fold belt. If this hypothesis is correct, it is possible that the re may have been some development of the triple-point texture then. Triple-point texture is indeed predominant in the older rocks Figure 16. SEM of Arkansas Novaculite from an area near a in the central "core" area, but adjoining younger Carboniferous units are covered pluton recognized by magnetic and gravity anomalies, also commonly reerystallized, whereas in adjoining outside portions of the from a quarry in the center of the SE'/i, sec. 8, T. 3 S., R. 16 W., region, all age-equivalent rocks that are exposed are cryptocrystalline. locality no. 16 in Figure 15, OM 3,000*. Research on the |5etrographic effects resulting from the depth of burial (and rise in temperature) on the sediments in the Ouachita Mountain fold belt has been relatively scanty. It is known, however, that the combined thickness of the rock units exceeds 50,000 ft (Morris, 1974; Stone and others, 1981), and that the diagenetic dewatering of the clays as proposed by Karlo (1977) was large and related to the over-all structural history of the orogenic belt. Jones and Knauth (1979) suggested from oxygen isotopic and petrographic evidence that stratigraphic burial depths for Arkan- sas Novaculite sedi ments were greater in the eastern and southern portions of the Ouachita Mountains, and this was generally confirmed by Morris (1974), Sholes and McBride (1975), and Stone and others (1981). The relative metamorphic effects from stratigraphic burial versus tectonic placement cannot be clearly evaluated, however, until details of the still controversial tectonic history of the Ouachita Mountains are clarified.

RESTRICTED AREAS OF Figure 17. SEM of Arkansas Novaculite at Broken Bow, HIGHLY ELEVATED TEMPERATURES Oklahoma, locality no. 17 in Figure 15, OM 1,000*.

In relatively localized areas on the fold belt, several episodes of more sedimentary rock may have predictive value for discovering otherwise intense metamorphism overprinted the regional-type texture with coarser hidden intrusions. triple-point recrystallization. At Magnet Cove, -24 km (15 mi) east of Hot At Broken Bow, Oklahoma, relatively coarse triple-point texture Springs, the sedimentary rocks are pierced by small portions of a wider, (Fig. 17) independently confirms earlier observations (Goldstein, 1975) buried, Cretaceous-age pluton. From a regional background texture of ~5 that temperatures of metamorphic intensity had been present. Neither the /um (talc grade) apparent diameter size in the novaculite 1,520 m (5,000 exact time nor the geologic cause of the thermal rise has been fully ft) from the exposed igneous rocks, the crystal sizes increase to >100 fim determined. (periclase grade) near the contact. Erickson and Blade (1963) indicated suites of metamorphic minerals in the sedimentary rocks encircling this AGENTS AND TEMPERATURES OF METAMORPHISM intrusion that further support these observations. Drilling operations also OF THE NOVACULITE have indicated that the pluton dips -45° beneath some of the sedimentary rock that exhibits local anomalously coarse crystallinity. Recrystallization of siliceous rocks during thermal metamorphism At the Potash Sulphur Springs intrusion, also Cretaceous in age, the represents a crystal-chemical response as the rocks move toward a lower novaculite in some xenoliths reerystallized to real diameters of the order of energy state of textural equilibrium within the metamorphic environment. 100 jum, as shown in Figure 14. Commercial vanadium and, inferentially, The higher total surface area and energy of smaller, anhedral crystals move possibly other deposits occur in these reerystallized and altered rocks at toward a state of lower total surface area and energy as the many smaller both Magnet Cove and Potash Sulphur Springs. crystals are replaced by fewer larger crystals. These larger crystals grow as Near the Tertiary Fall Line, in northeastern Hot Spring County, neoformed polygons that, as they compete for space, ordinarily meet at Arkansas, the Arkansas Novaculite appears to be anomalously coarsely triple-point junctions; hence, they develop new polygonal triple-point tex- crystallized to ~ 3 to 4 /xm (Fig. 16). Geophysical surveys of this region ture (Spry, 1969; Kretz, 1966). show a nearby magnetic and gravity anomaly that has been interpreted to Elevated temperature provides the kinetic energy to activate the indicate a buried pluton. To this point, G. W. Viele and D. Houseknecht change (metamorphism) and, as will be demonstrated, appears to be the suggested (1984, oral commun.) that Mesozoic igneous events located paramount agent inducing metamorphism. Geologic sources of elevated along the northeast-southwest structural zone approximately paralleling temperature in rocks include magmatic heat, heat-carrying fluids, and the present Fall Line are likely sources for the elevated temperature induc- geothermal heating over regional dimensions during either deep burial ing this metamorphic overprint. The observations of coarse triple-point beneath continued sedimentation or burial by tectonic thrusting and con- texture in surface rocks and presumed buried volcanism not only are sequent frictional heating at sites of rock movement. mutually confirmatory but also demonstrate that triple-point texture in Other agents also activating recrystallization in metamorphism are

mineralizing fluids and volatiles that may be either indigenous in pores or quartz crystals from veins in recrystallized novaculite in the Magnet Cove introduced (as from magmatic sources) and as products from metamorphic region, found a temperature gradient, from slightly >200 °C in quartz reaction and decomposition of the rocks, notably from carbonates, hydrox- 4,500 m (14,800 ft) from the pluton, to -444 °C, 50 m (164 ft) from the ides, and oxides. Chemical and mineral agents that, on the other hand, contact. It must be recognized, however, that the distances measured only may retard recrystallization include organic matter and aluminous com- horizontally on the surface from an exposed intrusion may be illusory, pounds, such as clay minerals, which occur within, or as coatings on, the because they do not necessarily take into account the possible horizontal grains. spread of the buried portion of the intrusive away from its exposed Intra-crystal strain within deformed rocks and minerals may possibly outcrop. facilitate recrystallization. From analysis of oxygen isotope ratios in novaculites, Friedman es- Although potential agents of metamorphism of the chert-novaculite timated temperatures of 140 °C for recrystallization of samples collected are numerous and quite diverse, as noted above, clear evidence from one ~ 100 m from the intrusion at Magnet Cove, and 50 °C for one sample to occurrence of metamorphosed chert in the Womble Shale delimits the the south from the Trap Mountains (Keller and others, 1977). These necessity of all agents except elevated temperature operating during the estimates of temperature may be subject to modification, however, because metamorphism. A thin, lenticular, disc-shaped silica concretion, ~40 cm the equations used in this calculation were formulated for a somewhat by 15 cm and 7 cm thick (Fig. 18), was collected (by William Hart) from higher range in temperature. the center of a section of the Ordovician Womble Shale, at least 200 m At the Isle of Skye, a temperature range of 309 to 406 °C in the thick, where the shale and novaculite are overturned ~30 km northeast of diopside-grade zone was calculated by Hoersch (1981) from the Hot Springs, Arkansas. Although the shale is tightly folded, the silica magnesian-calcite solvus. These temperatures were estimated to be too

concretion in the center of the section has been cushioned by the surround- low, however, in terms of their T-X (C02) equilibrium relationships. ing shale from deformation, as is substantiated by nondeformation of the Carpenter (1967) calculated a minimum temperature of 760 °C to gener- sponge spicules seen on the surface of the concretion. A SEM of a chip ate the mineral assemblage of periclase grade at the Crestmore, California, from the concretion, however, shows well-developed triple-point texture locality. (Fig. 19). It must be recognized that the temperatures calculated for both the As the sponge spicules were not deformed, we believe that intracrys- Skye and Crestmore localities are indicative of reactions that took place at tal strain in the quartz was not a compellingly active agent in this example low lithostatic pressures (250 to 1,000 bars). They therefore represent of metamorphism. Furthermore, because the concretion was embedded in, minimum temperatures for such equilibria, because, in general, the reac- and radially surrounded by, at least 100 m of relatively impermeable shale, tion temperatures increase with increasing pressure (~ 25-65 °C/km). the amount of mineralizing fluids externally delivered was minimal, and These temperatures thus can be applied only to samples that have also the pore fluids originally present in the chert and shale must have been equilibrated at low pressures. As will be shown in the following para- adequate to achieve the recrystallization. graphs, it is believed that low pressures did prevail in the Ouachita System, Additional evidence that mineralizing fluids are not important, at and that the above temperatures are valid. Additionally, other independent least in chert nodules, is also shown in some of the Skye samples. In measures of temperature produce similar results. approximately one-third of the nodules at appropriate grade, dolomite and With respect to metamorphism across the entire Ouachita System, quartz (chert) have not reacted to form diopside by the reaction: dolomite + quartz = diopside + COj. Apparently, either the clays and other impuri- ties found on the original surfaces of the chert nodules or the reaction rim that formed around the nodules reduced permeability into the chert. Water, which must act as a catalyst in the reaction, was prevented from reaching the interior of the nodules. Chert associated with this dolomite has, however, recrystallized to the triple-point texture typical of this grade, again showing that fluid was not the dominant agent that affected the recrystallization. We conclude that elevated temperature was the dominant and necessary agent that effected the metamorphism of both Skye and Arkansas cherts. H. Jackson and G. R. Nichols (1973, personal commun.), using homogenization of fluid inclusions to measure temperatures in smoky

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Figure 18. Siliceous sponge-spicule concretion from the Womble Shale, 30 km northeast of Hot Springs, Arkansas. Sponge spicules on the weathered surface of the concretion from the Womble Shale do Figure 19. SEM of Precambrian Onverwacht chert, South not show deformation. Africa (courtesy of A.E.J. Engel), OM 1,000*.

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Figures 20-25. Figure 20. SEM of Archean chert from the Soudan Iron Formation, Soudan Mine, Minnesota (specimen courtesy of R. L. Bauer), OM 1,000*. Figure 21. SEM of Proterozoic chert from the Biwabik Formation, northeastern Minnesota (courtesy of R. L. Bauer), OM 6,000*. Figure 22. SEM of Jemison chert, Talladega Slate Belt (courtesy of W. A. Thomas), OM 6,000*. Figure 23. SEM of chert from the Cretaceous Caledonia Formation, near a diorite intrusion, Christiansted, St. Croix, U.S. Virgin Islands, OM 1,500*. Figure 24. SEM of chert from peperite, Elwell Formation, California (courtesy of E. Brooks), OM 4,800*. Figure 25. SEM of Caballos Novaculite, 15 ft from a dike, 16 km southeast of Marathon, Texas (courtesy of E. F. McBride, Jr.), OM 4,800*.

Honess (1923), Miser (1959), Goldstein (1975), Denison and others generally no pressure corrections applied. Engel (1952) concluded that "a (1977), and others reported that regional metamorphism shows significant late middle Pennsylvanian age seems indicated for the quartz crystal increases in the rocks within the central part of the Ouachita Mountain deposits," and a temperature zonation was found by Bence (1969} in the fold belt of Arkansas and Oklahoma. Jackson (1977) examined the zeolite quartz veins in Arkansas that closely paralleled the regional metanc orphic minerals chiefly in the graywackes and tuffs of the Mississippian Stanley gradient and the triple-point textures of this report. Shale in Arkansas and determined that they were of diagenetic origin and Generalizing from these several independent observations, it may be thus exposed to, but not long sustained at, temperatures >300 °C. A inferred that the chert and novaculite formations in the Ouachita belt were maximum zeolite or low-temperature lower greenschist facies was thus regionally heated in the range from below 200 °C to somewhat above 300 seemingly documented for most of the rocks in this region. Engel (1952), °C. Locally, and also where triple-point texture is coarsest, the tempera- Bence (1964), Konig and Stone (1977), Pittenger and Konig (1977), and tures apparently exceeded 400 °C. In xenoliths, the temperatures ma y have Kurrus (1980) examined the thermometry of various liquid inclusions in exceeded 760 °C, comparable to Carpenter's (1967) observations. quartz and other crystals from veins in the Ouachita Mountains in Arkan- Other independent estimates of rock temperatures in the Arkansas- sas. A temperature was found that varied from 100 °C to 315 °C, with Oklahoma region using vitrinite reflectance in associated sedimentary

Arkansas: U.S. Geological Survey Professional Paper 425, 95 p. rocks, the color of conodonts, crystallinity of associated illite, and crystal- Flawn, P. T., Goldstein, August, Jr., King, P. B., and Weaver, C. E., 1961, The Ouachita System: University of Texas, linity textures in chert and novaculite using X-ray diffraction have been Bureau of Economic Geology, publication no. 6120,401 p. Goldstein, August, Jr., 1975, Geologic interpretation of Viersen and Cochran's 25-1 Weyerhaeuser well, McCurtain recently completed or are currently being investigated. Preliminary com- County, Oklahoma: Oklahoma Geological Survey Notes, v. 35, no. 5, p. 167-181. Goldstein, August, Jr., and Hendricks, Thomas A., 1953, Siliceous sediments of Ouachita fades in Oklahoma: Geological parisons between these techniques, using data from work in progress, Society of America Bulletin, v. 64, p. 421-442. indicate qualitative agreement for the general pattern of thermal belts and Gordon, MacKenzie, Jr., Tracey, Joshua I., Ellis, Miller W., 1958, Geology of the Arkansas bauxite region: U.S. Geological Survey Professional Paper 299, 268 p. halos (Houseknecht and others, 1985). Griswold, Leon S., 1892, Whetstones and the novaculites of Arkansas: Arkansas Geological Survey Annual Report for 1890, Volume 3,443 p. Haley, Boyd R., and others, 1976, Geologic map of Arkansas: U.S. Geological Survey and Arkansas Geological Commis- OCCURRENCES OF POLYGONAL TRIPLE-POINT sion, scale 1:500,000. Haley, Boyd R., and Stone, Charles G., 1981, Structural framework of the Ouachita Mountains, Arkansas: Geological TEXTURE OUTSIDE THE OUACHITA REGION Society of America, South-Central Section Meeting, Abstracts with Programs, San Antonio, Texas. Harrover, Robin D., Norman, David 1„ Saving, Samuel M., and Sawkins, F. J., 1982, Stable oxygen isotope and crystallite size analysis of De Long Mountain, Alaska, cherts: An exploration tool for submarine exhalative Polygonal triple-point texture occurs in chert likewise from distant deposits: Economic Geology Bulletin, v. 77, no. 7, p. 1561-1566. Hendricks, T. A., Knetchel, M. M., and Bridge, J., 1937, Geology of the Black Knob Ridge, Oklahoma: American localities and in ages ranging from the Precambrian to the Cretaceous, or Association of Petroleum Geologists Bulletin, v. 21, p. 1-29. Hoersch, A. L., 1981, Progressive metamorphism of the chert-bearing Durness limestone in the Beinn an Dubhaich younger, cited in the captions of Figures 20 to 25. We therefore urge that aureole, Isle of Skye, Scotland: A reexamination: American Mineralogist, v. 6, p. 491-506. Holbrook, Drew F., and Stone, Charles G., 1978, Arkansas Novaculite—A silica resource: 13th annual Forum on the geologic studies routinely include SEMs of chert from all occurrences in Geology of Industrial Minerals: Oklahoma Geological Survey Circular 79, p. 51-58. order to interpret the thermal history of those cherts from their textures. Hollingsworth, J. S., 1967, Geology of the Wilson Springs vanadium deposits, Garland County, Arkansas: Guide Book to central Arkansas economic geology and petrology compiled for Geological Society of America, prepared by Arkansas Geological Commission, p. 22-28. Honess, C. W., 1923, Geology of the southern Ouachita Mountains of Oklahoma: Oklahoma Geological Survey Bulletin POTENTIAL GEOLOGIC APPLICATIONS FROM no. 32, pt. 1, 278 p. TRIPLE-POINT METAMORPHIC TEXTURES Houseknecht, D., Guthrie, J., Ross, L., Keller, W., and Ethington, R., 1985, Correlation of organic and inorganic indicators of thermal maturity, Ouachita Mountains: Geological Society America Abstracts with Programs, v. 17, p. 162. Jackson, Kern C., 1977, Analcite as a temperature indicator of Arkansas Ouachita deformation, in Stone, C. G., and Inferences of geologic significance may be drawn from maps that others, eds., Ouachita Symposium, Volume 2; Arkansas Geological Commission, p. 63-64. show regional distributions of mean crystal sizes of triple-point textures. Jones, D. L., and Knauth, P. L., 1979, Oxygen isotopic and petrographic evidence relevant to the origin of the Arkansas Novaculite: Journal of Sedimentary Petrology, v. 49, p. 581-598, Thermal events may be recognized, such as occurred in the Ouachita Karlo, John F., 1977, Pore fluids as the primary rheologic control in the deformation in the eastern Ouachitas, in Stone, C. G., and others, eds., Ouachita Symposium, Volume 2: Arkansas Geological Commission, p. 65-70. Mountain fold belt, where the geothermal history is correlative with the Keller, Fred, Jr., Henderson, John R., and others, 1963, Aeromagnetic map of the Magnet Cove area Hot Spring County, tectonic and igneous histories of the region. Areas can be outlined where Arkansas: U.S. Geological Survey, Geophysical Investigations Map GP-409. Keller, Walter D., Viele, George W., and Johnson, Clayton H., 1977, Texture of Arkansas Novaculite indicates thermally geologic temperatures may be estimated as favorable to the conversion of induced metamorphism: Journal of Sedimentary Petrology, v. 47, p. 834-843. Keller, Walter D., Stone, Charles G., and Hoersch, Alice L., 1983, Textures of chert and novaculite: An exploration guide: kerogen to fluid hydrocarbons and alternatively where they were unfavor- Gulf Coast Association of Geological Societies Transactions (abs.), v. 33, p. 129. ably high. Possible sites of buried plutons, especially where correlated with King, Philip B., 1975, The Ouachita and Appalachian orogenic belts, in Stehli, F. G., and Nairn, Alan, eds.. Gulf of Mexico and Caribbean; The ocean basins and margins, Volume 3: New York, Plenum Publishing Corp. geophysical anomalies, may be delineated. Areas where mineral deposits Konig, R. H., and Stone, C, G., 1977, Geology of abandoned Kellogg lead-zinc-silver-copper mines, Pulaski County, Arkansas, in Stone, C. G., ed.. Vol. 2, Ouachita Symposium: Arkansas Geological Commission, p. 5-15. genetically associated with buried plutons, illustrated by those being oper- Kretz, R., 1966, Interpretation of the shape of mineral grains in metamorphic rocks: Journal of Petrology, v. 7, p. 68. ated near the exposed Magnet Cove and Potash Sulphur Springs intrusives, Kurrus, Andrew W., HI, 1980, Geochemistry, geothermometry, and mineralogy of quartz and base metal vein deposits, Montgomery County, Arkansas [M.S. thesis]: Fayetteville, Arkansas, University of Arkansas at Fayetteville, 84 p. may be outlined. The potential importance of making systematic collec- Ullie, R. J., Nelson, K. D , De Voodg, B., Brewer, J, A., Oliver, J, E., Brown, L. D., Kaufman, S., and Viele, G. W., 1983, Crustal structure of Ouachita Mountains, Arkansas: A model based on integration of COCORP reflection profiles tions and microtextural studies of chert-bearing formations in other regions and regional geophysical data: American Association of Petroleum Geologists Bulletin, v. 67, p. 907-931. of the world is indicated. Lowe, Donald R., 1974, Regional controls on silica sedimentation in the Ouachita System: Geological Society of America Bulletin, v, 85, p. 1123-1127. Matthews, Steven M., 1982, Thermal maturity of Carboniferous strata, Ouachita thrust fault belt [M. A. thesis]: Columbia, Missouri, University of Missouri-Columbia, 88 p. ACKNOWLEDGMENTS Miser, H. D., 1954, Geologic map of Oklahoma: U.S. Geological Survey, in cooperation with Oklahoma Geological Survey, scale 1:500,000. 1959, Structure and vein quartz of the Ouachita Mountains of Oklahoma and Arkansas, in Cline, L. M., and The SEM research was supported by National Science Foundation others, eds., Ouachita Symposium: Dallas and Ardmore Geological Societies, p. 30-43. Miser, Hugh D., and Purdue, A. H,, 1929, Geology of the DeQueen and Caddo Gap quadrangles: U.S. Geological Survey Grant EAR 76-1804-2. We wish to offer our sincere thanks to John David Bulletin 808, 195 p. Morris, R. C., 1974, Sedimentary and tectonic history of the Ouachita Mountains, in Dickinson, W. R., ed.. Tectonics and McFarland III, for his gracious assistance during the project. We have also sedimentation: Society of Economic Paleotologists and Mineralogists Special Publication 22, p. 120-142. benefited from discussions with J. Kaspar Arbenz; William V. Bush; Moody, C. L., 1949, Mesozoic igneous rocks of northern Gulf Coastal Plain: American Association of Petroleum Geologists Bulletin, v. 33, no. 8, p. 1410-1428. Rodger E. Denison; August Goldstein, Jr.; Boyd R. Haley; Robin D. Parks, Byran, and Branner, G. C., 1932, A barite deposit in Hot Spring County, Arkansas: Arkansas Geological Survey Information Circular 1,52 p. Harrover; Drew F. Holbrook; David W. Houseknecht; Clayton H. John- Pittenger, Gary C., and Konig, Ronald H., 1977, Geochemistry, geothermometry, and mineralogy of copper-lead-zinc and son; Earle F. McBride; Gary E. Smith; Charles T. Steuart; William A. antimony deposits of Sevier County, Arkansas, in Stone, C. G., and others, eds., Volume 2, Ouachita Symposium: Arkansas Geological Commission, p. 19-26. Thomas; Alan Thomson; George W. Viele; Bryan Willis; Jay Zimmer- Purdue, A. H., and Miser, H. D., 1923, Description of the Hot Springs District: U.S. Geological Survey Atlas: Hot Springs Folio 215, 12 p. man; and others. Sholes, Mark A., and McBride, Earle F., 1975, Arkansas Novaculite, in Briggs, Garrett, and others, eds., Sedimentology of Paleozoic flysch and associated deposits, Ouachita Mountains-Arkoma Basin, Oklahoma: Dallas Geological Society, Guidebook, April 1975, p. 69-87. REFERENCES CITED Spry, Alan, 1969, Metamorphic textures: New York, Oxford Press, p. 350. Sterling, Philip J., and Stone, Charles G., 1961, Nickel occurrences in soapstone deposits. Saline County, Arkansas: Arbenz, J. K., 1968, Structural geology of the Potato Hills, Ouachita Mountains, Oklahoma, in Cline, L. M., ed., Geology Economic Geology, v. 56, no. 1, p. 100-110. of the western Arkoma Basin and Ouachita Mountains, Oklahoma: Oklahoma City Geological Society Guide- Stone, C. G., Haley, B. R., and Viele, G. W., 1973, A guidebook to the geology of the Ouachita Mountains, Arkansas: book, p. 109-121. Arkansas Geological Commission Guidebook, 113 p. 1980, Fresh look at some Ouachita problems [abs.]: American Association of Petroleum Geologists Bulletin, v. 64, Stone, Charles G., and McFarland, John D., Ill, with the cooperation of Haley, Boyd R., 1981, Field guide to the p. 670. Paleozoic rocks of the Ouachita Mountain and Arkansas Valley Provinces, Arkansas: Arkansas Geological Bass, M. N., and Ferrara, G., 1969, Age of adularia and metamorphism, Ouachita Mountains, Arkansas: American Commission Guidebook 81-1, 140 p. Journal of Science, v. 267, no. 4, p. 491-498. Thomas, William A., 1977, Structural and stratigraphic continuity of the Ouachita and Appalachian Mountains, in Stone, Bence, Alfred Edward, 1964, Geothermometric study of quartz deposits in the Ouachita Mountains, Arkansas [M,S. C. G., and others, eds., Ouachita Symposium, Volume 1: Arkansas Geological Commission, p. 9-24. thesis]: Austin, Texas, University of Texas, 68 p. Viele, G. W., 1973, Structure and tectonic history of the Ouachita Mountains, Arkansas, in De Jong, K. A., and Scholten, Brooks, E. R., Wood, M. M., and Garbutt, P. L., 1982, Origin and metamorphism of peperite and associated rocks in the R., eds., Gravity and tectonics: New York, John Wiley & Sons, p. 361-377. Devonian Elwell Formation, northern Sierra Nevada, California: Geological Society of America Bulletin, v. 93, 1979, Geologic map and cross section, eastern Ouachita Mountains, Arkansas: Map summary: Geological Society p. 1208-1231. of America Bulletin, Part I, v. 90, p. 1096-1099. Wickham, John, Roeder, Dietrich, and Briggs, Garrett, 1976, Plate tectonic models for the Ouachita foldbelt: Geology, Carpenter, A. B., 1967, Mineralogy and petrology of the system Ca0-Mg0-C02-H20 at Crestmore, California: American Mineralogist, v. 52, p. 1341-1363. v. 4, no. 3, p. 173-176. Cline, L. M., 1969, Stratigraphy of the Late Paleozoic rocks of the Ouachita Mountains, Oklahoma: Oklahoma Geologi- Whetten, John T., 1966, Geology of St. Croix, U.S. Virgin Islands: Geological Society of America Memoir 98, cal Survey Bulletin 83, 113 p. p. 177-239. Denison, R. E., Burke, W. H., Otto, J. B., and Heatherington, E. A., 1977, Age of igneous and metamorphic activity affecting the Ouachita foldbelt, in Stone, C. G., and others, eds., Ouachita Symposium, Volume I: Arkansas Geological Commission, p. 25-40. MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 29,1985 Engel, A.E.J., 1952, Quartz crystal deposits of western Arkansas: U.S. Geological Survey Bulletin 973-E, p. 173-260. REVISED MANUSCRIPT RECEIVED MAY 9,1985 Erickson, R. L., and Blade, L. V., 1963, Geochemistry and petrology of the alkalic igneous complex at Magnet Cove, MANUSCRIPT ACCEPTED MAY 17,1985

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