Geology of the Ketcherside Mountain area, southeastern , and the source of Grassy Mountain Ignimbrite

J. RONALD SIDES Department of Geology, University of Texas at Arlington, Arlington, Texas 76019

ABSTRACT INTRODUCTION the uranium-lead method on zircons, are all 1,500 m.y. old. The Munger Granite The St. Francois Mountains igneous The St. Francois Mountains batholith is Porphyry, a unit in the western St. Francois complex is a shallow batholith about 1,500 located in southeastern Missouri at the Mountains, is about 1,408 m.y. old (Bick- m.y. old which has intruded a roof formed structural core of the Ozark Dome (Fig. 1). ford and Mose, 1975). by its own volcanic ejecta. It is similar to Volcanic rocks associated with the batho- Hamilton and Myers (1967) proposed other shallow-roofed batholiths such as the lith include .rhyolitic and quartz-alkali that the St. Francois Mountains complex is Boulder batholith. However, it has been trachytic ash-flow tuff, minor quartz latite, an epizonal batholith which intruded a roof tilted and beveled by erosion so that the and rare andesite. Intrusive rocks are formed by its own ejecta. This hypothesis is roof is available for study at a wide range of mostly granite, with some quartz supported by field, petrographic, and stratigraphic levels. The volcanic roof is monzonite and minor amounts of gabbro aeromagnetic evidence (Sides, 1980). Vol- composed mostly of ash-flow tuff sheets, and diabase. Several rock units, all dated by canic rocks crop out in the southwestern St. implying that caldera-collapse eruptions were the major mechanism of formation of the roof. Only two calderas have been pro- posed in the St. Francois Mountains thus far, the Taum Sauk caldera and the Butler Hill caldera. The southern ring fault of the Taum Sauk caldera is located in the Ketcherside Mountain area of southeastern Missouri. The fault is nearly vertical, strikes about N50°E, and is intruded by a small body of porphyritic granite with vertical flow folia- tion. Rocks in the down-dropped block are broken by high-angle faults as much as 3 km from the ring fault, but extracaldera units are little deformed. Associated with the ring fault are bedded tuffs and volcani- clastic sedimentary units, some of which are mineralized. These data support the hy- pothesis that the Taum Sauk caldera is a trap-door caldera with major faulting along the eastern and southern walls. There is lit- tle evidence of faulting along the western wall. The Butler Hill caldera occurs in the east- ern St. Francois Mountains and is centered approximately on the Butler Hill Granite. The caldera, one of the earliest features to K M develop, has been obscured by later struc- CD BUTLER HILL GRANITE tural modification, but the major collapse ash-flow tuff of the caldera, the Grassy VOLCANIC UNITS Mountain Ignimbrite, is well exposed. OTHER INTRUSIVE UNITS Analysis of flow directions of the ignimbrite indicates a source for the unit in the eastern St. Francois Mountains, as predicted by the Butler Hill caldera model. Figure 1. General geologic map of the St. Francois Mountains igneous complex.

Geological Society of America Bulletin, Part 1, v. 92, p. 686-693, 4 figs., 2 tables, September 1981.

686

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/9/686/3419324/i0016-7606-92-9-686.pdf by guest on 27 September 2021 SOURCE OF GRASSY MOUNTAIN IGNIMBRITE 687

Francois Mountains, and intrusive rocks The Taum Sauk Rhyolite, which occurs Ignimbrite (Fig. 2). The Taum Sauk caldera crop out to the northeast, implying that the in the western St. Francois Mountains (Anderson and others, 1969; Berry, 1975), batholith has been tilted to the southwest (Fig. 2), is at least 1,000 m thick and crops in the western St. Francois Mountains, pro- and beveled by erosion. This hypothesis is out over an area of 59 km- (Anderson, duced the Taum Sauk Rhyolite. supported by the following lines of evi- 1970; Berry, 1976), implying that its mini- dence: (1) Butler Hill Granite, the major mum volume is 59 km3. The Grassy THE TAUM SAUK CALDERA pluton of the batholith, changes from a Mountain Ignimbrite, which occurs in the fine-grained microgranite or granophyre in eastern St. Francois Mountains (Fig. 2), is Previous Work the southwest to a medium-grained hypidi- about 1,000 m thick and is exposed over an omorphic granite in the northeast (Sides, area of 32 km2, implying that its minimum R. E. Anderson (1962) showed that ash- 1980). (2) Aeromagnetic data indicate a volume is 32 km3. These volume estimates flow tuff units in the western St. Francois southwest-dipping contact between vol- do not include material which was probably Mountains dip radially inward toward canic and intrusive units (Allingham, 1960). originally present but has been eroded Taum Sauk Mountain (Fig. 2). He proposed (3) The first onlap of a basal away, material buried by younger units, that this structural depression is of volca- sandstone, the Lamotte Sandstone, is 500 m and material which may occur outside the no-tectonic origin. J. E. Anderson and lower in the southwestern St. Francois areas previously studied. The original vol- others (1969) proposed a collapse origin for Mountains than it is to the northeast umes of these units were probably over 100 the feature and named it the "Taum Sauk" (Snyder and Wagner, 1961). (4) Systematic km3. This volume corresponds to ash-flow caldera. Their evidence included arcuate chemical and mineralogical changes occur tuffs produced from other calderas (Smith faults, suggested by R. E. Anderson's map- in the Butler Hill Granite which indicate and Bailey, 1968; Steven and Lipman, ping, and Rb-Sr age data, which suggested that granite in the northeasternmost expo- 1976; Christiansen and others, 1977; that rocks within the caldera were 100 m.y. sure was emplaced well below granite ex- Smith, 1978). It has been proposed that younger than those which surround it. The posed to the southwest (Sides, 1977, 1980). these two major ash-flow tuff sheets were ring faults, however, are mostly covered by This unusual cross-sectional exposure has produced by two calderas. The Butler Hill rocks and alluvium, and exposed facilitated study of the processes of forma- caldera (Sides and Bickford, 1978; Sides rocks are little displaced across them. R. E. tion of the batholith and its extrusive cover. 1979), in the eastern St. Francois Anderson (1970) therefore concluded that This paper addresses the formation of the Mountains, produced the Grassy Mountain the Taum Sauk structure was not a caldera. volcanic roof of the batholith. The volcanic stratigraphy of the St. Francois Mountains is still not completely known, but the sec- tion is dominated by rhyolitic ash-flow tuff sheets, most of which are 100 m or more thick (Tolman and Robertson, 1969; An- derson, 1970; Berry, 1976; Sides, 1976, 1978; Pratt and others, 1979). The total stratigraphic section is about 6 to 7 km thick (Sides, 1980) and contains few inter- calated sediments, implying that rapid and almost continuous volcanic deposition oc- curred. The St. Francois Mountains vol- canic field is similar to other large volcanic fields, including the San Juan volcanic field (Steven and Lipman, 1976; Lipman and others, 1978), the Valles Caldera region (Smith and Bailey, 1966), and the Yel- lowstone region (Christiansen and Blank, 1972; Eaton and others, 1975). The St. Francois Mountains volcanic field may have been built, as were these three examples, by ash-flow tuff sheets produced during cal- dera collapse. If this hypothesis is true, sev- Taum Sauk caldera eral calderas may occur in the St. Francois Mountains. Kisvarsanyi (1980) has noted several possible cauldron-subsidence fea- tures, on the basis of subsurface data, and Cordell (1979) has identified a possible Figure 2. Generalized geologic map of the St. Francois Mountains, showing approxi- cauldron in the Hawn Park area, on the mate locations of the Taum Sauk and Butler Hill calderas. Stippled pattern = intrusive basis of gravity and magnetic data. Field units; gray pattern = Grassy Mountain Ignimbrite; dotted pattern = Taum Sauk Rhyolite. mapping has produced evidence for only Polygon outlines the area of Figure 3. X's mark locations of volcaniclastic sedmientary de- two calderas in the central St. Francois posits. J.S. = Johnson Shut-ins; K.G. = Ketcherside Gap; C.M. = Cuthbertson Mountain; Mountains. P.K. = Pilot Knob; I.H. = Ironton Hollow.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/9/686/3419324/i0016-7606-92-9-686.pdf by guest on 27 September 2021 688 J. R. SIDES

Moreover, U-Pb age determinations on zir- TABLE 1. COMPARISON OF PREVIOUSLY AND PRESENTLY USED STRATIGRAPHIC cons indicate that rocks in the western St. NOMENCLATURE, KETCHERSIDE MOUNTAIN AREA Francois Mountains are coeval, except for This study Bonham Tolman and Pratt and the Munger Granite Porphyry (Bickford (1948) Robertson others and Mose, 1975), negating the previous (1969) (1979) isotopic argument. Berry (1975) regarded the Taum Sauk Rhyolite as the major col- Ketcherside Ketcherside Stouts Creek Alkali-rhyolite Mountain Felsite Rhyolite lapse ash-flow tuff of the caldera and at- ignimbrite tributed the lack of ring fault displacement Glover Royal Gorge Royal Gorge Alkali-rhyolite to resurgent doming. However, the map- formation Felsite Rhyolite ping of Berry (1970, 1975, 1976) and R. E. Little Creek Little Creek Royal Gorge Rhyolite Anderson (1962, 1970) indicates a struc- formation Felsite Rhyolite tural depression rather than a dome. Grassy Mountain Royal Gorge Stouts Creek Grassy Mountain Arcuate structural discontinuities which Ignimbrite Felsite Rhyolite Ignimbrite occur in the central St. Francois Mountains are concave westward and are correctly positioned to be the eastern margin of the caldera. In the Lake Killarney Quadrangle, the Grassy Mountain Ignimbrite and other volcanic units terminate against younger rocks along an arcuate structural discon- tinuity (Fig. 2); (Sides, 1976, 1978, 1979; Sides and Bickford, 1978). These younger 90° 38' rocks include volcanic breccias, volcani- clastic sedimentary units, local rhyolitic + 37° 32' units which do not occur elsewhere, and a small intrusive unit. The Grassy Mountain Ignimbrite and other units terminate against younger units in the Iron Mountain Lake Quadrangle (Fig. 2). These younger units include a con- torted, brecciated zone with large slump blocks and ash-flow tuff breccias containing angular to subangular lithic fragments (R. L. Nusbaum, unpub. mapping). Nus- baum interprets these features as having formed along the topographic wall of a caldera. These discontinuities suggest an oval caldera with a southwest to northeast

diameter of roughly 20 km. Fa

Contact Problems with the Caldera Model Compaction or flow foliation •Ar Vertical foliation Difficult terrain and dense cover have © hampered study of the St. Francois Moun- Horizontal foliation tains to the extent that many problems as- Intrusive units

sociated with the Taum Sauk caldera are Cambrian and younger unresolved. Postbatholith structure, ero- Unassigned volcanic units sion, and sediment deposition have masked Bedded tuffs and vo I c oni cla s ti c many features which are commonly well units displayed in younger calderas. The exact lo- ketcherside Mountain Ignimbrite cation of the ring faults, as well as the na- Glover Formation I KM ture, timing, and amount of collapse, is still Little Creek Formation

unknown. The nonuniform collapse does Quartz bearing rhyolite not match the classic caldera pattern (Ste- Grassy Mountain Ignimbrite ven and Lipman, 1976; Christiansen, 1979). Minor faulting along the western Figure 3. Geologic map of the Ketcherside Mountain area. See Figure 2 for location of caldera margin and more intense faulting the major fault with respect to proposed ring structures.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/9/686/3419324/i0016-7606-92-9-686.pdf by guest on 27 September 2021 SOURCE OF GRASSY MOUNTAIN IGNIMBRITE 689

and brecciation along the eastern margin Ketcherside Mountain ignimbrite, Glover area: The Grassy Mountain Ignimbrite, and suggest a trap-door style of collapse similar formation, and Little Creek formation an unnamed rhyolite of limited extent. The to that of the Ute Creek caldera (Steven and (Table 1 and Appendix 1). These are infor- Grassy Mountain Ignimbrite (Sides, 1976, Lipman, 1976) and the Goodsight-Cedar mal field names devised for this study and 1978; Shuster, 1979) is an ash-flow tuff Hills volcano-tectonic depression (Seager, should not be considered formal nomencla- sheet which crops out extensively in the 1973). ture. Detailed descriptions of these units are eastern St. Francois Mountains but extends The Taum Sauk Rhyolite occurs within presented in Appendix 1, but brief descrip- no farther west than the Ketcherside the caldera and outside of it on the western tions are given here. All classification fol- Mountain area. The unit is remarkably ho- side. It does not, however, crop out in the lows Streckeisen (1973, 1979). mogeneous everywhere except in this area, eastern part of the caldera (Fig. 2), where Ketcherside Mountain ignimbrite is a where phenocryst content varies slightly. In rocks which are stratigraphically lower dark-gray, prophyritic, quartz-alkali this area, the elevation of the upper contact occur (Anderson, 1962, 1970; Berry, 1970, trachyte with phenocrysts of perthitic or- of the Grassy Mountain Ignimbrite varies, 1976; Pratt and others, 1979). The nonuni- thoclase (1.5%) and quartz (0.1%). The even though overall the unit dips gently and form distribution of this eruptive sheet does unit is characterized by its low phenocryst uniformly westward; thus, the upper sur- not conform to the classic caldera model; content, abundant small fiamme outlines, face of the unit probably represents an old however, the distribution is at least partly and small, white, bleached zones which im- erosion surface. explained by the tilt (approximately 11° part a mottled appearance to the matrix. The overlying, unnamed rhyolite, which SW; Sides, 1980) of the batholith. A tilt of Microscopically, shard outlines are well is 0 to 30 m thick, filled depressions in ir- 11° on a caldera block which is 20 km in preserved despite complete devitrification, regular paleotopography and consists of a diameter results in a 3.8-km difference in indicating that the rock is pyroclastic in na- dark gray porphyry with variable amounts the stratigraphic level of rocks which were ture. of perthitic-orthoclase (4.5%) and quartz originally at the same stratigraphic level but The Glover formation is a dark maroon, (2.5%) phenocrysts. The unit is generally on opposite sides of the block. Berry (1976) porphyritic, quartz-alkali trachyte with lithic rich, and some samples are volcanic described a volcanic stratigraphic sequence phenocrysts of white perthitic orthoclase breccias. However, pervasive recrystalliza- of comparable thickness underlying the (6.3%) and quartz (1.5%). The unit is tion and alteration of phenocrysts and ma- Taum Sauk Rhyolite (3,475 m summing characterized by an inhomogeneous matrix trix to sericite and clay minerals has de- average unit thicknesses). If the Taum Sauk which contains flow bands, lithophysae, stroyed the original structures of the unit. Rhyolite were originally deposited uni- and spherulites. The matrix is commonly Volcanic units are offset by several major formly within the caldera, subsequent tilt- recrystallized and has pods of secondary faults in the Ketcherside Mountain area ing and erosion would have removed it quartz (4.8%) which, together with quartz (Fig. 3). A major fault in the near Ketcher- from the eastern part. phenocrysts, give the rock a distinctively side Gap (Fig. 2) strikes N50°E, is nearly The location of the southern margin of quartz-rich appearance. The unit has tex- vertical, and has a minimum displacement the Taum Sauk caldera is a problem ad- tural features suggesting devitrification, and of about 180 m. It is downthrown to the dressed in this paper. Study of the Ketcher- it probably originated as a lava flow. north, as indicated by the absence of the side Mountain area was initiated because The Little Creek formation is a medium Grassy Mountain Ignimbrite and strati- previous studies predicted that the southern to dark gray porphyritic latite with buff- graphically lower units to the north and ring fault would crosscut the area. Detailed colored phenocrysts of plagioclase (8.8%) west of the fault. Volcanic units northwest mapping indicates a major fault in the pre- and perthitic orthoclase (10.2%). The ma- of the fault do not correlate with any previ- dicted position and orientation. trix is fairly homogeneous except for minor ously known volcanic stratigraphy and secondary quartz. The unit probably origi- therefore must lie above the Little Creek GEOLOGY OF THE KETCHERSIDE nated as an ash-flow tuff, although devit- formation. The combined thickness of the MOUNTAIN AREA rification has destroyed shard outlines. Little Creek formation and the unnamed Although the unit is mineralogically homo- rhyolite (183 m) therefore provides a mini- The Ketcherside Mountain area was first geneous, a thin, mappable zone occurs mum estimate of fault displacement, mapped by Bonham (1948), who named which is marked by the local occurrence of although the actual displacement is proba- and recognized some of the major units in finely laminated tuffaceous sediments and a bly much greater. This fault has the correct the area (Table 1) and correctly mapped a color change (marked by a contact line orientation and position to be the southern fault in and near Ketcherside Gap (Fig. 3). within the Ketcherside Mountain ignim- ring fault of the Taum Sauk caldera, a Bonham did not distinguish between pyro- brite in Fig. 3). Rock below this zone has a hypothesis supported by three other rela- clastic and lava-flow units and did not show black matrix and phenocrysts which are tionships: much detail. Bonham's data were incorpo- nearly black, making the rock appear 1. The porphyritic granite which crops rated into two more recent maps (Tolman aphanitic. This contrasts with the distinc- out near Ketcherside Gap (Fig. 3) is in- and Robertson, 1969; Pratt and others, tively porphyritic appearance of the Little truded along the fault and is bounded by 1979) in which unit names were changed Creek formation above the horizon. The the fault along its northern contact. The (Table 1), but no new map information was zone represents a hiatus in deposition of the granite, which is described more fully in added. Little Creek formation and separates two Appendix 1, is porphyritic with phenocrysts The Ketcherside Mountain area is domi- cooling units. of perthitic orthoclase which define a verti- nated by three major volcanic units (Fig. 3): Two other volcanic units occur in the cal flow foliation. The location and nature

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/9/686/3419324/i0016-7606-92-9-686.pdf by guest on 27 September 2021 690 J. R. SIDES

of this granite body is consistent with the tion 29, T. 34 N., R. 4 E.), and northwest of tailed study (Sides, 1978; Shuster, 1979) hypothesis that it is part of a ring dike Ketcherside Mountain, at Johnson Shut-ins has not indicated any cooling breaks despite localized along the ring fault. (section 16, T. 33 N., R. 4 E.) (Sides, 1978; its thickness of as much as 1,580 m. The 2. Volcaniclastic sedimentary rocks are Blades and Bickford, 1976). All of these rock is a black to dark-gray to dark-maroon abundant near the fault but rare elsewhere bedded deposits are along or near proposed porphyry with phenocrysts of pink, perthi- in the St. Francois Mountains. In the Ketch- ring faults of the Taum Sauk caldera tic, alkali feldspar (12.2% and quartz erside Mountain area, they generally consist (Fig. 2), suggesting that they had a common (8.5%) (Sides, 1976; Shuster, 1979). The of black, fine- to medium-grained sand- origin as intracaldera lake sediments. unit is remarkably uniform and generally stones and siltstones which are generally 3. North of the major fault, volcanic contains few fiamme or other lithic frag- well cemented with silica. Microscopically, rocks are offset by several nearly vertical ments. Although the rock is entirely devit- grains are angular fragments of quartz, faults which have relatively large vertical rified, the outlines of original shards are plagioclase, and alkali feldspar, with displacements (about 120 m minimum for preserved in some samples. The Grassy smaller amounts of polycrystalline quartz, two of the faults). These faults probably Mountain Ignimbrite overlies the Lake Kil- chert, and rare quartz pseudomorphs after occur in a 1- to 3-km-wide zone of chaotic larney Formation, a sequence of rhyolitic glass shards. The matrix is highly siliceous block faulting which originated during col- flows and ash-flow tuffs. The contact be- and contains sericite and chlorite. Angular lapse. The few faults which occur south of tween these units is probably conformable, grains and abundant feldspars demonstrate the ring fault have vertical displacements of although a paucity of consistent attitudes in immaturity. Shard outlines indicate that the less than 40 m, indicating that extracaldera the Lake Killarney Formation makes struc- sediments formed penecontemporaneously rocks were not strongly broken by collapse tural interpretation difficult (Sides, 1978). with pyroclastic activity, pos- of the caldera. The upper contact of the Grassy Mountain sibly in a caldera moat. In Ketcherside Gap, Ignimbrite is an angular unconformity; the sediments contain copper minerals, mainly SOURCE OF GRASSY MOUNTAEV unit is thus in contact with various overly- malachite. It seems significant that copper IGNIMBRITE ing volcanic units and has a variable thick- occurs along the proposed ring fault but ness (280 to 1,580 m in the Lake Killarney nowhere else within a 20-km radius of the The Grassy Mountain Ignimbrite is a Quadrangle). Because the unit is so thick, area. The only other copper prospects in the thick, widespread, rhyolitic, ash-flow tuff in widespread, and homogeneous, it has been entire western St. Francois Mountains lie the eastern St. Francois Mountains. De- proposed (Sides and Bickford, 1978; Sides, along one of Anderson's proposed ring faults (Anderson and others, 1969; Kisvar- sanyi and Kisvarsanyi, 1976). The copper probably originated from hydrothermal ac- tivity along the ring fault similar to the process proposed for ore deposition in the Creede caldera (Steven and Ratte, 1965).

Volcaniclastic and tuffaceous rocks also occur on Cuthbertson Mountain (Fig. 2), 2.6 km northwest of Ketcherside Gap, where a 90-m-thick section of bedded tuffs and bedded stromatolitic limestone occurs (Stinchcombe, 1976). The base of the sec- tion is a coarse conglomerate, with cobbles and boulders of volcanic material, which overlies a massive ash-flow tuff. Stinch- combe suggested that the depositional envi- ronment was marine or lacustrine, with nearby volcanic activity, and that the lime- stones may have resulted from algal growth near hot-spring activity. The tuffs have been mined for manganese (Kisvarsanyi and 4 KM others, 1976), which occurs as soft wad pyrolusite ore just above the limestone beds. The ore deposit is probably residual Cambrian and younger units and was produced by solution of the lime- stone. The ultimate origin of the manganese Misc. intrusive units may have been hydrothermal, though, as Butler Hill Granite 90° 40' pyrolusite has selectively replaced pumice Postcollapse units + 37° 30' lapilli in nearby ash-flow tuffs. Grassy Mountain Ignimbrite Similar bedded tuffs and volcaniclastic Lake Killarney Formation sediments occur (Figs. 2, 3) north and east of Ketcherside Mountain, at Ironton Hol- Figure 4. Geologic map of the eastern St. Francois Mountains. Numbered lines are the low (NWV4, SEV4, section 28, T. 34 N., locations of flow-direction samples and their lineations. Sample numbers correspond with R. 4 E.) and Pilot Knob (NWV4, SW'A, sec- those in Table 2.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/9/686/3419324/i0016-7606-92-9-686.pdf by guest on 27 September 2021 SOURCE OF GRASSY MOUNTAIN IGNIMBRITE 691

TABLE 2. SUMMARY OF FLOW DIRECTIONS FROM ACKNOWLEDGMENTS GRASSY MOUNTAIN 1GNIMBRITE The writer is very grateful to Bruce C. Sample Flow Chi-square Significance Miller, who did much of the actual flow- no. direction value (90% confidence) direction counting work. Many helpful 1 S61°E 224.96 Yes suggestions were provided by Dr. M. E. 2 S82°E 253.38 Yes Bickford and Robert Shuster. This project 3 N76°E 102.50 Yes was supported in part by a grant from the 4 S78°E 282.48 Yes 5 N58°E 3.25 No Faculty Research Fund, Western Michigan 6 N66°E 11.41 Yes University. 7 N49°E 11.04 Yes 8 N41°E 24.56 Yes 9 S58°E 66.48 Yes 10 S73°E 148.89 Yes 11 S61°E 93.45 Yes 12 S63°E 100.32 Yes 13 N59°E 45.24 Yes APPENDIX I. DETAILED 14 S81°E 174.10 Yes DESCRIPTIONS OF PRECAMBRIAN 15 S87°E 53.66 Yes UNITS IN THE KETCHERSIDE 16 N88°E 312.51 Yes MOUNTAIN AREA Note: Chi-square critical for these data is 4.61. Little Creek formation

In hand specimen, the Little Creek formation is a medium- to dark-gray porphyry with white to light-gray phenocrysts of alkali feldspar and pla- square test (Harrison, 1957); all but one 1979), that it was produced during collapse gioclase. The unit is fairly uniform over the ex- of the Butler Hill caldera into the underly- were significant (Table 2). Except for sam- posed area and does not contain pumice, flow ing Butler Hill Granite magma. Direct evi- ple B5, all 15 significant lineations (Fig. 4) banding, or eutaxitic texture. dence for the existence of this caldera will point radially eastward to a source within Microscopically, the unit may be described as a porphyritic latite. The rock is composed essen- be difficult to find for two reasons: (1) The the Butler Hill Granite, as predicted by the tially of plagioclase (8.8%), perthitic orthoclase Grassy Mountain lgnimbrite is low in the Butler Hill caldera model. Sample B5 was (10.2%), and opaque minerals (1.5%) in a ma- volcanic section, so that the caldera was one necessarily taken at the base of the unit, and trix of feldspar and quartz microlites (about of the first structural features to form in the so its flow lineation may have been affected 74.4%). batholith roof. The caldera must have been by local paleotopography. Plagioclase (An7 to Anl2) occurs as stubby, rectangular, euhedral crystals (mostly 2 to 4 mm obscured by later block faulting and magma long) and smaller crystal fragments of various intrusion. (2) Because the caldera is in the CONCLUSIONS sizes. Plagioclase grains are extensively altered to northeastern St. Francois Mountains, it is sericite, clay minerals, and rarely epidote; how- exposed at a relatively deep level, and many The existence of the Taum Sauk caldera ever, polysynthetic twinning and zoning are clearly visible. Orthoclase patchwork perthite features usually associated with calderas is supported by the occurrence of a major occurs as stubby, rectangular euhedra (about 2 to have been eroded away. fault at the location predicted for the cal- 4 mm long) and as smaller crystal fragments of It has been shown that ash flows gener- dera ring fault. A second caldera is various sizes. Perthite is extensively altered to ser- ally move outward from large calderas and suggested by the existence of the Grassy icite and clay minerals. Scattered single grains of perthite have a zone of well-developed albite that flow directions are often preserved in Mountain lgnimbrite, a thick, widespread, twining, indicating the possibility of metasomatic ash-flow tuff in the eastern St. Francois the resulting ash-flow tuffs (Elston and replacement of one feldspar by another. Opaque Smith, 1970; Rhodes and Smith, 1972). A Mountains. The ash-flow tuffs from these minerals are mostly magnetite and occur as flow-direction study of the Grassy Moun- two calderas compose nearly one-third of single, rounded, irregular grains (0.3 to tain lgnimbrite, on the basis of the method the 6- to 7-km-thick volcanic-batholith 0.03 mm), as dust in the matrix, and as small- grain aggregates associated with chlorite, epi- of Elston and Smith (1970; Appendix 2), roof. Other possible calderas are suggested dote, and a fibrous mineral resembling goethite. was undertaken in order to locate the by these data: (1) Most of the volcanic units These mineral aggregates are sometimes source of the unit. Sixteen carefully oriented in the batholith roof are relatively thick pseudomorphs after an unknown, prismatic, fer- samples of Grassy Mountain lgnimbrite ash-flow tuffs. (2) Numerous arcuate fea- romagnesian mineral, probably pyroxene. Epi- dote occurs in the mineral aggregates described were taken along a roughly north-south tures are apparent on satellite and airphoto above and as small, discrete grains within imagery of the St. Francois Mountains (Kis- traverse of about 27 km (Fig. 4), and the feldspars. phenocryst orientations in each were mea- varsanyi and Kisvarsanyi, 1976). (3) Sub- The matrix is an uneven intergrowth of quartz, sured to see if they defined a lineation surface geologic data suggest the presence feldspar, and opaque microlites (about 2 /x, but within the compaction foliation. Phenocryst of several circular granitic ring complexes locally larger because of secondary crystalliza- tion). Irregular zones of optically continuous re- orientations were grouped into 18 ten- (Kisvarsanyi, 1980). (4) Gravity and crystallized quartz (about 1 to 3 mm long) with magnetic anomalies suggest the presence of degree increments; in every case, the result- feldspar microlite inclusions are common. Shard ing unimodal frequency distribution re- at least one collapse feature (Cordell, outlines do not occur, possibly because of devit- sembled a normal distribution. The vector 1979). rification and recrystallization, and flow banding mean of each distribution was taken to be Caldera-collapse eruptions were proba- is not apparent. Snowflake texture is common. Although no shards occur, the paucity of flow the flow lineation. The resulting lineations bly a major, if not the dominant, process of lineations and the presence of numerous crystal were tested for significance at the 90%- formation of the roof the St. Francois fragments suggest that the units is an ash-flow confidence level, using the Tukey Chi- Mountains batholith. tuff.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/9/686/3419324/i0016-7606-92-9-686.pdf by guest on 27 September 2021 692 J. R. SIDES

Glover formation the unit often appears aphanitic. Fiamme and Quartz crystals (0.1 to 4 mm in diameter, typi- other inclusions (mostly volcanic) are abundant. cally 1.5 mm in diameter) are anhedral and dis- In hand specimen, rocks of the Glover forma- They are white to very light gray and often dis- play undulatory extinction. Magnetite occurs as tion are dark maroon to dark gray and porphyrit- color the matrix, imparting a white-spotted ap- discrete, subhedral grains (1 to 2 mm in diame- ic, with phenocrysts of quartz and light-gray pearance to the rock. ter) which are associated with epidote, chlorite, a alkali feldspar. The matrix of the rock is in- The unit may be described microscopically as a yellow fibrous mineral, and biotite. These as- homogeneous and displays variable amounts of porphyritic quartz-alkali trachyte. The rock is sociated minerals occur in clusters and may be flow banding. The unit is distinguished in the composed essentially of perthitic orthoclase break-down products of primary mafic minerals. field by the abundance of primary and secondary (1.5%) and quartz (0.1%) in a matrix of feldspar Epidote occurs as discrete grains about 0.3 mm in quartz. and quartz microlites (86.9%). The rock contains diameter in the mineral clusters and in feldspars. The unit may be described, microscopically, as varying amounts of fiamme (about 10.8%). Fibrous chlorite and an unknown, yellow, fibrous a porphyritic quartz-alkali trachyte. The rock is Orthoclase occurs as patchwork and vein mineral are intimately associated within the min- composed essentially of perthitic orthoclase perthite, as stubby, rectangular euthedral crystals eral clusters. Hematite occurs as thin grain coat- (6.3%), primary quartz (1.5%), and opaque (about 1 to 2 mm long), and smaller crystal frag- ings and fracture fillings. Fluorite is anhedral and minerals (0.5%) in a matrix of feldspar and ments. Feldspars are generally only slightly appears to be an interstitial, late-stage, primary quartz microlites (85.4%). Polycrystalline aggre- altered to sericite and cl^y minerals. Rarely, mineral. Biotite occurs as small discrete grains, gates of secondary quartz (4.8%) are common. however, feldspars are so altered that they cannot often largely chloritized. Muscovite is rare and Perthitic alkali feldspar occurs as euhedral to be identified. It is possible that some oi: these occurs within feldspars. subhedral, stubby, rectangular crystals (mostly altered grains were plagiqclase; However, no 0.5- to 2.2-mm-long) and as smaller (0.3- to polysynthetically twinned feldspars were ob- 0.1-mm-long) crystal fragments. Patchwork served. Quartz occurs as rounded to angular APPENDIX 2. FLOW DIRECTION perthite is prominent, and feldspar grains are grains (about 1 mm long) which, when euhedral, ANALYSIS PROCEDURES moderately to extensively replaced by sericite, display hexagonal, dipyramidal outlines. Quartz clay minerals, and rare calcite. Quartz occurs as also occurs as smaller crystal fragments. Opaque Elston and Smith (1970) detailed an excellent euhedral to rounded and embayed crystals (0.3 to minerals occur as rounded irregular grains and thin-section method for determination of ash- 2 mm long) and as ovoid and lenticular polycrys- grain aggregates of magnetite (0.1 to 0.5 mm) flow directions by analysis of ash-flow-tuff talline aggregates which are exceedingly variable and as dust in the matrix. The matrix is an even samples. The method is based on measurement of in size (typically 0.1 to 4 mm long). Quartz crys- intergrowth of feldspar and quartz microlbes and the orientation of phenocrysts, shards, and other tal fragments are conspicuous by their absence. opaque dust (about 4 to 20 fj.). Small zones of features. Mild recrystallization of the Grassy Opaque minerals, mostly magnetite, occur as polycrystalline quartz occur rarely (0.1 mm and Mountain Ignimbrite has destroyed most of the rounded- to irregular-grain aggregates (about 1.0 less in diameter). Despite complete devitrifica- delicate primary features; thus, only phenocryst to 0.03 mm long), and as small, discrete grains tion, flattened shard outlines (0.1 ot 0.6 mm orientations from slabbed samples were mea- (mostly 0.1 to 0.03 mm long). Epidote occurs long) are preserved. Felsic volcanic lithic frag- sured. rarely as small grains replacing feldspars and as ments occur in the matrix. Snowflake texture is Samples were collected only from outcrops large polycrystalline aggregates (1.5 to 0.5 mm conspicuous by its absence in the matrix. The where the compaction direction was uniform and long) which may be pseudomorphs after earlier outlines of pumice (about 2 to 20 mm long) are could be measured accurately. Before each sam- minerals. well preserved as fan-like intergrowths of quartz ple was collected, the compaction foliation was The matrix is a complex, very irregular inter- and feldspar which have snowflake texture. marked on the rock, along with a principal di- growth of quartz and feldspar microlites (0.1 mm Pumice inclusion centers are sometimes replaced rection, normally north. Samples could thus be and smaller) which contains abundant hematite by polycrystalline quartz, calcite, epidote, and oriented in the lab as they had been in outcrop. dust. Grain size varies considerably from sample white micas. The presence of fiamme, crystal Samples were slabbed along the compaction di- to sample, and recrystallized matrix micro- fragments, and shard outlines indicates that the rection with a diamond saw. Each sample was spherulites and lithophysae are common. Irregu- unit is an ash-flow tuff. oriented as it had been in the field, and the north lar pods of secondary polycrystalline quartz are direction was marked on the lower half of the common. The matrix often displays snowtlake Unnamed granite sample. The sample (lower half only) was texture, and it commonly has bands running sprayed with clear plastic. The orientation of through it which may have either greatly in- In hand specimen, the rock is a medium- to each phenocryst was counted, and each crystal creased grain size, fan-like intergrowths of quartz coarse-grained, porphyritic granite with pheno- was marked off to insure that phenocrysts could and feldspar, or both. These bands resemble flow crysts of alkali feldspar in a matrix of quartz, not be counted twice or skipped. The preferred bands more than they resemble fiamme. Com- alkali feldspar, and plagioclase. Microscopically, orientation of the phenocrysts in a given sample monly, irregular, rounded outlines of hematite the rock is a porphyritic, hypidiomorphic, granu- was interpreted as the flow lineation of the ash- dust occur, surrounding unusually coarse matrix, lar granite which contains perthitic orthoclase flow tuff if it proved statistically significant. phenocrysts, or normal matrix. The origin of (about 45%), quartz (about 30%), plagioclase these features is unknown, but they probably (about 10%), magnetite (about 2%), fluorite were formed by movement of hematite during re- (about 2%), and trace amounts of epidote, REFERENCES CITED crystallization of the matrix. chlorite, hematite, muscovite, and an uniden- The paucity of crystal fragments and the oc- tified, yellow, fibrous mineral. Accessory miner- Allingham, J. W., 1960, Interpretation of currence of what appears to be flow banding als include apatite and zircon. aeromagnetic anomalies in southeast Mis- suggest that this unit is a lava flow. The spherulit- Perthitic orthoclase occurs as stubby, lath- souri: U.S. Geological Survey Professional ic texture, fan-like intergrowths, and fine grain shaped, euhedral phenocrysts (as much as 3.5 cm Paper 400-B, p. 216-219. size of unrecrystallized matrix indicate devit- long, but typically 1 to 2 cm) and as anhedral Anderson, J. E., Bickford, M. E., Odom, A. L., rification of an originally glassy matrix. The ab- (typically 2 to 3 mm) matrix grains. Phenocrysts and Berry, A. W., Jr., 1969, Some relations sence of shard outlines supports the nonpyro- are not zoned and contain roughtly 50% ex- and structural features of the Precambrian clastic origin of the unit, but does not prove it, solved albite as patchwork perthite, as do matrix volcanic terrane, St. Francois Mountains, because devitrification and recrystallization orthoclase crystals. Matrix orthoclase may be southeastern Missouri: Geological Society could have destroyed shard outlines. zoned, although perthitic intergrowths may of America Bulletin, v. 80, p. 1815-1818. obscure the zoning. Orthoclase is extensively Anderson, R. E., 1962, Igneous petrology of the Ketcherside Mountain ignimbrite altered to clay minerals and some sericite. Plagi- Taum Sauk area, Missouri [Ph.D. thesis): oclase occurs as anhedral grains (typically 0.3 to St. Louis, Washington University. In hand specimen, the Ketcherside Mountain 2 mm long) which display polysynthetic twin- 1970, Ash-flow tuffs of Precambrian age in ignimbrite is a dark-gray porphyry with pheno- ning. Plagioclase is strongly zoned; however, southeast Missouri: Missouri Geological crysts of alkali feldspar and quartz. Phenocrysts alteration of plagioclase precluded accurate An- Survey Report of Investigation 46, 50 p. are small and scarce, and feldspar phenocrysts content determination. Plagioclase is partly Berry A. W., Jr., 1970, Precambrian volcanic are nearly the same color as the matrix, so that altered to clay minerals, sericite, and epidote. rocks associated with the Taum Sauk cal-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/9/686/3419324/i0016-7606-92-9-686.pdf by guest on 27 September 2021 SOURCE OF GRASSY MOUNTAIN IGNIMBRITE 693

dera, St. Francois Mountains, Missouri dimensional fabric analysis of till and en- 1980, Emplacment of the Butler Hill Gran- [Ph.D. thesis]: Lawrence, University of glacial debris containing particles from 3 to ite, a shallow pluton within the St. Francois Kansas. 40 mm in size: Journal of Geology, v. 65, Mountains batholith of southeastern Mis- 1975, The Taum Sauk caldera, in Lowell, p. 98-105. souri: Geological Society of America Bulle- G. R., ed., A field guide to the Precambrian Kisvarsanyi, E. B., 1980, Granite ring complexes tin, Part 1, v. 91, p. 535-540. geology of the St. Francois Mountains, Mis- and Precambrian hot-spot activity in the St. Sides, J. R., and Bickford, M. E., 1978, Possible souri: Big Rivers Area Geological Society, Francois terrane, Midcontinent region, calderas in the St. Francois Mountains 2nd Annual Field Conference, p. 43-48. United States: Geology, v. 8, p. 43-47. batholith, southeastern Missouri: Geologi- 1976, Proposed stratigraphic column for Kisvarsanyi, E. B., Robertson, C. E., and Stinch- cal Society of America Abstracts with Pro- Precambrian volcanic rocks, western St. comb, B. L., 1976, Road log number 3, in grams, v. 10, p. 25. Francois Mountains, Missouri, in Kisvar- Kisvarsanyi, E. B., ed., Studies in Precam- Smith, R. L., 1978, Ash-flow magmatism, in sanyi, E. B., ed., Studies in Precambrian brian geology of Missouri: Missouri Chapin, C. E., and Elston, W. E., eds., geology of Missouri: Missouri Geological Geological Survey Report of Investigation Ash-flow tuffs: Geological Society of Survey, Report of Investigation 61, p. 81- 61, p. 45-63. America Special Paper 180, p. 5—27. 90. Kisvarsanyi, G., and Kisvarsanyi, E. B., 1976, Smith, R. L., and Bailey, R. A., 1966, The Ban- Bickford, M. E., and Mose, D. G., 1975, Geo- Structural lineaments and mineralization in delier Tuff: A study of ash-flow eruption chronology of Precambrian rocks in the St. southeast Missouri, in Kisvarsanyi, E. B., cycles from zoned magma chambers: Bulle- Francois Mountains, southeastern Mis- ed., Studies in Precambrian geology of tin Volcanologique, v. 29, p. 83-104. souri: Geological Society of America Special Missouri: Missouri Geological Survey Re- 1968, Resurgent cauldrons, in Coats, R. R., Paper 165, 48 p. port of Investigation 61, p. 164-173. and others, eds., Studies in volcanology: Blades, C. L., and Bickford, M. E., 1976, Rhyo- Lipman, P. W., Doe, B. R., Hedge, C. E., and Ste- Geological Society of America Memoir 116, litic ash-flow tuffs and intercalated vol- ven, T. A., 1978, Petrologic evolution of the p. 613-662. caniclastic tuffaceous sedimentary rocks at San Juan volcanic field, southwestern Col- Snyder, F. G., and Wagner, R. E., 1961, Pre- Johnson Shut-ins, Reynolds County, Mis- orado: Pb and Sr isotope evidence: Geologi- cambrian of southeast Missouri: status and souri, in Kisvarsanyi, E. B., ed., Studies in cal Society of America Bulletin, v. 89, problems, in Hayes, W. C., ed., Guidebook Precambrian geology of Missouri: Missouri p. 59-82. to the geology of the St. Francois Mountain Geological Survey, Report of Investigation Pratt, W. P., Anderson, R. E., Berry, A. W., Jr., area: Missouri Geological Survey, Report of 61, p. 91-104. Bickford, M. E., Kisvarsanyi, E. B., and Investigation 26, p. 84-94. Bonham, L C., 1948, The geology of the south- Sides, J. R., 1979, Geologic map of exposed Steven, T. A., and Lipman, P. W., 1976, Calderas west part of the Ironton Quadrangle, Mis- Precambrian rocks, Rolla 1° X 2 ° Quad- of the San Juan volcanic field, southwestern souri [M.S. thesis]: St. Louis, Washington rangle, Missouri: U.S. Geological Survey Colorado: U.S. Geological Survey Profes- University. Miscellaneous Investigations Map 1-1161, sional Paper 958, 35 p. Christiansen, R. L., 1979, Cooling units and scale 1:125,000. Steven, T. A., and Ratte, J. C., 1965, Geology composite sheets in relation to caldera Rhodes, R. C., and Smith, E. I., 1972, Distribu- and structural control of ore deposition in structure, in Chapin, C. E., and Elston, tion and directional fabric of ash-flow the Creede district, San Juan Mountains, W. E., eds., Ash-flow tuffs: Geological So- sheets in the northwestern Mogollon Colorado: U.S. Geological Survey Profes- ciety of America Special Paper 180, p. 29- Plateau, Mew Mexico: Geological Society sional Paper 487, 87 p. 42. of America Bulletin, v. 83, p. 1863-1868. Stinchcomb, B. L., 1976, Precambrian algal Christiansen, R. L., and Blank, H. R., 1972, Vol- Seager, W. R., 1973, Resurgent volcano-tectonic and stromatolitic limestones canic stratigraphy of the rhyo- depression of Oligocene age, south-central in the St. Francois Mountains of southeast lite plateau in Yellowstone National Park: New Mexico: Geological Society of Ameri- Missouri, in Kisvarsanyi, E. B., ed., Studies U.S. Geological Survey Professional Paper ca Bulletin, v. 84, p. 3611-3626. in Precambrian geology of Missouri: Mis- 729-B, 18 p. Shuster, R. D., 1979, Chemistry and petrography souri Geological Survey Report of Investi- Christiansen, R. L., Lipman, P. W., Carr, W. J., of caldera-related ash-flows, St. Francois gation 61, p. 122-131. Byers, F. M., Orkild, P. P., and Sargent, Mountains, Missouri: Grassy Mountain Streckeisen, A. L., 1973, Plutonic rocks: K. A., 1977, Timber Mountain-Oasis Val- ignimbrite and Taum Sauk rhyolite [M. S. Classification and nomenclature recom- ley caldera complex of southern Nevada: thesis]: Lawrence, University of Kansas. mended by the I.U.G.S. Subcommission on Geological Society of America Bulletin, Sides, J. R., 1976, Stratigraphy of volcanic rocks the Systematics of Igneous Rocks: Geo- v. 88, p. 943-959. in the Lake Killarney Quadrangle, Iron and times, v. 18, p. 26-30. Cordell, L., 1979, Gravity and aeromagnetic Madison counties, Missouri, in Kisvarsanyi, 1979, Classification and nomenclature of anomalies over basement structure in the E. B., ed., Studies in Precambrian geology of volcanic rocks, lamprophyres, carbonates, Rolla Quadrangle and the southeast Mis- Missouri: Missouri Geological Survey Re- and melilitic rocks; Recommendations and souri lead district: Economic Geology, port of Investigation 61, p. 105-113. suggestions of the IUGS Subcommission on v. 74, p. 1383-1394. 1977, Trend-surface analysis of chemical the Systematics of Igneous Rocks: Geology, Eaton, G. P., Christiansen, R. L., Iyer, H. M., data across the St. Francois Mountains v. 7, p. 331-335. Pen, A. M., Mabey, D. R., Blank, H. R., batholith, southeastern Missouri: A test of Tolman, C., and Robertson, F., 1969, Exposed Zietz, I., and Gettings, M. E., 1975, Magma regional tilting: Geological Society of Precambrian rocks in southeast Missouri: beneath Yellowstone National Park: Sci- America Abstracts with Programs, v. 9, Missouri Geological Survey, Report of In- ence, v. 188, no. 4190, p. 787-796. p. 651. vestigation 44, 68 p. Elston, W. E., and Smith, E. I., 1970, Determina- 1978, A study of the emplacment of shallow tion of flow direction of rhyolitic ash-flow granite batholith: The St. Francois tuffs from fluidal textures: Geological So- Mountains, Missouri [Ph.D. thesis]: Law- ciety of America Bulletin, v. 81, p. 3393 — rence, Kansas, University of Kansas. 3406. 1979, The occurrence of calderas in the vol- MANUSCRIPT RECEIVED BY THE SOCIETY JUNE Hamilton, W., and Myers, W. B., 1967, The na- canic roof of the St. Francois Mountains 18, 1980 ture of batholiths: U.S. Geological Survey batholith of southeastern Missouri: Geolog- REVISED MANUSCRIPT RECEIVED DECEMBER 22, Professional Paper 554-C, p. 1 -30. ical Society of America Abstracts with Pro- 1980 Harrison, P. W., 1957, New technique for the grams, v. 11, p. 166. MANUSCRIPT ACCEPTED MARCH 2, 1981

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/9/686/3419324/i0016-7606-92-9-686.pdf by guest on 27 September 2021