Origin and Structural Implications of Upper Miocene Rhyolites in Kingston Canyon, Piute County, Utah

Origin and structural implications of upper Miocene rhyolites in Kingston Canyon, Piute County, Utah

PETER D. ROWLEY 1 THOMAS A. STEVEN I U.S. Geological Survey, Federal Center, Denver, Colorado 80225 HARALD H. MEHNERT J

ABSTRACT and volcanic domes of the rhyolite of Forshea Mountain, dated by K-Ar methods at 7.6 m.y. old. Those in the bottom of Kingston Kingston Canyon is one of the deepest antecedent canyons in the Canyon, the rhyolite of Phonolite Hill, are especially well exposed High Plateaus subprovince of the Colorado Plateaus. Here the East and provide spectacular examples of a pyroclastic cone whose base Fork of the Sevier River flows westward transversely across the is about at river level and a steep-sided volcanic dome emplaced gently east tilted Sevier Plateau, which is developed on a basin- into and through these deposits. The pyroclastic deposits, formerly range fault block uplifted more than 1,500 m along the Sevier fault 500 or more metres thick, consist of airfall, mudflow, and ash- zone on the west. Upper Tertiary rhyolites, uncommon in south- flow(?) material of rhyolite and foreign lithic fragments, especially western Utah, occur both on the northern rim and in the bottom of olivine basalt. The dome consists of flow-banded, mostly devitrified Kingston Canyon. Those on the northern rim consist of lava flows rhyolite as much as 500 m thick; it has been dated by K-Ar methods

114° 113° 1 12° 111°

UTAH

Area of Fig. 1

Figure 1. High Plateaus and adjacent areas of southwestern Utah. Stippled area is the Marysvale volcanic field. Axial part of Pioche- Marysvale igneous belt outlined by dashed line. Axis of Blue Ribbon lineament shown by dotted line. B, Beaver; C, Cedar City; M, Marys- vale; P, Paragonah; R, Red Canyon; S, Sevier; Sa, Salina.

Geological Society of America Bulletin, Part I, v. 92, p. 590-602, 15 figs., 3 tables, August 1981.

590

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at 5.4 m.y. In addition to the rhyolites, a dome and lava-flow com- mal energy and because some occur along sparse east-trending plex, the rhyodacite of Dry Lake, occurs near the northern rim and structural zones. For these reasons, three centers of rhyolite and is considered to postdate the rhyolite of Forshea Mountain and rhyodacite volcanism in the Kingston Canyon area were studied in predate the rhyolite of Phonolite Hill. detail during a regional study of the Marysvale volcanic field of The rhyolite of Forshea Mountain was deposited near basin- southwestern Utah. One of these, the rhyolite of Phonolite Hill, is a range faults, before the uplift of the Sevier Plateau and before the well exposed, spectacular example of a pyroclastic cone containing cutting of Kingston Canyon. Before uplift, a river flowed across the a steep-sided volcanic dome; it received special attention during the site of the present Sevier Plateau toward the east-southeast and study. In addition to providing information on volcanic processes, perhaps also across the Awapa and Aquarius Plateaus to the east. the rhyolites of the Kingston Canyon area allow us to bracket the The rhyodacite of Dry Lake was deposited during uplift and age of the major period of uplift, by basin-range faulting, of the perhaps before canyon cutting. During uplift, the river maintained southern Sevier Plateau. The oldest rhyolites, 7.6 m.y. old, cap the itself and cut Kingston Canyon. The rhyolite of Phonolite Hill was fault block and predate uplift by faulting, whereas the youngest deposited in this canyon, blocking the river flow, which probably rhyolites, 5.4 m.y. old, were erupted into Kingston Canyon and formed new outlets to the east. The Awapa and Aquarius Plateaus postdate the canyon cutting that resulted from uplift. This report later were uplifted along faults, disrupting the eastern part of the describes the rhyolites and related rocks and discusses the sig- river segment. The topography then took on its present appearance, nificance of these rocks to interpretations of the structural history and drainage was re-established through Kingston Canyon. There of the southern Sevier Plateau. has been little deepening since the reopening of Kingston Canyon. The first mention of the rocks in Kingston Canyon was by Dut- ton (1880), who showed a photograph of a sharp hill in Kingston INTRODUCTION Canyon with relief of about 500 m. The caption of the picture was "Phonolite," and the term was applied to the geographic feature Kingston Canyon is a 1,200-m-deep antecedent canyon of the (Fig. 2). Although Dutton tentatively identified the specimens he west-flowing East Fork of the Sevier River that cuts through the collected from the hill as phonolite, he noted that they were too southern Sevier Plateau. The Sevier Plateau is a major segment of weathered for definitive analysis. He also observed that the rock of the High Plateaus, in the Basin and Range-Colorado Plateaus Phonolite Hill postdates canyon cutting. Willard and Callaghan transition zone of southwestern Utah (Fig. 1). The plateau is de- (1962), in work on part of the northern side of Kingston Canyon veloped on a north-trending fault block about 20 km wide and 120 and areas farther north, locally distinguished a "calcic latite" phase km long; it was tilted about 3° to the east during uplift relative to of the Bullion Canyon Volcanics. Later mapping of the southern Sevier Valley to the west and to Grass and Johns Valleys to the east. Sevier Plateau, including Kingston Canyon, demonstrated that both Displacement along the Sevier fault zone at the western edge of the the "phonolite" and the "calcic latite" are rhyolites (Rowley, plateau was at least 1,500 m. Displacement along the Paunsaugunt 1968). In Rowley, Lipman, and others (1978), the name "rhyolite fault zone in Grass and Johns Valleys dropped the Sevier Plateau of Phonolite Hill" was used for some of the rhyolites. Later map- about 1,000 m relative to the Fish Lake, Awapa, and Aquarius ping (Rowley, 1979; Rowley, Cunningham, and others, 1979) re- Plateaus to the east. sulted in two new informal names, the rhyodacite of Dry Lake for a Upper Tertiary rhyolites are uncommon in southwestern Utah, more crystal-rich and less silicic sequence, and the rhyolite of yet they are of interest because some are associated with mineral Forshea Mountain for a crystal-poor rhyolite sequence. Following deposits of lithophile elements and with sites of potential geother- more detailed mapping in 1979 by Rowley and Steven, we now re- strict the term "rhyolite of Phonolite Hill" to those rhyolites within Kingston Canyon, centering on the vent at Phonolite Hill. The dis- tribution of the rhyolites as well as of mafic rocks of similar ages in the Kingston Canyon area is shown in Figure 3.

GEOLOGIC SETTING

Kingston Canyon provides an excellent geologic cross section of the southern Sevier Plateau. Rocks exposed in the canyon belong to either the middle Tertiary volcanic sequence (middle Oligocene through early Miocene) or the upper Cenozoic sedimentary and volcanic sequence (early Miocene to Pleistocene) of Rowley, Ste- ven, and others (1979). The older of the two sequences consists of thick calc-alkalic volcanic rocks that formed coalesced stratovol- canoes and sheets of ash-flow tuff. The younger sequence consists of relatively sparse biomodal volcanic rocks (rhyolite and mafic rocks, especially basalt) that are interbedded with basin-fill sedimentary rocks of the Sevier River Formation. The change from Figure 2. Kingston Canyon (right to center middleground), the older to the younger sequence marks the inception of exten- from the northwest, showing Phonolite Hill (arrow). Sevier Valley, sional (basin-range) tectonism, which in nearby parts of southwest- including Piute Reservoir and one of the prominent fault scarps of ern Utah began in middle Miocene time, about 22 to 21 m.y. ago the Sevier fault zone, in the foreground. Background shows Grass (Rowley, Steven, and others, 1979). Valley-Johns Valley area, beyond which are Awapa and Aquarius The rocks of the middle Tertiary volcanic sequence in Kingston Plateaus. Canyon are part of the Marysvale volcanic field, about 100 km

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112»15' 112°00' 111*45*

Figure 3. Rhyolite, mafic rocks, Sevier River Formation, and Quaternary sediments in the Kingston Canyon area. Modified from Rowley (1968, 1979); Rowley, Cunningham, and others (1979); and Williams and Hackman (1971). Rhyolite, dark stippled pattern; basalt or potassium-rich mafic rocks, crosshatched pattern; Sevier River Formation, line pattern; Quaternary sediments, light stippled pattern. Some faults, with bar and ball on the downthrown side, of the Sevier fault zone are on the west side of the mapped area and some faults of the Paunsaugunt fault zone are on the east side. Faults are dotted where concealed. Town names: A, Antimony; J, Junction; K, Kingston. Symbols for rhyolite masses: Tf, rhyolite of Forshea Mountain; Td, rhyodacite of Dry Lake; Tp, rhyolite of Phonolite Hill. Sample localities of radiometrically dated rock, showing age in millions of years, indicated by a dot.

across, that lies at the eastern end of the Pioche-Marysvale igneous The base of the middle Tertiary volcanic sequence in Kingston belt (Fig. 1). This belt, consisting of calc-alkalic rocks of mostly Canyon consists of the Needles Range Formation, a 30-m.y.-old, stratovolcanic and ash-flow origin, extends east-northeastward crystal-rich ash-flow tuff (Rowley, 1968). The Needles Range For- from the vicinity of Pioche in southeastern Nevada through Marys- mation is overlain by dacitic volcanic mudflow breccia and subor- vale, Utah, and nearby Kingston Canyon to Fish Lake and Awapa dinate lava flows, flow breccia, fluvial sedimentary rocks, and ash- Plateaus, 50 to 70 km farther east (Rowley, Steven, and others, flow tuff, all of the Mount Dutton Formation of 27 to 22 m.y. ago 1979). Near Kingston Canyon, the middle Tertiary volcanic se- (Anderson and Rowley, 1975). Two ash-flow tuff units, the Kings- quence rests on sedimentary rocks of the Claron Formation, a ton Canyon Tuff Member (26 to 25 m.y. old) and the Antimony fluvial-lacustrine sequence of mostly Eocene to middle Oligocene Tuff Member of the Mount Dutton Formation, are of regional ex- age (Fig. 4). tent. Another regional ash-flow tuff, the Osiris Tuff (23 to 22 m.y.

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from many scattered sources. Most rhyolite sources, however, AGE STRATIGRAPHY SEQUENCE occur in the Pioche-Marysvale belt. Some rhyolite sources, including those in the Kingston Canyon area, not only occur on the Pioche-Marysvale belt but also fall along a more restricted east- Sevier UPPER trending structural belt named the Blue Ribbon lineament (Rowley, CENOZOIC Lipman, and others, 1978), which has been the locus of recurring PLIOCENE River Formation SEDIMENT- igneous and fault activity from at least late Miocene until present Ù ARY AND times. Radiometric ages of rhyolite sources on the lineament range 1 a) in age from 21 m.y. in the southern Wah Wah Mountains, which VOLCANIC are 130 km to the west, to 5.4-4.8 m.y. for the rhyolite in Kingston SEQUENCE Canyon, which lies near the eastern end of the lineament (Fig. 1). x""™ / I * On the northern rim of Kingston Canyon, as many as ten dark gray and black olivine-bearing basaltic lava flows — interbedded MIOCENE z with tan, yellow, and light gray fluvial sedimentary rocks of the Basalt flows / Sevier River Formation — overlie the Osiris Tuff (Fig. 3). One of these flows probably correlates with a faulted basalt flow of similar Osiris Tuff lithology and stratigraphic position near Piute Reservoir (Rowley, 23-22 Cunningham,.and others, 1979) that has been dated (K-Ar) at 12.9 m.y. old (Damon, 1969; corrected according to Dalrymple's 1979 24 chart) and 12.7 m.y. old (Best and others, 1980). Lithologically similar potassium-rich mafic lava flows of 21 m.y. age (Best and \Antimony Tuff others, 1980), however, occur west of Piute Reservoir (Fig. 3) and \ Member MIDDLE could also be represented in the Kingston Canyon section. Above Mount \ TERTIARY the flows on the northern rim of Kingston Canyon is a covered in- Dutton ^ VOLCANIC terval apparently representing in part the Sevier River Formation Formation SEQUENCE and in part a basal tuff of the rhyolite of Forshea Mountain, which caps the rim. \Kingston Canyon ILI The southern rim of Kingston Canyon is capped by other 2 \ Tuff Member olivine-bearing basaltic flows, which overlie the Osiris Tuff (Fig. 3). ILI Locally the basaltic flows and Osiris Tuff are separated by thin, o poorly exposed rocks that probably belong to the Sevier River o \ 26-25 Formation. Best and others (1980) report a K-Ar age of 5.0 m.y. for CD basalt that forms a small west-tilted cuesta in Grass Valley between 27-22 Otter Creek Reservoir and Antimony (Fig. 3). The basalt may cor- _i relate with some of the flows on the southern rim of Kingston Can- Needles Range O yon, which are as little as 3 km to the west. The basalt flow, in turn, Formation probably is equivalent to one 5 km to the east (Fig. 3) that was 30 dated at 5.4 m.y. (Best and others, 1980); both have almost identi- cal chemical compositions (Best and others, 1980) and both occupy a west-tilted downthrown block west of the main range-front fault LOWER of the Awapa Plateau (Williams and Hackman, 1971). The rim of TERTIARY the Awapa Plateau, another 5 km to the east (Fig. 3), contains a Claron Formation SEDIMENT- 6.5-m.y.-old basalt (Best and others, 1980). ARY 38 The rhyolites of Kingston Canyon are described below from old- SEQUENCE est to youngest, with most emphasis given to the well-exposed EOCENE rhyolite of Phonolite Hill. The discussion of the rhyolites is fol- lowed by an interpretive section on the structural and geomorphic evolution of the Kingston Canyon area. Figure 4. Stratigraphy of the Kingston Canyon area. Symbols for rhyolite are those of Figure 3. Numbers refer to ages, in millions RHYOLITE OF FORSHEA MOUNTAIN of years. Age boundaries from Geological Society of London (1964). White, light gray to dark gray, and black, locally vesicular or amygdaloidal rhyolite lava flows and broad, flat domes, as much as 300 m thick, predate major faulting and form the resistant caprock old), overlies the Mount Dutton Formation (Anderson and Rowley, on the northern rim of Kingston Canyon as well as the backslope of 1975; Fleck and others, 1975; K-Ar ages modified according to the Sevier Plateau to the north and east (Fig. 3). These rocks under- Dalrymple, 1979). lie an area of about 45 kirr and are called the rhyolite of Forshea Bimodal volcanic rocks of the upper Cenozoic sedimentary and Mountain for a topographic prominence within the area. The volcanic sequence occur widely in western Utah. Mafic rocks, con- rhyolite of Forshea Mountain has extremely contorted flow layer- sisting of an older sequence of potassium-rich mafic rocks and a ing and is largely devitrified glass. In many places perlite or obsid- lithologically similar younger sequence of basalt, were erupted ian as much as 15 m thick occurs near the base of the flow se-

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TABLE 1. MODAL ANALYSES OF RHYOLITES AND BASALTS OF THE KINGSTON CANYON AREA, UTAH

Basalt on Rhyolite of Forshea Mtn. Rhyodai ite of Dry Lake north rim C72* 232* 78-175b 78-175d 1730* C65* C80* C113 78-732 78-821

Groundmass 88.6 98.6 98.2 97.8 76.6 72.0 75.6 74.2 75.6 70.2 Plagioclase 1.4 1.2 1.8 18.0 21.0 15.0 16.4 15.6 19.8 An% n.d. 14-16 14-17 15-17 36-39 44-46 32-38 34-40 36-44 38-44 Sanidine 0.3 0.2 0.2 tr. Quartz 0.2 0.6 0.2 Biotite tr. 1.6 1.6 0.8 1.0 1.8 2.0 Hornblende 3.2 3.6 5.8 6.4 6.0 5.6 Fe-Ti oxides tr. tr. 0.6 1.8 2.2 2.0 0.8 1.2 Olivine 11.4 Vole, rock frags. 0.3

Rhyolite of Phonolite Hill Cone deposits Basalt Dome Rhyolite south of river 345 s" 78-164 113* 114* 116* 120* 78-115 78-195 78-196

Groundmass 21.0 90.0 93.8 97.6 96.0 71.0 98.0 98.4 98.3 Plagioclase 0.6 0.8 5.2 2.2 2.8 3.0 1.8 1.3 1.1 An% n.d. n.d. 15-16 14-16 15-18 14-17 14-16 14-15 15-16 Sanidine 1.0 0.2 0.6 0.4 0.1 0.2 0.4 Quartz 0.6 tr. 0.1 Biotite tr. Hornblende tr. Fe-Ti oxides 0.2 0.1 tr. tr. Olivine 8.6 Vole, rock frags. 77.6 0.6 0.6 2.5.6 0.2

Note: 500 or 1,000 points counted on thin sections. • •= not present, tr. = trace, n.d. = not determined. An determinations by Michel-Levy method using curve for volcanic plagioclases of Slemmons (1962, Table 3). * From Rowley (1968).

quence. In some places the glassy rocks are underlain by flow brec- RHYOLITE OF PHONOLITE HILL cia of similar thickness, and even more locally the breccia is underlain by white and tan, soft airfall tuff and/or ash-flow tuff. The The rhyolite of Phonolite Hill consists of both pyroclastic de- upper part of the flow sequence is more vesicular and amygdaloidal posits and flow-foliated rhyolite in Kingston Canyon. These rocks and commonly contains products of vapor-phase crystallization. form Phonolite Hill, as well as scattered outcrops of similar rocks The rhyolite of Forshea Mountain contains small, sparse pheno- south of the East Fork of the Sevier River within 3 km southeast of crysts of plagioclase and sanidine (Table 1); it is somewhat more Phonolite Hill (Fig. 5). Hydrothermally altered rocks and minor potassic than the rhyolite of Phonolite Hill (Table 2). uranium and thorium mineralized rocks are related to emplacement Two concordant K-Ar ages were determined for the rhyolite of of the rhyolite of Phonolite Hill, but they do not approach the vol- Forshea Mountain (Table 3). One, a whole-rock age on obsidian ume and economic significance of altered and mineralized rocks re- from basal flow breccia, is 7.6 ± 0.4 m.y., and the other, an age on lated to emplacement of the Mount Belknap Volcanics in the sanidine from the same sample, is 7.6 ± 0.6 m.y. Marysvale area (Kerr and others, 1957; Cunningham and Steven, 1978, 1979a, 1979b; Steven and others, 1979, 1980, 1981). The RHYODACITE OF DRY LAKE work on altered and mineralized rocks near Phonolite Hill is con- tinuing. Pink, light gray, tan, and black, resistant flow-foliated, locally spherulitic rhyodacite domes and stubby lava flows occur near Dry Pyroclastic Deposits of Phonolite Hill Lake, west of Forshea Mountain (Fig. 3). Most of the rock was deposited on an eroded fault block dropped to the west; renewed White, light gray, and locally pink, soft, generally well layered faulting offset some of the rhyodacite as well. The unit overlies tuffaceous rocks are exposed from the level of the East Fork of the basalt lava flows probably correlative with those near Piute Reser- Sevier River to halfway up the slopes of Phonolite Hill. They lie un- voir, which have been dated by K-Ar methods as 12.9 and conformably on a highly irregular surface cut on gently (3°) east- 12.7 m.y. old. It probably is somewhat younger than the rhyolite of dipping rocks of the Needles Range Formation and Mount Dutton Forshea Mountain and predated cutting of Kingston Canyon. Formation. The unconformity has a local relief of at least 250 m The rhyodacite of Dry Lake has a maximum thickness of about where the rhyolite of Phonolite Hill is plastered against the north- 230 m and locally contains a black basal vitrophyre as much as 8 m ern wall of Kingston Canyon (Fig. 5, cross section). Clearly the thick. Phenocrysts of plagioclase and less abundant amphibole, pyroclastic rocks were deposited in the bottom of an ancestral can- biotite, Fe-Ti oxides, and quartz occur in a devitrified glass matrix yon whose position, depth, and configuration were similar to those containing crystallites (Table 1). The chemical composition is of present-day Kingston Canyon. Based upon their presence high rhyodacite to dacite (Table 2). on the eastern slope of Phonolite Hill, the pyroclastic deposits must

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TABLE 2. CHEMICAL ANALYSES AND CIPW NORMS OF RHYOLITES AND BASALTS OF THE KINGSTON CANYON AREA, UTAH

Rhyolite of Forshea Mtn. Rhyodacite of Dry Lake Rhyolite of Phonolite Hill Basalt, Phonolite Hill s 78-175b 78-175d 2+ 78-732 78-821 2* 113 ' 78-115 78-195 78-196 78-164

Si02 72.14 73.44 64.72 65.41 65.60 72.50 73.77 74.56 76.11 72.98 47.15 AI2O3 13.62 12.67 15.47 15.59 15.62 12.46 13.22 13.15 12.61 12.88 15.19

Fe2Os 0.83 0.59 2.13 2.07 4.36 1.40 1.09 0.39 0.91 0.64 3.09 FeO 0.06 0.27 2.18 1.90 0.05 0.15 0.07 0.19 0.06 0.30 6.39 MgO 0.02 0.02 1.39 1.17 1.17 0.18 0.07 0.02 0.02 0.02 9.20 CaO 0.48 0.54 4.02 3.31 3.56 0.66 1.03 0.55 0.44 0.51 8.44 Na20 4.16 3.86 3.73 3.88 4.13 4.52 5.07 4.21 3.90 4.33 3.32

K2O 4.49 5.01 3.32 3.01 2.68 4.17 4.03 4.22 4.73 4.20 1.76 Ti02 0.12 0.12 0.60 0.55 0.57 0.10 0.03 0.05 0.13 0.12 1.67 p2o5 0.02 0.02 0.22 0.20 0.21 0.01 0.01 0.02 0.03 0.02 0.54 MnO 0.10 0.11 0.15 0.76 0.08 0.09 0.10 0.12 0.10 0.11 0.16 H20( + ) 0.13 2.41 2.26 1.95 0.98 3.28 0.89 2.12 0.31 3.75 1.67

H2O(-) 0.13 0.33 0.06 0.14 0.28 0.31 0.36 0.88 0.22 0.34 0.52 Total 96.30 99.39 100.25 99.94 99.29 99.83 99.74 100.48 99.57 100.20 99.10 Th (ppm) 14 17 n.d. 8 9 n.d. n.d. 12 18 16 5 U (ppm) 6 6 n.d. 2 2 n.d. n.d. 6 5 5 1

CIPW NORMS Q 30.90 30.89 20.12 22.02 22.73 29.04 27.43 32.54 34.58 30.58 c 1.13 0.41 0.70 0.35 0.33 Or 27.55 29.79 19.57 17.80 15.95 24.68 23.88 24.82 28.07 24.77 10.50 ab 36.55 32.86 31.48 32.85 35.20 38.31 43.01 35.45 33.14 36.57 23.92 an 2.34 2.46 15.62 15.12 16.28 1.40 1.42 2.59 2.00 2.40 21.54 ne 2.40 wo 0.04 1.18 0.76 1.52 7.16 en 0.05 0.05 3.45 2.92 2.94 0.45 0.18 0.05 0.05 0.05 5.06 fs 0.02 1.53 2.29 0.17 0.03 1.48 fo 12.66 fa 4.09 mt 0.18 0.86 3.08 3.00 0.49 0.47 0.56 0.14 0.92 4.52 hm 0.74 4.40 1.07 0.77 0.82 il 0.24 0.23 1.14 1.05 0.28 0.19 0.06 0.10 0.25 0.23 3.20 tn 0.09 ru 0.39 ap 0.05 0.05 0.52 0.47 0.50 0.02 0.02 0.05 0.07 0.05 1.29 Total 99.73 97.25 97.69 97.93 98.76 96.41 98.76 97.03 99.47 95.93 97.82

Note: Published analyses determined by standard wet chemical techniques. Others determined by X-ray fluorescence techniques, by D. Hopping and V. McDaniel, U.S. Geological Survey, n.d. = not determined. • • = not present. Analyses of Th and U determined by delayed neutron activation, by H. T. Millard, Jr., C. Bliss, and C. McFee, U.S. Geological Survey. Samples 2 (Rowley, 1968), 78-115, 78-196, 78-175d, and 78-732 are obsidians. Sample locations: 78-175b, 38°13'N, 112°03'W; 78-175d, 38°13'N, 112°03'W; 78-732, 38°15'N, 112°09'W; 78-821, 38°16'N, il2°07'W; 78-115, 38°12'N, 112°05'W; 78-195, 38°10'N, 112°04'W; 78-196, 38°10'N, 112°04"W; 78-164, 38°12'N, 112°05'W. * From Rowley (1968). f From Willard and Callaghan (1962).

have been at least 500 m thick; if so, the cone they formed must the cone. The change in dip, shown in Fig. 5 by a dashed-crossed have spread over the canyon bottom. Most tuffaceous material in line, marks the position of the crater rim. the cone is soft and easily disaggregated, and erosion has removed Bedding types vary. Graded bedding, probably caused by depo- all but a few remnants of it. sition by airfall of larger fragments first, occurs locally. Where the Several types of deposits are represented in the pyroclastic cone. layered airfall tuff in the pyroclastic cone is thin bedded, crossbed- By far the most common rock type is poorly sorted, thin- to thick- ding (Fig. 6), in which upper beds cut out lower beds downdip, is bedded airfall lapilli tuff (Figs. 6, 7). Other depositional types in the widespread. This form of crossbedding may have formed by depo- cone include small quantities of volcanic mudflow breccia, sition following erosion by running water or slump, or some may moderately to well sorted airfall ash (Fig. 8), fluvial sandstone, and represent ground surge tuff. Abrupt differences in dip (Fig. 9) re- perhaps some nonwelded ash-flow tuff. sulted from irregularities on the depositional slope that also were Most of the pyroclastic cone deposits dip inward into Phonolite caused by local erosion or slump. Slump folding formed locally in Hill; these dips range from several degrees to as much as 40° and bedded deposits where material slid down the steep depositional represent initial dips on the inner walls of the pyroclastic cone. On surface and rumpled the moving beds and underlying beds the northeastern flank of the hill, however, beds that dip outward (Fig. 10). Slump folding is especially well displayed on the south- at angles of as much as 25° represent deposits on the outer flanks of western side of Phonolite Hill, where the dips of the pyroclastic de-

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112°05'

Figure 5. Phonolite Hill area (sees. 13, 14, 23, 24, T. 30 S., R. 2 V; W.) and cross section A-A' of the Phonolite Hill area, in the bottom part of the north side of Kingston Canyon. Topographic base from U.S. Geological Survey Phonolite Hill 7Va-min quadrangle, Utah. Map symbols: Tn, Needles Range Forma- tion; Tm, Mount Dutton Formation; Tmk, Kingston Canyon Tuff Mem- ber of the Mount Dutton Formation; Tma, Antimony Tuff Member of the Mount Dutton Formation; Tpc, pyroclastic cone deposits of the rhyolite of Phonolite Hill (light stippled pattern); Tpd, dome of the rhyolite of Phonolite Hill (dashed line pattern); Qt, talus and fan deposits; and Qa, alluvial deposits. Hydro- thermally altered rocks shown by + overprint. Downthrown side of faults shown by bar-and-ball symbol, dashed where approximately located and dotted where concealed. Contact between Tpc and Tpd is dotted where concealed. Crater rim is shown by a dashed-crossed line.

1 MILE

Phonolite Hill

posits are steepest. High-angle growth faults or tectonic faults, with type, but locally they predominate. The clasts range from lapilli size displacement less than 0.5 m in most places, locally cut the cone to blocks as large as 15 m across. The largest blocks now exposed deposits (Fig. 10). occur on the northern and northeastern side of the cone, especially The airfall lapilli tuff that forms most of the cone, as well as beds near the crater rim. The largest clasts are either of Mount Dutton of mudflow breccia, contain abundant angular clasts immersed in a Formation (Fig. 11) or olivine basalt (Fig. 12). One large block, finer grained matrix. Most clasts consists of rhyolite lava and about 3 m across, was broken on impact (Fig. 12). The larger clasts rhyolite pumice. Olivine basalt, with iddingsite replacing most within many air-fall beds fell into and deformed the finer grained olivine phenocrysts, is the next most abundant clast type. Frag- matrix. Some large clasts sank 1 to 20 cm into the unconsolidated ments of Mount Dutton Formation and Needles Range Formation, fine-grained matrix (Fig. 13). The resulting "bomb sag pits" locally ripped from the walls of the vent, are the third most abundant rock accumulated smaller clasts; these smaller clasts either washed or

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Figure 6. Thin-bedded airfall pyroclastic deposits and Figure 8. Well-sorted, thin-bedded pyroclastic deposits of ash- crossbedding due to truncation of older beds in a downdip direc- flow or airfall origin, inner wall of cone, northeast side of Phonolite tion (left), southwest side of Phonolite Hill. Hill.

Figure 9. A downdip (to right) increase in dip and downdip de- crease in thickness of thin-bedded pyroclastic deposits, southwest side of Phonolite Hill.

same age as this vent, however, are exposed no closer than 3 km from Phonolite Hill, on the northern and southern rims of Kingston Canyon. No evidence was seen to indicate that basalt magma was Figure 7. Coarse, thick-bedded airfall or mudflow deposits of present when the rhyolite was emplaced. There is no evidence for the outer wall, northeast side of Phonolite Hill. magma mixing as demonstrated at Yellowstone National Park (Wilcox, 1944; Hawkes, 1945) and in the San Juan Mountains (Lipman, 1975). For example, rocks of intermediate chemical composition, xenocrysts, resorbed inclusions of basalt in rhyolite, or rolled in from just upslope after the eruption or represent smaller glassy rhyolitic inclusions in basalt were not observed. clasts (with greater trajectories) that showered down and filled around the large, first-fallen clast during the same eruption. Dome of Phonolite Hill Because most foreign lithic clasts, including most of the largest blocks, are of olivine basalt, a sample of relatively fresh rock from Light to dark gray, black, and locally pink, resistant, locally the largest observed ejecta block was collected for dating. It has a spherulitic, crystal-poor rhyolite, interpreted to be an endogenous K-Ar age of 7.8 ± 0.5 m.y. (Table 3, Fig. 5), clearly older than the volcanic dome, forms most of Phonolite Hill. Generally, the rock is crater deposits. The presence of abundant clasts of older basalt flow foliated, containing alternating layers of light gray, vesicular, suggests that the vent for the rhyolite of Phonolite Hill partly fol- more devitrified and dark gray, less devitrified glassy rhyolite. The lowed a subsurface basalt vent that may have supplied the material rock tends to break parallel to the foliation and forms plates that for lava flows at the surface. Basalt lava flows that could be of the produce large talus fans. Locally the rock is a flow breccia, intrusive

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Figure 10. Folds (left of center and lower right) and faults related Figure 12. Corner of a large ejecta block of basalt that was to slumps in thin-bedded pyroclastic deposits on the southwest side cracked on impact. Crack is filled with fine-grained airfall matrix of Phonolite Hill. that was forced into the developing fracture on impact. From pyroclastic deposits of the outer wall of cone, northeast side of Phonolite Hill.

Figure 13. Bomb sag pit caused by a cobble that penetrated and deformed the underlying tuff bed. The rest of the impact crater was filled by finer-grained airfall material of the overyling bed. Pyro- clastic deposits, southwest side of Phonolite Hill.

breccia, or crumble breccia, some blocks of which consist of perlite. Clasts of Miocene crumble breccia, formed when talus material from the steep-sided dome flaked off during dome growth, are abundant in the Quaternary talus on the northeastern side of Phonolite Hill, but here any significant, mappable crumble breccia that may have been present was removed by erosion or is covered. The contact of the rhyolite with the enclosing pyroclastic cone deposits, as locally exposed on the flanks of Phonolite Hill, dips steeply inward or is vertical (Fig. 14). The rock of the dome in- cludes black or dark gray hydrated obsidian from 3 to 15 m thick Figure 11. Ejecta block about 6 m long of Mount Dutton For- adjacent to the contact. The obsidian grades inward into lighter mation, in the pyroclastic deposits near the crater rim, northeast colored, partly to completely devitrified rhyolite. Foliation of the side of Phonolite Hill. rhyolite is steep in most places and largely parallel to the contacts.

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TABLE 3. K-AR AGE DETERMINATIONS OF RHYOLITE AND BASALT FLOWS FROM THE KINGSTON CANYON AREA, UTAH

4 40 *Ar40 Sample Unit Material dated Location k2o Ar Age 10 (lat N, long W) (%) (IO" moles/g) (%) (m.y. ± 2cr)

78-115a Tpd Obsidian 38°11'45", 112°05'10" 4.25r 0.301 65.2 4.79 ± 0.23 78-115b do Sanidine 38°H'45", 112°05'10" 2.09, 2.13 0.164 52.2 5.42 ± 0.27 78-164 Tpb Basalt 38°11'52", 112°04'50" 1.85, 1.87 0.210 37.7 7.81 ± 0.45 78-175d Tf Obsidian 38°13'10", 112°03'27" 4.96, 4.97 0.541 37.3 7.55 ± 0.44 — do — do Sanidine 38°13'10", 112°03'27" 2.95, 2.95 0.325 25.5 7.63 ± 0.63

40 ,0 ,o 40 4 Note: Constants: K Xe = 0.581 x 10- /yr, XB = 4.962 x 10- /yr; atomic abundance: K /K = 1.167 x 10" . * Radiogenic argon; potassium determinations made with an Instrument Laboratories flame photometer with a Li internal standard. Unit symbols: Tpd, dome of the rhyolite of Phonolite Hill; Tpb, basalt in crater deposits of the rhyolite of Phonolite Hill; Tf, rhyolite of Forshea Mountain. Sample locations shown on Figure 4. f Determined by isotope dilution.

Commonly the cone deposits are "baked" and slightly hydrothermally altered at the contact. The shape of the rhyolite body suggests that it is a steep-sided endogenous volcanic dome (tholoid) or spine 0.8 x 1.2 km in diameter, elongated in the north-south direction. The inward dips (Fig. 14) of the marginal contact and the steep outward-upward- directed flow layering suggest that the dome probably enlarged upward into an inverted teardrop or fan shape. The dome was emplaced as a viscous mass of magma that moved upward through the pyroclastic cone deposits into the central crater at the end of the pyroclastic eruptions, perhaps in a manner similar to the well- documented emplacement mechanisms of the domes of the Lassen Peak area (Williams, 1932a, Fig. 37a; 1932b) and perhaps of the spine of Mt. Pelee (Jaggar, 1904; Lacroix, 1904; MacDonald, 1972). In thin section, the rhyolite contains several percent of small phenocrysts of plagioclase and minor amounts of sanidine and traces of quartz (Table 1) in a glassy to finely crystalline matrix. White layers are more spherulitic and vesicular than darker layers. Composition of the rhyolite of Phonolite Hill (Table 2), like that of the other rhyolites of Kingston Canyon, plots in the low- temperature trough of the Q-Or-Ab triangle (Fig. 15), as do typical rhyolites and granites, and thus could have formed by partial melting or fractional crystallization. A partial melting origin would require (Turtle and Bowen, 1958; Luth and others, 1964) that the rhyolites and rhyodacite originated at 2 to 5 kb (7 to 17 km depth), shallow to intermediate depths in the crust, which in the Kingston Canyon area is 35 to 40 km thick (Smith, 1978). The rhyolite of Forshea Mountain plots just below the isobaric minima and may have been derived from a somewhat more potassic source. Both obsidian and devitrified rhyolite from the rhyolites of Phonolite Hill and Forshea Mountain (Table 2) contain similar amounts of thorium and uranium. These contents, however, are less than half those for rocks of the Mount Belknap Volcanics; clearly the rhyolites in Kingston Canyon have less potential for economic mining of uranium deposits than the Mount Belknap Volcanics (Steven and others, 1980). Figure 14. Subvertical intrusive contact between gently inward Two K-Ar ages were determined from obsidian from the western dipping (to right) lighter colored pyroclastic deposits on the left and margin of the dome, one a whole-rock age of 4.8 ± 0.23 m.y., and steeply inward dipping flow-foliated rhyolite of the dome on the the other a sanidine age of 5.4 ± 0.27 m.y. (Table 3). The sanidine right, southwest side of Phonolite Hill. Height of outcrop shown is age is believed to be the most reliable indicator of the time of erup- about 10 m. tion of the rhyolite of Phonolite Hill.

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Other Rhyolite Bodies in Kingston Canyon cause structural events rarely are closely bracketed by datable rocks. The end of widespread calc-alkalic volcanism and the start Two masses of white, light to dark gray, and black rhyolite occur of compositionally bimodal volcanism (rhyolite and mafic flows) within about 1 km south of the river in Kingston Canyon (Fig. 3). and basin-fill sedimentation, dated at about 22 to 21 m.y. old, are Both masses are poorly exposed, and their relations to the pyro- suggestive of the start of basin-range extensional tectonism (Row- clastic cone deposits are unknown. The rock of both masses is ley, Anderson, and others, 1978), but are only circumstantial evi- lithologically similar to the rhyolite of the dome of Phonolite Hill dence. Although some high-angle faults date from 26 to 22 m.y., no (Tables 1, 2). known field evidence proves that these represent major basin-range The eastern body of rhyolite is a stubby north-northeast— faulting or that the major basin-range faulting took place from 22 trending dike(?) that is at least 600 m long and 120 m wide. Flow to 10 m.y. ago or perhaps even more recently (Rowley, Steven, and foliation is nearly vertical and strikes north-northeast. The rock is others, 1979). Yet some basin-fill sedimentary rocks date back to hydrothermally altered, especially along its southern margin. The the 22 to 10 m.y. period; the basins in which they were deposited western body of rhyolite underlies a small but steep-sided hill elon- probably represent extension, but, if so, they may have developed gated in a northerly direction; it has the shape of Phonolite Hill but more by broad warping than by faulting (Rowley, Steven, and is much smaller, being about 500 m long, 400 m wide, and 100 m others, 1979). Neither can the main period of basin-range faulting high. Black obsidian, at least 7 m thick, is exposed on the south- be late Quaternary, even though faults of this age are abundant in eastern side of the hill. Flow foliation is subvertical. southwestern Utah (Anderson, 1978), because most of these faults Poorly exposed massive pyroclastic rocks, similar to those at are of small displacement. Phonolite Hill, occur in a small outcrop about halfway between the Significant field data on the age of faulting occur in widely scat- two rhyolite masses and in another outcrop about 0.75 km east- tered areas. The eastern border fault of the basin surrounding southeast of the eastern dike. They may represent the remains of a Beaver, about 40 km west of Kingston Canyon (Fig. 1) was active significant volume of pyroclastic rocks, but whatever pyroclastic about 9 m.y. ago, the age of alunite along the fault at the Sheeprock rocks were not removed by erosion in these areas are largely cov- Mine (Steven and others, 1979, 1980, 1981). The southern Beaver ered by pediment deposits. We did not note attitudes for these de- basin contains basin-fill sedimentary rocks that range in age from posits. older than a 7.6-m.y.-old basalt flow (Best and others, 1980) to Pleistocene, but data are not available to date any faults in the basin STRUCTURAL AND GEOMORPHIC HISTORY as older than late Pleistocene. Near Sevier, Utah, 45 km north- northwest of Kingston Canyon (Fig. 1), 15- to 7-m.y.-old basin-fill The age of the main period of basin-range faulting in the High sedimentary rocks of the Sevier River Formation predate major Plateaus and eastern Basin and Range is poorly documented be- basin-range faulting (Steven and Cunningham, 1979; Rowley, Ste- ven, and others, 1979). The 6.4- to 5.0.-m.y.-old basalts in Grass Valley and the Awapa Plateau, mentioned above, predate the main uplift of the Awapa and Aquarius Plateaus along a fault with about Q 500 m of vertical displacement. At the western entrance to Red Canyon, just south of the Sevier Plateau (Fig. 1), 0.5-m.y.-old basalt (Best and others, 1980) flowed westward over a major fault scarp of the Sevier fault zone and was in turn downthrown 60 to 180 m westward by recurrent movement on this main fault; locally the basalt may have been extruded from a feeder along the surface of the main fault (Rowley, 1968, PI. 1; K. A. Sargent, 1980, oral commun.). Pleistocene basalts show similar relations between major faulting arid younger recurrent faulting in the Paragonah and Cedar City areas, 65 and 100 km, respectively, to the southwest (Anderson and Rowley, 1975). About 400 m of vertical displacement on the Hurricane fault near and south of Cedar City has occurred since 1 m.y. ago, whereas comparable or greater amounts of displacement are of Pliocene or early Pleistocene age (Anderson and Mehnert, 1979; R. E. Anderson, 1980, oral commun.). In none of the examples given above, however, can the age of the main period of basin-range faulting be as accurately dated as it can in Kingston Canyon. Specifically, the southern Sevier Plateau near Kingston Canyon was uplifted during a brief period of time 7.6 to 5.4 m.y. ago. The 7.6-m.y.-old rhyolite of Forshea Mountain and Figure 15. Normative compositions of rhyolites in the Kingston underlying 13(?)-m.y.-old basalts on the north rim of Kingston Canyon area plotted on the Q-Or-Ab face of the tetrahedron from Canyon form the upper part of the displaced stratigraphic section

the system Q-0r-Ab-H20. Data are from Table 2. Open circles, of the Sevier Plateau. Evidence to suggest that a paleo—Kingston rhyolite of Forshea Mountain; triangles, rhyodacite of Dry Lake; Canyon existed in the area when they were deposited has not been solid circles, rhyolite of Phonolite Hill. Isobaric curves from Tuttle recognized. Thus the rhyolite and basalt predate uplift and tilting of and Bowen (1958) and Luth and others (1964). the Sevier Plateau. Canyon cutting, which resulted from uplift of

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the Sevier Plateau, occurred after 7.6 m.y. ago. Kingston Canyon is occur farther west, under the basalt flows at the eastern entrance to an antecedent canyon (Dutton, 1880) formed when the river main- Kingston Canyon. Thus perhaps the ancestral stream in Kingston tained its course across the rising Sevier Plateau fault block during Canyon flowed east-southeast. Rhyolite clasts, however, were not its uplift. found farther east-southeast, under the 5.4-m.y.-old basalt flow or Some basaltic lava flows exposed south of Kingston Canyon in the sediments on the Awapa Plateau. flowed eastward from their vents toward a lowland (ancestral During the later uplift of the Awapa and Aquarius Plateaus, the Grass Valley) east of the Sevier Plateau. These flows, which may course of the river across these plateaus was destroyed, and the to- correlate with the 5.0-m.y.-old basalt (Best and others, 1980) in pography became similar to that of today. Little topographic ex- Grass Valley, overlie a wedge-shaped accumulation northwest of pression of the abandoned river course on the Awapa and Aquarius Antimony (Fig. 3), as much as 60 m thick, of fluvial sedimentary Plateaus remains; the volcanic sediments of the river have been rocks of the Sevier River Formation that also was deposited in this covered by younger basalt flows, partly removed by erosion, and lowland and pinched out westward. The slope beneath the wedge- offset by faults related to uplift of the plateaus. A new river, in shaped section of basalt flows and Sevier River Formation con- Grass and Johns Valleys, took the place of the old river. For a forms to the eastward tilt of older rocks in the Sevier Plateau, and while, the new river may have flowed north through Grass Valley both units were deposited after at least some uplift and tilting. (to Salina) or south through Johns Valley (to the Paria River or to The end of the main period of canyon cutting can be established Red Canyon), but if so, the route was captured early, for the pres- closely by dating the rhyolite of Phonolite Hill, which erupted ent lips on the three mentioned possible routes (Fig. 4) are 245 to through a vent at or near the bottom of the canyon and filled the 460 m above the present river level at Phonolite Hill. Alternately, lower several hundred meters of the paleocanyon with pyroclastic the new river may have ponded water or sediment against Phono- material. Thus the canyon had been cut to its present depth, sub- lite Hill. Headward erosion from the west through the dam at sequent to major uplift of the plateau, by 5.4 m.y. ago. Phonolite Hill or overtopping from the east led to re-establishment These interpretations lead to a conundrum for which we as yet of the river through Kingston Canyon and to excavation of Grass have no complete answer. If a canyon approximating the and Johns Valleys. But apparently there has been little uplift of the configuration of present-day Kingston Canyon were cut in only 2 southern Sevier Plateau relative to adjacent areas since eruption of m.y. in late Miocene time, about 7.6 to 5.4 m.y. ago, why has so the rhyolite of Phonolite Hill. little erosional modification of the same mountainous terrain occurred in the subsequent 5 m.y.? The pyroclastic cone of the rhyo- ACKNOWLEDGMENTS lite of Phonolite Hill is soft and easily eroded, and it would have been removed quickly if the drainage had maintained itself during The study reported here began as thesis research by Rowley at eruption or was reestablished after blockage. This relationship ap- the University of Texas under the direction of the late J. H. Mackin. pears to require that the area was characterized by tectonic stability Subsequent stages of the study have been carried out at the U.S. after the 7.6 to 5.4-m.y. period of uplift and downcutting. Perhaps Geological Survey. Valuable assistance throughout the course of also the river in Kingston Canyon was blocked by the volcanic the study from Mackin, J. J. Anderson, R. E. Boyer, R. J. Fleck, D. rocks and diverted elsewhere for part of Pliocene time, or possibly S. Barker, A. G. Everett, Eugene Callaghan, P. L. Williams, C. G. for Pliocene and part of Pleistocene time. Cunningham, and D. J. Pierson is appreciated. D. M. Cheney made Field evidence from Grass Valley and the Awapa and Aquarius mineral separations, and M. G. Nelson performed X-ray work. The plateaus allows us to fill in more details of the geomorphic de- manuscript benefited substantially from advice and thoughtful re- velopment of the area. The presence of the 6.4 to 5.0-m.y.-old views by M. A. Kuntz, J. H. Dover, M. G. Best, and L. A. Fernan- offset basalts on the Awapa Plateau and in Grass Valley indicates dez. that the main uplift of the Awapa and Aquarius plateaus took place after Kingston Canyon was cut and blocked. At least here basin- REFERENCES CITED range faulting is younger eastward, in keeping with relations suggested by the age of rhyolitic volcanism along the Blue Ribbon Anderson, J. J., and Rowley, P. D., 1975, Cenozoic stratigraphy of southwestern High Plateaus of Utah, in Anderson, J. J., Rowley, P. D., lineament (Rowley, Lipman, and others, 1978). Fleck, R. J., and Nairn, A.E.M., Cenozoic geology of southwestern On the Awapa and Aquarius Plateaus, Williams and Hackman High Plateaus of Utah: Geological Society of America Special Paper (1971) and Hackman and Wyant (1973) mapped volcanic sedi- 160, p. 1-52. ments of alluvial origin as much as 60 m thick. The sediments Anderson, R. E., 1978, Quaternary tectonics along the Intermountain Seismic belt south of Provo, Utah: Brigham Young University Geology largely underlie basalts of diverse ages, including Quaternary ba- Studies, v. 25, pt. 1, p. 1-10. salts and the 6.4-m.y.-old basalt. The sediments, which correlate Anderson, R. E., and Mehnert, H. H., 1979, Reinterpretaion of the history with the Sevier River Formation, apparently remain from channels of the Hurricane fault in Utah, in Newman, G. W., and Goode, H. D., or a shallow basin containing north-trending tributaries or stream eds., Basin and Range Symposium: Rocky Mountain Association of Geologists, and Utah Geological Association, p. 145-173. remnants north and northwest of Pollywog Lake and a southeast Best, M. G., McKee, E. H., and Damon, P. E., 1980, Space-time- channel or tributary along Big Swale (Fig. 4). 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Bulletin 1468, 34 p. zone within the Pioche mineral belt of southwestern Utah and eastern 1979b, Environments favorable for the occurrence of uranium within Nevada: U.S. Geological Survey Journal of Research, v. 6, p. 175 — the Mount Belknap caldera, Beaver Valley and Sevier River Valley, 192. west-central Utah: U.S. Geological Survey Open-File Report 79-434, Rowley, P.D., Cunningham, C. G., Anderson, J. J., and Steven, T. A., 1979, 15 p. Geologic map of the Marysvale SW quadrangle, Piute County, Utah: Dalrymple, G. B., 1979, Critical tables for conversion of K-Ar ages from old U.S. Geological Survey Miscellaneous Field Studies Map MF-1116. to new constants: Geology, v. 7, p. 558-560. Rowley, P. D., Steven, T. A., Anderson, J. J., and Cunningham, C. G., Damon, P. 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Rowley, P. D., Anderson, J. J., Williams, P. L., and Fleck, R. J., 1978, Age of structural differentiation between the Colorado Plateaus and Basin and Range provinces in southwestern Utah: Geology, v. 6, p. 51—55. MANUSCRIPT RECEIVED BY THE SOCIETY OCTOBER 29, 1980 Rowley, P. D., Lipman, P. W., Mehnert, H. H., Lindsey, D. A., and Ander- REVISED MANUSCRIPT RECEIVED MARCH 4, 1981 son, J. J., 1978, Blue Ribbon lineament, an east-trending structural MANUSCRIPT ACCEPTED MARCH 11, 1981

Printed in U.S.A.

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