Bear Valley Formation, an Oligocene- Miocene Volcanic Arenite

JOHN J. ANDERSON Department of Geology, Kent State University, Kent, Ohio 44240

Geology of the Southwestern High Plateaus of Utah: Bear Valley Formation, an Oligocene- Miocene Volcanic Arenite

ABSTRACT INTRODUCTION

The area which today comprises the south- It has been recognized since the days of ern High Plateaus of Utah was the locale dur- Dutton (1880) that the High Plateaus of ing the early and middle Tertiary of extensive Utah consist of tilted fault blocks made up volcanic activity which formed a volcanic largely of Mesozoic sedimentary rocks and pile many thousands of feet thick. During Cenozoic sedimentary and volcanic rocks; the time it took for this vast build-up of lava furthermore, it has been known that the flows, volcanic mudflow-breccias, and ignim- structures of the High Plateaus are transi- brites, there occurred, in the late Oligocene tional in nature from those of the Basin and and early Miocene, a cessation of regional Range Province on the west to those of the volcanic activity which was marked by the Colorado Plateau on the east (Fig. l). Few accumulation of volcanic arenite and associ- studies of the geology of the southern High ated clastic sediments in and around what is Plateaus have been published since this pio- today the northern Markagunt Plateau. The neer work, however, and on even the most rock stratigraphic unit thus formed is de- recent geologic maps much of the area of the fined here as the Bear Valley Formation. Markagunt Plateau and the Tushar Mountains The Bear Valley Formation was deposited is shown as undifferentiated Tertiary volcanic in an extensive structural and physiographic rock. basin within which faulting and volcanism Of the previous studies of the Markagunt were contemporaneous with sediment ac- Plateau which have been carried out, the first cumulation. Little volcanism accompanied were those of the Wheeler and Powell sur- the early stages of sand deposition; the later veys. The main geographic results of the stages, however, were marked by local erup- 1872 Wheeler Survey were embodied in a tive activity which admixed a considerable topographic map of the Markagunt Plateau quantity of glass shards in the sand and pro- and adjoining areas. The geologic investiga- duced local ignimbrite and tuff strata. Depo- tions of this survey were recorded by Wheeler sition of the arenite was accomplished mostly (1875), who outlined the drainage system by the wind, and took place under arid and noted the presence of several local lava climatic conditions. This is indicated by the flows. Gilbert (1875) and Howell (1875) re- mineralogy and texture of the sand and by corded additional geologic observations on the large-scale cross-bedding throughout maps and in accompanying texts, but these most of the section. were limited to the southern and western After deposition and subsequent burial by plateau. Dutton (1879) described lavas and renewed volcanism, the sand was cemented pyroclastics near Panguitch Lake and roughly by clinoptilolite. Today the Bear Valley For- mapped the region. The classic later paper by mation crops out over an area of more than Dutton (1880) presented a brilliant analysis 1000 sq mi and has a maximum exposed of the basic geology of the High Plateaus; it thickness in excess of 1000 ft. Typically, it still stands as an important reference for any may be described as moderately to well geologist concerned with the region. In this sorted, fine- to medium-grained, zeolite- paper, Dutton (1880, p. 188-210) gave the cemented submature to mature volcanic first summary statement of the geology of arenite. No fossils have been recovered from the Markagunt Plateau. He included obser- this rock unit. vations on the geology of the Bear Valley

Geological Society of America Bulletin, v. 82, p. 1179-1206, 17 figs., May 1971 1179

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SOUTHWESTERN UTAH PHYSIOGRAPHIC PROVINCES

MODIFIED AFTER RIDD, 1963

HIGH PLATEAUS SHADED

0 10 20 30 40

SCALE IN MILES

Figure 1. Location map. Probable depositional limits of Bear Valley Formation diagonally lined. region, and noted the ptesence there of "tuf- pioneer work, Gregory defined the Brian faceous beds" and "overlying lavas;" the Head Formation. According to Gregory, this "tuffaceous beds" are defined in this paper formation overlies the Eocene(?) Claron For- as the Bear Valley Formation. He did not mation (Leith and Harder, 1908, p. 41-43; observe the cross-bedding of the "tuffaceous the "Wasatch Formation" of Gregory), and beds," however, and, therefore, incorrectly is overlain, in turn, by andesitic lavas. He attributed their deposition to the agency of assigned a tentative Miocene(?) age to the water. Brian Head Formation, and stated (1945, p. Published geologic studies of the Marka- 985) that "its pyroclastic beds are the oldest gunt Plateau remained nil for some 60 yrs volcanics in the southern High Plateaus." after Dutton's time, until Gregory (1945, Thus included within the Brian Head Forma- 1949, 1950) published the first detailed geo- tion are strata of the Bear Valley Formation logic information, including geologic maps, as the latter is defined in the present paper. of parts of the southern Markagunt. In this Recent geologic investigations in south-

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western Utah carried out by Threet (I952a, Basin (Mackin, I960, p. 98-99), though in 1952b) have served to show that included in some areas volcanic flow breccias of local the Brian Head Formation as defined and origin intervene between the two. Bear Valley mapped by Gregory are volcanic units better Formation is proposed as the name of a stra- defined and understood as formations in their tigraphic unit defined to include this sand- own right. This led Threet (I952b) to sug- stone, and the lava flows, conglomerates, gest that the Brian Head Formation might volcanic mudflow-breccias, ignimbrites, and be abandoned as a formal stratigraphic unit. tuffs locally present within it; the name is My own work (Anderson, 1965, 1968) has derived from the locality at and near which led me to the same conclusion, which is dis- the formation is typically developed (Fig. 2). cussed below. The Bear Valley Formation as defined includes within it strata assigned to the Brian Field and Laboratory Methods Head Formation by Gregory (1945, 1949, The type section of the Bear Valley Forma- 1950). I must suggest, therefore, that the tion was measured by tape; field checks indi- name "Brian Head Formation" be abandoned cate that the section measurements can be or its definition modified. I do not do this, considered accurate to within +5 percent. however, because the way I want to define Away from the type section the thicknesses the Bear Valley Formation conflicts with of exposed sections were measured by pace, Gregory's definition of the Brian Head For- compass, and Jacob's staff; the accuracy of mation, since under such circumstances the these measurements is probably within +15 definition of the Brian Head would clearly percent. have priority. Instead, I base my suggestion Samples representing every lithologic vari- on my opinion that both Gregory's defini- ety recognizable in the field within the Bear tion and his usage of the term "Brian Head Valley Formation were collected for petro- Formation" violate the Code of Stratigraphic graphic study. The type section was sampled Nomenclature (American Commission on more intensively, however, so that at least Stratigraphic Nomenclature, 1961). Thus at three specimens of each lithologic variety various times Gregory defined the Brian within it were obtained. Head to include limestone, siliceous lime- The classification of the sedimentary rocks stone, volcanic ash, tuff, breccia, igneous of the Bear Valley Formation, including the grit, conglomerate, siltstone, sandstone, an- terminology used to describe grain size and desitic agglomerate, and "acidic and basic textural maturity, is essentially that of Folk lavas." This seemingly does not comply with (1951, 1954, 1956, 1961). Color terminology Article 6 of the Code which requires that a used in rock descriptions is that of Goddard formation be a body of rock "characterized and others (19-48). by lithologic homogeneity," nor does it Geology recorded on aerial photographs seem to fall within the limits of "extreme of 1:62500 and 1:20000 scales was transferred heterogeneity of constitution which in itself to U.S. Forest Service planimetric maps of a may constitute a form of unity compared to scale of 1:31680. The outcrop map of the the adjacent rock units," which is allowed Bear Valley Formation (Fig. 2) is based on under Article 6(a) of the Code. these geologic maps, but does not depict all It might be argued that the Brian Head available geologic data, and is not to be con- Formation does comply with Article 6(f) of sidered a final geologic map. the Code which allows "sedimentary rock and extrusive igneous rock that are intricately STRATIGRAPHY interbedded [to be] assembled into a forma- Definition tion under one name," but this does not In the area of the northern Markagunt appear to me to be the case. Recent geologic Plateau, the most widely exposed rock type, investigations in southwestern Utah by Threet except for the almost ubiquitous volcanic (I952b, 1963), me, and others have shown mudflow-breccias immediately overlying it, that included in the Brian Head Formation as is a tuffaceous sandstone of greatly varying mapped by Gregory are volcanic units, in- thickness and lithology. This sandstone gen- cluding regional ignimbrites, which have erally conformably overlies an ignimbrite themselves been elsewhere defined and map- sheet directly correlative with the Isom For- ped as formations. In other words, there is mation (Oligocene) of the nearby Great no question here of interbedding of volcanic

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and sedimentary strata so intricate that for One of these (Sample 167) is from a vitric mapping purposes they must be assembled ignimbrite assigned by Armstrong to the into one formation; instead, the Brian Head Rodger [sic] Park Breccia (Roger Park Brec- Formation clearly contains volcanic strata cia, Callaghan, 1939); it underlies the type which are better understood and defined as section of the Bear Valley Formation, sepa- formations in and of their own right rather rated from it by only a few feet of volcanic than as subdivisions of a stratigraphic unit of flow breccia of local origin. The age obtained such lithologic diversity and genetic com- is 25.8 + 0.5 m.y. An equivalent age of plexity as Gregory's Brian Head Formation. 25.2 + 0.4 m.y. for this rock unit has also The most telling argument in favor of been established by Robert Fleck of The abandonment or redefinition of the term Ohio State University (1970, written com- "Brian Head Formation," however, is that, mun.). The other age determination made by in both definition and application, Gregory's Armstrong is likewise from a vitric ignim- usage of this term seems to violate Article brite assigned to the Rodger [sic] Park Brec- 6(d) of the Code which states that "practic- cia (Sample 165). This rock unit is located ability of surface or subsurface mapping is 4 mi away from the type section, but occu- essential in establishing a formation." Thus pies a similar stratigraphic position; the age Gregory's definition of the Brian Head states of the sample is 27.2 + 0.6 m.y. Therefore that it includes pyroclastics which are "the the Bear Valley Formation is no older than oldest volcanics in the southern High Pla- late Oligocene. teaus" (Gregory, 1945, p. 985), yet in many Radiometric age determinations are also instances he contradicts his own definition available from a tuff and a vitric ignimbrite by mapping pyroclastic rock as "Wasatch." interbedded about half-way up the sandstone Furthermore, Gregory's geologic map of the section of the Bear Valley Formation at a west-central Markagunt Plateau (1950, PI. 2) locality 9 mi southwest of the type section. fails in many places, including the type The tuff age is 24.0 + 0.3 m.y.; the ignim- locality of the Brian Head Formation, to brite, 23.9 ± 0.4 m.y. (Robert Fleck, 1970, draw contacts between the Brian Head and written commun.). Deposition of the Bear the underlying "Wasatch Formation," and in Valley Formation thus can be demonstrated other places fails to draw a contact between to have continued into the Miocene. the Brian Head and the overlying Tertiary As of now there is no equally reliable estab- (latitic and andesitic) volcanic strata. Al- lished upper age limit of the Bear Valley though this does not necessarily mean that Formation. Deposition of the sandstone such contacts were impossible to draw even which makes up the bulk of the formation for Gregory, it does mean that it is impos- was ended apparently by a deluge of volcanic sible to check in the field the rock which flows and mudflow-breccias. Probably corre- Gregory considered to belong to each of lative rock in the Marysvale area to the north these stratigraphic units. has been dated on the basis of field relation- Of course, Gregory cannot be held respon- ships as Miocene(?) or Pliocene(?), or both, sible for failing to obey a code which had by Callaghan and Parker (1962) and Willard not yet been adopted at the time he pub- and Callaghan (1962), so by inference it is lished his findings, but the fact remains that probably tenable to assign an upper age limit the geologist of today is presented with in- of Miocene to the Bear Valley Formation. superable problems of definition, correlation, and mapping if he attempts to work within Type Section the Code and at the same time apply the term "Brian Head Formation." The type section of the Bear Valley Forma- tion is located north of Utah State Highway Age 20 about 1.5 mi west of its junction with U.S. Defining the stratigraphic position of the Highway 89. The formation is exposed in Bear Valley Formation in terms of direct cor- south-facing scarps developed on strata dip- relatives of the Isom Formation places a re- ping north at an angle of slightly less than liable lower limit on its age. Armstrong 20°. The line of the type section trends about (1970, Samples 165 and 167, p. 208-209) has N. 10° W. and extends from SEVt NW14 sec obtained radiometric ages on two samples of 5, T.33 S., R.5 W.( to NE 14 SE!4 sec 32, these Isom equivalents within the study area. T.32 S., R.5 W. (Fig. 2).

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1000

Moderately sorted fine to medium sandstone: zeolitic submature vitric-volcanic arenite. Color seen from distance is light to medium gray; "salt-and- pepper" appearance up close. Grain size typically varies between 0.1 and 0.5 mm, with the 900 16-84 % range between 0.1 and 0.4 mm or 0.2 and 0.5 mm. The median grain size is generally in the range of 0.275 to 0.3 E 352 inn. Composition is characterized by relatively high percentages of glass shards and quartz. Constituent grains are generally sub-round to round, except for subangular to angular glass shards and feldspar laths. 800

D 700 Sandy muddy pebble conglomerate: zeolitic immature volcanic P - »-J;S:f3>-^iri: & mudf low-breccia. D p±:.^=_3^.-t 63 Overall color is grayish pink to pale red. Included clasts, P «^^=- ZTZHT-S- mostly of volcanic origin, range in size to over 30 cm. E Moderately sorted fine to medium sandstone: zeolitic submature to mature vitric volcanic arenite. R "* "Sal t-and-pepper" gray in color. •600 Massive, cross-bedded. Laminae of bimodal moderately to poorly sorted coarse and moderately to well sorted fine sandstone common within individual cross-bed sets. Several lenses of pale red, massive volcanic mudflow breccia in upper part of unit. 223 Typical sandstone grain sizes range from 0.075 to 0.5 mm. The 16-84 % range varies between 0.125 to 0.225 mm and 0.125 to 500 0.250 mm. Composition is characterized by relatively high percentages of glass shards and feldspar. Except for shards and some feldspar laths, which are commonly subangular to angular, most sandstone grains are subround C to round. Moderately to well sorted fine to medium sandstone: zeolitic 400 submature to mature vitric volcanic arenite. 60 Massive and cross-bedded. Light to dark greenish gray color grading upward to light greenish gray. k covered interval . Moderately to well sorted fine to medium sandstone: zeolitic submature vitric-volcanic arenite. Yellowish gray in color. aoo 1Q6 Massive, cross-bedded. Composition is characterized by relatively high percentages of glass shards and feldspar. Glass shards generally subangular to angular; other detrital grains subround to round.

7 7 40 Covered interval . 200 ij^^^P&aiO;*'-! Moderately sorted fine to medium sandstone: zeolitic submature volcanic arenite; interbedded with conglomeratic sandy •rmisttr&'&r&s&irssi mudstone: zeolitic immature volcanic mudflow-breccia. L Several scour-and-fil 1 channel deposits in upper part of the B 123 unit. Clastc dikes common in mudflow-breccia beds. 0 ^•.^J/A-.^.-if^x^A-VfSf reddish brown. W 100 The unit contains one 9 cm thick ashfall bed, now altered to zeolite (cl inoptilolite) . E Covered interval, probably concealing strata largely similar R 7 M to underlying unit.

Al Figure 3. Columnar representation of Bear Valley Formation type section.

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Five lithologic units, designated in ascend- analysis a distinct sandstone lithology which ing stratigraphic order by the letters A characterizes, and in turn defines, the lower through F, are defined within the type section (informal) sandstone member of the Bear (Fig. 3). Two of these units, B and D, are Valley Formation, both at and away from present only in the immediate area of the the type section. Similarly sandstones from type section. Sandstone strata in Unit B, Units C and E, which in the type section are however, though different in detail, are litho- separated by a 63-ft-thick volcanic mudflow- logically very similar to sandstone of Unit A. breccia (Unit D), but are similar to each other For purposes of correlation of Bear Valley and different from the sandstones of the Formation sections geographically removed lower member, are combined to define a from the type section, the sandstones of sandstone lithology which characterizes, and, Units A and B are grouped together to define in turn, defines, the upper (informal) mem- by petrographic description and statistical ber of the Bear Valley Formation. Table 1 TABLE 1. STATISTICAL ANALYSIS OF THE MAJOR DETRITAL CONSTITUENTS OF UNITS A, B, C AND E, AND OF THE LOWER AND UPPER MEMBER LlTHOLOGIES, BEAR VALLEY FORMATION TYPE SECTION PERCENTAGE 10 20 30 40 50 60 70 80 UPPER VRF R=F3 Glass i •— i —• i Feldspar C^EU n =14 LITHOLOGY Quartz E \/nVKrr bPlac laSeS • 1 UNITE ' 1 I n = 3 Quartz 33* VRF 13D UNIT C Glass C3ED Feldspar EQ n =11 Quartz E LOWER \/DP 1 ' * 1 1 • 1fcJ 1 Glass Feldspar C3 : n = 7 LITHOLOGY Quartz 1 VRF __^ I • • 1 Glass UNITE •4 Feldspar JEr n * 3 Quartz 1 wnVKr Glass UNIT A Feldspar •« Quartz 1 n = 4 Mean percentage ------Standard deviation from the mean - - Confidence limits (95 %) on the mean n = Number of samples

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shows in graphic form the means, standard minerals and plagioclase laths in a commonly deviations, and confidence limits of the iron-stained hyaline or cryptocrystalline major mineralogic and lithologic constitu- groundmass; about one-third are partially ents of the sandstones from the four units of devitrified hyaline grains; and less than one- the type section and of the two sandstone tenth are grains of allotriomorphic-granular lithologies derived from these four units feldspar. The last-named may be of plutonic which define the two sandstone members of rather than volcanic origin. A few vitric the formation. ignimbrite fragments also are found in the Unit A. The exposed thickness of Unit A rock. of the type section is only a little more than The remainder of the clastic particles are 20 ft, but float indicates that much, if not all, mineral grains, chiefly plagioclase (andesine- of the overlying 64-ft covered interval is labradorite), augite, magnetite, and sanidine. similar rock. It consists of cross-bedded The majority of these grains are unaltered and sandstone the most striking characteristic of subround to round, with the exception of which in most samples is its bimodal grain- magnetite grains which, although generally size distribution (Fig. 4). The larger of the well rounded, invariably show some altera- two modes is coarse sand of peculiar pastel tion to hematite. colors—pale red (5R6/2 and 10R6/2), pale In general, then, the clastic particles in the blue (5PB7./2), and pinkish gray (5Y8/1). Unit A sandstone show little or no evidence The smaller mode is pale-olive (10Y6/2) and of exposure to lengthy or severe weathering yellowish-gray (5Y7/2) fine sand. From a and alteration processes. This suggests that distance the over-all color of the unit is deposition took place under arid climatic yellowish gray. The relative percentages of conditions, a conclusion which is supported the two modes vary considerably in different by the widespread sand-dune deposits of the beds, yet they almost always occur together. lower member. Petrographically, Unit A is for the most The bimodal size distribution of much of part (bimodal), moderately to well-sorted, the Unit A sandstone gives further evidence fine- and well-sorted coarse-grained, slightly of deposirion under desert conditions. Ac- calcitic, zeolite-cemented, mature-to-super- cording to Folk (1968), the main tendency mature volcanic arenite (Table 2, a-c). A few of wind in deserts is to remove the fine sand beds, however, are well-sorted (unimodal), fraction and leave behind a mixture of (a) fine-grained, zeolite-cemented mature vol- grains in the coarse to very coarse sand range canic arenite (Table 2, d). that are too large to saltate and (b) very fine About 60 percent of the rock consists of sand and silt that forms a coherent, smooth, subround-to-round detrital grains derived cohesive surface that is difficult to move by from a wide variety of volcanic rock types. wind action. As a result of such a sorting Of these, about half are grains exhibiting process, desert floor sediments tend to be- hyalo-ophitic texture, with ferromagnesian come strongly bimodal mixtures of reliable well-sorted coarse sand and very fine sand and silt. It seems probable that this sorting mechanism may have brought about the bimodality of the Unit A sand. Unit A sandstone is well cemented by zeolite, probably largely clinoptilolite, with some calcite. Discussion of the zeolite cement is deferred until later. Unit B. The type Unit B is a 123-ft thick sequence of over 200 alternating layers of sandstone and either conglomeratic mudstone or sandy mudstone, all ranging in thickness from 0.05 ft to 9 ft (Table 3). In general, the sandstone layers are slightly calcitic, zeolite-cemented submature volcanic Figure 4. Bimodal volcanic arenite, Unit A arenite, and the conglomeratic mudstone (lower member), type section. Plane-polarized layers are slightly calcitic, zeolite-cemented light. immature volcanic mudflow-breccias.

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TABLE 2. MODAL ANALYSES OF ROCK SPECIMENS, BEAR VALLEY FORMATION TYPE SECTION

Column a b c d e f g h i Member L L L L L L L U U Unit A A A A B B B C C Sample Number 63-136 64-311 64-312 (54-313 63-139 63-l40a 63-142 63-143 63-144 Matrix(l) 23.0 31.1 27.4 29.8 26.3 41.0 35.7 33.5 36.2 Calcite(2) P P 1.6 .. 0.5 0.2 VRF(3) 67.3 52.5 62.4 52.9 51.7 35.2 50.0 24.8 17.1 SRF(4) P 0.3 P 0.7 2.1 0.5 0.2 Glass(5) . . P P 1.0 ...... 20.7 27.1 K-feIdspar(6) 0.4 0.7 0.7 0.3 1.0 0.3 0.9 2.1 2.3 Plagioclase(7) 5.8 8.0 5.6 9.1 11.0 12.4 8.9 14.5 12.8 Quartz 0.5 0.2 0.6 0.3 0.5 0.9 0.6 0.1 0.8 Chalcedony > Augite 2.1 5.1 2.8 5.1 6.2 4.1 2.3 2.0 1.7 Hornblende 0.2 0.5 0.1 0.1 0.3 0.2 0.5 1.2 1.1 Lamprobolite P 0.1 P .. P .. P Magnetite 0.7 1.4 0.4 1.1 0.7 3.8 1.0 0.3 0.9 Hematite(8) P P P P P P P P P Biotite P 0.2 P 0.1 P P 0.1 0.4 P Opaque unknown P Olivine ? ? Apatite 0.1 ...... P P Zeolite (9) . . . . P Chlorite(lO) P P P P P P P P P Sericite(lO) P P P P P P P P P

(1) Zeolite (dinoptilolite) cement or zeolitized (5) Shards, angular to rounded. ash, or both, largely cement. (6) Largely sanidine, but may include minor (2) Cement; where present generally associated amounts of orthoclase and (very rarely) micro- with calcite-replaced feldspar and calcitic silt- dine. stone sedimentary rock fragments. (7) Andesine-labradorite. (3) Volcanic rock fragments; includes igneous (8) Includes limonite; alteration product, generally rock fragments of possible plutonic origin. of magnetite. (4) Sedimentary rock fragments; calcarenite or (9) Detrital grains. calcitic sandstone, or both. (10) Alteration product or authigenic, or both.

Sample j 63-136 Grain size: R: Silt—0.8 mm; M: 0.2 mm; S: 0.15- Classification: Well-sorted fine and well-sorted 0.3 mm. coarse sandstone: zeolite-cemented bimodal Grain shapes: Generally subround. supermature volcanic arenite. Sample # 63-139 Grain size, large mode: R: 0.3-1.5 mm; M: 0.7 mm; Classification: Moderately sorted medium sand- S: 0.6-0.9 mm. stone: slightly calcitic, zeolite-cemented sub- Grain size, small mode: R: Silt—0.3 mm; M: 0.125 mature volcanic arenite. mm; S: 0.1-0.8 mm. Grain size: Clay—1.2 mm; M: Medium sand; S: Grain shapes: both modes very well rounded. Moderate to good. (Note: Range, median and Sample t; 64-311 sorting vary slightly between different laminae.) Classification: Moderately sorted fine and well- Grain shapes: Generally rounded to well rounded sorted coarse sandstone: zeolite-cemented super- for all grains of fine sand size and larger; in mature volcanic arenite. smaller size range rounding varies from angular Grain size, large mode: R: 0.3-1.6 mm; M: 0.7 mm; to very round. S: 0.5-0.85 mm. Sample # 63-140a Grain size, small mode: R: Silt—0.3 mm; M: 0.15 Classification: Moderately sorted fine sandstone: mm; S: 0.1-0.3 mm. zeolite-cemented submature volcanic arenite. Grain shapes: Both modes rounded to well rounded. Grain size: R: Very fine to coarse sand; M: Lower Sample f 64-312 fine-sand range; S: 0.08-0.15 mm. Classification: Moderately sorted fine and well- Grain shapes: Generally subround. sorted coarse sandstone: zeolite cemented bi- Sample # 63-142 modal mature volcanic arenite. Classification: Moderately sorted interlaminated fine Grain size, large mode: R: 0.3-1.75 mm; M: 0.8 and medium sandstone: zeolite-cemented sub- mm; S: 0.5-1 mm. mature volcanic arenite. Grain size, small mode: R: Silt—0.3 mm; M: 0.18 Grain size, fine-grained laminae: R: Silt—0.8 mm; mm; S: 0.1-0.3 mm. M: Fine sand; S: 0.125-0.25 mm. Grain shapes: Both modes rounded to well rounded. Grain size, coarse laminae: R: Silt—1.25 mm; M: Sample # 64-313 Lower to middle medium-sand range; S: 0.175- Classification: Well-sorted fine sandstone: zeolite- 0.5 mm. cemented mature volcanic arenite. Grain shapes: Generally subangular to subround.

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TABLE 2. (Continued)

Column i k 1 m n 0 P q r Member U U U U U U U u U Unit C C C C C C C C C Sample Number 63-145 63-146 63-147 63-148 63-149 63-151 63-152 63-153 63-154 Matrix 33.7 35.1 31.7 27.9 32.9 44.4 29.9 38.8 45.9 Calcite 1.0 P VRF 23.7 12.1 19.4 20.1 11.1 11.1 20.3 15^6 19.2 SRF 0.2 P P 4.4 Glass 14.1 23.7 12.1 17.4 28.4 16.5 19.8 17.6 13.2 K-feldspar 4.1 2.9 2.3 3-9 2.6 0.4 0.6 0.6 0.7 Plagioclase 15.5 18.0 24.2 22.2 19.8 22.6 22.8 18.4 16.3 Quartz 0.7 3.2 4.1 3.4 2.4 0.4 1.6 0.4 0.1 Chalcedony Augite 2.6 1.4 2.4 2.6 1.7 3.8 3.2 2.4 2.6 Hornblende 2.2 P 1.0 0.6 0.2 0.3 0.5 0.4 0.6 Lamprobolite P 0.2 P P 0.1 0.1 P P Magnetite 2.1 3.6 2.6 1.8 0.9 0.4 1.2 1.4 1.4 Hematite P P P P P P P P P Biotite 0.1 P P 0.1 P P P P P Opaque unknown P P P P P Olivine Apatite P P Zeolite P P Chlorite P P P P P P P P P Sericite P P P P P P P P P Numbers in the table are percentages of a given Also given are brief petrographic descriptions of mineral determined by a 1000-point count of the the samples included in the table. In these descrip- sample studied; the symbol "P" denotes that a tions, the following abbreviations are used in the mineral is present but in an amount less than 0.1% grain size analyses: R = the size range of the clastic of the total composition; the symbol "?" indicates particles; M = the median size; and S = the sort- a probable, but not positive, identification; the ing, or to be more specific, the 16-84% range of the symbol " . . " indicates a mineral is not found in the clastic grain sizes. sample studied. Sample # 63-143 Sample # 63-147 Classification: Well-sorted fine sandstone: zeolite- Classification: Moderately sorted fine sandstone: cemented mature vitric-volcanic arenite. zeolite-cemented submature vitric-volcanic Grain size: R: Very fine sand—0.5 mm; M: Fine arenite. sand; S: 0.13-0.225 mm. Grain size: R: Very fine sand—0.45 mm; M: 0.175 Grain shapes: Most grains are subangular to angular, mm; S: 0.1-0.25 mm. and lathlike grains of plagioclase are common. Grain shapes: Many plagioclase grains are lathlike Sample ?'• 63-144 and angular; most other grains are subround to Classification: Moderately sorted fine sandstone: round. zeolite-cemented submature vitric-volcanic are- Sample § 63-148 nite. Classification: Moderately sorted fine sandstone: Grain size: R: Very fine sand—0.5 mm; M: 0.175 zeolite-cemented submature vitric-volcanic mm; S: 0.125-0.25 mm. arenite. Grain shapes: Most grains are subangular to sub- Grain size: R: Very fine sand—0.8 mm; M: 0.2 mm; round. S: 0.1-0.3 mm. Sample ;l 63-145 Grain shapes: Many angular glass shards and plagio- Classification: Well-sorted fine sandstone: zeolite- clase laths; most other grains are subround. cemented mature vitric-volcanic arenite. Sample # 63-149 Grain size: R: Very fine sand—0.5 mm; M: 0.175 Classification: Moderately sorted fine sandstone: mm; S: 0.15-0.25 mm. zeolite-cemented bimodal submature vitric- Grain shapes: Glass shards are subangular to angu- volcanic arenite. lar; other detrital constituents generally subround Grain size, large mode: A few grains of coarse to round, with the exception of plagioclase, sand size. which is commonly lathlike and angular. Grain size, small mode: R: Very fine sand—0.35 Sample f 63-146 mm; M: 0.2 mm; S: 0.125-0.3 mm. Classification: Moderately sorted fine sandstone: Grain shapes: Glass shards commonly angular; zeolite-cemenred submature vitric-volcanic other grains generally subround to round. arenite. Sample # 63-151 Grain size: R: Very fine sand—0.45 mm; M: 0.175 Classification: Slightly granular fine sandstone: mm; S: 0.1-0.25 mm. zeolite-cemented submature vitric-volcanic Grain shapes: Glass shards generally angular; other arenite. constituents generally subround to round. Grain size: R: Very fine sand—0.5 mm in thin

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TABLE 2. (Continued) Sample # 63-1:53 Classification: Pebbly moderately sorted fine sand- Column s t u stone: zeolitic submature vitric-volcanic arenite. Member U U U Grain size, sand fraction: R: Silt—0.3 mm; M: 0.2 Unit E E E mm; S: 0.1-0.25 mm. (Note: Larger grains, up to Sample Number 63-156 63-157 63-159 pebble size, are present in hand specimen.) Grain shapes: Mostly subround, but present are a Matrix 37.7 30.4 37.5 goodly number of lath-like subangular and Calcite angular glass shards. VRF 14.0 263 10.4 Sample # 63-1.54 SRF 0.1 Classification: Slightly granular sandy mudstone: Glass 33^7 17.0 13^6 zeolite-cemented immature vitric-volcanic arenite. K-feldspar 0.5 3.0 1.1 Grain size: R: Silt—1.95 mm; M: 0.125 mm; S: Plagioclase 10.5 20.8 23.2 0.75-0.3 mm. Quartz 0.6 1.3 7.2 Grain shapes: All degrees of rounding; most grains, Chalcedony however, are subangular to subround. Augite 1.6 0.6 3^6 Sample # 63-156 Hornblende P P 0.4 Classification: Moderately to poorly sorted medium Lamprobolite P sandstone: zeolite-cemented submature vitric- Magnetite 1.4 0.5 3.0 volcanic arenite. Hematite P P P Grain size: R: Silt—0.9 mm; M: 0.3 mm; S: 0.1- Biotite P P 0.4 mm. Opaque unknown P Grain shapes: Except for subangular to angular Olivine glass shards, most grains are subround to round. Apatite ? Zeolite Sample # 63-157 Chlorite P P P Classification: Moderately sorted medium sand- Sericite P P P stone: zeolite-cemented submature vitric-volcanic arenite. section, up to 25 mm in hand specimen; M: Grain size: R: Silt—1.2 mm; M: 0.3 mm; S: 0.2- Fine sand (thin section); S: 0.075-0.25 mm (thin 0.5 mm. section). Grain shapes: A very few angular laths; othetwise, Grain shapes: Generally subround. most grains are subround to round. Sample # 63-152 Sample # 63-139 Classification: Moderately sorted fine and well- Classification: Moderately sorted medium sand- sorted coarse sandstone: zeolite-cemented stone: zeolite-cemented submature vitric-volcanic bimodal mature vitric-volcanic arenite. atenite. Grain size, large mode: R: 0.6-2.5 mm; M: 0.9 mm; Grain size: R: Silt—0.75 mm; M: 0.275 mm; S: S: 0.7-1.2 mm. 0.15-0.4 mm. Grain size, small mode: R: Very fine sand—0.6 mm; Grain shapes: Subround to round for the most part M: 0.175 mm; S: 0.1-0.4 mm. with, however, some angular grains; these are Grain shapes: Generally subangular to subround. mainly glass shards.

On the outcrop, the most conspicuous be volcanic arenite dunes that accumulated differences between the two lithologies are in simultaneously with, but geographically re- resistance to weathering and in color. The moved from, the alluvial fan of the type conglomeratic mudstone is the weaker of the locality. two and weathers back between beds of The volcanic nature of the Unit B sand- more resistant sandstone (Fig. 5). The color stone, and of the conglomeratic mudstone difference is equally striking; the sandstone interbedded with it in the type section, is beds are generally pale yellowish brown evident from a study of their composition. (10YR6/2) or pale olive (10Y6/2), whereas There are no striking differences between the the conglomeratic mudstone beds are pale mineralogies of the two rock types, and both red (10R6/2) to pale reddish brown (10R5,4). closely resemble Unit A in this respect Unit B is interpreted as an alluvial fan (Tables 1 and 2, e-g). deposit that formed under arid or semi-arid Both the sandstone and the conglomeratic climatic conditions near the site of a stream mudstone are well cemented by a zeolite, debouching from a local highland to the probably clinoptilolite. The zeolite is prob- north. No similar sequence of interbedded ably an authigenic filling of pore space in the sandstone and conglomerate is known to sandstone, but in the conglomeratic mud- crop out elsewhere within the depositional stone, the zeolite is probably largely an area of the Bear Valley Formation, but sand- alteration product of primary volcanic ash. stone of the lower member lithology is wide- These conclusions will be discussed latet. spread. The latter deposits are interpreted to Texturally, the sandstone of Unit B is

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TABLE 3. THICKNESS MEASUREMENTS OF INTERBEDDED SANDSTONE AND CONGLOMERATIC SANDY MUDSTONES, UNIT B, BEAR VALLEY FORMATION TYPE SECTION

Top 9.0 -Ss 0.25-Cg 0.15-Ss 0.55-Cg* 0.2 -Cg 5.0 -Ch 0.2 -Ss 0.45-Cg 0.1 -Ss 0.55-Cg 1.4 -Cg 0.65-Cg 0.25-Ss 0.9 -Cg* 0.05-Ss 0.25-Ss 0.2 -Ss 0.7 -Cg 0.1 -Ss 0.4 -Cg* 0.2 -Cg 0.1 -Cg 0.5 -Ss 1.8 -Cg* 0.05-Ss 0.35-Ss 0.1 -Ss 0.9 -Cg 0.05-Ss 0.2 -Cg 0.75-Cg 0.1 -Cg 0.1 -Ss 0.9 -Cg 0.05-Ss 0.25-Ss 0.08-Ss 0.1 -Cg 0.1 -Ss 0.2 -Cg* 0.4 -Cg 0.15-Cg 0.2 -Ss 0.35-Cg 0.1 -Ss 0.1 -Ss 0.03-Ss 1.2 -Cg 0.35-Cg* 1.7 -Cg 1.9 -XX 0.2 -Cg 0.3 -Ss 0.1 -Ss 0.1 -Ss 0.35-Cg 0.05-Ss 5.5 -Cg* 0.7 -Cg 0.4 -Cg 0.45-Ss 0.95-Cg 0.03-Ss 0.25-Ss 0.05-Ss 0.4 -Cg 0.05-Ss 1.3 -Cg* 2.2 -Cg* 0.4 -Cg 0.05-Ss 0.3 -Cg 0.1 -Ss 0.1 -Ss 0.05-Ss 0.6 -Cg 0.03-Ss 1.3 -Cg* 1.35-Cg 1.9 -Cg 6.0 -XX 0.9 -Cg 1.3 -Cg 0.15-Ss 0.6 -Ss 0.3 -Cg 0.05-Ss 4.2 -Cg 0.35-Cg 0.15-Cg 0.65-Cg 0.05-Cg 1.0 -Ss 0.03-Ss 0.05-Ss 0.7 -Cg 0.05-Ss 0.8 -Cg* 0.3 -Cg 0.45-Cg 0.08-Ss 0.8 -Cg 0.2 -Ss 0.05-Ss 0.15-Ss 1.9 -Cg 0.05-Ss 0.9 -Cg* 0.45-Cg 0.65-Cg 0.9 -Ss 0.2 -Cg 0.2 -Ss 0.3 -Ss 0.75-Ss 0.4 -Cg 0.2 -Ss 4.6 -Cg* 0.55-Cg* 1.1 -Cg 2.0 -Cg 0.4 -Cg 0.25-Ss 0.05-Ss 0.2 -Ss 0.1 -Ss 0.1 -Ss 3.0 -Cg* 0.15-Cg 1.4 -Cg 0.1 -Ss o.i -Cg 1.2 -Ss 0.4 -Ss 0.3 -Ss 0.9 -Cg 0.05-Ss 0.65-Cg* 0.1 -Cg 2.0 -XX 0.15-Ss 0.25-Cg 0.8 -Ss 0.1 -Ss 0.1 -Ss 0.15-Cg 0.05-Ss 3.2 -Cg* 0.75-Cg 0.7 -Cg 0.2 -Ss 0.15-Cg 0.1 -Ss 0.2 -Ss 0.25-Ss 1.5 -Cg 0.15-Ss 0.75-Cg 0.3 -Cg 0.5 -Cg 0.05-Ss 0.4 -Cg 0.15-Ss 0.2 -Ss 0.2 -Ss 0.1 -Cg 0.1 -Ss 0.07-Cg 1.15-Cg 0.1 -Cg 0.2 -Ss o.i -Cg 0.1 -Ss 0.35-Ss 0.25-Ss 0.15-Cg 0.05-Ss 0.05-Cg 2.5 -Cg 0.5 -Cg 0.05-Ss 1-5 -Cg 0.15-Ss 0.4 -Sh 0.3 -Ss 0.2 -Cg 0.5 -Ss 1.0 -Cg 0.2 -Ss 3.8 -Cg 0.2 -Ss 0.3 -Cg 0.25-Ss 0.2 -Cg 0.3 -Ss 0.2 -Cg 0.25-Ss 2.7 -Ch 0.1 -Ss ' 0.45-Ss 0.2 -Cg 0.3 -Ss 0.9 -Cg* Bottom

Thicknesses in feet and tenths. Ss = sandstone, indicates a bed containing wedge-shaped crack Cg = conglomeratic sandy mudstone or mud- fillings. Stratigraphic interval marked (+) is illus- stone, Sh = bentonitic shale or zeolitite bed, Ch = trated on Figure 5. Read columns from lower right channel sands, XX = covered interval. Asterisk (*) corner for Stratigraphic succession. generally of fine- to medium-grain size. to very round. Two of the sandstone layers Horizontal lamination of different sand sizes, in the type section also exhibit scour-and-fill with laminae ranging in thickness between structure. 0.05 and 0.5 in., is developed in most of the The conglomeratic mudstone consists of a sandstone layers, and cross-bedding of the matrix of sand, silt, and zeolite generally same size range is present in others. The making up 40 to 50 percent of the rock which grains of a given sandstone bed may be encloses pebbles, cobbles, and even rare poorly sorted when the bed as a whole is boulders. Commonly the gravel fraction is considered, but within individual laminae, crudely graded (Figs. 5 and 6), but lamina- sorting is moderate to good. The rounding tion and cross-bedding are absent. The de- of sand grains of fine and larger sizes is gen- trital components of sand and larger sizes erally very good; even most plagioclase laths exhibit all degrees of rounding, but are sub- have had their corners rounded. In the very angular to round, in general. fine sand and silt sizes, however, the grains The interpretation of Unit B as an alluvial exhibit all degrees of rounding from angular fan deposit seems to explain best its unique

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Figure 5. Alternating beds of sandstone and volcanic mudflow-breccia, Unit B (lower member), type section.

characteristics. The interbedding of sand and Figure 6. Mudcrack fillings, Unit B (lower conglomeratic mudstone probably was caused member), type section. Rock slab shows plan by the deposition over varying periods of view of mudcracks. time of fluvial and air-borne sand spread over the fan; this process was interrupted at fairly yellow (5Y8/4) to dusky yellow (5Y6/4) regular intervals by mudflows which origi- when weathered; the sandstone of the upper nated on the nearby (volcanic) highland, part is a "salt and pepper" color combination flowed down the course of the stream which which appears from a distance to be light built up the fan, and then spread out as even gray (N7) to yellowish gray (5Y8/1). Some layers over the fan's surface. The conglom- of the strata alter to laminae of a greenish hue eratic nature of many of the sandstone beds, —light greenish gray (5G8/1) to dark green- another common feature of Unit B, can be ish gray (5G4/1), which grade into each attributed to intermixing of stream-carried other and into light-gray laminae. sand and gravel, and perhaps wind-blown Lesser thicknesses of sandstone similar to sand as well, on the surface of transportation and correlative with the sandstone of Unit C and deposition of the alluvial fan. The lami- are present throughout most of the area of nation of the sandstone can be explained also the present study. Unit C is interpreted as the as the result of separate episodes of sheet- rock record of the accumulation, probably wash transportation and wind-drifting of under arid climatic conditions, of massive sand. And lastly, the fact that many beds of cross-bedded eolian sand deposits over most mudflow-breccia that are overlain by sand- of the Bear Valley Formation's depositional stone preserve sand-filled mudcracks (Fig. 6) area; this accumulation was contemporan- provides another indication of a subaerial eous with explosive volcanism. The evidence environment of deposition such as an alluvial to support these conclusions is based on fan. three factors: (l) the ubiquitous massive Unit C. Unit C of the type section is a cross-bedding of the sandstone, (2) its min- massively cross-bedded sandstone, 337 ft eralogy, and (3) the shape of its component thick, containing numerous lenses of vol- detrital grains. canic mudflow-breccia in the upper 200 ft. The cross-bedding of Unit C sandstone is The sandstone is well-sorted, fine- to medium- typical of sand dune deposits (Fig. 7); the grained, and cemented with zeolite. Its color work of Hack (1941) records a modern ana- in the lower part of the unit is yellowish gray log of this type of depositional environment. (5Y7/2) on a fresh fracture and greenish According to the classification of cross-

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NORTH

Figure 7. Cross-bedding, Unit C (upper member), type section. somi stratification proposed by McKee and Weir Figure 8. Rose diagram of cross-bedding (1953), most of the Unit C cross-bedding is dip directions, Bear Valley Formation. 180 planar, although some ttough cross-stratifi- measurements plotted in 15° sectors. cation is also present, as well as varieties that are transitional between these two forms. To emphasize these compositional differences, According to the classification of Allen (1963), the term "vitric-volcanic arenite" is used by the cross-stratification is generally of the me to describe the Unit C sandstone, whereas omikron variety, together with some varieties the sandstone of Units A and B is termed transitional to, and in some cases including, simply "volcanic arenite." Reference is made pi-cross-stratification. Allen (1963, p. 110) to Table 1 and Table 2, h-r, for data relating attributes the origin of omikron-cross-strati- to the mineralogy and texture of the several fication to the migration of trains of large- samples of Unit C studied in thin section. scale asymmetrical ripples with essentially The major detrital constituents of Unit C straight crests. On the other hand, Allen are volcanic rock fragments, glass shards, and (1963, p. Ill) thinks that pi-cross-stratifica- plagioclase grains. The rock fragments are tion is the result of the migration of trains of chiefly hyalo-ophitic in texture; the included large-scale asymmetrical ripple marks with ferromagnesian minerals and feldspars within pronounced curved crests. such grains show little alteration, except for The individual sets of cross-strata within magnetite going over to hematite. The feld- Unit C of the Bear Valley Formation type sec- spar grains are also generally fresh. The glass tion average about 6 ft in thickness, but may shards, on the other hand, show varying reach double or even triple this dimension. degrees of devitrification, though for the most The individual beds or laminae within such part their hyaline nature is clearly discernible. sets range from one-sixteenth to 3 in. thick, Distinct differences in rounding are ex- with an average of about 1 in. Cross-bedding hibited by grains of different mineralogies. attitudes, both at and away from the type Glass shards show all degrees of rounding, section, indicate that the prevailing direction though they are for the most part subangular of the winds which transported and deposited to angular; many plagioclase laths are the the sand was from the south and west (Fig. 8). same. Most volcanic rock fragments, quartz, The mineralogic composition of the Unit and ferromagnesian grains, however, are sub- C sand is strikingly different from that of round to round (Fig. 9). Units A and B in that (l) angular glass shards, The striking green hue of Unit C in the all but absent in Units A and B, are a major type section which is characteristic of much constituent of Unit C; (2) the feldspar of the Bear Valley Formation elsewhere, owes (largely plagioclase) content of Unit C is its origin to the presence of streaks and appreciably higher than in either Unit A or irregular patches of a green mineral, too fine- Unit B; and (3) the volcanic rock fragment grained to be identified in thin section, which content of Unit C is considerably smaller is found between and coating rock and min- than that which characterizes Units A and B. eral fragments. This mineral probably was

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buried by moving sand. This would account for the angular nature of the shards and plagioclase laths in otherwise subrounded- to-rounded sand, and for the concentration or absence of glass shards in individual laminae or cross-beds, a commonly observed phenomenon. The dimensions of the volcanic mudflow- breccia lenses interlayered with the sandstone in the upper part of Unit C range from several tens to 150 ft in cross-sectional width and in maximum thickness from 3 ft to 15 ft. The upper surfaces of these lenses are flat (origi- nally horizontal), and the lower surfaces are Figure 9- Typical vitric-volcanic arenite, concave upward. Clearly the lenses are mud- Unit C (upper member), type section. Plane- flow-breccias which "filled in" a dune topog- polarized light. raphy. One further feature of Unit C is wotth formed by the alteration of the volcanic ma- noting. The sandstone is very commonly terials. Gregory (1949, p. 986) reported that marked by a cross-cutting, veinlike, or "box- chemical tests of a green, secondary mineral work," pattern of ridges which stands out in from sandstone sampled in Lime Kiln Gulch sharp relief on the outcrop. The nature and east of Panguitch in the southern Sevier origin of these features are problematical, Plateau (rock which probably underwent the and a thin sectiorl through one of them same history as the Bear Valley Formation, (Table 2, m) offers no clarification. A pos- even though they are not correlative) shows sible explanation is that these box-work that this is a potassium mineral; this fact, ridges represent fractures in the sandstone together with a moderately high birefringence, which were "healed" by cementation similar indicated to Gregory that the green mineral to, but more thorough than, that which is essentially similar in composition to cela- cemented the rock as a whole. The fracturing donite [KAl(Fe,Mg)Si4Oio(OH)2], a clay may have been attendant on the doming of mineral similar to illite (Deer and others, the strata at the type locality subsequent to 1962, p. 216). Rowley (1968), however, ob- their deposition. tained X-ray powder diffraction patterns Unit D. Unit D of the type section is a from slides of the oriented clay, with and 63-ft thick sandy muddy pebble conglomer- without the treatment of ethylene glycol, ate which undoubtedly originated as a vol- which indicated that most of the green clay canic mudflow. The unit is not well exposed, mineral is of the montmorillonite group, but it appears to be a single massive unit perhaps nontronite (Deer and others, 1962, rather than a number of separate lenses. The p. 226-241) with a minor amount of associ- color of the matrix lies between grayish pink ated illite. (5R8/2) and pale red (5R6/2), though the The large-scale cross-stratification of the color of the gravel fraction included in this Unit C sandstone, and the good rounding matrix is extremely variable. and sorting of most of its detrital grains The gravel fraction of Unit D makes up other than the glass shards and plagioclase about 40 percent of the rock and ranges in laths, both indicate eolian deposition. The size from granules to boulders. The sand glass shards and plagioclase are probably the fraction has a median in the fine-sand range principal ejecta of explosive volcanism which and is moderately sorted. Both gravel and took place contemporaneously with the de- sand fractions are characterized by a high position of the sandstone. The volcanic vents percentage of angular clasts; in the sand frac- from which probably the major portion of tion such angularity is especially noticeable the pyroclastic material was erupted are lo- in the feldspar laths. cated within the deposidonal area of the The paucity (about 1 percent) of glass Bear Valley Formation (see below). The shards and the relatively high percentages of ejecta apparently were erupted over much of volcanic rock fragments and feldspar (about the depositional basin and then were quickly 40 percent and 10 percent, respectively) in

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Unit D make it more similar to the conglom- Regional Distribution erates of Unit B than to those of Unit C. and Characteristics Unit E. Unit E is a single, massively cross- stratified sandstone, 352 ft thick. Typical General Statement. Tuffaceous sandstone outcrop appearance is shown in Figure 10. and interbedded volcanics of the Bear Valley In general the sandstone is medium-grained, Formation are exposed over an area in excess moderately sorted, and submature. No con- of 1000 sq mi. This area embraces the north- glomerate lenses or beds are included in the ern Markagunt Plateau and, to the west, the section measured. With the exception of neighboring Red Hills and Black Mountains glass shards and feldspar laths, both of of the Great Basin (Fig. l). which are commonly subangular to angular, In the northern Markagunt Plateau, the the constituent grains are generally subround Bear Valley Formation is exposed nearly con- to round. tinuously over an area of about 300 sq mi The color of the Unit E sandstone, when (Fig. 2). Within this area the northernmost viewed from a distance, is very light gray outcrops of the formation are essentially its (N8) to medium light gray (N6), but on northern limit of deposition. The southern- close investigation the "salt-and-pepper" ap- most outcrops, however, are overlain by pearance of much of Unit C is apparent, and younger volcanic flows that conceal the it can be seen further that the over-all color southern limit of deposition, but the fact is a function of the relative percentages of that these outcrops are only a few tens of feet dark and light grains present within a given thick indicates that the southern limit of stratum. deposition is probably at most only a few The rock is only fairly well indurated and miles further away. can generally be crumbled by hand fairly The eastern limit of deposition of the Bear easily. Locally present in Unit E, however, Valley Formation must lie under the alluvial are box-work ridges similar to those in Unit cover of the Sevier Valley. Gregory (1949, p. C which tend to make the rock less friable. 986) reported the occurrence of green tuf- The definition of Unit E is based on field, faceous sandstone in Lime Kiln Gulch of the not petrographic, relations. Units C and E southern Sevier Plateau that he believed to are separated in the field by the conglomerate be correlative with his Brian Head Formation of Unit D, but there is no similarly sharp dis- (Miocene?). More recent detailed study by tinction between their compositions (Tables Rowley (1968) has shown that these tufface- 1 and 2). Thus, although the mineralogic ous beds actually belong to the upper Claron composition of Unit E is somewhat different Formation (Oligocene?) and that no signifi- quantitatively from that of Unit C, similar cant thickness of arenaceous sedimentary minerals of similar appearance, but in differ- rock correlative with the Bear Valley Forma- ent amounts, are present in both. tion is found anywhere in the Sevier Plateau. Unit E records much the same history out- In the Red Hills, the first basin range west lined for Unit C, with the exception that no of the northern Markagunt Plateau, Threet mudflow-breccias were deposited. (I952a) described 100 to 500 ft of tuffaceous sandstone stratigraphically and lithologically correlative with the Bear Valley Formation which he named the "Cane Spring Forma- tion." North and west of the Red Hills, Rowley has traced the tuffaceous sandstone of the Bear Valley Formation into the Black Moun- tains (work in progress); here the formation

------is only a few tens of feet thick. This seemingly indicates that the westernmost limit of deposition of the Bear Valley Formation is concealed beneath the Escalante Desert (Fig. - 1). ------Markagunt Plateau. The Bear Valley For- Figure 10. Cross-bedding, Unit E (upper mation is well exposed throughout the whole member), type section. northern Markagunt Plateau; indeed, the

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Bear Valley sandstone is probably the most generally only a few feet thick. They de- observable and easily recognizable rock type crease in number higher up. in the whole of this area. But in the northern The rock of the upper 35 ft of the section Markagunt Plateau the Bear Valley is more consists mainly of massively cross-bedded than a thick blanket of sandstone; away from sandstone of "salt-and-pepper" appearance the type section the sandstone is interbedded (with the emphasis once again on "pepper"); with not only considerable amounts of vol- sandstone of yellowish to brownish hue is canic mudflow-breccia, but also thick and/or absent. Volcanic mudflow-breccia lenses and extensive lava flows, tuffs, and at least one beds are interlayered with the cross-bedded ignimbrite. Furthermore, the vents from sandstone. Also present are strata of pebbly which the tuffs and the ignimbrite, as well as sandstone up to 5 ft thick. some of the lava flows, were erupted are Elsewhere within this outcrop area, the located in the northern Markagunt Plateau. sandstone is generally bimodal, of lower About 2 mi northwest of the type section, member lithology, and "salt-and-pepper" in lowlands of relatively little relief have been color. developed on sandstone of the Bear Valley South of the type section, in the area Formation. However, in contrast to the type between the Sevier Valley on the east and the section where the sandstone is separated from line of mountains including Little Creek Peak the underlying Isom Formation equivalent and Bear Valley Peak on the west, and north by only a few feet of locally derived volcanic of about the latitude of Panguitch, lowlands flow breccia, the sandstone in this locality is and low hills have been developed on the separated from the Isom equivalent by 300 relatively weak sandstone of the Bear Valley to 500 ft of interbedded volcanic mudflow- Formation. The sandstone is generally mas- breccia and lava. This sequence of mudflow- sively cross-bedded and commonly some breccias and lavas is considered to be a local shade of yellowish brown or grayish green. member of the Bear Valley Formation. Prob- The thickness cannot be measured here, for ably it was deposited in a local down-faulted no complete section is exposed; incomplete basin or other lowland that developed not sections, however, are up to 200 ft thick. long after the eruption of the ignimbrite Samples from this area studied under the correlative of the Isom Formation. microscope are all of the upper member One section of the sandstone overlying the lithology, and are classified as moderately to local mudflow-breccia and lava member was well-sorted, fine- to coarse-grained, zeolite- measured as 115 ft thick; field study indi- cemented, submature to mature vitric- cates that this thickness is close to the maxi- volcanic arenite. mum for the whole of this outcrop area. About 4 mi east of Little Creek Peak, a Furthermore, field relations suggest that the vitric ignimbrite is found interbedded with sandstone does not extend more than a mile the sandstone of the Bear Valley Formation. or two further north, under the cover of This vitric ignimbrite, because of its strati- younger rock, from this, its northernmost graphic position, is a local member of the outcrop in the Markagunt Plateau. Bear Valley Formation and crops out in All of the sandstone of the measured sec- scattered places over an area only about 10 tion is of the lower member lithology. The sq mi in size; there is no evidence to indicate lower 80 ft of the section consist largely of that it was ever much more extensive. The massively cross-stratified sandstone exhibit- probable vent for this unit is located on the ing textural and color variations in different east side of a small north-south-trending val- cross-bed sets. Most common is fine- to ley in NE1/* NW1/* sec 17, T. 34 S., R. 6 W. medium-grained, moderately to well-sorted The geology of this area is chaotic, but the sandstone with an over-all yellowish-gray cross-cutting relationship of the ignimbrite (5Y8/1) to grayish-orange-pink (5YR7/2) feeder is readily discernible. Further study of color which is made up of grains of widely this vent is planned. differing, though generally pale colors. Per- The ignimbrite itself is a strong, local haps about half as common are cross-bed ledge-former, ranging in thickness from a sets of "salt-and-pepper" (with the emphasis maximum of about 20 ft, where it apparently on "pepper") sandstone. Interlayed with the filled topographic lows, to a feather edge, cross-bedded sandstone about halfway up where it pinched out against highs. Generally the section are a few conglomeratic beds the basal few feet are dark, almost black,

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vitrophyre, and the upper remainder of the and shortly after the eruption of the ignim- unit some shade of reddish-brown with a brite. A K/Ar age determination of biotite texture and appearance similar to that of un- separated from the tuff is 24.0 ± 0.3 m.y. glazed porcelain. (Robert Fleck, 1970, written commun.). Like According to the classification of Cook the ignimbrite, the tuff is another member of (1965), this local member of the Bear Valley the Bear Valley Formation. Formation is a vitric-crystal ignimbrite. Under The tuff member crops out discontinuously the microscope it can be seen to be made up over an area in excess of 25 sq mi and in of a groundmass of flattened and welded places reaches a thickness of over 150 ft. In shards which inclose uniformly distributed, some localities there are two tuff layers with small, generally subhedral phenocrysts (Fig. sandstone between them, while in other 11). The phenocryst content of the rock places the local ignimbrite is interbedded averages about 12 percent; in decreasing with the tuff. On the outcrop, the tuff is order of abundance they are plagioclase generally massive with only the crudest sort (andesine, about 8 percent), augite (14- of bedding developed and with no marker percent), magnetite (0.54- percent), and horizons recognizable. It is generally light in traces of hornblende, sanidine(?), and apa- color, ranging from light gray (N?) to pink- tite. Volcanic rock fragments can also make ish gray (5YR8/1). Commonly studded with up as much as 3 to 4 percent of the rock. small phenocrysts of quartz, feldspar, and The absolute age of this vitric-crystal ig- biotite, the rock also contains numerous nimbrite as determined by K/Ar dating of lithic fragments derived from older eruptive plagioclase separated from it is 23.9 + 0.4 rock; these fragments are commonly a few m.y. (Robert Fleck, 1970, written commun.). centimeters in size. The tuff is generally of At a distance of about 1.5 mi east-northeast relatively low density due to gas cavities that of the local ignimbrite vent just described, range in size from tenths of a millimeter to a there is a second eruptive volcanic vent (SW!4 few centimeters. NE14 sec 9, T. 34 S., R. 6 W.). This lattet In thin section, the tuff can be seen to vent was the source of rather large-scale consist of from 70 to 90 percent of a non- eruptions of a felsic tuff, eruptions which welded groundmass of glass dust and angu- apparently took place both shortly before lar glass shards. Alteration of the groundmass is ubiquitous. In places the alteration totally obscures any original shards which may have been present, although more commonly it obscures the boundaries between shards and dust. The remainder of the rock is made up of subangular to angular phenocrysts of quartz (commonly embayed), plagioclase, sanidine(P) and ferromagnesian minerals, averaging perhaps 0.5 to 0.75 mm in size, but ranging up to 3 mm or more; and of fine- to medium-grained, generally subround to round volcanic rock fragments (Fig. 12). Another possible eruptive vent is located near the two described above (SE!/4 NW/4 sec 5, T. 34 S., R. 6 W.). This vent is dikelike in appearance—a tabular, nearly vertical body about 500 ft long and 20 ft wide. From 50 to 75 percent of the rock of this dike is a pure vitric groundmass, characterized by a complex fracture pattern; the remainder of the rock is made up of sand grains of the lower sandstone member of the Bear Valley Forma- tion (Fig. 13). Apparently fluid magma welled up a fissure incorporating within it sand Figure 11. Local vitric-crystal ignimbrite grains of the lower sandstone member and member. Plane-polarized light. then cooled to form the present vitric dike.

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instances were quickly buried by the moving sand, thus preserving their delicate shapes. Three lines of evidence support the hypo- thesis that the shards and pyrogenic mineral grains of the upper sandstone member were of local origin. First is the fact, already al- luded to, that such fragile shards as are commonly found in the upper member would not have preserved their delicate shapes had they been subjected to prolonged surface transportation by wind or water. Second, the shard content of the Bear Valley sandstone increases markedly from the sandstone strata which underlie the local volcanic members to Figure 12. Local felsic tuff member from those which overlie them. Downwind (to- northern tuff vent (Fig. 2, SWV4 NE1/* sec 9, ward the north and east; Fig. 8) of the vents, T. 34 S., R. 6 W.). Plane-polarized light. and away from the outcrop area of the ignimbrite and tuff, the break between the lower No ignimbrite as such is associated with this and upper sandstone members is almost al- vent; it is possible, however, that explosive ways sharp, so sharp, in fact, that in some ejection of vitric shards took place from it. instances it can be seen within the limits of a Further study also will be made of this pos- single thin section (Fig. 14). Upwind (south sible vent. and west) of the vents, however, the contact In summary, three or more volcanic vents between the two members is usually grada- were active in the northern Markagunt Pla- tional. teau during the time of deposition of the The third line of evidence which suggests Bear Valley Formation and intercalated igne- that the shard content of the Bear Valley ous members with the formation's otherwise Formation is of local origin is that this shard largely sedimentary strata. It is probable that the volcanism which formed the igneous members also contributed the glass shards which serve to distinguish the lithology of the upper sandstone member of the formation from that of the lower sandstone member. From time to time, then, ejecta from these vents rained down on the dunes of the accumulating Bear Valley sand and in some

Figure 13. Volcanic rock fragments in vitric Figure 14. Contact between lower and upper groundmass from dike-like possible eruptive members. Upper member characterized by vent (Fig. 2, SEY4 NW54 sec 5, T. 34 S., R. 6 numerous glass shards, lower member by their W.). Plane-polarized light. absence. Plane-polarized light.

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content is of relatively local distribution. The percentage and angularity of the shards in the upper part of the formation are in roughly inverse proportion to the distance from the volcanic vents described above. Thus, for example, the Bear Valley sandstone on the western edge of the Markagunt Pla- teau, a distance of some 10 mi from the vents, contains a relatively high percentage of glassy clastic material, but none of it is angular; and the sandstone of the Red Hills, another 10 mi or so further west, contains virtually no shards at all. To return to a consideration of the regional distribution and characteristics of the Bear Figure 15. Local felsic tuff member from Valley Formation, it can be noted that the southern tuff vent (Fig. 2, SWV4 SE!4 sec 9, formation thins considerably toward the south- T. 35 S., R. 6 W.). Plane-polarized light. ern limit of its outcrop area. On top of the high Markagunt Plateau proper, in the general the southern tuff was erupted from a vent vicinity of Panguitch Lake, the exposed sec- within its outcrop area, located in or near tions are at most some tens of feet thick. The SWi/4 SE!4 sec 9, T. 35 S., R. 6 W.). The stratigraphic relationships which generally nature of the evidence is simple. The tuff is characterize the Bear Valley Formation con- thick (in the high tens of feet) at and around tinue to hold true even here, however, and this locale, but thins rapidly away from it. the lower and upper sandstone members can There is, then, an area some 3 mi wide be- be distinguished in their proper stratigraphic tween the two outcrop areas; within this area positions, but often the bulk of the sand- complete sections of the Bear Valley Forma- stone exposed is transitional between the two tion are exposed which contain no tuff. members, containing in roughly equal The mountains east of Bear Valley, includ- amounts both the volcanic rock fragments ing Bear Valley Peak, are made up largely of which characterize the lower member and the Bear Valley Formation. No complete sec- the glass shards which characterize the upper tion above the reference plane of the Isom member. Formation equivalent is present, but all in- Felsic tuff is interbedded with the sand- dications are that the sandstone is thickest in stone in the area north of Utah Highway 36 the central part of the mountain mass and and northeast of Panguitch Lake. Locally this thins toward the north and south. tuff is in the high tens of feet thick, though East of upper Bear Valley, the Bear Valley generally it is only a few tens of feet thick. It Formation conformably overlies the Isom crops out discontinuously over an area of equivalent; the lower sandstone member about 20 sq mi. makes up the lower 100 to 200 ft of the ex- It is unlikely that this tuff is the same unit posed section above the ignimbrite. The sand- that crops out further north and which has stone is commonly cross-bedded on a grand been discussed earlier. The two are similar in scale and is usually yellowish gray (5Y8. 1). hand specimen appearance and phenocryst Interbedded with the sandstone in the upper composition, but viewed under the micro- part of the member are about 50 ft of light- scope there are two marked differences be- greenish- gray (5G8 1) felsic tuff. tween them. First, the glass shard/dust ratio The upper member overlies the tuff and of the tuff in the Panguitch Lake area is dis- makes up the greater part of the mountain tinctly higher than that of the tuff to the mass. The upper member strata are at least north. Secondly, the shards of the tuff in the 500 ft thick; they are massively cross-bedded Panguitch Lake area are often only slightly and generally grayish yellow green (5GY7 2) altered, whereas the tuff further north is in- to yellowish gray (5Y8 1). They crop out at variably characterized by a high degree of the crest of the range east of Bear Valley, but alteration or devitrification, or both (Fig. 15). the plunge of the horst structure of the Furthermore, field evidence suggests that mountains brings them down to valley level

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east of lower Beat Valley and conceals beneath downfaulted to what is now the level of the the sutface any tock of the lower member valley floor. At some of these favorably sit- which may be present in the lower patt of uated exposutes the rock is strikingly bright the section. pale green (5G7, 2) to grayish green (10G4/2) Much of the mountain mass west of upper and is quarried for use as building stone and Bear Valley also is made up of the Bear Valley decorative gravel. Formation; here also the Bear Valley con- A much thinner incomplete section of the formably overlies the Isom Fotmation equiv- Bear Valley Formation caps the mountains alent. An apparently complete section of the west of uppermost Bear Valley. Lithologic Beat Valley Fotmation was measured at this evidence, however, indicates the fotmation locale as about 800 ft thick. Immediately thins in this direction, for sandstone of the above the reference plane of the ignimbrite upper member is found only 100 ft or so ate about 55 ft of interbedded sandy pebble above the Isom Formation equivalent. conglomerate, finely laminated silty sand, and The few, small outcrops of Bear Valley conglomeratic sandstone. Overlying these Formation sandstone ptesent in the moun- strata are about 225 ft of massively cross- tains west of lower Bear Valley overlie a bedded "salt-and-pepper" sandstone with an thick sequence of volcanic flow breccias over-all color of pinkish gray (5YR8/1) to which, in tutn, overlies the ignimbrite of light brownish gray (5YR6/1). This sand- Isom Formation equivalency. These sandstone is all of the lower member lithology. stone strata are all of the uppet member Interbedded in the lower part of the member, lithology or of lithology transitional between and filling in a dune topography, are dense those of the lower and upper members. basalt flows a few tens of feet thick. On the western edge of the notthern Mark- Sandstone of similar lithology, likewise agunt Plateau, bordering the Great Basin, the intetbedded with lava flows, is widespread in stratigtaphic picture is similar to that west of the lowlands developed on the western side lower Bear Valley, that is, sandstone overlies of the mountain mass west of upper Bear volcanic flow breccias which, in tutn, overlie Valley, and is present along the front of the the reference ignimbrite of Isom Formation mountains above the Isom Formation equiv- equivalency. At this locality the Bear Valley alent. Formation is only a couple of hundted feet Ovetlying the lower member in the mea- thick and consists entirely of sandstone. It is sured section, and widespread to the west of difficult to be sure of the mineralogy of the the mountain crest, is a massive, 165-ft thick sandstone by examination in thin section be- unit of fine-grained felsic tuff. This tuff is cause of very heavy alteration of most of its petrographically identical with that found constituents. It appears, however, as if all the east of Little Creek Peak, a distance of some samples studied are of the lower member, or 8 mi to the southeast, and in all probability perhaps of lithology transitional between the was erupted from the northernmost of the lower and upper members. tuff vents described earlier. Good exposures Red Hills. The Red Hills are the first basin of the tuff ate also found east of Beat Creek range west of the northern Markagunt Plateau; in the hilly cross-structure which separates the geology of this atea has been studied in uppet from lower Bear Valley. At this loca- detail by Threet (I952a). Threet described tion there are two tuff units, each about 50 ft extensive exposures of a "drab green, well thick, separated by a comparable thickness laminated, cross-bedded sandstone" in the of lower member sandstone. northern part of his area of investigation Massively cross-bedded sandstone 270 ft (Sees. 13, 14, 23-27, 35, T. 32 S., R. 9 W.). thick overlies the tuff in the measured sec- These strata were mapped by Thomas and tion. The lower hundred or so feet of this Taylor (1946) as "undifferentiated Cretace- unit are grayish-yellow-green (5GY7, 2) to ous," but Threet proved them to be Tertiary pale-olive (10Y6, 2), moderately to well-sorted instead. They occupy a stratigtaphic position fine to medium sand of lithology transitional confotmably overlying a regional ignimbrite between the lower and upper sandstone mem- of the Leach Canyon Member of the Quichapa bers. The upper part of the unit is light-green- Formation (Mackin, I960, p. 90-91; the ish-gray (5GY8/1) sandstone of similat tex- "Giant City Fotmation of Threet), which, in ture, but of the upper member lithology, turn, overlies the Isom Fotmation (Threet's Along the eastern edge of these mountains, "Gray Mountains Formation"); they are sandstone of the uppet member has been themselves overlain by volcanic flows and

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mudflow-breccias. Leach Canyon ignimbrites I960). Apparently sandstone was depositing have been dated by Armstrong (1970) as continuously in this area from late Isom time latest Oligocene to early Miocene. Thus the through Leach Canyon time; this deposition sandstone of the Red Hills is correlative with of elastics, near the western edge of the Bear the Bear Valley Formation. Valley depositional area, was interrupted only Threet applied the name "Cane Spring momentarily by eruptions, probably from Formation" to this Miocene(?) sandstone. vents in the Great Basin, of ignimbrites As this name was used by Threet only in his which spread across extensive areas of south- thesis and never by himself or others in sub- western Utah and eastern Nevada and had sequent publications, it has no status in their eastern margins near the western edge stratigraphic literature (American Commis- of what was later to become the Markagunt sion on Stratigraphic Nomenclature, 1961, Plateau. p. 653). Therefore I cannot suggest that the The lowest stratigraphic position of sand- name be "abandoned." I do suggest, how- stone correlative with the Bear Valley Forma- ever, that the strata described by Threet as tion is between the two uppermost cooling the ' 'Cane Spring Formation" in the Red Hills units within the Bald Hills Member of the area be included in the Bear Valley Formation Isom Formation; cooling units are assigned as the latter is defined in this paper. I do this to the Bald Hills Member on the basis of because the strata in question are lithologically detailed petrography recently completed by similar, if not identical, to, and occupy the Rowley (1969, written commun.). The re- same stratigraphic position as, the Bear Valley maining four stratigraphic positions of the Formation. sandstone are below, between, and above the Threet attributed the accumulation of the three cooling units within the Leach Canyon Bear Valley Formation strata in the Red Hills Member of the Quichapa Formation. The to deposition of probably wind-blown sand uppermost ignimbrite of the Leach Canyon in a local basin of standing water related in its Member is believed by Rowley to be local in origin to drainage obstruction by accumulat- areal extent; he has found it only in the ing volcanic rock in surrounding areas. He northern Black Mountains. Sandstone of reported that the greenish, laminated sand- Bear Valley lithology which may be con- stone can be traced southeastward and found sidered to belong to the Bear Valley Forma- to grade laterally into tuff which closely re- tion proper overlies this uppermost Leach sembles the sandstone, except for lack of Canyon unit and, in turn, is overlain by a lamination or other stratification. Threet thick sequence of volcanic flows and mud- noted the most striking feature of the sand- flow-breccias. stone in thin section—that it contains no Microscopic study of the different sand- glass shards or dust, but is made up of "well stones in the Black Mountains shows that sorted, rounded and angular grains, 0.1-lmm they may be rather closely correlated with the in diameter, of feldspar, quartz, augite, horn- two sandstone members of the Bear Valley blende, quartzite, red felsite, and black ob- Formation in the Markagunt Plateau. The sidian" (Threet, 1952a). Rowley (1969, writ- lower two sandstone units in the Black ten commun.), however, has found sand- Mountains, like the lower sandstone member stone of the Bear Valley Formation in the of the Markagunt column, are characterized Red Hills which does contain glass shards. by a high percentage of rock fragments (about Thus it appears that rock transitional be- 30 percent) and a negligible glass-shard con- tween the lower and upper sandstone mem- tent. The uppermost sandstone unit, on the bers of the Bear Valley Formation and of the other hand, is marked by the presence of a lower member proper are both present in the high percentage of glass shards (20 to 30 Red Hills area. percent) and a low percentage of volcanic- Black Mountains. Sandstone correlative rock fragments. The two middle sandstone with the Bear Valley Formation occupies at units are transitional between the lower and least five separate stratigraphic positions in upper units, though they are closer to the the Black Mountains, which lie immediately lower unit in that they contain a relatively to the north and west of the Red Hills. Thin high volcanic-rock fragment fraction and a (low tens of feet) beds of sandstone are relatively low glass-shard fraction. sandwiched between regional ignimbrites of The westernmost exposures of strata cor- the Isom Formation and the Leach Canyon relative with the Bear Valley Formation are Member of the Quichapa Formation (Mackin, found in the extreme southwestern Black

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Mountains, in the vicinity of Jako Wash sitional area of the Bear Valley Formation by (JSP/2 sec 36, T. 32 S., R. 12 W.). Here the the presence of the plutonic dome mentioned sandstone is found between two ignimbrites above. To the south and west there were ap- of the Bald Hills Member of the Isom For- parently no equally sharply defined deposi- mation. The sandstone may be traced north- tional boundaries; instead the formation ward from Jako Wash to the southern end of simply thinned to nothingness from its source Little Horse Valley (SE!4 sec 2, T. 32 S., area directions. R. 11 W.), where it is found interbedded with Proof of the age, the nature of the source strata of the Leach Canyon Member of the material, and the accumulation as a blanket Quichapa Formation. sand-dune deposit of the bulk of the Bear To the south and southwest, in the Mud Valley Formation has been discussed earlier. Spring Hills and Bald Hills, no strata cor- That the area of deposition was one of relative with the Bear Valley Formation are moderate relief is shown by the fact that there found, even though rocks of the correct are no significant, sharply defined differences stratigraphic position are exposed. In the ex- in the thickness of the formation over the treme western part of the Black Mountains, whole of its outcrop area. Instead, the for- only rock younger than the Bear Valley mation thickens more or less uniformly to- Formation crops out; this is true also for ward the north and east from its "feather most of the central Black Mountains. On the edge" southern and western limits. northern flank of the Black Mountains, an The northern and eastern depositional area which has been mapped in reconnaissance limits of the Bear Valley Formation are more by Erickson and Dasch (1963, 1968), rocks of complex, however (Fig. 16). Key to the de- the correct stratigraphic position are rarely scription and understanding of these bound- exposed. Rowley (work in progress) reports, aries is the presence of a large intrusion of however, that on the south side of Utah State middle Oligocene age immediately north of Highway 21 in Minersville Canyon (NW/4 the northern limit of the Bear Valley Forma- NW'4 sec 16, T. 30 S., R. 9 W.), the Bear tion. This intrusion, which today crops out Valley sandstone is about 10 ft thick, though over an area in excess of 20 sq mi (the extreme less than 1 mi to the north it is absent. Finally, southeastern corner of the outcrop area is Bear Valley Formation strata have not been shown on Fig. 2), is named the Spry Intrusion mapped to the north in the Mineral Moun- from the one-time crossroads community tains (Liese, 1957; Earll, 1957). near which it is located and according to local usage by which it is called the "Spry Depositional History Stock." A later paper will describe in detail The Bear Valley Formation formed during the Spry Intrusion, which is probably a lac- a relative lull in the volcanism which blan- colith, and the conglomerates and flow- keted south-central Utah during the Oligo- breccias derived from it. In general, it is cene and Miocene. Made up largely of multiple in nature, consisting of rock which reworked volcanic detritus and volcanic pyro- ranges from monzonite to dacite porphyry; clastic ejecta, it accumulated under arid cli- a K-Ar age of hornblende from the intrusion matic conditions as a wind-blown blanket has been determined to be 30.4 + 1.5 m.y. sand-dune deposit over an area of low to (Damon, 1968, p. 42). Furthermore, the Spry moderate relief in excess of 1000 sq mi. The Intrusion clearly formed a topographic high northern limit of deposition in the area of against which the ignimbrites which preceded what is today the Markagunt Plateau, which the deposition of the Bear Valley Formation also marked the site of maximum deposi- lapped and on which no deposition of Bear tional thickness, was a more-or-less latitudinal Valley sediments took place. This last con- fault scarp with relief in excess of 1000 ft. clusion is supported by field mapping which This fault was located immediately to the shows the absence of the Bear Valley Forma- south of a large plutonic dome of Oligocene tion and the ignimbrites in the vicinity of the (pre-Bear Valley Formation) age. Spry Intrusion. The eastern limit of deposition may have The uplift caused by the Spry Intrusion been either a longitudinal fault scarp or, more thus formed a natural obstacle to northward likely, a thick tongue of volcanic mudflow- extension of the sediments of the Bear Valley breccia derived from some source to the Formation blown from the south and west. north and deflected eastward from the depo- In addition, shortly after (or perhaps con-

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Figure 16. Geologic setting for accumulation of Bear Valley Formation type section current with) the emplacement of the Spry or zone of faults brought the Miocene(?) Intrusion, a fault, or zone of faults, formed breccias into contact with the Eocene(?) along east-west lines immediately south of Claron Formation. the intrusion (Fig. 2). The fault, itself down- As was stated earlier, no strata of the Bear thrown to the south, is today concealed, but Valley Formation are found in the Sevier its presence and sense of displacement are Plateau which lies only a few miles east of the certain. North of the type area, the north- Markagunt Plateau. It would seem that there dipping Bear Valley Formation is conform- must have existed some sort of barrier which ably overlain by a thick sequence of volcanic prevented the eastward spread of the wind- mudflow-breccias. These younger strata, in blown Bear Valley sediments. This barrier turn, are juxtaposed next to flat-lying strata was probably a thick accumulation of volcanic of the pre-Bear Valley Claron Formation flows and mudflow-breccias which spread (Eocene?) in what is in all probability a fault southward from a source north of the Spry contact. The Claron strata also are covered by Intrusion, but were deflected toward the east post-Bear Valley breccias, but there are no by the topographic high of the Spry intrusive Bear Valley strata between the two. It, there- dome. Mapping by Rowley (1968, Ph.D. fore, seems probable that the fault was active thesis, Univ. of Texas at Austin) in the Sevier prior to the deposition of the Bear Valley Plateau has shown such a thick sequence (in Formation, so that the wind-blown sand ac- excess of 1000 ft) of breccias and flows cumulated to its maximum thickness against occupying the stratigraphic position repre- the south-facing fault scarp, yet was not able sented by the Bear Valley Formation in the to lap onto the upthrown block to the north. Markagunt Plateau. This would indicate that the fault had a It also seems probable that some of the minimum throw equalling the thickness of early breccia flows were deflected to the west the type section, that is, more than 1000 ft. of the Spry dome. Evidence for this conclu- It is not certain, however, whether the mud- sion is found in the fact that between 300 and flow-breccias which buried the Bear Valley 500 ft of breccias underlie strata of the lower Formation flowed over this fault scarp or sandstone member of the Bear Valley Forma- whether later movement along the same fault tion about 2 mi northwest of the type section.

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Deposition of the eolian sandstone of the which zeolites are most readily derived Bear Valley Formation was ended during the (Deffeyes, 1959; Hay, 1966). Thin-section Miocene(?) by the inundation of the north- study of the sandstone with a high glass ern Markagunt under a great mass of volcanic shard content corroborates this; such study flows and mudflow-breccias. The exposed reveals that the shards and the zeolite cement thickness of these later volcanic strata is commonly grade into each other much as greatly varied in the northern Markagunt would be expected if the zeolite were derived Plateau and the adjacent area of the Great from surface dissolution of the shards. Fur- Basin; it ranges from zero to more than 2000 thermore, it is not to be expected that the ft. Much of the difference, however, is the sandstone, especially that displaying large- result of erosion; all indications are that the scale cross-bedding, ever contained a large original thickness was more than 1000 ft, and clay-sized fraction which was then altered to probably closer to 2000 ft. zeolite during diagenesis. Such fines were probably winnowed out and carried away by Cementation winds strong enough to impose cross-bedding The sandstone of the Bear Valley Forma- on the sandstone such as it now displays. tion is marked almost universally by the It might be noted, however, that the vol- presence of zeolite cement; the development canic mudflow-breccias which are now made of the cement varies from the merest trace to up of a very high percentage of impure zeolite the complete filling of all void spaces (Fig. probably did contain a high clay-sized detrital 17). Zeolitite beds that are altered ashfalls are fraction. Much of the present zeolite matrix found also within the formation. Three X-ray of the mudflow-breccias, therefore, may have analyses of the zeolite, two of them from been derived from the alteration of a primary specimens of zeolitite beds and one from mud, a large percentage of which probably cement separated from a sandstone, reveal was vitric volcanic ash. that the zeolite is clinoptilolite (NaAlSi.j.2-s Besides the presence of hyaline parent ma- OIQ.4-123.5-4H2O; formula standardized in terial, the second factor generally held to be a terms of a pure sodium end member having prerequisite for zeolite formation is alkaline one aluminum atom; Hay, 1966). water to dissolve the glass, from which so- The presence of this zeolite cement in the lution zeolite can then crystallize (Stringham, Bear Valley Formation certainly is not an 1952; Coombs and others, 1959; Morey and unexpected one; indeed, it would seem that Ingerson, 1937). Marshall (1961) noted that conditions more favorable for the formation such alkalinity is a normal result of the of a zeolite than those involving these strata hydrolysis of glass, which led Deffeyes (1959) could hardly be asked for. First, the sand- to point out that the possibility exists that stone of the entire formation contains a visibly the alkaline conditions necessary for dissolv- high percentage of glass shards and hyaline ing silica and producing zeolites can be pro- rock fragments, these providing the parent duced by the hydrolysis of glass after material generally agreed to be the one from deposition, and that zeolites are not necessarily indicators of alkaline depositional environments. In the case of the Bear Valley Formation, however, yet another source of water with a high pH can be postulated. According to Axelrod (1950, p. 242-243), the late Miocene and Pliocene epochs were characterized in the region of the Great Basin by an increas- ingly arid climate. Furthermore, with the beginning of large-scale block faulting during the same (or earlier) time span, this region became the site of restricted basins marked by internal drainage. The net result of these two factors seemingly would have been to produce local conditions of increasing alka- Figure 17. Zeolite (clinoptilolite) cement. linity and salinity, conditions which could Crossed nicols. have been reflected in the water which came

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into contact with the Bear Valley Formation sandstone members can be recognized within after its burial. the Bear Valley Formation; each of them Finally, one more theoretical aspect of sheds light on the geologic history of the zeolite formation should be mentioned. area of deposition. Both members record the Bramlette and Posnjak (1933), in a discussion accumulation under arid climatic conditions of zeolite alteration of pyroclastics, suggest of massive, cross-bedded sand deposits blown that clinoptilolite is an intermediate stage in in by winds from the south and west. The the process of alteration that finally results in lower member reflects the reworking of older a typical bentonite composed of a clay min- volcanic rock during a lull in the late Oligo- eral, though they do not imply that all cene and early Miocene volcanism of the bentonites have gone through such an inter- northern Markagunt region. The upper sand- mediate stage. They point out that the chem- stone member, on the other hand, records ical composition of clinoptilolite is close to continued accumulation of sand during the that of alkaline volcanic glass, whereas the Miocene contemporaneous with a renewal of composition of a bentonite composed of local volcanism. This volcanism provided montmorillonite shows relatively less silica glass shards and other pyroclastic debris and alkali and relatively more alumina and which became incorporated with reworked magnesia. From these chemical relations they volcanic detritus, thereby giving the upper conclude that there is theoretical support for sandstone member a lithology different from the idea that zeolite is an intermediate prod- the shard-free lower member; it also produced uct and that a clay mineral represents a later local ignimbrite and tuff members within the stage of more complete alteration. These Bear Valley Formation. theoretical considerations may have some Deposition of the Bear Valley Formation factual basis in the Bear Valley Formation, was ended by an inundation of at least 2000 ft for it may be that the green clay mineral of volcanic flows and mudflow-breccias. Fol- which commonly colors the sandstone of the lowing its burial, the sandstone of the Bear Bear Valley Formation is indeed an alteration Valley Formation was cemented in varying of zeolite to a member of the montmorillonite degrees by clinoptilolite, a zeolite formed by group. Further work is needed, however, be- the dissolution of the vitric components of fore this relationship can be demonstrated the formation and subsequent recrystallization. to exist. ACKNOWLEDGMENTS CONCLUSIONS The present paper is the first report on the During the early and middle Tertiary, the results of field and laboratory geologic in- tectonic forces which formed the structures vestigations of the Cenozoic geologic evolu- of what are the present-day southern High tion of the southern High Plateaus of Utah Plateaus of Utah were not yet operative. which my colleagues, students, and I have Instead, this region was apparently one of carried out during the years 1963-1970 and low relief. Volcanism began in the late Eocene which are continuing. This larger investiga- and continued intermittently through the tion began as a Ph.D. dissertation undertaken Pleistocene, covering the area with a blanket at the Department of Geological Sciences of of thousands of cubic miles of volcanic The University of Texas at Austin. Generous matetial. financial support for the field, laboratory, and A relative lull in volcanic activity marked academic phases of this work during the years late Oligocene and early Miocene time in the 1962-1965 was provided by the National southern High Plateaus. The Bear Valley Science Foundation, the Penrose Bequest to Formation, largely a blanket sand-dune de- The Geological Society of America, the Shell posit, accumulated during this lull over an Oil Company, and the Department of Geo- area of some 1000 sq mi, which today in- logical Sciences of The University of Texas at cludes the northern Markagunt Plateau and Austin. In the time since the completion of the adjacent portion of the Great Basin. the dissertation, further research has been Field studies reveal that the sand of the funded by the National Science Foundation Bear Valley Formation accumulated in an (Grants GA-1098 and GA-11081) as supple- extensive structural and physiographic basin mented by Kent State University. I wish to within which volcanism was contemporane- express my profound gratitude to these many ous with sedimentation. Two clearly defined generous benefactors.

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The late J. Hoover Mackin of the Depart- alteration of pyroclastics: Am. Mineralo- ment of Geological Sciences, The University gist, Vol. 18, p. 167-171, 1933. of Texas at Austin, supervised my graduate Callaghan, E. Volcanic sequence in the Marys- study and continued to offer aid and advice vale region in southwest-central Utah: in all phases of the since-expanded investi- Amer. Geophys. Union 20th Ann. Meeting Trans., Part 3, p. 428-452, 1939. gation until his recent untimely death. My Callaghan, E.; and Parker, R. L. Geology of debt to him is incalculable. the Delano Peak quadrangle, Utah: U.S. Peter D. Rowley of the U.S. Geological Geol. Survey Map GQ-153, 1962. Survey has been associated with me in the Cook, E. F. Stratigraphy of Tertiary volcanic High Plateaus project since its inception; rocks in eastern Nevada: Nevada Bur. Mines special recognition is due him for his contri- Rept. 11, 61 p., 1965. butions to the present study as well as all Coombs, D. S.; Ellis, A. J.; Fyfe, W. S.; and other aspects of the investigation. I also wish Taylor, A. M. The zeolite facies, with to express my gratitude to Professor Robert comments on the interpretation of hydrothermal syntheses: Geochim. Cosmochim. Fleck of the Department of Geology of The Acta, Vol. 17, p. 53-107, 1959. Ohio State University for the absolute age Damon, P. E.; and others. Correlation and determinations he has made of volcanic rock chronology of ore deposits and volcanic units within the Bear Valley Formation; and rocks: Ann. Progr. Rept. No. COO-689-100, to Robert L. Folk of The University of Texas Contract AT (ll-l)-689, Research Div., at Austin and A. Gordon Everett of the U.S. Atomic Energy Comm., 75 p. + ap- United States Department of the Interior for pendices, 1968. their counsel both during and since my dis- Deer,W. A.; Howie, R. A.; and Zussman, J. sertation study. Lastly, I wish to acknowledge Rock-forming minerals, Vol. 3, sheet sili- the research on the geology of the east-central cates: John Wiley & Sons, Inc., 270 p., New Yotk, 1962. Markagunt Plateau which has been carried Deffeyes, K. S. Zeolites in sedimentary rocks: out by Thomas livari, a graduate student in J. Sediment. Petrology, Vol. 29, p. 602- the Department of Geology of Kent State 609, 1959. University; it has been his efforts which have Dutton, C. E. Topographical and geological carried the mapping of the Bear Valley For- atlas of the High Plateaus of Utah: U.S. mation to its southern outcrop limit. Geog. Geol. Surv., Rocky Mtn. region, sheet 2, 1879. Dutton, C. E. Report on the geology of the REFERENCES CITED High Plateaus of Utah: U.S. Geog. Geol. Allen, J. R. L. The classification of cross- Surv., Rocky Mtn. region (Powell), 307 stratified units, with notes on their origin: p. 1880. Sedimentology, Vol. 2, p. 93-114, 1963. Earll, F. N. Geology of the central Mineral American Commission on Stratigraphic Range, Beaver County, Utah: Ph.D. thesis, Nomenclature. Code of Stratigraphic Univ. of Utah, Salt Lake City, Utah, 112 nomenclature: Amer. Ass. Petrol. Geol., P., 1957. Bull., Vol. 45, p. 645-665, 1961. Erickson, M. P.; and Dasch, E. J. Geology Anderson, John J. Geology of northern and hydrothermal alteration in northwestern Markagunt Plateau, Utah: Ph.D. thesis, Black Mountains and southern Shauntie Univ. of Texas at Austin, Texas, 206 p., Hills, Beaver and Iron Counties, Utah: 1965. Utah Geol. Min. Surv., Spec. Studies 6, Anderson, John J. Oligocene and Miocene (?) 32 p., 1963. volcanic arenite sedimentation, southern Erickson, M. P.; and Dasch, E. J. Volcanic High Plateaus, Utah [abstr.]: Geol. Soc. stratigraphy, magnetic data and alteration, Amer. Rocky Mountain Sect. Ann. Mtg. geologic map and sections of the Jarloose Prog., p. 24, 1968. mining district southeast of Minersville, Armstrong, R. A. Geochronology of Tertiary Beaver County, Utah: Utah Geol. Min. igneous rocks, eastern Basin and Range Surv., Map No. 26, 1968. Province, western Utah, eastern Nevada Folk, R. L. Stages of textural maturity in sedi- and vicinity, U.S. A. :Geochim.Cosmochim. mentary rocks: J. Sediment. Petrology, Acta, Vol. 34, p. 203-232, 1970. Vol. 21, p. 127-130, 1951. Axelrod, D. I. Studies in late Tertiary paleo- Folk, R. L. The distinction between grain size botany: Carnegie Inst. Washington Pub. and mineral composition in sedimentary 590, Contr. Paleontology, 323 p., 1950. rock nomenclature: J. Geol., Vol. 62, p. Bramlette, M. N.; and Posnjak, E. Zeolite 344-359, 1954.

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