View of Location# I at Phonolite Hill

ABSTRACT

Phonolite Hill is a late Miocene volcanic center that is composed of at least six vents (three are still visible), which ejected clast-rich pyroclastic material. These eruptions were followed by the intrusion of a rhyolite dome, which is the topographic feature that Phonolite Hill is named for today.

Previous work showed the important relationship between timing of eruptions at

Phonolite Hill and initiation of Basin and Range extension in the High Plateaus of southern Utah. The volcanology of this area is also important in understanding the evolution of small-silicic volcanic centers, and the relationship of these centers to

actively extending regions.

The pyroclastic deposits, which are adjacent to the rhyolite dome, give information about the eruptive and depositional processes that were once active at

Phonolite Hill. The margin of the rhyolite dome shows a variety of contact relationships

with the pyroclastic units, and demonstrates significant interaction between solidified

magma and country rock, with the formation of mixed breccia zones. This margin provides information on the mechanism of intrusion and the interactions that took place

between the rhyolite and the pyroclastic rocks.

The lithologies and bedding structures, in particular base surge deposits, visible

within these rocks indicate the presence of groundwater, which was interacting with

ii magma during the eruptive process. The groundwater fueled phreatomagmatic eruptions that formed tuff cones and possible tuff rings around the six volcanic vents.

The abundant lithic fragments found in the pyroclastic beds provide information about the pre-emptive stratigraphy that was present at Phonolite Hill. These clasts were analyzed geochemically and compared with known volcanic deposits in the Marysvale volcanic field. Clasts indicate the former presence of distinctive ash-flow tuffs as well as clasts comparable to known rock units.

The 9.00 Ma age of a vesicular basalt mega-clast on the northern side of Phonolite

Hill indicates the presence of a basalt lava flow at the bottom of Kingston Canyon 9 million years ago. This implies that Kingston Canyon had already been formed by that time. Kingston Canyon was cut during uplift of the Sevier Plateau, than the Sevier

Plateau and must, therefore, have been uplifted by 9 Ma.

iii

Figure I. I: Map showing the four major physiographic provinces in Utah. This study focuses on the Colorado Plateau Transition Zone. Actual location of this study, Kingston Canyon, is shown by the X.

112'15' 112'00' 111°45'

Awa pa

Plateau

legend 0l • Trp - Rhyolite of Phonolite Hill • Trd - Rhyodacite of Dry Lake • Trf - Rhyolite of Forshea Mountain

Aquarius

Plateau

0 5 10 kilometers 38'00'.______.__L..- ______, 3°'0 00' 112'15' 112'00' 111°45'

ift1 c"4W....___.11'-"3W'-'---''"'-12W:.:..;..._~ 42

...... ,. 41N _____

40N Figure 1.2: Map of late-Miocene, silicic, volcanic deposits found in Kingston Canyon. All three volcanic centers are located on the Sevier Plateau, however, the 39N Rhyolite of Phonolite Hill is the only center within Location of Kingston Canyon. From Rowley (1981). • ' larger map 38> 38N

Figure 2.1: Map of the High Plateaus region of southern Utah. The High Plateaus are part of the Colorado Plateau Transition Zone, and record evidence of late stage Basin and Range extension (<10 Ma). One of the major features in the High Plateaus is the Marysvale volcanic field, which is shown above outlined in black. Modified from Rowley (1994).

MONTANA

WYOMING

COLORADO

ARIZONA NEW MEXICO

Figure 2.2: Map showing southward sweep of the • Challis Magmatic Belt (53 - 43 Ma) four major mid-Cenozoic magmatic belts of the Tuscarora Magmatic Belt (43 - 37 Ma) western United States. From Christiansen and Sierra-Wasatch Magmatic Belt I (37 - 34 Ma) Yeats (1992) Sierra-Wasatch Magmatic Belt II (34 - 21 Ma)

110"

100 200 300 km

Montana

Oregon

Wyoming

Utah

Arizona

110 · 35'

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)0 Figure 2.3 : Map showing the extent and orientation of major Basin and Range faults in the western United States. From Christiansen and Yeats ( 1992). Cross-section at 39.5 from Wannamaker et al (200 I).

Stratigraphy of the Marysvale Volcanic Field

AGE SEQUENCE IGREAT BASIN I MARYSVALE VOLCANIC FIELD AND VICINITY 0 PLEISTOCENE 1.8

PLIOCENE

5 5.3 Localbasalt Sevier River Formatlon Local high-silica and other basin-fill r11yollle sedime tary rocks

15 MIOCENE

lava 5 0 onroe eak Calder Quichapa 23.0 Group Osiris Tuff U ana eak OW Turt' Member N~ W 25 O...Jcnu Q) u 0 f,r!innl~ Tulf Z;2:JZ u Mtrnh~ Albinus Can on W...JOW Q) c: r Uo<{UJ:J .s Q) lsorn Fm. 'jij 0 nree ree 5 UJ'ZO "

33.9 35 Brian Head Forma tion and related units

EOCENE 40 Claron Formation

45....L----....L------l.----'------l.------' Figure 2.4: Stratigraphy of the Marysvale volcanic field and adjacent areas. Modified from Rowley (1994)

16 Stratigraphy of Kingston Canyon

AGE SEQUENCE I STRATIGRAPHY I o PLEISTOCENE -1.8- Sevier River PLIOCENE Formation

I-- 5 --5.3- I li nYO I ~t ~~ nl tt A = Hill 'iij3~8 I l2aai II"UIYOIIWAO '> -5,25- Loca l 0°(/) tn-"'51 8a,s. of -Z~ 1---- KillgSlons:;:::::> Basalt 2~Ow C 8f1yOI'l Flows 10 - O>-W O ZO:::ZZ 1""Yoo , ~P~. .'> w~Qw O~--l~ O:::W~a r----- W~OW o..-O(/) 0..0::2: ~W- 1----- (/)lll r--- 15 - MIOCENE UiS<- .gai . c.::3 ' Potassium -rich ~~ Basaltic Rocks '8 ~ " - '" '" 20 ~~ ; '" --Osiris--- Tuff--- 1---23.0- 0 OW 1----::- Mount Dutton N?; W Formation 25 - O--l(/)O AntImoo TuftMeO'lDer z~~z IllJS~, ~~C~~,y on w--l0w ) 0Q."' ..2 '"

1---33.9- 1---- 35 - Claron >- oO:::w Formation o:::o~o wNZ z ~ow~ EOCENE OZ~a --lWCiw 40 - ow(/) (/)

Figure 2.5: Stratigraphy ofKingston Canyton, Utah. Modified from Rowley (1994) . 17

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Figure 4.2 Continued b: Location #6 Unit Lithology

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.;,. ... :.-:.·:.· :.· \ 0 ft Continued 35 Figure 4.2 Continued

c: Location #16 Stratigraphic Symbols 120 ft -, Unit Lithology Structures Lithofacies A: Mega-clast & tuff \ ~ ~.::~ ~ ~.~:~ ~ ~.~:~ ~ ~-~:~ ~ ~-~:~ ~ . breccia .. 8 9 ~n~nnnmim 15' B: Massive lapilli tuff,& lapillistone ... C· Planar-bedded tuff, lapilli tuff, · & lapillistone p'

• 0 26' B, C D: Cross-bedded lapilli tuff ~ ~;f!,ti~!i!~i I. ·-· - • -• 90 ft 8 Fine grained tuff and lapilli p ·o· ,•, . EZ2J tu ff . .~ .. ~ ... , ' 7 l·:: :-:_,_,;:::.,;_,_,_... ,.:, 4' 8 c l ;)~;( ;~:;~;~:j Medium to coarse grained :.:_:~{:>\A:Hf~ -. lapilli tuff [WJ.J Tuff breccia 60 ft 6 :~,'i'~[\i!J! 29' 8, C .. D Rhyolite breccia ::;: -~·:::;:_:·{:·.-:~·;. :.-·::·~: ~~ ~ ~', Rhyolite 5 (')/i:\(.// 6' 8 ~ Cross-bedding 14' B, C 30 ft 4 \it@,~'''~:·';. Planar-bedding . ___,..._ Ballistic clasts/sag pits 3 t·\: :'(! .:~\·\/: 6' 8 Grey obsidian rich-bed • 8 2 rAf'ii,:·rm;·') • 15' : • Angular polymict clasts Mud cracks . :·.·_:.:.:::·:: . 0 ft 1 3' 8 ~ Channel fill deposits

4.2.2 Location #6 (Southwest Side of Phonolite Hill)

The lower pot1ion of Location #6 (Figure 4.7) probably overlaps with the upper portion of Location #1 (~20-40 ' overlap), although beds cannot be traced between the two. The beds at this location are quite similar to those seen at Location # 1. There the degree of induration is much greater, and the surface has weathered to a light brown, which makes structures much less obvious than those seen at Location #1. The rocks consist of light brown, lithified, and poorly ~orted lapilli tuff, lapillistone, and tuff

36 Figure 4.3: View of Location# I at Phonolite Hill. The bedded pyroclastic sequences are dipping into the hill. This is the location of stratigraphic column A in figure 4.2. View is to the northwest.

Figure 4.4: Photo of anti-dune structure at Location # I. Beds thin drastically as they are traced right across the photo and over the crest of the dune. Crest is at arrow. Flow direction is Hammer for scale. l

37 ~ Ballistic Cl~ ·

Figure 4.5: Ballistic and entrained clasts within the pyroclastic deposit at Location # 1. Pencil and field notebook for scale

Figure 4.6: Small-scale faulting, and offset of laminer beds within the pyroclastic deposits at Location# I. Hammer handle for scale. l

Figure 4.7: Photo showing the outrop of Location #6. Location includes light colored deposits on the left side of photo (left ofred line), as well as the tree covered deposits in the center of the photo (right of red line). Figure 4.8, shows a close up photo of the contact between the the light and tree covered units. Person for scale.

Figure 4.8: Contact (red line) between tuffbreccia (Tvb), and the intra-

vent deposits (Tpc2) at Location #6. Bedding dips to the left (NW) in this photo (evident at arrow). Hammer for scale.

40 Figure 4.9: Near vertical beds at Location #8. Beds are composed of lapilli tu ff and lapillistone. View is to the north. Brunton for scale.

Figure 4.10: Large fine-grained lapilli tu ff block entrained in coarser lapilli tuff/ lapillistone at Location #8. Hammer for scale

Figure 4.11: Large ballistic clast (7') at Location #8. Beds on right side of clast show drag that formed by the impact of the clast. Chisel is approximately 6" long.

Figure 4.12: Near vertical bed (outlined) of lapilli stone cross-cut by fine grained tuff. In this photo the tu ff shows fine laminatiions. Theis bed also contains ripple-like structures. Hammer handle for scale.

Figure 4.13: View of outcrop of Location # 11 , showing alternating coarse and fine grained beds. View is to the south-southwest and dip is to the southeast. Photo is from the NE end of the outcrop. Person is 6' tall.

Figure 4.14: Coarse lapillistone bed within the deposit of Location # 11. In this photo the bed is inversely graded. Major clast is white/grey obsidian. Photo islfrom the SE end of the outcrop. Hammer for scale.

J Figure 4.15: Outcrop at Location #14 showing alternation ofcoarse and fine lapilli tuff. Unit is primarily massive, although some bedding is evident in the deposit. Field notebook is approximately 7" long.

Figure 4.16: Outcrop at location# 16 showing bedded and massive lapilli tuffs and lapillistone units. Beds are dipping to the east-northeast, view is to the west-southwest. Outcrop is part of the middle portion of stratigraphic column C in figure 4.2. Hammer for scale l

Figure 4.17: Ballistic clast possibly giving direction of vent to the west or southwest. Impacted bed outlined. Arrow showing impact direction. Hammer for scale.

Figure 4.18: Finer grained bedded deposits on the eastern side of Location # 16. These beds are primarily fine lapilli tuffs. Hammer for scale.

48 Figure 4.19: Bedded deposits at Location # 17. Thinly laminated beds with thicker massive lapillistone beds. Dominant clast is grey obsidian. Slumping is evident in the laminated beds in the center of the photo (arrow). Hammer for scale.

-----:... )

Figure 4.20: Massive to bedded coarse lapilli tuffto tuffbreccia. Clasts are almost completely composed of grey obsidian. Hammer for scale.

Figure 4.21: Low-angle cross-bedding at Location # I 8a (Truncation of bedding shown by arrow). Rocks are fine lapilli tuff with primarily grey obsidian clasts. Hammer for scale.

Figure 4.22: Mega-clast breccia deposit overlain by fine lapilli tuffbeds. Bedded units pinch out up and over larger red boulder (clast of Mount Dutton Fm approximately 8' high shown by arrow) in foreground.

52 Figure 4.23: Mega-clast breccia at eastern end of Location # l 8c. Majority of clasts are basaltic and over l' across. Hammer for scale.

Figure 4.24: Large mega-clast of Mount Dutton Formation in the deposit at Location # l 8d. This clast is the single largest clast found at this location. Bag is approximately 3' long.

Figure 4.25 : Photo showing the distribution of beds at Location #23. Most beds are under thin veneer of colluvium. View is to the east. Beds are dipping southeast. Hammer for scale.

Figure 4.26: Lithofacies A: Mega-clast Breccias and Tuff Breccia from the eastern edge of Location# l 8c. Clasts are primarily basalt. Person for scale.

Figure 4.27: Lithofacies A: Mega-clast breccias and tuffbreccias in Location #l8c. Here the clasts are more diverse in composition. The deposit is overlain by bedded tuffand lapilli tuffs. Person is 6' tall.

Figure 4.28: Lithofacies A: Tuffbreccia at Location #6. Unit is entirely massive and cross-cuts underlying bedded units (not seen). Note lack of abundant large clasts as compared with the mega-clast breccias. Large clasts that do occur (shown with arrows) are primarily basaltic. Person is 6' tall.

Figure 4.29: Lithofacies B: Massive lapilli tuffs (shown by arrows) in Location #1. Here this lithofacies is interleaved with bedded and cross-bedded lapilli tuffs. Hammer for scale.

Figure 4.30: Lithofacies C: Bedded tuffs and lapilli tuffs at Location # 18c. Here the thinly bedded tuffand fine lapilli tuffbeds are interbedded with coarser lapilli tuffbeds (finer grained beds protrude from unit). Hammer for scale.

Figure 4.31 : Lithofacies D: Low-angle cross-bedded deposits at Location #6. Obvious truncation of beds on lower side of photo by those on the upper part of outcrop (see arrow). Pencil for scale.

Figure 4.32: Lithofacies D : Cross-beds at Location # I showing truncation ofunderlying beds. Beds just above hammer are truncated by the overlying beds. 64

Type II Dunes

Type Ill Dunes

Flow Direction

Figure 4.33 : Type ll and Ill dune forms on an inner vent wall at Locations # I and 6 at Phonolite Hill. Flow direction in both the figures and photos are shown by the arrows (arrows point south in photos). Dune classification from Schmincke et al. (1973).

4.5 Rhyolite of Phonolite Hill

The rhyolite dome of Phonolite Hill, which forms the major topographic feature at this site, is elongated approximately north-south. Where seen in outcrop the rhyolite is usually a grey, resistant, aphanitic (almost glassy) devitrified rhyolite. The rhyolite forms thick obsidian layers and complex contact lithologies where exposed in contact with the 66 surrounding pyroclastic units. The rhyolite nearest the margins is typically flow-banded and devitrified ( devitrification usually occurs along flow-banding), and becomes more massive inward. The massive rhyolite near the margin usually forms columnar joints that are up to 3' across (Figure 4.34). The massive core of the dome is primarily massive aphanitic rhyolite. No phenocrysts are visible in hand sample. Contact relationships between the rhyolite and pyroclastic units will be described below.

Figure 4.34: Columnar jointing of flow-banded rhyolite at Location #22. Flow-banding is perpendicular to the Jong direction of the columns. Hammer for scale. 67

Figure 4.36: Massive lapilli tu ff near rhyolite contact. This deposit contains scattered larger clasts of various compositions. Major clast component is grey obsidian (see arrows). Hammer for scale.

Figure 4.37: Gradational contact between obsidian (left side of photo, white box) and flow-banded rhyolite (lighter colored rock on right side of photo, red box).

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Figure 4.38: Clast rich obsidian. Devitrified obsidian contain various lithic clasts (See Arrows). Hammer for scale.

Figure 4.39: Massive obsidian at Location# 17 showing distinct devitrification, most likely along internal flow-banding. Darker color is obsidian, lighter pink is devitrified obsidian.

Figure 4.40: Brecciated rhyolite contact between pyroclastic deposits and flow-banded rhyolite. Right side of photo show light colored rhyolite clasts in darker colored lapilli tuff. Arrow shows contact ofrhyolite and brecciated rhyolite. Hammer for scale.

Figure 4.41: Fractured rhyolite with infilling of rhyolite rich lapilli tuff. Pyroclastic deposits not visible at this locality. Rhyolite is darker flow-banded material, lapilli tuff, is lighter pink. Hammer for scale.

75 Figure 4.42: Photo showing devitrification along flow-bands within rhyolite at the obsidian/rhyolite margin (Location #3). Flow bands are very thin and are orange/pink in color. Hammer for scale.

Figure 4.43: Devitrified obsidian from the same locality as Figure 4.35. Devitrification occurs as orange/pink concentric structures. Most are less than I" across although some large ones occur. Hammer for scale.

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Figure 6.4: Spider diagrams for three rhyolites of Phonolite Hill. Normalized to whole crustal values (Taylor and McLennan, 1985) and chondrite values (Sun and McDonough, 1989).

... 12.7/12.9 Ma t• -13Ma .._ ._2.9Ma • Trp - Rtiyolite of Phonolte Hi1 • Tnl - ~eolDfyLaM • Tlf - RhyoliedForahNMo.nllin ~\ 21 . 1Ma_ . • Baalllows .!....__ Fault(a.rMdBlll -----...... onOowralw'ownSide)

Note: Basalts of unkown age are only shown on final map.

t•13-9Ma I

Figure 7. 1: Evolution of the volcanic centers in and adjacent to Kingston Canyon. Modified from Rowley et al. (1981 ).

t= - 13Ma: Prior to extension and uplift of the High Plateaus, numerous basalt flows were erupted as part of the bimodal high-K igneous sequence and the beginning stages of the late Cenozoic bimodal igneous sequence

t= 12-9Ma: Major extension has begun around the Sevier Plateau, as well as down cutting of Kingston Canyon. The rhyodacite of Dry Lake erupted sometime following the t•8-7Ma eruption of the 12.9 Ma basalt it overlies, but before faulting in that area (major faults cut the flow; Rowley et al. , 1981).

t= 9-8Ma: Eruption of a 9 .00 Ma basalt at the bottom of Kingston Canyon.

t= 8-7Ma: Eruption of the rhyolite ofForshea Mountain (7.6 Ma; Rowley et al., 1981).

t= 7-5Ma: Eruption of the pyroclastic deposits around Phonolite Hill (which brecciated and incorporated the 9.00 Ma basalt) and emplacement of the rhyolite of Phonolite Hill at the bottom of Kingston Canyon. Eruption of younger basalts, and activation of faults further east along and on the Arwapa Plateau.

103

deposited both by the East Fork of the Sevier River, and large alluvial fans which form near the canyon walls (Figure 8.1)

Figure 8.1: Photo of current alluvial fan within Kingston Canyon. View is from Phonolite Hill to the southeast. Road in foreground for scale.

The evidence of magma-groundwater fueled eruptions is suggested by the occurrence of high-velocity base-surge beds, which are common in phreatomagmatic deposits, and fractured rhyolitic material, which would have resulted from quenching at depth. The occurrence of bedding sags, formed by ballistic clasts, within a number of the pyroclastic beds implies that the beds were water saturated following eruption (Lorenz et 107

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