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. 2 112'15' 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 5 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). 8 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) 12 110" 100 200 300 km Montana Oregon 45 Wyoming Utah Arizona 3S 110 · 35' 0 10 20 )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). 14 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 10 15 MIOCENE 20 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 "<lJ Tuff Member ...JUt?UJ ""<p Ki"'iJMnnI':.t'1)"M o...J-cn '" TLJII'M.mbef OLIGOCENE E! ~ 00<{ (0'.0:; _U Ua. Needle$Range ~ Group Q;" BullionCanyonVolcanics 30 and RetRted Unlta 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 ) 0<X:w~ w . z o e --lOC)w '" c . Needles Range o--l-(/) .:.=c '"=' OLIGOCENE o~ Group -0 ~~ ~ ->'"0'" 30 - c..>Q."' ..2 '" 1---33.9- 1---- 35 - Claron >- oO:::w Formation o:::o~o wNZ z ~ow~ EOCENE OZ~a --lWCiw 40 - ow(/) (/) 45 Figure 2.5: Stratigraphy ofKingston Canyton, Utah. Modified from Rowley (1994) . 17 ) ·;;;;=····· ·· · ·· ······· ·~ - I r -1 ~ " .~ - ~--, l ./ .t............ ( \ ' ---.. -"·-·-····--...,_ ........ ·•··· ... Z£ ) ············ ..... "" ' ............ Figure 4.2 Continued b: Location #6 Unit Lithology , \ ~ - I ...... a: Location #1 Unit Lithology Structures Lithofacies 210 ft :~~i 'f: i i'.; ; :;~~~ :;;r 151 13 ·•97' A 50' B, C 180 ft 11~1~\j.~ii!i :;~ i'.: :l i;:~·( i1 1:. ii:~ it·~ i 240 ft 150 ft 14 22' B, C 1:~:~:~:1- ·~ 12 ' ':~, :;+·:~~;:. y=~i ~'? B, C 13 .. .. .. .. .. 5' B 210 ft 120 ft 11 B, C 12 37 B, C, D 180 ft r.;~![!~i~ril ·. .- ... ·.;:::.·. ·. :: 22' B, C 90 ft ;::!!~!!' : ~!'\:~:! 11 1.·~.i'~i>ji~\J!· 10 60 ft 9 !:;),;_~ B, C 8 .iit.. \ i1f 7 B, C 6 l-<'.:/'':.\{.:: ··.:.. ·.:::::.:- B. C 5 30 ft B, C 4 i~f&:11' ,\!~il~ 3 2 1 B, C, D 1 1r~r~:~i 1~~.ii 0 ft B, C 3 h;/;:;;·,'~ <:::: : ~:.. \'J 2 B, D .;,. ... :.-:.·:.· :.· \ 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 38 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.