GEOLOGY AND ERUPTIVE HISTORY OF THE LATE OLIGOCENE NATHROP VOLCANICS, CENTRAL

COLORADO VOLCANIC FIELD

William D. Emery

A Thesis

Submitted to the Graduate College of Bowling Green State

University in partial fulfillment of the requirements for the degree of

Master of Science

May 2011

Committee:

Dr. Kurt Panter, Advisor

Dr. Charles Onasch

Dr. Jeff Snyder

ii

ABSTRACT

Dr. Kurt Panter, Advisor

The Nathrop Volcanics consist of rhyolite lava and pyroclastic deposits located

on the eastern shoulder of the upper Arkansas Graben in south-central Colorado and are part of

the extensive late Eocene-Oligocene Central Colorado Volcanic Field. Deposits of the Nathrop

Volcanics at Ruby Mountain consist of a lower lithic-rich lapilli tuff (ca. 3 m thick) with multiple

layers that are reversely graded with respect to pumice clasts and are overlain by an

approximately 30 m thick lithic-poor tuff breccia containing pumice blocks up to 1 m in

diameter. The upper portion of the tuff breccia transitions up into a 5 m thick, moderately to densely welded tuff (vitrophyre), which in turn is overlain by a 20 m thick flow-banded rhyolite.

A similar stratigraphic sequence is found at Sugarloaf Mountain (<1 km to the NNE), and also portions of the sequence crop out as faulted and eroded blocks in the valley between the two mountains. These deposits have been interpreted as being formed by exogenic lava dome growth; pyroclastic facies (fall overlain by flow) followed by lava extrusion. This study considers three possible scenarios to explain the origin and geometry of these deposits. Pivotal to these scenarios is the explanation for the cause of welding and stratigraphic position of the vitrophyre. The three models are: 1) the flow-banded rhyolite was erupted as a lava immediately after the pyroclastic flow (tuff breccia) which caused the welding; 2) the tuff breccia and flow-banded rhyolite are not from the same eruptive episode and welding occurred in a thick pyroclastic flow that subsequently was eroded down to the level of the more resistant vitrophyre followed by eruption of the rhyolite as a lava flow; and 3) the whole sequence represents a single short-lived eruptive event in which the pyroclasts accumulated rapidly iii enough to weld and flow rheomorphically. This study evaluates all three models based on field relationships.

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ACKNOWLEDGEMENTS

I would like to acknowledge the following people who made this thesis possible. First

and foremost, I’d like to thank my parents, Cheryl and Bill Emery, for their support and

encouragement not just over the last 2 years of graduate school, but over the last 25 years of

my life.

I would like to thank Dr. Kurt Panter, for his patience and understanding during the

investigation and writing of my thesis. This is also extended to Dr. Jeff Snyder, and Dr. Charles

Onasch, for serving on my committee and offering many years worth of valuable insight into the Nathrop Volcanics.

Finally I would like to thank my peers at BGSU, especially Laura Webb, Megan Castles,

Colleen O’Shea, and Asako Kawatsura for the support they gave me during the course of my graduate work.

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TABLE OF CONTENTS

INTRODUCTION ...... 1 GEOLOGIC BACKGROUND ...... 2 Rio Grande Rift ...... 2

The Nathrop Volcanics ...... 4

Background on volcanic domes and their deposits ...... 5

METHODS ...... 10 RESULTS: DESCRIPTION OF VOLCANIC DEPOSITS ...... 12 Tnt – Tertiary Tuff ...... 12

Tntf - Lapilli tuff ...... 12

Tntb – Tuff breccia ...... 14

Tntv – Vitrophyre ...... 16

Tntvp – Perlitized vitrophyre ...... 17

Tnts – Volcanic sandstone ...... 17 Tnr – Tertiary Rhyolite lava ...... 19

DISCUSSION ...... 22

Lapilli tuff (Tntf) – Pyroclastic fall facies of Tnt...... 22

Tuff breccia (Tntb) – Pyroclastic flow facies of Tnt ...... 23

Vitrophyre (Tntv) – welded facies of Tntb ...... 26

Perlitized vitrophyre (Tntvp) altered vitrophyre facies of Tntv ...... 31

Volcanic sandstone (Tnts) – sedimentary facies of Tnt ...... 31

Tertiary Rhyolite (Tnr) – lava flows ...... 32

MODEL RECONSTRUCTIONS ...... 34 Model A: Lava-induced welding of pyroclastics ...... 34

Model B: Welding in a thick pyroclastic flow ...... 36

Model C: Welding by Rheomorphic Flow ...... 37

CONCLUSIONS ...... 39 vi

REFERENCES ...... 41 TABLES ...... 51 FIGURES ...... 53 APPENDIX ...... 88

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LIST OF TABLES

Table 1: Previously postulated ages for the Rio Grande Rift ...... 51

Table 2: Modified whole-rock geochemistry analysis by Honea (1955) ...... 51

Table 3: Modified bulk composition chemistry by Schooler (1982) ...... 52

Table 4: (Appendix) Details of samples collected in this study ...... 88

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LIST OF FIGURES

Figure 1: Basins of the Rio Grande Rift ...... 53

Figure 2: Basic volcanic dome shapes ...... 54

Figure 3: Particle relationships between pyroclastic surges/flows/falls...... 55

Figure 4: The Nathrop Volcanics and sample locations ...... 56

Figure 5: Geologic Map of the Nathrop Volcanics ...... 57

Figure 6: Stratigraphic section of Ruby Mountain ...... 58

Figure 7: Northeast Ruby Mountain area...... 59

Figure 8: Three subfacies of tuff ...... 60

Figure 9: Six subfacies of tuff ...... 61

Figure 10: Coarsening-up sequence of tuff ...... 62

Figure 11: Microscopic look at tuff ...... 63

Figure 12: Campground and Sugarloaf Mountain area ...... 64

Figure 13: Massive pyroclastic fall unit at the campground...... 65

Figure 14: Volcanic breccia ...... 66

Figure 15: Northern Ruby Mountain ...... 66

Figure 16: Pink volcanic breccia ...... 67

Figure 17: Angular formation of pink volcanic breccia ...... 68

Figure 18: Microscopic look at pink volcanic breccia...... 69

Figure 19: Interactions of white and pink tuffs of the campground...... 70

Figure 20: Microscopic comparison of white and pink tuffs of the campground ...... 71

Figure 21: Partially welded tuff...... 72

Figure 22: Microscopic look at vitrophyre...... 73

Figure 23: Perlite mine site ...... 74 ix

Figure 24: Reworked tuff area and closer look at sample...... 75

Figure 25: Microscopic look at reworked tuff sample ...... 76

Figure 26: Uniform dipping rhyolite lava...... 77

Figure 27: Lava lithophysae...... 78

Figure 28: Microscopic look: Ruby vs. Sugarloaf rhyolite lava ...... 79

Figure 29: Pumice/lithics vs. distance to vent ...... 80

Figure 30: Modified TAS diagram of the Nathrop Volcanics ...... 81

Figure 31: Evolution Model A: Lava induced welding of pyroclastics ...... 82

Figure 32: Evolution Model B: Pyroclastic erosion ...... 84

Figure 33: Evolution Model C: Rheomorphism ...... 86

1

INTRODUCTION The Nathrop Volcanics, located just east of Nathrop Colorado, represent a small portion of the late Eocene-Oligocene (38 – 29 Ma) Central Colorado Volcanic Field (McIntosh and

Chapin, 2004). The Nathrop Volcanics are found on the western margin of the Arkansas Graben which forms the northern part of the Late Cenozoic Rio Grande rift system. Previous studies on the Nathrop Volcanics include several Master’s theses (Honea, 1955; Schooler, 1982; Nickle,

1987; Bade, 1989), a state map publication by Keller et al. (2004), and three 40Ar/39Ar age determinations from three deposits are reported by McIntosh and Chapin (2004). Although descriptions of the main volcanic lithofacies are given in previous studies, none describe the overall stratigraphic sequence in the context of the physical growth of the volcanic edifice(s).

Furthermore, several key lithofacies have been overlooked that provide critical clues to help unravel syn-eruptive and post-eruptive events. This study incorporates both field and petrographic observations of all the layers to develop a reasonably coherent history to explain the origins of the primary volcanic deposits as well as secondary modifications of these deposits by vapor phase alteration as well as welding and hydration of glass-rich units. The deposits have been further modified by rift-related faulting and significant erosion since the Late Oligocene.

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GEOLOGIC BACKGROUND

Rio Grande Rift The Rio Grande Rift is a north-south trending continental rift zone that extends as far

north as central Colorado, and as far south as Mexico. The northern termination of the Rio

Grande Rift is enigmatic (e.g., Leonard et al., 2002). However some studies regard the Arkansas

Graben as the likely northern termination point for the extension of Rio Grande Rift during the

late Cenozoic (Leonard et al., 2002, Rasskazov et al., 2008). The Rio Grande Rift is composed of

a complex series of north-striking half graben structures that are separated from each other by

accommodation zones (Kellogg, 1999). The rifting occurred in two phases: the first phase occurred from the Middle Oligocene to the Early Miocene, leading to the formation of relatively shallow basins with low-angle normal faults and the occurrence of large-scale silicic magmatism, and the second phase occurring from the Middle Miocene into the Quaternary, with higher angle normal faulting and dominantly basaltic magmatism (Keller and Baldridge,

1999; Tandon et al., 1999).

The northern Rio Grande rift lies adjacent to and partially bisects two prominent volcanic fields; the Late Eocene to Early Oligocene (38-29 Ma) Central Colorado Volcanic Field

(CCVF, McIntosh and Chapin, 2004) and the Early to Late Oligocene (29-26 Ma) San Juan

Volcanic Field (SJVF) (Parker et al., 2005; Rasskazov et al., 2008). McIntosh and Chapin (2004) describe the CCVF as consisting of trachyandesite to trachydacite lavas, pyroclastics and volcanoclastic deposits spanning an area of approximately 22,000 km2, which were eroded and faulted during the Neogene leaving only “patches” of the extrusives exposed today. This is similar in size with the 25,000 km2 area covered predominately by andesite lavas and

volcanoclastic deposits as well as rhyolitic ash flows and domes that compose the SJVF (Lipman 3

and McIntosh, 2008; Parat et al., 2005). Like the SJVF, the CCVF have ignimbrite deposits

associated with major caldera forming eruptions (McIntosh and Chapin, 2004).

McIntosh and Chapin (2004) interpret the large expanse of the CCVF as evidence for a

broad area of low relief (high elevation) topography prior to Rio Grande rifting (Gregory and

Chase, 1992; Epis and Chapin, 1975), which allowed lavas and pyroclastic flows to cover large areas without being blocked by topographic barriers. The subsequent subsidence of the

Arkansas and San Luis basins between 30 and 26 Ma (McIntosh and Chapin, 2004), with the consequent rise of the rift-flank Sangre de Cristo Range and Sawatch Range provide the topography that is visible today. Erosion has since removed much of the volcanics and has exposed some of the source plutons west of the Arkansas valley (eg. Elk Mountains, McIntosh and Chapin, 2004; Grizzly Peak Caldera, Fridrich et al., 1991). The Thirty-nine Mile Volcanic field

is a subfield of the CCVF and lies to the immediate southeast of the study area. It covers an area of more than 2000 km2 in central Colorado, and its composition is mainly mafic to intermediate

lava flows and lahars that formed approximately 36 to 31 Ma (Wobus et al. 1990; McIntosh and

Chapin, 2004). The most voluminous and visible deposit within the Thirty-nine Mile Volcanic

Field is the appropriately named Thirty-nine Mile Andesite lava (Chapin and Wyckoff, 1969). In

some areas the deposits of the Thirty-nine Mile Volcanic Field overlie deposits of the Wall

Mountain tuff. The Wall Mountain tuff is a regional ignimbrite sheet that has a maximum

extent of 150 km from Nathrop to Castle Rock, CO. and a maximum thickness of 150 m

(McIntosh and Chapin, 2004). The source of the Wall Mountain tuff remains in question.

According to McIntosh and Chapin (2004), the previous interpretation that the intrusives

exposed at Mt. Princeton are a remnant of the source pluton for the Wall Mountain ignimbrite 4

(Chapin and Lowell, 1979) is incorrect due to the fact that dating of the intrusive (34.31 +/- 0.21

Ma) places it much younger than the tuff (36.69 +/- 0.09 Ma).

The Nathrop Volcanics The CCVF encompasses numerous volcanic remnants of Late Eocene to Early Oligocene age in central Colorado and studies have been carried out in the larger, more accessible areas.

However, smaller volcanic deposits are not as well studied. One area that is accessible is the

Nathrop Volcanics which is located along the eastern margin of the Arkansas Graben (Fig. 1).

The Arkansas Graben contains westward dipping basin fill that is truncated on the western margin by a master normal fault that has at least 3000 meters of offset (Kellogg, 1999).

According to Kellogg (1999) the graben began to subside during the Oligocene. The fill of the valley reaches a depth of approximately 600 m in the southwest portion of the graben and up to 1500 m on the western edge of the graben (Keller et al., 2004). This is shallow in comparison to other basins such as the San Luis immediately to the south which contains up to 5.6 km of valley fill (Kellogg, 1999). The Miocene to Pliocene age fill in the Arkansas Graben is called the

Dry Union Formation and consists of gray clastic sediments ranging from clay to gravel sized material (Keller et al., 2004). Gravel clasts are mostly volcanic but also include Precambrian igneous and metamorphic rocks as well as lower Paleozoic sedimentary rocks. There are also younger sedimentary deposits including Quaternary alluvium and glacial outwash deposits

(Keller et al., 2004).

The Nathrop Volcanics are located just west of the Thirty-nine mile volcanic field, east of the Sawatch Range, and west of the Mosquito Range (Fig. 1). The volcanic rocks exposed at

Ruby and Sugarloaf Mountains lie approximately 0.8 km east of U.S. Highway 285. Ruby 5

Mountain covers an area of ~0.24 km2, while Sugarloaf Mountain covers an area of ~0.84 km2.

Bald Mountain, located ~3 km north-east of Sugarloaf Mountain, covers an area that is ~1.5

km2, and is considered to be a part of the Nathrop Volcanics (Bade, 1989).

Piecing together the age of subsidence of the Arkansas Graben relative to the eruption

of the Nathrop volcanics is difficult because the timing of rifting is poorly constrained (Table 1).

In contrast, the Middle Oligocene age of the Nathrop Volcanics is relatively known based on

three 40Ar/39Ar dates from deposits on Ruby Mountain (30.08 ±0.08 Ma) and Bald Mountain

(30.35 ±0.08 and 28.88 ±0.21 Ma) by McIntosh and Chapin (2004).

The Nathrop volcanics were the subject of several Master’s theses. Honea (1955) performed one of the first geologic assessments of the area by composing a geologic map as well as petrographic observations and chemical analysis (Table 2). Schooler (1982) investigated

Ruby Mountain using chemical analyses for most of the layers (Table 3) and determining the role of magmatic gases in the development of the layers and post-emplacement mineralization.

Nickel (1987) determined that the Nathrop Volcanics have four distinct lithofacies: ash flow tuff, banded rhyolite lava, gray perlite, and vitrophyre, but focused his study mainly on the chemistry and alteration of the vitrophyre unit. Bade (1989) concentrated on the formation of

Bald Mountain. These previous studies lack complete descriptions of the pyroclastic facies exposed and do not detail the physical formation of the deposits, or the growth history of the volcanic edifice(s).

Background on volcanic domes and their deposits Ruby and Sugarloaf Mountains have been described as lava domes (Schooler, 1982).

Lava domes typically form after moderately intense eruptions (Carn et al., 2004), such as 6

Vulcanian eruptions, which can produce up to 106 m3 of tephra, with eruption columns reaching

as high as 10 – 20 km (Francis and Oppenheimer, 2004). These eruptions can be violent enough

to destroy parts of the volcano itself. The magma remaining in the conduits after the initial eruption has been degassed and extrudes as viscous lava to form a dome on the surface of the vent of the volcano. According to Francis and Oppenheimer (2004), four types of exogenic (i.e.,

“above ground”) lava domes are recognized: “low” domes, coulées, upheaved plugs, and

Peléean domes (Fig. 2). Low domes are flattish, and roughly symmetrical extrusions erupted on

level ground, while a coulée differs in that the extrusion appears on an uneven surface, giving it

a cross between a dome and a lava flow. Upheaved plugs are lava that has been extruded but

have high enough yield strength that they do not expand outwards. Peléean domes are a cross

between “low” domes and upheaved plugs in that the expanded lava dome has a spire of high

yield strength lava extruding above it. Another class of domes are endogenic domes (or

cryptodomes), which are lava domes that form underground and actively deform the surrounding strata but do not break through the surface (Riggs and Carrasco-Nunez, 2004).

Francis and Oppenheimer characterize two types of dome eruptions, Meripi and Peléean.

Meripi eruptions constitute an eruptive event as a result of a gravitational dome collapse,

releasing pressure on magma to instigate the eruption. Peléean eruptions are explosive events

produced from within the lava dome itself.

Exogenic lava domes are generally small, ranging from hundreds of meters to a few

kilometers in diameter (e.g., Cerro Pizarro, Mexico; Mt. St. Helens, USA; Cimini Volcanic

Complex, Italy). Lava domes have been identified that have varying composition including the

rhyolitic (e.g., Cerro de Vidrio; Bissig et al., 2002), dacitic (e.g., Mt. St. Helens; Rowe et al., 7

2008), and andesitic (e.g., Soufriere Hills; Carn et al., 2004) types. Domes are active for several

years to a few centuries (Carrasco-Nunez and Riggs, 2008) and can either be monogenetic (only

one period of activity), or as Carrasco-Nunez and Riggs (2008) demonstrate, domes can be

polygenetic (resumed activity after periods of repose).

Lithologically, volcanic domes can contain lavas, pyroclastic deposits, and vitrophyres.

Although domes are small scale features, their growth can be complex due to both magmatic processes and the rheology of the material that are erupted. Given the rates of deposition and viscosity of the material, volcanic domes can become victims of their own growth, becoming unstable with increasingly steep slopes that often lead to dome collapse (Meripi-type

eruptions). Dome collapse causes explosive activity resulting in deposits of pyroclastic fall and flow. For the purpose of this study, pyroclastic flows (or pyroclastic density currents – PDCs) refer to the density flow of volcanic clasts and gas that can form from the collapse of an eruption column or from a lateral volcanic blast.

Pyroclastic flow deposits are characterized by polymodal (poorly sorted) grain sizes ranging from microns to meters, with a composition of pumice or scoria, with lithic fragments

derived from conduit walls or surface bedrock (Freundt et al., 2000). Pyroclastic falls are

produced when volcanic material precipitates down through the atmosphere to the Earth’s

surface from an eruption column. The deposits that are created by such an event evenly

blanket the landscape and are well sorted relative to pyroclastic flow deposits and often show

grading in clast size, be it normal, reverse, or oscillatory grading (Houghton et al., 2000). A third

deposit of pyroclastic origin is a pyroclastic surge deposit. Surges are distinct from flows

because their movement is characterized by having a higher concentration of gas than particles 8

(with the gasses inside being more dense than the ambient air surrounding the surge), classified

by Valentine and Fisher (2000) as containing less than 0.1 to 1% of solids by volume during

transport. Their deposits display thin (centimeter – decimeter) crossbeds. Surge deposits are

better sorted and contain smaller maximum clast sizes than flows (Valentine and Fisher, 2000).

Along with pyroclastic flows, surges are classified as a dilute form of PDC. The relationships

between particle concentration and movements of PDCs as well as pyroclastic falls are shown in

Figure 3. A specific type of deposit formed by PDCs associated with dome collapse is called a

block and ash deposit (Lockwood and Hazlett, 2010). Block and ash deposits are characterized

by a lack of pumice with abundant angular blocks derived from source domes. These deposits

rarely exceed 1 km3 in volume and spread only a few kilometers from the source vent

(Lockwood and Hazlett, 2010). Block and ash flow deposits differ from ash flow deposits in that ash flows are pumice-rich (aka. ignimbrite), with varying amounts of lithics (Lockwood and

Hazlett, 2010). The proportion of lithics to pumice in a pyroclastic flow deposit can vary due to density segregation. For example, lithic-rich portions (e.g., lag breccias) of pyroclastic flow

deposits can be found in channels while pumice-rich segregates formed from the same PDC can

form levees on the sides of the channels (Lockwood and Hazlett, 2010, Fisher and Schmincke,

1984).

Volcanic dome sequences may contain vitrophyres, which are glassy volcanic deposits.

Vitrophyres can form one of two ways: ash and pumice deposited remain warm enough to weld

into a dense groundmass containing no crystalline structure (Freundt et al., 2000) or

vitrophyres can form from lavas that have cooled so quickly upon reaching the surface (in air or

water) that no crystalline groundmass can form. Freundt et al. (2000) defined welding of 9

pyroclasts as the cohesion, deformation, and eventual coalescence of vitriclasts at high

temperatures under loading stress. A balance between pressure (load bearing on top or within

the pyroclastic deposit) and heat from the newly emplaced pyroclastic flow gives rise to

different degrees of welding. Freundt et al. (2000) explain that the higher the emplacement

temperature and the lower the clast viscosity, the more intense is the compaction under a

given load. The flattened remnants of pumice, known as fiamme, in a welded pyroclastic

deposit are used as indicators of the degree of welding based on aspect ratio. The intensity of

welding determines if it will undergo a secondary rheomorphic flow. Rheomorphism is defined

by Lockwood and Hazlett (2010) as the welding of a pyroclastic deposit to the point where the

interlocking material weakens, lubricated by internal films of molten fluid, to instigate

secondary motion. The rheomorphic flow is greatly aided by gravity on a steep slope or by

gravitational spreading if the deposit is sufficiently thick (Bachmann et al., 2000). The resulting

features from rheomorphism can result in a flow banded texture in hand samples.

The compaction of the pyroclastics release volatiles, which can mobilize and locally alter the deposit (Freundt et al., 2000). Deuteric vapor-phase alteration and precipitation of high

temperature secondary minerals (alkali feldspar, cristobalite, tridymite, etc) can occur within

remaining pore spaces (Freundt et al., 2000, Lockwood and Hazlett, 2010). Another secondary

form of alteration that takes place later, at lower temperatures, is the perlitization (hydration)

of volcanic glass. This process involves the diffusion of external water into the glass to create

perlite (Friedman et al., 1966, Denton et al., 2009), which is characterized by abundant

concentric microfractures. 10

METHODS Field Investigations took place between June 8th and June 24th, 2009. GPS points were

taken and samples selected based on lithology and stratigraphic position using an Xplore tablet

PC and a Bluetooth Teletype GPS receiver. The coordinates were entered into a GIS database

and were used to create both a sample and geologic location map of the area (Figs. 4 and 5).

Pictures taken using a scale card for reference always had the arrow pointing in the North

direction, for both scale and direction. Formation identification and nomenclature were based

on the geologic map produced by Keller et al. (2004), which designate three units that

encompass deposits of the Nathrop Volcanics: Tertiary tuff (Tnt), Tertiary vitrophyre (Tnv), and

Tertiary rhyolite (Tnr). Based on significant changes in lithology this study has subdivided the

Tertiary tuff into five distinct lithofacies, which includes reassigning the Tertiary vitrophyre as a

subunit of the tuff.

The samples collected from Ruby Mountain and the campground area (Fig. 4) were analyzed and classified by grain size and composition using the criteria of Schmid (1981). It should be noted for clarity that the campground area is in a valley between Ruby and Sugarloaf

Mountains, bounded to the west by the Arkansas River, and to the east by the Precambrian granite. Samples that were very fine grained or needed further analysis were cut into billets using the rock saws in the BGSU Geology Department, and 20 thin sections were prepared.

Samples that were poorly lithified or weathered were impregnated with an epoxy before being

cut, and one pumice sample was stained with a blue dye to give an idea of porosity. The slides

were analyzed using standard petrographic techniques and photographed.

The following description of the volcanic units in the Ruby Mountain area will be presented in stratigraphic order from the lowermost unit to the uppermost unit. A generalized 11 stratigraphy of Ruby Mountain is shown in Figure 6. Due to erosion, limited exposure and disruption from rift-related faulting, the thicknesses of some of the units are approximate.

12

RESULTS: DESCRIPTION OF VOLCANIC DEPOSITS

Tnt – Tertiary Tuff The tuff deposits on Ruby Mountain and in the campground area form the lower half of

the Nathrop Volcanics stratigraphy. Outcrops and eroded remnants of this formation are found at the southernmost end of Ruby Mountain and are exposed discontinuously northward to the lower southeastern slopes of Sugarloaf Mountain (Figs. 4 and 5). While this is a large stretch of outcrops, the exposures between Ruby and Sugarloaf Mountains are poor, being obscured by

Quaternary alluvium and/or vegetation. Keller et al. (2004) and several Master’s theses

(Schooler, 1982; Nickle, 1987; Bade, 1989) classify the formation as a single pyroclastic unit.

Here it is described as a sequence of volcanoclastic deposits with two primary volcanic lithofacies and three additional facies that were formed by secondary processes which modified the primary facies.

Tntf - Lapilli tuff On the eastern side of Ruby Mountain (Fig. 7), the lowest ~3 meters consists of a salt-

and-pepper colored, vitric-lithic lapilli tuff that exhibits multiple graded layers that vary in

thickness, grain size and in the abundance of lithic (ca. 15% by volume) and pumice clasts (ca.

50%). Within the outcrop there are areas where these layers can be easily identified, while in

others areas they are obscured or absent, making it difficult to decipher their true lateral

extent. The best exposed outcrop reveals up to six layers that extend approximately 32 m

horizontally but are interrupted by slumps of the overlying pyroclastic breccias (Figs. 7, 8, and

9). The unit provides very few bedding surfaces to measure attitude and only two measurements of dip were made that reveal angles of 37° and 50° to the WSW. 13

Each of the layers, which range from 30 to 100 cm thick, is reversely graded with respect

to pumice clasts, coarsening-upward from coarse ash to lapilli (Fig. 10). The boundaries

between layers are difficult to determine due to the gradational nature of the grain size but were picked based on horizons that had the most distinct change in grain size, and/or where a distinct ‘line’ could be drawn at boundaries where lithic content changed significantly. Within these bedded layers, occasional block-sized pumice clasts are found. The pumice blocks do not

appear to disrupt the surrounding bedding.

Hand sample and petrographic analysis reveal pumice and lithic clasts set within a fine grained matrix. Lithic clasts of rhyolite lava are up to 5 cm as identified in outcrop (Fig. 10) and in thin section average about 3 mm in diameter. Pumice clasts in thin section also average about 3 mm. The matrix is mostly altered with veins of calcium carbonate giving it a brown color at low magnification (Fig. 11). Small plagioclase feldspar minerals no greater than 0.1 mm are rare. Schooler (1982) determined the lithic clasts of rhyolite lava located in the lower subunits were 100% crystalline consisting of anhedral quartz and sanidine.

A 5 m thick exposure of a massive, moderately sorted, lapilli tuff is found within the campground area (Figs. 12 and 13). This deposit is similar to the tuff at the base of Ruby

Mountain in that it is reversely graded but with respect lava clasts rather than pumice. This deposit may be equivalent but the distinctive multiple graded layers are absent here.

The relatively well sorted nature of the lapilli tuff and the occurrence of multiple layers of even thickness observed at Ruby Mountain indicate a likely origin from pyroclastic fallout.

The multiple reversely graded layers would suggest several episodes of activity or multiple pulses from a single eruption (Houghton et al., 2000). 14

Tntb – Tuff breccia Overlying the lapilli tuff on the northeast side of Ruby Mountain is a ~28 m thick

exposure of vitric tuff breccia (Fig. 7). The tuff breccia is exposed to a lesser extent elsewhere

on Ruby Mountain, in the campground area, and on the lower eastern slopes of Sugarloaf

Mountain. The color of this layer varies from white to khaki to pinkish-red in color depending

on location. The deposit is characterized by lapilli (2 cm in diameter) to block-sized (up to 1 m in

diameter) subrounded pumice clasts set in a medium ash matrix, giving it a very poorly sorted

appearance (Fig. 14). The deposit is characterized by an overall coarsening-up in pumice clast

size, and lava clasts are absent. The upper portion of this unit and contact with the overlying

deposit is covered by colluvium, but in places the tuff breccia displays a gradual change

characterized by decreasing pumice clast size from block to lapilli and a change in color from

white to pink to dark purple. The change in color, and to a lesser extent texture, is ascribed to

post-depositional modification that will be discussed below.

The mineralogy determined by Schooler (1982) is similar to that of other pumice

analyzed in this study, being composed of ~2 vol.% phenocrysts of unzoned oligoclase (~25%)

and alkali feldspar (~75%). Using determinations from the ICP technique for major elements,

Schooler concluded that the composition of the pumice in the tuff breccia and lapilli tuff units were identical.

An area of the tuff breccia deposit which exhibits a distinctive pink color is located on the northern side of Ruby Mountain (Fig. 15). Of particular interest in this area is a well defined, irregular and elongated structure that consists of highly angular pumice fragments (Fig. 16). The structure is approximately 0.8 m wide and its exposure extends over 4.5 meters diagonally, dipping to the northeast (Fig. 16). The margins of this structure are marked by a color change 15

from reddish-pink to light pink. Pumice within this structure range from 1 to 6 cm in size, and is

typically smaller, more angular, and a deeper pink than the pumice in the surrounding tuff breccia (Fig. 17). Petrographically, angular pumice range from 0.4 mm to 2 cm in size and phenocrysts of plagioclase are highly fractured (Fig. 18).

The tuff breccia is also found in the campground area to the north of Ruby Mountain

(Figs. 4 and 12). The exposures in this area are limited due to cover and erosion but several places show complex contact relationships with other deposits. For instance, a thin (~1 m) exposure of the tuff breccia lays comfortably on top of the 5 m thick lapilli tuff discussed earlier

(Fig. 13).

Approximately 10 m to the northwest of this outcrop is an exposure of pink reworked tuff juxtaposed to a white lapilli tuff forming an unconformable vertical contact (Fig. 19), which is unlike other areas where the color change is gradual. The pink tuff (Tnts – see description

below) is stratified with a southwest dip of between 8° and 20° and the white tuff is massive

and shows reverse grading both in size and occurrence of rhyolite lava clasts (Fig. 20). Lava

lithics observed in thin section are up to 2.4 mm in diameter but the dominant size is about 0.8

mm. Petrographically, the white tuff shows a lower degree of brown-colored weathering in plane polarized light than the pink unit, as well as more angular phenocrysts and lava clasts (Fig.

20). Lithics and crystals in the pink unit are moderately well-sorted, with a mixture of subhedral and anhedral feldspars that range from 0.08 mm to 1.6 mm. Pumice and lithics of rhyolite lava in the white unit are 0.4 mm to 10 mm in size and are more poorly-sorted and angular than the pink unit. Biotite is an accessory mineral in both units and shows alteration around crystal edges. The two samples are similar microscopically with the exception of the abundance of lava 16

lithics and weathering (Fig. 20). The key differences between these two units are macroscopic, stemming from their color and bedding structures within the red unit and the dominance of much larger, lapilli-sized rhyolite lava clasts within the white unit (Fig. 19). The pumice-rich and

lava clast-poor breccia has characteristics definitive of an ash flow deposit that was emplaced

on top of the earlier erupted lapilli tuff fall deposit.

Tntv – Vitrophyre The vitrophyre that is found on Ruby and Sugarloaf Mountain as well as the campground

area is widespread, but not continuous. Some of the outcrops of vitrophyre are dark purple and

dense with elongate purple and black fiamme, while other outcrops have a light purple color

and less compacted with a texture in between that of relic pumice and fiamme (Fig. 21). In this

study, any glass-rich deposit at this stratigraphic level that shows signs of welding is considered

part of the vitrophyre. The vitrophyre lacks any relic surface to get a measurable strike and dip,

but the whole layer, when viewed at a distance, dips to the SW at a relatively high angle (~20-

30°).

The underlying tuff breccia (Tntb) transforms upward on the slopes of Ruby Mountain from a more inflated pumiceous texture, white in color to a partially welded (dark pink/light purple hue, pumice slightly elongated in one direction) and then to a strongly welded (dark purple/black) vitrophyre. The vitrophyre is composed of relict pumice that on average can range from 1 – 5 cm in length.

A representative thin section of the vitrophyre is shown in Figure 22. Other material within the section includes a brown carbonate and about 5% anhedral feldspars that show albite twinning. Overall, the vitrophyre is composed of 50% relic pumice, 49% ash, and 1% 17

crystals. The phenocrysts studied here were similar to those found in previous studies; primarily

plagioclase and sanidine based on the interference figures and twinning observed within thin

section.

The vitrophyre represents variably welded tuff breccias. The possible cause of the welding will be discussed below.

Tntvp – Perlitized vitrophyre The perlite unit is best exposed in a mining prospect pit located on the northeast side of

Ruby Mountain above the tuff breccia (Figs. 7 and 23), and is bounded on either each side by

dark purple vitrophyre. The contact boundary between the vitrophyre and perlite is complex

and varied. In some areas the vitrophyre grades into perlite, while other areas show the contact

as being abrupt and easily discernable. The rock has a characteristic gray color, with a chonchoidal fracture that shows little to no relic primary textures. Dispersed within the perlite

are spherules of obsidian or “Apache Tears”. The black to translucent brown glass are spherical

shaped and can vary in size from 0.1 – 1 cm. Though no petrographic analysis was done on the

perlite in this study, Keller et al. (2004) confirms that “sparse” amounts of phenocrysts within

are composed of quartz, sanidine and plagioclase, supporting the idea that the perlite

represents hydrated vitrophyre. The alteration of the vitrophyre may not be Tertiary in age but

for consistency the perlite will be designated as a subunit of the vitrophyre (Tntvp).

Tnts – Volcanic sandstone Bedded volcanoclastic units occur within the campground area and just east of

Sugarloaf Mountain (Figs. 5 and 24). Figure 24A shows a 10 m thick volcanic sandstone outcrop

near Sugarloaf Mountain. Overall the unit dips 25° to the southwest, similar in attitude to the 18

pink bedded deposits in the campground area (Fig. 19A). The texture and mineralogy is similar

to pyroclastic deposits with the exception of three fine-grained resistant layers found at three

different levels within the deposit. These layers are horizontally continuous with a relatively

uniform average thickness of 15 cm (Fig. 24B).

The makeup of one of the resistant layers, based on thin section analysis, consists of

15% crystals of feldspar and biotite, with 10% clasts of rhyolite lava that range in size from 0.08

mm to 4 mm and less than 5% clasts of pumice. Alteration minerals consist of calcite and

chlorite (after biotite). The sample also contains a large perlitized lithic clast ~3 cm in length

(Fig. 25). Overall, the lava, pumice and perlite clasts vary from angular to well rounded. The

remaining ~70% is a brown glassy fine-grained matrix (Fig. 25). The lesser amount of pumice, by

comparison to other pumice-bearing units, and higher amount of biotite (also recognized by

Schooler, 1982) as well as secondary alteration minerals calcite and chlorite, distinguish this deposit from other pyroclastic deposits in the campground area. There is about 20% less pumice in this unit relative to the pyroclastic layers at Ruby Mountain.

To further distinguish reworked pyroclastic units from primary units it was noted that the stratified pink reworked unit juxtaposed to the white lapilli tuff discussed earlier (Fig. 19A) has more rounded crystal fragments than the white counterpart and petrographically exhibits a red alteration halo around coarser grained lava clasts that is not seen within the white tuff unit.

When considering any bedded deposit that consists of nearly 100% volcanic material an assessment of whether or not it is a result of direct volcanic activity or reworking of volcanic material is necessary. Dilute PDCs (surges) have cross-bedded structures and a well sorted facies within deposits that are comparable to those formed by sedimentary processes. However 19 erosion by water or wind tends to round grains more efficiently and would more likely contain clasts from the surrounding bedrock (granite). Although non-volcanic clasts were not noted in deposits of volcanic sandstone, the crumbling consistency of most of the primary pyroclastic deposits of the Nathrop Volcanics demonstrate how easily erodible they have been since the time of their deposition.

Tnr – Tertiary Rhyolite lava The rhyolite lava forming the top of Ruby Mountain is massive, approximately 20 meters thick and flow banded. The basal contact between the rhyolite lava and the tuff (Tnt) on Ruby

Mountain is covered by colluvium but the upper-most units of Tnt that outcrop below the lava are the vitrophyre (Tntv) and perlite (Tntvp). Large, non-fractured outcrops show a relatively uniform dip of 45° – 61° to the southwest (Fig. 26), comparable again to the orientations of the underlying units and those in the campground area. Sugarloaf Mountain is also capped by flow- banded rhyolite lava with similar orientation as deduced by previous studies. Major talus piles derived from the rhyolite lava occur on the steep western slopes of both Ruby and Sugarloaf

Mountains. In addition to the distinctive flow banding, the lava deposits on Ruby and Sugarloaf

Mountains are also characterized by the occurrence of lithophysae (Fig. 27). Generally, the lithophysae are larger (max size ~10 cm) in large boulder blocks (2 – 4 m in diameter) of rhyolite lava float found near the base of the mountain. But overall, lithophysae and vesicles show a wide range in size from 5 mm to 10 cm and comprise anywhere from 5-20% of the lava by volume. “Rock hunters” frequently search for topaz and garnet crystals that formed as vapor phase minerals in some of the lithophysae. 20

Petrographic analysis of the rhyolite lava collected at the northern portion of Ruby

Mountain is slightly weathered with a brown banded texture. Some hand samples have a weathered coating of whitish powdery caliche. Petrographic analysis shows that the lava is composed of ~85% crystalline groundmass with ~10% glass and 5% phenocrysts. Vesicles are abundant and average ~0.2 mm in diameter. Some of the phenocrysts are large enough

(average size of 0.2 mm) to show albite twinning indicative of plagioclase feldspar, which was determined through petrographic observation to be oligoclase in composition by Schooler

(1982). Quartz phenocrysts are less common than plagioclase crystals. Banding in the rhyolite is created by color and grain size. Some bands have an average larger grain size that varies from

0.16 to 2.8 mm with subhedral crystals, while other bands are finer grained. Another distinguishing factor is the alteration between bands. Some grains of feldspar show a higher degree of alteration, replaced with very fine grains that exhibit undulatory extinctions that are most likely zeolites. Bands can also be separated by their crystal orientation, with some bands having an elongated orientation parallel to the bands while others do not.

Two lava samples were collected from the Sugarloaf Mountain, one from the northern part of the campground area and another from a large boulder at the bottom of the northwest end of Sugarloaf Mountain. Both samples exhibit less banding at the macroscopic level, but it is evident on cut surfaces. These samples are coarser grained than their Ruby Mountain counterparts, exhibiting larger feldspar phenocrysts and less obvious banding (Fig. 28). The banding difference is obvious at both microscopic and macroscopic scales. The banding that is present is less linear and is folded over in a “whirling” texture. An outcrop near the southern base of Sugarloaf Mountain (Figs. 4 and 5) is part of the same outcrop of lava that extends to 21

the summit of the Sugarloaf Mountain. Outcrops of rhyolite lava are notably absent in the

campground area between Ruby and Sugarloaf Mountains.

Keller et al. (2004) states that the rhyolite lavas on both Ruby and Sugarloaf mountains

consist of flow bands and lithophysae, some of which host millimeter size crystals of garnet and

topaz. Within the field study it was confirmed that both rhyolite units from Ruby and Sugarloaf

Mountains have these accessory minerals and show flow banding to infer that these units were

solidified from similar, if not the same rhyolitic lava flows. Schooler’s (1982) chemical analysis

of major element oxides for one lava sample (RM-4) is high in SiO2 (76.4 wt.%) with high K2O

(4.63 wt.%) but a low Na2O + K2O/Al2O3 ratio (< 1), making it a high-K rhyolite but not

peralkaline (Table 2).

Keller et al. (2004) postulates that the banded rhyolite unit is 1,000 ft thick or more, stating that the true thickness is unknown because the rest of the unit is buried beneath the

Arkansas valley fill. This remains speculation as no subsurface data exists. Based on exposures in the study area it is likely that the rhyolite lavas do not exceed 50 – 60 m in thickness.

22

DISCUSSION Approximately 30 million years have passed since the eruption of the Nathrop Volcanics

and this section will attempt to piece together the volcanic, tectonic and erosion processes that

formed and modified the volcanic deposits over that time period. First, each unit and subunit

will be discussed in terms of the specific processes responsible for its formation. Second, the

sequence of the deposits observed in the stratigraphy at Ruby Mountain will be used to

reconstruct the eruptive history and deduce post-depositional modifications related to rifting

and erosion.

Lapilli tuff (Tntf) – Pyroclastic fall facies of Tnt. The lapilli tuff is the lowest and oldest exposed volcanic unit. The lapilli tuff contains an

abundance of rhyolite lava clasts that have similar chemical make up as the rhyolite lava that forms the youngest volcanic unit (Tnr), and both are predominantly crystalline. This indicates that pre-existing older lavas, either from an older eruption sequence at the same vent or, less likely, from a different origin altogether, were fragmented by explosive activity, incorporated into the eruption column, and deposited as part of the pyroclastic fall. The overall layering of this unit, as well as the reverse grading within individual beds and variation in the abundance of lava lithics from layer to layer, suggest dynamic conditions during this phase of eruptive activity or possibly changes in wind direction or speed (Houghton et al., 2000, Fisher and Schmincke,

1984). Deposition of the lapilli tuff from a single steady pulse is unlikely, since that would most likely produce normal grading and only a single layer, barring any strong controls by wind. It is more likely that the layered lapilli tuff was formed by multiple eruptive pulses, each one increasing in intensity as to permit reverse grading in each layer (Bryan et al., 2000). The limited 23

exposure of this unit prevents the production of any isopach/isopleths maps that would help

determine eruption trajectory and/or wind direction.

The source vent for the erupted material may have been between Sugarloaf and Ruby

Mountains. This idea is tentatively based on the greater thickness of the pyroclastic fall deposit

located within the campground area (~ 5 m) relative to the deposit at the base of Ruby

Mountain (~ 3 m). However, the true thickness of either of these deposits is uncertain given

that their bases are not exposed. One other piece of supportive evidence, however, is the size

difference between lava clasts that make up the pyroclastic fall units. Larger lapilli-sized lava

clasts occur in the campground area deposit, which if from the same eruption (and vent) would

suggest that this unit was deposited generally closer to the vent simply because large clasts are

not transported as far as small clasts (Houghton et al., 2000, Fisher and Schmincke 1984).

Tuff breccia (Tntb) – Pyroclastic flow facies of Tnt Pyroclastic flow deposits produced by small scale lava dome collapse are mostly of the

block and ash type (Francis and Oppenheimer, 2004). Block and ash deposits consist mostly of

block sized lithic clasts interspersed in a finer grained ash matrix. Deposits of this type are

typically matrix-supported. Block and ash deposits contrast with lithic-poor and pumice-rich

pyroclastic deposits called ignimbrites that are commonly formed from more energetic Plinian- type eruptions associated with eruption column collapse (Sparks, 1976). The pyroclastic flow deposits of the Nathrop Volcanics are pumice-rich and more typical of ignimbrites. Gomez et al.

(2009) also characterize block and ash deposits from dome collapse as having no internal stratification when deposited on open, flat surfaces and the Nathrop deposits are indeed unstratified, but this again is a common characteristic of ignimbrites. The Nathrop Volcanics, 24 however, were deposited on a surface with at least some minor topographical relief as evidenced by elevation differences of 10 to 20 m at the contact with the underlying

Precambrian granite. A vertical contact between the rhyolite and granite exists at the northern end of Sugarloaf Mountain. The origin of this contact is debated as a depositional contact or a fault. This should be addressed in future studies, as this study did not examine the northern sections of Sugarloaf. Original surface roughness leaves open the possibility that the flow could have become segregated with respect to clast density. That is, the pumice-rich breccia may preserve a deposit that had previously lost its lava lithics (i.e. pumice levee) and a complimentary clast-rich lag breccia may exist elsewhere (Lockwood and Hazlett, 2010, Fisher and Schmincke, 1984). However, without any evidence for the existence of a lag breccia in the area, this theory is tentative at best.

Another feasible scenario is that an intermediate-sized silicic eruption could produce a pumice-rich flow deposit. Bachmann et al. (2000) theorizes that a flow unit rich in pumice could be formed from a low-energy pyroclastic fountaining. Ekren et al. (1984) cite high eruption temperatures from their study of ash-flow tuffs in Idaho as the deciding factor that forms eruptions with negligible convection components. Ekren et al. (1984) and Branney et al. (1992) both describe welded ash flow tuffs as lava-like, rheomorphic structures. They cite high emplacement temperatures from low eruption columns as the factor in producing these textures. These models will be incorporated into the reconstruction section.

Even though the volcanic activity in the area is 30 million years old and has been significantly eroded, the lack of any outcrop of volcanic material between that of Bald

Mountain and Sugarloaf-Ruby Mountain, or anywhere else to the north or south, is a small but 25

still relevant piece of evidence supporting a location for the vent(s) near the preserved volcanic

deposits. This, as well as the large size of the pumice clasts found within the breccia (≥ 1 m) and

the thickness of the deposits (> 30 m), support a vent location proximal to Ruby and Sugarloaf

Mountains. Freundt et al. (2000) made the comparison between maximum clast size (pumice

and lithics) versus distance from vent for several deposits including the Taupo Ignimbrite (New

Zealand), Rattlesnake Tuff (Oregon), Mount St. Helens blast deposit (Washington), and Ito flow

deposit (Japan). Pumice clasts of the size found at Ruby Mountain are predicted to have a

maximum transport distance of ~1 km based on results presented in Freundt et al. (2000).

Another study conducted by Walker and Croasdale (1970) suggest similar findings that are

quantified in Figure 29. Therefore Bald Mountain, which has been hypothesized as the source

vent for all of the Nathrop Volcanics, cannot be the vent location.

Some studies (Honea, 1955; Bade, 1989) have concluded that the three edifices:

Sugarloaf, Ruby and Bald Mountains, were part of one large volcanic dome, while others

(Schooler, 1982) have suggested that they share a magma chamber only, but extruded at

different localities. The chemical similarities (Schooler, 1982; Bade, 1989) and the recent

determinations of the timing of these eruptions (McIntosh and Chapin, 2004), and

determinations of pumice clast size transportation (Freundt et al., 2000; Walker and Croasdale,

1970), are more factors in the geographic origins of Ruby and Sugarloaf Mountain that suggest

a local vent for Ruby and Sugarloaf mountains, though the same magma system could have also

supplied eruptions at Bald Mountain.

The red/pink areas in the tuff breccia found at Ruby Mountain (Figs. 15, 16, and 17) formed from a process called vapor phase alteration/crystallization. Vapor phase alteration is 26

the process by which superheated volatiles follow permeable areas through an unwelded newly

deposited pyroclastic flow, causing oxidation of Fe-bearing minerals (e.g., biotite, magnetite)

and precipitation of minerals from vapor (e.g., topaz, tridymite) (Freundt et al., 2000; Lockwood

and Hazlett, 2010). The vapor sources may be internal (magmatic volatiles) or external; water

incorporated from atmosphere and/or moist substraight (Roche et al., 2001). It is common for

areas affected by this type of alteration to be intermittent within a deposit (Fisher and

Schmincke, 1984). Fluidization provides enough force to counter the force of gravity and move

particles vertically upward and most undisturbed pipes are vertical (Roche et al., 2001). This

dark pink zone (Figs. 16 and 17) represents a brecciated conduit by which violently streaming

vapors escaped vertically from the newly emplaced pyroclastic flow. These structures are

known by many names including gas segregation pipe, lapilli pipe, breccia pipe, fluidization pipe, and fossil fumarole. Given the secondary brecciation of the angular pumice within the pipe, this study will refer to areas of gas alteration as breccia pipes. As seen in Figure 16, the breccia pipe is inclined (50° SW) and therefore has been tilted from its original vertical position, in agreement with the overall dip of the entire volcanic sequence. The structure may have appeared on the surface as a fumarolic vent (Kodosky and Keith, 1995).

Vitrophyre (Tntv) – welded facies of Tntb As described earlier, the vitrophyre exhibits varying degrees of welding that range from

a deep red, slightly welded tuff, to a light purple, moderately welded tuff/vitrophyre (Fig. 21), to a dark purple vitrophyre with barely recognizable relic pumice textures. This gradational transition takes place in the upper part of the volcanic breccia below the vitrophyre (Fig. 15) indicating an origin for the vitrophyre by welding of the volcanic breccia. Also, as described 27

earlier, the contact between the vitrophyre (or perlitized vitrophyre) and the overlying lava is

obscured but in places by only 5 m. This short distance between the vitrophyre and the lava has important implications for the origin of the deposit. The process of welding in a single pyroclastic deposit (simple cooling unit) takes place in the portion of deposit that is not subjected to the chilled bedrock below or the cool atmospheric air above, but in the central portion that retains enough latent heat after deposition to compress and weld original vitriclasts together (Wallace et al., 2003; Francis and Oppenheimer, 2004; Lockwood and

Hazlett, 2010). The area prone to welding is typically located in the lower third portion of the pyroclastic deposit, closer to the underlying bedrock than the atmosphere. Since there is no exposed recurrence of the unwelded pyroclastic unit between the vitrophyre and the lava (nor is there any pyroclastic float found above the vitrophyre) the problem arises: Where is the rest of the unwelded pyroclastic deposit that is predicted to be stratigraphically above the vitrophyre? To explain the presumed absence the author envisages three scenarios:

1) The emplacement of the pyroclastic flow was initially much thicker than what is seen

today and welding to form the vitrophyre occurred in the lower third of the freshly deposited

pyroclastic flow. A period of erosion then reduced the upper unwelded portion of the deposit

to the more resistant vitrophyre, which was covered later by the lava flow.

2) The pyroclastic flow was overridden by the lava flow (Tnr) soon after its deposition

thereby adding heat (+ pressure) that was sufficient enough to weld the upper portion of the

pyroclastic breccia into the vitrophyre leaving no unwelded pyroclasts immediately below the lava. 28

3) The vitrophyre is a result of a syn- or post-emplacement processes know as

rheomorphic flow, which is the internal remobilization of a still hot pyroclastic layer. In this

scenario, the overlying flow banded rhyolite may not be lava after all, but may represent an

upper remobilized portion of the welded pyroclastic deposit.

The first scenario is based on observation and experimental evidence for the formation

of lenticular welded zones within ignimbrite deposits (Keating, 2005; Russell and Quane, 2005).

In order to fully evaluate this scenario the original thickness of the pyroclastic flow must be

estimated. The total thickness of the tuff breccia plus vitrophyre at Ruby Mountain is a little less

than 40 m (Fig. 6). Assuming that the position of the welded zone is similar to a classic welded ignimbrite and occurs in the lower 1/3 of the deposit, then the approximate pre-erosion ignimbrite would have to be on the order of 120 m thick or greater. This assumes that erosion halted at the upper boundary of the vitrophyre, likely due to the dense and more resistant makeup of the vitrophyre. However, if portions of the vitrophyre were eroded as well as the pyroclastics above it, then the deposit could be somewhat thicker. Thick, welded ignimbrites are most often found in widespread deposits originating as outflow sheets from caldera collapse and can cover areas 100’s to 1000’s km2 (Lockwood and Hazlett, 2010). Eruptions capable of producing this amount and extent of ejecta are often of the Plinian or super-Plinian

type (column heights typically reach about 15 – 20 km, with deposits reaching up to 1000 km3,

Lockwood and Hazlett, 2010), and are not from smaller localized activity typical of Vulcanian type eruptions (column heights typically reach about 5 – 10 km, with deposits less than 1 km3,

Francis and Oppenheimer, 2004). Smaller scale eruptions could produce thick deposits with little lateral extent (like that found at Ruby Mountain) if there were sufficiently high 29 topographic barriers and valleys for pyroclastic flows to pond. Such a paleotopography might have existed in this area but previous studies of central Colorado infer that, in general, the region consisted of a relatively flat topography, referred to as the late Eocene Erosional Surface

(Gregory and Chase, 1992). A local outlier to this study can be found on the north end of

Sugarloaf, where the rhyolite lava forms a vertical contact with the Precambrian granite.

Topography aside, high temperature, low column eruptions have been described previously as being able to produce these deposits. Other pieces of evidence that might substantiate the erosion of a much thicker ignimbrite would be a surface-of-erosion marked by uneven topography and heterolithic sediments deposited upon it. Although the contact is covered, it appears planar (Figs. 5, 7, 12 and 15), and sediments are not found in the float covering the contact.

The second scenario involves the idea that the emplacement of thick lava on top of a hot, freshly deposited pyroclastic flow can bring about welding. Very few studies have suggested this (Christiansen and Lipman, 1966; Smith and Cole, 1996). Christiansen and Lipman

(1966) describe the pyroclastic and lava flows that comprise the Comb Peak rhyolite, located near the Fortymile Canyon in Nevada, as a basal ash fall tuff, covered by a pyroclastic flow containing areas of “fused” or welded vitrophyre, and capped with a flow banded lava. The pyroclastics can be as thick as 160 m near the vent, and as thin as 30 m at the farthest radial deposit. The lava flow was found to have a maximum thickness of 250 m. The vitrophyre/fused tuff located just under the foliated lava was interpreted by Christiansen and Lipman (1966) as forming in response to the overflow of lava at high velocity in a confined valley, with super heated gas from the flow acting as a catalyst for welding. Smith and Cole (1996) describe the 30

Woolshed Creek ignimbrite (New Zealand) as being a rhyolitic, porphyritic, pyroclastic flow that

was welded into a black glassy vitrophyre (~20 m thick). The Somers rhyolite lava that tops the

vitrophyre consists of domes that are between 300 – 500 m thick, and lava flows that are ~100

m thick. It was Smith and Cole’s deduction that the Somers lava fused the lower Woolshed

Creek ignimbrite during its emplacement.

To evaluate the third scenario it is necessary to review investigations that describe the

rheomorphic flow of pyroclastic deposits. Rheomorphic flow can occur during different stages

of pyroclastic deposition (Sumner and Branney, 2002 and references therein). Sumner and

Branney (2002) summarized four different models which can result in a rheomorphic flows. The first model involves a decelerating pyroclastic density current that slows to become a rheomorphic flow, the second model describes a laminar boundary between a rheomorphic flow and a still moving pyroclastic flow above it, the third model describes a rheomorphic flow stemming from remobilization of a hot stationary ignimbrite, and the fourth model describes rheomorphism occurring from agglutination and deformation at/within the base of a PDC.

Bachmann et al. (2000) describes possible occurrences of rheomorphic flow in small scale pyroclastic flows and explains how a smaller scale eruption can create pumice-rich, lithic-poor deposits, which may be applicable to the Nathrop Volcanics. At the southern end of the campground, a single round globular block-sized lava was found encased within the upper

portions of the vitrophyre, which may support rheomorphic interaction between a fresh, hot

pyroclastic deposit and a newly emplaced lava flow. 31

Perlitized vitrophyre (Tntvp) altered vitrophyre facies of Tntv Nickle (1987) preformed an in-depth study of the perlite using chemical and

petrographical analysis and concluded that the hydration of the vitrophyre was caused by slow

percolation of meteoric water through contacts between the vitrophyre and rhyolite unit.

Because of heterogeneities of permeability expected through volcanic deposits as well as areas

of high permeability associated with joints/faults and depositional contacts, the hydration of

the vitrophyre was not uniform and the contact between perlite and vitrophyre is patchy and

sporadic and in some places sharp and interfingering but in most places it is gradational.

Volcanic sandstone (Tnts) – sedimentary facies of Tnt The difference between reworked and primary pyroclastic deposits is that volcanic

sedimentary rocks have been weathered and eroded from the original primary deposit that

leads to rounding and sorting of clasts as well as possible incorporation of foreign materials

before being redeposited (McPhie et al., 1993). Assuming that the outcrops of the Nathrop

Volcanics are more or less showing their original extent, not much transport would have occurred. For the outcrop of volcanic sandstone shown in Figure 24, the only evidence for sedimentation is the resistance due to secondary cementation and a high amount of rounded pumice clasts within the 15 cm thickness of the facies. The less resistant layers appear to be near primary. Therefore, this author envisions the formation of the resistant layers by means of interaction with slow-moving water that had sifted through the fresh tephra during, or soon after, the emplacement of the pyroclastic fall. The three reworked, resistant layers in the deposit then could be explained by intermittent water ponding events, perhaps related to transient obstructions to small stream flow due to volcanic activity. 32

Tertiary Rhyolite (Tnr) – lava flows The rhyolite lavas located at the top of Ruby and Sugarloaf Mountains have been

described and agreed upon by previous works to have come from the same magma source,

some even stating the same chamber that produced the deposits on Bald Mountain (Honea,

1955; Schooler, 1982; Nickle, 1987; Bade, 1989). Previous petrographic and geochemical

studies have shown these lavas to be rhyolite, while the pyroclastic, vitrophyre, and perlite

deposits are mostly rhyolite with a few variations being dacite (Tables 2 and 3; Fig. 30). As

observed in this study, however, there are textural differences between Sugarloaf and Ruby

Mountain. Lavas on Sugarloaf Mountain frequently show a coarser-grained texture with a higher proportion and size of phenocrysts. Lithophysae consistency and size vary throughout both formations, but are typically elliptical and flattened parallel to flow band direction. The amount of float and alluvium that now cover the area, as well as the tectonically tilted beds have impeded this and previous studies from finding a vent based on lava flow bands and crystal size.

The lava itself is brought into question, however, by the rheomorphic flow scenario described previously. It has been shown that welded pyroclastic flow deposits can sometimes resemble lavas both texturally and mineralogically (Pioli and Smith, 2005; Smith and Cole, 1997;

Sumner and Branney, 2002; Manley, 1996; Geissman et al., 2010; Self et al., 2008; Bachmann et al., 2000). Both Manley (1996) and Smith and Cole (1997) compared textural features between pyroclastic deposits and lava flows. Outcrop-scale diagnostic features such as responses to topography and thickness of the flow that could be used to decipher rheomorphic ignimbrites from silicic lavas cannot be evaluated well at Ruby and Sugarloaf Mountains given the poor exposure and limited extent of outcrop. No outcrops of lava were found that exhibited basal 33 breccias to evaluate whether the sequence was a rheomorphic flow or a lava flow. Hand sample and smaller scale diagnostic features include the presence of internal breccias (common in lava flows), the orientation of lithics (ex: lens-shaped cavities around a rheomorphically rotated lithic), and phenocryst/pumice textures (broken and attenuated infer rheomorphic alteration).

Lack of internal breccias (pro-rheomorphic model), no rotated lithics (pro-lava flow model), and the existence of stretched pumice and broken phenocrysts (pro-rheomorphic model), leaves the debate open to a lava flow welded model or a rheomorphic flow model. Studies have been conducted to confirm the rapid rate at which rheomorphic flow features form (Geissman et al.,

2010), the characteristics of rheomorphic flows and in what ways that they can resemble lava flows (Manley, 1996; Sumner and Branney, 2002), and studies that show how syn-depositional structures in rheomorphic flows can be due to the momentum of overriding PDCs (Pioli and

Rosi, 2005). Future studies could investigate the origins of the Tnr further with attempts to find evidence of features mentioned above.

34

MODEL RECONSTRUCTIONS Using the results and discussion portions of the paper, this section will reconstruct the

formation, secondary alteration, and erosion and structural modification of the Nathrop

Volcanics. Three model reconstructions (Figs. 31, 32, and 33) are presented using the first three

scenarios given for the formation of the vitrophyre.

Model A: Lava-induced welding of pyroclastics Stage I – Pre/Early Volcanic Activity (Fig. 31A)

It is known from the geologic background and previous studies that the general area had less relief than present day. This does not rule out some topography, but suggests nothing more extreme than rolling hills. The presence of lava lithics within the lowermost and essentially oldest unit, Tntf, proves the pre-existence of volcanic material in the area before the

eruption recorded by the lapilli tuff.

Stage II – Main Volcanic Events of the Nathrop Volcanics (Fig. 31B and C)

Initial dome collapse that caused the release of an ash cloud would form the Tntf

pyroclastic fall deposit containing lava lithics (Fig. 31B). The location of the vent being closer to

present-day Sugarloaf is in conjunction with the overall decreasing size of the clasts of the

deposit as the deposit continues south to present-day Ruby Mountain. Not shown in the figure

but implied is the varying intensity of the eruption to form the various sublayers of reverse

grading in the tuff unit. Also shown in Figure 31B is the formation of the reworked pyroclastic

deposit, Tnts. Figure 31B shows a proposed water source interacting with the fresh ash fall

deposits. 35

Figure 31C details the resulting pyroclastic flow that followed the fall deposit. This would eventually form the pyroclastic breccia (Tntb), vitrophyre (Tntv), and perlite (Tntvp). An external water source might have provided the vapor that formed the pink breccia pipe on the

NE side of Ruby Mountain.

Stage III – Emplacement of Lavas and welding – Tntr (Fig. 31D)

The lava flow that followed the fall and flow deposits covered the area with a hard, resistant top layer that would become the youngest primary unit of the Nathrop Volcanics at

Ruby Mountain. The weight and heat from the overlying lava coupled with the still fresh and hot pyroclastic flow deposit was adequate enough to weld the upper portion of the tuff breccia into the purple vitrophyre (Tntv). The further the pyroclasts from the heated vent, the less intense the welding, hence the gradational contact from white to pink to purple seen in some outcrops.

Stage IV – Modifications by Faulting and Erosion (Fig. 31E and F)

Since the eruptions that produced the volcanic at ~30 Ma, hydration of the vitrophyre

(Tntv) began to form the perlite (Tntvp). The onset of rifting after the volcanism formed north- south trending normal faults along the eastern margin of the Arkansas Graben. This eventually caused the layers to incline from near horizontal to the current dip of the units that range from

~15° to ~65° in a WSW direction. Following the principle of lateral continuity and observing the offset of the layers between the Ruby and Sugarloaf Mountains and the campground area, it is likely that several oblique-slip faults extend east-west through the Nathrop Volcanics (Figs. 5 36

and 31E) accommodating the N-S structure of the rift. The area of the present day campground

was lowered relative to Ruby and Sugarloaf Mountains, allowing it to become more susceptible

to erosion (Fig. 31E). Finally, Figure 31F shows the results of the erosion and faulting, which is

how the Nathrop Volcanics appear today. Unlike previous studies (Schooler, 1983; Nickle,

1987), this study proposes that both Sugarloaf and Ruby Mountains, as well as the smaller

Dorothy Hill (a rhyolite knob to the west of the Arkansas river shown in Figs. 4 and 5) are not

three separate dome units, but one larger dome unit that has been altered through faulting to

change the present appearance of the area.

Model B: Welding in a thick pyroclastic flow Stage I – Pre/Early Volcanic Activity (Fig. 32A and 32B)

This stage of the pyroclastic erosion model starts very similar to the previous model. The

low relief area and pyroclastic fall (Tntf) units are the same, as are the reworking of the Tntf

units in the campground into the volcanic sandstone (Tnts) units seen today.

Stage II – Main Pyroclastic Event (Fig. 32C and 32D)

This stage differs with the previous model in that the resulting pyroclastic flow came from an eruption column collapse, not a volcanic dome collapse, releasing a much greater quantity of tephra that formed a tuff-breccia deposit of at least 120 m thick (Fig. 32C). Welding in the lower third of the deposit formed the vitrophyre (Tntv) immediately after deposition (Fig.

32D). The basal and upper portions of the flow remained unwelded due to cooling against the

ground and atmosphere, respectively.

37

Stage III – Erosion, Lava Extrusion, and modifications by faulting and alteration/erosion (Fig. 32E and 32F)

A period of little to no volcanic activity of unknown length occurred allowing the thick tuff breccia to be eroded down to the vitrophyre. At this point, volcanic activity resumed, and a lava flow covered the exposed vitrophyre, creating an unconformity (Fig. 32E). From this point on, the model resembles that of Model A (perlitization, faulting, erosion, and alluvium deposits)

(Fig. 32F).

Model C: Welding by Rheomorphic Flow Stage I – Pre/Early Volcanic Activity (Fig. 33A and 33B)

Like previous models, the early volcanic activity of a pyroclastic fall (Tntf) was followed by the reworking of the tephra (Fig. 33A). The difference here is the eruption style, producing a lower eruption column than in Model B. The collapse of a low eruption column caused a rapid accumulation of the tuff breccia near the vent.

Stage II – Rheomorphic Flow (Fig. 33C and 33D)

The hot pyroclastic flow compacts and welds under its own gravitational weight. The basal portion of the tuff breccia is cooled against the ground but the upper portion welds and flows producing lava-like textures.

Stage III – Alteration, Faulting, and Erosion (Fig. 33E) 38

Like Model A, the perlitization of the interior vitrophyre, as well as the transform faulting and erosion of the lava-like rheomorphic pyroclastic flow reveal the present day appearance of the Ruby and Sugarloaf Mountain area.

39

CONCLUSIONS The Nathrop Volcanics have been extruded, altered, faulted, and eroded to an extent

that the reconstruction of its history is challenging. This study has taken all previous viewpoints

and ideas and tried to combine with the results from this study to derive three visual models that detail the eruptive history as well as post-volcanic landscape evolution. This study has found evidence that the Nathrop Volcanics have undergone small amounts of pyroclastic reworking, welding of a volcanic breccia to form a vitrophyre, the subsequent hydration of that vitrophyre to form a perlite, the vapor phase alteration of a breccia, faulting and erosion. This study has gone further to explain different scenarios of how these events occurred, citing processes of pyroclastic welding in classic ignimbrites, welding pyroclasts induced by lava extrusion overtop, as well as considering a model that involves rheomorphic flow to explain welding and possible pyroclastic origin of what has been considered to be a lava flow.

Further study would help to decipher between the three models proposed here. For example, the geology and stratigraphy of Sugarloaf Mountain should be examined in detail and evaluated relative to the stratigraphy established in this study. Finding and describing the contact relationship between the ‘lava’ and vitrophyre is paramount to resolving the origin of each. Even if the contact cannot be found, the careful examination of the rhyolite ‘lava’ in outcrop, hand sample and thin section may shed more light on its origin. Radiometric dating may or may not be able to resolve age differences in the deposits, but a more detailed geochemical study, including the measurement of trace elements and mineral chemistry, may provide information on magma genesis and magma system dynamics. Mathematical and modeling studies could be performed to determine the realism of the interpretations of the 40 pyroclastic fall and flow deposits based on the resulting vent radius, exit velocity, volatile content, vent opening, etc.

41

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51

TABLES

Table 1: Summary of estimates for the age of the Rio Grande Rift

Table 2: Whole rock geochemistry determined by Honea (1955). 52

Table 3: Whole rock geochemistry determined by Schooler (1982). 53

FIGURES

Figure 1: Basin Geometry of the northern portion of the Rio Grande Rift (Modified from Karnuta, 1995). Inset: Map showing layout of the Nathrop Volcanics modified from Nickel, 1987.

54

Figure 2: Varying types of volcanic domes (Modified from Francis and Oppenheimer, 2004).

55

Figure 3: Diagram of particle concentration in the depositing pyroclastic material compared to the trajectories of the depositing clasts (Modified from Wilson and Houghton, 2000). Inset diagrams: Examples of bedforms from each pyroclastic process (Modified from Lockwood and Hazlett, 2010).

56

Figure 4: Aerial photograph depicting the Nathrop Volcanics, as well as the sample locations. Sample labels are ordered in chronological order from A-Z on the day collected (ex: Sample #2 [B], collected on June 21st = B621). 57

Figure 5: Geologic Map of the Nathrop Volcanics for the area shown in Figure 4. 58

Figure 6: Generalized stratigraphic section of the Ruby Mountain volcanics. 0-3 m: Sublayers of the fall deposit, Tntf. 3-21 m: Volcanic breccia, Tntb, formed from the pyroclastic flow. A gradational contact is between the breccia and perlite/vitrophyre unit. 21-29 m: Perlite layer, Tntvp, located on Ruby Mountain, but not a uniform layer. Breccia unit at exposed areas grade directly into vitrophyre (Tntv). 29-40 m: Dark purple vitrophyre with relic pumice textures in some areas. 40-60 m: Banded rhyolite lava (Tnr). 59

Figure 7: Overall area of observed pyroclastic unit on Northeast Ruby. Boxes outline the area of other figures. Unit labels follow the labels on Figure 4. Dotted lines show approximate contacts. 60

Figure 8: Three identified subunits of the Tntf based on coarsening up sequences and lava clast abundance. 61

Figure 9: Six identified subunits of the Tntf based on coarsening up sequences and lava clast abundance. 62

Figure 10: Coarsening up sequence between pumice and lava clasts shown in one of the Tntf subunits on NE Ruby. 63

Figure 11: Thin section from a sample of a Tntf subsequence in plane polarized light (A) and crossed polarized light (B) located at the Northeast side of Ruby Mountain. 64

Figure 12: View from Ruby Mountain to the North showing the layout of the Sugarloaf and campground area. 65

Figure 13: Massive white pyroclastic outcrop (Tntf) in the campground. 66

Figure 14: Tuff breccia deposit (Tntb) on the northeast side of Ruby Mountain.

Figure 15: View of the northern side of Ruby Mountain, with respect to observed units. Height from base of mountain to top is ~80 m. 67

Figure 16: Closer view of the red brecciated tuff. Red/pink boundary clearly identified. 68

Figure 17: Detail of Figure 16. The secondary breccia within the tuff breccia (Tntb) located on the north slope of Ruby Mountain. Note the angular shape of the pumice. Cap of red pen is ~4 cm.

69

Figure 18: Sample from the pink breccia pipe, showing the physical brecciation caused by gas escape. 70

Figure 19: Campground area. Pink Tnts and white Tntf are juxtaposed and show distinct contact between pink reworked pyroclastic deposit with bedding and the lapilli tuff. 71

Figure 20: Microscopic comparison between the pink and white juxtaposed tuff units in the campground. Upper images in plane polarized light. All images are at the same magnification. Scale bar shown in A. 72

Figure 21: South East side of Ruby Mountain, showing a sample of welded tuff breccia (vitrophyre – Tntv). Orientation of photograph relative to pumice clasts is perpendicular to flattening direction. 73

Figure 22: Thin section of the vitrophyre showing pumice under plane light (A) and crossed polarized light (B). 74

Figure 23: Outcrop of Perlite (Tntvp) at the mining pit on NE Ruby Mountain (for location refer to Fig. 7). Note overall concentric fracture defining outcrop texture. 75

Figure 24: (A) Image showing the overall stratigraphic layout of the sequence of pyroclastic deposits and the few resistant subunits identified as reworked pyroclastic units (Tnts). (B) The thickest reworked layer observed (Tnts), showing the thickness of this subunit as well as the textural differences of the pyroclastic deposits above it. 76

Figure 25: Perlite lithic within reworked pyroclastic layers (Tnts). 77

Figure 26: Rhyolite lava (Tnr) on southern summit of Ruby Mountain facing south. Scale 5 x 7 inch scale marker in middle foreground. Note the west dipping layers. 78

Figure 27: Lithophysae found within lava blocks of Tnr on Ruby Mountain. 79

Figure 28: Sample G624Rhy came from a rhyolite outcrop on Ruby Mountain, while G622Rhy came from an outcrop at Sugarloaf. Note the slightly coarser crystals in the Sugarloaf rhyolite. Both images in crossed polars. 80

Figure 29: (A) Maximum clast size of pumice found relative to distance to vent. The star symbolizes the maximum pumice size found at Ruby Mountain in the flow unit. (B) Maximum clast size of lithics found relative to distance to vent. Modified from Walker and Croasdale (1970). 81

Figure 30: A TAS Diagram showing the alkali vs. silica relationship of the Nathrop Volcanics using Schooler’s (1982) and Honea’s (1953) geochemistry data. Calculated with Le Bas et al. (1986) TAS model. Unit labels correlate with Tables #2 and #3. 82

Figure 31: Proposed evolution of the Nathrop Volcanics 83

Figure 31, continued: Proposed evolution of the Nathrop Volcanics. A: The pre-eruptive environment of the Nathrop Volcanics. B: Initial eruptions of varying intensity creates ash-fall deposits, some of which are reworked by local water sources. C: A pyroclastic flow covers the fall unit, while a migrated water source sets up the location for a gas escape structure. D: Lava caps the area, allowing the welded origin of the vitrophyre. E: Millions of years result in the gradual hydration of areas of the vitrophyre to form perlite, as well as the formation of faults and dips due to the onset of the Rio Grande Rift. F: Resulting erosion from weakened fault zones gives the appearance of the Nathrop Volcanics as seen today. 84

Figure 32: Pyroclastic Erosion Model 85

Figure 32, continued: Note the main differences between Model A and Model B. Instead of the small scale eruption seen previously, a larger column collapse creates a larger pyroclastic flow (Tntb) (C). D shows a traditional welding within the pyroclastic unit itself, and E shows the pyroclastics eroded down to the vitrophyre layer, with a fresh extrusion of lava on top. F shows the present day volcanics. 86

Figure 33: Rheomorphism Model

87

Figure 33, continued : A and B show a similar start to the volcanics similar to Model A, but C shows a low-column collapse that forms a slow, hot, pumice-rich pyroclastic flow that eventually becomes rheomorphic (D), forming an interior vitrophyre, with an intensely welded carapace that resembles a lava flow. E shows the repeated alteration, tectonics, faulting, and erosion.

88

APPENDIX

Table 4: Details of samples collected in this study.