Magmatic Insights from a Sedimentary Sequence in a Dynamic Volcanic

Center, Black Mountains, Arizona REU: Before and After a Supereruption

Scott Williams 12/1/2014

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Table of Contents

Abstract…………………………………………2

Introduction……………………………………..3

Geologic Setting……………………………….5

Methods………………………………………...7

Results………………………………………….10

Map……………………………………………..15

Discussion……………………………………..16

Conclusions……………………………………20

Acknowledgments…………………………….21

References…………………………………….22

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Magmatic insights from a sedimentary sequence in a

dynamic volcanic center, Black Mountains, AZ

Scott H. Williams

REU: Before and After a Supereruption, Vanderbilt University, Nashville, TN 37235,

Occidental College, Los Angeles, CA 90041

ABSTRACT

The >640km3 (DRE) Peach Springs Tuff (PST) and its source, the Silver Creek

Caldera, are a well exposed example of a supereruption. In this study, we sought to

understand more fully the magmatic, environmental, and topographic evolution of this

supervolcano by examining a sedimentary section 5km north of the caldera rim in the

Black Mountains of northwestern Arizona (Union Pass quadrangle). Our mapping along

a 4km long north-south transect followed an erosive unconformity that coincides in time

and stratigraphy with the locally absent PST. A welded tuff overlying the unconformity is

17.8 Ma, and the youngest date from the pre-unconformity trachytes is 18.9 Ma,

indicating that the 18.8 Ma PST outflow is missing (Lang et al 2008). XRF, SEM, and

thin section analyses of sediments and lavas from this sedimentary basin show that the

unconformity marks a boundary, dividing trachytic lavas and their derivative sediments

from rhyolitic, and mostly explosive, volcanism. This indicates a shift in magmatism near

the time of the PST eruption. In one location, the post-PST unit fills a channel in the top

of the pre-PST sandstones at the contact, and in most locations the pre-PST

sandstones become friable. In some locations they grade up into the post-PST strata.

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These observations indicate an erosional, rather than nondepositional unconformity.

Though nondeposition cannot be ruled out merely by the presence of an erosive surface

where we expect to see the PST, the unconformity may represent removal of the PST

via erosion shortly after eruption. This relatively quick turnaround from a sedimentary

basin to an erosive surface could suggest tectonic activity or resurgent uplift at or near

the time of the PST eruption.

INTRODUCTION

The Peach Spring Tuff is an 18.78 Ma ignimbrite unit found over a wide area of

the Southwest, from the edge of the Colorado Plateau to an area near Barstow, CA,

over 150 km from the caldera (Nielson et al 1990). Its wide coverage was a main reason

that Glazner et al. initially chose it as their correlative marker unit for studying Tertiary

extension in this region (Glazner et al, 1986). Later interest in the PST came from

studying it as a “supereruption,” defined as a silicic eruption that produces over 1000

km3 of material (Buesch, 1992). The exact source location of the PST was not known

until 2012 when Ferguson et al. identified the Silver Creek Caldera in the Black

Mountains of northwestern Arizona.

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Petrologic studies in recent years have focused on finding

triggers for the PST eruption that may suggest causes for other supereruptions (Frazier

2013, McDowell, 2014, Pamucku et al., 2013). In particular, these authors have followed

a line of evidence for a mafic injection into the mostly crystallized PST magma body that

heated and mobilized it.

The PST is a prominently thick and wide unit throughout the eastern Mojave

Desert, yet it is missing in the Secret Pass Canyon area less than 5km away from the

Silver Creek Caldera. The main motivation for my study was an attempt to understand

why the PST is missing so close to its source. The three most obvious causes could be:

1. Nondeposition, likely as a result of topography blocking the pyroclastic outflow.

2. Erosion: removal of the PST after eruption and deposition in this area.

From Pamucku et al 2013: Extent of the Peach Spring Tuff,

with inset of Silver Creek Caldera.

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3. Tectonic/structural removal by normal and/or strike-slip faulting: given that this is

a highly extended region, the PST and other units of the same age could have

been displaced.

My field partner Jake Lee and I mapped and established a detailed stratigraphy in the

Secret Pass area in order to learn which of these possibilities is more likely.

GEOLOGIC SETTING

The PST is the most voluminous eruption in a flareup of volcanism that coincided

with the initiation of the Colorado River Extensional Corridor (CREC) in the Black

Mountains area. This extension swept northward during the Miocene, preceded by

magmatism by a margin of 1 to 4 m.y. CREC extension is believed to have been caused

by cessation of the subduction zone of the Farallon under the North American plate,

followed by initiation of a transform boundary along the San Andreas Fault (Faulds et

al., 2001).

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The >640km3 (DRE) PST is compositionally zoned. Intracaldera and outflow

material are the same age and demonstrate compositional overlap, but intracaldera is

more trachytic (65%–68% SiO2) while outflow is rhyolitic (68%–76% SiO2) (Ferguson et

al., 2012). Due to its volume, the PST is a significant unit throughout the Kingman, AZ

area, so its absence in part of the Black Mountains is intriguing (see map above).

From Ferguson et al. 2008: Map

of Silver Creek Caldera. Secret

Pass Canyon is in the NE corner.

Units that we studied are

“clastics” and “dacitic volcanics”

on this map in dotted orange.

Unconformity can be seen in the

cross section below.

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The Black Mountains are a fault block tilted ~30°NE

(Lang et al., 2008). Lang describes the Secret Pass

stratigraphy as dominated by volcanics and tuffs, with

minor sediments (at right):

“consists of a trachydacite (unit Td) that interfingers with

volcanogenic sediments and tuffs (unit Tst), overlying

volcanogenic sediments and tuffs (unit Tvs), rhyolitic

breccias (unit Tbr), and lava flows (unit Trf).”

In the field, we broke out many new units within Lang’s

units Tst and Tvs in order to understand the stratigraphy

around the unconformity in greater detail. The sedimentary

sequence is capped by 17.8 Ma welded tuff and overlies

18.9 Ma trachyte lava (Ages from Lang et al 2008). These

sediments and the unconformity within them straddle the

age of the 18.8 Ma PST and could therefore hold the

answer to why it is missing.

METHODS

My project was conducted in two stages. In May of

2014 I mapped and sampled in the Black Mountains with Jake Lee and, intermittently,

Calvin Miller and Nick Lang. In June, we analyzed these samples in laboratories at

Vanderbilt University and Middle Tennessee University. My research was tied closely

with Lee’s. In addition to mapping together, we collected one set of samples, which we

Stratigraphy from Lang et al.

2008: Generalized stratigraphic

column of the Secret Pass Canyon

volcanic center. Tbr—rhyolite

breccias; Td—trachydacite; Ti—

felsic dikes and sills; Tir—rhyolite

dome; Trf—lava flows; Tst—

volcaniclastic sediments and tuffs;

Tvs—volcaniclastic sediments;

Xg—Proterozoic granite.

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later divided up based on research focus. Lee chose to analyze mostly trachytic

sandstone and interbedded lava samples, while I focused on analytically comparing the

trachytic to the pumiceous sandstones. Because our research was so interrelated, we

conducted our XRF, SEM, and thin section preparations and analyses together,

collecting the data as one set and dividing them later.

Field Work

Lee and I sought to understand more precisely the magmatic, environmental, and

topographic evolution of this supervolcano by examining a sedimentary section 5km

north of the Silver Creek Caldera rim. We chose the sedimentary package found below

Secret Pass Canyon to find evidence for what happened to the PST around the Silver

Creek Caldera because by nature sediments record a continuous record through time

unless interrupted by unconformity. Therefore, the Pumiceous and Brown Sandstones

ought to contain the PST because they contain the time of its eruption within their

stratigraphy.

We worked with Lang’s general stratigraphy from 2008 and broke out new units

within his “Tst” and “Tvs.” The 4 km-long section that we mapped along a northwest-

southeast transect records at least two unconformities (see schematic). We were

especially interested in the lower unconformity found near the top of the main

sedimentary package in the area because the PST was erupted during the time that it

erased.

Mapping was conducted in the southeast corner of the Union Pass 1:24,000

scale 7.5’ USGS quad over the course of 10 days. We followed the lower unconformity

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(“the” unconformity in this paper) along the extent of its exposure between two wide

washes where these units are covered by Quaternary colluvium and alluvium. Our

mapping was much more detailed than previous work by Lang and Ferguson, so a

major part of our time in the field was spent breaking out new units and describing the

stratigraphy of the Secret Pass volcanogenic sediments. During the mapping process

we identified the exact stratigraphic position of the unconformity and its relationship to

minor sedimentary strata that widen and then pinch out along the transect path. As we

mapped, we collected samples from the pumiceous sandstones above the unconformity

and the sandstones and lavas that interbed and mingle with them below the

unconformity. All samples and unconformity locations were recorded with GPS

coordinates and marked on the map.

Laboratory Analyses: XRF, SEM, Thin Section

In the lab, we hoped to find clasts or phenocrysts in the Secret Pass sandstones

that were definitely eroded from the PST, which would be strong evidence that the PST

was eroded during the unconformity time and redeposited in these pumiceous

sandstones.

Thin sections: The 10 thin sections were especially valuable for examining textures,

microstructures, and interactions between the components of our sediment and lava

samples. We identified lithic clasts found within the sandstones, confirmed the presence

of pumice in the pumiceous sandstones, and estimated pumice abundance to look for

variation within the unit.

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XRF: The five successful XRF analyses provided data for concentrations of major and

selected trace elements in our samples. The purpose of these XRF data was to develop

a generalized picture of how the overall composition of the material found in this

paleovalley changed as it deposited over time, especially with respect to the

unconformity and potentially the PST eruption.

SEM: The 10 SEM mounts were used to calculate precise major element concentrations

of phenocrysts, lithic clasts, pumice, and groundmass. We scanned wide crystal-poor

areas for estimates of groundmass composition and scanned most of the larger crystals

in each sample. Results from SEM analyses were similar to those from XRF, but more

localized and specific to particular crystals and clasts within our samples. They were

therefore useful for distinguishing the chemistry (and interpreting the origin) of the

various lithic clasts, pumice, and crystals found within our sandstone samples.

RESULTS

Map and Stratigraphy of Secret Pass Canyon

Mapping revealed that the unconformity is an erosional disconformity between

Brown Sandstone (Tvs) and Pumiceous Sandstone (Tps) (see map and field photo). We

found two erosive channels large enough

to map. The first is the “lower” Pumiceous

Sandstone unit that cuts down into Brown

Sandstone (yellow Tps on map and

schematic). This essentially forms a

“sandwich” relationship with Brown

Erosive channel at the unconformity contact

between Brown Sandstone and Pink Sandstone

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Sandstone between these two Pumiceous Sandstone units. The unconformity forms the

contact between the upper Pumiceous and Brown sandstones, so the lower Pumiceous

Sandstone was below the unconformity in the stratigraphy that we established.

The second channel is a finger of Rhyolite that cuts down through Pumiceous

Sandstone and the unconformity, and makes contact with Brown Sandstone, indicating

the presence of another unconformity after the one which we focused on in this study.

This channel was approximately 100m wide and cut down through 20m of stratigraphy.

Brown Sandstone (Tvs) is a lithic clast-rich, brown to purple, thickly bedded to

massive pebbly volcanogenic sandstone. It is feldspar-rich and quartz-poor, with

angular to sub-rounded grains and occasional cobble-sized clasts (in some cases they

may be bombs) and channel structures. This unit was broken out of Lang’s unit Tvs.

Schematic stratigraphy and NW-SE cross section of map area Tr: various rhyolite tuffs and lavas (post-PST) Ti: ignimbrite unit at the base of the rhyolite package (17.8 Ma) Tps: pink pumiceous sandstones, and reworked tuffs immediately overlying the unconformity Tvs: Brown trachytic sandstones underlying unconformity Tsl: Red unit made of lava and wet sediment interaction, coeval with brown sandstones Tt: trachyte lavas and related volcanogenic sediments (~18.9 Ma). Age from Lang et al, 2008 Xg: 1.4 Ga basement gneiss/Proterozoic granite. The PST is missing here, so it would fall between Tt and Ti if it were present in this area. Though two unconformities are shown, we focused on the lower one because of this age relationship.

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Pumiceous Sandstone is a poorly consolidated pink pumiceous sandstone; its

individual laminations and beds are likely reworked tuff deposits. This unit is also broken

out from Lang’s Tvs. In some places, we found that Brown graded up into Pink

Sandstone (see field photo). The contact in these places has four facies as seen below,

described bottom to top:

1. Brown Sandstone appears

normal: medium-grained, massive,

well-lithified and coherent.

2. Brown Sandstone is coarse-

grained, poorly-sorted, and extremely

friable. The contact between First and

Second facies is irregular.

3. Friable Brown Sandstone

grades up into Pumiceous. First,

Brown Sandstone acquires pumice,

which increases upward in abundance.

Then other characteristic Pumiceous Sandstone clasts appear.

4. Pumiceous Sandstone appears as it does in many places, but moderately friable.

Trachyte Lava (Tt) is a trachyte lava flow

package with interbedded volcanogenic sediments.

This unit is the same as Lang’s Td, and Lee and I

confirmed that it is interbedded with the base of Brown

Sandstone. The contact between Brown Sandstone

Trachyte Lava showing flow banding and autobreccia

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and Trachyte Lava flows is very irregular on both the map and the outcrop scales. In

addition, Trachyte Lava often mixes, mingles, and interbeds with the lower 10-20m of

Brown Sandstone.

Two units are interbedded within the Brown Sandstone: a Sediment-Lava mixture

unit (Tsl) and a finger of pink to white pumiceous sandstones 10m below the top of

Brown Sandstone. I have labeled this unit Tps on the map and stratigraphy because in

the field this finger was lithologically identical to the Pumiceous Sandstone (Tps) above

the unconformity, yet appeared below the unconformity. This lower Pumiceous

Sandstone pinches out after about 100m

and does not reappear anywhere else

along the transect. Sediment-Lava is

made up of two mafic lava peperite flows

that are interbedded with Brown

Sandstone and in some locations show

evidence of lava-wet sediment interaction

(Lee et al., 2014). Sediment-lava was

brick-red and had baked the underlying

sandstone, and in some places the

sandstone was indistinguishable from

lava or surrounded by tongues or blobs of lava. Sediment-Lava maintains a consistent

thickness along the transect until it pinches out in the northern end.

Rhyolite (Tr) is the cliff-forming package of rhyolite tuffs, lavas, and block-and-

ash flows at the top of the stratigraphy in Secret Pass Canyon. This unit forms the cap

Sediment-Lava interaction in an outcrop of Tsl.

Note some blobs/clasts have discrete boundaries

while some appear to have disintegrated.

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of the mountains in my field area as well as the top of the stratigraphy. Ignimbrite (Ti) is

a 5m thick welded ignimbrite at the base of Rhyolite. Lang et al. dated this unit at 17.8

Ma in 2008, and Ignimbrite and Rhyolite are broken out from Lang’s Tbr.

A single fault cuts across the package at the northern end with a 20m

displacement. The sedimentary package dips northeast between 10 and 30 degrees

along the extent of the transect. One large finger of Trachyte Lava extends nearly to the

unconformity line, but is covered by 10m of Brown Sandstone before the unconformity.

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Map of Secret Pass Canyon area:

Sample locations are shown with white diamonds. Units are labeled the same

way on the schematic diagram. The Unconformity is marked by the bold pink

line.

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Laboratory Analyses (Results)

Our thin sections were used to reinforce our field hand sample observations. In

Brown Sandstone, we found a majority lava and some basement gneiss clasts, many

plagioclase and biotite grains, and some secondary calcite. Lee found textural evidence

for wet sediment-lava mingling in Sediment-Lava (Lee et al., 2014). Pumiceous

Sandstone was made up of 40-60% pumice, with common sanidine, biotite, and

plagioclase, and a few fine-grained felsic intrusive lithic clasts. Most grains other than

pumice were angular, and some crystals showed evidence of shattering during eruption.

We have whole-rock

chemistry from the XRF broken

down by SiO2, Al2O3, Fe2O3, MnO,

MgO, CaO, Na2O, K2O, TiO2, and

P2O5, plus ppm amounts of Sr, Zr,

Rb, Y, Nb, and Ba for four samples

from below the unconformity and

one from above it in Pumiceous

Sandstone (See graph).

Our SEM scans supported

what we saw in our thin sections, but more concisely and definitively: we found more

sodic and calcic plagioclase and trachyte lithics in Brown Sandstone, while Pumiceous

Sandstone was rich in pumice, potassic feldspars, biotite, and some quartz.

XRF Results: Brown points represent pre-

unconformity samples. Pink point represents sample

from Pumiceous Sandstone.

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DISCUSSION

My work reveals a paleovalley near the caldera that filled with trachyte lavas,

trachyte-derived sandstones, pumiceous sandstones, and finally rhyolite tuffs and lavas.

The accumulation of this material was interrupted by at least two unconformities. My

study focused on the lower unconformity between Brown Sandstone and Pumiceous

Sandstone because of the ages that these units span.

Evidence for my interpretation that there is an erosive unconformity in the Secret

Pass Canyon include the following:

1. Erosive channels on outcrop and map scales

2. Pumiceous Sandstone unit found below the unconformity that pinches out

3. Brown Sandstone that suddenly becomes friable and grades up into Pumiceous

Sandstone

4. Absence of Peach Spring Tuff

5. Presence of younger unconformity in which Rhyolite fills a channel in Brown

Sandstone

The presence of graded contacts along the presumed unconformity line initially

contradicted our unconformity hypothesis, but after further investigation it is apparent

that the top of Brown Sandstone was eroded and redeposited on top of the

unconformity, mixing with Pink Sandstone and forming a graded contact as it deposited.

Evidence for this comes from the outcrop mentioned above that contained a well-

cemented section of the top of Brown Sandstone, overlain by a very friable, coarse-

grained Brown that grades up into Pumiceous Sandstone.

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The irregular contact between the Trachyte Lava and overlying Brown

Sandstones and the lava-sediment mixing and interbedding both indicate that this

contact represents continuous deposition between the two units. Therefore, there was

not a break in deposition (unconformity) of Trachyte Lava before Brown Sandstone

began to deposit. The presence of Sediment-Lava shows that the basin remained an

active volcanic area even as it filled with volcanogenic sediments.

On the other hand, my mapping revealed several buttresses and irregularities in

the top of Trachyte Lava. This is to be expected in a volcanic terrain in which lavas do

not adhere to the lateral continuity that sediments do, but it also shows that the

paleotopography was in rapid flux throughout the time recorded by these lavas and

sediments around Secret Pass.

Laboratory analyses essentially summed up to one conclusion: pre-unconformity

rocks--and thus pre-PST--are more mafic and trachytic than post-unconformity. I did not

use the XRF trace element data and focused on SiO2, Al2O3, CaO, Na2O, and K2O to

link the sediments to the type of rock they eroded, i.e. trachyte or rhyolite.

Our XRF results were somewhat weak because our samples returned with total

elemental values too low to be considered precisely accurate. Our totals ranged from

85% to 95%. These inaccuracies are most likely caused by the presence of H2O and/or

CO2, which the XRF does not record and are present in sedimentary and altered

volcanic rocks. All rocks had high Ca counts (~4%-6.5%) which is to be expected in

trachytic rocks but is unusual in rhyolite. This could simply represent the presence of

secondary carbonates from fluid alteration.

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The trend that can be seen from the samples hints that material below the

unconformity was much more mafic (67-69%SiO2) in comparison to afterwards in

Pumiceous Sandstone (77%SiO2).

The exception to this trend is the short Pumiceous Sandstone below the

unconformity that pinches out. I interpret this “sandwich” unit to be filling a wide channel

in Brown Sandstone, indicating another (third) small unconformity that may have been

erased mostly by the main one. This also means that Pumiceous material was being

deposited as it cut down into Brown, but more Brown Sandstone was deposited before

the rest of Pumiceous was. In the overall story of this paleovalley, however, this third

unconformity is essentially part of the same kind of uplift that took place between the

two dated units. I cannot determine which unconformity may have removed the PST, so

it is reasonable to refer to them as a biconformity that essentially makes up a single

geologic event.

I had hoped to use the SEM data to find a chemical signature of the PST, but this

method proved inconclusive in that respect; we have data that show a distinct change in

magmatic composition after the PST eruption, but lack data that confirms that the PST

makes up some part of Pumiceous Sandstone clastic material. It remains a possibility

that the PST was eroded and redeposited to form Pumiceous Sandstone. The

unconformity line itself marks a distinct compositional change, which would be expected

when the Peach Spring magma chamber emptied its volume into a supereruption and

then collapsed. A new generation of petrologically distinct material would follow the

eruption, and I found just that following the unconformity.

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As mentioned above, Lang et al. provided dates from Trachyte Lava below

(~18.9 Ma) and Ignimbrite above (17.8 Ma) the sediments and unconformity. Based on

these dates, I know that if the 18.8 Ma PST were present here it would be somewhere

between the Trachyte Lava and the dated Ignimbrite, either interbedded with the

sediments or within the unconformity time. The PST is absent here, so I look to the

unconformity, inferring that the time erased by the lower unconformity includes the age

of the PST eruption. This is significant because it implies that the PST was eroded

during the unconformity. The observation that the unconformity is erosive supports this

theory, and the Pumiceous Sandstone could be reworked PST based on its

components in the field.

Therefore, I claim that the Peach Spring Tuff was deposited when it erupted

18.78 Ma, and subsequently eroded during the unconformity between Brown Sandstone

and Pumiceous Sandstone. Furthermore, it is likely that the PST makes up some or all

of the clastic material above the unconformity in the Pumiceous Sandstone.

One important question (ripe for further study) remains: Why did this paleovalley

alternate so rapidly from a basin collecting lava and sediments to an erosive surface,

and back to a sedimentary basin again? One likely cause could be uplift and doming

caused by the PST magma body’s fluctuations. There would have been a short swelling

or resurgent doming event during the unconformity time, followed by a period of

subsidence after the supereruption in which tuff-derived sediments flooded the area.

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CONCLUSIONS

My research showed that the mappable sub-units in the Secret Pass area are

useful for understanding the nature and timing of the unconformity that coincides with

the Peach Spring Tuff eruption. This unconformity temporally and spatially divides

effusive trachytic lava and its derivative sandstones from explosive rhyolite and its

derivative pumiceous sandstones. The dynamic cycles of uplift, erosion, subsidence,

deposition, and eruption that formed this stratigraphy provide important information for

the understanding of supereruptions. It is apparent that this area was turbulent both

before and after the PST eruption, not only during it. The causes for these cycles are

important questions for further study.

ACKNOWLEDGMENTS

I would like to thank Calvin Miller, Aaron Covey, Susanne McDowell, Lily

Claiborne, Charles Ferguson, J. Warner Cribb, Brandon Browne, Jake Lee, my fellow

REU researchers, and my advisor Margaret Rusmore for their generous and patient

guidance in the field, the classroom, and the laboratory. In addition, I would like to

acknowledge the support of the National Science Foundation for the REU grant EAR-

120523, Middle Tennessee University for XRF analyses, and Occidental College for its

support.

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REFERENCES

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