Ubehebe GSA Final Final - Geological Society of America

Volcanology of Ubehebe Craters, Death Valley, California: Geochemistry and Size-Frequency of Basalt Fragments in Ejecta
Shereena S. Dyer1, Eugene I. Smith2
University of Nevada, Las Vegas, Department of Geoscience
dyers@unlv.nevada.edu1, gene.smith@unlv.edu2
Abstract
Ubehebe Craters, in Death Valley National Park, consist of a group of maar
craters formed during Holocene time. The biggest and youngest of these is
Ubehebe Crater (800 m wide, 235 m deep). Little Hebe Crater, which is
about 100 m wide, lies just south of Ubehebe Crater. Earlier studies
suggested that the ejecta blankets (covering 15 square km) were mainly
composed of pyroclastic surge deposits. Although these earlier studies
established style and sequence of eruption, little is known about the
geochemistry of basalt clasts in ejecta, details of an eruption, or the origin
of distal ejecta deposits. For this study, 32 samples were collected from four
sites: three are 0.2 to 0.4 km west of the main crater and the fourth is Little
Hebe Crater, 0.2 km south of the main crater. All of the samples are older
than those associated with the eruption of Ubehebe Crater. After collection,
each sample was washed, photographed, and the size of clasts digitally
measured. Basalt clasts are less than 10 mm in size and are identical in
chemistry to a lava flow exposed in the wall of Little Hebe Crater. These
clasts, therefore, represent ripped up bedrock and not new magma.
Chemically, Ubehebe basalt is similar to basalts in the Greenwater Range
and volcanoes near Yucca Mountain. Yogodzinski and Smith (1995)
proposed the Amargosa Valley Isotope Province (AVIP) outlining basalt fields
with enriched Nd and Sr isotopes and distinctive trace-element signature.
Originally the AVIP included volcanoes near Yucca Mountain and those in
the Greenwater Range east of Death Valley. Basalt from Ubehebe Craters is
identical in chemistry to AVIP, suggesting that the AVIP should be enlarged
to include Ubehebe Craters. The sizes of over 2000 clasts were measured
and median and Inman sorting coefficients were calculated for each sample.
Compared to size-sorting data from other volcanic fields and work by Crowe
and Fisher (1973), the sampled deposits are more typical of pyroclastic
air-fall deposits than surge. Variations in median size and sorting values
represent deposition of ejecta from a series of eruptions each having a
different energy. In summary, eruptions at Ubehebe were dynamic, ripping
up basalt with AVIP chemical signatures into fragments usually less than
10 mm in size and depositing them during numerous pyroclastic air-fall
events.
Geochemistry
Ubehebe Craters erupted basalt tephra and flows. Chemically lavas are
potassic trachybasalts.
The basalt in the ejecta geochemically matches the basalt of a flow at the
base of Little Hebe Crater (Site 4 Layer 1). This suggests that these clasts
represent ripped up bedrock and not new magma.
Amargosa Valley Isotope Province (AVIP)
Yogodzinski & Smith (1995) defined the AVIP as an area of the mantle that
has higher water content or higher temperatures (compared to the
surrounding mantle). Therefore, the AVIP represents an area of the mantle
that is more likely to melt, and this implies a greater risk of future volcanic
activity within the AVIP.
The AVIP is geochemically defined by volcanoes with:
• Low εNd (-4 to -10) and moderate initial 87Sr/86Sr (0.705 to 0.708).
Basalt Classification after LaBas et al., (1986)
• Enrichment in light rare earth elements (LREE’s) and large ion
lithophile elements (LILE’s) compared to ocean island basalt (OIB)
(such as La and Rb).
• Depletion in high field strength elements (HFSE’s) (such as Nb and Ta).
Ubehebe basalt and mafic volcanic rocks within the AVIP contain identical
geochemical signatures, suggesting that the previously defined area of the
AVIP should be enlarged to include Ubehebe Craters.
Base Surge or Air-Fall?
Crowe and Fisher (1973) sampled ejecta at Ubehebe Craters. They measured
the sizes of clasts in ejecta and then calculated the median diameter (MdΦ)
and Inman (1952) sorting coefficients (σΦ (phi deviation)). They then plotted
these data on a diagram created by Walker (1971) which distinguishes
base surge from air-fall deposits.
Implications for Variations in Eruption Energies
Crowe and Fisher (1973) concluded that the majority of ejecta at Ubehebe
Craters was deposited by base surge and not air-fall. My measurements
show, however, that a significant amount of ejecta at Ubehebe Craters was
deposited by air-fall (Figure 5). Also, variations in median diameter and sorting
coefficient up section suggests eruption energy varied considerably during
eruption (Figure 2 and Figure 3).
Determining the style of deposition of ejecta at Ubehebe Craters is important,
because it provides insight into future styles of deposition if an eruption
were to occur again.
Figure 2 Diagram showing a variation in the median diameter of ejecta
sampled at Site 2. The base of the section is represented by Site 2 Layer 1
(S2L1), and the top of the section is represented by Site 2 Layer 8 (S2L8).
Photographs
Trace Element Geochemistry
Note: S4L1 is
the flow at the
base of Little
Hebe Crater.
Style of Deposition
Figure 5 Diagram showing the median diameter and phi deviation of samples.
Plot originally designed by Walker (1971), but used by Crowe and Fisher (1973).
This plot is adapted from Crowe and Fisher (1973) and shows that the
samples collected in this study are air-fall and not base surge deposits.
Ubehebe Craters
Conclusions
Figure 1-A View of eastern crater
wall of Ubehebe.
Alkaline vs. Subalkaline
Figure 1-B Having fun while hard
at work.
Figure 3 Diagram showing a variation in the Inman (1952) sorting coefficient
of ejecta sampled at Site 2. The base of the section is represented by Site 2
Layer 1 (S2L1), and the top of the section is represented by Site 2 Layer 8
(S2L8).
Goals
1. To determine and compare the geochemistry of basalt fragments in ejecta.
2. To determine the nature of eruption and the origin of basalt fragments
in ejecta.
Crater Flat
Histograms
Greenwater Range
Map of Site Locations
Figure 1-C Sampling ejecta at Site 1
from the base to the top.
Figure 1-D View of the section at
Site 3.
• Ubehebe basalt has the same geochemical signature as mafic volcanics of
the Amargosa Valley Isotope Province (AVIP). This suggests that the AVIP
should be enlarged to include Ubehebe Craters.
• Basalt in ejecta matches the chemistry of a basalt flow at the base of Little
Hebe Crater indicating that these clasts represent ripped up bedrock and not
juvenile magma.
• Size-sorting data show that the ejecta at this locality was erupted as
pyroclastic air-fall rather than as base surge as originally suggested by
Crowe and Fisher (1973).
• Variations in the median size and sorting values (phi deviation) between
sampling sites probably represent a series of eruptions with differing
energies.
• Overall, the basalt sampled at Ubehebe Craters represents ripped up
bedrock, with AVIP chemical signatures that was deposited as pyroclastic
air-fall.
References & Acknowledgements
Sodic vs. Potassic
Crowe, Bruce M., Fisher, Richard V., 1973, Sedimentary structures in base-surge
deposits with special reference to cross-bedding, Ubehebe Craters, Death
Valley, California, Geological Society of America Bulletin, v. 84, p. 663-682.
Map of Amargosa Valley Isotope Province (AVIP)
Inman, Douglas L., 1952, Measures for describing the size distribution of
sediments, Journal of Sedimentary Petrology, v. 22, no. 3, p. 125-145.
Figure 1-E View of northern wall of
Little Hebe Crater.
Figure 1-F Sampling ejecta at Site 2
from the base to the top.
LeBas, M.J., LeMaitre, R.W., Streckeisen, A., and Zanettin, B., 1986, A chemical
classification of volcanic rocks based on the total alkali silica diagram,
Journal of Petrology, v. 27, p. 745-750.
Walker, G.P.L., 1971, Grain-size characteristics of pyroclastic deposits, Journal of
Geology, v. 79, p. 696-714.
Figure 1-G Macro view of lapilli in
outcrop.
Figure 1-H View of southwestern wall
of Little Hebe Crater.
Figure 4 Set of histograms from Site 2 (Layers 1, 3, 5, & 7) which show the
size-frequency (calculated in phi) of basalt fragments in ejecta. Site 2 Layer 1
represents the base of the section, and Site 2 Layer 7 represents the second
to last layer at the top of the section.
Yogodzinski, G.M., Smith, E.I., 1995, Isotopic domains and the area of interest
for volcanic hazard assessment in the Yucca Mountain area: Eos (Transactions,
American Geophysical Union), v. 76, no. 46, p. 669no.
We acknowledge the help of S. Robinson, Judy Costa, Brandy Viscaino, and
Racheal Johnsen, the National Park Service for issuing a collecting and study
permit, and Inyo County, California for partial funding.