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A 10 million year old ash deposit in the Ogallala Formation of the Texas Panhandle.

Abstract. -- A vitric ash bed on the east bank of West Amarillo Creek, Potter County, of the Texas Panhandle occupies a portion of a paleovalley in the Ogallala Formation of late Tertiary age. Major element and selected trace element analyses and comparison of similarity coefficients with other known ashes in the western United States suggest a source in the Twin Falls Volcanic Field of southern Idaho. A K-Ar age determination of 9.5 Ma is consistent with this correlation.

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Late Cenozoic silicic ash fall tephra are well documented across the Great Plains. The youngest of such tephra include the family of "Pearlette" ashes which occur throughout the Great Plains (Izett & Wilcox 1982). The "Pearlette" tuffs were generated by major explosive eruptions in the Yellowstone Plateau Volcanic Field at the active end of the Yellowstone hotspot track between 2.1 Ma and 0.64 years ago (Christiansen 2002). Older late Cenozoic silicic tuffs are also widespread in the Great Plains where they are commonly reported from the middle to late Miocene Ogallala Group (Swineford et al. 1955; Skinner & Johnson 1964; Swinehart & Diffendal 1997). As shown by ongoing research at the University of Utah (Perkins et al. 1995; Perkins 1998; Perkins & Nash 2002) the silicic ash fall tephra in the Ogallala Group are dominantly from eruptions in older Yellowstone hotspot volcanic fields located across southern Idaho.

While there are a number of known occurrences of Pearlette family ash beds in Texas (Izett & Wilcox 1982) the authors are aware of only two reported occurrences of middle to late Miocene age silicic tephra in Texas. These are (1) a late Miocene tephra which entombed the Hemphillian fossils of the Coffee Ranch local fauna in the Texas Panhandle (Voorhies 1990; Passey et al. 2002) and (2) a middle Miocene tephra along the Pecos River in southwestern Texas (Powers & Holt 1993). Analyses done by the junior author indicate that both of these tephra are from Yellowstone hotspot sources and correlate with known tephra in the Basin and Range sections of Perkins et al. (1998). Thus, the new tephra occurrence reported here is a significant addition to the sparse catalog of known Miocene tephra in Texas.

LOCATION AND EXTENT

Detailed geologic mapping along West Amarillo Creek, Potter County, Texas, revealed a small outcrop of volcanic ash on the east bank of the creek and a few hundred meters north of Wildcat Bluff (Fig. 1). The latitude and longitude of the volcanic ash outcrop are 35[degrees]14'38" N, and 101[degrees]56'36" W. The volcanic ash is light gray with a maximum exposed thickness for the relatively pure ash of 1.2 m and a maximum horizontal extent of about 12 m (Fig. 2). The average present-day density of the clean ash is 1.7 gm/cc. The base of the clean ash layer is approximately 7 m above the bed of West Amarillo Creek and at the level of the highest and oldest of three fluvial terraces formed by the Creek (Cepeda 2001). The layer of clean ash (Fig. 3), is overlain by two ledges of light pink quartz sandstone of the middle part of the Ogallala Formation. Each of the sandstone beds is approximately 1 m thick and contains a mixture of ash and quartz grains (Fig. 4) suggesting some reworking of the ash deposit and mixing with stream sand during deposition of the Ogallala Formation. Percentage of ash particles in the sandstone decreases with increasing distance above the clean ash layer and is not seen in samples more than 4 m above the ash layer.

The location of the outcrop of ash is within a broad, shallow paleovalley in the Ogallala Formation that trends in a generally northwest to southeast direction. The exact trend of the paleovalley is difficult to determine because erosion has removed the Ogallala Formation to the north and west.

MATERIALS AND METHODS

Samples of the clean volcanic ash layer and of the mixed sand/ash beds above this layer were collected at 1 meter stratigraphic intervals. Thin-sections were prepared for each of the samples and these were used for description and classification of the ash. A 0.5 kg sample was shipped to the United States Geological Survey (USGS) Tephrochronology Laboratory at Menlo Park, California. Volcanic glass was separated from the sample and analyzed by electron-microprobe for major elements. Initial comparison of the analytical results with the USGS database was conducted by Andrei Sarna-Wojcicki. An additional analysis of the volcanic glass by electron microprobe as well as by X-ray fluorescence spectrometry was carried out at the University of Utah. Finally, a glass concentrate from this ash was prepared and analyzed by a commercial laboratory using the Potassium-Argon method to obtain an age for the sample.

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RESULTS AND DISCUSSION

In thin-section the ash consists of 85 to 90% incipiently to moderately altered, highly elongate glass shards to 0.3 mm in the longest dimension (see Fig. 3). The majority of the shards are bubble wall shards, with a small percentage (<5%) exhibiting bubble wall junctions. The remainder of the ash consists of equidimensional quartz and feldspar microphenocrysts to 0.2 mm in diameter and altered rock fragments with a possible trace of biotite. The ash is classified as a light gray to yellowish gray, silt-sized vitric ash.

The electron microprobe analyses (Table 1) of the volcanic glass shards reveal that the ash is a highly silicic, moderately potassic rhyolite. These compositions are typical of many of the late Cenozoic tephras of western North America. However, the composition of the glass shards of the West Amarillo Creek (WAC) tephra lie within the compositional field of the 15.2 to 7.5 Ma Stage 2 Yellowstone hotspot tephra (Perkins & Nash 2002) and outside of the compositional range of other major sources of middle to late Miocene tephra of the western U.S (Perkins et al. 1998). So the WAC tephra is likely one of the more than 100 tephra erupted during Stage 2 explosive volcanism along the hotspot track.

More particularly the WAC tephra lies within the composition field of the chemical group A of tephra of the Trapper Creek section (Perkins et al. 1998). The Trapper Creek Section lies on the southern edge of the Snake River Volcanic Province in southern Idaho. Fallout tuffs within the Trapper Creek section originated from the nearby Twin Falls volcanic field to the north or from the more distant Bruneau-Jarbidge volcanic field 100 km to the west (Perkins et al. 1995). The group A tephra, with their characteristically high Ba (mostly 1000-1200 parts per million by weight [ppmw]) and low Rb (150-180 ppmw) relative to other stage 2 tephra, occur in the upper part of the Trapper Creek section with most being younger than [approximately equal to]11 Ma. The group A tephra of Trapper Creek are mostly interbedded with ashflow tuffs from the Twin Falls volcanic field of the Yellowstone hotspot, and basal ashfall associated with these [approximately equal to]10.5 to 8.5 Ma ashflow tuffs are mostly group A type tuffs. Thus, the [approximately equal to]10.5 to 8.5 Ma Twin Falls volcanic field was the most likely source for the WAC tephra.

The correlation to ash of the Trapper Creek Section is the best match if the alkalies (Na and K) are removed from the comparison. Loss of alkalies in volcanic glass by post-depositional alteration and devitrification has been well documented (Noble 1970; Scott 1971). The Trapper Creek Section lies on the southern edge of the Snake River Plain in southern Idaho just north of the boundaries between the states of Idaho, Nevada and Utah. The ashes in this stratigraphic section are believed to be derived from one or more of the Snake River Plain calderas formed by the trace of the Yellowstone hotspot. These calderas are now covered by younger Snake River Plain basalts.

Analysis of the West Amarillo Creek volcanic ash for selected trace elements is shown in Table 2. A similarity coefficient (SC) using the elements Fe, Ca, Ba, Mn, Rb, Sr, Ti, Zr, and Th revealed a good match with either the 10.2 Ma Opal Canyon 3 ash bed of Perkins et al. (1998), or the 10.7 Ma Ibex Peak 19 ash bed of Perkins et al. (1995). The SC values are 0.97 for the match with the Opal Canyon ash and 0.96 for the match with the Ibex Peak ash. Ages and chemical compositions for northern Basin and Range ash beds between 9 and 11 Ma are also given in Table 2 for comparison purposes.

A whole-rock potassium-argon age determination on a sample of the West Amarillo Creek ash yielded an age of 9.5 [+ or -] 0.3 Ma. The 9.5 Ma age is not unexpected for a presumed 10.2 to 10.7 Ma ash bed, considering the susceptibility of volcanic glass to loss of argon. A loss of argon from the glass would result in an analytical result younger than the true age of the sample. The analytical results of the age determination are presented in Table 3.

The stratigraphic position of the West Amarillo Creek ash within the late Tertiary Ogallala Formation, its potassium argon age date of [approximately equal to]10Ma, and its major element and trace element composition of the glass shards suggest an origin in the Twin Falls Volcanic Field of southern Idaho. Providing a specific correlation of the WAC tephra to dated group A tephra is problematic. Individual group A tephra are, within analytical uncertainty, all quite similar. In particular individual group A tephra commonly have compositional ranges, and these ranges show considerable overlap between different group A tephra. This overlap is most apparent with electron probe analysis, but overlap also occurs to a lesser degree with the higher precision XRF analyses of group A tephra. When electron probe analysis of the WAC tephra are compared with tephra derived from the Yellowstone hotspot (the sections of Perkins & Nash 2002), 10 or so potential correlations, using methods of Perkins et al. (1998), are found with tephra in the [approximately equal to]11 to 9.5 Ma age range.

Trace element comparisons using XRF analyses further the possible correlations. All of the possible correlatives to the WAC ash are within the same general age range. However, one of two tephra, the 10.74 [+ or -] 0.10 Ma Ibex Peak 19 tephra and the 10.18 [+ or -] 0.10 Ma Opal Canyon 3 tephra of Perkins & Nash (2002), is the most likely correlative of the WAC tephra. A specific correlation of the WAC tephra to a specific northern Basin and Range ash bed may be possible, but would require additional analyses beyond the scope of this investigation. It has not yet been determined if this ash is correlative with other [approximately equal to]10 Ma ash beds that have been identified in the Great Plains and mid-continent region. Until such correlations are made, the full extent and volume of the ash plume cannot be determined. However, the fine particle size of the glass shards suggests that they rest near the distal end of an ash plume--consistent with the 1250 km distance between the likely source in southern Idaho and West Amarillo Creek.

ACKNOWLEDGMENTS

Research was funded in part by the Killgore Research Center. Special thanks to the members of the Board of Wildcat Bluff Nature Center, especially Pam Allison, who assisted in the field work. Thanks to Charles Meyer, Andrew Brownstone and Drew Eriksson at the United States Geological Survey Tephrochronnology Laboratory, Menlo Park for separation of the volcanic glass, preparation of the sample and electron-probe analysis. Andrei Sarna-Wojcicki, USGS-Menlo Park, provided the initial comparisons of this ash with the USGS database. We also thank Calvin Barnes for a thoughtful and constructive review of the manuscript.

LITERATURE CITED

Cepeda, J. C. 2001. A 10 Ma silicic fallout tuff in the Texas Panhandle: comparisons with the Pearlette Ash. Geological Society of America, Abstracts with Programs 33(6):A397: http://gsa.confex.com/gsa/2001AM/finalprogram/session_1186.htm

Christiansen, R. L. 2001. The Quaternary and Pliocene Yellowstone Plateau volcanic field of Wyoming, Idaho, and Montana. U.S. Geological Survey Professional Paper, 729-G, 145 p.

Izett, G. A. & R. E. Wilcox. 1982. Map showing localities and inferred distributions of the Huckleberry Ridge, Mesa Falls, and Lava Creek Ash Beds (Pearlette Family ash Beds) of Pliocene and Pleistocene Age in the Western United States and southern Canada: Miscellaneous Investigations Series, Map I-1325.

Noble, D. C. 1970. Loss of sodium from crystallized comendite welded tuffs of the Miocene Grouse Canyon Member of the Belted Range Tuff, Nevada: Geological Society of America Bulletin, 81(9):2677-2688.

Passey, B. H., T. E. Cerling, M. E. Perkins, M. R. Voorhies, J. M.Harris & S. T. Tucker. 2002. Environmental change in the Great Plains: An isotopic record from fossil horses. Journal of Geology, 110:123-140.

Perkins, M. E. 1998. Miocene ash beds and Miocene mammals in the intermontane west and Great Plains, USA. Journal of Vertebrate Paleontology, 18:70A

Perkins, M. E., F. H. Brown, W. P. Nash, W. McIntosh & S. K. Williams. 1998. Sequence, age, and source of silicic fallout tuffs in middle to late Miocene basins of the northern Basin and Range Province. Geological Society of America Bulletin, 110:344-360.

Perkins, M. E. & B. P. Nash. 2002. Explosive silicic volcanism of the Yellowstone Hotspot: the ash fall tuff record. Geological Society of America Bulletin, 114:367-381.

Perkins, M. E., R. F. Diffendal, Jr. & M. R. Voorhies. 1995. Tephrochronology of the Ash Hollow Formation (Ogallala Group)--Northern Great Plains. Geological Society of America Abstracts with Programs, 27:79.

Powers, D. W. & R. M. Holt. 1993. The upper Cenozoic Gatuna Formation of southeastern New Mexico, 44th Field Conference, Carlsbad Region, New Mexico and west Texas, New Mexico Geological Society Guidebook, p. 271-282.

Scott, R. B. 1971. Alkali-ion exchange during devitrification and hydration of glasses in ignimbrite cooling-units. Journal of Geology, 79:100-110.

Skinner, M. F. & F. W. Johnson. 1984. Tertiary stratigraphy and the Frick collection of fossil vertebrates from north-central Nebraska. Bulletin of the American Museum of Natural History, 178:215-368.

Swineford, A., J. C. Frye & A. B. Leonard. 1955. Petrography of the late Tertiary volcanic ash falls on the central Great Plains. Journal of Sedimentary Petrology, 25:243-261.

Swinehart, J. B. & R. F. Diffendal, Jr. 1997. Geologic Map of the Scottsbluff 1[degrees] x 2[degrees] Quadrangle, Nebraska and Colorado. U. S. Geological Survey Geological Investigations Series Map, I-2545.

Voorhies, M. R. 1990. Vertebrate Biostratigraphy of the Ogallala Group in Nebraska. Pp. 115-151, in T. C. Gustavson, Geologic Framework and Regional Hydrology: Upper Cenozoic Blackwater Draw and Ogallala Formations, Great Plains, Texas Bureau of Economic Geology Publication, 153pp.

JCC at: Jecepeda@mail.wtamu.edu

Joseph C. Cepeda and Michael E. Perkins

Department of Life, Earth and Environmental Sciences

West Texas A & M University, Canyon, Texas 79016 and

Department of Geology and Geophysics

University of Utah, Salt Lake City, Utah 84112
Table 1. Major Element Electron Microprobe Analyses of Volcanic glass
shards from West Amarillo Creek Ash. USGS analysis at the
Tephrachronology Laboratory, Menlo Park, CA. University of Utah analysis
by Mike Perkins. Water content calculated from difference between
measured and stoichiometric oxygen content assuming all Fe as
[Fe.sub.2][O.sub.3].

 Univ. of Recalc.
 USGS Raw Recalculated Utah Raw to 100%
Constituent Probe Data to 100% Probe Data water-free

Si[O.sub.2] 70.74 76.45 73.69 76.23
[Al.sub.2][O.sub.3] 11.35 12.27 11.80 12.21
Fe as Fe as 2.20 2.38 2.43 2.51
[Fe.sub.2][O.sub.3]
MgO 0.099 0.11 0.130 0.134
MnO 0.035 0.04 0.040 .041
CaO 0.805 0.87 0.820 0.848
Ti[O.sub.2] 0.309 0.33 0.320 0.331
[Na.sub.2]O 2.774 3.00 2.43 2.51
[K.sub.2]O 4.222 4.56 4.67 4.83
Cl - - .030 0.031
F - - 0.25 0.26
[H.sub.2]O - - 4.85
Total 92.531 100.01 101.30 100.00

Table 2. Ages and chemical composition of glass by X-ray fluorescence
spectrometry for the Wildcat Bluff Ash and 9 to 11 Ma Rhyolitic Ash Beds
of the Northern Basin and Range. Data for Northern Basin and Range ash
beds from Perkins et al. (1998; 1995) [Fe.sub.2][O.sub.3] and CaO given
in weight percent, all other data in ppm.

Ash Age
Bed (Ma) [Fe.sub.2][O.sub.3] CaO Ba Mn Nb Rb Sr Ti

West 9.5 1.57 0.63 1106 222 39 180 48 1782
Amarillo
Creek
Mink 9.24 1.40 0.55 1004 250 46 158 49 1982
Creek
Sheep 9.46 0.49 0.34 373 309 17 137 59 568
Dip
Opal 9.52 1.35 0.51 970 212 43 170 42 1546
Canyon 6
Section 9.70 1.44 0.56 971 276 45 160 52 2002
26
Celatron 9.73 0.78 0.54 542 220 30 146 60 897
Quarry 9.73 0.72 0.46 566 200 28 146 45 820
G 9
Hazen 9.81 1.43 0.51 1066 216 40 180 44 1804
Opal 10.19 1.57 0.59 1114 232 40 168 48 1785
Canyon 3
CPT XV 10.45 1.56 0.47 1008 233 45 179 35 1592
Opal 10.54 1.65 0.56 1098 257 46 168 50 1635
Canyon 2
Ibex 10.70 1.54 0.54 1130 210 36 168 44 1680
Peak 19

Ash
Bed Y Zn Zr La Nd Th Ce

West NA 46 447 80 61 28 148
Amarillo
Creek
Mink 57 41 456 74 61 24 138
Creek
Sheep 19 17 84 37 27 20 68
Dip
Opal 48 32 389 82 62 26 160
Canyon 6
Section 55 39 482 71 60 24 134
26
Celatron 45 30 175 75 57 20 142
Quarry 43 27 143 74 57 21 144
G 9
Hazen 59 40 445 74 54 26 140
Opal 60 46 449 66 57 26 146
Canyon 3
CPT XV 64 52 475 81 65 26 160
Opal 71 60 449 73 64 26 162
Canyon 2
Ibex 50 45 465 90 50 28 140
Peak 19

Table 3. Results of Whole-rock Potassium Argon Age Determination.
Constants used: [[lambda].sub.[epsilon]] = 0.584 x [10.sup.-10]
[yr.sup.-1], [[lambda].sub.[beta]] = 4.72 x [10.sup.-10], [yr.sup.-1],
[.sup.40.K]/K = 1.193 x [10.sup.-4] gm/gm. Analysis by Geochron
Laboratories, September 1999.

 [.sup.40*.Ar]/Total
Sample Number Material Analyzed [.sup.40.Ar],ppm [.sup.40.Ar]

WB-1 Glass conc. 0.002045 0.194
 80-200 mesh 0.002082 0.165

 [.sup.40*.Ar]/
Sample Number %K Avg. %K [.sup.40.K],ppm [.sup.40.K]

WB-1 3.079 3.112 3.712 0.000556
 3.144
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Author:Cepeda, Joseph C.; Perkins, Michael E.
Publication:The Texas Journal of Science
Geographic Code:1U7TX
Date:Feb 1, 2006
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