The Bishop Conglomerate Ash Beds, South Flank of the Uinta...

131

INTRODUCTION

Extremely large volcanic eruptions, those that are 1,000 to 3,000 times as large as the 1980 Mount St. Helens eruption, are rare occurrences in the Earth’s history, happening only once every few hundred thousand years. For example, in the last 2 million years, only 4 eruptions of this magnitude have occurred in North America (3 from sources near Yellowstone National Park and 1 from near Mammoth, California). The devastation caused by this type of eruption is enormous and covers a wide area. One group of ancient eruptions of this magnitude has been studied for a number of years by the faculty and students at Brigham Young University. These are the tuffs of the Needles Range Group, called the Cottonwood Wash Tuff, Wah Wah

Springs Formation, and Lund Tuff. Their ages range from about 27 to 30 Ma (Best, 1987; Best and others, 1987a; Best and others, 1987b; Best and others, 1989; Best and Christiansen, 1991; Maughan and others, 2002). The source for these tuffs was the Indian Peak caldera located along the Utah–Nevada border west of Cedar City, Utah (Best and others, 1989). An adjacent volcanic center, the Central Nevada caldera complex, erupted even larger volumes of mostly rhyolitic tuffs from about 35 to 22 Ma (Best and others, 1989; Best and others, 1993). Even though these eruptions and their products, the pyroclastic fl ows, have been carefully studied, dated, and analyzed near their sources, only one previous study (Blaylock, 1998) has attempted to look at pyroclastic fall deposits from these eruptions far from the source area. Blaylock (1998) found that an

The Bishop Conglomerate Ash Beds, South Flank of the Uinta Mountains, Utah: Are They Pyroclastic Fall Beds from the

Oligocene Ignimbrites of Western Utah and Eastern Nevada?

Bart J. Kowallis*, Eric H. Christiansen*, Elizabeth Balls*, Matthew T. Heizler†, and Douglas A. Sprinkel‡

ABSTRACT Two ash beds from the Bishop Conglomerate along the south fl ank of the Uinta Mountains have given 40Ar/39Ar laser fusion eruption ages of 30.54 ± 0.22 Ma (ash of Diamond Mountain Plateau) and 34.03 ± 0.04 Ma (ash of Yampa Plateau). Age, mineral composition, and zircon morphology of the ash of Diamond Mountain Plateau strongly suggest that it is correlative with the Cottonwood Wash Tuff of the Needles Range Group in western Utah/eastern Nevada. The Cottonwood Wash Tuff eruption has also been suggested as the source for the Upper Whitney ash bed in the Brule Formation of Nebraska and Wyoming (Blaylock, 1998). The presence of widespread pyroclastic fall deposits from the Cottonwood Wash Tuff, but not for other more voluminous eruptions of the Needles Range Group (Lund and Wah Wah Springs tuffs) may suggest that eruption conditions were different for the Cottonwood Wash Tuff compared to the other eruptions. The ages obtained from the Bishop Conglomerate suggest that it is time-equivalent to the Starr Flat Member of the Duchesne River Formation. Because these two units are also lithologically similar, the Starr Flat Member of the Duchesne River Formation should perhaps be eliminated and all of these sediments placed under the formal name of the Bishop Conglomerate.

*Department of Geology, Brigham Young University, Provo, UT [email protected]†New Mexico Bureau of Geology & Mineral Resources, New Mexico Tech, Socorro, NM 87801-4796‡Utah Geological Survey, Salt Lake City, UT 84114-6100

Kowallis, B.J., Christiansen, E.H., Balls, E., Heizler, M.T., and Sprinkel, D.A., 2005, The Bishop Conglomerate ash beds, south fl ank of the Uinta Mountains, Utah: Are they pyroclastic fall beds from the Oligocene ignimbrites of western Utah and eastern Nevada?, in Dehler, C.M., Pederson, J.L., Sprinkel, D.A., and Kowallis, B.J., editors, Uinta Mountain geology: Utah Geological Association Publication 33, p. 131-145.

ash layer in western Nebraska and eastern Wyoming matched the age, chemical composition, mineralogy, and other characteristics of the Cottonwood Wash Tuff; however, he did not fi nd ashes correlative with the Wah Wah or Lund Tuffs even though the Cottonwood Wash Tuff eruption was the smallest of the three from the Indian Peak caldera (Best and others, 1989). This may be explained by several alternative hypotheses: 1) the Lund and Wah Wah Springs eruptions did not have signifi cant plinian ash clouds that would have produced widespread pyroclastic-fall deposits even though the ash-fl ows from these eruptions were larger; 2) beds from the Wah Wah Springs and Lund eruptions were deposited but have subsequently been eroded away; 3) wind conditions were unfavorable for deposition of theWah Wah Springs and Lund ashes in the areas of study; or 4) the Wah Wah Springs and Lund pyroclastic fall deposits are present, but not enough detailed sedimentary sections have been measured in order to recognized them. At the present, any of these hypotheses could be correct; however, recent work in the Brule Formation of Nebraska has identifi ed additional ash beds that may correlate to the Wah Wah Springs or Lund tuffs (Emmett Evanoff, personal comm. 2004). Alternatively, Ross and others (2002) have shown that the Cottonwood Wash pyroclastic fl ow was preceded, at least locally, by a pyroclastic surge, which could have been associated with a plinian ash cloud eruption, while the Wah Wah and Lund ash-fl ows do not have signifi cant surge deposits beneath the ignimbrites and, therefore, may not have had large plinian ash clouds. Earlier workers have reported that the Bishop Conglomerate may merge basinward with the Starr Flat Member (uppermost member) of the Duchesne River Formation (Hansen, 1986) based on their similar lithology and stratigraphic positions. However, existing radiometric ages from these two units do not strongly support this idea (Winkler, 1970; Hansen and others, 1981). In order to better constrain the age of the Bishop Conglomerate and to further test the hypotheses listed above concerning the absence of the Wah Wah Springs and Lund tuffs in the earlier study by Blaylock (1998), we have collected and analyzed samples from pyroclastic fall beds in the Oligocene Bishop Conglomerate near the eastern end of the Uinta Mountains of northeastern

Utah. On preliminary examination, these ash beds appear to be about the right age and mineralogy for correlation with the pyroclastic fl ows from the Indian Peak caldera complex. This location is about 450 km from the source calderas, but considerably closer than the 950 to 1000 km distance to the localities in Nebraska studied by Blaylock (1998).

PREVIOUS WORK A broad erosional surface developed during the Oligocene across and around the Uinta Mountains. This surface, called the Gilbert Peak erosional surface, has been known for many years (Bradley, 1936; Winkler, 1970; Hansen, 1986). Remnants of the Gilbert Peak erosional surface can be found on both the north and south fl anks of the range and are capped by Bishop Conglomerate (fi gure 1), which was deposited on this widespread surface (Hansen, 1986). The Bishop Conglomerate ranges in thickness from a maximum of 244 m near the Uinta River at Jefferson Park to a “thin skin” or absence in other localities depending upon pre-exisitng topography on the Gilbert Peak erosional surface and post-Bishop erosion (Hansen, 1986). As the formation name implies, it is composed conglomerate at the type section, but in many localities, sandstone, siltstone, and tuff beds predominate over conglomerate (Hansen, 1986; Sprinkel, 2002). Hansen and others (1981) dated a tuff bed from Diamond Mountain Plateau (SW¼ section 13, T2S, R23E) by conventional K-Ar methods. They obtained an age of 29 Ma (biotite 29.50 ± 1.08 Ma; hornblende, 28.58 ± 0.86 Ma) on a light gray, compact tuff that was “fl ecked with abundant euhedral biotite, much of which was coarser than 0.5 mm across,” and suggested southern Utah or central Nevada as possible sources. This tuff is about the right age to correlate with the Needles Range Group. The large biotite crystals in this ash are similar to biotites in the Cottonwood Wash Tuff (Best and others, 1989), although the age of the Cottonwood Wash Tuff (30.81 ± .18 Ma, from A. L. Deino, written communication published in Blaylock, 1998) seems a bit too old to correlate to their dated sample. Winkler (1970) collected samples from two tuffs in the Bishop Conglomerate and had them dated by Damon (1970). The fi rst sample came from the Diamond Mountain Plateau (near the center

132

The Bishop Conglomerate Ash Beds,...Are They Pyroclastic Fall Beds from the Oligocene Ignimbrites...? B.J. Kowallis, E.H. Christiansen, E. Balls, M.T. Heizler, and D.A. Sprinkel

of section 14, T2S, R23E) about 1 km west of the sample dated by Hansen and others (1981). This sample gave a conventional K-Ar age of 26.2 ± 0.7 Ma on biotite. The second sample dated by Winkler (1970) was collected on the Yampa Plateau (in the NW¼ section 22, T6N, R25E) and gave an age of 41.3 ± 1.1 Ma (Note: These ages have not been corrected using new K-Ar decay constants). Damon (1970) indicates that the biotites are “brownish black and slightly resinous”, which often indicates incipient alteration (Mauger, 1977; Hansen, 1986).

SAMPLE COLLECTION, PREPARATION, AND DESCRIPTION

Three ash samples were collected from the Bishop Conglomerate in October 2002 (fi gure 1). Sample DJ-1 comes from a road cut along the Diamond Mountain Plateau highway near the center of section 35, T2S, R23E. The ash, which we call the ash of Diamond Mountain Plateau, is of variable thickness but is up to 0.5 m thick, gray-green in color, and contains abundant, large biotite fl akes (up to 4 mm across). The Bishop Conglomerate is about 15-20 m thick at this locality on Diamond Mountain Plateau and the ash bed (DJ-1) is about 1.5 m below the unconformity with overlying Pleistocene gravels (fi gure 2). The second sample, DJ-8 was collected along the Echo Park Road about 0.3 km from the junction with the main Harper’s Corner Road in the NW¼ of section 15, T6N, R25E on the Yampa Plateau. This bed, which we call the ash of Yampa Plateau, is white, about 10 cm thick, with small biotite grains visible in the fi eld. We collected an additional sample of this ash bed, DJ-9, nearby (about 30 m up the road north of DJ-8). Even though both samples were from the same layer, DJ-9 was more consolidated, breaking into 3-4 cm sized chunks (fi gure 3), while DJ-8 was composed mostly of softer clay. The difference in consolidation is probably due to localized differences in secondary cement. The thickness of the Bishop Conglomerate at this locality is again about 15-20 m. The ash of Yampa Plateau was collected about 6 m above the basal contact of the Bishop Conglomerate, which is well exposed in the road cut. The lowest unit in the Bishop Conglomerate at this locality appears to be a debris fl ow unit with large blocks (up to 1 m) of

layered red siltstone in a matrix of smaller clasts and mud. In the laboratory, the samples were crushed and sieved. Using a combination of magnetic and heavy liquid methods, biotite, amphibole, zircon, and sanidine were separated from the ashes. All three samples contain quartz, plagioclase, biotite, sanidine, apatite, and zircon, but only DJ-1 contains amphibole. The relative abundances of mineral phases in the sample residues have been affected by secondary alteration, particularly with respect to plagioclase. In the ash of Diamond Mountain Plateau (DJ-1) plagioclase = quartz > biotite > amphibole > sanidine, whereas in the ash of Yampa Plateau (DJ-8 and DJ-9) sanidine > quartz > biotite > plagioclase. DJ-1 also has some remnant, chunky glass shards that commonly show broken bubbles along the edges. DJ-8 and DJ-9 have signifi cant secondary carbonate cement, but both ashes appear to have very few rounded grains that are obviously detrital.

MINERAL CHEMISTRY

Because minerals are more resistant to chemical alteration than fi ne-grained glassy ash, their compositions are useful in distinguishing ash beds from one another and in correlating them with their source ignimbrites. Below we compare the mineral compositions of the Bishop Conglomerate ashes with three very large volume ignimbrites erupted from caldera complexes in the southern Great Basin. These three Oligocene ignimbrites have ages similar to those of the ash beds of the Bishop Conglomerate—the Windous Butte Tuff has an age of 31.30 ± 0.03 Ma (1σ standard error), the Cottonwood Wash Tuff is 30.81 ± 0.18 Ma and the Wah Wah Springs Tuff is 29.95 ± 0.18 Ma (unpublished laser fusion 40Ar/39Ar ages of M.G. Best and A.L. Deino as given in Blaylock, 1998). The Windous Butte Tuff was erupted from the Central Nevada caldera complex and the other two from the Indian Peak caldera complex.

Hornblende

Of the two ash beds studied in the Bishop Conglomerate, only DJ-1 contains hornblende. The hornblende is relatively fresh and only unaltered

2005 Utah Geological Association Publication 33

133

Dehler, Pederson, Sprinkel, and Kowallis, editors

134


Figure 1. Index map and generalized geologic map of the eastern Uinta Mountains (modified from Winkler, 1970). The rock units are labeled as follows: pCu = Precambrian undivided, Pu = Paleozoic undivided, Mu = Mesozoic undivided, Tu = Tertiary undivided, Tb = Bishop Conglomerate, Tbp = Browns Park Formation. Quaternary sediments not shown. Sample localities for ash samples dated are shown by the (*) with the sample numbers at the side.

grains were analyzed. The magnesiohornblendes (Leake and others, 1997) of this ash bed are signifi cantly different those in the Wah Wah Springs and Windous Butte formations, but they are indistinguishable from those in the Cottonwood Wash Tuff. Figure 4 shows the compositions in terms of Fe/Fe+Mg, Mg, and Al. The pressure calculated from the Al-in-hornblende geobarometer (Johnson and Rutherford, 1989) averages about 2.5 +/- 0.7

kb, indistinguishable from the average pressure calculated from hornblende in the Cottonwood Wash Tuff (2.4 +/- 0.5 kb)

Biotite

Both the Yampa and Diamond Mountain Plateau ash beds contain biotite, although crystals are larger and more abundant in DJ-1 than in DJ-8 and DJ-9. The compositions of biotites analyzed in the two beds are signifi cantly different, especially in terms of Fe/(Fe+Mg) ratios (fi gure 5). The biotites in the Yampa Plateau ash have higher Al

total, Mn, F/Cl ratios, and

Fe/Mg ratios, suggesting they are from a more silicic


135


Figure 2. Ash bed DJ-1, gray-green in color, in road cut on the Diamond Mountain road near the center of section 35, T2S, R23E in Uintah County, Utah. As can be seen in the photo, the Bishop Conglomerate is fine-grained at this locality.

Figure 3. Ash bed DJ-9 exposed in road cut along Echo Park road about 0.2 km from the junction with the Harper’s Corner road in NW3 section 15, T6N, R25E, about 3 km west of the Colorado-Utah border. The ash is white and broken into small walnut-sized pieces. Individual pieces are fairly well preserved, even though the outcrop is near the surface and quite weathered.

Feto

tal/(

Feto

tal +

Mg

)

Al - pfu1.0 1.2 1.4 1.6 1.8 2.0 2.2

0.28

0.33

0.38

0.43

0.48

0.53

0.58

AMPHIBOLE

Cottonwood Wash TuffBishop Cong. DJ-1 AshUpper Whitney Ash

Wah Wah Springs Tuff

Windous Butte TuffLower Whitney Ash

Fetotal/(Fetotal + Mg)

Mg

- p

fu

2.0

2.2

2.4

2.8

3.2

3.4

0.25 0.350.30 0.40 0.45 0.50 0.55 0.60

2.6

3.0

3.6

AMPHIBOLE


Wah Wah Springs Tuff Windous Butte TuffLower Whitney Ash

Figure 4. Chemical variation diagrams for biotites from the Bishop Conglomerate ash beds compared to ash beds from the Brule Formation in Nebraska and to tuffs of the same age from eastern Nevada/western Utah. Symbols are as follows: Bishop Conglomerate ashes, DJ-1 ( ) and DJ-8 ( ); Brule Formation ash beds, Upper Whitney ( ) and Lower Whitney ( ); and the Wah Wah Springs Tuff ( ), the Cottonwood Wash Tuff ( ), and Windous Butte Tuff ( ).

or less oxidized magma than the one that erupted to form the ash of Diamond Mountain Plateau. As for hornblendes, the biotites in DJ-1 (ash of Diamond Mountain Plateau) are very similar in composition to the biotites from the Cottonwood Wash Tuff. However, the biotites in the ash of Yampa Plateau (DJ-8) are unlike any in the comparison set from the southern Great Basin. Although it has overlapping Ti concentrations, DJ-8 biotite is distinctive in Fe, Mg, Al, and Mn. For example, it has Fe/(Fe+Mg) ratios between the Cottonwood Wash and the Windous Butte and it has higher Al than any biotite in the comparison set. Temperatures calculated from the Ti/Fe ratios of the biotites, using the equations of

Luhr and others (1984) give an average temperature for the ash of Yampa Plateau of 747°C +/- 17 and for the ash of Diamond Mountain Plateau of 783°C +/- 9.

Feldspars

Both ash beds have co-existing plagioclase and sanidine. The sanidines in both samples are similar with average compositions of Ab

21An

0.7Or

78Cn

1.3

for DJ-1 and Ab24

An0.7

Or75

Cn0.9

for DJ-8. The only signifi cant difference is in the lower content of Ba in DJ-8, consistent with the notion that DJ-8 was erupted from a more evolved rhyolitic magma that was more depleted in Ba as a result of extended separation of sanidine before eruption. Plagioclase in DJ-1 (ash of Diamond Mountain Plateau) is moderately zoned and varies from about An

50 to

An40

. Two feldspar temperatures calculated from an average of the most sodic plagioclases and the average sanidine are about 770°C (calculated at 2.5 kb using the solution model of Ghiorso, 1984). The average hornblende-plagioclase temperature using the same plagioclase and pressure estimate is 764°C. These temperatures compare closely with those calculated for the Cottonwood Wash Tuff (758° to 788° with an average of 776 +/- 13°C for 13 samples, K. Ross and E.H. Christiansen, unpublished data).

ZIRCON MORPHOLOGY Zircon morphology has been shown to be a useful and consistent characteristic of altered volcanic ash beds (Kowallis and others, 1989; Blaylock, 1998, Kowallis and others, 2001). Zircons were extracted from two of the ash samples collected in the Bishop Conglomerate (DJ-1 and DJ-8). As was mentioned, sample DJ-9 was collected only a few m away from DJ-8 in the ash of Yampa Plateau. Zircons were mounted and photographed with a scanning electron microscope at the Brigham Young University Electron Microscopy Laboratory (fi gure 6). The photographed grains were then examined and classifi ed using the zircon classifi cation system of Pupin (1980). Figure 7 shows zircons from the two Bishop Conglomerate ash beds plotted against zircons from the three ignimbrites of the Needles Range Group. As was seen in the geochemical data,

136


Ti - pfu

Fe/(

Fe +

Mg

)

0.25

0.30

0.35

0.45

0.50

0.55

0.60

0.40

0.65

0.270.26 0.28 0.30 0.31 0.330.29 0.32

Bishop Cong. DJ-8 Ash

BIOTITE




Fetotal - pfu

Mg

- p

fu

0.90.9

1.7

1.7

1.8

1.1

1.1

1.0

1.0

1.2

1.2

1.4

1.4

1.6

1.6

1.3

1.3

1.5

1.5

BIOTITE

Bishop Cong. DJ-8 Ash




Figure 5. Chemical variation diagrams for amphiboles from the Bishop Conglomerate ash beds compared to ash beds from the Brule Formation in Nebraska and to tuffs of the same age from eastern Nevada/western Utah. Symbols are as follows: Bishop Conglomerate ash DJ-1 ( ); Brule Formation ash beds, Upper Whitney ( ) and Lower Whitney ( ); and the Wah Wah Springs Tuff ( ), the Cottonwood Wash Tuff ( ), and Windous Butte Tuff ( ). Note that ash DJ-8 does not have amphibole.

sample DJ-1 is very similar to the Cottonwood Wash Tuff, while sample DJ-8 is unlike any of the Needles Range Group tuffs.

ISOTOPIC DATING Three K-feldspar bearing samples were dated using both single and multi-crystal 40Ar/39Ar laser fusion methods (fi gure 8). The details of the methods are given in table 1 and the isotopic data are compiled in table 2. The single crystal method is most desirable for identifying grains that are anomalously old or young compared to the entire population of data, however because the crystals are small and relatively young, analytical uncertainties are high (table 2). Multi-crystal fusion analyses benefi t from larger argon signal sizes that yield more precise analyses, but suffer from possible mixed age populations (which are common in reworked ash layers). Combining the methods provides the ability to identify relatively old grains despite the high

errors and in turn provides better insight towards interpreting the more precise, but skewed age distributions given by the multi-crystal analyses.

Ash of Diamond Mountain Plateau (DJ-1) Sample DJ-1 yielded single-crystal laser fusion (SCLF) dates between about 30 and 635 Ma, whereas the multi-crystal laser fusion (MCLF) dates range between about 30 and 25.5 Ma (fi gure 8a, table 2). Assuming that the only disturbance to the population of dates results from incorporation of crystals that are too old, we have selectively removed analyses from the weighted mean calculation until the youngest group of dates defi nes a normal distribution (e.g. Singer and others, 1998; Campbell and others, 2000). For the SCLF results, the youngest 8 dates yield a weighted mean age of 31.80 ± 0.46 Ma (fi gure 8a). The two youngest MCLF analyses have a weighted mean of 30.47 ± 0.25 Ma that just overlaps the weighted mean SCLF age within 2σ error. Notably, several of the single crystals from DJ-1 have 36Ar/39Ar and 37Ar/39Ar values that are negative (table 2). This indicates that the measured 36Ar and 37Ar data have been overcorrected for blank contributions, but


137


Figure 6. Scanning electron photos of zircons from Bishop Conglomerate ash beds (a) DJ-1 and (b) DJ-8. Note that in general the DJ-1 zircons have large (100) prism faces and small (110) prism faces, while DJ-8 zircons have these two prism faces close to the same size.

0 100 200 300 400 500 600 700 800

800

700

600

500

400

300

200

100

Pyramid Index

Pris

m In

dex

Lund TuffDJ-8 Ash

Cottonwood Wash Tuff and DJ-1 Ash


Figure 7. Zircon classification diagram based upon the relative sizes of prism and pyramid faces (Pupin, 1980). The Bishop Conglomerate ash beds, DJ-1 (open circle) and DJ-8 (closed square) are plotted with the three Needles Range Group tuffs. The 1F errors for each sample analyzed are approximately the size of the symbols.

detailed inspection of the data does not identify any blank outliers that may have been applied during initial data reduction. Adjusting the 37Ar/39Ar ratio to be typical of sanidine (i.e. 0.005-0.01) causes no signifi cant change to the single-crystal laser fusion ages. Forcing the youngest population of data to 100% radiogenic (i.e. 36Ar = zero) and propagating

the initially measured errors reduces the weighted mean age from 31.80 ± 0.46 Ma to 30.74 ± 0.44 Ma. This adjusted age is similar to the MCLF age and is interpreted to be the best age for the single-crystal analyses. Combining the youngest populations of the adjusted single crystal with the multi-crystal data yielded a weighted mean age of 30.54 ± 0.22 Ma (MSWD = 0.40) which represents our preferred eruption age of this ash.

Ash of Yampa Plateau (DJ-8 and DJ-9)

Sample DJ-8

The SCLF data from DJ-8 are imprecise and except for a young outlier, yield a normally distributed data set (MSWD=1.01) with a weighted mean age of 34.46 ± 0.26 Ma (fi gure 8b; table 2). Like DJ-1, several analyses are greater than 100% radiogenic; however, unlike DJ-1 several are less than 100% radiogenic and, therefore, a major systematic blank correction bias does not appear to exist. In detail, there is a correlation between radiogenic yield and apparent age as older dates correspond to higher radiogenic yields. This indicates a slight bias towards over correction of the measured 36Ar intensities. The MCLF data show minor skew towards older ages with one of the analyses yielding a much older age of 235 Ma (table 2). This indicates minor contamination; however, the 11 youngest MCLF dates yield a normal distribution with a well defi ned weighted mean age of 34.00 ± 0.06 Ma. Combining the normal distributions of the SCLF and MCLF data yields a preferred eruption age of 34.07 ± 0.04 Ma for sample DJ-8.

Sample DJ-9 DJ-9 is from the same ash as DJ-8, but from a separate proximal locality. SCLF and MCLF analyses from DJ-9 yield indistinguishable weighted mean ages of 33.81 ± 0.19 Ma and 33.73 ± 0.11 Ma, respectively (fi gure 8c). No old contaminants were detected in DJ-9; however, one low radiogenic and imprecise result from the MCLF data (analysis 64) has been omitted from fi nal age calculations (table 2). The results from DJ-9 are analytically the same as those from DJ-8 and provide confi dence in the overall results of the two samples. A best

138


50100150

0

20

40

0

20

40

0

20

40

50100150

50100150

33.81 ± 0.32, MSWD = 0.1933.73 ± 0.11, MSWD = 1.34

10 15 20 25 30 35 40 45 50

Age (Ma)

31.80 ± 0.46, MSWD = 1.0830.47 ± 0.25, MSWD = 0.60

34.46 ± 0.26, MSWD = 1.0134.00 ± 0.06, MSWD = 1.35

DJ-1 K-feldspar

DJ-8 K-feldspar

DJ-9 K-feldspar

Multi-crystal

Single-crystal

A

B

C

Figure 8. Probability diagrams for 40Ar/39Ar single crystal and multi-crystal laser fusion data. Single crystal data are shown in black, whereas multi-crystal data are in gray. Solid symbols are data used to generate the solid distributions whereas the dashed data also contain the open-symbol data. Preferred populations result from sequential removal of oldest apparent age until a normal distribution of the youngest data is determined. Old apparent ages are interpreted to reflect contamination by detrital grains.

eruption age for the ash sampled by DJ-8 and DJ-9 can be obtained by combining the populations of the SCLF and MCLF data from both samples. This combination yields an eruption age of 34.03 ± 0.04 Ma (MSWD = 1.54).

DISCUSSION

The mineral assemblages and mineral compositions of the 30.54 ± 0.22 Ma pyroclastic fall bed of Diamond Mountain Plateau (DJ-1) suggest its parent magma was broadly dacitic to rhyolitic, whereas the parent magma of the 34.07 ± 0.04 Ma ash of Yampa Plateau (DJ-8/9) was more rhyolitic. The ash of Diamond Mountain Plateau is similar to those in the Cottonwood Wash Tuff in its mineral assemblage, calculated temperature, large and abundant biotite, zircon morphology, and its radiometric age. Blaylock (1998) already showed that the Upper Whitney ash bed in the Brule Formation of Nebraska and Wyoming correlates with the Cottonwood (fi gures 4 and 5). On the other hand, we fi nd no close correlative for the 34.03 ± 0.04 Ma ash of Yampa Plateau (DJ-8/9) among the analyzed tuffs of the southern Great Basin. Mineral analyses do not exist for several potential candidates from the Central Nevada caldera complex, including the tuff of Pritchards Station (34.0 Ma), Pancake Summit (34.8 Ma), and Caetano (33.5 Ma) (Best

and others, 1989; Best and others, 1993). However, the ash of Yampa Plateau is too young and has the wrong composition to correlate with the rhyolitic tuffs of the 35.3 Ma Stone Cabin Formation. Other eruptive sources should also be investigated among the Oligocene pyroclastic rocks of Colorado. The new ages reported here provide better constraints on the age of the Bishop Conglomerate, which ranges at least between the 34.03 ± 0.04 Ma and 30.54 ± 0.22 Ma ages from our two tuff beds, placing it in the early Oligocene. Earlier published ages of 29.50 ± 1.08 Ma on biotite and 28.58 ± 0.86 Ma on hornblende (Hansen and others, 1981; Hansen, 1986), probably come from the ash of Diamond Plateau and are broadly correlative with our new age for this ash of 30.54 ± 0.22 Ma. Hansen (1986) described the ash as having large and conspicuous biotite phenocrysts, a prominent feature of our ash of Diamond Mountain Plateau. However, the biotite ages of 26.2 ± 0.7 Ma and 41.3 ± 1.1 Ma obtained by Winkler (1970) do not relate to either of the ash beds we have dated. As has been previously stated here and by Hansen (1986), these ages are suspect and do not represent eruption ages of any ash bed in the Bishop Conglomerate. The sedimentology of the Bishop Conglomerate is poorly known, partially because it is poorly exposed and partially because of a lack of study. Where it is exposed, it is composed of conglomerate, sandstone,


139


Sample preparation and irradiation:

Mineral separates provided by Bart Kowallis.Separates were loaded into machined Al discs and irradiated for 7 hours in D-3 position, Nuclear Science Center, College Station, TX (NM-172)Neutron flux monitor Fish Canyon Tuff sanidine (FC-2). Assigned age = 27.84 Ma (Deino and Potts, 1990) relative to Mmhb-1 at 520.4 Ma (Samson and Alexander, 1987).

Instrumentation:

Mass Analyzer Products 215-50 mass spectrometer on line with automated all-metal extraction system.Single and multi-crystal sanidines fused by a 50 watt Synrad CO2 laser.Reactive gases removed during a 1 or 2 minute reaction with 2 SAES GP-50 getters, 1 operated at ~450°C and 1 at 20°C. Gas also exposed to a W fi lament operated at ~2000°C and a cold fi nger operated at -140°C.

Analytical param:

Electron multiplier sensitivity averaged 1.50x10-16 moles/pA.Total system blank and background = 100, 2.2, 0.4, 0.9, 0.54 x 10-18 moles for masses 40, 39, 38, 37, 36, respectively.J-factors determined to a precision of ± 0.1% by CO2 laser-fusion of 6 single crystals from each of 4 radial positions around the irradiation tray. Correction factors for interfering nuclear reactions were determined using K-glass and CaF2 and are as follows: NM-172: (40Ar/39Ar)K = 0.0002±0.0003; (36Ar/37Ar)Ca = 0.000280±0.000005; and (39Ar/37Ar)Ca = 0.00070±0.00002.

Table 1. Analytical methods used in isotopic dating

140


ID 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 39ArK K/Ca 40Ar* Age ±1s

(x 10-3) (x 10-15 mol) (%) (Ma) (Ma)

DJ-1 Sanidine, single-crystal, L1:172, J=0.0007545±0.10%, D=1.00484±0.00092, NM-172, Lab#=54461

07 22.10 -0.041 -2.037 0.166 - 102.7 30.64 0.81

08 22.16 -0.125 -4.027 0.071 - 105.3 31.5 1.8

04 22.96 -0.055 -1.366 0.150 - 101.7 31.52 0.91

13 23.03 -0.060 -1.634 0.132 - 102.1 31.7 1.0

05 24.29 -0.120 -2.219 0.062 - 102.7 33.6 2.2

01 23.29 -0.091 -6.239 0.098 - 107.9 33.9 1.4

09 23.14 -0.215 -7.432 0.049 - 109.4 34.1 2.7

06 23.09 -0.209 -9.498 0.056 - 112.1 34.9 2.4

# 02 25.52 -0.132 -7.113 0.079 - 108.2 37.2 1.7

# 11 21.91 -0.098 -43.77 0.015 - 159.0 46.8 8.8

# 14 132.8 -0.001 3.885 0.222 - 99.1 170.9 1.3

# 03 217.5 -0.043 3.301 0.182 - 99.6 272.9 1.9

# 12 382.7 -0.036 3.806 0.253 - 99.7 456.4 2.8

# 10 560.9 -0.083 6.087 0.130 - 99.7 634.9 5.5

Mean age ± 1s n=8 MSWD=1.08 31.81 0.46

DJ-1 Sanidine, multi-crystal, L1:172, J=0.0007545±0.10%, D=1.00484±0.00092, NM-172, Lab#=54462

63(6) 22.44 -0.025 0.014 0.401 - 100.0 30.28 0.35

56(3) 22.67 -0.010 -0.148 0.414 - 100.2 30.66 0.34

# 51(4) 24.88 -0.008 0.572 0.481 - 99.3 33.32 0.32

# 54(3) 25.50 -0.005 1.258 0.336 - 98.5 33.88 0.42

# 60(8) 30.46 0.002 1.632 0.448 267.7 98.4 40.35 0.35

# 57(5) 41.07 0.005 0.936 0.569 108.9 99.3 54.68 0.32

# 53(5) 46.15 0.005 1.357 0.761 105.6 99.1 61.22 0.27

# 58(5) 59.51 0.002 5.533 0.306 251.3 97.3 77.11 0.63

# 52(8) 80.16 0.008 0.936 1.447 63.7 99.7 105.58 0.28

# 64(7) 103.3 0.009 1.027 1.266 57.2 99.7 135.05 0.40

# 62(10) 115.8 0.016 7.567 1.153 32.7 98.1 148.27 0.49

# 59(8) 168.1 0.009 1.669 1.114 53.8 99.7 214.80 0.57

# 50(12) 173.9 0.015 2.312 2.680 34.8 99.6 221.63 0.45

# 61(4) 189.9 0.008 5.091 0.309 64.0 99.2 239.8 1.3

# 55(5) 201.8 0.013 3.305 0.593 40.5 99.5 254.56 1.00

Mean age ± 1s n=2 MSWD=0.60 30.47 0.25

Table 2. 40Ar/39Ar laser fusion isotopic data and ages


141



(x 10-3) (x 10-15 mol) (%) (Ma) (Ma)

DJ-8 Sanidine, single crystal, L7:172, J=0.0007504±0.10%, D=1.00484±0.00092, NM-172, Lab#=54465

# 16 29.10 0.006 49.93 0.070 89.6 49.3 19.3 2.6

02 24.91 0.016 3.832 0.072 31.6 95.5 31.9 1.9

07 25.52 -0.003 2.972 0.085 - 96.6 33.1 1.6

17 25.55 -0.001 0.705 0.269 - 99.2 33.98 0.51

05 25.23 -0.048 -0.738 0.126 - 100.8 34.1 1.1

15 25.70 -0.009 0.704 0.320 - 99.2 34.18 0.44

14 25.86 -0.082 0.949 0.062 - 98.9 34.3 2.1

04 25.19 -0.074 -1.579 0.083 - 101.8 34.4 1.6

03 25.53 -0.080 -1.649 0.082 - 101.9 34.9 1.6

10 25.74 -0.030 -1.088 0.165 - 101.2 34.93 0.81

06 25.19 -0.063 -3.061 0.086 - 103.6 35.0 1.5

01 25.28 -0.006 -3.497 0.083 - 104.1 35.3 1.7

09 25.77 -0.077 -3.744 0.090 - 104.3 36.0 1.4

13 25.83 -0.119 -4.732 0.069 - 105.4 36.5 1.9

11 26.20 -0.040 -3.729 0.064 - 104.2 36.6 2.0

12 25.53 -0.032 -10.274 0.060 - 111.9 38.3 2.1

08 25.67 -0.118 -10.890 0.060 - 112.5 38.7 2.2

Mean age ± 1s n=16 MSWD=1.01 34.46 0.26

DJ-8 Sanidine, multi-crystals, L7:172, J=0.0007504±0.10%, D=1.00484±0.00092, NM-172, Lab#=54466

51(5) 25.21 0.008 0.512 0.431 63.0 99.4 33.61 0.32

62(8) 25.40 0.011 0.680 1.207 47.4 99.2 33.79 0.15

61(8) 25.46 0.005 0.659 1.106 107.4 99.2 33.88 0.15

64(10) 25.56 0.008 1.030 1.902 64.5 98.8 33.88 0.12

56(15) 25.48 0.009 0.668 1.917 60.0 99.2 33.901 0.010

57(12) 26.43 0.050 3.679 1.441 10.2 95.9 33.99 0.13

59(10) 25.56 0.012 0.681 1.747 43.8 99.2 34.01 0.11

60(10) 25.64 0.008 0.635 1.729 67.8 99.3 34.14 0.11

53(8) 25.67 0.009 0.715 1.070 55.7 99.2 34.14 0.17

50(10) 25.74 0.009 0.628 1.149 57.6 99.3 34.27 0.15

65(6) 25.87 0.006 1.046 0.736 80.9 98.8 34.28 0.21

# 66(20) 25.69 0.008 0.416 5.369 65.6 99.5 34.281 0.070

# 52(8) 25.61 0.013 0.139 0.749 39.3 99.8 34.29 0.21

# 55(8) 25.98 0.012 0.834 0.766 41.0 99.1 34.51 0.21

# 54(4) 26.06 0.001 0.601 0.371 982.9 99.3 34.70 0.37

# 63(8) 185.2 0.003 0.228 1.620 196.9 100.0 234.63 0.95

Mean age ± 1s n=11 MSWD=1.35 34.00 0.06

Table 2. 40Ar/39Ar laser fusion isotopic data and ages (continued)

tuffaceous sandstone, siltstone, and a few tuff beds. The color of the formation ranges from dark red and reddish-orange in the conglomerate, sandstone, and siltstone beds to light gray or greenish-gray in the tuffaceous sandstone and tuff beds. Bryant and others (1989) describe the Starr Flat Member of the Duchesne River Formation as “pale-red, moderate-red, reddish brown, and grayish-red boulder conglomerate, sandstone, and minor claystone” in regions near the Uinta Mountains, becoming fi ner-grained and changing color to yellow, gray, and

greenish-gray out into the Uinta Basin. Because of their similar character, previous workers have been unable to agree on where the contact should be placed between them (Rowley and others, 1985; Bryant and others, 1989) or even if both formations are present (Huddle and McCann, 1947; Huddle and others, 1951; Bryant and others, 1989). Bryant and others (1989) dated several tuff beds from the Starr Flat Member of the Duchesne River Formation along the south fl ank of the Uinta Mountains. Their zircon fi ssion-track ages for

142



(x 10-3) (x 10-15 mol) (%) (Ma) (Ma)

DJ-9 Sanidine, single crystal, L11:172, J=0.0007543±0.10%, D=1.00484±0.00092, NM-172, Lab#=54468

04 27.09 -0.115 8.951 0.029 - 90.2 32.9 4.4

16 25.63 -0.019 2.572 0.135 - 97.0 33.52 0.97

11 25.64 0.086 2.641 0.043 5.9 97.0 33.5 3.0

02 25.33 0.011 1.524 0.285 44.5 98.2 33.54 0.49

06 25.75 0.017 2.573 0.128 29.3 97.1 33.7 1.0

15 25.44 0.012 0.607 0.141 43.8 99.3 34.05 0.93

05 25.59 -0.001 0.605 0.169 - 99.3 34.26 0.77

14 25.71 -0.045 0.003 0.106 - 100.0 34.6 1.2

Mean age ± 1s n=8 MSWD=0.19 33.81 0.32

54(6) 25.22 0.001 1.149 0.723 403.7 98.7 33.55 0.22

52(6) 25.54 0.008 1.985 0.333 66.7 97.7 33.65 0.42

53(6) 30.20 0.145 17.38 0.223 3.5 83.0 33.81 0.63

61(5) 25.89 0.005 2.723 0.247 101.6 96.9 33.81 0.55

50(8) 25.59 0.015 1.672 0.747 34.6 98.1 33.83 0.22

65(8) 26.70 0.057 5.392 0.428 9.0 94.1 33.86 0.34

57(6) 25.66 0.009 1.580 0.353 58.3 98.2 33.96 0.40

63(6) 26.90 0.066 4.195 0.588 7.8 95.4 34.59 0.26

Mean age ± 1s n=15 MSWD=1.34 33.73 0.11

Notes:

Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions.Ages calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory standard at 27.84 Ma.Errors quoted for individual analyses include analytical error only, without interfering reaction or J uncertainties.Mean age is weighted mean age of Taylor (1982). Mean age error is weighted error of the mean (Taylor, 1982), multiplied by the square root of the MSWD where MSWD>1, and also incorporates uncertainty in J factors and irradiation correction uncertainties.Decay constants and isotopic abundances after Steiger and Jager (1977).# symbol preceding sample ID denotes analyses excluded from mean age calculations. D= 1 AMU DiscriminationCorrection factors: (39Ar/37Ar)Ca = 0.00070 ± 0.00002 (36Ar/37Ar)Ca = 0.000280 ± 0.000005 (40Ar/39Ar)K = 0.0002 ± 0.0003

Table 2. 40Ar/39Ar laser fusion isotopic data and ages (continued)

this member range between 30 to 37 Ma (with 2σ errors of 1.5 to 3 Ma). If these ages are valid, then our dating suggests that the Bishop Conglomerate and the Starr Flat Member of the Duchesne River Formation are time equivalent and perhaps the stratigraphy of these two formations should be reconsidered in light of these new ages. Additional high-precision ages from the Starr Flat Member and Bishop Conglomerate would help to resolve this issue.

CONCLUSIONS

The new data presented here support the hypothesis that ash from a pyroclastic fall of the Cottonwood Wash Tuff was far traveled, reaching into eastern Utah and as far as Nebraska. Pyroclastic fall deposits from some larger eruptions of nearly the same age and from the same source area (the Wah Wah Springs and Lund ignimbrites) have not been found. More work is needed before we can determine if this is a general case or simply due to undersampling. Our age data suggest that the Bishop Conglomerate and Starr Flat Member of the Duchesne River Formation are time-equivalent units. Additional ages are still needed, but it appears that the Starr Flat Member of the Duchesne River Formation may be a facies of the Bishop Conglomerate that was deposited farther out in the depositional basin rather than part of an older, underlying formation.

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