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Core OL-92 from Owens Lake, southeast California

Age and Correlation of Tephra Layers in Owens Lake Drill Core OL-92-1 and -2

Andrei M. Sarna-Wojcicki
Charles E. Meyer
Elmira Wan
Stan Soles
U.S. Geological Survey, Menlo Park, California

Contents:

Abstract

 Tephra layers present in the ~320-m-deep bore hole in Owens Lake, Calif., were sampled, petrographically described, and the volcanic glass shards were analyzed by electron-microprobe to chemically characterize the layers, and correlate them to tephra layers that have been previously identified and dated. The purpose of this study is to provide additional age control to paleolimnologic and paleoclimatic studies of the lake sediments.

Tephra layers identified are: the Bishop ash bed, ~760 ka, at depths of ~320 to 304 m; the Dibekulewe ash bed, estimated to be ~470 to ~610 ka, at ~224 m; and one of the "Walker Lake" ash beds, estimated to be 60 to 85 ka, at ~50.7 m. Other tephra layers, the ages of which are poorly constrained, have also been identified. Age constraints provided from other lines of evidence in this study, such as a sedimentation rate curve based on dry bulk density and magnetostratigraphy, provide new age constraints to the undated or poorly dated tephra layers. An average sedimentation rate for the 1992 Owens Valley core is 0.41 mm/yr for the last ~665 ka, compared to 0.35 mm/yr for a core drilled previously into Owens Lake sediments in 1957.

Introduction

Tephra (volcanic ash) layers present in cores from the composite 320-m- deep bore hole drilled in 1992 in Owens Lake, east-central California, were sampled and chemically analyzed to provide correlation and age control to a paleolimnologic study of upper Quaternary lake sediments in this basin. The primary focus of this study is to characterize the history of climatic fluctuations during the latter part of Quaternary time. The volcanic glass shards of tephra samples present in the core were separated and chemically analyzed. Glass compositions were then compared to those of previously analyzed tephra layers, some of them of known age. Correlations were identified on the basis of similar petrographic characteristics, glass chemical composition, and stratigraphic sequence in the Owens Lake drill hole, and at various localities in the Western U.S. where the age, correlation, and stratigraphic sequence of potentially correlative tephra layers have been previously documented.

Methods

 Approximately 1 to 3 cm3 (cubic centimeters) of tephra were sampled from macroscopic tephra layers present in the drill cores, and the volcanic glass shards from these were separated and analyzed using methods described by Sarna-Wojcicki and others (1984). In brief: samples were wet- sieved with water in plastic sieves fitted with nylon screens, and the 200 to 100 mesh size fraction (~80 to 150 mm, respectively) was retained for separation of glass shards. This fraction was placed in an ultrasonic vibrator in water, then treated with a 10% solution of HCl acid for a few minutes, to remove authigenic carbonate adhearing to the glass particles, and with an 8% solution of HF acid for about 30 seconds to one minute, to remove other coatings or altered rinds that may have been present on the glass shards. The glass shards were then separated from other components of the tephra sample using (1) a magnetic separator and/or (2) heavy liquids of variable density made from mixtures of methylene iodide and acetone. The glass separate was mounted in epoxy resin in shallow holes drilled into plexiglass slides, and the slides were ground-down and polished with several size grades of diamond paste. The polished sample was then coated with carbon, and individual shards of some samples were analyzed using a 9-channel SEMQ electron-microprobe and, subsequently, the remaining samples with a JEOL electron-microprobe, both instruments at the U.S. Geological Survey in Menlo Park, Calif.1 See Sarna-Wojcicki and others, 1984, and Sarna-Wojcicki and others, 1985, for specifics of analytical conditions.

The SEMQ electron microprobe, a 9-channel instrument, was replaced with a JEOL electron microprobe, a five-channel instrument, in January of 1993. Standards and duplicate tephra samples analyzed by the two instruments indicate that the same results have been obtained with the two instruments.
Samples of volcanic glass shards were analyzed for nine elements: SiO2, Al2O3, FeO, MgO, MnO, CaO, TiO2, Na2O, and K2O. Widespread tephra layers, the products of the most voluminous and violent pyrolcastic eruptions, are generally silicic in composition, containing high concentrations of SiO2, Al2O3, K2O, and Na2O. Volcanic glasses of such layers generally contain lower concentrations of FeO and CaO than those of intermediate or basic composition, but still high enough to be measured with precisions of about 2 to 5%. Silicic tephra contains lower concentrations of MgO, MnO, and TiO2, often close to detection limits for these oxides. Silicic tephra erupted from sources in the Cascade Range of the western U.S. (Washington, Oregon, and northern California) have higher concentrations of these latter elements than tephra erupted from interior sources within the craton of the western U.S., such as the Long Valley Caldera and the Mono Craters (east-central California), the Yellowstone National Park caldera complex (northwestern Wyoming and east-central Idaho), and the Jemez Mountains caldera complex (northern New Mexico)(Sarna-Wojcicki and Davis, 1991). Consequently, comparisons of tephra layers for the purpose of correlation were made with several different combinations of elements, excluding those elements that are present in small quantities close to the detection limit in comparisons of some groups of samples.

Results of chemical analysis of the volcanic glass were compared with our current data base of approximately 3,100 previously analyzed samples of volcanic glass from late Neogene tephra layers collected within the conterminous western U.S. and sediments of the adjacent Pacific Ocean bottom. The best matches were identified using numerical and statistical programs (SIMANAL and RATIONAL; Sarna-Wojcicki and others, 1984). The best matches were then examined for similarities in petrographic characteristics and stratigraphic position and sequence (Sarna-Wojcicki and Davis, 1991). Correlative layers were identified on the basis of these three main criteria: (1) chemical composition of volcanic glass, (2) petrographic characteristics, and (3) stratigraphic position and sequence. Because some of the tephra layers identified in this study have been previously dated at other sites, we are able to assign correlated ages, or at least age estimates, to these layers in the Owens Lake core.

This report summarizes results to date. Of 20 samples submitted for analysis, 17 had a sufficient quantity of volcanic glass shards that they were considered to be air-fall or reworked tephra layers, or ashy sediments. The remaining three samples had very few glass shards, in amounts typical of ambiant concentrations commonly present in fluvial or eolian sediments in this area. The rare glass shards from these three samples were not analyzed. Three of the samples analyzed were bimodal; two different compositions for each of these samples are reported. After the initial analyses were completed and preliminary identifications were made, additional samples were collected and examined from specific depth intervals, where additional tephra layers might be expected to occur based on known stratigraphic sequences and ages from other sites in the western U.S. The latter work was undertaken to see if cryptic, disseminated tephra is present at these depth intervals in the core. Work on 10 additional such samples continues.

Results

Information regarding the analyzed samples is given in Table 1. Results of electron-microprobe analysis are presented in Table 2. Lithologic and petrographic descriptions of the samples analyzed are given in Appendix I. Individual comparisons for each analyzed tephra sample with the 30 closest matches is given in Appendix II, together with annotations regarding the locations and age information of tephra layers that match most closely with those in the Owens Valley cores.

Our study indicates that the following widespread, dated tephra layers were identified in the Owens Lake bore hole (OL92-1 and -2): the Bishop ash bed in the interval 320.01 to 303.94 m (~0.76 Ma); the Dibekulewe ash bed at 224.15 m (>0.40, <0.665 Ma), and a tephra layer at 50.66 to 50.69 m attributed to early eruptions of the Mono Craters, a correlative of which has been found in core samples from Walker Lake, Nev., estimated to be between 60 and 85 ka based on age control at the latter site. In addition, several zones of what appears to be reworked Bishop ash or chemically similar ash, are present at several different levels in the Owens Lake bore hole, and suggest that the Bishop ash bed was repeatedly reworked within the Owens Lake drainage basin.

The Bishop ash bed:

Abundant coarse- to fine-sand-sized tephra layers are present within the lowermost ~16 m of the core. Several zones within this interval are massive, and the entire interval consists dominantly of tephra that was either direct airfall material or tephra reworked locally by streams and wind within the Owens Valley drainage basin, and deposited in the lowest part of the Owens Lake basin--or probably a combination of these two possible origins. In terms of its composition, the volcanic glass of this tephra is chemically homogenous, and probably represents the products of a single large eruption that was either repeatedly reworked within the Owens Valley drainage system, or of several eruptions spaced closely in time, emanating from the same eruptive source, probably also reworked over some unknown but relatively short period of time.

In terms of their chemical composition, the volcanic glass shards of this material are most similar to a set of Quaternary tephra layers that had their source in the Glass Mountain-Long Valley Caldera area of east- central California, ~155 km to the north of Owens Lake. These layers are referred to as the Glass Mountain set of ash beds (Glass Mountain D, G, and U, plus several other unnamed ash layers from this source), and the Bishop ash bed, erupted from the nearby Long Valley Caldera (Gilbert, 1938; Bailey and others, 1976; Izett, 1981; Sarna-Wojcicki and others, 1984, 1991; Izett and others, 1988). These layers were erupted during the period ~1.2 to ~0.76 Ma. There are other, older tephra layers that had their source at or near Glass Mountain (for example, the Tuff of Taylor Canyon), but these can be distinguished fairly easily by their glass chemical composition from the younger set. The youngest, coarsest, and by far the most voluminous tephra layer of the younger set (1.2 to 0.76 Ma) is the Bishop ash bed, the distal air-fall and reworked air-fall ash of the proximal Bishop Tuff.

The proximal Bishop Tuff, including the basal air-fall pumice and overlying ash-flow tuff, as well as distal air-fall ash, have been redated recently, yielding an average age of 0.758 ±0.002 Ma by the laser-fusion 40Ar/39Ar method on sanidine crystals separated from this tephra (Sarna- Wojcicki and Pringle, 1992). Previous recent, generally accepted ages of the Bishop Tuff and ash bed were between about 0.73 and 0.74 Ma (Dalrymple, 1980; Izett and others, 1988). The Brunhes Normal-Matuyama Reversed Magnetic Chron Boundary is situated one to several meters below the Bishop ash bed, when both are found in the same stratigraphic sections (Dalrymple and others, 1965; Eardley and others, 1973; Hillhouse and Cox, 1976; Mankinen and Dalrymple, 1979; Colman and others, 1986; Sarna-Wojcicki and others, 1987, 1991). The previous recently estimated age of the Brunhes-Matuyama boundary was 0.73 Ma (Mankinen and Dalrymple). Recent work, and the ages and stratigraphic relations of the Bishop ash bed and the boundary that are discussed above, indicate that the age of the boundary must be older than its previously accepted age. Recently obtained revised ages are 0.775 ±0.005 (Sarna-Wojcicki and Pringle) and ~0.780 to ~0.790 ± ~0.010 Ma (Baksi and others, 1992; McDougall and others, 1992; Obradovich and Izett, 1992). In any case, the Bishop ash bed is normally magnetized, is situated close above the Brunhes- Matuyama boundary, and thus must have been deposited during the Brunhes Normal Magnetic Chronozone. The younger Glass Mountain tephra layers, by contrast, range in age from about 1.0 to 1.2 Ma (although some are probably as young as ~0.9 Ma; Sarna-Wojcicki and others, 1984), are situated stratigraphically below the Brunhes-Matuyama boundary, and are all magnetically reversed (Liddicoat, 1993).

We propose that the thick interval of tephra at the base of Owens Lake drill hole OL92-2, in the cored interval of 303.94 to 320.01 m, is the Bishop ash bed, for the following reasons:

  1. The tephra in this basal interval is thick, coarse, and massive, typical of a tephra layer from a major eruption close to its eruptive source area. The Bishop ash covered a very large area of the western and central U.S., and was particularly thick near its source, the nearby Long Valley Caldera. Large areas underlain by the air-fall pumice and ash-flow tuff are still exposed at the north end of Owens Valley in the Volcanic Tableland. Direct airfall ash into Owens Lake at the time of eruption, as well as reworking of the ash and tuff from the northern end of Owens Valley, from the entire Owens Valley Drainage system north of the sill between Owens Valley and China Lake, as well as from drainage systems tributary to Owens Valley (Long Valley, Chalfant Valley, Adobe Valley) must have brought large volumes of the tephra into Owens Lake shortly after the eruption, and for some period of time afterward. No other eruption from the Glass Mountain-Long Valley Caldera was as voluminous and widespread as that of the Bishop Tuff.

  2. The shapes of the shards and the mineralogy of the tephra within this basal, 16-m-thick interval is compatible with that of the Bishop ash bed (Appendix I). The tephra is composed of dominantly pumiceous glass shards, with subordinate bubble-wall and bubble-wall-junction shards, and contains sanidine and plagioclase feldspar, quartz, biotite, hornblende, some pyroxene (both ortho- and clino-), as well as minor amounts of allanite and zircon. Some grains of all of these minerals are found with some glass adhering their surfaces. This mineralogy is typical of the Bishop Tuff and ash bed, but is also typical, however, of some of the Glass Mountain tephra layers erupted during the interval ~0.9 to 1.2 Ma. These layers, however, were produced by eruptions of considerably smaller volume, extent, and were consequently much finer, thinner, or absent at distal sites.

  3. The chemical composition of the volcanic glass of this 16-m thick interval in the Owens Valley core is the same as that of the Bishop ash bed (table 2; Appendix II). Some of the ash beds erupted from the Glass Mountain source, however, are chemically similar and cannot be distinguished on the basis of electron-probe analysis alone. Follow-up analysis by X-ray fluorescence and neutron-activation analysis is desirable.

  4. The entire cored section, from the base of the massive tephra interval to the top where Holocene sediments are present, is normally magnetized, and thus suggests that this entire interval was deposited during the Brunhes Normal Magnetic Chron (Glen and others, this volume). A short distance beneath the massive tephra interval, near the base of the drill hole, magnetic inclinations begin to shallow and vary, indicating that a magnetostratigraphic boundary is present. This transition is identified as part of the Brunhes-Matuyama boundary by Glen and others (this volume), and suggests that the lower part of this transition, with fully reversed inclinations, lies beneath the lowermost recovered cores from this drill hole.

  5. A sedimentation rate curve derived from dry bulk density of the cored sediments, and an assumption that the tephra at the base of the Owens Lake bore hole is the Bishop ash bed (Bischoff, this volume), agrees well with an independent chronology derived from magnetostratigraphy for a number of brief magnetic excursions observed in this bore hole above the Bishop ash bed, in Brunhes time (Glen and others, this volume). That is, the age- depth curve derived from dry bulk density passes through the age-depth points defined by the magnetostratigraphy, within the published age errors of the latter.

Based on these five main arguments, we conclude that the thick tephra interval at the base of the Owens Lake core is the Bishop ash bed. This includes samples OL92-1021, at 303.94 m, through OL92-1030, at 320.01 m.

Ash bed of sample OL92-1020 and ash bed RVS-BL-1

The ash bed at 296.04 m in the Owens Valley bore hole contains 90% glass shards. Chemically, it matches well on the basis of its glass chemical composition with a tephra layer (RVS-BL-1) from the Borrego Badlands of southern California, west of the Salton Sea (R.V. Sharpe, written commun., 1988). No independent age is available for this ash bed. The eruptive source of this tephra is not known. The Bishop ash bed is found close to the latter ash bed in the Borrego Badlands, but their stratigraphic relationships there are not known. Results of the current Owens Lake study now provide a stratigraphic context (close above the Bishop ash bed) and age estimate (>0.665; <0.758 Ma) for this ash bed. Using the sedimentation rate curve of Bischoff (this volume), the age of this layer is estimated to be about 740 ka.

Ash of sample OL92-1019

Ashy diatomaceous clay at 266.43 m in the bore hole contains about 25% glass shards. These are chemically identical to those of the Bishop ash bed and the younger ash layers of Glass Mountain. This sample is most likely the Bishop ash, reworked upward in the section.

The Dibekulewe ash bed

This clayey, very fine grained ash layer, containing 55% glass shards, is present at a depth of 224.15 m in the core. Chemically, the volcanic glass shards of sample OL92-1016 match well with those of the Dibekulewe ash bed of Davis (1978). The latter tephra layer is found at a number of localities in Oregon, California, and Nevada, where it commonly overlies the Lava Creek B ash bed, and is overlain, in turn, by the Rockland ash bed (Davis, 1978; Izett, 1981; Sarna-Wojcicki and others, 1985, 1991). The age of the Dibekulewe ash bed is not known, but the underlying and overlying layers have been dated. The Lava Creek B ash bed is ~665 ka (Izett and others, 1992), and the Rockland ash bed is ~400 ka (Meyer and others, 1991) or ~470 ka (Alloway and others, 1993). Rieck and others (1992) have estimated the age of the Dibekulewe ash bed to be about 610 ka, based on its position close above the Lava Creek B ash bed in the Tulelake core in north-central California, and paleoclimatic inferences made regarding the sedimentation rate in that basin. Data from this study, based on dry bulk density (Bischoff and others, this volume) and magnetostratigraphy (Glenn and others, this volume), however, suggest that the age of the Dibekulewe ash bed is younger, closer to ~500 ka.

Sample OL92-1015

Disseminated glass shards in diatomaceous and calcareous, fine-grained sediment, at a depth of 216.54 m in the core, comprised only about 5% of the very fine-grained, diatomaceous, calcareous sediment. These shards are chemically identical to the Bishop ash bed and the Glass Mountain tephra layers. Because of the large volume and wide dispersal of the Bishop ash bed, and because the glass shards in this sample constitute such a small percentage of the sediment sample and are situated considerably higher in the core than the main body of the Bishop ash bed, are most likely reworked from the latter, and represent a sporadic background contamination.

Sample OL92-1003

This sample, at a depth of 52.25 m in the core, consists of 50 to 55% glass shards. Of these, most are clear, colorless, and 2 to 3 % are brown in color. The chemical composition of shards from this sample are generically similar to rhyolite flows erupted from Mammoth Mountain (~155 km north of Owens Lake), that are dated between 50 and 150 ka by the conventional K-Ar method (Bailey and others, 1976). None of the latter units, however, are sufficiently similar to OL92-1003 to be considered as a correlative. Iron in particular is higher in the source rocks. The similarity does, however, suggest common provenance. OL92-1003 is most similar to a tephra layer (UCSB-FS-89-6-6BC) present in river terrace sediments of the Owens River near Fish Slough, about 18 km north of Bishop, but we have no independent age or stratigraphic control on this latter layer.

Sample OL92-1003 is also similar to several tephra layers that were exposed along a causeway connecting Negit Island to the north shore of Mono Lake in 1982, when the lake was at its lowest historic level due to diversion of water by the Los Angeles Department of Water and Power. This set of tephra layers unconformably underlies the oldest of the Wilson Creek ash beds (~12 to ~36 ka), and thus is older than about 40 ka (Lajoie, 1968). Tephra layers of this chemical type (that is, similar in glass chemistry to OL92-1003), are intercalated in the causeway section with tephra layers similar in chemical composition to those erupted from Mono Craters, but are somewhat different from both the Wilson Creek beds and Holocene tephra layers derived from Mono Craters. These intercalated beds are considered to be the oldest known beds erupted from the Mono Craters.

The Wilson Creek ash beds are present in a section of lake beds that were deposited during the last high stand of Mono Lake, or pluvial Lake Russell (Lajoie, 1968), a period closely coincident with the last major glaciation and the deep-sea oxygen-isotope stage 2 (Shackleton and Opdyke, 1973; Imbrie and others, 1984). The Wilson Creek lake beds are underlain by a gravel indicating that lower lake levels preceded deposition of the Wilson Creek beds. This lower level probably correlates to the deep-sea oxygen-isotope stage 3. The Negit causeway section underlies both the Wilson Creek beds and the basal gravel, and thus probably correlates with the next older glaciation, and with deep-sea oxygen-isotope stage 4, approximately 60 to 80 ka BP, or with an older high stand of the lake. The available age control is more compatible with the former choice.

Sample OL92-1001

This sample, obtained from a depth of 50.69 m in the core, contains about 75% glass shards. Chemically, this sample matches well with the group of early, Mono Craters-like tephra layers found in the Negit Island causeway section (see above), as well as with tephra layers that are present at depths of about 63 to 79 m in a bore hole in Walker Lake, Nev., about 260 km N. of Owens Lake. At Walker Lake, these beds are estimated to be between 60 and 85 ka, based on radiocarbon ages in the upper parts of the core, several correlated ages in the core obtained by tephra correlations, and uranium series ages in the middle part of the core (Sarna-Wojcicki and others, 1988; John Rosholt, cited in Benson, 1988).

Sample OL92-1

This sample, obtained from a depth of 50.6 m in the Owens Lake core, contained about 75% glass shards, the remainder being crystalline material, both clastic detrital and pyrogenic minerals. This sample was obtained from the first hole drilled in 1992, and consequently cannot be related directly to the stratigraphy in the second core. Two replicate analyses were run on this sample. Chemically, the glass shards of this tephra layer match most closely with tephra layers obtained from depths of ~54 to ~62 m in the Tulelake core, in northern California. The interpolated ages of these layers, based on age control in this core (Rieck and others, 1992) range from about 165 to 550 ka (see Appendix II). Inasmuch as most of the tephra layers of this particular chemical composition in the Tulelake core were erupted from a local source, probably the Medicine Lake Volcano, and have not been found outside of north-central California and south-central Oregon (beyond a radius of about 100 km), they are not likely to be present in Owens Lake, 670 km to the southeast. The ages of these layers in Tulelake core also are not compatible with the chronology and correlations derived on the basis of other tephra units in the Owens Valley core, or the sedimentation rate curve of Bischoff (this volume).

The tephra of this layer is also generically similar to tephra erupted from the Mono Craters, specifically to tephra layers collected by J. O. Davis in the vicinity of Walker Lake, Nev., that range in age from latest Pleistocene to Holocene, but have no precise age control.

Sample OL92-S

This is a surface sample obtained from a quarry in lake gravels east of the bedrock ridge that extends to the Death Valley-Lone Pine Road. Chemically, this tephra layer matches with the Long Valley-Glass Mountain suite of tephra layers, 1.2 to 0.76 Ma in age. Most of the best matches are to the Bailey ash bed, but several good matches are also to the Bishop ash bed. This unit may thus correlate to the Bishop ash bed at ~304 to ~320 m in the Owens Lake core, or it may correlate to an older tephra layer derived from Glass Mountain, such as the 1.2 Ma Bailey ash bed. We cannot discriminate among these on the basis of probe analysis of the glass shards or petrographic criteria alone. Additional studies that could pinpoint the identity of this layer are X-ray-fluorescence and instrumental neutron-activation analyses of the volcanic glass, and paleomagnetic study of the layer and associated sediments. Normal polarity of the tephra layer and associated sediments would indicate that the layer was most likely the Bishop ash bed.

Discussion and Conclusions

The 16-m-thick interval of tephra at depths of ~304 to ~320 m in the Owens Lake core is the Bishop ash bed, based on a combination of several independent lines of evidence. Among these are (1) the exceptional coarseness and thickness of this tephra interval suggest that it was produced by an eruption of exceptional magnitude; (2) its petrographic and chemical (in terms of glass composition) similarity to the Bishop ash bed and its homogeneity; (3) its position within a normally magnetized section that extends to the historic sediments at the top of the core, but close above a paleomagnetic transition from normal to intermediate and scattered directions at the very base of the core, indicates that it is younger than, but close to, the age of the Brunhes-Matuyama boundary (775-790 ka), and in agreement with an age of ~760 ka determined on surface outcrops of the proximal and distal Bishop Tuff and ash bed; (4) the position, size and geometry of the Owens Lake basin and its topographic relation to tributary basins such as Long, Adobe, and Chalfant Valleys, together with the presently known distrubution of the Bishop Tuff and ash bed, suggest that a large volume of Bishop tephra must have fallen on, and subsequently been reworked into, the depositional center of the Owens Lake basin; and (5) an independent age curve based on dry bulk density and an assumption that the 16-m-thick interval is indeed the Bishop ash bed provides reasonable agreement for the ages and positions of identified tephra layers present higher in the core--the Dibekulewe ash bed, and the Walker Lake ash bed (Bischoff, this volume). The same curve is in agreement with the ages and positions of short magnetic reversals and excursions within the Brunhes Normal Chron determined independently by Glenn and others (this volume).

Identification of the 16-m-thick interval near the base of the Owens Valley bore hole as the Bishop ash bed provides a reasonable explanation for the great thickness of tephra at this site. The Owens Lake drainage basin and its tributary basins would be completely blanketed with tephra after the eruption of the Bishop Tuff, because this basin and its tributaries are situated within the fallout region of the Bishop ash bed (Izett, 1981; Izett and others, 1988; Sarna-Wojcicki and Davis, 1991). The streams within these basins would not be transporting anything but tephra for some time after the eruption because their basins would be overloaded with sediment, and that sediment would be loose, easily erodable tephra. The effective density of particles of the tephra, composed dominantly of pumice and glass shards (see Appendix I), is lower (~1.2 to 2.5 g/cc) than the normal clastic load underlying the tephra blanket. The latter load would not begin to move until most of the tephra had been cleared from the channels and slopes of the drainage basin, not only because the tephra is lighter, but because it overlay the normal clastic load.

The winds would undoubtedly be continually moving the tephra around from place to place, building huge drifts and dunes of the material at sheltered sites, only to rework them as wind patterns shifted. The local sink for such eolian dust would be the Owens Lake, for once the ash fell into a lake, it could not be picked up again by the wind (providing the lake, like Owens, stayed wet). The Bishop tephra would continue to be reworked sporadically from thicker, more protected areas within the Owens Lake drainage basin during exceptionally heavy storms, thus accounting for the zones of reworked Bishop tephra that are present higher in the borehole.

The basal layer of Bishop-like tephra at ~320 m in the hole consists of coarse-sand-sized pumiceous shards, but also has a higher component of comagmatic mineral grains than the other samples in this 16-m-thick interval: 20% feldspars and quartz, and 20% biotite, compared to the samples higher in this 16-m interval. This basal zone probably corresponds to the initial direct airfall component from the eruption, the high percentage of the minerals at the base being due to their higher density, and more rapid settling velocity.

How rapidly was the tephra of the Bishop ash bed eroded from the landscape of the Owens Lake drainage basin after the eruption? The 16-m-thick interval near the base of the Owens Lake bore hole is composed dominantly if not entirely of tephra of Bishop-like chemical composition and mineralogy, and this interval corresponds to the major period of reworking following the eruption. At ~297 m, a change to finer grain size, clay and silt, and change in sediment composition, mark a return to "normal" clastic sedimentation in the basin. A meter above this change, at a depth of ~296 m, a younger tephra layer (OL92-1020) of different chemical composition than the Bishop ash bed is present in this finer interval.

The reworking of the Bishop tephra from the landscape probably was quite rapid, despite the large mass of material that was deposited, because the tephra is loose, light, and consequently presents little resistance to erosion. The rate of erosion must have been much more rapid than the average erosion rates calculated from the entire thickness of sediment in the core, or from the sedimentation rate calculated from dry bulk densities (Bischoff, this volume). Eolian deposition was probably also accelerated at this time, because a very large surface area of loose ash was exposed to the wind. A guess would be that the drainage system was largely cleared of ash within several centuries to several thousand years, but probably not more than 10 ka. It is important to obtain more precise estimates on the rates of such a clearing process, because the presence of thick and extensive dust veils for periods longer than several millenia should show up as paleoclimatic anomalies in the stratigraphic record, and would document the presence of an additional mechanism for climate forcing, rather than just the earth's precessional parameters alone.

The abovementioned age curve derived from bulk density data (Bischoff, this report) provides a reasonable age estimates for the Dibekulewe ash bed of ~500 ka. Previously, this ash bed has been bracketed between ~620 and ~400 ka, and its age was estimated to be ~610 ka based on its close stratigraphic position to the Lava Creek B ash bed in the Tulelake core (Rieck and others, 1992), and paleoclimatic inferences regarding the relationship of these two units. Recent analyses provide ages of 400±60ka (Meyer and others, 1991) and 470 ±40 ka (Alloway and others, 1992) for the Rockland ash bed, and ~665 (Izett and others, 1992) for the Lava Creek B ash bed. Both the current (~500 ka) and previous (~610 ka) age estimates for the Dibekulewe ash bed are thus compatible with currently available data; we do not know which of the two estimates is closer to the actual age.

The correlated age of the Walker lake tephra identified in the Owens Lake core (sample OL92-1001), ~60 to ~85 ka, is compatible with the ~70 to ~80ka age estimated from the sedimentation rate curve based on dry bulk density (Bischoff, this volume).

A previously-drilled hole in Owens Lake (Smith and Pratt, 1957) encountered the Lava Creek B ash bed at a depth of ~234 m in the hole, and disseminated shards of what was presumed to be the Bishop ash bed, at ~262m. We were not able to find the Lava Creek B ash bed as a macroscopic tephra layer in the cores obtained in 1992. Work to try to find it as disseminated shards is still in progress. If the sedimentation rate curve derived from dry bulk densities is accurate, corresponding units are deeper in the 1992 hole than in the previous one. The depth of the Lava Creek B ash bed in the older hole corresponds to an estimated age of ~520ka in the 1992 hole, considerably younger than the determined age of the Lava Creek B ash bed. Calculated sedimentation rates down to the estimated position of the Lava Creek ash bed, assuming the sedimentation rate curve of Bischoff (this volume) is correct, are 0.35 mm/yr for the old hole, and 0.41 mm/yr for the 1992 hole.

Tables

  1. Data on tephra samples studied from the Owens Lake core

  2. Electron-microprobe analyses of volcanic glass shards from tephra layers in Owens Valley Cores OL92-1 and OL92-2, and one surface sample in the vicinity of Owens Lake. Oxides are given in weight percent, recalculated to 100 percent (fluid-free basis). (S) - SEMQ 9-channel probe; (J) - Jeol 5-channel probe.

  3. Data on the thick, coarse tephra-bearing interval near the base of Owens Lake core OL92-2. The table shows sample number, depth, color, qualitative grain size, and percentage of glass present.

References

U.S. Department of Interior, U.S. Geological Survey
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