Kansas Geological Survey, Guidebook 1, originally published in 1976
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The Succession of Late Cenozoic Volcanic Ashes in the Great Plains: A Progress Report

by John Boellstorff

Nebraska Geological Survey

Introduction

Scores of Pleistocene volcanic ash lentils have been located in the Central Great Plains since the early 1800's when G.P. Merrill first recognized ash in southwestern Nebraska. The potential value of these deposits as a tool for regional correlations was soon recognized because of the occurrence of ash throughout the Great Plains, both in the glaciated and non-glaciated area.

Several extensive studies of the ash deposits were undertaken for the purpose of determining the number of ash horizons and their stratigraphic positions. These early studies were culminated by that of Frye, Swineford, and Leonard (1948). They concluded that the various Pleistocene ash deposits could be considered a datum of early Yarmouthian age (Fig. 1), even though in 1946 Swineford and Frye concluded there might be as many as three Pleistocene ash falls--all of about mid-Pleistocene age. The interpretation of a single ash became the general consensus of Pleistocene stratigraphers in the area, and the "Pearlette ash" of late Kansas or early Yarmouthian age was used as a key bed in making regional correlations and in the development of classification schemes for Pleistocene deposits in the Central Great Plains.

Figure 1--Previous correlation and classification of various stratigraphic units from the Missouri Valley area into the Central Great Plains. The stratigraphic position of the "Pearlette" volcanic ash is shown by large dots. (Used by permission of the University of Chicago Press from John C. Frye, Ada Swineford, and A. Byron Leonard, "Correlation of Pleistocene deposits of the Central Great Plains with the glacial section," Journal of Geology, Vol. 56, No. 6 [Nov., 1948], p. 520. Copyright 1948 by the University of Chicago. All rights reserved).

It was not until the 1960's that new evidence was presented indicating that the "Pearlette ash" might be multiple in nature. Young and Powers (1960) and Miller and others (1964) suggested that as many as four separate ash falls comprised the Pearlette. Reed and Dreeszen (1965) presented evidence for at least two Pleistocene ashes in Nebraska--one of Kansan or early Yarmouthian age and another of possibly late Nebraskan age.

More recently, chemical, mineralogical, and geochronological studies have verified earlier interpretations of more than one Pleistocene ash in the Great Plains and have shown that some of the ashes are greatly separated in time (Izett and others, 1970, 1972; Naeser and others, 1973; Boellstorff, 1972, 1973a, b). With further studies, the known number of Pleistocene volcanic ashes is increasing. Data to be discussed later indicate that at least six ashes from three volcanic source areas may be present in the Great Plains.

The concept of a single ash--the "Pearlette ash" of late Kansan or Yarmouthian age--was generally accepted and the "Pearlette ash" was used as a marker bed in making regional correlations. During the construction of classification schemes for Pleistocene deposits, strata and fauna at widely separated localities in the Great Plains have been considered equivalents because of their close association with the "Pearlette Ash." Thus, evidence showing that the various ash deposits used in making these correlations and classification schemes may differ greatly in age necessitates a reevaluation of the classification schemes and age assignments given to some sediments and faunas.

Fission-Track Dating Method

In nature, fission tracks are formed in terrestrial materials when the nucleus of an U²³⁸ atom spontaneously splits into two fragments having a high positive charge (Fleischer and others, 1965). The atoms adjacent to the trajectory of the two fission fragments are ionized--and by mutual repulsion are forced into the surrounding structure. The damage channel formed by the two fission fragments is about 20 to 30 microns long and 50 angstroms in diameter. These damage channels are preferentially etched by hydrofluoric acid which enlarges them, making them visible with an optical microscope (Price and Walker, 1962).

Fission tracks can also be induced in glass by bombardment with thermal neutrons in a reactor. These induced tracks are formed when thermal neutrons cause a portion of the U²³⁵ in the sample to undergo fission, forming tracks in the same manner as already outlined.

The density of spontaneous or fossil fission tracks is proportional to the age and original concentration of uranium. The density of induced fission tracks is proportional to the neutron dose and concentration of uranium still present in the sample. The age of the sample, in years before present, is proportional to three measured variables--neutron dose, spontaneous track density, and induced track density.

A simplified method for fission track dating volcanic ash shards (Boellstorff, 1973c) was used in this study and will now be briefly described. The coarser-grained shards sieved from air-dried samples are cleaned in water with an ultrasonic bath, wet sieved to remove loosened particles, and then air dried. These shards are split into two portions which are packaged in separate polyethylene vials. One portion is reserved for determining the density of fossil or spontaneous fission tracks (p_s); the other is reserved for irradiation with thermal neutrons in a reactor, after which the density of fission tracks induced by the neutron bombardment (p_i) is determined.

Vials of "unknown" ashes along with three splits of an ash standard repeatedly dated at approximately 2.0 m.y. (Borchers Ash) and five pieces of a glass dosimeter are placed in a polyethylene jar and simultaneously irradiated with thermal neutrons in the graphite section of the Georgia Institute of Technology Research Reactor. A recent improvement in the technique is the rotation of the jar at a rate of 1 rpm during the irradiation period (30 minutes). This led to a significant improvement in the precision of the dose measurements and in the calculated ages of the ashes.

After irradiation and determination of the neutron dose (φ), separate 1 cc splits of the irradiated and nonirradiated shards are etched in continuously agitated hydrofluoric acid (24%) at 23° C for equal periods of time--generally two minutes. The etching process enlarges the tracks, making them visible with a microscope and removes a rind of glass thicker than the etchable range of a fission-track (about 12 microns in natural glasses). Fission tracks due to any surficial uranium contamination are thus removed. The etched shards are placed in water on slides and scanned at 500 magnification in transmitted light. The density of spontaneous fission tracks (p_s) in the nonirradiated portion and the density of spontaneous plus induced fission tracks (p_s+i) in the irradiated portion are determined by counting the number of tracks on shard surfaces that fill an area outlined by a circular eyepiece reticle of an appropriate size. After the fission-track densities are determined, the age (A) in years before present is calculated using the equation below (after Fleischer and Price, 1964).

A = 14.95 x 10⁹ log {1 + (9.50 x 10^-18) (φ) [p_s / (p_s+i - p_s]}

The Borchers Ash is used as an intralaboratory standard because it has been repeatedly dated at about 2.0 m.y. by several investigators and can be easily dated. It has been dated at 1.95 ± 0.31 m.y. (F-T zircon, Naeser and others, 1973); 1.97 ± 0.25 m.y. (F-T glass, Boellstorff, 1973a, b); and 1.93 m.y. (F-T glass, Seward, 1974). In addition, this ash is correlated with the Huckleberry Ridge Tuff, which is the probable source for the ash (Naeser and others, 1973). The Huckleberry Ridge Ruff has a K-Ar (sanidine) age of 1.96 ± 0.25 m.y.

After the samples are irradiated and the neutron dose determined, the age of the Borchers Ash standard is measured using the technique outlined above. If this newly measured age is within the limits of the age as known from previous data, it is assumed that the dose measurement is accurate and the unknowns are then dated. The best estimate of the age of the Borcher's Ash is 1.96 ± 0.22 m.y. This value is the mean of Naeser and others (1973), Seward's (1974), and my determinations (Table 1).

Table 1--Fission-track (glass) age data for the Borchers Ash.

Splits of the Borchers Ash have been included and reanalyzed with eight separate batches of unknowns. In all, 33 age determinations have been made on glass from the Borchers Ash, using the technique outlined here. These determinations yielded an average age of 1.96 ± 0.20 m.y. (one σ). As stated earlier, rotation of the sample jar during irradiation has increased the precision of the calculated dose and measured age. The latter 21 of the 33 age determinations on the Borchers Ash were from samples rotated during irradiation. The average of these 21 determinations is 2.01 ± 0.13 m.y. (one σ).

The use of the Borchers Ash as an interlaboratory standard was informally proposed at the IXth INQUA Congress in Christchurch, New Zealand, (Boellstorff, 1973d) and is reiterated here. The use of a standard ash, such as the well-dated Borchers Ash, permits making more confident comparisons of radiometric dates between experiments and between experimenters because the effects of possible systematic errors in dose measurement, track recognition criteria, and knowledge of decay constants are minimized.

If samples of the Borchers Ash are irradiated and dated along with unknown samples, the measured ages for the unknowns could then be expressed in terms of "Borchers Equivalents" as well as in radiometric years. This would permit future adjustments of the "radiometric years" values to accommodate changes in procedures making possible more accurate dose measurements and changes in values for physical constants (mainly the spontaneous decay rate for U²³⁸. These changes would not affect the relative ages of the ashes nor their age as expressed in terms of "Borchers Equivalents." In essence, with this system each experimenter would date each unknown relative to the Borchers Ash standard.

Samples of the Borchers Ash standard have been dated along with all of the ashes reported on in this paper. I consider the relative ages of these various ashes to be reliable because the same procedures and values for physical constants have been used throughout and because the Borchers Ash standard has yielded similar dates (see Table 1). The absolute ages presented in this report are considered valid if the values for the physical constants and the neutron dose are accurate. Evidence indicating that the dose measurements and physical constants used in this report are accurate is the close correspondence of fission-track dates determined on the Borchers Ash (=2.0 m.y.) with K-Ar dates determined by J.D. Obradovich on sanidine from the Huckleberry Ridge Tuff (2 m.y., Christiansen and Blank, 1972; 1.96 ± 0.05 m.y., Naeser and others, 1973), which is the probable source for the Borchers Ash (Naeser and others, 1973).

Fleischer and Hart (1972) have developed an age equation that does not require knowledge of the absolute neutron dose. As shown below, the age of the sample is given in terms of three track densities and a proportionality factor (ζ).

A = ζ(p^s/pⁱ)p^d

The three track densities measured in dating an unknown are: the spontaneous track density (p^s) in the sample, the induced track density (pⁱ) in the sample, and the induced track density (p^d) in the dosimeter glass. According to Fleischer and Hart (1972) the proportionality factor (ζ) takes into account nuclear properties and properties of the dosimeter glass. This proportionality factor can best be determined by measuring p^s, pⁱ, and p^d for a sample whose age (A) is independently known. The proportionality factor is then given by:

ζ = (Apⁱ/p^sp^d).

This value for the proportionality factor can be used thereafter to determine the ages for other samples, provided that the same dosimeter glass is always used with the same etching and counting procedure. Since the counting procedure is somewhat arbitrary and varies from operator to operator, each operator should determine his own proportionality factor even if using the same dosimeter glass. This age equation is simpler than that of Fleischer and Price (1964) in that it does not require the determination of the absolute neutron dose. However, it does require the availability of a sample whose age is independently known in order to determine the proportionality factor (ζ).

Pleistocene Ashes

In an earlier study (Boellstorff, 1973a), fission-track dates were determined on glass shards from eleven volcanic ash deposits (Fig. 2 and Table 2) considered to be important in dating and reevaluation of the classification framework of glacial deposits in eastern Nebraska and adjacent areas. Except for the ash in Nuckolls County, Nebraska (Fig. 2, loc. 7), all of these ashes were formerly considered "Pearlette" and used as a datum of late Kansan age. The ash in Nuckolls County, Nebraska, was recognized as a non-Pearlette ash by Miller and others (1964) and was correlated with the Bishop Tuff of California by Izett and others (1970) and termed Bishop Ash. Unlike the others examined in this early study, the Bishop Ash contains phenocrysts of biotite. Boellstorff (1973a, c) adopted from Izett and others the term Bishop Ash for the deposit in Nuckolls County, Nebraska; but I here recommend dropping the term because it now appears that additional Bishop source area ashes may be present in the Great Plains (see Table 3). The name Mount Clare Ash, from a nearby settlement of the same name, is herein applied to the Bishop source ash in Nuckolls County, Nebraska (Fig. 2, loc. 7).

Figure 2--Location of some radiometrically dated Pleistocene ashes. Hachured line approximates maximum extent of continental glaciation (hachures on glaciated side). Location numbers correspond to those in Table 2.

Table 2--Fission-track (glass) age data for some midcontinent Pleistocene ashes.

As shown in Table 2, ashes formerly considered Pearlette consist of four significantly different ages of ash--the Pearlette Ash (restricted), 0.61 ± 0.04 m.y.; the Hartford Ash, 0.74 ± 0.04 m.y.; the Coleridge Ash, 1.21 ± 0.05 m.y.; and the Borchers Ash, 1.97 ± 0.25 m.y. The terminology used here is based on both age and chemical criterial (see Table 2 and Fig. 3) and differs from that of Izett and others (1972) and Naeser and others (1973). Succinctly, the Pearlette Ash (restricted) and the Hartford Ash are both referred to as type O by Izett and others (1972) and Naeser and others (1973). These ashes are nearly identical chemically, but are chronologically distinct (see Table 3). The Coleridge Ash = Pearlette type S, and the Borchers Ash = Pearlette type B of Izett and others (1972). The ash terminology used in this report is summarized in Table 3.

Figure 3--Relationships of the concentration of selected chemical elements in glass shards from some radiometrically dated Pleistocene ashes. Dots are mean values with solid lines showing standard deviations (one σ). Numbersx correspond to those in Table 2.

Table 3--Terminology for the succession of Pleistocene ashes from the Great Plains.

Volcanic vents in the Yellowstone National Park region of Wyoming and Idaho are the probable source area for some of the Pleistocene ashes in the Great Plains (Jack and Carmichael, 1969; Wilcox and others, 1970; Izett and others, 1970). Based on chemical and mineralogical data, Izett and others (1971 and 1972) concluded that ashes herein termed Borchers, Coleridge, and Pearlette (restricted) were derived from the Yellowstone National Park area. The volcanic stratigraphy of the Yellowstone National Park area has been outlined by Christiansen and Blank (1972, see Fig. 4). Naeser and others (1973) correlated the ash at Borchers Ranch (Borchers Ash) in Meade County, Kansas, with the Huckleberry Ridge Tuff and the ash at the Cudahy Ash Mine in Meade County, Kansas, with the Lava Creek Tuff. The Coleridge Ash correlates with the Mesa Falls Tuff. The Hartford Ash (about 0.74 m.y.) may correlate with the Mount Jackson Rhyolite.

Figure 4--Summary of stratigraphic nomenclature of rhyolites from the Yellowstone National Park area. (From R.L. Christiansen and H.R. Blank, Jr., 1972, Volcanic stratigraphy of the Quaternary Rhyolite Plateau in Yellowstone National Park: U.S. Geol. Survey Prof. Paper 729-B).

The succession of Pleistocene ashes in the Great Plains is summarized in Table 3. Additional chemical and mineralogical data is needed to assign source areas to some of these ashes. Table 3 should be considered a "state of the art" summary. It is likely that the number of ashes will increase as more ash deposits are studied.

Stratigraphic Positions of the Ashes

The Borchers Ash

At its reference locality, the Borchers Ash is associated with early Pleistocene fluvial sediments overlying the Ogallala Formation (NW NE sec. 21, T. 33 S., R. 28 W., Meade County, Kansas--Stop 4 of this Guidebook). The ash is overlain by sediment containing a vertebrate fauna, the Borchers local fauna (Hibbard, 1941). Hibbard noted that some of the forms in the Borchers fauna were previously known only from the late Pliocene. However, in 1949, Hibbard assigned a Yarmouthian age to the Borchers fauna based on Frye and others (1947) study of the "Pearlette Ash," in which they concluded that the age of formations containing the ash is latest Kansan and Yarmouthian. More recently, Hibbard (1972) assigned an Aftonian age to the Borchers local fauna based on evidence that the "Pearlette Ash" in reality consists of at least two ages of ash.

Hibbard (written communication, March 6, 1973) considered the gravel (Stump Arroyo Member of Crooked Creek Formation) underlying the silt sequence (Atwater Member of Crooked Creek Formation) containing the Borchers local fauna and Borchers Ash to be the same age as the Angell Member of the Ballard Formation--late Pliocene or earliest Pleistocene. Hibbard (1972) considered the Angell Member to have been deposited during, or after, Alpine glaciation but prior to the Nebraskan Glaciation.

The Borchers Ash is not known with certainty to occur in the glaciated region. However, an ash occurring near the base of a sequence of unconsolidated fluvial silts and sands believed to be of Nebraskan age was discovered in 1952 in a test hole near the village of Hartington, Nebraska during the course of a groundwater investigation program in cooperation with the U.S. Geological Survey (Test hole #40-A-52 located in SE SW sec. 33, T. 31 N., R. 1 E., Cedar County, Nebraska; Fig. 2, loc. 11). The ash-bearing sequence is overlain by a till assigned a Kansan age (probably early Kansan) and is underlain by the Ogallala Formation (late Tertiary). These age assignments were based on extensive regional surface and subsurface studies that led to a revision of the Pleistocene classification of Nebraska (Reed and Dreeszen, 1965). The authors (page 8) state, "...volcanic ash has been noted in the samples from two test holes at a level below a till correlated as early Kansan and one or two exposures of volcanic ash in Nebraska do not seem to duplicate the petrographic characteristics of the Pearlette. Therefore, it appears that there may have been an earlier ash fall in the Pleistocene, possibly of late Nebraskan age, which is imperfectly and rarely preserved." The ash in the test hole near Hartington is one of the sites that prompted the above statement.

The ash near Hartington, provisionally termed the Hartington ash, was recently cored for dating purposes. Four fission-track age determinations on glass shards from this ash yielded a date of 2.08 + 0.18 m.y. Since this ash is devoid of biotite phenocrysts, and because this date is virtually identical with the Borchers Ash, the Hartington ash probably correlates with the Borchers Ash. Both age and chemical criteria, however, are needed for conclusive correlation of isolated ash occurrences. As chemical data are presently lacking for the Hartington ash, its correlation with the Borchers Ash remains tentative. Nevertheless, the stratigraphic positions and dates on the Borchers Ash and the Hartington ash indicate that the climatically defined Pliocene/Pleistocene boundary as used in Kansas and Nebraska may be about 2.0 m.y. old (Fig. 7). However, neither of these ashes occur in direct association with the oldest recognized continental glacial sediments, and thus do not conclusively date the beginning of continental glaciation. Also, if the Pliocene/Pleistocene boundary is placed at 1.8 m.y. in the stratotype area (Berggren and Van Couvering, 1974), these ashes would be late Pliocene. Until some general agreement on the age of this boundary is reached, the date of about 2.0 m.y. is here used as a tentative local working boundary.

The Coleridge Ash

Near David City, Nebraska (Fig. 2, loc. 9), the Coleridge Ash (1.2 m.y.) occurs near the top of a thick sequence of silt and clay assigned to the Fullerton Formation (late Nebraskan, Reed and Dreeszen, 1965; Fig. 5) and is overlain by two tills assigned to a Kansan age (Dreeszen, 1970). Near Coleridge, Nebraska (Fig. 2, loc. 10), this ash occurs near the top of a silt sequence believed to be the Fullerton Formation. The silt sequence is overlain by till that correlates mineralogically and lithologically (Boellstorff, 1973c) with both the lower till (pre-Kansan of Bain, 1896 and Chamberlin, 1896) near Afton, Iowa, and the till assigned an early Kansan age in eastern Nebraska by Reed and others (1966).

Figure 5--Classification of Pleistocene Deposits in Nebraska. (From V.H. Dreeszen and E.C. Reed, 1965, Revision of the classification of the Pleistocene deposits of Nebraska: Bull. Nebr. Geol. Survey, 23).

The correlations outlined above indicate that the Coleridge Ash occurs near the top of sediments assigned a late Nebraskan age in Nebraska and below a till correlated as the early Kansan till in Nebraska and as the pre-Kansan till near Afton, Iowa. As shown in Figure 7, these relationships indicate that the Nebraskan sequence of Reed and Dreeszen (1965) is older than about 1.2 m.y. old and that the pre-Kansan till near Afton, Iowa, is younger than this and is Kansan in age in terms of Reed and Dreeszen's classification (1965).

The Coleridge Ash occurs also at the top of the Sappa Formation (late Kansan, Reed and Dreeszen, 1965) at its type locality (Fig. 2, loc. 8). The relationships of the Coleridge Ash to glacial and nonglacial sediments at Coleridge and David City indicate that the Sappa Formation at its type locality is older than late Kansan (Fig. 7) and is probably late Nebraskan or Aftonian following Reed and Dreeszen's (1965) classification.

The interpretation of Illinoisan till in northeastern Nebraska (Condra and Reed, 1950) and southeastern South Dakota (Flint, 1955) was based largely on the occurrences of till overlying volcanic ash near Coleridge, Nebraska, and Hartford, South Dakota (Fig. 2, locs. 10 and 6, respectively). These tills were assigned an Illinoisan age because it was then believed that only one age of volcanic ash was present in the Central Great Plains--the "Pearlette Ash" of late Kansan age. However, it now appears that the ash at Hartford is about 0.74 m.y. old and the ash at Coleridge about 1.2 m.y. old. These ashes are also chemically distinct (Fig. 3). Ash deposits in the Central Great Plains range in age from early through middle Pleistocene; therefore, an Illinoisan age for deposits overlying volcanic ash can no longer be assumed. Pleistocene deposits overlying ash may be as old as early Nebraskan.

On May 15, 1973, C.W. Hibbard discovered an ash overlying the Borchers Ash in Meade County, Kansas. This is the only known locality (Fig. 2, loc. 11) in the Great Plains where two ashes occur in superposition in Pleistocene sediments. This new ash, herein provisionally termed the Upper Borchers ash, is separated from the Borchers Ash by 21.5 feet of clayey silts. A sample of this ash, which I collected with Hibbard in June, 1973, yielded a date of 1.22 ± 0.18 m.y. This date is virtually identical to that for the Coleridge Ash. Unlike the Coleridge Ash, however, the upper Borchers ash at Borcher's Ranch is not a Yellowstone-source ash as evidenced by the presence of biotite phenocrysts. The presence of these phenocrysts suggests a possible Bishop, California, or Jemez mountains, New Mexico, source area (Table 3). Nevertheless, the age equivalency of the Coleridge Ash and the Upper Borchers ash (≈1.2 m.y.) along with the stratigraphic relationships outlined for the Coleridge Ash and the Hartington ash suggests that the 21.5 feet of sediment between the two ashes at Borcher's Ranch represents the Nebraskan Stage as used by the Nebraska Geological Survey. The sediments underlying the Borcher's Ash, which have been assigned at different times to the Meade and Crooked Creek Formations of late Kansan age, may be Pliocene.

The Mount Clare Ash

The only known locality of this ash is in Nuckolls County, Nebraska (Fig. 2, loc. 7), where it is interpreted to be overlain by Illinoisan age deposits (Miller and others, 1964).

The Hartford Ash

Near Hartford, South Dakota (Fig. 2, loc. 6), the Hartford Ash overlies two glacial tills and is overlain by glacial till and yellow-brown silts (Fig. 6). The tills underlying the ash have been considered Nebraskan Till overlain by Kansan Till (Flint, 1955; Steece and others, 1960). These age assignments were largely based on concepts rather than actual correlations with Nebraskan and Kansan deposits in their stratotype area.

At the time these age assignments were made, the general consensus was that the isolated lentils of ash were all late Kansan in age (the "Pearlette Ash") and that the Nebraskan and Kansan glacial stages were each represented by a single glacial till. Thus, if two tills were present below the "late Kansan Pearlette ash", the upper one would be considered Kansan and the lower, Nebraskan.

Attempts to correlate the tills at the Hartford locality with tills in the Kansan Till and Nebraskan Till stratotype areas by means of mineralogical and petrological data were not conclusive (Boellstorff, 1973c). Based on these analyses, I believe that the tills below the ash at the Hartford locality are younger than the Elk Creek Till (early Nebraskan, Fig. 5) and are probably Kansan, following the terminology of Reed and Dreeszen (1965).

Even the age of the silts overlying the Hartford Ash (Fig. 6) cannot be determined with certainty. Flint (1955) assigned these silts to the Loveland Formation (Illinoisan), whereas Steece and others (1960) assigned them to the Sappa Formation, assignments that probably were influenced by concepts--the "Pearlette ash of late Kansan age" and the existence or nonexistence of the Iowan till.

Figure 6--Relationships at the Hartford Section, South Dakota. (Diagram prepared by South Dakota Geological Survey and published by the Nebraska Academy of Sciences in INQUA Guidebook ("C", 1965).

At the Little Sioux locality, Harrison County, Iowa (Fig. 2, loc. 5), the Hartford Ash occurs in a fluvial silt sequence overlain by Loveland Loess (Illinoisan). The silt sequence overlies sand and gravel that in turn overlies till which correlates mineralogically and petrologically (Boellstorff, 1973c) with till assigned an early Kansan age in Nebraska (Reed and others, 1966) and with the lower (pre-Kansan) till of the Afton, Iowa area.

The relationships discussed above only limit the stratigraphic age of the Hartford Ash to post early Kansan pre-Loveland Loess (Illinoisan). The Hartford locality probably has the best potential for accurately determining the stratigraphic position of the Hartford Ash.

The Pearlette Ash (restricted)

At the Dam 7 locality in Seward County, Nebraska (Fig. 2, loc. 4), the Pearlette Ash (restricted) overlies sand and gravel assigned a medial Kansan age (Reed and Dreeszen, 1965) and is overlain by deposits assigned medial and late Illinoisan ages. Here also, the concept of the "Pearlette ash of late Kansan age" probably played an important role in making the age assignments. In the past, sediments below the "Pearlette" ash would be late Kansan or older if overlain by the "Pearlette" ash and the overlying deposits could not be older than latest Kansan.

At the Elk Creek, Nebraska, ash site (Fig. 2, loc. 2), the Pearlette Ash (restricted) is directly overlain by a brown silty clay with a paleosol (Yarmouth) developed in the upper part (Reed and Dreeszen, 1965). The silt sequence containing the ash is underlain by the Grand Island sand and gravel (late Kansan) which is channeled into Elk Creek till.

At the Guthrie County, Iowa, ash site (Fig. 2, loc. 3), the Pearlette Ash (restricted) overlies a glacial till which, based on mineralogical and petrological studies (Boellstorff, 1973c), appears to correlate with the till assigned an early Kansan age in Nebraska (Reed and others, 1966). Although tenuous, these relationships suggest a late Kansan or Yarmouthian, or possibly an early Illinoisan age for the Pearlette Ash (restricted).

The reference section of the Pearlette Ash (restricted) is the Cudahy Ash Mine in the SW sec. 2, T. 31 S., R. 28 W., Meade County, Kansas (also Fig. 2, loc. 1, Stop 7 this Guidebook). The ash at Big Springs Ranch (Stop 6, this Guidebook) dated at 0.62 ± 0.02 m.y. is probably the Pearlette Ash (restricted). This correlation is tentative since chemical data are not available, but the ash at these localities is probably late Kansan, or Yarmouthian in age, following the classification of Reed and Dreeszen (1965).

Guaje Ash

The Guaje Ash (Izett and others, 1972) is the upper of two ashes exposed in sediments of the Blanco Formation at its type locality Crosby County, Texas (Table 3). Izett and others (1972) obtained a fission-track date of 1.4 ± 0.2 m.y. on glass shards from the Guaje Ash. I obtained a date of 1.77 ± 0.44 m.y. on shards from this ash.

Evans (1948) concluded, "The Blanco beds evidently represent a period of relative humidity preceded and succeeded by periods of relative aridity. This condition suggests a glacial stage, and the faunal evidence precludes any but the first glacial as the time of the Blanco deposition." The Guaje Ash is herein considered Pleistocene because it is younger than the Borchers Ash and Hartington ash (≈2.0 m.y.) which occur in Pleistocene sediments. The sediments associated with the Borchers Ash were assigned to medial Pleistocene largely because of the presence of the ash that was thought to be the "Pearlette ash of late Kansan age." This was not the case in assigning an early Pleistocene age to the sediments associated with the Hartington ash. The early Pleistocene assignment of the sediments directly associated with the Hartington ash was based on the sediments having an aspect more akin to nonglacial Pleistocene sediments than to "latest Pliocene" sediments of the Ogallala Group.

Green Mountain Reservoir Ash

The Green Mountain Reservoir Ash (Izett and others, 1970) occurs in Summit County, Colorado. It is known to be younger than the Pearlette "Type-O" ash of Izett and others (1970) and is estimated to be about 0.5 m.y. old (Izett and others, 1972). This ash has not been found in the Great Plains as yet. if it occurs here, it most likely will be found in early Illinoisan sediments.

The Classification of Pleistocene Deposits

Chemical, mineralogical, and geochronological data and the occurrence of two ashes in superposition at Borchers Ranch show that the concept of a single Pleistocene ash datum--the "Pearlette ash of late Kansan age"--in the Great Plains is no longer tenable. Furthermore, the geochronological data indicate that ashes previously termed "Pearlette" and considered late Kansan in age (Figs. 1 and 7) are significantly different in age. Consequently, the premise of a single ash would lead to errors in correlations of sediments and, hence, in their age assignments and classification.

Figure 7--Comparison of Pleistocene chronologies.

An excellent example of such errors is the correlation of the Sappa Formation in Nebraska with the Crooked Creek Formation in Meade County, Kansas (Stop 4, this Guidebook). As shown in Figure 7, the ash in the Sappa Formation (Coleridge Ash) is approximately 1.2 m.y. old, whereas that in the Meade Formation is about 2.0 m.y. old. The late Kansan age assigned to these sediments was based on relationships of the "Pearlette ash" to glacial and nonglacial sediments in the Missouri Valley region of South Dakota, Nebraska, and Iowa--principally at the County Line or Little Sioux, Iowa, locality and at the Hartford, South Dakota, locality. The ash at these localities (Hartford Ash) is about 0.7 m.y. old.

As shown in Figure 7, the ash in the Sappa Formation (Coleridge Ash) is nearly 0.5 m.y. older than the ash in the Missouri Valley region from whence the late Kansan stratigraphic age for the Sappa Formation was determined. Stratigraphic relationships of the Coleridge Ash to glacial sediments in Nebraska indicate that the Sappa Formation at its type locality is late Nebraskan following Reed and Dreeszen's (1965) terminology (Fig. 5). The ash in the Crooked Creek Formation (Borchers Ash) at its type locality is about 0.8 m.y. older than the ash in the Sappa Formation (Coleridge Ash) at its type locality and about 1.3 m.y. older than the ash (Hartford Ash) in the Missouri Valley region from whence the late Kansan stratigraphic age for the Sappa Formation was derived.

Correlations of glacial tills by means of heavy mineral and pebble type analyses (Boellstorff, 1973c) along with the chronology of volcanic ashes suggest that the scope of nomenclatural problems may be larger than indicated above. As shown in Figure 7, the Nebraskan Stage as used by Reed and Dreeszen (1965) is older than the Kansan and pre-Kansan tills in the Afton, Iowa area (Bain, 1896 and Chamberlin, 1896), and the Nebraskan Till near Florence, Nebraska (Shimek, 1909). Thus, deposits assigned a Nebraskan age by Reed and Dreeszen (1965) appear to be pre-Nebraskan in terms of the classic type areas of the Kansan, Aftonian, and pre-Kansan deposits near Afton, Iowa, and Nebraskan deposits near Florence, Nebraska.

The usefulness of the terms Kansan and the Nebraskan as stage names becomes more confusing when the inferred chronologies for the Central Plains (this study) and Gulf of Mexico (Beard, 1969) are compared. As illustrated in Figure 7, this comparison indicates the term Nebraskan has been applied to sediments ranging in age from ≈1 m.y. to ≈3 m.y. and the term Kansan to sediments ranging in age from ≈0.6 m.y. to ≈1.7 m.y.

During the development of the classification of early Pleistocene deposits in North America, it was believed that each glacial stage was represented by single drift. Over the last decade, much evidence has been presented that shows that the glacial history of the early Pleistocene is probably as complex as that of the Wisconsinan. In view of this complexity of early Pleistocene glaciations, the restriction of the stage names Nebraskan, Aftonian, and Kansan to their original usage for the tills and interglacial deposits near Afton, Iowa would serve no useful purpose. Indeed, such a restriction may necessitate abandoning these terms as stage names.

Any redefinition of the terms Nebraskan, Aftonian, and Kansan must take into account that many, if not most, students of the Pleistocene in North American have the concept of Nebraskan as the first major episode of Pleistocene continental glaciation, the Aftonian as the first interglacial episode, and Kansan as the second episode of continental glaciation. Indeed, this concept was probably followed by Beard (1969) in naming the lowermost Pleistocene cold cycle detected in the Gulf of Mexico sediments Nebraskan and the next younger cold cycle Kansan.

Reexamination and redefinition of these stage names must also recognize the problems involved with defining the Pliocene-Pleistocene boundary. The Calabrian Stage of southern Italy was originally assigned to the Pliocene by Gignoux (1910) and was reassigned to the Pleistocene by the XVIIIth International Geological Congress (London, 1948). In 1965 the base of the Calabrian Stage was accepted as the base of the Pleistocene by the General Assembly of the 7th INQUA Congress.

Data on rates of sedimentation presented by Selli (1967) and paleomagnetic data presented by Nakagawa and others (1970) indicate that the base of the Calabrian is about 1.8 m.y. old. This date is about the same as that presented for the base of Kansan in the Gulf of Mexico (Beard, 1969). The correlation of the basal Calabrian with the base of Beard's Kansan in the Gulf of Mexico is supported by faunal evidence (Poag, 1972).

If the base of the Calabrian is generally accepted as the base of the Pleistocene and if the correlations between Italy and the Gulf of Mexico are correct, then Beard's data indicate that the Pleistocene in the northern Gulf of Mexico--and presumably central U.S.A.--would consist of three glacial stages, not four.

The record of early Pleistocene events in the central U.S.A. must certainly be closely related to the depositional history of sediments in the northern Gulf of Mexico. Because the record of these events is probably more nearly complete there than in any part of the glaciated region, the chronology of glacial and interglacial stages can probably most accurately be determined from detailed studies of the sediments on the northern Gulf of Mexico.

Late Tertiary Ashes

A summary of preliminary fission-track ages (glass) determined on some late Tertiary ashes is presented in Figure 8. A detailed discussion of the succession of late Tertiary ashes is beyond the scope of this report, but the succession of radiometric dates presented here may be a useful tool in making stratigraphic and faunal correlations, age assignments, and interpretations as to conditions and events leading up to Pleistocene time.

Figure 8--Succession of some late Cenozoic ashes.

Fission-track dates obtained so far from the succession of late Tertiary volcanic ashes (mainly from Ogallala Group sediments from Nebraska) show that alternative interpretations of the stratigraphic and paleontologic record may be in order.

In Nebraska, sediments of the Ogallala Group have long been considered Pliocene, even though Lugn (1935) thought that this age assignment should be tentative because of the lack of agreement among stratigraphers and paleontologists as to the criteria used to differentiate Miocene from Pliocene. The Ogallala Group sediments have been considered to represent the "latest" Tertiary aggradation of the Central Plains prior to uplift and associated climatic changes that inaugurated the Pleistocene (Schultz and Stout, 1945, 1948; Schultz, Tanner, and Martin, 1972). Furthermore, the Ogallala was considered to span the entire Pliocene epoch (Schultz and Stout, 1948; Tanner, 1975).

The upper formation in the Ogallala Group, the "Kimball Formation", and its contained fauna have been considered latest Pliocene (Schultz and Martin, 1970; Schultz and Stout, 1961; Schultz, Schultz, and Martin, 1970; Tanner, 1967). Schultz and Stout (1948) stated "the mammals found in the Kimball-Sidney sediments are of a distinctive Pliocene type (closely related to those found in the underlying Ash Hollow Formation), and they differ markedly from the mammals found in the later Broadwater-Lisco deposits of the same area." (Barbour and Schultz, 1937; Schultz and Stout, 1948). The Broadwater Fauna was considered early Pleistocene in age. Apparently, because Schultz and Stout (1948) considered the Ogallala Group sediments to span the entire Pliocene, they assigned a late Pliocene age to sediments and faunas at the top of this lithostratigraphic unit.

Schultz and Martin (1975) stated that "the Kimballian fauna from Nebraska differs from the Hemphillian fauna in that most of the known forms are markedly more advanced, but definitely pre-Blancan (Early Pleistocene)." This statement and the fission-track dates on volcanic ashes overlying "Kimballian" fauna and Hemphillian fauna (Fig. 8) are in contradiction. [Note: Kimballian was informally introduced as a North American provincial mammal age by Schultz and Stout (1961).]

Schultz and Stout (1948) recognized the existence of a significant lithologic and faunal break between the finer-grained carbonate-rich latest Ogallala sediments (Kimball Formation) and the coarse-grained crystalline gravel deposits of the earliest Pleistocene (Broadwater Formation). As a measure of the temporal gap between these two sets of sediments, they concluded that "the interval apparently is not quite as long as the time which elapsed between Early and Medial Pleistocene."

The "Kimballian" has also been viewed as a period of climatic change and extinction because of the profound difference between the "Kimballian" faunas ("latest Pliocene") and the early Pleistocene Broadwater fauna (Schultz and Stout, 1948; Schultz, Schultz, and Martin, 1970; Schultz, Tanner, and Martin, 1972). Schultz and Stout (1948) state, "The faunal break between the Kimball and Broadwater formations is the most important one paleontologically in the Upper Tertiary and Pleistocene of western Nebraska (and Great Plains). It is the writers' opinion that this is the Pliocene-Pleistocene boundary."

As shown in Figure 8, fission-track dates indicate that Ogallala Group sediments, previously considered latest Pliocene in western Nebraska, are older than about 6.5 m.y. and that a hiatus of approximately 3 to 4 million years duration exists between these sediments and those considered early Pleistocene in eastern Nebraska. These dates indicate that sediments assigned to the Kimball Formation (Stout, 1971), the Ash Hollow Formation (type locality), and the Ogallala Formation (type locality--Elias, 1936) are contemporaneous.

This conclusion is supported by lithologic studies. Swinehart (1974) stated, "Attempts to correlate Ogallala sediments with the Kimball and Sidney units as established on outcrop have not proven feasible. These units do not appear to be valid lithostratigraphic units because they are not lithologically distinct and do not occupy consistent stratigraphic relationships." All of these sediments, perhaps with the exception of local caliche deposits at or very near the topographic surface, are Miocene in age in terms of a Miocene/Pliocene boundary at 5.0 m.y. as indicated by Berggren and Van Courvering (1974).

As shown in Figure 8, ashes overlying "Kimballian" faunas are older than 7 m.y. and some are as old as 9.3 ± 0.8 m.y. (Fig. 8). Ashes overlying the type Hemphillian fauna at Coffee Ranch, Hemphill County, Texas, and fauna identified as Hemphillian in Knox County, Nebraska (personal communication, Dr. Michael Voorhies, 1975), date at 5.3 ± 0.4 and 5.0 ± 0.2 m.y. respectively. Apparently, some "Kimballian" faunas must be Hemphillian or older.

An ash near the top of the stratal span of the Mount Blanco Fauna at its type locality is dated at 2.8 ± 0.3 m.y. This date indicates the Mount Blanco Fauna and its equivalents, such as the Broadwater and Rexroad faunas, may be of a similar Pliocene age. The time gap between the Broadwater Fauna (equivalent to the Mount Blanco, and early Pleistocene in age, Schultz and Stout, 1948; Schultz and Martin, 1970, 1975) and some "Kimballian" faunas may be 4 m.y. or more.

The rapid extinction and climatic changes postulated by Schultz and Stout (1948) and considered "one of the events which marks the end of the Pliocene Epoch" (Schultz,, Tanner, and Martin, 1972) was based on the marked differences between the "Kimballian" faunas and the "early Pleistocene" Broadwater Fauna. Fission-track ages suggest that a 4 m.y. hiatus may exist between these faunas. Thus, the significance of the marked differences between these faunas needs to be reevaluated.

Recently, sediments representing the 4 million year hiatus between sediments assigned a latest Pliocene age and those assigned an early Pleistocene age in Nebraska (Fig. 8) have been identified in central and northeastern Nebraska by means of fission-track dating. Faunas from these sediments should prove to be younger than those assigned to the "Kimballian" from western Nebraska.

Interpretations of conditions leading up to continental glaciation in Nebraska have been based largely on interpretations of faunas and sediments from what was considered to be a relatively complete sequence from latest Pliocene time ("Kimball Formation and Kimballian faunas") to early Pleistocene time (Broadwater Formation and Broadwater Faunas and their equivalents). The presence of a lengthy hiatus between these sediments and faunas was unrecognized. Deposits representing this hiatus have not been recognized in western Nebraska and may not be recorded there.

In terms of radiometric dates for the Pliocene/Pleistocene and Miocene/Pliocene boundaries (Berggren and Van Couvering, 1974) the Broadwater Fauna and equivalents would now be considered to be late Pliocene in age and the "Kimballian" faunas late Miocene.

It now appears that the sedimentary and faunal record spanning time from latest Miocene (> 5.0 m.y.) through the Pliocene (< 5.0 m.y. > 2.0 m.y.) and into the early Pleistocene (< 2.0 m.y.) is present in the central to eastern parts of Nebraska. It is here that studies of changing conditions leading up to Pleistocene time in Nebraska should be centered.

Acknowledgments

Gerald E. Schultz of West Texas State University provided samples of the two ashes from the Mt. Blanco locality, Texas, and later showed me the relationships there. Morris Skinner of The American Museum of Natural History, New York, provided the samples of ash from Elias' (1936) type Ogallala locality and from the Coffee Ranch locality, Hemphill County, Texas. Claude W. Hibbard guided me around Meade County, Kansas, and assisted in the collection of ashes there. Jim Spellman of the Nebraska Geological Survey assisted in the analyses of the ashes. A grant from the Earth Sciences Section National Science Foundation (NSF Grant DES 74-23535) supported the coring and dating of the Hartington ash from Cedar County, Nebraska.

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Kansas Geological Survey, Guidebook 24th Annual Meeting Midwestern Friends of the Pleistocene
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