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Volcanic Ash Falls
The use of petrographically distinctive volcanic ash lentils for the establishment of time lines has been demonstrated in the Great Plains region (Frye, Swineford, and Leonard, 1948). As this technique represents a new approach to the problems of intra-Ogallala correlation, it merits special attention in a discussion of the stratigraphy of the formation. The petrography and stratigraphic occurrence of volcanic ash in the Miocene and Pliocene of western Kansas and Nebraska have recently been described by Swineford, Frye, and Leonard (1955) and the following descriptions are chiefly a summary of this detailed work.
All the Ogallala volcanic ash deposits are predominantly vitric tuffs. They have been differentiated petrographically by the character of the glass shards, including average value and range of index of refraction; thickness, shape, and color of the shards; presence or absence of small second-order vesicles in the glass; and chemical composition of the glass. Most of these characters (i.e., refractive index, color, and chemical composition) are more or less affected by weathering of the glass and therefore must be evaluated with caution.
Some ash falls are characterized by shards that are simple and so thin that iridescence is observed in a few fragments. Other falls contain coarse glass shards 20 microns thick or thicker. Glass in some deposits shows a small radius of curvature of the bubble walls, or frothy to fibrous aspect caused by numerous bubbles, or multiple bubble junctures in one shard. The chemical composition of most of the samples analyzed is typically rich in silica, or approximately that of rhyolite.
Ten petrographically distinctive volcanic ashes in the Ogallala formation of northern Kansas have been described and six of these have been judged to warrant the application of formal bed names. Of these ten ash falls, eight are definitely placed in the lower three-fifths of the Ash Hollow member, the stratigraphic position of one (represented by a single exposure) is not certain, and the other ash fall is placed in the upper part of the Valentine member. Fortunately, regional correlation is possible because two of these falls are also judged to be represented by lentils in Nebraska that occur in the Valentine and Ash Hollow type sections. In all, lentils of Ogallala volcanic ash from 28 localities in northern Kansas were described and the stratigraphic relations of 13 of these lentils are given in the measured sections included with this report. The distribution of known Ogallala volcanic ash localities is shown on the map in Figure 1c, and the field appearance of some typical exposures is shown on Plate 4.
The four unnamed ash falls known from the northern Kansas Ogallala have little or no stratigraphic significance and will be mentioned only briefly. Only one of these is known from more than one locality. Samples of this ash were collected in the NW NE sec. 12, T. 3 S., R. 22 W., Norton County, and the SW NE sec. 3, T. 2 S., R. 19 W., Phillips County, from a bed judged to be about one-third of the distance above the base of the Ash Hollow member. Another lentil in the lower third of this member is exposed in the NE NE sec. 19, T. 2 S., R. 18 W., Phillips County. An ash lentil exposed in the SE SW sec. 25, T. 2 S., R. 25 W., Norton County, is judged to lie stratigraphically at approximately the middle of the member.
A volcanic ash of uncertain stratigraphic position is exposed in the NE NE NE sec. 2, T. 7 S., R. 24 W., Graham County. The details of the local stratigraphy are given in the measured section with this report.
In the following paragraphs the named ash beds in the Ogallala of northern Kansas are briefly described. The Calvert bed is in the Valentine member; the Rawlins, Fort Wallace, Dellvale, Reager, and Reamsville beds occur in the lower three-fifths of the Ash Hollow member.
Calvert bed
The Calvert ash bed was named from exposures in the commercial ash mine at Calvert (NW SW sec. 25, T. 3 S., R. 22 W.), Norton County. In this area the bed attains a maximum thickness of 22 feet, which is the maximum known thickness of any Ogallala volcanic ash. The Calvert bed has also been identified in exposures in the SE SE sec. 25, T. 3 S., R. 25 W., Norton County, and the NW NW sec. 33, T. 3 S., R. 34 W., Rawlins County. As this bed has been correlated with partly altered ash occurring 185 feet above the base of the type section of the Valentine member, Cherry County, Nebraska, it has been of value in the delineation of the Valentine-Ash Hollow boundary in the Ogallala of northern Kansas. The Calvert ash bed is in the upper one-fifth to one-sixth of the Valentine member and although the thickness of the Valentine varies from place to place this relative position is judged to hold generally.
Rawlins bed
The Rawlins ash bed is known from seven localities in northern Kansas and the type section of the Ash Hollow member in Nebraska. The Rawlins ash was described from exposures at the center of the west line of the SW sec. 4, T. 4 S., R. 34 W., Rawlins County, where the bed is 3 1/2 feet thick (see measured section for stratigraphic association). Its stratigraphic relationships are well illustrated in measured sections, included here, from Norton County (W2 sec. 16, T. 2 S., R. 21 W.), Phillips County (NW NE sec. 30, T. 1 S., R. 19 W.), Rawlins County (SE SE sec. 20, T. 5 S., R. 36 W.), and Wallace County (SE SE SE sec. 8, T. 14 S., R. 38 W.). This bed has also been studied at exposures in the NE SE sec. 27, T. 4 S., R. 23 W., Norton County, and NE NW sec. 10, T. 5 S., R. 29 W., Decatur County.
The Rawlins bed lies within the lower part of the Ash Hollow member. The correlation of this bed with ash in the type Ash Hollow section, and its association in northeastern Norton County with the Calvert bed, which is correlated with ash in the type Valentine section, provide new evidence for the placement of the boundary between these two members in northern Kansas, as shown in Plate 1. As will be described, the evidence of these two volcanic ash falls is in harmony with paleontological and stratigraphic evidence.
The succeeding ash falls, progressively higher within the Ash Hollow member, aid in correlation within Kansas, but as they are not known from the Nebraska type section, they have little value in definition of units.
Fort Wallace bed
The Fort Wallace bed is known only from Wallace County. Its stratigraphic relations are shown by the measured section at its type locality in the west line of the SW sec. 7, T. 14 S., R. 38 W. It was also sampled from a one-foot bed in the NE NE sec. 16, T. 14 S., R. 40 W.
Dellvale bed
The Dellvale bed is also relatively local in its known geographic extent. The associated stratigraphy is shown by two measured sections in Norton County, the type locality of the bed southeast of Dellvale (NW NW sec. 2, T. 4 S., R. 24 W.), and the Almena section (W2 sec. 16, T. 2 S., R. 21 W.). It has also been described from exposures in the NE NE sec. 19, T. 2 S., R. 18 W., Phillips County.
Reager bed
The Reager bed is known from four localities distributed over a distance of approximately 70 miles. It was named from exposures in an abandoned pit in the SW SE sec. 2, T. 3 S., R. 25 W., southeast of Reager, Norton County. It was also studied in the SE SE sec. 16, T. 1 S., R. 21 W., Norton County, NE NE sec. 15, T. 3 S., R. 30 W., Decatur County, and SW SE sec. 2, T. 3 S., R. 33 W., Rawlins County. In Rawlins County the measured section shows the close stratigraphic relation of this bed with the next higher, Reamsville ash bed.
Reamsville bed
The Reamsville bed is the highest named volcanic ash bed in the Ogallala formation, (Pl. 1) and is known from four localities distributed over an east-west distance of more than 100 miles. Its stratigraphic association is shown by three measured sections; the type locality south-southwest of Reamsville in Smith County (center of the west line of the SW sec. 32, T. 1 S., R. 14 W.), a section in Decatur County (SE SW sec. 8, T. 4 S., R. 29 W.), and in the same section with a Reager ash bed in Rawlins County (SW SE sec. 2, T. 3 S., R. 33 W.). The bed was also studied in the NW SW sec. 31, T. 1 S., R. 19 W., Phillips County.
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Kansas Geological Survey, Geology
Placed on web Aug. 4, 2011; originally published March 1956.
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