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1982 Regional Implications of the Geology of the Ogallala Group (Upper Tertiary) of Southwestern Morrill County, Nebraska, and Adjacent Areas R. F. Diffendal Jr. University of Nebraska-Lincoln, [email protected]

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Diffendal, R. F. Jr., "Regional Implications of the Geology of the Ogallala Group (Upper Tertiary) of Southwestern Morrill County, Nebraska, and Adjacent Areas" (1982). Robert F. Diffendal, Jr., Publications. 32. http://digitalcommons.unl.edu/diffendal/32

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R. F. DIFFENDAL, JR. Conservation and Survey Division, IANR, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0517 Regional implications of the geology of the Ogallala Group (upper Tertiary) of southwestern Morrill County, Nebraska, and adjacent areas

R. F. DIFFENDAL, JR. Conservation and Survey Division, IANR, University of Nebraska-Lincoln, Lincoln. Nebraska 68588-05/7

ABSTRACT INTRODUCTION major erosional event was responsible for carving the Ogallala paleovalleys. Virtually A part of a filled. Ogallala Group (upper Five generalizations which have been all of the subsequent history of the Ogallala Tertiary) paleovalley system exhumed by proposed over the years about the Ogallala is said to represent a gradual filling of these recent stream erosion in southwestern Mor­ Group, either regionally or specifically in paleovalleys interspersed with short periods rill County, Nebraska, and adjacent areas western Nebraska, will be tested in this of equilibrium, during which paleosols contains steep gradient gullies filled with report. The first postulates the shape of formed, and with short periods of stream locally derived sediments. Younger chan­ paleovalleys in which Ogallala sediments erosion producing valleys with relatively nels filled with granitic gravels derived by were deposited. The few available studies low relief. The third generalization in recent stream erosion of the Rocky Mountains to and maps describing and depicting the literature deals with maximum clast size the west and southwest cut across and shape of the paleovalleys show them to have and clast mineralogy in the Ogallala and through these older filled gullies, indicating smoothly sloping sides and few tributaries. Quaternary deposits. Quaternary gravel that there were at least two periods of The second generalization is that a single clasts are claimed by some authors to be downcutting followed by periods of filling during the development of this system. SOUTH DAKOTA Individual clasts from younger Ogallala 43 gravels are often as large as or larger than clasts from Quaternary gravels transported comparable distances by streams. This fact supports the idea that streams of Ogallala age were often at least as competent as their Quaternary counterparts. In a tributary to Greenwood Creek, six major volcanic ash lentils occur in superpo­ sition. These ash deposits range in geometry C> I from tabular to channel-shaped and were Z deposited on land surfaces or in ponds and gullies at different times during the deposi­ tion of the Ogallala Group. Some of the lentils grade laterally into diatomites. Caliches are common throughout the Ogallala Group. Major caliches are devel­ oped in older rocks directly beneath the Ogalalla and in sediments beneath the eroded upper surface of the group. Caliche horizons occur at many horizons within the group and have "tepee," honeycomb, and other typical structures. The preceding observations are in part directly opposed to observations made by earlier workers for the Ogallala in general COLORADO or specifically for the study area. The Ogal­ 20 40 60 KILOMETERS I I I lala Group should be re-examined to see whether the earlier models or those offered I Figure 1. Location of the study area and principal physiographic features in western here better explain its geologic pistory. . Nebraska (base from DeGraw, 1971).

Geological Society of America Bulletin, v. 93, p. 964-976, 13 figs., 1 table, October 1982.

964 GEOLOGY OF OGALLALA GROUP, NEBRASKA 965

larger than comparable Ogallala gravels, been studied in detail at many locations the Ogallala Formation, as they called and mineralogy is claimed to be distinctive. on the Great Plains. The Ogallala is it, into the Valentin'e; Ash Hollow, and The fourth concept is that only one or two dominantly a fluvial unit consisting of Kimball Members on the basis of "average volcanic ash beds occur in any single super­ interbedded conglomerates, sandstones, silt­ lithologic characters that are sufficiently posed sequence of Ogallala strata in the stones, and pebbly sandstones and their distinctive to distinguish them one from southern Nebraska Panhandle. Fifth, a uncemented equivalents, and air-fall vol­ another in a general way, but which are not single terminal caliche development is pre­ canic ash, diatomites, and caliches. The adequate to permit a sharp delineation of sumed to have occurred at the end of Ogal­ Arikaree Group consists of friable, largely members in any particular section." I agree lala deposition, completing a trend toward volcanically derived eolian and fluviatile with the last part of this assertion and, increasing aridity throughout Ogallala time. sands and sandstones, siltstones containing therefore, at this time have chosen not to The validity of each of these ideas will be concretions of variable shape, and conglom­ assign formation or member names to the examined. erates made up of clasts of reworked con­ Ogallala Group exposed in the study area. The area of study for this investigation cretions. The Whitney Member of the Brule However, differences in faunal elements includes extreme southwestern Morrill Formation of the White River Group is allow time-stratigraphic subdivision of the County, Nebraska, and adjacent portions of primarily a volcanically derived eolian silt­ sequence, and this subdivision has been Banner and Cheyenne Counties (Fig. I). stone containing concretions at some loca­ attempted. Seven sites have been chosen to illustrate tions. The Whitney also includes some specific points. These sites, all on private widespread very pure volcanic ash beds. GENERALIZATIONS EXAMINED land, are precisely located in Appendix I. Lugn (1939) redefined the Ogallala Group Excluding Quaternary alluvium, collu­ in Nebraska and subdivided it into the Val­ Generalization 1. Simple Paleovalley Shape vium, and eolian silt and sand, rocks of entine, Ash Hollow, Sidney, and Kimball three groups are exposed in the study area Formations. Such subdivision continued to Middle and late Quaternary streams (Fig. 2). Rocks belonging to the Ogallala be used by Schultz and Stout (1961), Stout carved out the broad east-west-trending Group of late Tertiary age were probably (1971), and Schultz and Falkenbach (1968, valley of Pumpkin Creek (Diffendal, 1981) first reported on in western Nebraska by p. 411) in Nebraska. Frye and others (1956, lying primarily just to the north of the study Stansbury in 1852 and since that time have p. 47), working in Kansas, have subdivided area. The south side of this valley forms the

o 5 KILOMETERS IL-~ __~ __-L __ -L __ ~I Figure 2. Generalized bedrock geologic map of the study area. Base map is a U.S. Geological Survey 1:250,000 topographic map with 100-t't contours. The Ogallala Group of late Tertiary age is shaded. The Arikaree Group of Miocene age and the Whitney Member of the Brule Formation of Oligocene age are shown in white. 966 R. F. DIFFENDAL, JR .

Figure 3. Stream-eroded gully with an overhanging wall carved into the Brule Formation.

steeply sloping Cheyenne Tablelands es­ carpment. Modern tributaries draining from the tablelands to Pumpkin Creek have cut deep, often narrow valleys through the Ogallala Group strata that cap the table­ lands and into the underlying rocks of the Arikaree Group and Whitney Member of the Brule Formation. Some of these deeply incised valleys carved into the Whitney con­ tain falls, overhangs (Fig. 3), and vertical '':\~': '" - 'f"'-. , walls having reliefs of more than 35 m. ,,*;J' ~----... Logs of oil and gas tests in the southern Figure 4. (a) Older Ogallala gully fill. The gully was carved into the Whitney Member of Panhandle of Nebraska allowed DeGraw the Brule Formation. Man is 1.9 m tall. (b) Illustration of 4a showing relationships. (1971) to produce a map of the pre­ Oligocene topography and drainage pattern developed there. The pattern is extremely complex and resembles the modern drain­ All of these attempts, whether based on leys were recognized from at least one loca­ age network of the area. test holes or surficial studies, depicted sim­ tion in Nebraska as early as 1957, when Previous attempts to depict the Ogallala­ ple, broad paleovalleys with 90-120 m of Morris Skinner (unpub. field notes of filled paleovalley systems developed on the relief, smooth sides, and only a few tributar­ August 4, 1957) of the American Museum top of the Whitney Member of the Brule ies. This image of the paleovalley system in of Natural History observed a gully fill of Formation in the Nebraska Panhandle which Ogallala-aged sediments were depos­ Ogallala age along U.S. Highway 26 in include maps by Lugn and Lugn (1956), ited was also repeated in the literature on southeastern Morrill County. In 1975, after Smith's map for Cheyenne County (1969), Ogallala deposition and geologic history for obtaining new faunal evidence, Skinner the map and geologic sections for Kimball many areas of the High Plains (Frye and determined the age of the fill to be Claren­ County by Smith and Souders (1971), and others, 1956; Frye and Leonard, 1957; donian. Swinehart and Diffendal (1978) col­ for Banner County by Smith and Souders Walker, 1978). I question whether the Ogal­ laborated in mapping a large area of Morrill (1975). Breyer (1974, 1975) produced a geo­ lala paleovalley systems in western Ne­ and Garden Counties, Nebraska, and dem­ logic map and configuration of the pre­ braska are really that simple. onstrated the nature of the filled gully sys­ Ogallala surface for the southern Nebraska In the study area and at adjacent sites, tem of Clarendonian age at the site worked Panhandle and extreme southeastern Wyo­ Ogallala paleovalley systems are not simple, earlier by Skinner. Swinehart (1979) ex­ ming and a geologic section along Green­ smooth-walled features. Basal Ogallala panded that work and found that the base wood Creek in Morri.!1 County. Group gully fills tributary to main paleoval- of the Clarendonian fill was more than 46 m GEOLOGY OF OGALLALA GROUP, NEBRASKA 967

A A'

(f) a:: ILl 1300 I- ILl ::;;:

Z

ILl a ::::l I- 1250 I- ....J <{

o .5 I Kilometer o .5 I Kilometer LI______LI ______-....J 1 A , , 8

8 8'

(f) a:: ~ 1300 "'WYY,'Y'" Figure 5. (A) Location of site 2, ILl ::;;: Appendix I, showing positions of z geologic profiles. Base map is a U.S. Geological Survey 7W topographic ILl a map. Contour interval is 30 m. (B, C) ::::l Transverse geologic profiles at site 2. l- t: 1250 Tmo represents rocks of the Ogallala ....J <{ Group; Tow, rocks of the Whitney Member of the Brule Formation. Qal is Quaternary alluvium. Tmo) is Clar­ endonian in age; Tmo2 is Hemphil­ .5 I Kilometer V. E.' 12.4 X Iian. Dashed lines show inferred oLI ______LI ______-....JI contacts. (D) Longitudinal geologic c profile at site 2 showing the primary dip of the beds in Tmo) and the steep gully slope. c C' (f) a:: ILl 1300 I- ILl ::;;: and Ogallala drops more than 18 m. The Z Ogallala sediments above the contact were

ILl deposited in deep, steep-sided, often narrow a ::::l gullies like those forming in the area to­ I- day. As there is no evidence of faulting of 1250 I- ....J surficial strata in this area to explain these <{ features, their origin is probably solely erosional. An even more telling example of deep gully development on the pre-Ogallala sur­ o .5 IKilometer V.E.'12.4X LI ______LI ______~I face in the study area can be seen at site 2 (Fig. 5A). Here, the topography is "in­ o verted" with the ancient branching gully below the base of the adjacent rocks of the illustrate the relationship between the basal system, filled with sediment less easily Ogallala Group containing Hemphillian portion of the Ogallala Group and the eroded than the adjacent Brule Formation, fossils. underlying Whitney Member of the Brule left standing topographically high. The sed­ Similar relationships have also been Formation at site I. In a distance of < 0.16 iment filling the system consists of inter­ found in the study area. Figures 4a and 4b km, the erosional contact between the Brule bedded gray, green, and brown sandstones Figure 6. (a) Ogallala gully fill of site 2. Beds have approximately 4° primary dip to the southeast shown by dashed line. (b) Lithic conglomerate in the gully fill. Clasts are derived from erosion of the underlying formations up the paleoslope from this site. The hammer is ~ 40.6 cm long. (c) Partially exhumed Hemphillian gravel-filled channel at site 3. Channel sides shown by dashed lines. (d) Coarser lag covering the surface of the channel fill in 6c. (e) Sandstone on top of a coarse sand and gravel at site 4. GEOLOGY OF OGALLALA GROUP, NEBRASKA 969

'- . ! '-'-l o 2 4 5 KilOMETERS L._-,-_-1.'. -,---,-'----',

Figure 7. Locations of Ogallala Clarendon ian channel fills and the direction of slope of channels shown by dark shading and arrow points. Base map is a U.S. Geological Survey 1:250,000 topographic map with 100-ft contours. and conglomerates that have primary dips developing today. Figure 7 shows the loca­ no more than a thin veneer of Ogallala to the southeast and south-southeast of tion and direction of slope of all such Ogal­ sediments. Once the veneer was breached, about 4° (Fig. 6a). Gravel clasts (Fig. 6b) lala tributary paleogullies. The change in Quaternary stream erosion proceeded to include siltstones and reworked concretions slope directions seen on the map is a reflec­ differentially erode the region, leaving the eroded from the Arikaree and Brule tion of the change in direction of the long Ogallala exposed on the faces of high escarp­ Formations higher up the paleoslope to the axis of the major paleovalley which these ments at this location where it can be easily north. The conglomerates may be massive, smaller valleys entered. examined. Third, reliable 7'h' topographic parallel-bedded, or trough cross-bedded. If the tributary system during Ogallala maps for the southern Panhandle of Ne­ Where troughs occur, they plunge to the time was similar to the modern system, as braska have been available only since southeast or south-southeast. One longi­ has been illustrated, then why have so few 1980. Thus, lack of elevation and location tudinal and two transverse profiles through reports demonstrated the fact? A combina­ control made detailed analysis difficult. a part of the gully system illustrate its shape tion of factors has contributed to the and general relations to adjacent units simpler interpretations made in the past. Generalization 2. Single Cycle of (Figs. 5B-50). These profiles show that the First, and probably most important, this Erosion and Deposition system is shallow and narrow at its up­ study area, in contrast to most others, is stream end but rapidly deepens with increas­ along and subparallel to the partially ex­ The commonly accepted picture of Ogal­ ing slope to the south-southeast. The shape humed valley side of an Ogallala paleovalley lala Group erosional and depositional his­ of this paleovalley system is similar to those system rather than in a drainage developed tory throughout the Great Plains is, of Recent gully systems cut into the Whit­ along a paleovalley axis where tributaries perhaps, best typified by Frye and Leo­ ney along the sides of stream valleys. would be more difficult to discern. If nard's (1957) diagram shown in part in Fig­ The two Ogallala paleo gullies and gully ancient tributaries exist, the paleogeogra­ ure 8. Their description and illustration of fills just described are the best-known phic position of this area is clearly the best the deposition of the Ogallala (p. 3-5) examples in the study area for illustrating place to look for them. Second, the paleo­ stressed the conformable and gradational the point that the gullies along the sides of valley side here rises rapidly to the north nature of contacts between rock units. They major valleys during Ogallala time were to an upland underlain by pre-Ogallala further stated (1959, p. 27-29; 1965, p. 205- similar in size, shape, and gradient to gullies sediments probably covered originally with 207) that the oldest part of the Ogallala is 970 R. F. DIFFENDAL, JR.

S N sequent valley-filling during the Clarendon­ ian and Hemphillian portions of Ogallala time. During the youngest of these erosional episodes, stream erosion produced a valley system cut through older Ogallala deposits ~LOWER OGALLALA Figure 8. Frye and Leon­ and below the levels of the earlier periods of ard's model of deposition of erosion. Seni (1980) studied the deposi­ the Ogallala Group through tional history of the Ogallala deposits in time (modified from Frye Texas using irrigation well records com­ and Leonard, 1957). Dotted bined with surficial investigation and pro­ ~MIDDLE OGALLALA lines represent time lines. posed that the Ogallala was deposited as coalescent, wet alluvial fans with low gra­ dients. He identified three fan lobes in his Texas study area which overlapped one , . . . another and which were progressively younger toward the south. Each of the three ~UPPER OGALLALA depositional lobes was preceded by a period found in the lowest parts of pre-Ogallala Two recent studies of the Ogallala Group of stream erosion. valleys and that the valleys were then filled stand out as exceptions to this simple model In southwestern Morrill County, Nebras­ completely, producing a nearly featureless of general alluviation through time after an ka, a second period of erosion occurred dur­ depositional plain. This theme was used initial period of valley development. Skin­ ing which streams cut valleys across the many times thereafter by other authors, ner and others (1977) described the strati­ valley fills previously described and into the such as Schultz and Stout (1961), Kent graphy of Cenozoic deposits in central underlying rocks of the Arikaree and upper (1963), and Breyer (1975), to explain the Sioux County, Nebraska, mapped their dis­ White River Groups. These streams pro­ depositional history of the Ogallala in parts tribution, and worked out cycles of erosion duced steep-sided, often narrow, valleys of western Nebraska and by Walker (1978) and deposition. These workers were able to that were later filled with a series of very in a study of the Ogallala in Texas. establish three periods of erosion and sub- coarse-grained granitic sand and gravel

o 4 5 K ILOMETERS I~~ __~ __-L __ -L I __~ I Figure 9. Location of the principal older Hemphillian channel fills in the study area. Outcrops shown in darker shading. Base map is a U.S. Geological Survey 1:250,000 topographic map with 100-ft contours. GEOLOGY OF OGALLALA GROUP, NEBRASKA 971 deposits derived from erosion of the Rocky northeast. Breyer (1975) attempted to show thickness of sediment in three sand and Mountains, interbedded with finer-grained the relationships at Greenwood Canyon gravel bodies separated from one another sediments. Figure 6c is an aerial view of one where this northern channel fill is well by finer-grained sands and silts. Three other of these filled valleys at site 3 that has been exposed, but, unfortunately, he did not use stratigraphically higher sand and gravel partially exhumed by recent erosion. Some an irrigation well record from the west side bodies in this fill, also separated by sands of the sand and gravel filling the valley is of Greenwood Canyon that clearly indi­ and silts (Fig. 6e), are exposed by surficial very coarse (Fig. 6d). This valley is part of a cated that the filled Ogallala paleovalley erosion west of the irrigation well. Fossils channel system incised into the bedrock in just described was present in the area. Fig­ from this filled paleovalley system indicate the Morrill County study area shown in ures lOA and lOB show Breyer's and this that it is Hemphillian in age, and the fossils Figure 9. The principal northernmost chan­ author's interpretations of the geology of found in the older fills described earlier are nel of this system, preserved best at sites 4 Greenwood Canyon. The northern paleo­ Clarendonian or older (Appendix 2). and 5, trends west to east to the vicinity of valley is ~ 1.6 km wide at its base and in the In southwestern Morrill County, the his­ Greenwood Canyon, where it turns east- subsurface is filled with at least a 45-m tory of the Ogallala group clearly includes A SOUTH NORTH 1300

en Ir Figure 10. (A) Breyer's w I- 1250 interpretation (1975) of the w ::E geology along east side of z Greenwood Canyon, Mor­ rill County, Nebraska. (B) w Geologic profile section 0 :::> along west side of Green­ l- I- 1200 wood Canyon. Tma? is prob­ ...J « ably part of the Arikaree Group of Miocene age. See explanation of Figure 5 for other symbols.

v. E.' 20.2 0,--, __--I..? __ --ll Kilometer B SOUTH NORTH 1300 1300

en Ir Volcanic en w ash Ir I- 1250 w 1250 I- w w ::?: ::?: z Well z w 0 w • 0 :::> I- :::> l- I- 1200 1200 ...J I- ...J « «

.5 I Kilometer o1.... __...... 1 __---" 972 R. F. DIFFENDAL, JR.

at least two major periods of erosion, pre­ should be taken in designating a gravel ogy comparisons made on gravels from Clarendonian and pre-Hemphillian, follow­ from western Nebraska as Pleistocene in distinctly different source areas far removed ed by periods of filling during part of the age because it has an abundance of from one another and transported variable Clarendonian and part of the Hemphillian, anorthosite. distances are of questionable value except respectively. During the second of these cut­ Measurements made on the ten largest to establish possible drainage directions and and-fill episodes, stream erosion produced a clasts by Breyer (1974, 1975) and by Swine­ changes in these directions through time. If deeper cut that eliminated major portions of hart (1979) generally are lower than those Breyer's maximum sizes for the Ogallala the earlier cut and fill. This view of the ero­ reported earlier by Stanley and Wayne gravels at Greenwood Canyon are com­ sional and depositional history of the Ogal­ (1972) at the same sample sites. A field pared with the maximum sizes for the early lala Group in the study area is clearly closer check at the sites supported the values cited Pleistocene gravels northeast of Lisco, Ne­ to the sequence of events outlined by in the later reports. A comparison of those braska, making the distance from each Skinner and others (1977) than to that out­ values with measurements made on clasts at major source area comparable, the generali­ lined by either Breyer (1974, 1975) or Frye other previously unreported localities near­ zation on size and competence made by and Leonard (1957). Furthermore, Souders by revealed that in some cases gravels from Stanley and Wayne (1972) is reversed. Max­ (1981, personal commun.) believes that the Ogallala Group had greater mean imum sizes of clasts carried in some Ogal­ there may have been three or four major intermediate diameters than did gravels lala streams were greater than their early cut-and-fill cycles in the depositional his­ from early Pleistocene sites. For example, Pleistocene counterparts, suggesting that tory of the Ogallala Group in other parts of Breyer (1975, p. 5) reported mean long and these Ogallala streams were more compe­ western Nebraska. intermediate diameters for the ten largest tent than some early Pleistocene streams. clasts at the early Pleistocene Broadwater Generalization 3. Distinctive Gravel Sizes Quarries as 16.0 and 7.7 cm, respectively. Generalization 4. One or Two Ash Falls and Mineralogies South of Broadwater in the SWV4NWV4 NEV4 sec. 33, T. 18 N., R. 48 W., an Until 1971, no more than two volcanic Stanley (1971) and Stanley and Wayne Ogallala gravel has mean long and interme­ ash lentils had been reported in superposi­ (1972) believed that Ogallala gravels could diate diameters of the ten largest clasts of tion in a single exposure of the Ogallala be separated from post-Ogallala gravels in 16.6 and 11.5 cm. This Ogallala gravel is Group in the southern Nebraska Panhan­ western Nebraska, and they made a number close enough to the type Broadwater area to dle. Lueninghoener (1934) reported two of assertions related to the general topics of demonstrate that generalizations on separa­ volcanic ash deposits in the Ogallala at Ash sediment dispersal, source areas, and stream tion of gravel ages in the southern Panhandle Hollow in Garden County and two volcanic competence for Ogallala and early Pleisto­ of Nebraska based on differences in maxi­ ash beds at Greenwood Canyon in Morrill cene gravels. Stanley and Wayne (1972) mum size may be of questionable value. County. Stout (1971) illustrated five ash stated that Ogallala gravels in western The main source areas of Ogallala gravels lentils, four of which were in superposi­ Nebraska .are generally free of anorthosite, and the major drainage patterns for the tional proximity, at Greenwood Canyon that with a few exceptions the presence of streams carrying these gravels have been but reported none from Ash Hollow. Boell­ anorthosite and larger clast size may be described by Blackstone (1975). The heads storff (1978) reported two ash beds from used to separate early Pleistocene from of these streams were in the vicinity of the each of the two localities. Diffendal ( 1980a, Ogallala gravels, and that the main early Continental Divide in Colorado at least 67 1980b) noted at least five separate ash beds Pleistocene drainage way in the Nebraska km southwest to west-southwest of the Lar­ in superposition in Ash Hollow and estab­ Panhandle was through the ancestral Pump­ amie Anorthosite body. Size and mineral- lished that there are at least four major ash kin Creek Valley. They also concluded that lentils in superposition in the Greenwood Pleistocene streams were more competent TABLE I. COMPOSITION OF Canyon area in addition to those reported than Ogallala streams in western Nebraska. ANORTHOSITIC OGALLALA earlier by Stout. The locations of these len­ Breyer (1974, 1975) reported anorthosite­ GRAVEL, BANNER COUNTY tils in the Ogallala Group at site 6 in bearing gravels in the Ogallala Group from southwestern Morrill county (Fig. II) sup­ the Greenwood Canyon area of Morrill Clast type Percentage of sample port the earlier proposal of Swineford and County at site 4. This type of gravel also others (1955) that the Ogallala sequence occurs in the Ogallala sequence to the south Quartz 7.7 contains many ash beds. (NWV4SEV4NWV4 sec. 12, T. 17 N., R. 50 Granite 49.8 The shape of the lentils at Greenwood W.) and west (SWV4SWV4NWV4 sec. 17, T. Anorthosite 21.4 Canyon varies from broad tabular bodies to Rhyolite 2.6 18 N., R. 52 W.) in Morrill County and in narrow gully fills (Fig. 12). The tabular Metamorphics 3.8 Banner County (NEV4SWV4SEV4 sec. 21, T. Silica varieties 5.4 bodies may appear as outcrops for several 18 N., R. 53 W. and NWV4NWV4NWV4NEV4 (flint, jasper) kilometres and be as much as 6 m thick, and sec. 23, T. 18 N., R. 53 W.), where anortho­ Sandstone 3.4 the gully fills may be < 15 m wide and as site is abundant (Table I). Such occurrences Quartzite 3.3 much as 6 m thick. Limestone 2.6 indicate that tributaries draining the Lara­ The sedimentary characteristics of the 100.0 mie Anorthosite area in Wyoming were ashes vary. They may be virtually pure active, if minor, contributors to the Ogallala masses of ash shards and other associated Note: Location of sample site in NEY4SWY4- depositional system during Hemphillian SEY4 sec. 21,1'.18 N., R. 53 W. Sample obtained volcanic debris, or they may contain varia­ time in parts of southwestern Nebraska; ranges from -4 to -5 . ble quantities of nonvolcanic silt, sand, and because of that, I suggest that great care even gravel. Ash lentils may be entirely or GEOLOGY OF OGALLALA GROUP, NEBRASKA 973

T. 18 N.

Figure II. Location of ash lentils at site 6. Base map is a U.S. Geological Survey 7'12' topographic map. T. 17 N

R. 50 W. .5 o Kilometer I partially cemented by calcite or silica. When The tabular ash bodies may grade later­ ronment of deposition became progressively the ash is partially cemented, usually the ally into white calcareous diatomaceous more arid through time, culminating in a upper portions contain the greatest amount deposits commonly containing shells or terminal Ogallala caliche formation across of cement and frequently form hard gray molds of gastropods and ostracods as well the Great Plains. Near the top of the Ogal­ ledges. In many cases, the ash is less perme­ as vertebrate remains. This gradation sug­ lala sequence in southwestern Morrill Coun­ able than the surrounding sediment and gests that the ash was deposited in these ty, coarse granitic sand and gravel deposits produces local perched water tables with cases in ponds that were probably peren­ were laid down in subparallel channels attendant small springs and seeps. Bedding nial. The diatoms may have formed algal trending east to east-southeast (Fig. 13) is variable and may be planar, cross­ blooms after silica-rich waters formed in the which have been exposed by recent stream stratified, or convoluted. Some lentils are ash-laden ponds. erosion. These gravels contain trace quanti­ massive; others are thinly laminated and ties of anorthosite. At site 7, the median have ripple marks. Shard diameters vary Generalization 5. Increasing Aridity and long and intermediate diameters of the ten from coarse-sand to silt size. Fossils asso­ Caliche Formation largest clasts are 12.8 cm and 7.7 cm, respec­ ciated with the ash include "seeds" of tively. After these and associated finer­ grasses and borages, and bones and teeth of Frye and Leonard (1965) and other grained beds were deposited, stream erosion vertebrates. workers have stated that the Ogallala envi- began, followed by development of thick Figure 12. Volcanic ash deposited in a gully carved into older Ogallala sediments of Hemphillian age at site 6. Gully side shown by dashed line.

'- . I " I " I '-" o 4 5 KILOMETERS LI__ ~ __L-~ __~I __~I

Figure 13. Younger Hemphillian channel fills at and near site 7. Outcrops shown in darker shading. Base map is a U.S. Geological Survey I :250,000 topographic map with 100-ft contours. GEOLOGY OF OGALLALA GROUP, NEBRASKA 975

caliches in the surficial deposits beneath an 6. Ash lentils and coarse terrigenous clas­ K. Pabian, J. R. Thomasson, V. L. Souders, irregular erosion surface. These caliches tics deposited in gullies at the base of and and J. B. Swinehart, II for their helpful have pisolitic structures in some places. within the Ogallala sequence indicate that comments during the preparation of this While this terminal Ogallala caliche de­ gUllying was a common phenomenon dur­ paper. J. J. Gottula accompanied and velopment is important, caliches with hon­ ing Ogallala time. aided me during portions of the field work. eycomb and "tepee" structures occur locally 7. Caliches occur throughout the Ogallala Special thanks are extended to N. Alex­ throughout the Ogallala section in south­ sequence and are not merely restricted to a ander, T. Coop, R. Corman, M. Flamig, R. western Morrill County (Stout, 1971) and terminal Ogallala event. Harless, C. Lapaseotes, D. Lease, M. Lund, may even be as strongly developed at some These conclusions should be tested in the Mrs. E. Marrin, K. Nelson, J. Newkirk, R. horizons as the caliches capping the High future throughout the Ogallala outcrop belt Rodgers, and D. Wright, all of whom Plains surface. Breyer (1974, 1975) has doc­ in the Great Plains to see if they more ade­ allowed me to study rock exposures on their umented the occurrence of a caliche devel­ quately reflect the true history of the group properties. M. R. Voorhies identified fossils oped in the uppermost older Tertiary rocks than do previously accepted models. and made valuable suggestions. I thank past directly beneath the Ogallala exposed along and present workers who have studied the the escarpment at the head of Greenwood ACKNOWLEDGMENTS Ogallala Group across the Great Plains. Canyon (Fig. 10). This caliche can be traced Their stimulating ideas prompted me to south along this escarpment for about 3 km I am very indebted to R. G. Corner, L. G. examine this sequence of continental depos­ and also occurs at the heads of the canyons Tanner, M. P. Carlson, R. R. Burchett, R. its and to test previous hypotheses. to the west of Greenwood Canyon. The prominence of caliches throughout the Ogallala sequence in southwestern Morrill APPENDIX I. LOCATIONS OF STUDY SITES County suggests that the environment may have been similar throughout Ogallala time I. SEY.NEY. sec. 36, T. 18 N., R. 52 W. Morrill County in this area. 2. SEY. sec. 28, T. 18 N., R. 51 W. Morrill County 3.SEY.SEY. sec. 20 and NEY.NEY. sec. 29, T. 18 N., R. 51 W. Morrill County 4. SWy. sec. 28 and SEY. sec. 29, T. 18 N., R. 50 W. Morrill County 5. NWY. sec. 4 and NEY. sec. 5, T. 17 N., R. 50 W. Morrill County CONCLUSIONS 6. Sections 2, 3, 10, II, T. 17 N., R. 50 W. Morrill County 7. SEY. sec. 21, T. 17 N., R. 50 W. Cheyenne County Seven conclusions can be drawn from studying the Ogallala Group exposures in southwesern Morrill County: I. If an ancient Ogallala valley side is APPENDIX 2. FAUNAL LIST OF exposed by erosion, it will have a set of FOSSILS FOUND IN THE tributary gullies and larger tributary valleys STUDY AREA feeding into it that are just as complex as those developing along Recent valley sides. (Derived in part from Kent, 1963) 2. These tributaries were filled with sedi­ ment derived primarily from local sources Clarendonian or older Fossils higher up the old paleoslope. Merychippus cf. insignis 3. These tributaries and their contained Pseudhipparion gratum deposits were later cut through in places by Cormohipparion occidentale streams during a second major period of Calippus sp. Amphicyonidae erosion during Ogallala time and then were Hemphillian Fossils· filled with coarse gravel and sand derived Osteoborus sp. from distant sources, as, in this case, pri­ Megatylopus sp. marily from the southern Rocky Mountains Hemiauchenia sp. of northern Colorado. Dinohippus sp. Astrohippus sp. 4. Ogallala sand and gravel deposits, Neohipparion sp. when viewed from proximity to their ulti­ Nannippus sp. mate source area, may contain clasts as Teleoceras sp. large as or larger than those in adjacent .. Prosthennops" serus Barbourolelis sp. Quaternary stream deposits. This compara­ Hesperotestudo spp.-Thin and thick shelled bility of maximum sizes indicates that some turtle Ogallala streams were at least as competent Antilocapridae as the most competent Quaternary streams. Rodentia 5. A large number of volcanic ash lentils ·Including the Kimballian of Schultz and are found in superposition in the Ogallala if Stout (1961) and Schultz (1981). a thick sequence of beds is studied. 976 R. F. DIFFENDAL, JR.

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