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Robert F. Diffendal, Jr., Publications Natural Resources, School of

1985 Cenozoic Paleogeography of Western Nebraska James B. Swinehart University of Nebraska - Lincoln, [email protected]

Vernon F. Souders University of Nebraska - Lincoln

Harold M. DeGraw University of Nebraska - Lincoln

Robert F. Diffendal Jr. University of Nebraska-Lincoln, [email protected]

Follow this and additional works at: http://digitalcommons.unl.edu/diffendal Part of the Geology Commons, Geomorphology Commons, Hydrology Commons, and the Stratigraphy Commons

Swinehart, James B.; Souders, Vernon F.; DeGraw, Harold M.; and Diffendal, Robert F. Jr., "Cenozoic Paleogeography of Western Nebraska" (1985). Robert F. Diffendal, Jr., Publications. 28. http://digitalcommons.unl.edu/diffendal/28

This Article is brought to you for free and open access by the Natural Resources, School of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Robert F. Diffendal, Jr., Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Cenozoic Paleogeography of Western Nebraska

Cons .... r lation & ~II \} I r i'l; i~n Insthde 0 '( I - -9urce 1:16 lr,V2·',t " - ,"':';,rj

RE-print \..;Eme..l .J:t; S-~ James B. Swinebart

"eroon L. Sonders

Harold M. DeGraw

Robe .. t F. Diffendal, Jr.

Rocky Mountain Section, SEPM Rocky Mountain Paleogeograpby Symposium 3 Cenozoic Paleogeograpby of West-central United States R.M. Flores and S.S. Kaplan, Editors Denver, Colorado, 1985 ROCKY MOUNTAIN SECTION-S.E.P.M. CENOZOIC PALEOGEOGRAPHY OF WEST-CENTRAL UNITED STATES R. M. FLORES AND S. S. KAPLAN, EDITORS DENVER, COLORADO 1985

CENOZOIC PALE

James B. Swinehart Vernon L. Souders Haro1d M. DeGraw Robert F. Diffenda1, Jr. Conservation and Survey Division-IANR University of Nebraska-Lincoln Lincoln, NE 68588-0517

ABSTRACT It also presents some new interpretations on the role of structure in influencing regional depositiona1 and The Cenozoi c strata of western Nebraska are an erosional patterns in this area. Lugn and Lugn (.1956) extensive sequence of continental deposits that extend presented the first attempt at a regional synthesis of eastward from the Hartville, Laramie, and Front Range Tertiary pa1eogeography of Nebraska. Since 1956, many up1 ifts and southeast from the Black Hi 11s. The outcrop studies and pa1eonto10gic investigations have oldest Cenozoic sediments (Chadron Formation, White modified understanding of the geology of western River Group) are Early 01igocene alluvial valley Nebraska. In addition, analysis of a large amount of fills. Subsequent to filling of these drainages and subsurface information has provided new insights into continuing for about the next 7 m.y., landscape devel­ the stratigraphy of the region and furnished a data opment in western Nebraska was domi nated by eo1 i an base for pa1eogeographic interpretations which would deposition of tremendous volumes of rhyo1itic volcanic be impossible to achieve from outcrop studies alone. ash derived from western eruptions. A plain of low relief, with only an occasional narrow drainage The surface of Cenozoic rocks in the study area heading in the western highlands, was maintained forms an eastward sloping plain from the Laramie during most of this period. Uplift in the Rocky Range-Hartville Uplift in Wyoming (Fig. 1) that has Mountains and Great Plains (pre-Gering Formation, been deeply eroded in the North P1atte River valley Arikaree Group) caused erosion and brought epic1astic and along the Pine Ridge (Fig. 2). Dune sand covers det ritus into the area about 28 m.y. ago. Eo 1 i an most of the east central part of the area. Subsur­ sediment consisting mostly of pyroc1astic detritus face data used in this report came from electric logs continued building the plains during and after Gering of approximately 11,600 oil and gas tests and from alluvial deposition until about 19 m.y. ago when samp 1es and elect ri clogs of about 500 tes t ho 1es Ari karee deposition ceased. About thi s time, western volcanic activity declined for several million years and was followed by a marked decrease in the volume of rhyo1itic vo1canism for the remainder of the Cenozoic. At the end of Arikaree deposition in western Nebraska, a major pulse of erosion (pre-Runningwater Formation, Oga11a1a Group) was followed by a fundamental change in depositiona1 style and landscape evolution, charac­ terized by a heterogeneous mixture of epic1astic valley fills. Sands and gravels from Rocky Mountain sources were first deposited in a major valley in the northern half of the area and later in valleys to the south. Episodic regional and local structural move­ ments influenced the size and position of many Ogalla1a valleys. For the past 5 m.y. degradation, in response to major regional uplift, has greatly exceeded aggradat i on as the domi nant f actor affect i ng landscape evolution in western Nebraska. KANSAS

INTRODUCTION vs This report is a general outline of successive Figure 1. Index map of study area and major regional changes in the Cenozoic landscape of western Nebraska. uplifts and basins.

209 J. B. SWINEHART, V. L SOUDERS, H. M. DEGRAW, AND R. F. DIFFENDAL,JR.

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15 " 'I I! 15 30 KILOMETERS

[.~oJ Ogollolo Group T. While River Group 1,,1111111 Foull (down-thrown Side nocnured) • A"koree Group o Cretaceous rocks

Figure 2. Generalized geologic map of western Nebraska. Post-Ogallala rocks (Pliocene and younger) not shown. Cretaceous rocks refer to Pierre Shale in the north and Fox Hills Sandstone in Scotts Bluff County. Only known faults with apparent throws in excess of 100 ft (30 m) are shown.

210 CENOZOIC PALEOGEOGRAPHY OF WESTERN NEBRASKA

dri lled by the Conservation and Survey Division and SOUTH DAKOTA SOUTH DAKOTA its cooperators (F i g. 3). Dri llers' logs of i rri ga­ tion wells were also utilized. 9 2 I 2 10 ~ GENERAL STRATIGRAPHY AND LITHOLOGY OF CENOZOIC ROCKS 6 4 !:'" I,- '"z 4 Rocks of the Paleocene or Eocene series have not o'" 76 0'" 0 >-1---- 64 L,-- >- 5 been identified in western Nebraska and the oldest ~ 330 8 4 ". 4 2 Cenozoic rocks known belong to the Oligocene Chadron ,L-- Formation (Fig. 4 and Table 1). An oxidized zone (the 410 240 "37' 3 5 5 10 4 Interior Paleosol) occurs throughout much of western COLORADO COLORADO Nebraska on truncated Cretaceous and Jurassic rocks ELECT R 1CLOG S OF SAMPLES AND ELECTRIC and represent s a pre-Chadron and Chadron weatheri ng OIL AND GAS TESTS LOGS OF CONTROLLED SHALLOW DRILLING horizon. The Cenozoic deposits of western Nebraska Number of tests per SINCE 1945 can be divided into two major packages: an older, lOO mi. 2 (260 km 2) Number of holes per relatively homogeneous set of fine-grained volcani­ lOO mi.2 (260 km 2) clastic rocks (White River and Arikaree groups, Table 2) primarily of eolian origin and a younger, coarser grained and more heterogeneous set (Ogallala Group and SOUTH DAKOTA younger rocks, Table 2) composed mostly of epiclastic debris of alluvial origin. Deposition for much of the 5 6 White Ri ver and Ari karee was remark ab ly cont i nuous 10 ,-- 21 over 1arge areas (F i g. 5). The Oga 11 ala and younger '"z 82 I,- '"z rocks consist of a much less continuous set of complex 3 '"0 61 '"o valley fills of widely variable geometries and litho­ >-1-- 43 L,-- >­ logies. ". 31 17 12 ". ,L-- 36 White River Group 30 ¥ 83 COLORADO

Traditionally, the White River Group has been REGISTERED IRRIGATION WELLS GEOLOGIC MAPPING divided into three stratigraphic units in western Number of we/Is per • 7.5' quadrangle detail Nebraska, the Chadron, Ore 11 a, and Whitney (Schu ltz lOO mi. 2 (260 km 2) and Stout, 1955). We recogni ze three depositional ~ 7.5' quadrangle reconnaissance sequences in the group, based on subsurface data vs D 2° quadrangle reconnoissance supp 1emented by out crop st ud i es, wh i ch do not corre­ spond to the above mentioned units. The depositional Figure 3. Index maps showing the density of the sequences (Fig. 6) consist of the following strati­ three categories of subsurface information and graphic intervals (oldest to youngest): 1) the base the distribution and level of geologic mapping. of the Chadron Formation to the unconformity in the Orella Member of the Brule Formation; 2) the unconfor­ mity to the top of the Whitney Member of the Brule Formation; and 3) the Brown Siltstone beds of the Oga 11 ala Group Brule Formation (a new informal unit). This upper sequence previously was not recognized in Nebraska, or Considerable controversy also has surrounded the was either identified as various units of the Arikaree stratigraphy of the Ogallala Group (e.g., McKenna, Group or included in the Whitney Member. From south­ 1965; Galusha, 1975). Our usage expands the Ogallala wes t to northeas t, these sequences become more d i f­ Group to include the Runni ngwater through Ash Hollow ficult to distinguish because of lack of control, formations (Fig. 4 and Table 1). The lithologic and absence of marker beds and increasing lithologic simi­ geometric complexities characteristic of the Ogallala larity. make correlations withi n and between formations d if­ ficult even in small areas of good exposures (Skinner Arikaree Group and others, 1977; Diffendal, 1982). In the subsurface these difficulties are compounded and, with little or The stratigraphy of the Arikaree Group has been no accompanyi ng surf ace cont ro 1, recogni t i on beyond the subject of much controversy since the 1930s (see the group level is not always possible on the basis of discussions by McKenna, 1965; Skinner and others, reconnaissance drilling (Fig. 5, C-C' and E-E'). 1977; Hunt, 1981). Based on lithologic and mineralo­ gic similarities and E-log characteristics (Fig. 7), PALEOGEOGRAPHY we break the Arikaree Group into three units (Fig. 4): 1) the Gering Formation; 2) Monroe Creek-Harrison Pre-Chad ron formations; and 3) the Upper Hasrison beds. In the northeast, the Arikaree Group is generally finer Paleocene and Eocene rocks, if deposited, were grained and browner (Table 1), making it so lithologi­ eroded before deposit ion of the Chadron Formation. cally similar to the White River Group that differen­ Evidence of intense we~t~ring (Interior Paleosol) tiation is difficult. duri ng the Pal eocene and for Eocene is common in the study area (Schultz and Stout, 1955; Pettyjohn, 1966). Thi s pal eoso 1 seri es may also have cont i nued to form

211 J. B. SWINEHART, V. L SOUDERS, H. M. DEGRA W, AND R. F. DIFFENDAL, JR.

during initial deposition of the Chadron Formation (Retallack, 1983). A variety of landforms was pro­ duced by pre-Chadron erosion but they are difficult to interpret from the map of the base of the Cenozoi c (Fig. 8) because of later structure. The general pre-Chadron landscape appears to have been an east­ northeastward sloping plain with moderate local re- 1i ef. The more res i stant Upper Cretaceous Fox Hill s Sandstone, Lance Formation, and Transition zone of the Pierre Shale (Fig. 9) were eroded to a more dissected topography in the southwestern part of the study area (DeGraw, 1969). Upli ft along the Chadron Arch from Late Cretaceous into Early Cenozoic time caused the erosion of as much as 1800 ft (549 m) of Late Cretaceous rocks from the arch. Sparse data allow only a very generalized reconstruction of paleoto­ pographic features along the arch. Chadron Formation-Lower Part of Orella Member of the Brule Formation Significant erosion, completely removing the Interior Paleosol from some lowland areas, estab­ lished a drainage system prior to deposition of Va lentine Chadron-lower part of Orella (Early Oligocene) sedi­ Formotion' ments. The i ni t i a 1 phase of depos it i on was domi nated by alluvi al processes as the valleys were fi lled. w z Basal sands with some gravels overlain by bentonitic w u clays (mostly altered pyroclastic material) typify o 15 deposits of the valleys. Clays and interbedded sands ::. fill paleotopographic lows if coarse clastics are absent. Similar clay beds, generally thinner and probably representing paleosols (Retallack, 1983), occur on the uplands. Figure 10 is a reconstruction >- of the Early Oli gocene paleogeography at the end of 0:: this fluvial phase (Chadron Formation, Fig. 6). The cl major drainage feature was a west-east through-flowing t­ o:: valley about 25 mi (40 km) wide entering present day W Nebraska in northwest Sioux County and turning south­ t- east in western Dawes County. Net sandstone thick­ nesses exceed 200 ft (61 m) in northeast Sioux County and may represent deposition in locally subsiding basins or in inner channels (Schumm, 1977). A major tributary paleovalley cuts across the southwest part of the study area parallel to the subcrop contact be­ tween the Transition'zone of the Pierre Shale and the Pierre Shale (Fig. 9). This paleovalley has sand­ fi lled tributaries originating in uplands developed on Gering the Upper Cretaceous Fox Hi lls Sandstone and Lance Formation Formation. The major paleovalley diverges east of Cheyenne County (Fig. 9). A tributary drainage system also trends from eastern Dawes into Sheridan County

but it cannot be extended southeastward because of w lack of control. z 30 w u 0 - -- Fi lling of paleovalleys was followed by deposi­ Orello ~ tion of pyroclastic air-fall debris, including dis­ ....J , 0 crete ash beds (M ash, Fi g. 6), over most of western Member -- --

Chadron Figure 4. Time stratigraphic chart of Cenozoic units 35 in western Nebraska. Units positioned on bases of Formation superposition, fossil mammals and available age dates. Width of unit box indicates approximate extent of unit from south to north across study area. Slanted lines indicate a hiatus. I and equivalents

212 CENOZOIC PALEOGEOGRAPHY OF WESTERN NEBRASKA

Table 1. Dominant lithologic characteristics of Cenozoic strati graphic units in the study area

Strati graphic Maximum Uni t Lithologic Characteristics Thickness Sands and gravels along the North and South Platte rivers and beneath Quaternary adjoining terraces; sands, silts, and minor amounts of gravel from local Extreme l.v depos its sources along other streams; sand dunes; and loess. variable Broadwater Sands, gravels, and some silt and diatomite lenses along north side of 300 ft Formati on North Platte River valley from Wyoming border to Garden County, more silts (91 m) eastward. North part--gray, olive, and olive-brown fine- to coarse-grained sand- stones, some sandy western-source gravels (abundant igneous and metamorphic clasts derived from western plutonic areas), and locally occurring ash Ash Hollow beds. South part--gray, brown, and reddish-brown fine- to coarse-grained 600 ft Formati on sandstones, silty sandstones, sandy western-source gravels, siltstones, (183 m) and locally occurring ash beds. More poorly sorted and more carbonate cement than in north part. Deposits from two parts mingle in Garden and Arthur counties.

Runningwater Gray, greenish-gray, and brown medium- to fine-grained sandstones, coarse 350 ft sands, sandy siltstones, western-source gravels, and locally occurring (107 m) Formati on clayey silts and ash beds. Some fine-grained volcaniclastics in west.

Upper Brown volcaniclastic sandy siltstones. Grayish-brown to gray silty fine- grained sandstones and locally occurring coarser-grained sandstones at the 300 ft c. Harri son base. Generally grades from (91 m) ::s beds Silica-cemented horizons common in west. 0 grayish-brown silty sandstones in west to brown siltstone in northeast. s- C!l Gray, brownish-gray, and grayish brown volcaniclastic silty very fine- QJ Monroe Creek- QJ grained sandstones. Generally finer-grained (sandy siltstones) and 420 ft s- Harri son formati ons browner northeastward. Carbonate-cemented horizons ("pipy concretions") (128 m) -'"'" common • .~ s- e( Gray, brownish-gray, and grayish-brown volcaniclastic fine- to medium- Geri ng grained sandstones, silty sandstones, brown sandy silts, and locally 350 ft Formati on occurring beds of ash, coarse sand, and fine gravel. Generally finer- (107 m) grained and browner northeastward. Brown Brown volcaniclastic sandy siltstones and silty very fine grained sand- stones. Mudstones and fine- to medium-grained sandstones occur locally 450 ft s:: Si ltstone c. 0 and generally at or near base. Regionally correlative zone of ash beds (137 m) ::s beds (Nonpareil ash zone, new informal name) occurs in lower part. 0 :;::; s- '"E Cl s- Brown volcaniclastic siltstones. Mudstones and fine-to medium-grained 0 Whitney 300 ft s- lL. sandstones occur locally. QJ Contains two regionally correlative ash beds QJ Member (91 m) > (Upper Ash and Lower Ash) and several less continuous ash beds. .~ '; a:: s- ea QJ Brown to greenish-gray volcaniclastic mudstones and siltstones. Fine- to ...., Ore 11 a medium-grained sandstones and thinly bedded mud stones occur in upper part 400 ft .~ ..s:: Member throughout broad areas. Regionally correlative ash bed (M ash, new (122 m) 3: informal name) occurs in lower part.

Chadron Gray and greenish-gray bentonitic claystones and mudstones. Throughout 300 ft much of the area fine- to coarse-grained sandstones and locally occurring Format ion conglomerates underlie the claystones-mudstones. (91 m)

213 J. B. SWINEHART, V. 1. SOUDERS, H. M. DEGRAW, AND R. F. DIFFENDAL, JR.

A ... A' SOUTH "... \..)'" ... NORTH I ..'" ~ ... Ir- .. et: ~I r-a ... ~I ::::J", \..)" ... a~

Ul ;:: a Cl:: w a w w a r- lL w " ::0 w e w ::::J 0 r- ::::J r- r- --' r- ~ --' ~

a a a ... '" C ~ Cl: " ~ C' SOUTH ~'" ., Hills Cl: NORTH

a r- a- w a'" w 0 -

JS

Figure 5. Geologic sections across study area. Brule Formation, Arikaree Group and Ogallala Group are subdivided in selected areas as follows: A-Brown Siltstone beds; B-Gering Formation; C-Monroe

214 CENOWIC PALEOGEOGRAPHY OF WESTERN NEBRASKA

D WEST D' EAST o o '"

I/) a:: w I­ W ::lE w - 0 Transition zone, ::> I­ I- ...J Pierre Shale

Pierre Shale

E WEST E' EAST

I/) a:: w I­ w w o ::lE ::> I- w o ::> I-

Pierre Shale

N Vertical exaggeration x 106 o 40 MILES s' South Dakota C' I I I o 40 KILOMETERS

E l:'·.<:."j Quaternary deposits .. Ankaree Group Lower Ash of Whitney Member, Brule Formation ~ Pllacene deposits Brule Formation o Ogallala Group ~ Chadron Formation Fault Arrows Indicate direction of re/olive movement. Dashed D' when! Inferred. A S Colorado

Creek-Harrison formations; D-Upper Harrison beds;E-Runningwater Formation; F-Sheep Creek and Olcott formations; G-Ash Hollow Formation.

215 J. B. SWlNEHART, V. L SOUDERS, H. M. DEGRAW, AND R. F. DIFFENDAL,JR.

SOUTHWEST NORTHEAST SE -NE-SW sec.4 NE -SW sec. 27 T. 29 N.- R. 48 W. T. 33 N.-R.43 W. FEET Alt. G.L. 4120 It. Alt. K.S. 3784 ft. METERS 700- NW-NW sec. 20 ~ 36mi.---- T.23 N.-R.51 W. 57.9 km. 1 Alt. K.S. 4422 It. so. OAK. NE ;9- 42 mi. -----~l/ C- SE -NE sec.20 600- Ogalial a Glo~9 . 01" T. 18 N. - R. 53 W. __ t>-I'" :2 All. K.S. 4653 It. ~

500- <>- 150 SW COLO.

400-

300- SE-SE-NE sec. 14 T. 14 N.-R. 55 W. Alt. K.S. 4868 It. 200- -50

100-

O--r~~-----+r-~~------+;~-~~------~~~~ ---t~---O

100-

-50 200-

300- Cretaceous and Jurossic rocks -100

400- R scole 01 other logs Chodron fJ Arch

500- _ Alluviol deposits predominant -150

600- vs Figure 6. Southwest to northeast correlation of White River Group based on electric logs of selected oil and gas tests, western Nebraska. Three deposit iona1 sequences are bracketed and numbered. Note the number of regiona11y persistent correlations that can be made within the members of the Bru1e Formation. This figure can be viewed as a chronozone diagram since several of the correlation lines are drawn on volcanic ash beds.

Nebraska producing a plain of low relief. Soil devel­ Upper Part of Orella Member through Brown Siltstone opment and some reworking of this air-fall material Beds of the Bru1e Formation must have occurred, but no significant amount of coarser epic1astic detritus has been observed in the Initiation of regional downcutting prior to depo­ i nterva 1 between the M ash and the Ore 11 a i nt raf or­ sition of the upper part of the Orella through Brown mationa1 unconformity. Si1tstone interval (Middle 01igocene) created a series of valleys (Fig. 6) which had less relief and were smaller than those of the Chadron Formation. Only

216 CENOZOIC PALEOGEOGRAPHY OF WESTERN NEBRASKA

SOUTH NORTH ALTITUDE 25-UNW-76 ALTITUDE NE cor. sec.1 (FEET) 29-UNW-76 (METERS) T.26N.-R.48W. sw cor. sec. 31 4000- T. 26 N.- R. 47 W. 33-UNW-76 -1220 SE-NE sec. 14 23-UNW-76 / T.26N.-R.47W. sw cor. sec. 31 T. 28 N.-R. 47 W.

28-UNW-76 NE cor. sec. 24 T. 25 N.-R. 48W. -1200 post­ Box Upper Butte Horrison­ ~ pre- Box Fm. Butte 3900- rocks

-1180

Upper Harrison beds -1160 3800-

-1140

3700- Harr i son fms.

-1120

Brown Siltstone beds

-1110

3600-

Resistivity ~ -1080

No electric log curve or poor quolity curve so. OAK. o 2 3 Miles I ! I L o ~ 4 Kilometers 3500- g I \---- "' f-- 'r-- -1060 r'--- f-----1 COLO.

vs

Figure 7. Correlation of Arikaree Group strata showing the profile of an upland-valley sequence in Box Butte County. Electric logs and samples from Conservation and Survey Division test holes.

217 J. B. SWINEHART, V. L SOUDERS, H. M. DEGRA W, AND R. F. DIFFENDAL, JR.

o 15 30MILES I 1 I I1 I I o 15" 30 K 1LOMETE RS

Figure 8. Configuration of the base of Cenozoic rocks in western Nebraska. Contour interval is 200 ft (61 m). This surface primarily represents the Late Cretaceous to Early Oligocene unconformity.

218 CENOZOIC PALEOGEOGRAPHY OF WESTERN NEBRASKA

,''' ·s -- ~J! t!. ....L.Q..A~r ______..... -.!..- ______·~~-i) I . ,

UPLANO

e J B TTE , Kp ,I l-' ---T------.:------'-' I I , rr I i I I'1- --

t ,y I c--,I ____ _ HO HO co

Figure 9. Pre-Cenozoic geologic map of western Figure 10. Paleogeography at the close of Chadron Nebraska. Jurassic System: Jm-Morrison Formation; Formation deposition (Early Oligocene), western Cretaceous System: Kd-Dakota Group; Kb-Benton Nebraska. Unshaded areas were predominantly well­ Group; Kn-Niobrara Formation; Kp-Pierre Shale; drained rolling uplands with some local ponds. Ktp-Transition zone, Pierre Shale; Kfh-Fox Hills Shaded areas represent poorly drained areas of Sandstone; Kl-Lance Formation. paleovalleys dominated by swamp, lacustrine, and flood plain environments. Stippled areas were pri­ mary sites of fluvial sand deposition within paleovalleys. Contour interval of net sandstone thickness, 100 ft (30 m). Note the difficulty of Table 2. Percentages ex ±

219 J. B. SWINEHART, V. L SOUDERS, H. M. DEGRA W, AND R. F. DIFFENDAl, JR.

" ~ ,. ~-- -~- -,.- -- ~;.!'i ~ ~Aj-"'r -- : ;------~------'~ : I , ,--, I , .... rl ~ , ,. J'-- I, ".,~,..J~ ~\l I, .~"'".r~J /~ /D ...... / I )/-- )- ",.!It ~ T - NOT R[C~II[O ,. i ,. '" £ A P ., INSUFFICIENT O.tA , AHO DIF'ICULTIES ~ IN It[COGIroI'tION ~ ,

, 1------T Ft", III ~

I El A ~ '" r ft I

It-- - - _ _ _ _ r' ------

I _,,,, i HEY ' •• -~ .... I I( t.t 6" L H' I ~ [ 'H f .... ""'+ I I I' --1 ___ --- t----~-COL O~AOO --_J'_-i-r HO,JS HO. vs • • I

. ~

Figure 11. Occurrence of the Lower Ash of the Whitney Figure 12. Occurrence of the Nonpareil ash zone, Member in western Nebraska and configuration of its Brown Si1tstone beds, in western Nebraska and con­ base. Contour interval is 200 ft (61 m). The figuration of its base. Contour interval is 200 ft Lower Ash contains 85 to 95 percent rhyo1itic glass (61 m). Thickness typically ranges from 30 ft shards and is biotitic. Thickness typically ranges (9.1 m) to 50 ft (15 m) and the zone may contain up from 5 ft (1.5 m) to 12 ft (3.6 m) and the ash to 3 discrete biotitic, vitric ash beds. Maximum thins toward the east-northeast. observed thickness of a single ash bed is 20 ft (6.1 m) in southern Dawes County.

across much of western Nebraska (Fig. 6). The dis­ western Nebraska and that southwesterly winds predomi­ tribution and configuration of the Lower Ash of the nated during the deposition of the White River Group. Whitney Member (Fig. 11) and the Nonpareil ash zone (a new informal name; Fig. 12), although modified by sub­ Ari karee Group sequent structural movements, document the absence of significant relief. Braided and ephemeral streams did The essentially continuous building of a succes­ cross this plain, however, and deposited sediments sion of low-relief landscapes in western Nebraska by dominated by epic1astic material (STan1ey, 1976). the aggradation of pyroc1astic eo1ian material was Although vo1umetrica11y small in relation to the interrupted in the Late 01igocene when the region was eo1ian deposits, this alluvial facies has been encoun­ subjected to widespread fluvial erosion. This erosion tered locally throughout the ent ire i nterva 1 of the produced the unconformity whi ch separates the Geri ng White River Group. Valleys were probably cut less Formation (Arikaree Group) from older Cenozoic units than 50 ft (15 m) deep and were narrow. A pa1eova11ey (Figs. 4 and 7). Two major drainage systems and at in the Brown Si1tstone beds contains up to 200 ft (61 least one smaller one were developed (Fi g. 14). m) of fill and crosses north-central Sioux County from Maximum depths of cutting ranged from about 300 ft (91 west to east paralleling the Pine Ridge in Dawes m) in the north to about 100 ft (30 m) in the south County. (Fig. 5). The general west-east trend of these drain­ ages represents a significant shift from the south­ The White River sediments and landscapes are easterly trend of the major Early 01igocene pal eo­ overwhelmingly products of the deposition of fine­ valley (Fig. 10). Sediments within the Gering grained pyroc1astic material. This material, typified pa1eovalleys reflect multiple cuts and fills and in­ by glass shards (Table 2), was derived from western­ clude some eo1ian deposits. Tracing the small source rhyo1it ic and andesit ic volcanic centers (Fig. southern pa1eova11ey is facilitated by the unique 13). The distribution and northeast slope of the occurrence of pumice pebbles in one of the fills. The ashes (Figs. 11 and 12) and northeast thinning of the pumi ce was transported by a Geri ng ri ver with head­ Lower Ash and many White River Group stratigraphic waters in a volcanic field in north-central Colorado units (Fig. 6) indicate that volcanic centers in (Izett, 1975; Fig. 13). Colorado were the most probable sources for ash in

220 CENOZOIC PALEOGEOGRAPHY OF WESTERN NEBRASKA

Following the filling of these valleys, the accu­ mulation of pyroclastic material once again resumed and eolian deposits of the Upper Harrison blanketed much of western Nebraska. As in preceding intervals dominated by eolian processes, deposition greatly exceeded erosion. The principal differences between the pyroclastic materials in the Arikaree Group and those of the White River Group are the larger grain size and smaller percentage of glass shards in the Arikaree (Table 2). However, the total pyroclastic contribution to the eolian deposits of the Arikaree Group exceeds 75 percent (Vicars and Breyer, 1981; 'Hunt, 1981). The textural and mineralogic changes may reflect a shift in volcanic source areas (Fig. 13), a change in high-altitude wind patterns (Stanley, 1976), or eolian reworking of older pyroclastic deposits in present day northern Colorado. No widespread ash marker beds have been identified in the Arikaree Group, although a number of ashes are known to occur, especially within the Gering Formation. Oga 11 a 1a Group

Landscape development in western Nebraska under­ went a major change in the Middle Miocene beginning with the period of erosion separating the Arikaree and Ogallala groups (Figs. 4 and 15). Widespread eolian Figure 13. Generalized distribution of rhyolitic and andesitic rocks between 35 and 18 m.y. old (black) in the western United States (after Stewart and Carlson, 1978) and the mi nimum probab le extent of the White River (shaded; after Robinson, 1972) and the Arikaree Group (stippled).

Direct air-fall and wind blown pyroclastic detri­ tus (Monroe Creek-Harri son format ions) succeeded the deposition of the complex set of alluvial fills (Gering Formation) and an aggrading plain of low re­ lief again dominated the western Nebraska landscape. The southern part of the area may not have recei ved any sediment accumulation during this time (Fig. 13). Minor amounts of volcaniclastic alluvial deposits occur at a number of horizons within the Monroe Creek-Harrison unit and indicate that small local drainages were present. Vicars and Breyer (1981) noted such a deposit associated with eolian dunes in central Sioux County. A second peri od of regi ona 1 erosion, typ ifi ed by valleys of low relief and low-angle side slopes, re­ sulted in an unconformity that separates Monroe Creek-Harrison sediments from the Upper Harrison beds (Fig. 4). This erosion, while not producing as much down cutting as pre-Gering erosion, was sufficient to remove thin accumulations of Monroe Creek-Harrison deposits from divides between Gering paleovalleys in severa 1 areas so that Upper Harri son beds res t directly upon rocks of the White River Group (Fig. 7). Figure 14. Inferred paleodrainage systems (arrows) of Hunt (1981) and Vicars and Breyer (1981) described one the Gering Formation, Arikaree Group, western of these basal Upper Harrison valley fills in central Nebraska. Known and inferred extent of the for­ Sioux County that contains a maximum of about 40 ft mation is shaded. In drainage divide areas (e.g. (12 m) of fine sandy volcaniclastic alluvial deposits. central Sioux County), some thin Monroe Creek­ Our studies indicate that a coeval valley fill with up Harrison alluvial deposits may be included in the to 60 ft (18 m) of somewhat coarser material occurs in Gering. Present distribution of Arikaree Group northern Sheridan County. shown by even-weight line, dashed where inferred.

221 J. B. SWINEHAR T, V. 1. SOUDERS, H. M. DEGRA W, AND R. F. DIFFENDAL, JR.

o 15 30 MILES " I! ! I r I i " o 15 30 K I LOMETER S

Figure 15. Configuration of the base of the Ogallala Group, western Nebraska. Contour interval is 200 ft (61 m). Shaded where Ogallala eroded.

222 CENOZOIC PALEOGEOGRAPHY OF WESTERN NEBRASKA

brate fossils in northeast Colorado just south of Kimball County (Galbreath, 1953) and in southeast Wyomi ng 30 mi (48 km) west of Kimball County (Cassi li ano, 1980). The erosional remnants of a complex set of Sheep Creek and Olcott paleovalley fi lls in south-central Sioux County (Figs. 16 and 5, A-A') were mapped by Skinner and others (1977). Galusha (1975) described the narrow fills of the Box Butte Formation in Dawes and Box Butte counties. Immediately preceding Ash Hollow deposition, sev­ era 1 valleys were developed in present day southern Box Butte County and the southwest part of the study area (Fig. 16). Subsequent fills, like those of the older Ogallala Group, consist of a number of separate cut and fi 11 sequences exhibiting frequent lateral facies changes over short distances. These fills, particularly those containing coarse sediments, can be separated from one another at outcrop sites and the age relationships determined (Fig. 17; Diffendal, 1982). However, the paleovalleys are often so narrow, less than 1 mi (1.6 km) wide, and have such sinuous, irregular geometries, that their discrimination and correlation in the subsurface is difficult without close spaced drilling. In marked contrast to the White River and Arikaree groups, the Ogallala Group contains much less

A ~es of po leovalleys at base of Primarily upland areas during deposition admixed pyroclastic debris (Table 2; Stanley, 1976). Ogollolo Group of Ogallo la Group The group does have a significant number of rhyolitic Slroligrophic Units ashes (Figs. 17 and 18; Skinner and others, 1977; CD Ql @ ® @ Runnlrlgwoter Formation Box Suite-Sheep Creek- Valentine Formation Ash Hollow FormatIOn Upper Ash Hollow Diffendal, 1982). The areal extent of these ashes is oleott formations and equivalents sand and grovel much 1es s than those of the White River Group (F i gs. Th(;' oldest slr(1!lgrl)plllc /Imf In fhe poleorolley is indicated by IIIe lIumOer. If l/lere IS /llIcerlonlly N'gordl1l9 fhe Idl!flliflCOlfQII 01 fhe /llIIf, more 11101/ one III/mber IS shown 11 and 12). No single Ogallala ash is known to extend over an area larger than 5 mi 2 (13 km 2) and none has Figure 16. Postulated distribution of Ogallala paleo­ so far proved useful for regional subsurface correla­ valleys. tion. Potential source areas for Ogallala ashes younger than 17 m.y. are different from those whi ch yielded White River and Arikaree pyroclastic material (compare Figs. 19 and 13). This 17 m.y. date was used deposition gave way to restricted alluvial deposition as a geologically important division point for in valleys. During the last two thirds of the Miocene Cenozoic igneous rocks in the western United States by a wide variety of drainage systems of different sizes, Stewart and Carlson (1978) because it is approximately the larger heading in the mountains of Wyoming and the low point of a significant lull in igneous acti­ Colorado, were developed and subsequently filled with vity and marks a change from eruptions of predomi­ epiclastic material. The base of the Ogallala Group nantly calc-alkalic rhyolites and andesites to bimodal (Fig. 15) rests on a composite surface eroded at dif­ assemblages of basalt and rhyolite. ferent times and places. Figure 16 illustrates in a general way the time transgressive nature of this sur­ We have not attempted to correlate the paleo­ face. valley systems and fi lls in the eastern part of the study area (Figs. 5 and 16) because of the absence of The largest of the older Ogallala paleovalleys is exposures and sparse subsurface data. Thi s region, located in the northern half of the study area and is now overlain mostly by sand dunes, was a major area of fi lled with alluvi al sediments of the Runningwater Ogallala deposition. Formation. The paleovalley is about 15 mi (24 km) wide in Dawes and Box Butte counties where maximum Broadwater Formation and Quaternary Deposits cutting was about 300 ft (98 m) (Fig. 5, B-B'). Yatkola (1978) and Souders and others (1980) described During the last 5 m.y., erosion greatly exceeded the complex cuts and fills of the Runningwater Forma­ deposition in western Nebraska and there is little tion within this paleovalley in eastern Sioux, south­ sedimentary record available over most of the area. ern Dawes and northern Box Butte counties. Sediments The modern North Platte River from the Wyoming­ that may be age equivalents of the Runningwater Nebraska state li ne to Garden County follows a trend through Valentine formations occupy parts of a smaller that was establ i shed near the end of the Miocene by a paleovalley system in Kimball and Cheyenne counties. valley in the uppermost part of the Ash Hollow (Figs. This age assignment is made on the basis of strati­ 16 and 20). During the Pliocene and Quaternary a suc­ graphic position and lithologic similarities between cession of valleys maintained this trend but shifted these fills and valley fills containing dated verte- progressively to the southwest (Fig. 20). Remnants ,of

223 J. B. SWINEHART, V. L SOUDERS, H. M. DEGRA W, AND R. F. DIFFENDAL, JR.

R. 55 W. R. 56 W. R. 54 W. 11>11 f----,----'I

T. 18 N.

OGALLALA GROUP - ASH HOLLOW FORMATION BRULE FORMATION V-Volcanic Ash ~ Trend of Major • Young Old ~ Tributary Paleovalley Fill Fi II Fill D F-Faunal Site Figure 17. Distribution and age relations of Ash Hollow Formation paleovalleys in Banner County. Undifferenti­ ated Ash Hollow of wide age range indicated by stipple pattern but not shown where it covers old fill.

coarse-grained inner channel fills, similar to those deposited. Ski nner and Johnson (1984) prepared a described by Schumm (1977), occur at the base of the similar map for the stratigraphic equivalent of the (Pliocene) Broadwater Formation (Swi nehart , 1981) and upper part of the Ogallala Group. Deposits of thi s define a paleovalley axis. In eastern Morrill and interval were spread more widely across the plains Garden counties the Broadwater paleovalley trends than those of preceding intervals and regionally the eastward and a complex and widespread sequence of sand and gravel was deposited in the southern Sand Hills (Fig. 5, C-C') and on to the northeast. 4100' ~MAP Development of the ancestral Pumpkin Creek drain­ t±::llj SITE age sys tern, head i ng in the southern Larami e Range, began no later than the Early Pleistocene (Corner and ~ 18 Diffendal, 1983) and joined the ancestral North Platte =E N. drainage in southeastern Morrill County (Fig. 20). ~ ~ D The ancestral Pumpkin Creek was pirated by tributaries "'; 4000' of the North Platte River immediately west of the .. Wyoming-Nebraska state line and in south central "'~ c > Morrill County in Nebraska late in the Pleistocene "'o (Oiffendal,1984). The Lodgepole and Niobrara drain­ ..'" ~ B age systems are probably Middle to Late Plei stocene ~ "' features. The present South Platte River valley (Fig. : 3900' 20) has been the location of a major drainage way from the beginning of the Pliocene, a situation similar to th at des cri bed by Scott (1982) in northeasternmost Colorado. Ahlbrandt and others, (1983) documented episodic eolian deposition during arid intervals of the Holocene that formed the Sand Hills (Fig. 5, C-C', 3800' E-E') . SEQUENCE OF PALEODRAINAGE DEVELOPMENT RD The locations of paleovalleys for five post­ Eocene stratigraphic intervals and the relationship of Figure 18. Map of a part of Morrill County showing the paleovalleys to nearby positive elements are shown six superposed and areally restricted ash lenti ls in Figure 21. West to east drainage predominated in the Ash Hollow Formation. Topography from USGS except for the Chadron Formation. Major drainageways F airchi ld Ranch It' quadrangle. The position of of the Geri ng Formati on a()d" the lower part of the the ashes with respect to sand and gravel deposits Ogallala Group were located in the northern half of shown at left. More ashes appear to occur where the study area but had shi fted southward by the time the formation is thicker and contains significant sediments of the upper part of the Ogallala Group were amounts of coarse-grained deposits.

224 CENOZOIC PALEOGEOGRAPHY OF WESTERN NEBRASKA

Front Range became more prominent as a source of sedi­ Chadron paleovalleys (Fig. 21, Chadron and Gering for­ ment. Position of Broadwater and equivalent (Plio­ mations). Normal faults, especi ally along parts of cene) paleovalleys outside the study area are from the Pine Ridge (Fig. 2), are suspected to have con­ Swi nehart (1981) and Scott (1982). trolled the location of some of the Geri ng pal eo­ valleys. CENOZOIC STRUCTURE AND PALEOGEOGRAPHY Further regional uplift, accompanied by local Deposit ion of the Lance Formation at the conclu­ faulting and folding in the study area, post-dates sion of the Cretaceous probably occurred only in the deposition of the Upper Harrison beds. The White Clay southwest part of the study area (Fig. 9) at eleva­ Fau lt (F i gs. 2 and 22) is post-Upper Harri son and has tions slightly above sea level. The present maximum vertical displacements ranging from less than 300 ft altitude of the Lance Formation is 4800 ft (1463 m) (91 m) to as much as 700 ft (213 m). Ash Ho 11 ow fl u­ above sea level (Fi gs. 8 and 9) and much of thi s up­ vial deposits occupy the downthrown block of the fault lift probably occurred during the Paleocene (Trimble, (compare Fi gs. 2 and 22) and 1 ie at the base of an 1980). By Early Oligocene (basal Chadron Formation) escarpment capped by Upper Harri son beds. The Agate up to 1800 ft (549 m) of Mesozoic age rocks had been and other faults (Hunt, 1981) in Sioux County (Figs. 2 eroded from parts of the Chadron arch and an east­ and 5, A-A') a 1 so cut Upper Harri son beds. Vert i ca 1 northeast sloping plain was established. di sp 1acement across the Agate F au lt is 230 ft (70 m) in one locality. Collings and Knode (1984) report Regional uplift occurred in the south part of the that the White River Fault (Fig. 22) has a vertical study area sometime in the Oligocene prior to deposi­ displacement that ranges from 100 to 400 ft (30 m to tion of the Gering Formation (Figs. 5, A-A', B-B' and 122 m). The fault cuts rocks of the White River Group 8). A structural hinge in the vicinity of the present in western Dawes County and rocks of the White River North Platte River (Fig. 22) was established and as and Arikaree groups to the west in Sioux County. much as 400 ft (122 m) of uppermost White River Group These faults, and those inferred to occur along the rock s were eroded from part s of the up 1ands to the Pine Ridge and the Niobrara River, are a continuation southwest. Gering deposition was concentrated in the of the Whalen trend in Wyoming (Hunt, 1981). The northern part of the study area and the paleodrainages principal Runningwater paleovalley appears to have have a distinctly different orientation than do developed along one faulted zone of this trend (compare Figs. 16 and 22).

...... ,.': .'. . ... ,,'. ',:. ., .... ' ... : : : '.

, ' , ... ' .. : .

.'. " .' :' ..... '.... ,' .. .:','" ...... ' ..

',' .

• " :: ...... '. ... :' .', : ~ :: .. : ...... :. ," .' ,':.', ... '. , , , ' 2'0 Kilomefers Minimum former extent --~ ~ of Ogollolo Group Axes of paleovalleys or Recent upland areas madern drainages Solid line where scorp bounded Stratigraphic Units JS ® ® ill ® Upper Ash Hallow Broadwater Formatian Unnamed Pleistocene Unnamed Holocene Figure 19. Generalized distribution of rhyolitic, sand and gravel A-earlier alluvium alluvium dacitic and andesitic volcanism between 17 and 5 B-Iater m.y. old (black) in the western United States (after Stew art and Carlson, 1978; Smith and Luedke, Figure 20. Axes of Upper Ash Hollow and younger 1984) and the minimum former distribution of the paleovalleys in the southern part of western Ogllala Group. Nebraska • .~

225 J. B. SWINEHART, V. L SOUDERS, H. M. DEGRAW, AND R. F. DIFFENDAL,JR.

Regional uplift, probably centered in the Front Range (Fig. 1), caused deep incision of valleys prior .".... , ""'" .... " ! to deposit i on of the Oga 11 ala Group (F i gs. 5, A-A', 0-0' and 15) and completely removed the White River Group in one locality. The Rush Creek structural feature and the North Platte hinge funneled sediments of the Oga 11 ala Group into a narrow gap in southeast :~~'F'~'-=u~,-~,~\: Morri 11 County and di rected them to the northeast. , The Rush Creek feature remained active into the Plio­ , \ JurasslC and older rocks cene (compare Figs. 20 and 22; Oi ffend a 1, 1980). Up - 7'" t..- -·-1 NEBRASKA \ to 400 ft (122 m) of Ogallala alluvial deposits occupy , the subsiding part of the Big Springs feature (Fig. 5, ApprOllmate distribution CaslleRock· I---;~~------of While River Group 0-0'). Pre-Oga11ala Cenozoic rocks dip relatively Cgl I 0 100 Miles steeply and uniformly toward the structural low at the COLORADO I 6 do Kilomelef5 vs east edge of the study area (Figs. 5, E-E', and 22) Chodron Formation while the present land surface flattens. The low con­ tains at least 700 ft (213 m) of post-Arikaree clas­ tics, one of the thi ckest sect ions of these rock sin the Great Plains. The flattening indicates post­ Ogallala uplift along the Chadron Arch. Other post-Ogallala epeirogenic and local move­ ments affected deposition of the Broadwater Formation and determi ned the courses of. some of the younger drainages. The Broadwater drainage was diverted northeastward by the Rush Creek structure (compare Figs. 20 and 22). Later in the Pliocene the anticli­ nal part of this structure was breached and the ances­ tral North Platte River assumed its southeastward Minimum former eden! of Arlkaree Group course through the eastern part of the study area. Structural movements late in the Pleistocene probably COLORADO vs caused pi racy of the ancestral Pumpki n Creek inme­ Gering Formation diately west of the Wyoming-Nebraska state line. To the north, the Niobrara River parallels the Runningwater paleovalley in southern Oawes County (F i g. 16) and then di verges to the northeast across the crest of the Chadron Arch (F i g. 22). Late Quaternary alluvial fills occur high above the river across Sheri dan and Cherry count i es and the Ni obrara flows in entrenched meanders near the White Clay Fault (Fig. 22). These features indicate that Late Quaternary structural activity influenced the drain­ age and seismic activity along the arch continues Jurosslc and older rocks today (B urchett, 1979). In Oawes County, a modern stream (the White River) parallels two faults for much Minimum former ellent of its length (Fig. 22). of Runnlngwoter, 80l Bulte, Sheep Creek, and Olcoll formations COLORADO cnd e ul~olen's vs Ogollolo Group - lower port Figure 21. Regional setting of Cenozoic paleodrain- ages.

Jurasslc cnd older roells

Minimum former edent Minimum former utenl of Valentine Gnd Ash of Broodwaler Formation Hollow lormatlons and equivalents and equivalents COLORADO vs JS,RD Ogollolo Group-upper port Brood water Formation

226 CENOZOIC PALEOGEOGRAPHY OF WESTERN NEBRASKA

grees of structural movements in the Rocky Mountains to the west. The approximate times and relative mag­ nitudes of post-Larami de up 1i ft are represented in an oversimplified, qualitative fashion by the negative slopes on the cumulative graph (Fig. 23). Trimble (1980) suggests a simpler picture of Cenozoic tectonic history of the Southern Rocky Mountains and Great Plains. Figure 23 furthermore illustrates the impor­ tance of pyroclastic air-fall material in the con­ c... <0 struction of the High Plains of westernmost Nebraska. .off '90 ,"+ :.t- STRUCTURAL LOW Climatic conditions and internal sedimentary con­ trols (Schumm, 1977) can greatly influence erosion and sedimentation, so every instance of downcutting within the Cenozoic cannot be attributed to tectonic activi­ ty. Nevertheless, we believe structural movements, combined with pre-Ogallala volcanic activity to the west, were the primary determining factors in shaping the past and present landforms of western Nebraska.

ACKNOWLEDGEMENTS We gratefully acknowledge Frank Smith, Conser­ vation and Survey Division, for his hard work in

" obtaining test hole information in western Nebraska. ____ 1_ - vs We also thank John Boellstorff, formerly of the Conservation and Survey Division, for his ideas and .~ y( .J,. j,.. Y" Up ..l-.l.- ~Up eff ort s both in the fi e 1d and 1ab. Morri s Skinner Synclrne Antrcllne Normal foul! wIth apparent Hinge line. LrneDtchQngein and Richard Tedford, American Museum of Natural throll' greQter thon 100 regional strike and dipal feet (30m) Dashed base of CenOlOIC rocks .. here inferred History, and Mi chae 1 Voorhi es and Robert Hunt, Uni vers ity of Nebrask a State Museum, part i ci pated in many stimulating discussions on the Cenozoic of the Great Plains with the authors over the past decade. Figure 22. Significant known and suspected Cenozoic structural features of western Nebraska primarily compiled from overlays of maps of the base of the REFERENCES CITED Cenozoic (Fig. 8), Ogallala Group (Fig. 15), and from the configuration of Lower Ash of the Whitney (Fig. 11). Ahlbrandt, T. S., Swi nehart , J. B., and Maroney, D. G., 1983, The dynamic Holocene dune fields of the Great Plains and Rocky Mountain basins, U.S.A., in Brookfield, M. E. and Ahlbrandt, T. S., eds., As outlined above, local structural control of Eol i an sediments and processes: Elsevier Pub­ post-Ogallala drainages is apparent in parts of the lishers, Amsterdam, p. 379-406. study area. The magnitude of regional uplift is indi­ Burchett, R. R., 1979, Earthquakes in Nebraska: cated most strongly by the large amount of erosion Nebraska Geologi cal Survey Educational Ci rcular that has occurred in the region since deposition of No. 4, p. 1-18. the Ogallala Group. Scott (1982) noted a similar Cassiliano, M., 1980, Stratigraphy and vertebrate situation in northeastern Colorado. The North Platte paleontology of the Horse Creek-Trail Creek Area, River in central Scotts Bluff County is at least 1000 Laramie County, Wyoming: University of Wyoming ft (305 m) lower than the Ogallala-capped tablelands Contribution to Geology, v. 19, p. 25-68. on either side of the valley (Fig. 2). In the north Collings, S. P., and Knode, R. H., 1984, Geology and near the Wyomi ng border, the Pi ne Ri dge escarpment discovery of the Crow Butte uranium deposit, rises 1200 ft (366 m) above Pierre Shale hills (Fig. Dawes County, Nebraska, in Practical Hydromet 5, A-A') and relief gradually decreases to about 400 '83: 7th Annual Symposium on Uranium and Precious ft (122 m) at the east end. All Tertiary rocks were Metals, American Institute Metallurgical eroded from the area north of the ridge while most of Engineers, p. 5-14. the Ogallala Group and much of the Arikaree Group were Corner, R. G., and Diffendal, R. F. Jr., 1983, An removed from the western part of the upl ands between Irvingtonian fauna from the oldest Quaternary the Pine Ridge and the North Platte River valley (Fig. alluvium in eastern Pumpkin Creek Valley, Morrill 2) • and Banner counties, Nebraska: University of Wyoming Contribution to Geology, v. 22, p. Cenozoic structural events and associated geo­ 39-43. graphi c features are certainly more compl i cated than DeGraw, H. M., 1969, Subsurface relations of the discussed here. Generally speaking, episodic regional Cretaceous and Tertiary in western Nebraska: uplift and local adjustments took place throughout the University of Nebraska-Lincoln, Conservation and Cenozoic and presumably are expressions of varying de- Survey Division open-file report, 137 p.

227 J. B. SWINE HART, V. L SOUDERS, H. M. DEGRA W, AND R. F. DIFFENDAL, JR.

MILLION YEARS BEFORE PRESENT 40 35 30 25 20 15 10 5 o 2000 600 Dashed portion of line symbolizes 1= 1800 complex cut-and-fill sequences w which are not reflected in the bars w below. ~ 1600 (f) (f) (f) w 1400 (f) z I~ I , ~ " " 400 ~ U " ~ I 1200 " \~ U I- " I~ I " 1<1> ::!: " I- ::J 1000 I 300 ::!' ::!: I ::J x I ::!' « 800 I x ::!: \ « w 200 ::!' > 600 w I- > « I­ --l 400 ::J « ::!: 100 :3 ::J ::!' U 300 ::J U

0 -L----~~~------~O t-= u. 800 (f) (f) 200 w 600 z ~ ~ I 400 I- 100 ::!: ::J 200 ::!: x « 0 0 .; vs ::!: Q) c c ~ =° ..0 c 0 ~ E .2 E E -~Q) "'" . (/) Q) u. u. ~::;;: 0 Q)1Il E u. e°.o :>.- ~ "0 u.. a.E o ~ -; 1-:; 1Il ~ Q) (/) ~~ E ~ u..~Q) E ~cQ)Q) ~ t:c::!:: u.. U c Q)c 0 ~ cQ)::;;: c -u; I° 0'1 0 - "- CQ) III = Q)O ~ ° o~ a...c: 0> c:~ -;u °~ ~.~ ~Oo c O(/) ~ '';:'0 -0 C - CD a. c > I ..., Q)1Il ...,-= 1ii~ ~1Il .... Q)(/) ·c ~ Q).- -0 0..., Q) ~-c 0"0 ~ c~ a...., -"0 -::;) ..c: 0a. Q) C:'-><~ °0 .cc~ c.. c: '- Q) 00 Q.Q) ~ 0 O..c ~ 00- (/) ~ ::;) Q) UoO ::J ° CD..o (!) ::!'I ::J..o 0:: u.. CD (f) 0 >Q) « CD 0-0 [WHITE RIVER GROUP] [ ARIKAREE GROUP ][ OGALLALA GROUP ]

~ Predominantly volcaniclastic 1::>.:.1 Predominantly volcaniclastic c=J Predominantly alluvial deposits, alluvial deposits eolian deposits qenera Ily with less than 15 % volcaniclastic material Figure 23. Time-maximum thickness and cumulative curve diagrams for the Cenozoic of the western part of the study area. Patterns of deposition are best shown by time-sediment volume graphs but sufficiently accurate volumes for the stratigraphic intervals used here are not known.

Diffendal, R. F. Jr., 1980, The Rush Creek-Lisco Diffendal, R. F. Jr., 1984, Evidence for Quaternary Structural Basin, Garden and Morri11 counties, pi racy of Pumpki n Creek, south-central Morri 11 Nebraska: Transact ions of Nebraska Academy of County, Nebraska: Transactions of Nebraska Sciences, v. 8, p. 123-130. Academy of Sciences, v. 12, p. 65-69. Diffendal, R. F. Jr., 1982, Regional impl ications of Galbreath, E. C., 1953, A contribution to the Tertiary the geology of the Oga11ala Group (Upper Ter­ geology and paleontology of northeastern tiary) of southwestern Morrill County, Nebraska: Colorado: Uni versity of Kansas Paleontological Geological Society of America Bulletin, v. 93, p. Contribution 13 Vetebrata, article 4; p. 1-119. 964-976.

228 CENOZOIC PALEOGEOGRAPHY OF WESTERN NEBRASKA

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