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Robert F. Diffendal, Jr., Publications Natural Resources, School of
1984 Cenozoic Geology of the Southern Nebraska Panhandle: Old Valleys, Volcaniclastics, and Vertebrates: 1984 Fall Field Trip, Nebraska Geological Society James B. Swinehart University of Nebraska-Lincoln, [email protected]
Robert F. Diffendal Jr. University of Nebraska-Lincoln, [email protected]
Mary Rebone University of Nebraska-Lincoln
Robert Hunt 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.; Diffendal, Robert F. Jr.; Rebone, Mary; and Hunt, Robert, "Cenozoic Geology of the Southern Nebraska Panhandle: Old Valleys, Volcaniclastics, and Vertebrates: 1984 Fall Field Trip, Nebraska Geological Society" (1984). Robert F. Diffendal, Jr., Publications. 47. http://digitalcommons.unl.edu/diffendal/47
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 GEOLOGY OF THE SOUTHERN NEBRASKA PANHANDLE
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OLD VALLEYS, VOLCANICLASTICS, AND VERTEBRATE!
1984 Fall Field Trip Nebraska Geological Society
LEADERS: JAMES SWINEHART MARY REBONE R.F. DIFFENDAL, JR. -
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Contour Interval 200 feet LOCCfho't) or Smps NEBRASKA GEOLOGICAL SOCIETY 1984 FALL FIELD TRIP LIST OF PARTICIPANTS Damon Brown Jim Kearney 812 W. ~1yrtle 4226 Locust St. Fort Collins, CO 80521 Lincoln, NE 68516 Colorado State University SCS George F. Engelmann Mi ke Leite Geog/Geol, UNO 1635 F St., No.6 Omaha, NE Lincoln, NE 68508 UNO - Geology UNL - Geology Al Fagerstrom Nan Lindsley-Griffin 420 Morrill Hall 433 Morrill Hall UNL - Geology Lincoln, NE Lincoln, NE 68588 UNL - Geology Robert Goodwin Dave Loope 433 Morrill Hall 1263 So. 20th, #2 Lincoln, NE 68588 Lincoln, NE UNL - Geology UNL - Geology John R. Griffin Greg Ludvi gson 4920 Woodland Ave. 123 North Capital St. Lincoln, NE 68516 Iowa City, IA 52240 Geo 1ogi c Consul tant Iowa Geological Survey Ann Guhman Ginny Maciel P.O. Box 94814 6208 Parker St. Lincoln, NE 68509 Omaha, NE68104 UNL - Geology UNO - Geology Joel Haverland Dave May 830 N' 36 601 E. Eagle Heights Lincoln, NE Madison, WI 53705 UNL - Geology University of Wisconsin Eric Hubbard Michael T. McCawley 328 West St. 4035 S. 30th Fort Collins, CO 80521 Lincoln, NE 68502 Colorado State University USDA - SCS Bob Hunt Mary Rebone 433 Morrill Hall 433 Morrill Hall Lincoln, NE Lincoln, NE 68588 UNL - Geology and UNSM UNL - Geology Denny L. Jorgenson Richard S. Rhodes II 4948 N. 113th 2014 Rochester Ave. Omaha, NE 68164 Iowa Ci ty, IA 52240 UNO - Geology University of Iowa Christin Scholting 4510 South 18th St. ()naha, NE 68107 UNO - Geology Rob Skolnick W436 Nebraska Hall Lincoln, NE 68588 UNL - Vert. Paleo. Mark C. Spencer 1302 Marbee Dr., #6 Omaha, NE 68124 UNO - Geology Jim Swinehart 113 Nebraska Hall, UNL Lincoln, NE 68588-0517 Conservation & Survey Div. LLoyd Tanner RR#2, P.O. Box 469 North Platte, NE 69101 130~ fI:t~e5c.St Wayne Vanek Scottsbluff Soil Survey Office 4502 Ave. I,· Scottsbluff, NE 69361 Soil Conservation Service Mike Voorhies W428 NH, UNL Lincoln, NE 68588 Univ. Neb. State Museum Michael Walter 3474 S. 82nd St., Apt. #1 Omaha, NE 68124 UNO - Geology Stephen White ASH 228, UNO . ()naha, NE UNO - Geology Janet L. Wright 3715 So. 33rd St. Lincoln, NE UNL - Geology NEBRASKA GEOLOGICAL SOCIETY FIELD TRIP CENOZOIC GEOLOGY OF THE SOUTHERN NEBRASKA PANHANDLE Sept-ember 8, 1984
Cumulative Mi leage 0.0 7:00 a.m. assemble at Bell Diner, Bridgeport. Drive north on U.S. 26 0.2 Cross North Platte River 0.7 Jct of U.S. 26, 385 and Union Pacific Railroad, keep right 14.9 Broadwater 15.2 Jct U.S. 26 and Neb. 92, continue southeast on 26 20.9 Road to Rush Creek Land and Cattle Company, Foremans Ranch, of the Duer Ranch locality. 22.1 STOP 1 - Duer Ranch. Park on north si de of road east of sand dr aw and ins i de gate.
DUER RANCH LOCALITY, MORRILL COUNTY, NEBRASKA James B. Swinehart and R. F. Diffendal, Jr. Conservation and Survey Division University of Nebraska-Lincoln Lincoln, Nebraska 68588-0517
The Duer Ranch locality contains some of the finest and most easily accessible examples of different styles of alluvial cuts and fills in the Cenozoic of Nebraska. It offers a unique area in which to examine the geometries and alluvial fills of several Miocene and Pliocene age paleovalleys and paleogullies. Good exposures of eolian volcaniclastic siltstones and a regionally important volcanic ash of the Oligocene age Brule Formation are also present at the Duer Ranch locality. Deposits of Oligocene through Quaternary age are exposed along the deeply incised intermittent stream valleys on the Duer Ranch (fig. D-1 and 2). The oldest of these units is the Whitney Member of the Bru 1e Formation, White River Group. The Whitney is usually considered the uppermost of the two Brule members recognized in western Nebraska (the other being the Orella). However, recently Souders et al (1980) have presented evi dence that another uni t, i nforma lly named the Brown
2. Siltstone beds, can be recognized above the Whitney in many places in western Nebraska.
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Figure 0-1. Ouer Ranch locality. Base is from U.S. Geological Survey Tar Valley SW (contour interval 20 ft) and Broadwater (contour interval 10 ft) 7.5' quadrangles. The Whitney Member is a massive to crudely bedded volcaniclastic siltstone containing abundant smectite and carbonate cement. There are at least two prominent vitric ash beds (tuffs) exposed in the Whitney (fig. 0-2). The lowest of these ashes, easily visible from Highway 26 in the NE! sec. 10, T. 18 N., R. 47 W., is the Upper Ash of the Whitney, a regional marker bed for much of the Nebraska Panhandle. The Lower Ash of the Whitney (figs. 0-2 and 3), an even more extensive marker bed, was identified in test hole 24-A-53. Siltstones of the Whitney Member are very well sorted and contain an average of 50% relatively unaltered rhyolitic glass shards (fig. 0-4) and an estimated 30% volcanically derived plagioclase and rock fragments in the coarse silt and very fine sand fractions. These characteristics combined with the general lack of stratification and scarcity of fluvial sequences suggest that most of the Whitney was deposited by the wind and accumulated on upland surfaces where organisms and pedogenic processes > a:: 8 Ash Hol low Format ion ~ w z w u o· I o Duer Ra nch beds '. :E ~- AR IKAREE GRO UP 0 Tbr Har rison and Monroe Creek formations, undifferentiated 0 ~- w ' . . ' Zw { WH ITE RIVER GROUP uo 8"""F bw.':" (.!) " ::::i Bru le Formation , Wh itney Member o Contact exposed (dos!led where opproKimole) Volca ni c- ash bed Sand and grovel channel (on ow indicoles flow direclion) AI lA' Line of geo log ic sect ion ~ Loca tion af mea su red secti on Figure 0-2. Geologic map of the Ouer Ranch locality. destroyed most stratification. The pyroclastic material was derived from volcanic vents in the western U.S. with the San Juan volcanic field of southwestern Colorado one of the most probable sources. Regional studies of the Whitney indicate that it is a blanket with the Upper and Lower ash beds present over thousands of square kilometers of western Nebraska. These studies add additional strength to the concept of a primarily loessic upland origin for the Whitney. The upper 15 to 30 m of Whitney Member mapped in the western quarter of the locality (figs. 4 SOUTH NORTH A A' Tbr. .... Broadwater Formation Loess Tbrr ... Broadwater Formation, Remsburg Ranch beds 122Q 4000 Tah .... Ash Hollow Formation Td ...... Ouer Ranch complex ....J ~ 1190 To ..... Arikaree Group 3900 L;:! w > ....J W oCt ....J W oCt en 1160 3800 W w en > 5t'... /4!.. Fo .. no,Q-r 10 "" W o > CD I3 rouo"" S, I+:sione ~s: o : 1130 3700 ~ a:: w I I W W W u.. ::::!: 1100 +++Upper Ash . Tbw Brule Formation 3600 Whitney Member 1070 3500 Vertical ..a99&ralion X 20 A o I MILE ~I------~,----~I o I KILOMETER Present land surface -. 1220 4000 ~ 1190 3900.....J > W w > .....J UJ ....J Volcanic aah bed and 1070-c~~~~~==~~==~~~ ______~.~~~ca~n~ic~la~st~ic~s"~im~.~nt~ ______~ 3500 o I MILE 13 rl------,,----~I o I KilOMETER Figure 0-3. A. Geologic section along line A-A'. Lower Ash of Whitney projected from Conservation and Survey Division test hole 24-A-53, 1.2 kilometers west of section; B. Restored geologic section. 0-2 and 3A) may be part of the Brown Siltstone beds of the Brule described above. The Arikaree Group is represented by the undifferentiated Harrison and Monroe Creek formations that disconformably overlie the Brule. These yellow brown to yellow grey, massive, very well sorted volcaniclastic 5 * Broadwater Formation QUARTZ • Ogallala Group A- Ash Hollow Formation D-Duer Ranch beds .i. Arikaree Group • Whitney Member o Whitney ashes PYROCLASTIC ce FELDSPAR GRAINS Figure 0-4. Ternary plot of quartz, feldspar and pyroclastic grains in the very fine sand fraction of selected samples from the Ouer Ranch locality. Pyroclastic grains include glass shards, rhyolitic to andesitic volcanic rock fragments and glass-mantled quartz, plagioclase and heavy minerals. This class represents the minimum amount of volcanically derived material in any.>samp1e. Samples were sieved and treated with 10% HC1 to remove carbonates and with 3% HF to remove authi geni c clay (smect ite) grain coat i ngs. si lty sands with calcareous "pipy" concretions occur only in the north western part of the locality (fig. 0-2). Poorly defined horizontal bedding and locally abundant vertical tubules (burrows?) are the only common structures present in the Arikaree. The mineralogy of the Arikaree, like that of the Brule, is dominated by volcanically derived grains (fig. 0-4). The Arikaree at the Ouer Ranch locality is interpreted to be primarily an eolian 10essic deposit similar to the underlying Brule. Unconformably overlying both the Arikaree and Brule is a fluvial sequence informally called the Ouer Ranch beds (figs. 0-2 and 3) and included within the Ogallala Group. The Ouer Ranch beds consist of rock types ranging from claystones to coarse conglomerates. Brown, silty, very fine to medium grained sands are the most common lithology. These materials fill several paleogul1ies cut a minimum of 15 m below the Upper Ash of the Whitney Member and 50 m below the base of the Ash Hollow Formation. In the NEt sec. 4, T. 18 N., R. 47 W. (fig. 0-2) the 6 Duer Ranch paleogullies trend easterly but less than 1 km east they trend southerly. Coarse sediments, mostly colluvial deposits, are more common in the lower 15 meters of the Duer Ranch and also near the edges of the gullies (fig. D-3B) where clasts of Brule siltstone and Arikaree sandstone up to 0.6 m in diameter are locally abundant. Pebbly lenses are generally less than 30 cm thick and very limited in extent. Indistinct horizontal stratification is the most common sedimentary structure in the Duer Ranch, although some crossbed sets up to 0.5 m thick are present. Crossbed orientations essentially agree with the gully trends defined by gully boundaries. At some sites near these boundaries Duer Ranch strata have primary dips of up to 7° and the gully sides dip as steeply as 20°. Gradients up to 20 m per km (.02) occur along some of the paleogullies tributary to the main complex. The fine-grained sediments in the Duer Ranch beds are similar in color and general appearance to the sandy silts of the Brule Formation but the two units are very distinct mineralogically. Silty sands of the Duer Ranch contain less than 5% glass shards and less than 15% total volcani cally derived grains (fig. 0-4). The Duer Ranch beds represent a significant change from the eolian dominated environments of the Brule and Arikaree. The complex probably represents a combination of fluvial and mass wasting processes. The absence of pedogenic structures (paleosol horizons) and unconformities within the fills suggest rapid sedimentation and the Ouer Ranch beds are interpreted to have been deposited by ephemeral streams. Vertebrate fossils collected from the Ouer Ranch beds place its deposi tion during the early part of the Clarendonian Land Mammal Age (about 10 to 12 m.y. B.P.). These fossils include two horses, Pseudhipparion and a primitive Pliohippus, a camel, cf. Protolabis, and an antilocaprid, Cosoryx. The Ash Hollow Formation rests disconformably on the Ouer Ranch beds and is characterized by a heterogeneous assemblage of sands, sandstones, sandy siltstones, sand and gravels, calcretes, silts and clays and volcanic ashes. The typical carbonate cemented sandstones and siltsto nes ("mortar beds") of the type Ash Holloware common. Ash Hollow sandstone are arkoses to subarkoses, usually containing less than 15% volcanically derived grains (fig. 0-4). The 16 to 32 mm size clasts of gravels are characterized by 80% granitic (including feldspar and quartz) pebbles and 10% rhyolitic to andesitic volcanic pebbles. In this locality, the cuts and fills of the Ash Hollow, in contrast to the Duer Ranch beds, are not deeply incised into older beds and lack steep gradients (figs. D-2 and 3). There is an excellent exposure of a 10 m thick sand and gravel filled Ash Hollow channel in the SWi SEi sec. 33, T. 19 N., R. 47 W. A light grey volcanic ash bed (vitric tuff) up to 6 m thick occurs in the SEi SWi sec. 34, T. 19 N., R. 47 W. (fig. 0-2) and forms the base of a prominent vertical face. The abundance of sili ceous rhizolith (= root cast) horizons and calcretes indicates a slower rate of deposition for the Ash Hollow than for the Duer Ranch beds. 7 Diagnostic vertebrate fossils from the Ash Hollow Formation at this locality include two horses, Dinohippus and Astrohippus, and a rhino ceros, Teleoceras. These fossils allow assignment of an early Hemphillian Land Mammal Age (7 - 9 m.y. B.P.) to this unit. The Broadwater Formation of Pliocene age provides an example of a third style of alluvial deposition at the Duer Ranch locality. The formation consists primarily of sands and gravels within a major paleovalley that can be traced along the North Platte River Valley for over 150 km. The Broadwater Formation, as originally defined by Schultz and Stout (1945) from exposures in sections 20 and 21, T. 19 N., R. 47 W., was composed of three members, a basal gravel, a middle finer grained unit (the Lisco) and an upper gravel. Subsequent field work and test drilling have demonstrated that there is more than one fine-grained unit in the main body of the Broadwater (fig. 3). In this guide the Broadwater Formation is divided into the lower Remsburg Ranch beds (a new informal name) and an upper, generally finer grained alluvial fill representing the main body of the-Broadwater. The Remsburg Ranch beds were deposited primarily in the deep, narrow, anastomosing bedrock-incised inner channels of the Broadwater paleovalley (figs. 0-2 and 3). Breyer (1975) first suggested that these deposits were part of the Broadwater Formation but only used indirect evidence to support this idea. Superposition of the Remsburg Ranch beds and the main body of the Broadwater now can be demonstrated in the SW! SW! sec. 35, T. 19 N., R. 47 W. The Broadwater increases in thickness from about 18 m in the northwestern corner of the study area to about 80 m in sec. 35 (fig. D~2). In sections 33 and 34 the Remsburg Ranch beds occur as remnants within inner channels 160 to 330 m wide and up to 46 m deep (fig. 3). During the incision of the inner channels, several bedrock islands were formed. The morphology of these deep narrow cuts is clearly visible from the air and resembles closely the inner channels of valleys described by Shepherd and Schumm (1974). Both the Remsburg Ranch beds and the Broadwater as origi nally defi ned are composed primari ly of sand and gravel interbedded with fi ner grai ned fluvial and lacustrine deposits. The channel margin of the Remsburg Ranch beds is cemented locally with carbonate but otherwise the lack of consolidation makes for few exposures of this unit. Where exposed, the dominant facies is a horizontally stratified, matrix-supported gravel. Crossbedding is generally absent in these sediments but flow directions can be estimated from imbricated cobbles. The maximum size of western source clasts (primari ly quartzite derived from the Medicine Bow Mountains of southeastern Wyoming) allows dif ferentiation of the two parts of the Broadwater. The intermediate diameters of the ten largest clasts from local sites within the Remsburg Ranch beds average 11 to 14 em while the same value for the main body of the Broadwater Formation is 7 to 8 em. In addition, the Remsburg Ranch beds locally contain more mafic plutonic clasts and fewer sandstone and siltstone clasts than the upper part of the Broadwater. The Broadwater Formation is interpreted to have been deposited by a major braided river system. 8 The age of the Broadwater Formation based on fossil vertebrates is early to middle Blancan (2.5 - 3.5 m.y. B.P.). Fossils from sites along the outcrop belt of the Remsburg Ranch beds indicate that this unit is somewhat older than the original Broadwater Formation. REFERENCES Breyer, J. 1975. The classification of Ogallala sediments in western Nebraska: University of Michigan Papers on Paleontology, v. 3, no. 12, p. 1-8. Schultz, C. B., and Stout, T. M. 1945. Pleistocene loess deposits of Nebraska: American Journal of Science, v. 243, no. 5, p. 231-244. Shepherd, R. G., and Schumm, S. A. 1974. Experimental study of river inclslon: Geological Society of America Bulletin, v. 85, p. 257-268. Souders, V. L., Smith, F. A., and Swinehart, J. B. 1980. Geology and groundwater supplies of Box Butte County, Nebraska: Conservation and Survey Division, University of.Nebraska, Nebraska Water Survey Paper No. 47, 205 p. NOTE: This text is modified from a manuscript submitted to the GSA as a DNAG Field Guide Cumulative Mileage 22.1 Depart STOP 1 and return to Jct U.S. 26 and 385 north of Bridgeport 44.0 Jct U.S. 26 and 385. Merge with 385 and continue north west. 45.2 Whitney Member, Brule Formation exposures to northeast capped by gravels probably post-Broadwater. 48.9 Whitney Member exposures to northeast cut into by Broadwater Formation inner channel gravels. 50.6 Intsct with graveled county road. Turn east. 51.6 Int sct with Northport Canal road. Turn north along canal. 52.7 Cross siphon. Continue north up Indian Creek. 9 Cumulative Mi leage 53.3 STOP 2 INDIAN CREEK EARLY MIOCENE MAMMALS AND LITHOSTRATIGRAPHY OF THE INDIAN CREEK DRAINAGE, NEAR BRIDGEPORT, MORRILL COUNTY, WESTERN NEBRASKA Marry Rebone and Robert Hunt Department of Geology and State Museum University of Nebraska-Lincoln Lincoln, Nebraska 68588 FOSSIL MAMMALS The Bridgeport Quarries produced great quantities of fossi 1 mammal remains in the 1930s, when fiel~ parties from the University of Nebraska worked several sites in sections 9, 10, and 16, T. 21N, R. 50W, Angora SE 7.5 1 topographic quadrangle (1965 edition) (fig. B-1). Most abundant are bones of Early Miocene rhinoceros, accompanied by horses, camel, dromomerycid deer, rodents, four species of carnivores (including the largest North American sample of the rare primitive ursid ce1halogale). A fluvial origin for the principal bone deposits in section a is indi cated by the abraded condition of many bones from the quarries; these fossils also show evidence of scavenging by carnivores, indicating a history of environmental processing before sediment burial. The mammals from the Bridgeport Quarries comprise the largest geographically loca lized fauna of Miocene age from the north rim of the North Platte valley in Nebraska, yet the lithostratigraphy of the area has not been care fully worked out. LITHOSTRATIGRAPHY Three mappable nonmarine Cenozoic rock units have been identified during field work in summer 1983 in sections 9, 10, and 16 (figs. B-1 and 2). This area was mapped at a scale of 1 mile = 8 inches, using the Angora SE quadrangle as a base. The three units are informally designated as (1) brown volcaniclastic siltstone (BVS), (2) gray volcaniclastic sandstone (GVS), and (3) light-toned clastics (LTC). The stratigraphically lowermost unit is BVS, with a maximum local thickness of about 100 feet. GVS overlies BVS in the map area, and is 25 to 45 feet thick. GVS is overlain by LTC with interspersed micrites; LTC thicknesses range from 50 to 170 feet. The brown volcaniclastic siltstone is uniform in texture and laterally continuous in the map area. BVS contains at various levels a number of well developed carbonate-cemented paleosols that are traceable for hundreds of feet. These paleosols are quite level over great distances. 10 Figure B-1 .. Indian Creek Drainage. Angora SE and Angora 7.5 1 quads. BVS is rich in volcanic glass shards which make up a major fraction of the rock, and suggest a significant pyroclastic contribution to the unit. The unit has produced no mamnalian fossi ls within the map area. The gray volcaniclastic sandstone overlies BVS in sections 9 and 10, and locally incises the BVS. A pebble to cobble conglomerate locally occurs at the base of GVS. Age is problematic, since GBS is not fossiliferous in the map area, but its lithology is similar to Upper Arikaree rocks in Sioux County, Nebraska, in the type area of the Harrison and Monroe Creek Formations~ Excellent exposures of GVS can be seen along a ranch road in section 25, T. 21N, R. SOW, Angora SE quadrangle, in the drainage of Upper Dugout Creek; here the relationship between conglo merate and fine sand facies within the GVS unit are clearly seen. J I Feet 200~------~ LTC 150 GVS 100 50 BVS o~------~ No horizontal scale Figure B-2. General lithostratigraphic relationships in Indian Creek. A series of fossilferous light-toned clastics (including claystone, siltstone, sandstone, and conglomerate) with rare interbedded micrites overlies and cuts into GVS and BVS within the map area. These rocks seem to represent a major paleovalley fill traversing the southern part of the map area from west to east, where incision can be easily observed, and is most pronounced. Pebble to cobble conglomerates are found at the base and along the sides of this paleovalley in several locations, and can be shown to be distinct stratigraphically from conglomerate found at the base of GVS. Diagnostic mammalian fossils from the basal paleovalley fill clearly establish a maximum age of Late Arikareean for the LTC unit. Fossil mammals were found throughout the vertical extent of this fill, and when studied, should identify an age span for the stratigraphic extent of the LTC unit in the map area. The Bridgeport Quarries are stratigraphically high in the LTC unit, and will serve as an important age datum for the final phase of deposition of these beds. The fauna from these light toned clastics indicates an Early Miocene age for the paleovalley fi 11 from its base to the level of the Bridgeport Quarries, a vertical inter val of about 140 feet. Since the first reconnaissance of the Arikaree by N.H. Darton of the United States Geological Survey, published in 1899, intra-Arikaree conglomerates have been periodically observed in the North Platte valley escarpments by various authors (Darton, 1899; Schultz and Stout, 1961:51; Vondra, 1962:23f.; Schultz, Falkenbach, and Vondra, 1967:50-51; Vondra, Schultz, and Stout, 1969:13 and geologic map [Tmh-c]). These conglomerates are spectacular in outcrop, because of the often thick accumulation of cobble-sized clasts of Arikaree carbonate-cemented concretions that comprise the bulk of the lithofacies. In the past, these conglomerates were believed to be a basal facies of the Harrison Formation, although confirming evidence for this conclusion has never been discussed in print. For this reason, the discovery of these conglomerates at two definite stratigraphic levels in the Indian Creek map area is Significant. Here these conglomerates are certainly asso ciated with incised drainages filled with sediments assigned both to the GVS and LTC units. Fortunately chalicothere remains discovered by 12 Rebone in association with conglomerate at the base of the LTC unit firmly dates this valley fill as Late Arikareean to Early Hemingfordian. An Arikareean age for the GVS unit is probable on the basis of strati graphic position between BVS and LTC, despite lack of fossils from the unit. Whether the GVS unit is a lithostratigraphic equivalent to the Harrison Formation in its type area in Sioux County remains to be decided. The conglomerates are, in a sense, the result of the great textural disparity between the parent Arikaree fine sands and silts, and the pebble gravel-sized clasts made up of carbonate-cemented concretions. These concretions developed during th~ diagenesis of Arikaree sediments in the Early Miocene, probably through ground water flow. Later stream incision into the fine-grained Arikaree rocks left the concretions behind as a lag gravel in the base and along the margins of ephemeral drainages. Preliminary observations reveal no preferred orientations, which would suggest alignment by stream action. We believe that this suggests these conglomerates represent in situ lag deposits that have undergone little or no transport, due to-their considerable clast size. These cobbles exceeded the carrying capacity of ephemeral streams in the local area. Stratigraphic relationships of the BVS, GVS, and LTC units are shown both diagrammatically (fig. B-1) and also to scale in figures B-3 and 4. South North A AI Feet 4100 I MILE o,-, ___ -'I Verlical exaggeralion 4X Figure B-2. Section A-AI. 13 South North 8 81 Feet 4200 LTC 4100 Indian Creek l- I 4000 BVS o I NILE 1 , Vertical exagveration 4X Figure 3. Section B-BI. REFERENCES Darton, N. H. 1899. Preliminary report on the geology and water resources of Nebraska west of the one hundred and thi rd meri di an. U.S. Geol. Surv. 19th Ann. Report, 1897-1898. Pt. 4 (Hydrography), pp. 719-785. Schultz, C. B., and T. M. Stout. 1961. Field Conference on the Tertiary and Pleistocene of Western Nebraska. Spec. Publ. No.2, University of Nebraska State Museum, July 1961, 55 p. [See p. 51, Stop 23] __, C. H. Falkenbach, and C. F. Vondra. 1967. The Brule-Gering con tact in the Wildcat Ridge area of western Nebraska. University of Nebraska State Musuem Bull. Vol. 6(4), pp. 50-51. Vondra, C. F. 1962. The Stratigraphy of the Gering Formation in the Wildcat Ridge in Western Nebraska. Doctoral dissertation, University of Nebraska-Lincoln, 155 p. [See p. 23f., and cross- sections] __.' C. B. Schultz, and T. M. Stout. 1969. New members of the Gering Formation (Miocene) in Western Nebraska. Nebraska Geol •. Surv. Paper No. 18: p. 13 and geologic map. Cumulative Mileage 53.3 Depart STOP 2 and return to Bridgeport for lunch 62.3 Jct U.S. 26 and 385. Continue south on 385 to Bridgeport 62.8 Cross North Platte River 14 63.4 Jct U.S. 26, 385 and Nebraska 92. Reassemble here after 1unch ANCIENT ALLUVIAL DEPOSITS IN PUMPKIN CREEK VALLEY AND BENEATH THE CHEYENNE TABLELANDS, NEBRASKA Robert F. Diffendal, Jr. Conservation and Survey Division, UN-L Lincoln, Nebraska 68588-0517 The youngest pre-Quaternary deposits beneath the Cheyenne Tablelands to the south of Pumpkin Creeek Valley are conglomerates, sandstones, siltstones, caliches, diatomites, volcanic ash, and their uncemented equivalents belonging collectively to the Ogallala Group. A major paleovalley filled with Ogallala sand and gravel and/or conglomerate, characterized by rhyolite and metamorphic clasts, has a slightly sinuous course along or just south of the tablelands escarpment (fig. P-l). The principal source of this deposit was in the vicinity of North Park in Colorado. o 10 KILOMETERS '-1____ -'· Figure P-l. Areal distribution of key Cenozoic features in Pumpkin Creek and vicinity. Numbered patterns are as follows: 1. Sand- and gravel-filled channel of Ogallala Group. 2. Early Pleistocene Wright alluvial fill of ancestral Pumpkin Creek. 3. Complex remnant alluvial fan deposit (covering older alluvial fill in places). 4. LaGrange alluvial fill with remnant tributaries. 5. Bull Canyon alluvial fill. 6. Wi ldcat Ridge. Af'-Ie~ DI~Y\cb\ and Corner C\98~). The valley of Pumpkin Creek began to form as late as the Early Pleistocene when the. creek headed in the Laramie Range in Wyoming. Gravel in alluvial fills deposited during the earliest stage in valley /5 development by ancestral Pumpkin Creek contains large quantities of anorthosite as do more recent fills (figs. P-1 and 2). During the Quaternary Pumpkin Creek shifted to the north of its original position and deepened its valley several times leaving strath terraces capped with anorthosite-rich deposits which mark the former locations of the creek. Tributaries to the creek draining the Cheyenne Tablelands to the south of the developing valley cut headward into the Ogallala-filled paleo valley mentioned earlier. These gravels were reworked in part by the Quaternary tributaries and carried north into ancestral Pumpkin Creek. Remnants of tributary fills of several ages containing the reworked Ogallala gravel and sand cap elongate south to north trending hills and a broad "alluvial fan/! along parts of the south side of Pumpkin Creek Valley today. Stops 3, 4, and 9 are at sites where ancestral Pumpkin Creek alluvium (Wright fill) derived from the Laramie Range is preserved. Stops 6 and 7 are at good exposures of the Ogallala fills. Stop 8 is at a remnant Quaternary tributary of the LaGrange fill capping a Brule (Oligocene) high. This site is a good example of the inverted topography frequently associated with such fills. Stop 5 has an ancestral Pumpkin Creek fill (anorthosite rich) overlain by tributary fill sediments slightly anorthositic). Cumulative Mi leage 63.4 Jct. U.S. 26, 385 and Nebraska 92 in Bridgeport; continue south on 385 and 92. 63.8 Jct. U.S. 385, Nebraska 92 and 88; keep right and go south on 88. 63.9 Cross Burlington Northern tracks. 65.2 Note sand dunes. Why are they here at this altitude? 67.3 Note hill to east crossed by gravel road; remnant alluvial fill caps hill. 67.5 Note Brule Formation exposed in road cut. 68.6 Note road on right to Jail and Courthouse rocks. 69.1 Cross Pumpkin Creek. 69.2 Highway 88 turns right (west). 69.5 Good view of Jail and Courthouse rocks on right. Contact between Arikaree and White River groups visible. Now dri vi ng on Late Quaternary complex alluvial fill. 16 PUMPKIN CREEK TRIB. ALLUVIAL FILLS AlW\J. CD ~ c: CD .. W .. U - ~ o ~ en -o .".. r"::,:'~ Bull Canyon >- :x: :<.::.:. Alluvial Fill North en I-- I~"J' ,I South A a:::>- A' ..c F77?l LaGrange --- 1400 North South 8 8' c 1500 t500 ... Tmo" ...l; '" 1450 .: 1450 ...e z.. 1400 ~ 1400 (I) VI LaGrange a: ~ a: ... Alluvial Fill ....OJ >- ~ 1350 ""' OJ 1350 ~ Bull Canyon ~ :::; ~ Alluvial Fill t", Tab 1300 ! 1300 ".".~:~.. ;;;.. .;.:;.:;" Tab 1250 1250 North South C C' 1550 1550 ....c 1500 ~ 1500 ... Tmo SToP 9 '" 1450 1450 oJ. VI ." ~ a: Wright a: ... ~ A lIuvial Fill ...... 1400 1400 :;:; OJ ~ ~ t ~ ~ LaGrange ~ Bull Canyon Alluvial Fill 1350 1350 Alluvial Fill + Tab ~,. ~ ! .;1 Tob 1300 -'~'....:...... ~';"'';';'~'; 1300 0, 2, K .. Figure P-2. North-south cross sections showing principal rock units and alluvial fills in Pumpkin Creek Valley. Qall and Qa12 are gully-fill deposits in tributaries equivalent in age to two Pumpkin Creek alluvial fills. Note the change in altitude of Qa12 from section 8-8 1 to section C-CI. Lines of sec tion shown on Figure 2P-1. Affey Dlr~ndal ahd COY n el'" (1963) 17 Cumulative Mileage 77.1 Note Quaternary sand draw deposits in roadcut. 78.1 Redington. 79.7 Cross Pumpkin Creek. 80.4 Cross Pumpkin Creek. 80.5 Cross Pumpkin Creek. 81.3 Cross Pumpkin Creek. 83.0 Turn left on gravel road just west of mile marker 17. 83.5 Cross auto gate. Open range. Drive carefully. 83.8 Note sand dunes. 84.4 Cross auto gate~ 84.9 Turn left (east) on gravel road and cross auto gate. Open range. 85.6 STOP 3. Gravel Pit. This is the highest and oldest Quaternary alluvial fill (Wright fill, fig. P-1) preserved in this part of Pumpkin Creek Valley. Abundant anorthosite gra vel from the Laramie Range supports the idea that the fill was deposited by a stream formerly draining that mountain area to the west. The fill is Early Quaternary in age. Mammoth and horse fossils from here indicate that the fill was deposited during the Irvingtonian Land Mammal Age (Corner and Oiffendal, 1983). The two small conically shaped hills to the southeast are,capped by anorthositic gravel. The north south trending hill on the horizon to the east is capped by Quaternary sand draw deposits and anorthositic gravel. Turn around and return to the Highway 88. 87.7 Nebraska Highway 88. Turn left (west). 89.2 Enter Banner County. 90.3 Note dissected surface capped by a younger Quaternary allu vial fill. 98.7 Peacekeeper on left. Slow down for stop just ahead. 98.8 STOP 4. Turn left on gravel road and park. This is the gra vel pit sampled by K. O. Stanley and W. J. Wayne (1972). Note that this anorthosite-rich gravel is considerably below the highest point on the valley floor to the southeast. This fill (LaGrange) is younger than the first site we visited. Return to highway and turn left (west). \8 Cumulative Mileage 99.3 STOP 5. Park carefully on shoulder. Cross highway and exa mine roadcut on south side of road. An anorthosite-rich sand and gravel is overlain by a tan sand containing gastropods. The sand in turn is overlain by sand and gravel with traces of anorthosite gravel. 100.1 Observe sand and gravel in road cuts. From this point west to the Highways 71 and 88 junction the gravels contain either no anorthosite or rare anorthosite. These sand and gravel deposits are part of an "alluvial fan" and are reworked from an Ogallala paleovalley fill outcropping along the edge of the Cheyenne Tablelands to the south. 102.1 Mile marker 3. Turn left (south) on gravel road. 103.1 Intersection. Continue south (straight ahead). 104.2 Intersection. Turn left (east) across autogate. Open range. Drive carefully. 104.9 Autogate. 106.3 STOP 6. Faden Ranch Conglomerate. Leave cars and walk to prominent outcrop to southeast (fig. P-3). This is an excellent example of an Ogallala conglomerate containing megaclasts. The basal part of the conglomerate is composed of many large local clasts. The upper conglomerate includes cobbles of granite, rhyolite, and other rocks eroded from the southern Rocky Mountains. Note the numerous sedimentary structures in this unit and the megaclasts. The conglomerate fills a paleovalley with an irregular base having up to 20 m of relief. Turn around and return to Ranch Road turnoff. T. II N. OGALLALA GROUP - ASH HOLLOW FORMATION BRULE FORMATION ~ Compl.. I;, "1 Old • Yount -+ T,ibula" V-VolcaniC Ash [..:.:..:J Fill Fill Fill Fill D F-Faunol Site Figure P-3. Ogallala (Ash Hollow Formation) paleovalley relationships in part of Banner County. 108.4 Continue straight ahead (west) on county road. 19 Cumulative Mileage 111.3 Route 71. Turn left (south). 113.8 Turn left on county road. Autogate. Open range. 114.0 STOP 7. Gravel Pit. This is the same deposit that we just saw at Faden Ranch (fig. P-3). It is largely uncemented at this site. Return to Highway 71. 114.2 Stop. Turn right (north) on route 71. 116.8 State Spur 4A intersection. Turn left (west) toward Harrisburg. 118.5 STOP 8. This is a Quaternary sand draw deposit, tributary of the LaGrange Fill. It caps a Brule ridge sloping north. Note the general lack of anorthosite in the gravels of this deposit. 119.6 Spur 4A curves to left (south). Leave the paved spur and continue straight ahead on a gravel roa~ Be prepared to stop. 119.8 Stop. Turn right (north) on gravel road. 121.0 Note flat-topped, gravel capped hill. 121.8 STOP 9. Walk to west roadcut and examine the thin anorthosite-rich gravel capping the hill top, a remnant of the Wright fill. Continue north to T intsct. 124.8 Stop. Turn right (east) on gravel road. Wildcat Ridge to north. 127.9 Stop. Highway 71~ Turn left (north) on highway. 128.1 Cross Pumpkin Creek. 129.1 Funnel rock to west. Upper Whitney Ash near base of expo sures. White River-Arikaree visible on next ridge to west. 131.5 Enter Wildcat Hills State Recreation Area 131.6 White River-Arikaree Group contact in first roadcut 132.5 Campground and scenic via turnout to right (northeast). Wildcat Ridge divide. 133.6 White River-Arikaree Group contact visible on either side of road. 138.2 Carter Canyon Road turnoff to west. Continue north 139.2 Roubadeau Pass Road turnoff to west. Continue north Cumulative Mileage 139.4 Hwy 71-Business 71 Junction. Take Business 71 into Gering and Scottsbluff 142.0 Stop right. Intsct Hwy 71 and M Street Gering, Nebraska. M Street continues west to Scottsbluff Monument (2.6 miles to Muesum parking lot). Assemble at Museum parking lot at 8 a.m. Sunday morning Sept. 9. SELECTED REFERENCES Adams, G. I. 1902. Geology and water resources of the Patrick and Goshen Hole quadrangles in eastern Wyoming and western Nebraska. United States Geological Survey Water Supply Paper, 70:1-50. Babcock, H. M., and F. N. Visher. 1952. Reconnaissance of the geology and ground-water resources of the Pumpkin Creek area, Morrill and Banner counties, Nebraska. Circular of the United States Geological Survey, 156:24+iv p. Corner, R. G., and R. F. Diffendal, Jr. 1983. An Irvingtonian fauna from the oldest Quaternary alluvium in eastern Pumpkin Creek Valley, Morrill and Banner counties, Nebraska. Contributions to Geology of the University of Wyoming. 22:39-43. Darton, N. H. 1903a. Preliminary report on the geology and water resources of Nebraska west of the one hundred and third meridian. Professional Paper of the United States Geological Survey, 17:1-69. ___ ~. 1903b. Camp Clarke folio. United States Geological Survey Folio, 87:1-4. ______,. 1903c. Scotts Bluff folio. United States Geological Survey Folio, 88:1-5. Diffendal, R. F., Jr., and R. G. Corner. 1983. Asymmetrical distribu tion of Quaternary alluvial fills, Pumpkin Creek drainage basin, western Nebraska. Bulletin of the Geological Society of America, 94:720-729. ______• 1984. Evidence for Quaternary piracy of Pumpkin Creek, south central Morrill County, Nebraska. Transactions of the Nebraska Academy of Science, xii:65-69. Rapp, J. R., F. N. Visher, and R. T. Littleton. 1957. Geology and ground-water resources of Goshen County, Wyoming. United States Geological Survey Water-Supply Paper, 1337:1-145. Smith, F. A., and V. L. Souders. 1975. Groundwater geology of Banner County, Nebraska. Nebraska Water Survey Paper, 39:96+iv p. 21 Stanley, K. O. 1971a. Tectonic implications of Tertiary sediment dispersal on the Great Plains east of the Laramie Range. Guidebook to 23rd Field Conference of the Wyoming Geological Association, 65-70. __.1971b. Areal and temporal differences in Plio-Pleistocene gravel composition, Nebraska. In T. M. Stout, H. M. DeGraw, L. G. Tanner, K. O. Stanley, W. J. Wayne, and J. B. Swinehart. Guidebook to the Late Pliocene and Early Pleistocene of Nebraska. Lincoln, Nebraska, Conservation and Survey Division, University of Nebraska:22-27. , and W. J. Wayne. 1972. Epeirogenic and climatic controls of --Early Pleistocene fluvial sediment dispersal in Nebraska. Bulletin of the Geological Society of America, 83:3,675-3,690. DAY 2 NEBRASKA GEOLOGICAL SOCIETY 1984 FALL FIELD TRIP 8 a.m. Assemble at Scotts Bluff National Monument Museum parking lot. Drive up Monument road to summit parking area. LATE CENOZOIC GEOLOGY, SCOTTS BLUFF NATIONAL MONUMENT, NEBRASKA James Swi nehart Significance of Locality The exposures of the Arikaree and White River group along the summit to Musuem Trail (fig. 5-1) are certainly the most accessible of any in western Nebraska. A wide vari ety of sedimentary structures, di age net i c features,trace fossils and volcaniclastic sediments are well displayed along the trail. An excellent panoramic view of the North Platte River Valley is provided from the north overlook (fig. 5-2) and one can see why the bluff--an erosional outlier of Wildcat Ridge to the south like nearby Chimney Rock--was an important landmark along the Oregon Trail. Lithostratigraphy The rocks exposed on the monument belong to four nonmarine stratigraphic units (OrellaJWhitney; Gering; Monroe Creek-Harrison) of Tertiary age (figs. 5-3 and 4) that can be placed into two groups--the White River and overlying Arikaree. 22 T. 21 N. QUADRANGLE LOCATION E::a:=~==C:::=E+3===1:.~=:Je---a==3:=:::efF""+3====O~======~; MILE lOOOe;s=S=r::=Esi:0===lOOO~=;=~23000===3=OOO~==~4000c:==500il ~O ==~60003:1~==7~~ FEET lEE33::::EE33:::::EE3:=·i:5=:EE33:=:EE3=r:::::i?:=. ======3; KILOMETER CONTOUR INTERVAL 10 FEET Figure S-l. Topographic Map of Scotts Bluff National Monument. ~ nn~ U~<; ScoHsbl\.toff 50\4+1--) ,'I,.' qu.acl ranel~. 2.3 '~' '.00 ' COIItoul' 'n,.na. 50 , ..t 10 ' ..t ....,. do"ed Figure S-2. Detail of sUll11lit trail and main hiking trai1. Letters refer to stops along hiking trail. Base map USGS Scottsbluff South 7.5' quadrangle. The Orella Member of the Brule Formation consists of 'tOlcaniclastic siltstones and mudstones with interbedded local, thin sandstones (Unit 1, fig. S-4). They form the small Badlands area in the northeast part of the monument. Nearby subsurface i nformati on fAdi cates that the base of the White River Group. is about 250 ft (76m) below the lowest Orella exposures. The Whitney Member is a massive appearing pinkish brown, volcaniclastic, slightly clayey siltstone with blocky to platey fracture (Unit 2, fig. S-4). It contains two vitric ash beds (tuffs) of regional extent. There is about a 20 ft (6 m) thi ck sequence of interbedded fine-grained sandstone and siltstone below the Lower Ash on Sentinel and Eagle rocks (fig. S-I). Refer to STOP 1 for further comment on the Brule. The White River-Arikaree group contact is easily recognized on the monu ment by the color change (pinkish brown to gray-brown) and style of weathering. A significant part of the basal Arikaree unit, the Gering, is missing at the monument (fig. S-3). 24 :::Ii." ~ ~ LITHOLOGY AND ROCK UNITS )oW I- >- 1 1 .Z1 J. -L J -L 1 J a: I- Brown'Slltstone ~ LU beds ~ c: a: z '""" -. 1-30 0 LU LU U ~:: >-- ;;;;rr;;., - ~ :::J a Member 0 f Q.. ~ ... E C) --- - :;) CD ... Orella ...J If 0 0 ~~ Member a: C) a: I------LU -> I- a: ~ Chadron a: z loJ w 0 ~ CD a: 35 Formation - ::E 0 J: w cC ~ ::E I J: ~ U c( oJ ... woJ a: 7777Y77!7! o Figure S-3. Time stratigraphic Figure S-4. Diagrammatic geologic chart of a portion of the Cenozoic. section on NE face of Scotts Vertical bars indicate approximate Bluff. Letters refer to points of stratigraphic intervals exposed interest along the hiking trail. at Scotts Bluff National Monument. The Gering (unit 7, fig. S-4) is about 65 ft (20 m) thick and consists of pale brown very fine- to medium-grained volcaniclastic sandstone, thi nly bedded and lami nated at top and thi cker bedded at base. It con tains several ash lentils including a remarkably continuous 1 in. (2.S em) thick ash in the thin-bedded sequence. Placement of an upper contact for the Gering is subject to some debate as the Gering typically has a gradational upper contact in many localities. The thin bedded sequence is present at other sites in the Wildcat Ridge and apparently occurs above the more typical fluvial cut and fill sequences of the Gering. I have shown the overlying pale brown and light gray silty very fine to fine-grained sandstones as a combined Monroe Creek and Harrison unit because the criteria for differentiating the two formations outside their type areas in the Pine Ridge of northwest Nebraska have not been established. Previous criteria, such as presence of lithic conglomer ates at the base of the Harrison, have not proven consistent discrimi nators as we observed at STOP 2. North Overlook on Summit Trail The Orella badlands are visible from the North Overlook {fig. S-2} and a number of vertebrate fossils were collected from this area prior to 1910 when the area was incorporated into the monument and fossil collecting was prohibited. The overlook also provides a panoramic view of the fertile cropland in the North Platte River valley, which is about 6 miles {9.7- km} wide at this location. Above this point'2the North Platte River drains an area of 24,330 square mi les (63 000 km ) ,including some snow-capped peaks in the Rocky Mountai ns. Before reservoirs were bui lt to retai n spri ng _ snowmelt for irrigation during the summer, the flow of the river was much more variable and its channel was very much wider than it is now. Regulation of the river's flow by means of reservoir releases, together with irrigation-seepage returns to th'e river, results in flow during all seasons. Today the channel seldom runs dry, as it formerly did before the upstream dams were built~ River dis§harge ranged from 449,000 to 1,700,000 acre-feet {0.55 km to 2.10 km }, averaging 822,000 acre-feet (1.01 km3) duri ng the 10-year period 1967-76. Most of the water used for irrigation within the North Platte valley is obtained from canals that di vert water from the North Pl ate Ri ver in Wyomi ng and Nebrask a. A relatively small acreage is irrigated with groundwater. We will have a brief discussion of the geologic history of the North Platte Ri ver valley before start i ng down the mai n hi ki ng trail to the museum. Geology along the Summit to Museum Trail The trail begi ns between Summit Trail markers 12 and 13 (fi g. S-2) and ends at the Museum, 1.6 miles (2.55 km) to the south. STOP A {Start of Trail} Carbonate cemented "pipy" concretions typical of the massive volcaniclastic fine-grained sandstones of ~he Arikaree. Why this shape? The Arikaree Group volcaniclastics are coarser than those of the White River Group and possibly have a higher propo~tion of crystal and lithic pyroclastic grains vs glass shards (vitric ash). So what? STOP B (at first switchback) Note knobby "potato" concretions in lower part of unit 11 (fig._ S-4). Also note low angle bedding (?) or joint (1) planes. dQUe' pasa? STOP C (at second switchback) About 45 ft (14 m) down trail from museum Signpost is a 2 to 6 in (5-15 em) thick pinkish altered ash lentil. Note cross-bedding above and below ash. ,.,,. STOP 0 (at concrete steps on trail, fig. S-5). These tabular cross bedded sandstones were interpreted to have been formed by migrating transverse and 1inguoid bars in active channels of a braided river (Stanley and Fagerstrom, 1974). They interpreted the rhyolitic ash Figure S-5. Cross-bedded Ari karee sandstone at STOP O. The somewhat out of shape, former Nebraska track star is 6 ft (1.82 m) tall. lenses to have accumulated in local depressions in abandoned channels of the braided streams. It mi ght prove interesting to exami ne the cross beds in some detail as there is some indication that many of them may have been formed by accretion of migrating wind ripples (refer to Mi sce 11 aneous Figures Append i x) • Other interesting features of these strata and the enclosed ash lentils are the many trace fossil s (burrows and bi oturbated layers) present. Stanley and Fagerstrom (1974 and figs. S-6, 7, and 8) hypothesized that many of the burrows could have been formed by beetles as similar popula tions of burrows occur in modern Platte River sediment where they are made by several kinds of beetles (fig. S-9). Fagerstrom has described a new type ichnogenus Ancorichnus coronus from this locality (Frey et a1, 1984) and suggested thi s men; scate burrow may have been produced by an arthropod that preferred moist or wet nonmarine substrates. STOP E (major bend in trail with museum sign post) Note lenticular bed of ash about 12 ft (3.7 m) above trail. STOP F (first of three switchbacks on trail along northeast-facing bluff) Strata here are near middle of unit 8 (fig. S-4) in massive eolian? volcaniclastic sandstone. STOP G (at third of three switchbacks) Outcrops here are part of unit 17 (fig. S-4) and consist of 35 ft (10.7 m) of silty sandstone beds, 1/4 inch to 1 inch (6.4 to 25.4 mm) thick. Small-scale cross stratification is locally common. Several thin (less than 2 in [5.1 cm]) ash layers are present. One of them may extend continuously 27 LOCALI TV LOCALITY Z I II ;=,Q ;1 ~O" a: ...CI .~ 0.=" ..... "-.. ~ ..u ... Q ~"'''COi:'~AAOO L-----::-----7 ..Z KANSAS CI:. Z ;=Q C :. ..ell ... und ...z a: ~ ... .. ~ P.r."el stratified Nnd "'Lj~~'I::~ a: RhYOlitic •• h 1\,'1 " '" ~ ~ 1 /1' /' "- .. c~ ",',1,1 "'.~~~ ... CI ,'\1 \' I ~ =: ",,," \ Figure s-!. OC~'~S of trace fossils at Scotts Bluff National Monument. Locality I is n~p'per tunnel on summit road and II is on hiking trail . ./'" Volcuic utic Sud "-...J Figure S-7. Shapes of shelter burrows preserved in Arikaree Group sediments. 35 Vertical Burrows in 30 Platte River at 25 Valley. Nebras ka Horizontal Burrows in 25 Ve rt ica I Burrows in Monroe Creek Formation 20 20 Monroe Crnk Formation 15 N:386 15 N:325 10 10 o o I t 3 4 5 6 1 • 9 10 11 o .. /"1M" B \0 OIAMETER IN MILLIMETERS Figure S-8. Size frenqllency distributions of burrows in Monroe Creek ash lenses at Locality II and vertical burrows in the Platte River. Note: Figures S-7,8 and 9 after Stanley and Fagerstrom (1974). 28 for over 0.6 miles (1 km) around the monument. If true this puts some serious constraints on possible depositional environments for this unit. Also note the almost total absence of bioturbation. Observe the vertical cliff to the northeast (fig. S-9). The stra tigraphy along the trail is nicely displayed in one vertical section. The sandstones below the thin bedded strata display a number of good examples of soft sediment deformation structures along the trail to the next stop. STRATIGRAPHIC POSITION OF TRAIL SITE @ Figure S-9. Cliff to northeast of trail stop G. STOP H (at entrance to tunnel) Note the ash bed above the tunnel and the cone1ike features projecting downward from it into the underlying thin-bedded gray sandstone. Bart (1975) described these projecting more than 3.3 ft (1 m) into the host sand and interpreted that these long projections were formed by animal and plant activity. He interpreted the smaller cone1ike structures to have probably formed by thixotropic flow through ripple troughs. 29 The contact of the Arikaree Group-Gering Formation with the underlying Whitney Member of the Brule Formation can be seen across the canyon from the south end of the tunnel. STOP I (at head-of-canyon switchback) Exposures of the siltstones of the Whitney Member. The Upper Whitney Ash crops out 30 ft (9.1 m) above the trail. STOP J (700 ft [213 m] down trail from Stop I) The remains of a large rock fall that occurred in October 1974 can be seen near the end of the ridge that the tunnel passes through. Note the nature of the Arikaree Bru le contact above and west of the rock fall. The Lower Whitney Ash can be seen above the trail to the west. STOP K (Scotts Spring) The spring water comes from fractures in Whitney si ltstones. END OF TRAIL - Museum REFERENCES Stan 1ey, K. O. and Fagerstrom, J. A. 1974. Miocene i nvertebrate t~race fossils form a braided river environment, western Nebraska, USA: Palaeogeog., Palaeoclim., Palaeoecology, vol. 15, p. 63-82. Frey, R. W., Pemberton, S. G., and Fagerstrom, J. A. 1984. Morphological, ethological and environmental significance of the lchnogenera Scoyenia and Ancorichnus: Jour. of Paleo., vol. 58, p. 511-528. Bart, H. A. 1975. Downward injection structures in Miocene sediments, Arikaree Group, Nebraska: Jour. of Sed. Pet., vol. 45, p. 944-950. MIS CE LLAN EDUS 31 ------r-r CASPER ARCH ------.,..- ~ .. - -; PLAINS -, .., - I i\ : :-~ --~--7 ) ---r--- I ! !, OVINCE --.~ .' - - .- ~ _. - .. , / i i I _.i __ ,__ . / :/-! -/J f '" l.... ____ ;_ ~ ------..-.-I I Bas/hS ctMd Upl/f-l:; H) -rhL 6'y~; Ph/Vt-S qhd Celf'l-1t-1:1.1 Roc..k,"es fyOl'lll 6a:>loj" J}HOb of {(ocl"1 ('\oc:a" -tar"1 Rt~(·cM (ZMt'r(j . 1912- Table 1. Characteristics of basic types of eolian stratific:ation Depositional Chatxter of Type of Dip anale Thickness Sea:rea:ation Packina Form of strata process depositional stratific:ation of strata of a:rain types surface Sharpness Size a:radinS of con_ Subcritkally Stratification: low Thin Distinc:t dimbina (typic:ally 0-20", (typic:ally 1-10 mm, Inverse translatent maximum - 30") maximum -5 em) Close Tabular, Stratification Depositional surface: Sharp, erosional planar similarly low Supercritiailly Stratification: Intermediate Disline:! Tabular, dimbinll variable (0-90") (typic:ally 5-15 mm) Inverse except Oose commonly translatent Depositional surface: Gradational in contae:! zones curved Rippled stratific:ation intermed. (10-25°) Trac:tional deposition RippJe.foreset Relative to Oose Tabular, c:t'OSS- translatent Individual laminae: con<:ave-up lamination stratification : Thin or sismoidal intermed. (5-20") (typic:ally 1-3 mm) I ndividual laminae Sharp or and sets of laminae: Ripple/'orm Generalized : a:radational, Indistinc:t Close Very tabular, lamination intermediate non-erosional Normal and wavy (typiallly 10-25") inverse. Smooth Planebed Low Sets of laminae: neither sreatly Oose Very tabular, lamination (typic:ally 0-1$0, Intermediate predominating planar max.?) (typkally 1-10 em) Sharp or aradationa!, LarseJy Smooth Grainlall Intermediate nonerosional Intermediate Very tabular, arainfaU lamination (typkally 20-30", follows deposition min.O° pre-exislent max. - 40") toposraphy Grainllow Marked by Sandtlow Hip (anale of Thick Distinc:t to Cone-shaped, deposition avalanches <:ross- repose) (typic:ally 2-5 em) indistinc:t tonsue-shaped, stratification (typically 28-34") Sharp, Inverse except Open or rouahly erosional or near toe tabular noncrosiOnaI Eolian stratification ~ RIppled surface ~; )~.~~I, , ~alpfoc(' With ovolon(h~i CJ Smooth sur face Floor acrOSS whICh ~- - dune :S mov Ing FIg. 1. Schematic block diagram of part of a sinuous transverse dune, showing typical places of occurrence of rippled surfaces, smooth surfaces, and slipfaces marked by avalanches. T ronsiolenl SIr 010 Ripplefarm lamlnoe 1---,..--1f------.--- ~ Subcrificolly climbing transloten! strOlo o oS. .~ " .~u_ Cf Critically climbing Complete lronslolent slrolo ripple - foresel crosslominoe ~ ~ ~ ~ SupercrificOily climbing Complele rippleform lomlnoe lranslolenl slrOlO '------_._---_._----- Fig, 4. Types of stratification potentially composing climbing-ripple structures at subcritical and supercritical angles of ripple climb. 33 WIND rs::;;;?~~::l WATU 32· ",.. 'mu", dip WIND t~ii~~i~~ cll ...... 'n' rlpp'. lalfttnetJon WIND ."oelo"., bcwn4l1"" euri._ WAnIt (Ioe) typo b WINO ''''e'e .... of ".. " 'nd .. ,._a.. WAnIt Curvecl Opolttlottel beMa"',.. .u'f.c... ~ • ''''''''' ,n,cMd I ..... ' .... WATU rlppl. for ... temfteMa I .. - • _tura' ft.~ftetty (GO...... n.., or ... of ... ,,,u'. rt,. •• -.conunuou •• I,.,..,._ bound .... lurf.c•• PUJYlAL·I!OLIAN LOW.ANeLil TO HIet'I·ANeLil EO UAN PIO. 12.-Schcmatic diagram showing areal distribution and stratigraphic relationships of low-angle eolian IUd Ihcct deposits to other types of deposits. Interdunes are the main type of low-angle or horizontal eolian dcpolit between dunes in the main dune mass. Other low-angle eolian deposits of the sand sheet occur peripheral to the main dune mass. and form a transitional lithofacies which intercalates with high-angle dune deposits as wen as "extradunal" (Lupc and Ahlbrandt; 1975). non-colian deposits. ~ fi,.~~~ ~h'" »"" .. di (J9IJI) - Ii-":::. T: ~;'::/~.~-~vi::;+~~~~;~~ ::~::~.~~~,-~.~.", ' ...... '" -.: ':-" '. .... ". ", .... ", CRY INTEAOUNE DUNE WET INTERDUNE FlO. I.-Diqram of commonly observed bioturbation traces in. eolian deposits (block diagram is not to scale): A. Beetle burrow. B. Burrowing and disruption of sediment by ants. C. Wolf spider burrow with web coDar and reinforced burrow walls. D. Sand-treader camel cricket burrow. in. slipface deposit. with backfilled entrance. E. iller beetle larva burrow. F. Aestivating gastropods. G. From left to right. two trial burrows. a nesting burrow. and a slccpina burrow of a sand wasp. H. Toad burrow. I. Crane fly larva burrow. 1. Root JIlolds (dikalta). K. Gopher burrow with disruption of sediment by plant roots . .M. A second type of crane fly larva burrow. f'rOWl ,4}' I b rt:tII\ct:I e.t ~ (19 7S) 34