• Abstract and Figures
• Public Full-text

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Robert F. Diffendal, Jr., Publications Natural Resources, School of 1983 Asymmetrical Distribution of Quaternary Alluvial Fills, Pumpkin Creek Drainage Basin, Western Nebraska R. F. Diffendal Jr. University of Nebraska-Lincoln, [email protected] R. George Corner University of Nebraska State Museum, [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 Diffendal, R. F. Jr. and Corner, R. George, "Asymmetrical Distribution of Quaternary Alluvial Fills, Pumpkin Creek Drainage Basin, Western Nebraska" (1983). Robert F. Diffendal, Jr., Publications. 34. http://digitalcommons.unl.edu/diffendal/34 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. Asymmetrical distribution of Quaternary alluvial fills, Pumpkin Creek drainage basin, western Nebraska R. F. DIFFENDAL, JR. Conservation and Survey Division. IANR. University of Nebraska-Lincoln. Lincoln. Nebraska 68588-05/7 R. GEORGE CORNER University of Nebraska State Museum. Lincoln. Nebraska 68588-0514 ABSTRACT Purposes Detailed areally restricted studies of the general geology have been done by Vondra A remnant alluvial fill of early Pleisto­ The purposes of this report are threefold: (1963), Vondra and others (1969), and Aad­ cene age exposed in Pumpkin Creek Valley, to outline the general sequence of Quater­ land (1959) for the Wildcat Ridge, and by Banner and Morrill counties, Nebraska, has nary erosional and depositional events in Breyer (1974, 1975, and 198 I) and Dif­ yielded fossils of Mammuthus meridionalis the Pumpkin Creek drainage basin; to fendal (1980b, 1982) for parts of the (Nesti) and Equus sp. cf. E. scotti Gidley. establish a general age for the oldest Qua­ Cheyenne Tablelands. Stanley (1971 a, Younger fill remnants of trunk and tribu­ ternary deposits in the eastern part of the 1971b, 1976) and Stanley and Wayne (1972) tary streams allow a refinement of earlier valley, using vertebrate fossils as evidence in have reported on variations in sediment views on the development of the Pumpkin support of this age; and to stimulate others characteristics and source areas of sedi­ Creek drainage basin during the Quaternary to examine the Quaternary sequences along ments deposited in Nebraska and south­ Period. Ancestral Pumpkin Creek both streams in western Nebraska and adjacent eastern Wyoming, including the Pumpkin shifted to the north and entrenched its val­ states more carefully for fossils and other Creek drainage basin. ley several times during the Quaternary materials, which can provide better dates Darton's geologic maps and sections of Period leaving alluvial fills at three levels or for important Quaternary events in the the Camp Clarke (l903b) and Scotts Bluff more south of the present creek. Piracy of area. (l903c) quadrangles, and the sections pre­ the headwaters of ancestral Pumpkin Creek pared by Smith and Souders (1975), show took place after the last of these erosional GEOLOGIC SETTING the general relationships between the older events, probably in middle or late Pleisto­ Tertiary formations at the surface of and cene time. The Quaternary geo,logic history of beneath the drainage basin of Pumpkin Asymmetrical distribution of alluvial fills Pumpkin Creek Valley has been described Creek. On the Wildcat Ridge part of the occurs in Pumpkin Creek Valley, along the in a general way by Darton (1899), Adams drainage basin in Nebraska, the oldest rocks north side of parts of the North Platte River (1902), Darton (1903a, 1903b, and 1903c), exposed belong to the Whitney Member of in western Nebraska, and along streams east Wenzel and others (1946), Babcock and the Brule Formation, a part of the White of the Black Hills. The asymmetrical distri­ Vis her (1952), Rapp and others (1957), and River Group of Oligocene age. The Whitney bution of these deposits may have been due Smith and Souders (1975). in this area is primarily a siltstone but may to structural warping, which could have caused these streams to shift their courses TABLE I. AVERAGE COMPOSITIONS OF GRAVELS (16 TO 32 mm) IN PERCENT laterally and to entrench their valleys repeatedly. Older Quaternary Pumpkin Creek tributary alluvium alluvial fills INTRODUCTION Qal2 QalJ Qal2 QalJ W L L L Composition 1 2 3 4 5 6 7 8 Pumpkin Creek (Pumpkinseed Creek of some older reports) is an underfit stream Quartz 8.8 7.4 3.7 5.2 5.2 3.8 6.0 5.9 that today heads in a poorly drained area in Granite 61.7 73.2 78.7 73.2 55.3 56.8 36.9 53.9 · . · . · . · . · . 0.2 0.2 0.5 southwestern Scotts Bluff County, Nebras­ Graphic Granite Orthoclase 1.4 · . 0.4 · . · . 0.4 1.3 2.0 ka. The creek, a tributary of the North Anorthosite · . 4.7 T 6.3 28.8 29.2 38.8 25.7 Platte River, flows through parts of Scotts Volcanics 4.5 4.7 5.3 3.3 2.0 1.5 2.1 1.8 Bluff, Banner, and Morrill counties (Fig. I) Quartzite 0.7 3.8 0.8 0.9 · . 1.5 and enters the North Platte about 8 km Other metamorphics 20.4 6.2 8.9 8.7 0.5 4.3 1.9 7.1 Silica varieties southeast of Bridgeport, Nebraska. Some Flint, Jasper, etc. 0.9 T · . · . 3.5 1.5 5.5 0.8 reaches of the creek are either intermittent Sandstone 0.8 T 1.I 1.5 2.5 0.4 3.4 0.8 or ephemeral, but others are perennial. The Carbonate · . T · . 1.0 .. 1.9 0.3 drainage basin of Pumpkin Creek also Shale/ Siltstone · . · . · . 0.9 · . 1.I 0.2 0.3 · . · . .. includes parts of easternmost Laramie and Unknown 0.9 1.I 1.2 0.7 1.5 Goshen counties, Wyoming. Note: T = Observed at site but not collected in samples. For legal descriptions of sample sites, see Geological Society of America Bulletin, v. 94, p. 720-729, 7 figs., 4 tables, June 1983. 720 QUATERNARY ALLUVIAL FILLS, NEBRASKA 721 also contain sandstones and conglomerates lies and larger paleo valleys, are also com­ a number of workers. These surfaces have filling channels within the siltstone. Uncon­ mon in the Ogallala. Some of these deposits been called terraces by Darton (1903a), formably above the Brule are rocks belong­ occupy channels and channel remnants Adams (1902), and Wenzel and others ing to the Arikaree Group of late Oligo­ exposed at several sites along the edge of the (1946); pediments by Babcock and Visher cene(?) to Miocene age. The Arikaree tableland escarpment forming the south (1952); and pediment and terrace surfaces primarily consists of fluvial and aeolian vol­ side of Pumpkin Creek Valley. These grav­ by Smith and Souders (1975). These varia­ caniclastic sands and friable sandstones els and conglomerates are not homogene­ tions in terms applied to the same features containing abundant volcanic glass shards. ous. In fact, as Table I indicates, there are probably reflect differing opinioris on their Calcium carbonate concretions of various major compositional differences among origin and also reflect the numerous and shapes and sizes occur within the group as them, reflecting the fact that streams cross­ often confusing classifications of terraces in well as in the upper part of the Whitney. ing the area during Ogallala time flowed the literature. Some of the variability in Above the Arikaree in at least two places on from different source areas in the Rocky descriptions and classifications of features the Wildcat Ridge are remnants of the Ogal­ Mountains, and that sometimes these similar to those preserved adjacent to lala Group. All three of these groups also streams were either locally tributary to one Pumpkin Creek may be seen by studying may contain sediments derived from local another or were reworking older gravel and comparing the works of. Howard stream erosion of older beds and from deposits and incorporating these gravels (1959), Howard and others (1968), Leopold stream erosion of igneous, metamorphic, into their own sediments. and Miller (1954), Mackin (1937), de la and sedimentary rocks exposed in the The Whitney Member of the Brule For­ Montagne (1953), Leopold and others Rocky Mountains to the west. mation directly underlies the Quaternary (1964), Cotton (1940), Schultz and Stout The rock groups exposed on the surface deposits beneath the valley floor of Pump­ (1948), and Ritter (1978). and buried directly beneath the Cheyenne kin Creek. Springs flowing from volcanic In Pumpkin Creek Valley, remnants of Tablelands to the south of Pumpkin Creek ash beds and fractured zones within the stream-eroded bedrock surfaces are directly are the same as those from the Wildcat Whitney, as well as along the contacts overlain by sand and gravel resting uncon­ Ridge. Breyer (1974, 1975), Smith and between the other formations previously formably upon siltstones of the Whitney Souders (1975), and Diffendal (1982) have described, may have helped to widen the Member of the Brule Formation. The described parts of this sequence in detail. In valley by spring sapping (Babcock and Whitney-Quaternary contact can be seen in contrast to the exposures along the Wildcat Vis her, 1952). only a few places. The contact surface Ridge, the Ogallala Group generally is appears to be a nearly smooth, gently slop­ QUATERNARY SURFACES IN thicker to the south beneath the Cheyenne ing erosion surface, or strath terrace of PUMPKIN CREEK VALLEY Tablelands, whereas the Arikaree Group is some of the classifications referred to pre­ commonly either thin or absent. Loose sand Remnants of flat-surfaced alluvial fills viously.

Load more

  11

Copy Link