Sedimentology of the Whiteclay Gravel Beds (Ogallala Group) in northwestern Nebraska, USA: Structurally controlled drainage promoted by Early Miocene uplift of the Black Hills Dome
Abstract The newly recognized Whiteclay Gravel Beds (WGB) of the Miocene Ogallala Group crop out as a narrow, discontinuous ribbon of sands and gravels in Dawes and Sheridan Counties, northwestern Nebraska, USA. The WGB are exposed in a series of municipal gravel quarries and natural exposures that define a linear trench in underlying strata at least 20 m deep and up to 300 m wide, with short, southeast-trending reaches separating generally longer east-trending sections. This gravel-filled trench can be recognized from the Nebraska–South Dakota border near Whiteclay, Nebraska southeastward to east of Gordon, Nebraska, a distance of ∼ 30 km. The outcrop belt of the WGB is coincident in location and trend with the Whiteclay Fault Zone. Where exposed in quarries, the walls of the trench are steep-sided, vertical, or locally overhanging. Polished surfaces, slickensides, and parallel joint sets are common in the walls of the trench near Whiteclay, but uncommon in those to the east. The narrow belt defined by this trench is filled by stratified gravel ( The WGB are interpreted to have formed in response to tectonic upheaval associated with uplift of the Black Hills of South Dakota in Early Miocene times. Fault rupture topography facilitated formation of a steep-sided canyon, or valley, up to 20 m deep, being virtually straight with sharp bends at intervals of several km. An alluvial channel belt developed in the floor of the valley, filling the available accommodation space with coarse sand and gravel via aggradational stacking of the deposits of successive channels and channel belts. Channel belts were probably braided, with individual channels up to 4 m deep and a few tens of metres wide. The multi-storey character of the deposit indicates multiple episodes of cutting and filling. The coarse grain-size of the fill suggests energetic discharge with frequent bankfull flows, even though the system had a relatively low gradient (0.004). An abundance of reworked fossil debris is derived from several stratigraphic units, clasts of which have been identified in the fill. The presence of a contemporary merychippine horse and a primitive species of the oreodont Brachycrus constrain formation of the WGB to a short interval within the Early Miocene (c. 17.5 Ma). The mammal fauna suggests that this stream was a valuable source of water, while fragments of aquatic organisms such as turtles and fish indicate perennial discharge. The WGB provides a crucial window into a pluvial period in the Miocene that is largely unpreserved elsewhere in the basin, facilitated in part by fault rupture topography.
Depositional processes in the distal part of a large alluvial fan's feeder channel in Himalayan foothills, India
This paper presents results of detailed facies and architectural element analyses of the sedimentary fill of the Ranibag valley in the Himalayan foothills of north‐western India. Forming a buffer zone between the source and sink of the Ganga foreland basin's Gola alluvial fan, the Ranibag valley is filled with coarse gravels comprising five distinct facies. These are clast‐supported massive gravel, horizontal‐bedded gravel, crudely bedded gravel, sandy gravel and matrix‐supported disorganized gravel facies. The specific assemblages and sequences of these facies, defined by fifth‐ and fourth‐order bounding surfaces, belong to mutually associated channel (CH) and gravel bar and bedform (GB) architectural elements. Both the elements have accreted only during floods by combined fluvial and debris‐flow processes, but dominantly by the fluvial processes under conditions of high water‐to‐sediment ratio. Depositional processes in the valley are akin to those of the proximal alluvial fans of the Ganga Basin. Nonetheless, voluminous alluviation of coarser gravels in this extreme distal feeder channel valley has affected the fan processes downstream. It reduced the sediment budget and thus increased water‐to‐sediment ratio downstream to favour fluvial processes on the Gola fan. Copyright © 2016 John Wiley & Sons, Ltd.
An Allostratigraphic Correlation of the Muskiki and Marshybank Formations (Coniacian-Santonian) in the Foothills and Subsurface of the Alberta Basin
ABSTRACT The Muskiki (Coniacian) and Marshybank (Coniacian--early Santonian) formations were studied in 19 outcrop sections in the Foothills of Alberta and British Columbia, in eight cores and in 1510 well logs from the Plains. The Muskiki Formation, plus unit A of the Marshybank Formation consist mainly of shale and siltstone arranged in a series of subtle upward-coarsening facies successions, the bounding surfaces of which are essentially parallel. These rocks comprise a transgressive systems tract. Units B-F of the Marshybank Formation each comprise one or more upward-coarsening facies successions, which downlap onto the top of unit A from west to east. Units E and F contain shoreface sandstones in the west. In the northwest, unit F is overlain by up to 16 m of nonmarine strata. Units B-G are disconformably overlain by a second package of marine strata, comprising units H-L. Unit H onlaps F and G and pinches out toward the west where it is overstepped by unit I. Units H, I and K comprise upward-coarsening successions of shelf siltstones and sandstones. Units J and L consist of erosive-based shoreface sandstones, which pinch out toward the east. In the far northwest of the study area, unusual conglomeratic sandstones are considered to be the proximal equivalents of units H-L. As a whole, units B-L comprise a progradational highstand systems tract. The Marshybank and Muskiki are truncated by a northeast-dipping erosion surface (sequence boundary) with a ridge and swale topography of several tens of metres. In the eastern part of the area, oolitic sandstones of the Bad Heart Formation (Santonian) rest disconformably on this surface and pass laterally westward into siltstones along a well-defined northwest-southeast-trending line. RESUME Les formations Muskiki (Coniacien) et Marshybank (Coniacien a Santonien inferieur) ont ete etudiees dans 19 coupes d'affleurement dans les piemonts de l'Alberta et de la Colombie britannique, dans huit carottes et dans 1 510 diagraphies de forage de la region des plaines. La formation Muskiki, en plus de l'unite A de la formation Marshybank consistent surtout de schiste argileux et de microgres disposes sous forme de series de successions subtiles de facies dont la granulometrie augmente vers le haut. Les surfaces limite de celles-ci sont essentiellement paralleles. Ces roches forment une zone de systemes transgressifs. Chacune des unites B a F de la formation Marshybank comprend une ou plusieurs successions de facies dont la granulometrie augmente vers le haut, qui chevauchent le haut de l'unite A d' ouest en est. Les unites E et F renferment des gres de la zone infratidale dans l'ouest. Dans le nord-ouest, l'unite F est recouverte par jusqu'a 16 m de couches continentales. Les unites B a G sont recouvertes en discordance par une deuxieme serie de couches marines, qui comprennent les unites H a L. L'unite H chevauche F et G et disparait progressivement en biseau vers l'ouest ou elle est erodee et recouverte par l'unite I. Les unites H, I et K comprennent des successions de microgres et de gres deposees sur un plateau continental et dont la granulometrie augmente vers le haut. Les unites J et L sont formees de gres de la zone infratidale reposant sur une surface d'erosion, et disparaissent progressivement en biseau vers l'est. Dans l'extreme nord-ouest de la region etudiee, des gres conglomeratiques peu communs sont consideres comme etant l'equivalent a proximite du rivage des unites H a L. Dans leur ensemble, les unites de B a L forment une zone de systemes qui progresserent vers la mer a un moment ou le niveau marin etait le plus eleve. Les formations Marshybank et Muskiki sont tronquees par une surface d'erosion inclinee vers le nord-est (limite de sequence dans le sens de "sequence boundary"), avec un relief de crete et de bas-fond de plusieurs dizaines de metres. Dans la partie est de la region, des gres oolithiques de la formation Badheart (Santonien) reposent en discordance sur cette surface et se changent lateralement vers l'ouest en microgres le long d'une ligne bien definie orientee nord-ouest a sud-est. Traduit par Marc Charest. End_Page 288------------------------