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.
Sedimentary environment and geomorphic development of the uppermost Siwalik molasse in Kumaun Himalayan Foreland Basin, North India
An area of about 250 sq. km. around Ramnagar and Garjiya within the Siwalik molasse in the Kumaun foothills was mapped for geomorphic evolution and sedimentation pattern. The area, bounded by Himalayan Frontal Thrust (HFT) in the south and Main Boundary Thrust (MBT) in the north, consists of the sediments of uppermost part of the Pinjor and overlying Boulder Conglomerate Formations. Sedimentologically, three facies associations (FAA–FAC) were recognized with their order of superposition as FAA → FAB, FAA → FAC, FAB → FAC and FAC → FAB with variability in depositional setting. Lithofacies FAA–FAC were seemingly deposited in sand‐dominated fluvial channel belt, gravelly braided channel belt and cut‐off channel settings, respectively. The sediment dispersal in the basin was influenced by active basinal tectonics that caused frequent avulsions during the Bounder Bed times. The compositions of the clasts of these lithofacies reveal a shift in the primary provenance from pre‐Tertiary Himalayan hinterlands during the emplacement of FAA (of the Pinjor Formation) to evolving frontal Siwalik highlands during the emplacement of FAB and FAC (of the Bouldder Conglomerate Formation). This temporal shift in the provenance indicates exhumation of the older Siwalik sequences during the later phases of Himalayan orogeny. A number of subordinate faults, oblique to above mentioned intracrustal boundary thrusts and formed after the sedimentation was seized, were likely responsible for displacement between the sediments of different associations. The neotectonic rejuvenation is also reflected by the upliftment of two sister basins in opposite direction, rapid uplift in V‐shaped valleys, fault escarpments, three levels of terrace development and displacement of rock types along the subordinate faults as well as deflection and entrenchment in the course of lower order streams. Copyright © 2017 John Wiley & Sons, Ltd.
Velocity moments in alongshore bottom stress parameterizations
The time-averaged alongshore bottom stress is an important component of nearshore circulation models. In a widely accepted formulation the bottom stress is proportional to , the time average of the product of the instantaneous velocity magnitude |u→| and the instantaneous alongshore velocity component υ. Both mean and fluctuating (owing to random, directionally spread waves) velocities contribute to . Direct estimation of requires a more detailed specification of the velocity field than is usually available, so the term is parameterized. Here direct estimates of based on time series of near-bottom currents observed between the shoreline and 8-m water depth are used to test the accuracy of parameterizations. Common parameterizations that are linear in the mean alongshore current significantly underestimate for moderately strong alongshore currents, resulting in overestimation of a drag coefficient determined by fitting modeled (with a linearized bottom stress) to observed alongshore currents. A parameterization based on a joint-Gaussian velocity field with the observed velocity statistics gives excellent overall agreement with the directly estimated and allows analytic investigation of the statistical properties of the velocity field that govern . Except for the weakest flows, depends strongly on the mean alongshore current and the total velocity variance but depends only weakly on the mean wave angle, wave directional spread, and mean cross-shore current. Several other nonlinear parameterizations of are shown to be more accurate than the linear parameterizations and are adequate for many modeling purposes.