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.
Rivers and Floodplains: Forms, Processes, and Sedimentary Record
1. Introduction. 2. Overview of River Systems. 3. Fundamentals of Water Flow. 4. Fundamentals of Sediment Transport. 5. Bed forms and Sedimentary Structures. 6. Alluvial Channels and Bars. 7. Floodplains. 8. Along--valley Variations in Channels and Floodplains. 9. Channel--belt movements across floodplains. 10. Long--term, Large--scale Evolution of Fluvial Systems. 11. Fossils in Fluvial Deposits. Appendix 1. Methods of Measuring Bed Topography, Water flow, Sediment Transport, Erosion and Deposition in Rivers. Appendix 2. Methods of Describing and Interpreting Sedimentary Strata. References
Indirect Response of the Peace River, Florida, to Episodic Sea-Level Change
Rivers must respond to base-level change but the timing, nature, and location of the response varies with the river. This response includes incision and aggradation as recorded in the flood-plain sediment and soil and calibrated by radiocarbon dates. Stream response to sea-level variations in a karst terrain is identified along the Peace River in central Florida. Incision and aggradation of this small river, more than 35 km upvalley from its mouth, primarily respond to changes in water-table elevation. This elevation in turn responds to both sea-level and climate changes. As a consequence, river response to sea-level change is indirect and nonsynchronous with sea-level variations and with that of other non- karst streams that respond more directly to sea-level fluctuation. Incision of the Peace River occurred prior to 22 ka, during the initial decline of sea level and dry climate associated with the late Wisconsin glacial stage. These conditions caused the depression of the water table and hence the potential incision for the Peace River. As the river incised, upstream fluvial and eolian erosion of dry upland surfaces increased sediment load and subsequently caused aggradation of the Peace River, despite a declining sea level. The present incision of the river occurred within the last few millennia when sea level was at or near its present high stand and the climate was humid, thus resulting in a shallow water table. The wet vegetated ground surface restricted erosion therefore reducing sediment load of the Peace River which then began to incise during a sea-level maximum.