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.
Alluvial Valleys and Alluvial Sequences: Towards a Geomorphic Assessment
Criteria for identifying alluvial-valley fills in the geological record have commonly been derived from coastal areas, where high-magnitude glacioeustatic fluctuations generate predictable systems tracts. In contrast, inland alluvial valleys formed in response to climatic, tectonic, and intrinsic events of varied magnitude and duration. Sequence boundaries and architecture reflect a variety of causes, typically mediated through changes in the river’s equilibrium profile and equilibrium floodplain height. We evaluate valley and sequence models using Quaternary examples from southern Asia and Australia. For these areas, proxy records are well established for climate and sea level, but the scarcity of records for tectonic activity and intrinsic channel changes hamper an interpretation of stratigraphic events. On the large alluvial plains of northern India and central Australia, climatic changes have radically affected sediment and water discharge on timescales as short as 10 to 10 years. Periods of incision and aggradation generate discontinuity-bounded sequences (aggradational–degradational rhythms), and reaches may vary between valley and channel morphology over short periods and distances. Across Asia, changes in monsoonal precipitation have caused near-synchronous incision and aggradation in varied tectonic settings. However, mature interfluve paleosols correlative with valley bases may be local or absent on these large plains, where small interfluve channels, eolian dunes, and lakes aggrade while mainstem valleys incise. Gullied floodplains, adjacent to valleys and tributaries, and terrace deposits are useful indicators of incised systems. Climatic effects appear to be underrepresented in interpretations of ancient alluvial sequences in general. Angular unconformities, channel diversion, and incised and tilted fans near major faults reflect tectonic controls on architecture, and soft-sediment deformation and rare surface ruptures testify to earthquake events. In smaller basins such as the Rio Grande Rift of southwestern USA, alluvial architecture reflects both tectonic and climatic controls. Channel-body amalgamation may not be a good proxy for tectonically induced subsidence, especially for megafans at upland exits where a low degree of amalgamation may reflect rapid accumulation rather than rapid subsidence and accommodation creation. Multistory valleys and extensive multistory and single-story sheets are present in inland and coastal alluvial areas. However, below the Australian continental shelf, seismic profiles identify entrenched channels and channel belts rather than the valley fills that are commonly assumed to form under such conditions. This example shows that sea-level fall and river extensions need not generate deeply incised fluvial valleys in coastal settings.
Formation and Strength of Magnesia Whiskers
The formation of single-crystal magnesia whiskers during the compressive deformation of single crystals at room temperatures is described. These whiskers can have strengths of the order of millions of pounds per square inch. They are formed during fracture as a result of the discontinuous propagation of cracks which are mechanically relieving internal stresses resulting from plastic flow