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.
The Effect of Drainage Reorganization and Climate On the Sedimentologic Evolution of Intermontane Lake Systems: The Final Fill Stage of the Tertiary Ebro Basin (Spain)
Abstract Stratigraphic and sedimentologic analyses and geochemical data from the middle and late Miocene record in the southwestern-central sector of the Ebro Basin have allowed the evolution of its Neogene final fill stage to be characterized and its controlling factors to be discerned. The focus of these analyses was the distinct depositional environments that occurred over time (tecto-sedimentary units T6, T7, and T8). Twelve main associations of lacustrine, palustrine, fluvial, and alluvial facies revealed that the sedimentary processes leading to the formation of the units occurred in different environments in units T6 and T7 (middle and late Aragonian, i.e., Langhian–Serravallian) and in unit T8 (Vallesian–Turolian?, i.e., Tortonian). The depositional environments of units T6 and T7 encompassed a closed, shallow carbonate lake system with wide palustrine fringes. The lake was fed by small sandy and gravelly channels flowing from the southwest (Iberian Range) and a sandy fluvial system from the north (Pyrenees). Inputs were also received from a southeastward-flowing axial fluvial system (AS). The mouths of these systems commonly formed small deltaic sandy lobes on the northern lake margins. The evolution of unit T6 and in particular that of unit T7 was greatly influenced by Pyrenean tectonics. The depositional environments of unit T8 included the occurrence of the AS, but this system was soon replaced by an open, shallow carbonate lake surrounded by wide and shallow oncoid-bearing channels and palustrine tufaceous areas on its west-southwest margin. Distinct depositional events from the south and west, which involved intra-T8 erosion, resulted in minor coarse alluvial deposits. The carbonate system of unit T8 occurred in an open depression created after local erosion of the pre-T8 deposits and subsequent northward shifting of the AS since early deposition of unit T8, the origin of which was related to late contractile deformation in the northwestern Iberian Range. Bicarbonate- and calcium-rich water from the Mesozoic carbonate rocks in the Iberian Range fed the T8 system. Sedimentologic, δ13C, δ18O, and trace-element data support the change from closed to open drainage, and strongly suggest a general climate trend toward wetter and cooler conditions, which is consistent with the middle–late Miocene climate evolution of other European regions. Despite the general timing coincidence between unit T8 deposition and the opening process of the Ebro Basin, the attributes of the T8 deposits are inconclusive to support the previous hypothesis that the fluvial–lacustrine–palustrine system could be directly connected to the emptying drainage of the basin into the Mediterranean Sea. Instead, the results of this study demonstrate the combined influence of tectonics and climate on the evolution of the final fill stage of this part of the Ebro Basin. These results may be useful for assessing the influence of such allocyclic factors on the sedimentologic, hydrologic, and geochemical evolution of other intermontane lake basins.
Atmospheric drag effects on the KOMPSAT-1 satellite during geomagnetic superstorms
We examine the atmospheric drag on the low earth-orbiting satellite, KOMPSAT-1 in a sun-synchronous orbit at ~685-km altitude starting in 1999, during a 3-month (October-December) period in 2003. This 3-month interval includes the October 29-30 and November 20 magnetic superstorms and weak to moderate storms. We observed that the daily KOMPSAT-1 drag acceleration transiently responses to transient storm-time disturbances. That is, there is an one-to-one correspondence between the KOMPSAT-1 drag accelerations and the storm events. We find that the drag acceleration correlates strongly with the level of geomagnetic disturbance. This indicates that the trajectory of KOMPSAT-1 is significantly perturbed during extremely disturbed intervals because of atmospheric density increase. The main contributor to the density increase is Joule heating associated with the geomagnetic activity rather than the solar EUV radiation, as reported by previous studies. We suggest that understanding how the upper atmosphere responses to the geomagnetic-associated heating is important to predict space weather impacts on low earth-orbiting satellites.