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The role of eolian-fluvial interactions and dune dams in landscape change, late Pleistocene–Holocene, Mojave Desert, USA
This study focuses on the geomorphology and geochronology of dunes formed on three sandy barrier systems at Clark, Europe and Kangaroo Lakes in Wisconsin's Door Peninsula. The Lake Michigan shoreline in the peninsula contains abundant evidence for fluctuations in lake level with paleo-shoreline features that lie up to ~7 m above the present shoreline. Dunes are not very common along the Lake Michigan shoreline in Wisconsin, but the three bay barriers studied contain beach ridges that were buried by varying depths of eolian sand in the form of low relief sandsheets as well as parabolic and transverse dunes that have relief of up to 21 m. The purpose of this study was to document when the barriers formed and when the subsequent eolian activity occurred. The chronology presented here for barrier emplacement and dune development is based on 65 optically stimulated luminescence (OSL) samples which were collected from littoral sediment in the barriers (n = 17) and the overlying eolian sand (n = 48). Sediment samples were collected using bucket augers or a vibracoring device at depths ranging from 0.5 to 4.1 m below the ground surface. The OSL ages show that barriers in each of the study sites were constructed between ~5.9 and 3.9 ka, corresponding closely to the Nipissing high lake phase. OSL ages falling between 3.3 and 2.5 ka at the Kangaroo Lake site suggest the portion of the barrier closest to Lake Michigan formed during the Algoma phase. The majority of the eolian ages fall into two primary groups that overlap with or are slightly younger than the ages acquired from the barriers. These results suggest eolian activity ended between 4.5 and 3.7 (n = 20 ages) and 2.5 and 1.8 (n = 11 ages) ka. Both geomorphic and geochronological evidence suggests that dune development occurred rapidly when sand supply increased as lake levels fell following these two transgressive events.
Sand Point is a small cuspate foreland located along the southeastern shore of Lake Superior within Pictured Rocks National Lakeshore near Munising, Michigan. Park managers' concerns for the integrity of historic buildings at the northern periphery of the point during the rising lake levels in the mid-1980s greatly elevated the priority of research into the geomorphic history and age of Sand Point. To pursue this priority, we recovered sediment cores from four ponds on Sand Point, assessed subsurface stratigraphy onshore and offshore using geophysical techniques, and interpreted the chronology of events using radiocarbon and luminescence dating. Sand Point formed at the southwest edge of a subaqueous platform whose base is probably constructed of glacial diamicton and outwash. During the post-glacial Nipissing Transgression, the base was mantled with sand derived from erosion of adjacent sandstone cliffs. An aerial photograph time sequence, 1939–present, shows that the periphery of the platform has evolved considerably during historical time, influenced by transport of sediment into adjacent South Bay. Shallow seismic reflections suggest slump blocks along the leading edge of the platform. Light detection and ranging (LiDAR) and shallow seismic reflections to the northwest of the platform reveal large sand waves within a deep (12 m) channel produced by currents flowing episodically to the northeast into Lake Superior. Ground-penetrating radar profiles show transport and deposition of sand across the upper surface of the platform. Basal radiocarbon dates from ponds between subaerial beach ridges range in age from 540 to 910 cal yr B.P., suggesting that Sand Point became emergent during the last ~1000 years, upon the separation of Lake Superior from Lakes Huron and Michigan. However, optically stimulated luminescence (OSL) ages from the beach ridges were two to three times as old as the radiocarbon ages, implying that emergence of Sand Point may have begun earlier, ~2000 years ago. The age discrepancy appears to be the result of incomplete bleaching of the quartz grains and an exceptionally low paleodose rate for the OSL samples. Given the available data, the younger ages from the radiocarbon analyses are preferred, but further work is necessary to test the two age models.
Drought drove forest decline and dune building in eastern upper Michigan, USA, as the upper Great Lakes became closed basins
From buttes to bowls: Repeated relief inversion in the landscape of the Colorado Piedmont
Abstract Mesas and buttes of the central Colorado Piedmont are composed of at least twodistinct rock types, which differ in their cohesiveness and resistance to erosion. Thelower parts of the exposed stratigraphic section are poorly cemented, Upper Cretaceousto Middle Eocene sandstones of the Dawson Formation. The caprocks arecomposed of one or more resistant formations of Late Eocene age: the Castle RockConglomerate, Wall Mountain Tuff, and the conglomerate of Larkspur Butte. Theseformations were originally deposited in topographic lows, but due to their resistance,they now cap prominent buttes and mesas of the Colorado Piedmont. Erosion of thecaprock through progressive retreat of the butte scarp produces colluvium that has ahigher resistance to erosion than the poorly cemented underlying sandstone. Once the caprock of a butte has been removed by erosion, the underlying weaklycemented Dawson Formation is readily eroded. Ultimately, the armored lower slopesof the former butte remain as a circular ridge standing as much as 100 m above thesurrounding topography. This process produces a topographic low surrounded byrelict faceted slopes where the flat top of the butte once stood. Prominent alluvial fans are associated with some of these annular features, andthey record the main phases of butte removal and excavation of the central part of thearmored slopes. Multiple generations of alluvial fans contain coarse- and fine-grainedfacies that represent changes in effective stream power and record alternating phasesof aggradation and erosion. The degree of soil development in the fan alluvium andheight of the fan surfaces above streams indicates the oldest preserved gravel fandeposit is of late-middle Pleistocene age. The youngest luminescence (optically stimulatedluminescence) dated alluvial fans were deposited during the late Pleistoceneabout the time of the Pinedale glacial maximum in Colorado, ca. 21,000 yr B.P. Keywords: Colorado Piedmont, talus flatiron, talus flatiron ring, inverted topography.
Late Pleistocene through Holocene landscape evolution of the White River Badlands, South Dakota
Abstract Badlands are common arid and semiarid landscapes long recognized in slope development and erosion rate studies by preeminent geomorphologists including Gilbert, Davis, and Schumm. The trip described here will examine in detail Quaternarystrata and landscape evolution in arguably the most famous badlands, the White River Badlands of South Dakota, which were pivotal during development of vertebrate paleontology in North America. Geologists have collected fossils from the White River Group there nearly every field season since the mid-1800s; however, until recently, little work was reported on the extensively exposed Quaternary strata. The White River Badlands are also a proposed dust source for the widespread PeoriaLoess of the Central Great Plains. The research highlighted on this trip includes (1) luminescence and radiocarbon ages from late Pleistocene through Holocene eolian sand, (2) radiocarbon ages from Holocene eolian cliff-top deposits, (3) luminescenceages from late Pleistocene fluvial silts, (4) radiocarbon ages of late Holocene fluvial silts, and (5) cosmogenic ages on ventifacts from the adjoining upper prairie. These new studies will facilitate discussions, including (1) late Quaternary paleoenvironments,(2) late Quaternary fluvial incision rates and episodes, (3) up-wind sediment supply of late Quaternary nonglaciogenic loess, (4) landscape evolution spanning late Pleistocene tableland through late Holocene sod table development, and (5) modern erosion-pedimentation rates. Keywords: Badlands, fluvial, eolian, geochronology, geomorphology.