Fluvial Terrace Deposits Research Articles

ブータンヒマラヤの地形 山間盆地の河岸段丘に関するいくつかの問題

Characteristics of fluvial terraces in intermontane basins of the Bhutan Himalaya are investigated from the viewpoint of the geomorphic evolution during the Quaternary.Main rivers in the Bhutan Himalaya generally flow in remarkably deep valleys with steep gradients from northern to southern areas. Intermontane basins with several fluvial terraces, however, are developed in the middle reaches of these rivers, where gradients of river beds are small. In view of chronological sequence, the wider distribution of the Lower terrace surface (L2 surface) at Wangdiphodrang, central Bhutan, may be caused by the increase in sediment load due to the advance of glaciers in the upper reaches during the Little Ice Age. The characteristics of fluvial deposits of the middle terrace surface (M2 deposits) suggest catastrophic flows which can result from englacial or proglacial lake bursts. The development of fluvial terraces and deposits in the Bhutan Himalaya has been mainly affected not only by climatic changes but also by occasional catastrophic floods like glacial lake outbursts.The bedding planes of the older fluvial terrace deposits at Wangdiphodrang dip upstream. It suggests the Quaternary uplift of the southern Bhutan Himalaya, that is, the front of the crystalline nappe. Nevertheless the rear side of the nappe front are less eroded. Moreover Quaternary fluvial terraces and deposits in the intermontane basins seem to be rather less developed in the Bhutan Himalaya than in the Nepal, Kumaun and Punjab Himalayas. These show possibilities whether in the Bhutan Himalaya the rate of uplift might be smaller or the initiation of rapid uplift might be rather later than that in the central and western Himalayas.

Stratigraphic effects of change from internal to external drainage in an extending basin, southeastern Nevada

Observations from southern White River Valley, southeastern Nevada, shed light on sediment dispersal and geomorphic development during and after extensional faulting that formed the valley. Fine-grained lacustrine and alluvial sediments of the late Neogene White River Narrows Formation were deposited in a subsiding half graben, while coeval coarse-grained deposits were confined to within 1 km of the active fault. The syntectonic deposits are overlain along an erosional unconformity by post-tectonic, uniformly coarse-grained, alluvial-plain and fluvial-terrace deposits. This sequence conforms to a recent proposal that fine-grained intervals in extensional basins are deposited syntectonically, whereas coarse-grained intervals are deposited during post-tectonic progradation of marginal clastic wedges. The stratigraphy and geomorphology of the study area suggest that post-tectonic progradation of coarse clastic sediment resulted from incorporation of the White River Valley into the Colorado River drainage system. Meadow Valley Wash also links previously closed, young but presently quiescent, extensional basins to the Colorado River system. By contrast, most nearby basins that contain Quaternary scarps remain internally drained. Maintenance of internal drainage by active subsidence therefore may have been important in ponding of coarse-grained sediments next to the fault. After tectonic subsidence ceased, the basin drainage became southward. This action resulted in post-tectonic progradation of coarse-grained clastic deposits as sediment-dispersal systems responded to the new external base level. Regional comparisons, however, indicate that, depending on local rates of tectonic subsidence and erosion, a basin may become externally drained syntectonically, which can cause syntectonic deposits to be coarse grained. In addition, a basin may remain closed long after tectonic activity ceases, causing fine-grained deposits to continue to accumulate in a quiescent basin. We conclude that the interplay between internal and external drainage, which depends not only on the rate of local fault activity, but also regional tectonics, climate, and other factors, is a significant factor in the stratigraphic evolution of continental extensional basins.

Paleoenvironmental Analysis of Insects and Extralimital Populus from an Early Holocene Site on the Arctic Slope of Alaska, U.S.A.

Analysis of pollen and plant and insect macrofossils from 9400-yr-old fluvial terrace deposits along the Ikpikpuk River on the Arctic Slope of Alaska permit reconstruction of the diversity of habitats present at that time, as well as an assessment of surface moisture conditions and summer temperatures relative to those of today. Consideration of the modern ecological requirements for the total fossil flora and fauna shows that the variety of habitats was similar to the total variety seen today. However, the beetle fauna is dominated by species indicative of dry, open grasslands, suggesting that dry, well-drained habitats were more widespread than now. Warmer summer temperatures are indicated by extralimital taxa including Populus balsamifera and seven beetle species that reach their present northern limits in the boreal forest zone of north-central Alaska and adjacent Canada. Consideration of the modern climatic tolerances of the beetle taxa suggests that early Holocene mean July temperatures at the fossil

Age and significance of fossil white spruce ( Picea glauca), Tunica Hills, Louisiana-Mississippi

Radiocarbon dates indicate a Farmdalian through Woodfordian age for fossil white spruce ( Picea glauca) in the Tunica Hills, a greater time span than previously documented. These dates, furthermore, require revision of the ages of fluvial terrace deposits in the region. Spruce-bearing Terrace 1 sediments, previously regarded as late Woodfordian to Holocene age, are of Farmdalian to late Holocene age. Terrace 2 sediments, previously assigned a Sangamonian or Farmdalian age, are of probable Altonian age. Plant fossils in Terrace 1 sediments represent two climatically and temporally distinct assemblages: a cool-temperate, Farmdalian-Woodfordian assemblage dominated by white spruce but including deciduous hardwoods suggestive of an oak-hickory association; and a warm-temperate Holocene assemblage composed of southern hardwoods and nonarboreal species representative of the modern flora of the Tunica Hills. Locally, fossils of white spruce have been reworked into younger Terrace 1 sediments containing the warm-temperate plant assemblage. The transition from the cool-temperate flora to the modern warm-temperate flora in the Tunica Hills probably occurred around 12,000 yr B.P.

Recognition and Description of Soil-Paleosol Relationship, Palos Verdes Estates, California: Mappable Time Horizons for Engineering Geology

A very dark gray contemporary clay soil (Diablo adobe) exhibiting an “A–C” profile overlies a moderately well-preserved paleosol on a portion of the No. 2 terrace (elev. 150 to 200 ft) at Palos Verdes Estates, California. The buried soil (paleosol) consists of a distinct dark yellowish brown “B b ” horizon and is generally in abrupt contact with a distinct, calcified “C b ” horizon. Upper Pleistocene fluvial, colluvial and marine terrace deposits generally underlie the soil sequence and rest unconformably on a wave-cut platform in the highly folded Middle Miocene Altamira Shale member of the Monterey Formation. At least two episodes of alluviation and ensuing land mass stability on the No. 2 terrace are represented by the surface and buried soils. Such soil-paleosol relationships have not been adequately described in the southern California region and can be useful as mappable time horizons applicable to determining recency of faulting, recency and extent of mass movement, depth and extent of artificial fill, erosional rates, geologic history and paleoenvironments.