Bedform and stratification characteristics of some modern small-scale washover sand bodies   总被引：3，自引：0，他引：3  

ROBERT K. SCHWARTZ 《Sedimentology》1982,29(6):835-849

Washover sand bodies commonly develop along microtidal coastlines in beach/barrier island or spit settings. Wave runup, usually in conjunction with an abnormally high water level, may overtop the most landward berm of the beach and the foredune crest, if one exists, to produce overwash and subsequent runoff across the more landward subaerial surface. Two main elements of the resulting deposit are the washover fan and runoff channel. Newly formed, small-scale washover deposits were examined along the Outer Banks, North Carolina, near Pt Mugu, California, and at Presque Isle (Lake Erie), Pennsylvania. The fans were formed in response to unidirectional landward transport, and the runoff channels in response to unidirectional flow usually in a landward direction, but sometimes in shore-parallel then seaward direction. Where overwash carried across the fan surface and entered a pond or lagoon, a small-scale delta (microdelta) developed. In this case, the washover fan consisted of two subfacies, the wetted, but ‘subaerial’ part of the fan and the subaqueous washover delta. Flow associated with the development of the fan and runoff channel produced distinctive sets of bedforms and internal stratification. High velocity discontinuous surges moving across the fan surface resulted in the development of a plane bed and subhorizontal to low-angle (landward dipping) planar stratification which comprised the major part of the fan. Similarly, rhomboid forms were produced by high velocity sheet flow across the fan surface. Where flow carried into a standing body of water, delta-type foreset strata developed. For this case, the lateral structural sequence was subhorizontal, planar stratification merging landward into landward dipping, delta (tabular) foreset strata. In the runoff setting, where flow became channelized and continuous, both upper-flow and lower-flow regime currents were typical. Upper-flow regime bedforms included antidunes, standing waves, and plane beds. The most commonly observed lower-flow regime bedforms included microdelta-like bars, low-amplitude bars, linguoid ripples, and sinuous-crested current ripple trains. The sets of sedimentary structures comprising modern washover sand bodies provide criteria for the identification of similar deposits in ancient sediments and for more specific interpretation of the environment.  相似文献   

Neogene non-marine sedimentation and tectonics in small pull-apart basins of the San Andreas fault system, Sonoma County, California   总被引：1，自引：0，他引：1  

ROBERT J. McLAUGHLIN  TOR H. NILSEN 《Sedimentology》1982,29(6):865-876

The San Andreas fault system in northern California forms an 80–90 km wide zone of right-lateral shear. Extensional tectonism within this broad shear zone is indicated by both Neogene silicic volcanic rocks that gradually young in the direction of shear propagation to the north-west and by numerous Neogene faultbounded structural basins filled with thick non-marine sequences. The Little Sulphur Creek basins, three well-exposed 1·5–2 km wide pull apart basins within this shear system, have sedimentation patterns analogous to those of much larger pull-apart basins. They were formed and subsequently deformed by east-west extension and by north-west to south-east-orientated right-slip concurrently with basin filling. Palaeocurrent and maximum-clast size data indicate both lateral sediment transport from fault-bounded basin margins and longitudinal transport down the basin axes. The basins are filled primarily with coarse alluvial-fan and streamflow deposits derived from a surrounding igneous, sedimentary, and metamorphic provenance. Two of the basins contain basin-plain-type lacustrine turbidites that grade laterally into distal alluvial fan, fan-delta, and sublacustrine delta deposits. Talus deposits along the south-west margin of the basins contain megabreccia indicative of active uplift. Structures indicative of dewatering, liquefaction, and slumping suggest penecontemporaneous tectonism.  相似文献