Bedform and stratification characteristics of some modern small‐scale washover sand bodies

Abstract

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.

Sulfate-Rich Eolian and Wet Interdune Deposits, Erebus Crater, Meridiani Planum, Mars

“…However, the 1-2 m thick deposits of low-angle to planar lamination are unlikely to form subaqueously given the thickness of this deposit. In a search of the literature, 33 deposits of upper-flow-regime plane-bed lamination were found with a thickness less than a meter (Williams 1971;Shaw 1972;Schwartz 1982;Dardis 1985;Cheel and Middleton 1986;Langford and Bracken 1987;Røe 1987;Chakraborty and Bose 1992;Bal and Lewis 1994;Sohn et al 1999;Plint and Wadsworth 2003); six deposits were found with a thickness between 1 and 2 meters (Shaw 1972;Røe 1987;Bal and Lewis 1994); and three deposits were found with a thickness of 2 meters or greater (Shaw 1972;Andteron 1976;Bristow 1993).…”

El Impacto De Eventos Catastrã“ficos Costeros en El Litoral Del Golfo De Cãdiz

“…La morfología y estructura interna de los abanicos de derrame depende de múltiples factores como la granulometría de los sedimentos, la pendiente, o bien si se depositan en un medio inundado o expuesto. Suele consistir en una estratificación horizontal o de bajo ángulo inclinada hacia tierra y estratificación cruzada (delta foreset) en la zona más distal (Hobday y Jackson, 1979;Schwartz, 1982). Cada lámina, marcada por un lag de minerales más pesados, generalmente oscuros (como ilmenita, hematite o rutilo), corresponde al paso de una ola.…”

“…Cada lámina, marcada por un lag de minerales más pesados, generalmente oscuros (como ilmenita, hematite o rutilo), corresponde al paso de una ola. Su extensión varía mucho dependiendo de la altura del cordón litoral y de la energía del oleaje, que a su vez puede estar condicionada por la pendiente de la costa (Schwartz, 1982;Jelgersma, Stire y van der Valk, 1995). El espesor de los abanicos es muy variable, pero no suele sobrepasar los 0,4 a 0,6 m y se acuñan hacia tierra o al alejarse de los levees estuarinos o canales de marea.…”

“…Se han realizado catas de 30x30x30 cm en abanicos de derrame de Camposoto (San Fernando, Cádiz). La estratificación horizontal y de delta foreset es típica de este tipo de depósito (Schwartz, 1982 El abanico de derrame se debe a la rotura del cordón litoral de la zona de Sancti Petri, cuyas dimensiones han sido transformadas antrópicamente, lo que impide conocer su altura original. Presenta una longitud aproximada de 50 m, morfología lobular (orientada de Oeste a Este) y se extiende sobre una pequeña marisma parcialmente colmatada.…”

“…La morfología y estructura interna del abanico es coherente con la definida por Schwartz (1982) en distintos abanicos de derrame. La estratificación en delta foreset coincide con un medio inundado en la zona distal del abanico.…”