Large-scale Gravelly Foreset Beds: A Gravelly Spit-Offshore Gravel Bar Model.

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  • 大規模れき質フォーセットベッド　　れきし‐沖合れき州モデル
  • ダイキボレキシツ フォーセットベッド レキシ オキアイレキシュウ モデル
  • A Gravelly Spit-Offshore Gravel Bar Model
  • 礫嘴-沖合礫州モデル

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Abstract

A gravelly spit-offshore gravel bar model convincingly explains the depositional process of gravel beds in a transgressive systems tract. According to this model, large-scale gravelly foreset beds are deposited on the slope of a spit platform, which is the subaqueous portion of a gravelly spit. Massive, horizontally stratified, or planar cross-stratified subhorizontal sets (topset beds) overlie the foreset beds, and exhibit a foreset and topset structure. The topset beds include nearshore deposits on the spit platform and offshore gravel bar deposits, which are reworked sediments from the pre-existing spit-beach gravels that had formed during a transgression. Large-scale foreset beds of gravelly spits have a high preservation potential, because gravelly spits are likely to form in fluvially incised valleys, in which gravelly spit deposits are preserved from truncation by shoreface erosion during shoreface retreat. The gravelly spit provides a new example of large-scale foreset beds. The foreset beds of the gravelly spit are easily differentiated from those of fluvial gravel bars and tidally influenced dunes in a marine environment, but are similar to those of Gilbert-type deltas. Sequencestratigraphic interpretations differ depending on the depositional model that is applied to the foreset beds : a delta during a highstand, lowstand, or shelf-margin systems tracts; a gravelly spit in a transgressive systems tract. At this point we need to find a criterion to differentiate the foreset beds of gravelly spits from those of deltas.

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