Gravelly spit deposits in a transgressive systems tract: the Pleistocene Higashikanbe Gravel, central Japan

The Pleistocene Higashikanbe Gravel, which crops out along the Pacific coast of the Atsumi Peninsula, central Japan, consists of well‐sorted, pebble‐ to cobble‐size gravel beds with minor sand beds. The gravel includes large‐scale foreset beds (5–10 m high) and overlying subhorizontal beds (0·5–3 m thick), showing foreset and topset structure, from which the gravel has previously been interpreted as deposits of a Gilbert‐type delta. However, (1) the gravel beds lack evidence of fluvial activity, such as channels in the subhorizontal beds; (2) the foresets incline palaeolandwards; (3) the gravels fill a fluvially incised valley; and (4) the gravels overlie low‐energy deposits of a restricted environment, such as a bay or an estuary. The foresets generally dip towards the inferred palaeoshoreline, indicating landward accretion of gravel. Reconstruction of the palaeogeography of the peninsula indicates that the Higashikanbe Gravel was deposited as a spit similar to that developed at the western tip of the present Atsumi Peninsula, rather than as a delta. According to the new interpretation, the large‐scale foreset beds are deposits on the slopes of spit platforms and accreted in part to the sides of small islets that are fragments of the submerging spit during relative sea‐level rise. The subhorizontal beds include nearshore deposits on the spit platform topsets and deposits of gravel shoals or bars, which are reworked sediments of the spit beach gravels during a transgression. The lack of spit beach facies in the subhorizontal beds results from truncation by shoreface erosion. Dome structure, which is a cross‐sectional profile of a recurved gravel spit at its extreme point, and sandy tidal channel deposits deposited between the small islets were also identified in the Higashikanbe Gravel. The Higashikanbe Gravel fills a fluvially incised valley and occupies a significant part of a transgressive systems tract, suggesting that gravelly spits are likely to be well developed during transgressions. The large‐scale foreset beds and subhorizontal beds of gravelly spits in transgressive systems tracts contrast with the foreset and topset beds of deltas, characteristic of highstand, lowstand and shelf‐margin systems tracts.     

Possible evidence for underlying non‐linear dynamics in steep‐faced glaciodeltaic progradational ­successions

Steep‐faced glaciodeltaic progradational successions are often studied in order to reconstruct the behaviour of the glacial feeder system, or changes in the sediment sink. This paper analyses the magnitude and frequency of depositional events associated with steep‐faced glacier‐fed progradational successions recorded in Scandinavia and Ireland. The successions exhibit depositional patterns that may be interpreted as a function of underlying non‐linear dynamics. A number of the sequences display fractal scaling in the frequency and thickness of foreset units. Other successions demonstrate chaotic patterns and strong relationships between delta‐front angle and bed thicknesses, suggesting that the progradation of such sequences is self‐organized, and to an extent occurs independently of forcing by the feeder system that provides sediment to the delta front. These patterns of sedimentation appear to be a function of the steepness of the delta front and/or the textural characteristics of the sediment. This paper provides further evidence for the simultaneous presence of order and chaos in Earth surface processes and calls into question the extent to which palaeoenvironmental reconstructions may be made from steep‐faced progradational successions. Copyright © 2000 John Wiley & Sons, Ltd.     

东营凹陷古近纪沙三中期超深水湖泊的研究

利用地震剖面和连井剖面,结合岩心和测井等资料,通过恢复三角洲前积层的高度,探讨东营凹陷古近纪沙三中时期 三角洲沉积时的最大古水深。结果表明,东营凹陷沙三中时期发育了很好的建设性三角洲,尤以在牛庄洼陷一带前积层非常 发育;五个地震剖面的统计结果揭示了三角洲前积层的平均高度在485 m,校正后反演的覆水深度在681 m,说明东营三角洲 在形成时期的湖泊古水深可达485~681 m;三角洲前积层主要为一套细粒沉积,以灰色粉砂岩和暗色泥岩为主,粉砂岩中常含 漂浮泥砾,是一种沿三角洲前缘斜坡发育的砂质碎屑流和浊流沉积,进一步反映了深水的沉积环境。     

地震进积结构的判别标志与地质意义

进积结构在地震剖面上的分辨极限不同于Widess薄层分辨率。前者约为l.5λ左右,后者约λ/4。本文根据世界上各种水体的深度,计算了不同深度下斜交进积单元的分辨极限。结果表明:浅海浅湖中的斜交进积单元,即令在一秒深度的地震剖面中也难以被识别。由此判断地质文献中某些作为古水流标志的进积结构,值得商榷。