An evaluation of shape indices as palaeoenvironmental indicators using quartzite and metavolcanic clasts in Upper Cretaceous to Palaeogene beach, river and submarine fan conglomerates
An evaluation of shape indices as palaeoenvironmental indicators using quartzite and metavolcanic clasts in Upper Cretaceous to Palaeogene beach, river and submarine fan conglomerates
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利用上白垩统至古近纪海滩、河流和海底扇砾岩中的石英岩和变火山碎屑评价作为古环境指标的形状指数
DOI：
10.1111/j.1365-3091.1992.tb02128.x
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发表时间：
1992
期刊：
影响因子：
3.5
通讯作者：
J. Howard
中科院分区：
文献类型：
--
作者：
J. Howard
使用来自加利福尼亚西南部的11条河流、6条海底扇和4条海滩砾岩的样本，评估了使用定量形状测量来区分来自不同沉积环境的碎屑种群的可行性；这些起源是以前通过相分析确定的。通过修正Wentworth圆度（Rw）、最大投影球度（δp）、扁长指数（OPI）和长轴比(L)、中轴比(I)和短轴比(S)等指标对石英岩和变火山碎屑进行了表征。这些指标与先前记录的现代砾石的指标进行了比较。结果表明，在多个采样点取样和采用统计方法的情况下，某些指标是有用的古环境指标，尽管它们的形状由于纹理而存在遗传差异。统计上，δp和S/L是最有效的形状指标，采用Zingg分类（I/L vs. S/I）效果较好；利用石英岩碎屑也取得了较好的效果。OPI可用于区分滩砾岩和河砾岩，这两种砾岩分别主要由扁砾岩和长砾岩组成。叶片状碎屑的相对丰度是沉积物成熟度的一个有用指标，河流碎屑的相对丰度最大，海底扇碎屑的相对丰度最小。后者以球形粒子为主。古环境与Rw之间没有相关性，因此，所研究的海滩砾岩中丰富的圆盘状碎屑可能是在冲浪带沉积之前，在河流搬运过程中通过悬浮物和沉积物的选择性搬运。在浅海环境中，滚动体（球体和棒状）在再沉积之前通过牵引进行选择性输运，这可能是海底扇砾岩中球形颗粒占主导地位的原因。
The feasibility of using quantitative shape measurements to discriminate between clast populations from different depositional settings was evaluated using samples from 11 fluvial, six submarine fan and four beach conglomerates from south-west California; these origins had been established previously by facies analysis. Quartzite and metavolcanic clasts were characterized by the following indices: modified Wentworth roundness (Rw), maximum projection sphericity (δp), oblate-prolate index (OPI) and long (L), intermediate (I) and short (S) axial ratios. These indices were compared with those documented previously for modern gravels. The results show that certain indices are useful palaeoenvironmental indicators, despite inherited differences in shape due to texture, provided that multiple sites are sampled and a statistical approach is used. Statistically, the most effective shape indices are δp and S/L which give good results with the Zingg classification (I/L vs. S/I); better results are also obtained using quartzite clasts. The OPI is useful for discriminating between beach and river conglomerates, which consist largely of oblate and prolate clasts, respectively. The relative abundance of blade-shaped clasts is a useful index of sediment maturity, being greatest for river clast samples and smallest for submarine fan clast samples. The latter are dominated by spherical particles. No correlation between palaeoenvironment and Rw is observed, hence the abundance of disc-shaped clasts in the beach conglomerates studied is attributed to selective transport in suspension and sediment by passing during fluvial transport prior to deposition in the surf zone. Selective transport of rollers (spheres and rods) by traction in a shallow marine setting, prior to redeposition by mass transport, may be responsible for the dominance of spherical particles in submarine fan conglomerates.