Characteristics and Depositional Processes of Large-scale Gravelly Gilbert-Type Foresets in the Miocene Doumsan Fan Delta, Pohang Basin, Se Korea
Characteristics and Depositional Processes of Large-scale Gravelly Gilbert-Type Foresets in the Miocene Doumsan Fan Delta, Pohang Basin, Se Korea
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DOI：
10.1306/d4268513-2b26-11d7-8648000102c1865d
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发表时间：
1997
影响因子：
2
通讯作者：
Y. Sohn;S. B. Kim;I. Hwang;J. Bahk;M. Choe;S. Chough
中科院分区：
文献类型：
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作者：
Y. Sohn;S. B. Kim;I. Hwang;J. Bahk;M. Choe;S. Chough
韩国东浦项盆地中新世斗山扇三角洲发育高150 m以上、倾角约20°的大型砾质吉尔伯特型林带。前丛由6个沉积相组成：具有不同级配模式和层状几何形状的中至厚层状砂砾石矿床（相A），薄至中层状砂砾石矿床（相B），薄片状，通常是反级配的卵石砾石矿床（砾片；相C），薄片状，几粒厚的卵石砾石（砾透镜），透镜状，几粒厚的卵石卵石（砾透镜）；相D)，薄层状砂带超大型碎屑（相E），以及趾套区非常厚层状（bb10 - 10米厚）无组织砾石矿床（相F）。这些相代表了无黏性碎屑流（相A和B）、碎屑落流（相C和D）、湍流（相E）和与大规模森林破坏（相F）有关的非常厚的碎屑流的沉积。除F相外，这些相相互联系密切，在单个沉积单元内垂直叠加或横向并置。这表明这些相起源于一系列演化的沉积物重力流。由于大碎屑优先向上漂移和细粒物质的表面转化作用，在上缘-前缘边界或前缘斜坡中部形成的无黏结性泥石流将沉积物分离为富含卵石的下部和砂质稀疏的卵石-卵石碎屑的上部。砾石下部因摩擦冻结而就位，形成A相层。另一方面，上部沉积物以砂质湍流和砾质碎屑的形式持续移动，形成E相和D相沉积物下坡。无黏结性的碎屑流偶尔会通过沿流顶部的卷波发育转变成一系列较薄的流，在a相层的边缘形成B相层。无黏性碎屑流通过向下渗透去除间隙砂并剥落到环境水中，依次转化为颗粒组合碎屑落和单颗粒碎屑落。碎屑落落形成砾片（相C）和砾透镜（相D）。在这种流动转化过程中，卵石到卵石大小的碎屑和砂质物质被选择性地向下坡移动，导致前三角洲沉积物的结构双峰性突出，其中包括孤立的大型碎屑和砂质背景物质中卵石到卵石砾石的透镜状沉积物。
ABSTRACT The Doumsan fan delta in the Miocene Pohang Basin (SE Korea) includes large-scale gravelly Gilbert-type foresets that are more than 150 m high and dip at about 20°. The foresets consist of six sedimentary facies: medium- to thick-bedded sandy gravel deposits with variable grading patterns and bed geometries (Facies A), thin- to medium-bedded, commonly inversely graded sandy gravel deposits (Facies B), sheet-like layers, a few grains thick, of pebble gravel (gravel sheets; Facies C), lensoidal layers, a few grains thick, of cobble to boulder gravel (gravel lenses; Facies D), thin-bedded sand with outsized clasts (Facies E), and very thick-bedded (> 10 m thick) disorganized gravel deposits in the toeset area (Facies F). These facies are indicative of deposition from cohesionless ebris flows (Facies A and B), debris falls (Facies C and D), turbulent flows (Facies E), and very thick debris flows that are related to large-scale foreset failure (Facies F). These facies are in close association with one another, except for Facies F, and are either vertically superposed or laterally juxtaposed within single sedimentation units. This suggests that these facies originated from a series of evolving sediment gravity flows. A cohesionless debris flow generated at the topset-foreset boundary or on the middle of the foreset slope segregated its sediments into a pebble-rich lower division and a sandy upper division with sparse cobble-to-boulder clasts by preferential upward drift of large clasts and surface transformation of fine-grained material. The pebbly lower division was emplaced by frictional freezing, forming Facies A beds. On the other hand, the sediments in the upper division continued to move as a sandy turbulent flow and bouldery debris fall, resulting in Facies E and D deposits downslope. The cohesionless debris flow occasionally transformed into a series of thinner flows by development of roll waves along the top of the flow, producing Facies B layers on the margin of a Facies A bed. Otherwise the cohesionless debris flow transformed into grain-assemblage debris fall and single-grain debris fall consecutively by removal of interstitial sand via downward percolation and stripping into ambient water. The debris falls produced gravel sheets (Facies C) and gravel lenses (Facies D). During this flow transformation, cobble- to boulder-size clasts and sandy material were selectively transported farther downslope, resulting in prominent textural bimodality of the prodelta deposits, which comprise isolated large clasts and lensoidal deposits of cobble-to-boulder gravel set in sandy background material.