Recent debris-flow processes and resultant form and facies of the dolomite alluvial fan, Owens Valley, California
Recent debris-flow processes and resultant form and facies of the dolomite alluvial fan, Owens Valley, California
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DOI：
10.2110/jsr.68.800
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发表时间：
1998-09-01
影响因子：
2
通讯作者：
McPherson, JG
中科院分区：
文献类型：
--
作者：
Blair, TC;McPherson, JG
1984年8月22日，一场午后雷雨引发的多相泥石流沉积在加利福尼亚州欧文斯山谷的白云岩冲积扇上。在这次事件中积累了两个10-300厘米厚的区域，覆盖了68,000米(2)(26%的扇形)，类似于50,000米(3)的沉积物。这两个区域由近端扇面上高100-300厘米的多对巨石堤坝和远端扇面上20-100厘米厚的多个相连的叶组成。每条堤叶通道都是由一个单独的涌浪建造的，水流路径的切换是由于随后的涌浪堵塞了前方的巨石鼻口而造成的。堤坝由碎屑支撑到基质支撑、泥泞、卵石、鹅卵石、巨石砾石组成，显示出发育良好的径向排列的a轴碎屑组构。裂隙主要由基质支撑的泥质卵石砾石组成，碎屑支撑的裂隙边缘富含鹅卵石，沿两侧显示出强烈的倾斜平行的a轴组构，在口吻处转变为倾斜垂直的组构，大多数富含碎屑的裂隙的远端被10-30厘米厚的碎屑覆盖并被剥离。大堤的中央地带和大部分小叶表面被退耕阶段的水槽冲刷，将风化的颗粒、沙子和泥浆从扇面上移走。从近端的大堤到小叶的径向变化在形态上表现为两个相交的坡段，上一段倾斜9-12度，下一段倾斜3-5度。这种组成形式与斜率值之间的直接关系也是1984年前沉积的特征，表明1984年的事件是白云岩扇构造的主要过程的典型。从堤坝到洞穴的显著径向变化及其表现为坡度拐点，是由于经过的泥石流涌流对大多数巨石的选择性损失造成的。巨石被从巨浪中分拣出来，因为它们的浮力更大，这使得它们优先集中在海床的一部分，而这部分后来被剪断成堤坝。这种选择性的巨石去除耗尽了提供侧向支持的碎屑流，促进了流的扩张和叶的开始发育。因此，所形成的相组合是集水区堆积物中可用的特定沉积物粒度套件的函数，以及这种混合物在转化为碎片时的运移特征，以及在罕见雷暴期间快速增加暴雨的作用。
A multiphase debris flow triggered by an afternoon thunderstorm was deposited 22 August 1984 on the Dolomite alluvial fan of Owens Valley, California. Two tracts 10-300 cm thick accumulated during this event, together covering 68,000 m(2) (26% of the fan) with similar to 50,000 m(3) of sediment. Both tracts consist of numerous paired boulder-rich levees 100-300 cm high on the proximal fan, and multiple attached lobes 20-100 cm thick on the distal fan. Each levee-lobe pathway was built by an individual surge, with flow-path switching caused by clogging of the frontal bouldery snout of the ensuing surge. The levees consist of clast-supported to matrix-supported, muddy, pebbly, cobbly, boulder gravel displaying a well-developed, radially aligned a-axis clast fabric. The lobes mostly consist of matrix-supported, muddy, pebbly cobble gravel, The clast-supported lobe margins are enriched in cobbles displaying a strongly preferred slope-parallel a-axis fabric along the sides that changes to a slope-perpendicular fabric at the snout, The distal ends of most of the clast-rich lobes are overlain and offlapped by clast-poor debris-how lobes of granular mud 10-30 cm thick. The central zones of the levees and most lobes were surficially washed by recessional-stage water hows that moved winnowed granules, sand, and mud off the fan.The radial change from levees in the proximal fan to lobes distally is morphologically expressed as two intersecting slope segments, the upper one sloping 9-12 degrees and the lower one 3-5 degrees. This direct relationship between the constituent forms and slope values also is characteristic of pre-1984 deposits, suggesting that the 1984 event is typical of the primary processes constructing the Dolomite fan. The pronounced radial change from levees to lobes, and its manifestation as a slope inflection, was caused by the selective loss on the upper fan of most of the boulders from the passing debris-flow surges. Boulders were sorted from the surges because of their greater buoyancy, which caused them to be preferentially concentrated in the part of the how that subsequently was sheared off as levees. This selective boulder removal depleted the flow of the clasts that provided lateral support, facilitating flow expansion and the onset of lobe development. Thus, the resultant facies assemblage is a function of the specific sediment grain-size suite available in the catchment colluvium, and the transport characteristics of this mix when transformed into a debris how by the rapid addition of heavy rainfall during infrequent thunderstorms.