Resedimented conglomerates in the bottomsets of Gilbert-type gravel deltas
Resedimented conglomerates in the bottomsets of Gilbert-type gravel deltas
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吉尔伯特型砾石三角洲底部的再沉积砾岩
DOI：
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发表时间：
1985
期刊：
影响因子：
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通讯作者：
T. Roep
中科院分区：
文献类型：
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作者：
G. Postma;T. Roep
摘要描述和解释了西班牙东南部维拉盆地上新世埃斯皮里图桑托组（Espiritu Santo Formation）的一个以质量流为主的海相扇三角洲层序及其再沉积砾岩层。所研究的层序从底部向上包括：1）陆棚泥岩（Cuevas组）; 2）40 m厚的砂砾岩片层序，主倾角最大为10°（三角洲下部斜坡，LDS）; 3）40 m高的平面至切线三角洲前缘，主倾角最大为25°（三角洲上部斜坡，UDS）。大多数LDS砾岩的基质为石灰泥（现已重结晶）、粉砂、砂、颗粒和卵石，其中含有分散的、弱叠瓦状的卵石。在顺流方向上，结构变化不大，但晶粒尺寸略有减小。厚于2 m的砾岩层主要是由于汞齐作用，从这些层中出现的泥层位的残留物推断。砾岩主要是不分级的，尽管在底部，可能存在几厘米薄的反分级带。有些层在其远段显示出被扩散分层砾岩覆盖的正常级配的粗尾。良好的暴露表明，砾石沉积开始刚刚超过坡折（水跃区）。颗粒组构表明塑性粘性砾石体（以摩擦强度为主的泥石流）的层流行为。砾石层顶部平坦、被侵蚀，以及鹅卵石大小的叠瓦状碎屑的存在，表明可能在沉积时受到了强烈水流的改造。因此，建议文件LDS砾岩起源于三角洲前缘滑坡的转化，要么转化为高密度湍流和泥石流（下部）的组合，要么转化为最初完全湍流。在后一种情况下，泥石流可能源于湍流的重力转化（Fisher 1983）。在这两种情况下，快速移动的浊流增强了泥石流的流动性。LDS砾岩层夹有局部层状（悬浮沉积）的无结构（含砾）泥岩层。无结构泥岩可能源于生物扰动、脱水、再沉积和覆盖砾石沉积物流的变形。UDS砾岩包含碎屑和基质支持的床，主要是不分级。基质由一些（重结晶的）石灰泥、粉土、砂和小卵石组成。根据陡峭（25°）的沉积坡度推断，这些岩层可能是由类似于LDS的泥石流沉积而成，但强度值相对较高。许多UDS前积层的切向几何形状已被解释为流动转换（稀释）和加速（流动粘度降低），增强其流动性。LDS砾岩层经常出现在底积细粒中，是经典吉尔伯特型三角洲的一种变形。因此，术语底置修正吉尔伯特型三角洲被应用于说明主要三角洲前缘故障的存在。
ABSTRACT A marine, mass-flow-dominated, fan-delta sequence (Espiritu Santo Formation, Pliocene, Vera basin, southeastern Spain) and its resedimented conglomerate beds are described and interpreted. The studied sequence from the base upward contains 1) shelf mudstones (Cuevas Formation); 2) an up to 40-m-thick sequence of sand and conglomerate sheets with primary dip of up to 10° (lower delta slope, LDS); 3) up to 40 m high, planar to tangential delta foresets with primary dip of up to 25° (upper delta slope, UDS). Most of the LDS conglomerates have a matrix of lime mud (now recrystallized), silt, sand, granules, and pebbles containing scattered, weakly imbricated cobbles. There is little change in texture in the downcurrent direction, but the grain size decreases slightly. Conglomerate beds thicker than 2 m are mostly due to amalgamation, as inferred from remnants of mud-layer horizons occurring within these beds. Conglomerates are dominantly nongraded, although at the base, a few centimeters of thin, inversely graded zone may be present. Some beds show in their distal segments coarse-tail, normal grading covered by diffusely stratified conglomerate. Good exposures indicate that gravel deposition started just beyond the slope break (zone of hydraulic jump). The grain fabric suggests laminar-flow behavior of a plastic-viscous gravel mass (debris flow with dominantly frictional strength). The flat, eroded top of the gravel beds, and the presence of cobble-sized, imbricated clasts there suggest reworking by a strong current, probably at the time of their deposition. It is therefore suggested that file LDS conglomerates originate from the transformation of a delta-front slide either into a combination of high-density turbulent flow and debris flow (lower part), or into an initially fully turbulent flow. In the latter case, the debris flow would have originated from gravity transformation of the turbulent flow (Fisher 1983). In both ca es, the faster-moving turbidity current enhances the mobility of the debris flow. The LDS conglomerate beds are intercalated with dominantly structureless (pebbly) mudstone beds which are locally laminated (suspension sedimentation). The structureless mudstones probably originate from a combination of bioturbation, dewatering, resedimentation, and deformation of overriding, gravelly sediment flows. The UDS conglomerates contain clast- and matrix-supported beds which are dominantly nongraded. The matrix consists of some (recrystallized) lime mud, silt, sand, and small pebbles. These beds are inferred to have been deposited by debris flows similar to those of the LDS, but with relatively higher strength values, as inferred from the steep (25°) depositional slope. The tangential geometry of many UDS foreset beds has been explained by flow transformation (dilution) and acceleration (decrease of flow viscosity), enhancing its mobility. The regular occurrence of LDS conglomerate beds within bottomset fines is a modification of the classical Gilbert-type delta. Therefore, the term bottomset-modified Gilbert-type delta is applied to illustrate the presence of major delta-front failures.