Point-Bar Deposits, Old River Locksite, Louisiana
Point-Bar Deposits, Old River Locksite, Louisiana
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路易斯安那州 Old River Locksite 的 Point-Bar 矿床
DOI：
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发表时间：
1961
期刊：
影响因子：
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通讯作者：
A. Osanik
中科院分区：
文献类型：
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作者：
D. E. Frazier;A. Osanik
在密西西比河沿着老河（Old River）的一条如今已被废弃且部分填满的蜿蜒环路上，为一个通航船闸进行了一次大型挖掘，在船闸底部周围暴露出40英尺长的点坝砂垂直剖面，这些点坝砂沉积在平均低潮期以下。来自美国陆军工程兵团钻孔的数据使确定这种沙洲的极限及其与封闭冲积沉积物的关系成为可能。裸露的棒状矿床为岩屑砂岩，由分选良好的中至细粒砂组成，其中包含薄层卵石、浸渍的植物碎屑和稀疏的重矿物浓度。在暴露部分的上部有带装甲的泥球；木头碎片随处可见。整个暴露剖面的特点是花状交错层理——这是唯一存在的层理类型。花状交叉层的形成是对准同生沙洲沉积过程中伴随沙波向下游迁移的冲淤作用的响应。这个地区的河流发展历史是众所周知的；因此，有可能将测量到的沉积物的方向性与沉积时河流的当前方向联系起来；交错层的平均倾斜方向为东经南纬40°，与当地河道走向S非常吻合。晶粒取向为东南26°E。
ABSTRACT A large excavation for a navigation lock, dug in a now-abandoned and partly filled meander loop of the Mississippi River along Old River, exposed around its base a 40-foot vertical section of point-bar sands that had been deposited below mean low-water stage. Data from U. S. Army Corps of Engineers borings made it possible to determine limits of this bar sand and its relationship to enclosing alluvial deposits. The exposed bar deposit is a lithic arenite and is composed of well-sorted medium to fine-grained sands incorporating thin layers of pebbles, macerated plant debris, and sparse heavy-mineral concentrations. Armored mudballs are present in the upper portion of the exposed section; wood fragments are common throughout. The entire exposed section is characterized by festoon crossbedding--the only type of bedding present. The festoon crossbeds formed in response to penecontemporaneous scour-and-fill processes accompanying downstream migration of sand waves during bar deposition. History of river development in this area is well known; it was therefore possible to relate measured directional properties of the sand deposits to current directions of the river at the time of deposition; the average dip direction for the crossbeds was S. 40° E., in close agreement with local channel alignment--S. 45° E. Preferred grain orientation was S. 26° E.