Formation of sedimentary structures in flume experiments under upper-flow-regime conditions

为了在上流态条件下生产平面床，我们在东京大学海洋研究所用15厘米宽、700厘米长的水力水槽进行了多次试验。我们成功地产生了两种类型的平面床：1)在近临界或亚临界流动(弗劳德数Fr=0.94；类型1)下，低起伏不对称波纹迁移形成并埋藏的纹层；2)在超临界流动(Fr=1.98；类型2)下，不形成任何低起伏床型的纹层。在制作了这两种类型的平面床后，研究了碎屑颗粒沿每种平面床的叠置情况。砂粒大小的石英沉积物(平均直径0.72 mm)在类型2中表现出高的叠置角度(高达30ﾟ)，而在类型1中表现出很低的叠置角度(接近0ﾟ)。两种类型平面床的叠置角度的差异可能有助于推断野外观测到的平面床的个别形成过程。

In an attempt to produce plane beds under upper-flow-regime conditions, we have conducted a number of runs (experiments) using a hydraulic flume of 15 cm wide and 700 cm long, at the Ocean Research Institute, University of Tokyo. We have succeeeded in producing two types of plane beds : 1) laminae formed and buried by the migration of low-relief asymmetric ripples under near- or sub-critical flow (Froude Number Fr=0.94 ; type 1), and 2) laminae formed without causing any low-relief bedform but under super-critical flow (Fr=1.98 ; type 2). After making these two types of plane beds, imbrication of detrital grains along each plane bed was investigated. The sediments of sand-sized quartz grains (mean diameter 0.72 mm) show a high angle of imbrication (up to 30ﾟ) in type 2, whereas very low angle of imbrication (close to 0ﾟ) in type 1. Differences of imbrication angle between the two types of plane bed may be useful to infer the individual formational process of plane beds that are observed in the field.