The Influence of Bedrock Geology on Knickpoint Development and Channel-Bed Degradation along Downcutting Streams in South-Central Indiana
The Influence of Bedrock Geology on Knickpoint Development and Channel-Bed Degradation along Downcutting Streams in South-Central Indiana
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DOI：
10.1086/629519
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发表时间：
1991-07
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影响因子：
--
通讯作者：
Jerry R. Miller
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文献类型：
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作者：
Jerry R. Miller
通过比较印第安纳州中南部在岩性“均质”碳酸盐单元和层状硅质碎屑地层中发育的下切河流，阐明了对拐点发育的地质控制和河道床退化的机制。单个岩性单元中形成的断点呈阶梯状；每个台阶均由一个小点面和胎面组成。每个台阶的高度等于床层厚度并由床层厚度控制。在空间上，“均质”地形中的拐点仅限于以下倾斜的节理地层：(1) 上游，或 (2) 下游，角度小于河道梯度。在岩性层状地层中观察到的缺口点的特征是单一台阶，其中抗性节理岩石覆盖并保护较弱的单元。总的缺口点浮雕受到较弱地层厚度的影响。层状硅质碎屑地层中的拐点在空间上仅限于因河道切割速率不同而产生的基岩河段。拐点分布还受到节理频率、岩性分层以及地层倾角与水流方向之间的关系的影响。河道切口是由于基岩地层沿着层理面不连续性的侵蚀而发生的，并且通常沿倾角方向进行。对退化的地质控制导致了独特类型的河床形态。人们普遍认为，下切流是不平衡系统，表现出快速演变的地貌。然而，下切河道似乎形成了一种平衡形式（由基岩地质决定），这种平衡形式一直维持到发生基准面降低的长期（数百年）剖面调整为止。
Geologic controls on knickpoint development and the mechanics of channel-bed degradation were eluci-dated by comparison of downcutting streams in south-central Indiana developed in lithologically "homoge-neous" carbonate units and layered siliciclastic strata. Knickpoints formed in a single lithologic unit are stepped; each step consists of a knickpoint face and tread. The height of each step is equal to and controlled by bed thickness. Spatially, knickpoints in "homogeneous" terrain are limited to jointed strata that dip: (1) upstream, or (2) downstream at angles less than the channel gradient. Knickpoints observed in lithologically layered strata are characterized by a single step in which resistant jointed rocks overlie and protect weaker units. Total knickpoint relief is influenced by thickness of the weaker strata. Knickpoints in layered siliciclastic strata are spatially limited to bedrock reaches produced by differential rates of channel incision. Knickpoint distribution is also influenced by joint frequency, lithologic layering, and the relations between strata dip and stream flow direction. Channel incision occurs by the erosion of bedrock strata along bedding plane discontinuities and typically proceeds in the direction of dip. Geologic controls imposed on degradation lead to distinctive types of channel bed morphology. It is generally agreed that downcutting streams are disequilibrium systems that exhibit rapidly evolving landforms. However, the downcutting channels appear to develop an equilibrium form (determined by bedrock geology) maintained until longer-term (hundreds of years) profile adjustments to base level lowering can occur.