Crevasse splays from the rapidly aggrading, sand‐bed, braided Niobrara River, Nebraska: effect of base‐level rise
Crevasse splays from the rapidly aggrading, sand‐bed, braided Niobrara River, Nebraska: effect of base‐level rise
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DOI：
10.1046/j.1365-3091.1999.00263.x
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发表时间：
1999-12
期刊：
影响因子：
3.5
通讯作者：
Bristow;Skelly;Ethridge
中科院分区：
文献类型：
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作者：
Bristow;Skelly;Ethridge
Niobrara河上基准面上升1.22·35 m，导致河道带淤积和最近的一次撕脱。漫滩地区已被上升的地下水淹没，1993年至1997年期间形成了8个以上的裂缝。位于Niobrara西部和东部的两个裂缝扇已经使用探地雷达（GPR），浅钻孔和地形测量进行了研究。振动孔和探地雷达剖面提供了一个接近三维的决口扇沉积物结构视图。东扇于1993/94年冬季启动，现已扩大至覆盖面积约200 m x 1000 m，沉积物厚度达2.5 m。1995年秋季，一条决口通道通过一个堤坝打开，形成了西向扇，面积为150 m × 250 m，一年内沉积了多达1.2 m的沙子。Niobrara扇体以砂为主，其特征是在5-30 m宽、0.5 -2 m深的水道内推移质沉积，并发育有滑面，在滑面处扇体进入静止水体。岩心中的沉积构造包括水平纹层、涟漪纹层和交错层理组。坡折带有轻微变粗的趋势，但坡折带内的个别岩层经常变细。Niobrara河上大量的裂缝与其他辫状河漫滩形成对比。在Niobrara，决口扇形成后，河道带内的加积，这是对基准面上升的反应。决口扇、河道加积和基准面上升之间的联系对古辫状河和洪泛平原序列的解释具有重要意义。建议决口扇沉积应是加积河流沉积物的重要组成部分，因此，应保存在岩石记录。在这种情况下，加积和裂缝与基准面高程的上升有关，建议在辫状河受基准面上升影响的地方，例如在海侵和古河谷填充期间，应保留类似的沉积物。
Base‐level rise of ≈2·35 m on the Niobrara River has resulted in aggradation of the channel belt and a recent avulsion. Overbank areas have become flooded by rising groundwaters, and more than eight crevasse splays have formed between 1993 and 1997. Two crevasse splays, situated on the west and east sides of the Niobrara, have been studied using ground‐penetrating radar (GPR), shallow boreholes and topographic surveys. The vibracores and GPR profiles provide a nearly three‐dimensional view of the architecture of crevasse splay deposits. The east splay was initiated in the winter of 1993/94 and has expanded to cover an area ≈200 m by 1000 m, with sediment up to 2·5 m thick. The west splay, which was initiated by the opening of a crevasse channel through a levee in the autumn of 1995, covers an area ≈150 m by 250 m, with up to 1·2 m of sand deposited in a single year. The Niobrara splays are sand dominated and characterized by bedload deposition within channels, 5–30 m wide and 0·5–2 m deep, with the development of slipfaces where splays prograde into standing bodies of water. Sedimentary structures in cores include horizontal lamination, ripple lamination and sets of cross‐stratification. There is a slight tendency for splays to coarsen up, but individual beds within the splays often fine up. The abundance of crevasse splays on the Niobrara River contrasts with other braided river floodplains. In the Niobrara, crevasse splay formation followed aggradation within the channel belt, which occurred in response to base‐level rise. The link between crevasse splays, channel aggradation and base‐level rise has important implications for the interpretation of ancient braided river and floodplain sequences. It is suggested that crevasse splay deposits should be an important component of aggrading fluvial sediments and, hence, should be preserved within the rock record. In this case, the aggradation and crevassing have been tied to a rise in base‐level elevation, and it is suggested that similar deposits should be preserved where braided rivers are affected by base‐level rise, for instance during transgression and filling of palaeovalleys.