Bristow CS, Bailey SD, Lancaster N
School of Earth Sciences, Birkbeck College, University of London, UK.
Nature. 2000 Jul 6;406(6791):56-9. doi: 10.1038/35017536.
Linear sand dunes--dunes that extend parallel to each other rather than in star-like or crescentic forms--are the most abundant type of desert sand dune. But because their development and their internal structure are poorly understood, they are rarely recognized in the rock record. Models of linear dune development have not been able to take into account the sub-surface structure of existing dunes, but have relied instead either on the extrapolation of short-term measurements of winds and sediment transport or on observations of near-surface internal sedimentary structures. From such studies, it has not been clear if linear dunes can migrate laterally. Here we present images produced by ground penetrating radar showing the three-dimensional sedimentary structure of a linear dune in the Namib sand sea, where some of the world's largest linear dunes are situated. These profiles show clear evidence for lateral migration in a linear dune. Moreover, the migration of a sinuous crest-line along the dune produces divergent sets of cross-stratification, which can become stacked as the dune height increases, and large linear dunes can support superimposed dunes that produce stacked sets of trough cross-stratification. These clear structural signatures of linear dunes should facilitate their recognition in geological records.
线性沙丘——彼此平行延伸而非呈星形或新月形的沙丘——是沙漠中最为常见的沙丘类型。但由于人们对其形成过程及内部结构了解甚少，它们在岩石记录中很少被识别出来。线性沙丘形成模型未能考虑现有沙丘的地下结构，而是依赖于对风和沉积物搬运的短期测量推断，或对近地表内部沉积结构的观测。基于这些研究，线性沙丘是否能够横向移动尚不清楚。在此，我们展示了由探地雷达生成的图像，这些图像呈现了纳米比亚沙海（世界上一些最大的线性沙丘所在地）中一个线性沙丘的三维沉积结构。这些剖面图清晰地证明了线性沙丘存在横向移动。此外，沿着沙丘蜿蜒的脊线移动会产生不同方向的交错层理组，随着沙丘高度增加，这些交错层理组可能会堆叠起来，大型线性沙丘还能支撑叠加沙丘，从而产生叠加的槽状交错层理组。线性沙丘这些清晰的结构特征应有助于在地质记录中识别它们。
