Lee-face Airflow, Depositional Types and Its Significance
Field measurement of surface airflow, sand transport and sedimentary p rocesses on the modern dunes (barchan dune, the main ridges and secondary ridges of networks dune) was conducted in southeastern Tengger Desert, to refine aeoli an cross-strata as an indicator of paleaowind direction. It has been found that the v elocity and direction of lee-side secondary airflow varied with dune morphology and primary wind direction and their interactions. Three types of lee-side seco n dary flow occurred in this example, separated flow, attached flow, and deflected flow. These flow is controlled at least two factors, lee-slop morphology and primary wind direction relative to dune crestline. Separated flow, characterized by lower wind velocity, typically occurred on the dunes with steeper (>20°) lee -slop in the conditions where primary wind direction is perpendicular to dune cres tl ine. Attached flow with higher velocity occurred on the gentle lee slop (<20 °) . The attached flow could be deflected in the oblique primary wind environment but not deflected in transverse flow conditions. The direction of the lee-side defl ec ted flow is in accordance with the direction of crestline of dunes regardless of the inci dent angle values of oblique wind. Further analysis has also found that the magn itude of lee-side secondary wind velocity is the cosine function of primary wind di r ection relative to dunes, which can be expressed by natural log cosine relations hip:Ul=Uc{K*ln〔cos(I)〕+A}.0<I<90 This agrees with the result of Tsoar and Sweet on longitudinal dune and barchan dune,respectively. According to measurement of sand transport rate and observation of sedimentar y processes and stratification types, the relation between primary wind directio n and aeolian cross-bedding attitude was also discussed. In transverse flow con d itions, lee-slops of dunes with steeper lee-slop controlled by separated flow and avalanching of sands is the only mechanism for lee slop deposits. Also, the resulting deposition would consist grainflow cross-strata, and direction of dune a dvance would be perpendicular to the lee face. In this case, the dip direction of cross bedding would be the same to the primary wind direction. In oblique flow condit ions, the deflected flow controlled the lee-slops and sands from stoss-slop not deposited but transported along the lee-slop. As a result, migrating wind rippl e or smaller dunes (in the case of dune lee-slop with enough size) were the only processes operating on the lee-slop except the narrow belt of upper slop near dune crest. The deposition resulting from such flow conditions would consist of cl imb ing ripple cross-lamina or compound cross strata with grainflow cross-strata s ep arated by second-order bounding surfaces. The dip direction of climbing ripple l amina is a function of underlying slops so that could not reflect the primary wi nd direction in most cases. The dip direction of the cross-strata resulting fro m superposed dunes on larger ones indicates only lee face secondary flow directio n and does not the primary wind direction. The dips of the second-order boundin g surface is in accordance with the orientation of lee slop could indicate primary wind direction in the case of complex transverse deposit but not to longitudinal or other complex bedforms. Accordingly, because of the variation of regional flow is filtrated by dune and flow interaction, the dip directions of most aeolian cross-strata except simple grainflow cross strata resulting from simple transve rse dunes could not accurately reflect paleaowind direction.
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