LATE MIDDLE JURASSIC AEOLIAN DEPOSITS NEAR BEIPIAO,LIAONING

In accordance with our study,the ugper part of the middle Jurassic, the Nanling Group, in Beipiao district of Liaoning, is apparently- of Aeloian feature,either on sedimentary structure or texture, and it is affirmed that the Group is formed under a desert environment, mainly of aeloian, and in addition, carrying-water deposition.The deposit assemblage that is a multiple terrestrial detritus formation, may be subdivided into three members--the lower one, the middle one and the upper one. The lower member and the upper member consist of sandstone and conglomerate of multiple composition, which are mainly wadi deposits with lower maturity and intercalated with Aeolian matters. The middle member is composed of well-sorted quartz sandstone of Aeolian dune deposit, locally intercalated mith mudstone of small oasis deposit. On sequent, these constitute a symmetrical texture with the middle member as its main part, On lithololy, this Group is composed primarily of purplish red, dark purple and grey-white quartz sandstone, and secondarily, the detritus arkose, arkose and conglomerate, involving a great quantity of ventifacts within it. The composition texture maturity of quartz sandstone is relatively high, often showing a mono-mode and a bi-mode distribution. In general, it consists of 3-4 grades, mainly the medium-fine ones. The surface of quartz sand graitt in smooth, coated with desert varnish, and the ventifaces is possessed of pitted surface. All results determined from the cumulative probability curve, the sorting coefficient, the grade analysis and the roundness and sphericity, show that the depositional character is identical with that of modern desert.Sandstone of middle member develops tabulate and trough-shaped cross-bedding of big size (thickness of a set may be of 10-30m), The fine beds dip gently(10-15° in general) and there are also steeper ones, but the direction of inclination is variable. The foresets are steep in the upper part and gentle in the lower part.They tend to be horizontal near the bottom of a set and some fine beds expand radially in bundle form. There is often lagging ventifact deposit in the bottom of a set.Then, this area is a small erg ettvironment and a good continental drainage area formed on active tectonic background of the Nantianmen fault. It is situated in the tropicsubtropic zone(the paleo-equator being situated in 45-50° of modern north latitude). Owing to the effect of northeastward monsoon, it has formed hot desert environment of frequent storm. According to the development character(i.e. the progressive decrease in thickness and scale of cross-bedding sets etc. )of each members of the upper \anling Group, it shows that the climate of this area approaches wet and the desert environment is progressively replaced by a lake condition at the end of middle Jurassic。