Quantitative characterization of hydrodynamic conditions in the scour zones of the lower Zhiluo Formation in Shenfu south area

Palaeochannel scouring zone is the key geological factor causing water damage in coal mining. However,there are few studies on the quantitative characterization of palaeohydrodynamic conditions of palaeochannel scour zone worldwide,which limits the indepth understanding and accurate prediction of the formation process of palaeochannel scour zone in the coalbearing rock series. Taking zhiluo Formation in Shenfu South area of northeastern Ordos Basin as an example,this paper quantifies the paleohydrodynamic conditions of the formation of the palaeochannel scour belt by means of grain size analysis. It is found that four sedimentary cycles a,b,c and d can be identified in the lower member of Zhiluo Formation from bottom to top,from a cycle to d cycle. The average particle size of histogram is gradually distributed and concentrated. The probability accumulation curve is dominated by two sections,and the suspended total content increases gradually. The particle size(Mz)is mainly fine sand,with a small amount of very fine sand and coarse sand. The standard deviation(σ)ranges from 0.50 to 1.16,indicating that the sorting is medium to good. The skewness(Sk)is mainly symmetrical and positive skewness. Kurtosis(Kg)is mainly moderate to sharp. On C-M graph it corresponds to PQ and QR segment. The a cycle back to c cycle is the distributary channel deposits in the plain of the braided river delta,while d cycle is the distributary channel deposits in the plain of the meandering river delta. The calculation of paleohydrodynamic conditions with grain size parameters and sedimentary environment as constraints shows that the average starting velocity of gravel deposition at the bottom of a cycle is 1.78 m/s,and the Froude number is 1.05,which belongs to the highflow fluvial transport. The starting velocity of sandstone deposition in a cycle back to d cycle is 0.27,0.26,0.25 and 0.25 m/s respectively,the Froude number is 0.16,0.15,0.14 and 0.14,belonging to low flow state river transport. The conglomerate deposition period of the lower member of Zhiluo Formation is the key period for the formation of channel scour zone,and the sandstone deposition period is the filling of the ancient valley formed in the early stage.