TREND SURFACE ANALYSIS OF THE CONTROL OF PALEOTECTONICS TO COAL ACCUMULATION OF YANAN FORMATION IN ORDOS BASIN

The result of trend surface analyses of two geological variables， the general thickness of strata and the one of coal beds of Yanan formation， indicates that forming and developing of coal-bearing basin in early-middle Jurassic were controled mainly by the paleotectonic pressure stress field of Yanshan movement． The general spread direction of coalbearing basin was NE35°， which was in accordence with the one of regional coal-rich belts． A evident separate line existed from Wuzhong， Yanchi， to Suide （near north latitude 38°）：in the north of it， the spread direction of secondary tectonics evidently in a line with the one of secondary thicking and thinning belts of coal beds， and in the south of it， the relationship between them was more complexed， which reflected the important influence of paleotopographic features formed by paleotectonic pattern of Indo-China movement．