Late Jurassic clastic deposits typically assigned to the Tuchengzi or Houcheng formations are widespread in the Yanshan belt of North China. This paper questions the validity of Yanshan-wide stratigraphic correlation of "Tuchengzi/Houcheng" units and the underlying "Tiaojishan/Lanqi" and overlying "Zhangjiakou/Donglingtai" volcanic units. Recently published 40Ar/39Ar and U-Pb (zircon) ages, which other workers have assigned to these stratigraphic units, have considerable overlap as follows:Tiaojishan/Lanqi, ca.175～147 Ma; Tuchengzi/Houcheng, ca.156～139 Ma; Zhangjiakou, ca. 147～127 Ma. These ages suggest problems in stratigraphic correlation, indicate that there is considerable diachroneity in the timing of volcanism and sedimentation in the Middle and Late Jurassic Yanshan orogen, and cast doubt on the traditional interpretation that the Tiaojishan-Tuchengzi sequence is bounded by regional unconformities dated at ca. (165±5) Ma and (135±5) Ma. Although some Tiaojishan volcanism and Tuchengzi/Houcheng sedimentation accompanied thrust faulting, both north- and south-directed, in general the 30～35 Ma time interval during Tiaojishan-Tuchengzi deposition appears to have been tectonically rather quiescent. This conclusion is based on the common description of the contacts between and within "Tiaojishan" and "Tuchengzi" units as being disconformable and concordant. The author accepts recent interpretations based on an analysis of Tuchengzi strata in the Chengde basin that the Chengde thrust must have a considerably smaller displacement than that proposed by Davis, et al. (2001). He disagrees that Tuchengzi sedimentation in the Chengde area was primarily in response to major south-directed thrusting north of Chengde. Instead the current Chengde synclinal basin is almost certainly the consequence of footwall deformation below the north-directed Chengde County thrust as indicated by (1)its northward-overturned geometry, and (2) the influx into the southern part of the basin of coarse sediment whose protolith appears to be the Chengde County allochthon. Structural and/or tectonic controls on all Tuchengzi/Houcheng sedimentation are not required. Such sedimentation followed a 20～25 Ma-long period of Tiaojishan/Lanqi volcanism and was accompanied by plutonic emplacement (ca.158～141 Ma) in some Yanshan areas. The Middle to Late Jurassic Yanshan magmatic arc must have had considerable relief, and this topography would have been quite favorable for rapid erosion and coarse clastic sedimentation. Finally, there is no basis for correlation of Tuchengzi/Houcheng strata in the Yanshan fold-thrust belt with Daqingshan Formation strata in the Yinshan belt as has been proposed.%土城子组/后城组为广泛分布在中国北方的燕山褶皱冲断带和盆地中晚侏罗世的典型碎屑岩沉积.本文主要是针对目前在燕山地区的通行的有关土城子组/后城组、及其之下的髫髻山组/ 蓝旗组,和上覆的张家口组/东岭台组火山岩的相关对比方法提出质疑.其他同行近期发表相关的氩-氩法和铀-铅法同位素测年数据指出髫髻山组/蓝旗组年龄为175～147 Ma、土城子组/后城组年龄为156～139 Ma、张家口组/东岭台组年龄为147～127 Ma,显而易见,上述地层组的年龄是相互重叠的.这些测年数据说明以往的地层对比是有问题的,燕山造山带在中、晚侏罗世所发育的火山岩和沉积岩地层是穿时的.因此,传统上用(165±5) Ma 和(135±5) Ma之间的区域不整合来作为划分髫髻山组和后城组的层序界限是值得商榷的.尽管一些髫髻山组的火山岩和土城子组/后城组的沉积岩是与向南或向北的冲断作用相伴生的,但在髫髻山组和土城子组/后城组沉积之间的30～35 Ma的时间间隔内却是相对的构造平静期.这一结论是基于以往的髫髻山组和土城子组之间为假整合或平行不整合的观点所得出的.新近基于对承德盆地土城子组地层形成研究分析认为承德冲断层的实际位移距离应小于Davis等2001年所提出的位移距离,笔者接受这一观点.但笔者并不同意在承德地区土城子组的沉积主要是受控于承德北部的向南冲断作用.现今承德向形盆地主要是由于向北冲断的承德县冲断层下盘变形的结果, 主要是(1) 它向北发生倒转;(2) 盆地南部的粗碎屑沉积的物源主要是来源于承德县的异地体.土城子组/后城组的沉积没有必要完全受控于构造作用.土城子组/后城组的沉积是紧随着在燕山部分地区发生的,持续了20～25 Ma 的髫髻山组/蓝旗组火山及岩浆活动.在中、晚侏罗世期间,燕山地区的岩浆活动必定导致地形的起伏,这就为快速剥蚀及粗碎屑的沉积提供了有利条件.最后需要指出的是,从前所提及的有关燕山带的土城子组/后城组和阴山带的大青山组的地层对比的依据并不存在.
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