西藏当雄九子拉地区石炭系诺错组中震积岩的发现及意义

西藏当雄九子拉地区石炭系诺错组地层序列中,首次发现了一套非正常沉积的粉砂岩和灰岩的组合.岩系中可识别出非正常的沉积构造,即微同沉积断裂、火焰状构造、同生变形构造及振动液化泄水构造等,这一软沉积变形构造具震积岩的识别标志特征,代表灾变事件震积的地质记录.在剖面结构上大致显示为7个单元的震积序列,自下而上由下伏弱震层(A)—液化脉(B)—震褶岩(C)—震裂岩(D)—阶梯状微断层(E)—火焰状构造(F)—上覆未震层(G)的渐变过程及震积作用逐渐减弱的趋势呈不同形式的组合出现,有ABG、ACDG、AEG、ABCG和ABFG等组合.本研究分析认为晚古生代诺错组早期存在一个强烈的构造活动时期,结合震积岩的分布及区域地质背景研究表明,九子拉北侧一带可能为近东西向同生断裂带,该断裂带的强烈构造活动是本区诺错组震积作用的直接诱发因素.从而佐证了九子拉地区正处于构造转换阶段.本研究对分析晚古生代的构造演化、沉积盆地类型奠定了基础,同时保存完好的震积事件地层记录对推断古断裂、古环境也具有较大意义.     

Sedimentological analysis of two drill cores through the crater moat‐filling breccia,Flynn Creek impact structure,Tennessee

Sedimentological (line‐logging) analysis of two drill cores, FC77‐3 and FC67‐3, situated, respectively, in the northwestern and southeastern quadrants of the Flynn Creek impact structure's crater‐moat area reveals that the ~27 m thick crater moat‐filling breccia consists of three subequal parts. These parts, which were deposited during early modification stage of this marine‐target impact structure, are distinguished on the basis of vertical trends in sorting, grain size, and counts of clasts per meter in comparison with other well‐known marine‐target impact structures, namely Lockne, Tvären, and Chesapeake Bay. The lower part is interpreted to represent mainly slump deposits, and the middle part is interpreted to represent a stage intermediate between slump and marine resurge, that is, a traction flow driven by overriding suspension flow. The upper part (size graded, and relatively well sorted and fine grained) is interpreted to represent marine resurge flow only. The upper part is capped by a relatively thin and relatively fine‐grained calcarenite to calcisiltite deposit.