黔中织金下寒武统牛蹄塘组黑色页岩中的白云岩及其沉积环境意义

THE DOLOSTONE IN THE LOWER CAMBRIAN NIUTITANG FORMATION IN THE ZHIJIN REGION OF CENTRAL GUIZHOU AND ITS SEDIMENTARY ENVIRONMENT SIGNIFICANCES

黔中织金地区以黑色页岩为特征的下寒武统牛蹄塘组中部发育有一套厚约9.2m的白云岩,且在白云岩中由下至上发育有较强水动力成因的小型冲刷构造、小型槽状交错层理以及水动力相对较弱的低角度交错层理和水平层理,表明黔中地区早寒武世牛蹄塘组沉积期曾经出现过短暂的、碳酸盐浅滩相的沉积环境,同时也揭示了早寒武世牛蹄塘组沉积期并非一直处于滞留、低能的还原环境。另外,牛蹄塘组底部黑色页岩之下的紫红色泥岩以及明心寺组底部的粉砂岩整合于牛蹄塘组顶部的黑色页岩,表明牛蹄塘组沉积早期和晚期水体也相对较浅。因此,黔中织金地区牛蹄塘组中白云岩的发现,表明了在牛蹄塘组沉积期至少存在二次以上的三级海平面升降变化,为研究全球海平面的变化提供了新的材料。

A 9.2 m thick dolostone unit is present in the middle Niutitang Formation （lower Cambrian）, which is characterized by black shale, in the Zhijin region of central Guizhou province. Small-scale scour structures, trough cross bedding, and lower angle cross bedding and horizontal bedding in this dolostone unit indicate deposition in a carbonate shoal facies during the Niutitang time, rather than low energy reduc- ing environments. In addition, the occurrence of purple mudstone underlying basal Niutitang black shale and siltstone in the overlying Mingxinsi Formation suggest that the lower and upper Niutitang Formation were also deposited in relatively shallow water depths. The presence of dolostone in the Niutitang Forma- tion in the Zhijin region of Central Guizhou shows that there are at least two sea-level changes in the Niuti- tang depositional stage, and provides new data constraining global sea-level changes in the Cambrian Period.