湘西松柏场地区寒武系车夫组风暴岩的沉积特征及其地质意义

TEMPESTITE IN THE CAMBRIAN CHEFU FORMATION IN THE SONGBAICHANG AREA OF NORTHWESTERN HUNAN PROVINCE

湘西松柏场寒武系车夫组为一套浅海相碳酸盐沉积,通过层序界面识别和沉积特征分析,将车夫组自下而上分为3个三级层序,其中第2个层序的低水位体系域和海侵体系域、第3个层序的海侵体系域中发育典型的风暴岩沉积。风暴沉积含有冲刷—充填构造、风暴砾屑层、风暴层理构造(丘状或洼状交错层理、粒序层理、浪成波痕和平行层理、水平层理等),包括5种岩相单元:砾屑灰岩段(A段)、发育水平层理的砂屑灰岩段(B段)、发育丘状交错层理的含砂屑粉屑灰岩段(C段)、发育水平层理或沙纹层理或波痕构造的粉屑灰岩段(D段)和薄层状泥晶微晶灰岩段(E段)。这些岩相单元组成ABCDE、ABDE、AD、ADE、AB、ABD、ACD共7个近源风暴岩类型和1个远源风暴岩类型DE。风暴岩的识别与发现表明该地区在寒武系车夫组沉积时期位于低纬度的风暴作用带,对湘西北寒武纪时期的古纬度与古板块演化、古地理及沉积学研究具有重要意义,同时为湘西北寒武纪地层的划分与对比、探讨该区风暴沉积模式的空间变化等提供了重要依据。

The Cambrian Chefu Formation in the Songbaichang are in northwestern Hunan Province consists of shallow-water carbonate deposits. On the basis of sequence stratigraphic analysis, the Chefu Formation can be divided into three third-order sequences, Tempestites have been found in the second and third third-order sequences. The tempestites are characterized by scour filling structures, storm bedding structures （e.g., hummocky cross stratifications, graded bedding, ripple marks, parallel bedding, horizontal bedding, etc）. They mainly consist of five types of lithofacies： A-calcirudite with scour filling structures; B-calcarenite with graded bedding or parallel bedding; C-calcarenite with homogeneous bedding; D-calcisiltite with horizontal bedding, ripple cross lamination, or ripple marks; E-micrite with homogeneous bedding. Eight facies associations have been recognized, including proximal storm （ABCDE, ABDE, AD, ADE, AB, ABD, and ACD） and the distal-storm （DE） facies associations. The discovery of the storm deposits indicates that northwestern Hunan Province was located in the low-latitude tropical area during the Cambrian Period. This interpretation has important significance for the palaeogeography and environments of South China during the Cambrian Period.