湖南下寒武统层序地层特征与古环境演化变迁
Characteristics of Sequence Stratigraphy and Plaeoenvironmental Evolution of Lower Cambrian strata in Hunan Province
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摘要: 提要：湖南下寒武统发育完整，湘西北和湘中地区主要发育一套从黑色岩系到碳酸盐岩的地层序列，湘南地区相变为一套以杂砂岩和炭质板岩不等厚韵律层为主的地层序列，整体构成一个完整的二级层序，可进一步划分为6个三级层序，对应6个有序的海进-海退沉积旋回，其中三级层序的沉积体系域受沉积古环境的影响由“凝缩段（CS）+高位体系域（HST）”和“海进体系域（TST）+凝缩段（CS）+高位体系域（HST）”两种垂向叠加形式组成，下寒武统的层序界面类型主要为Ⅱ型层序界面和淹没不整合型层序界面。在空间上，湘西北和湘中地区为由NW向SE的缓坡环境，随着沉积环境逐渐加深，下寒武统沉积厚度逐渐变薄，至湘南地区沉积厚度增厚，沉积深海相厚层杂砂岩夹板岩，其相序变化系统记录了湖南省早寒武世复杂的被动陆缘古地理环境演化和变迁，其演化和变迁也反映了古环境演化对成矿作用的制约。Abstract: Abstract：Lower Cambrian strata are well developed in Hunan. With the main sedimentary formations of black shale and carbonates in western and central Hunan and a suite of greywacke and carbonaceous slate with different thicknesses of rhythmic layers in southern area, the Lower Cambrian in the study area constitutes a second-order sequence, which can be further divided into six third-order sequences, corresponding to six orderly transgression-regression cycles. Impacted by palaeosedimentary environment, the depositional system tracts of the third-order sequences are formed by the vertical sequences of “CS + HST” and “TST (transgression system tract) + CS + HST”, with the major sequence boundaries being II type sequence boundary and the drowning-type sequence boundary. The Lower Cambrian sedimentary formation resulted from continental slope-ramp in the northwestern and central parts of Hunan; from northwest to southeast, with the gradual deepening of the sedimentary environment, the total thickness of the Lower Cambrian sedimentary formation becomes thinned. In the southern Hunan, in contrast, a suite of greywacke and carbonaceous slate of abyssal facies was deposited and becomes thickened remarkably. The sequence-stratigraphic framework records the complex palaeoenvironment evolution of passive margin in the Early Cambrian in Hunan and also reflects the restriction of the metallogenesis from palaeoenvironment evolution.
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