The Early Triassic Sequence Stratigraphic Framework in Southwestern Margin of Yangtze Platform
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摘要: 扬子地台西南缘早三叠世地层被划分为4个沉积层序。通过综合研究已取得的牙形石生物地层资料,与Haq等人提出的三叠纪牙形石生物时带进行了对比,初步定出了各沉积层序及体系域的界面年龄(层序1底界年龄251Ma;层序2底界年龄245.2Ma;层序3底界年龄243Ma;层序4底界年龄240.5Ma,顶界年龄239.4Ma),从而建立了区内分辨率较高的早三叠世地层格架。格架中层序组合特征在碳酸盐岩台地边缘显示得非常清楚,其中,层序1-层序2高水位体系域的台地边缘依次向陆后退,呈退积或超覆(overlap)型组合关系;层序3-层序4高水位体系域的台地边缘则依次向海推进,呈进积或退覆(offlap)型组合关系。这种组合特征主要受沉积盆地的构造沉降及2级海平面升降亚旋回的海面变化的控制,构造沉降加速和海面上升期形成的层序呈退积型组合关系;构造沉降平缓及海面下降期形成的层序呈进积型组合关系。此外,还研究了斜坡地带(贵阳改毛)层序1-层序3底部的碳氧同位素组成变化规律:海侵体系域-高水位体系域碳、氧同位素组成变化不大,δ13C主要为正值,但在高水位体系域顶部明显负向偏移;低水位体系域碳、氧同位素组成变化剧烈,且均为负值。这为地层划分提供了地球化学方面的依据。Abstract: The early Triassic lithostratigraphy in southwestern margin of Yangtze Platform is divided into 4 depositional sequences by studying the structure and stacking pattern of stratigraphic bodies, the features of unconformity-bound. The depositional sequences are subdivided into lowstand system tract, transgressive system tract and highstand system tract. After researching the data about conodonts obtained, and correlating them with Triassic conodont biozones established by Haq et al., the boundary ages of sequence and system tract are determined initially (lower boundary age of sequence 1 is 251Ma; lower boundary age of sequence 2 is 245.2Ma; lower boundary age of sequence 3 is 243Ma; lower boundary age is 240.5Ma, upper boundary age is 239.4Ma in sequence 4). Thus early Triassic stratigraphic framework with higher resolution is established in this area. The characters of sequence-combination in framework are displayed clearly in carbonate platform margin.The platform margin retreated successively toward continent during highstand system tracts of sequence 1-2, showing combinatorial relation overlapped. While platform margin advanced orderly in highstand systems tract stage of sequence 3-4, showing combinatorial relation offlaped. The characters of sequence-combinations mentioned over were controlled by the tectonic subsidence of sedimentary basin and the change of up-down subcycle of 2 order sea level. The sequences showed retrograding combinatorial relation when the tectonic subsidence was accelerated and sea level rised; on the contrary, the sequence appeared prograding combinatorial relation when the tectonic subsidence was gentle and sea level declined. In addition, the change law of carbon-oxygen isotope researched in sequence 1-bottom sequence 3 in slop zone (Gaimao, Guiyang) is as follow: The change of carbon-oxygen isotope composition isnot large in transgressive system tracts-highstand system tracts. δ13C is mainly positive, but it move obviously to negative direction in top highstand system tracts. The change of carbon-oxygen isotope composition is very big and negative number in lowstand system tracts. These geochemical characters provide basis for dividing stratigraphy.
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Key words:
- sequence stratigraphy /
- boundary age /
- stratigraphic framework /
- carbon-oxygen isotope
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