豫西登封地区寒武系第二统朱砂洞组生物成因的豹斑构造

Biogenic Leopard Patch Structures from the Zhushadong Formation(Cambrian Series 2),Dengfeng Area,Western Henan

豫西登封地区寒武系第二统朱砂洞组发育了大量形态极具特色的豹斑状构造。豹斑为灰-深灰色,呈管状、斑块状或网状分布,主要成分为半自形-自形的白云石;而基质为灰白-浅灰色,主要成分为微晶方解石,与豹斑界限明显。根据其形态及发育特征,这些豹斑构造可分为条带状、花斑状、网格状3种类型。根据白云石及方解石含量,含有这3类豹斑构造的岩性分别定名为云斑状灰岩、灰质斑状白云岩、斑状白云岩。研究表明,豹斑是由Thalassinoides造迹生物对沉积物扰动改造形成复杂的潜穴系统,并在成岩期优先发生白云岩交代作用所致。朱砂洞组发育的豹斑自下而上从条带状经花斑状演化为网格状,反映其沉积时的水动力逐渐降低、环境更加稳定,潜穴系统趋于复杂化、生物扰动强度增大,后期白云化程度也逐渐增强。

Abundant distinctive leopard patch structures occur in the Zhushadong Formation from Dengfeng area, western Henan. The leopard patches, gray to dark gray in color, display tubal, piebald and grid distribution and mainly consist of subidiomorphic and idiomorphic dolomite. The matrix, grey-white to light gray in color, mainly consists of microcrystalline calcite and has the clear boundaries with leopard patches. The leopard patch structures can be divided into three types： banded, piebald and grid structures based on their morphological and development characteristics. The three types of leopard patch structures can respectively belong to dolomitic patchy limestone, calcareous patchy dolostone and patchy dolostone. The study shows that the leopard patch structures were produced by the diagenetic dolomite replacement which preferentially began within the complex burrow systems built by Thalassinoides trace-makers. The leopard patch in the Zhushadong Formation evolved from the banded structures via piebald into grid structures from the bottom to up, reflecting the gradually reducing water energy, more stable environment, more complex burrow systems, increasing bioturbation intensity and enhancing diagenetic dolomitization.