豹皮(斑)状碳酸盐岩分类与成因初探:以鄂尔多斯盆地马家沟组四段为例

Classification and genesis of leopard skin(spot)carbonate rock:a case study of the Member 4 of Majiagou Formation in Ordos Basin

豹皮(斑)灰岩是华北寒武系—奥陶系广泛发育的一种特殊形貌的海相碳酸盐岩,因具有显著的识别特征和成因机制而备受关注。鄂尔多斯盆地中奥陶统马家沟组四段发育丰富的豹皮(斑)碳酸盐岩,目前已在其中识别出遗迹化石7属8种,分别是Thalssinoides network,Thalassinoides boxwork,Planolites isp.,Palaeophycus tubularis,Phycodes isp.,Chondrites isp.,Skolithos isp., Zoophycos isp.,其以Thalassinoides的2种形态类型(水平迷宫形network和三维箱状结构boxwork)占主导,属于固底底质条件的Glossifungites遗迹相。通过野外露头、岩心的宏观和微观观察,系统调研前人关于生物扰动成因碳酸盐岩的类型,依据生物扰动、生物潜穴形态特征和方解石/白云石相对含量,笔者提出了生物扰动成因豹皮(斑)状碳酸盐岩的分类方案,将其主要分为豹皮灰岩和豹斑云岩2个大类,分别对应Thalassinoides network(水平二维迷宫)和Thalassinoides boxwork(三维箱状结构)2种形态类型。鄂尔多斯盆地马家沟组四段的豹皮(斑)状碳酸盐岩主要受生物扰动和高频海平面升降下的早期成岩作用耦合控制,表现为在固底底质条件下,生物潜穴为富Mg2+卤水的白云石化流体提供了良好的运移通道,使得潜穴内部优先发生白云石化作用,而不同类型的生物潜穴及准同生和埋藏白云石化作用,最终形成了豹皮灰岩和豹斑云岩这2种类型的生物扰动成因豹皮(斑)状碳酸盐岩。

Leopard skin(spot)limestone,a type of marine carbonate rock with unique morphology,is widely distributed in the Cambrian-Ordovician strata of North China.It has attracted significant attention due to its distinct characteristics and formation mechanism.Abundant leopard-skin(spot)carbonate rocks developed in Member 4 of the Middle Ordovician Majiagou Formation in the Ordos Basin.Eight species of 7 genera have been identified,including Thalssinoides network,Thalassinoides boxwork,Planolites isp.,Palaeophycus tubularis,Phycodes isp.,Chondrites isp.,Skolithos isp.,?Zoophycos isp.Two morphologic types of Thalassinoides(horizontal enigmatic network and three-dimensional boxwork)are predominant,belonging to the Glossifungites ichnofacies under firm ground condition.Based on detailed macroscopic and microscopic characteristics of outcrops and drilling cores,this paper systematically summarizes the types of carbonate rocks caused by bioturbation.By examining the morphological characteristics of bioturbation and burrow,as well as the calcite/dolomite content,this paper proposes a classification for leopard-skin(spot)carbonate rocks formed by bioturbation.These rocks are categorized into leopard-skin limestone and leopard-spot dolomite,corresponding to the Thalassinoides network(two-dimensional horizontal mystery palace)and the Thalassinoides boxwork(three-dimensional box-like structure),respectively.The leopard-skin(spot)carbonate rocks in Member 4 of the Majiagou Formation in the Ordos Basin are primarily controlled by the interaction of bioturbation and early diagenesis during high-frequency sea level fluctuations.The bioturbated burrows in the firm substrates creat pathways for Mg2+rich brines,promoting dolomitization within the burrows.Two distinct types of leopard-skin(spot)carbonate rocks,induced by bioturbation,were generated under varying bioturbation types,parasyngenesis,and buried dolomitization.