冀中坳陷束鹿凹陷古近系沙河街组三段泥灰岩成因探讨

Discussion on origin for marls of the Member 3 of Shahejie Formation of Paleogene in Shulu Sag of Central Hebei Depression

冀中坳陷束鹿凹陷中部古近系沙河街组三段下部(简称沙三下)泥灰岩属一类特殊的岩石类型组合,既非典型的碳酸盐岩,又非经典的碎屑岩。在进行了详细的岩心、薄片观察和各种分析化验资料的综合分析基础上,认识到研究区岩石为混积岩类,将混积岩(广义的)、混合沉积的概念和体系引入沉积物类型和岩相的划分中,为泥灰岩的科学定名提供了依据。据此对混积岩的类型进行了细致的划分,共划分出3大类型:结构混合、互层混合和夹层混合。每一类可以再细分为多种,其中结构混合包括①陆源组分为主的混积岩,②以内源组分为主的混积岩。互层混合成因的混积岩是最重要的岩石类型,本区所谓的纹层状泥晶灰岩大部分属于这一沉积类型,主要由灰质泥岩或泥灰岩与泥晶灰岩组成明暗相间的条带,有时含丰富的有机质。互层出现的形式是多种多样的,依据其岩石中原地组分和异地组分的含量不同,可进一步划分为4种类型。夹层混合沉积,表现为巨厚的碳酸盐岩质砾岩层夹前两类混积岩。构造、气候及物源等是控制混积岩形成的主要因素。

A special type of rock suite was discovered in the lower part of the Member 3 of Shahejie Formation of Paleogene in the Shulu Sag of Central Hebei Depression, Hebei Province, which is neither typical carbonate rocks nor classic clastic rocks. Based on detailed observations of cores and thin sections combined with comprehensive analyses of testing data, it is known as the hunji rock. Hunji rock （in broad sense） as well as the concept and system about mixing deposits were introduced into the classification of sediment type and lithofacies and thus provide proofs for scientific definition of marl. According to their origin, hunji rock can be divided into three types： textural mixing, interbedded mixing and interlayered mixing deposits. Each type can be subdivided into several kinds： textural mixing deposits include rocks mainly composed of terrigenous components and rocks that mostly consist of intrabasinal composition. Interbedded mixing deposits is the most important types of rocks. Laminated microcrystalline limestones are mainly composed of limy-mudstone, marl and micrite which make up light and dark ribbons with organicrich materials sometimes. The arranges of lamina are various and can be subdivided into 4 types in terms of the different contents of composition in situ and allogene composition. Interlayered mixing deposit consists of thickly-bedded conglomerate （carbonate breccia） intercalated with the former two types of hunji rock Tectonics, climate and provenance are the principal factors for controlling the formation of Hunji Rock.