一种新的错时相沉积物——巨鲕及其环境意义

Origin and Environmental Significance of Giant Ooids in the Early Triassic:a new kind of anachronistic facies

华南下三叠统缓坡背景的地层中广泛发育高能滩相鲕粒灰岩,其中含相当数量的巨鲕,粒径大部分在2～6mm之间,最大可达12 mm。巨鲕的核心类型以多晶粒状方解石和球粒为主,具明暗相间的同心纹层,沿切线方向定向排列的晶体结构表明其原生矿物可能为文石。含巨鲕灰岩与生物碎屑灰岩交互沉积,指示了当时海平面的相对变化。巨鲕在新元古代地层中广泛发育,但在显生宙和现代海洋环镜中超过5 mm的鲕粒相当少见。巨鲕在二叠纪末灭绝事件后大量出现,随着中三叠世生态的复苏、海洋环境趋于正常而又消失。这些特征说明巨鲕可以作为一种错时相沉积物,它的形成无疑与当时的生态和海水环境有关,代表着一类特殊的沉积学现象。分析了巨鲕产出背景、微观结构和沉积特征,对于了解其成因、探索二叠纪—三叠纪之交地质突变期异常的生态状况和特殊的海洋环境具有重要意义。

Oolitic beach facies of high-energy was widely distributed in the Lower Triassic Strata of South China under the background of carbonate ramp,particularly within a certain quantity of giant ooids.Examined the sizes of giant ooids are mostly between 2mm and 6mm,some up to 12mm,and the nucleus predominantly consist of granular calcite and peloid.All giant ooids exhibit concentric laminae and tangentially oriented crystals,suggesting their originally aragonitic mineralogy.Unites of giant ooids-bearing oolitic and bioclastic limestone deposited alternately,reflected the relative sea-level changes at that time.The occurrence of such giant ooids which diameters over 5mm are commonly developed in the Neoproterozoic,however,were extremely uncommon after the Cambrian.Numerous giant ooids were widespread occured after the end-Permian mass extinction,yet eliminated following the radiation of the Mesozoic biotas and the normalization of marine environment.It is implied that giant ooid is a new kind of anachronistic facies,and its occurrence apparently relate to biotas and marine environment at that time.This paper is based on detailed analysis of production background,microscopic structure and sedimentary characteristics of giant ooids,therefore,it is available to aid our understanding of their origin,explore the harsh marine environment and the particular ecosystems under the Paleozoic-Mesozoic transitional events.