辽东大连金石滩新元古代碳酸盐岩臼齿构造形态及其沉积环境指示意义

Morphology of molar-tooth structures in Neoproterozoic and its indication significance for the depositional environment of Jinshitan area in Dalian,eastern Liaoning Province

辽东大连金石滩新元古代兴民村组上部灰岩段发育了一系列形态复杂多样的臼齿构造(Molar-tooth struc-ture,简称MT)。通过野外及室内综合研究鉴别出条带状、短直杆状、蠕虫状、细丝状、圆斑状(气泡状、瘤状)和碎屑状6种主要类型,其中以条带状最为发育。MT发育段主要由含砂屑或粉屑灰岩、泥晶灰岩、纹层状泥晶灰岩和含泥泥晶灰岩的韵律组成,系多个向上沉积动力减弱、水体变浅的潮下-潮间带沉积旋回序列。兴民村组MT发育的沉积环境具有一定水深和频繁波动的水动力特点,MT形态受宿主岩石性质的影响和控制。MT主要发育在浅潮下带-潮间带下部沉积微相。不同形态的MT及其组合分别宿主于不同的沉积岩相或沉积韵律,表现了MT形态的指相意义。

The term ＂molar tooth＂ （MT） was used by Baueman in 1885 to describe a special sedimentary structure which was first found in carbonates of Belt Supergroup,North America. Since then,geologists all over the world have conducted researches in this aspect by means of physics,chemistry and biology,with many genetic hypotheses put forward. In regard to morphology of MT,scientists has paid much attention to the relationship between lithology,morphology,environment and micro-fabrics rather than described lithology,morphology and environment simply. They have also shown concern about the problem whether different morphologies can reflect different original depositional environments or not. Exemplified by MT from Neoproterozoic Xingmincun Formation of Jinshitan in Dalian,this paper studied its morphological characteristics and analyzed its indication significance for sedimentary environment. A series of MT with complicated morphologies are developed in limestone of Upper Xingmingcun Formation. On the basis of a detailed observation,molar teeth are divided into ribbon,short-straight bar,worm,filamentous,dotted and detritus forms,with the ribbon form being dominant. The ribbon form molar teeth are subdivided into straight ribbon,bended ribbon,broken ribbon and spinous ribbon according to the difference in width,sinuosity and degree of crushing. As for sedimentary structures,there exist graded bedding,parallel bedding,horizontal bedding,stylolite and erosion surface in limestone. Some sedimentary rhythms composed of silt-carbonates,micrite-carbonates,laminated carbonates and mud-carbonates were identified,such as cycles of micrite-carbonates and laminated carbonates and cycles of silt-carbonates and laminated carbonates,which represent subtidal-intertidal cycles with upward abating sedimentary dynamics and upward shallowing water. In other words,molar teeth were formed in the lower part of shallow subtidal-intertidal microfacies,in an environment with a certain water depth and frequently fluctuating hydrodynamic force. Furthermore,the morphology of MT was constrained by the characteristics of host rocks. It is thus concluded that different sets of morphology represent different lithofacies or rhythmic units,showing the indication significance for the morphology of MT.This study partly reflects the relationship between morphology and depositional environment of MT in the whole Precambrian. With further researches,the opinion and method held by the authors could be used to study different formations in different areas,and a relationship model between morphology and depositional environment of MT can be established,which will also contribute to the researches on the genesis of MT.