川西北甘溪地区吉维特阶核形石特征、成因及地质意义

川西北龙门山前甘溪地区吉维特阶发育一套厚2~3m灰-深灰色微生物岩,由直径0.1~4.0cm的核形石组成,其核心皆由早期葛万藻及其他生物碎屑组成,包壳由具同心圈层结构的亮色和暗色含葛万藻的纹层组成,并可分为球状、帽状和变形状3类核形石微生物岩,垂向上可分4个旋回,从底到顶核形石密度和粒径总体变密、变大,平面上呈不均匀发育。核形石样品具不同程度的δ~(13)C、δ~(18)O值负偏移和~(87)Sr/~(86)Sr正偏移特征,表明核形石微生物岩层遭受过古表生期大气淡水的影响,其中亮色纹层与球状核形石微生物岩受大气淡水影响最为强烈,核形石是在葛万藻发育、水流扰动及大气淡水参与的环境下,葛万藻包覆于生屑的外缘而成,藻类越发育、水体能量越高、沉积环境越稳定,形成的核形石粒径越大、丰度越高。核形石微生物岩在纵、横向分布特征,表明在全球中泥盆统艾菲尔期—吉维特期海平面上升背景下,龙门山地区吉维特金宝石组末期表现为至少6期次脉动式且逐渐强烈的构造抬升作用,造成该地区金宝石组末期海平面大幅下降而暴露于地表遭受大气淡水的淋滤作用。

Characteristics,Origin and Geological Significance of Oncolites of Givetian(Middle Devonian) in Ganxi Area,Northwestern Sichuan

A set of Givetian (Middle Devonian) microbial rocks with 2-3 m thickness developed in Ganxi of Longmengshan area,northwestern Sichuan,China.They are composed of oncolites which are 0.1-4.0 cm in diameter,sphere,cap-shaped or deformed.The cores of oncolites are mainly composed of biodetritus including earlier girvanella,and the coatings are composed of light-colored micro-sparry calcite and dark gray layer.Four cycles were recognized according to the abundance and size of these oncolites which are bigger and denser upward,with irregular horizontal distribution.Compared with the fossil brachiopods from the same horizon,negative δ18O and δ13C isotope values and positive 87Sr/86 Sr ratios are recognized in the microbialites,show an influences of atmospheric fresh water.Negative δ18Oand δ13C isotope values are especially obvious in the spherical oncolites and the light-colored coatings.Then,the forming environment and formation pattern of oncolites are further discussed.At last,the distribution pattern of microbial oncolites in the studied area indicates that more than six structural uplifts happened during the final stage of deposition period of the Jinbaoshi Formation.Thus exposure and leaching of the Jinbaoshi Formation developed under the significant sea level rise during Eifelian and Givetian (Middle Devonian).