意大利Mt.Faito地区白垩纪早阿普第期浅水碳酸盐岩沉积微相及其环境演化

Microfacies Analysis of Lower Aptian (Cretaceous) Shallow-water Carbonate in the Mt.Faito Section,Italy

通过对白垩纪早阿普第期Mt.Faito野外露头剖面宏观特征描述及室内岩石薄片分析,根据岩性及沉积特征变化,将其划分为A、B、C三个地层单元:A以潮下带粒泥灰岩为主,时有生物碎屑粒泥灰岩-泥粒灰岩夹层沉积;B以潮下带双壳类漂浮岩沉积为主,基质多为生屑泥粒灰岩;C与下伏地层呈不整合接触,该单元岩性变化复杂,整体以潮下带-潮上带沉积环境为主,偶尔发育潟湖沉积。根据三个地层单元所发育的骨架生物颗粒类型,可将地层A、B和C划分为"Chlorozoan"、"Chlorozoan&Foramol"、"Foramol&Microbial"三种不同组合的生物颗粒沉积。其沉积环境演变如下:A为暖水贫营养的水体环境,B为暖水富营养环境,而C则主要为温水富营养环境。基于Mt.Faito剖面的生物沉积特征及碳同位素数据可知,由温度、CO2及水体营养物质增加所引起的早白垩世生物沉积及其环境演化具全球性。

Three distinct lithostratigraphic units A,B,and C have been recognized according to field work and thin section observations. The crudely stratified Unit A deposited in subtidal environments is characterized by monotonous wackstone intercalated with bioclastic wackstone-packstone. The well-bedded Unit B consists largely of bivalve floatstones in a matrix of bioclastic packstone deposited in subtidal conditions. Unit C,disconformably overlying Unit B,is composed of complex lithofacies associations that deposit in variable environments from subtidal to supratidal settings,sometimes in lagoon environments. According to the biota range chart and stable carbon isotope data from the Mt. Faito section,which can be used to analyze the biosedimentary diversity,different climatic and sea water conditions can be inferred as well. Unit A is characterized by a"Chlorozoan"association,which indicates warm and oligotrophic conditions;Unit B is dominated by"Chlorozoan"and"Foramol"associations that indicate relatively warm and mesotrophic conditions;and Unit C is a combination of"Microbial"and"Foramol"associations,hence mesotrophic and temperate thermal conditions. Therefore,local early Aptian biosedimentary shifts and paleoenvironmental turnovers exerted by the increasing temperature,CO2,and nutrient contents have been interpreted on a global scale by analyzing the biosedimentary evolvement and correlating the C-isotope curve with those of other early Cretaceous carbonate platforms and basins worldwide.