黔西水城晚二叠世长兴期钙藻和有孔虫及其古环境意义

Changhsingian(Late Permian)calcareous algae and foraminifers from Shuicheng,western Guizhou,South China and their paleoenvironmental significance

在黔西水城地区的K576井长兴组共鉴定钙藻3属3种,包括Gymnocodium bellerophontis、Permocalculus sp.和Tauridiumkurdistanensis;有孔虫8属10种,其中(虫筳)类2属2种,有孔虫动物群主要由Reichelinasp.indet.、Nankinella sp.、Pachyphloia schwageri、Pachyphloia sp.、Geinitzina sp.、Nestellorella sp. indet.、Howchinella sp.、Hemigordius aff. saranensis、Hemigordius sp.和Midiella sp. indet.组成。将本井按照生物特征分为有孔虫-钙藻-介形虫组合、有孔虫-腕足类-介形虫组合、介形虫-双壳类-腹足类组合、有孔虫-钙藻-双壳类组合、有孔虫-腕足类-介形虫组合、有孔虫-钙藻-双壳类组合和介形虫组合等7个组合。按照层序地层划分、垂向沉积序列特征和测井资料的分析,有孔虫-钙藻-介形虫组合(SQ3-3)和有孔虫-腕足类-介形虫组合(SQ3-4)时期地层为三角洲前缘环境。介形虫-双壳类-腹足类组合(SQ4-1)时期地层为混合坪环境。有孔虫-钙藻-双壳类组合(SQ4-2)、有孔虫-腕足类-介形虫组合(SQ4-3)和有孔虫-钙藻-双壳类组合(SQ4-3)时期地层为潟湖环境,介形虫化石组合(SQ4-3)时期地层为砂坪环境。整体上黔西水城地区长兴期发育三角洲和障壁-潟湖的沉积体系,为一个低能、平静和温暖的近岸浅海环境。此外,在上述化石组合分析中发现,当在潟湖环境同时出现有孔虫和钙藻时,生物多样性增加明显。

Three species in three genera of calcareous algae,including Gymnocodium bellerophontis,Permocalculus sp.,and Tauridium kurdistanensis and ten species in eight genera of foraminifers,including two species in two genera of fusulinids,are identified from Well K576 near Shuicheng,western Guizhou,South China.The foraminifers mainly consist of Reichelina sp.indet.,Nankinella sp.,Pachyphloia schwageri,Pachyphloia sp.,Geinitzina sp.,Nestellorella sp.indet.,Howchinella sp.,Hemigordius aff.saranensis,Hemigordius sp.,and Midiella sp.indet.The fossil fauna is divided into seven assemblages:Foraminifer-Calcareous alga-Ostracod,Foraminifer-Brachiopod-Ostracod,Ostracod-Bivalve-Gastropod,Foraminifer-Calcareous alga-Bivalve,Foraminifer-Brachiopod-Ostracod,Foraminifer-Calcareous alga-Bivalve,and Ostracod assemblages.Based on sequence stratigraphy of the study area,characteristics of the vertical sedimentary sequence,and analysis of log data,the palaoenvironment of the deposits that yield the Foraminifer-Calcareous alga-Ostracod assemblage(SQ3-3)and Foraminifer-Brachiopod-Ostracod assemblage(SQ3-4)is a delta front.The strata that yields the Ostracod-Bivalve-Gastropod assemblage(SQ4-1)was deposited on a tidal flat.The paleoenvironment of the Foraminifer-Calcareous alga-Bivalve(SQ4-2),the Foraminifer-Brachiopod-Ostracod(SQ4-3),and the Foraminifer-Calcareous alga-Bivalve(SQ4-3)assemblages is a lagoon.The strata that yields the Ostracoda assemblage(SQ4-3)was deposited on a sand flat.A low-energy,calm,and warm nearshore delta and barrier lagoon depositional systems were generally developed during the Changhsingian in Shuicheng area of western Guizhou.In addition,analysis of the fossil assemblages indicates that biodiversity increases significantly when both foraminifers and calcareous algae are present in the lagoon environment.