广西隆安都结剖面下石炭统都安组上部鲕粒类型及其地质意义*

鲕粒是一类特殊的沉积颗粒,为古气候和古海洋环境的重要指示器。为了深入认识此类特殊颗粒的成因机制、形成环境及地质意义,对广西隆安地区都结剖面下石炭统都安组上部含鲕粒地层开展了古生物学、沉积学和岩相学研究。研究区共识别出5种主要的鲕粒类型: 放射状纹层鲕粒(O1)、规则同心放射状纹层鲕粒(O2)、不规则同心放射状纹层鲕粒(O3)、泥晶鲕粒(O4-A和O4-B)和复合鲕粒(O5)。各类鲕粒的显微组构和沉积环境指示其具有不同的形成过程,其中水动力条件影响和控制着鲕粒的发育和分布情况。研究区含鲕粒地层形成于维宪期末—谢尔普霍夫期,恰好对应早石炭世晚期冰川作用的开始。受冰川作用影响,全球海平面频繁波动,研究区地处低纬度地区并以浅滩和潮坪沉积环境为主,为鲕粒的形成提供了适宜的水体条件,即温暖、动荡的浅水环境。此外,含鲕粒岩层内广泛发育钙质微生物和微生物席,说明微生物活动在研究区较为常见,可能与鲕粒的形成过程具有一定的关联。

Ooids types of the upper Du'an Formation of lower Carboniferous in Dujie section of Long'an area in Guangxi and their geological significance

Ooids,a typical type of sedimentary grains,are an important proxy for paleoclimate and paleoceanography. To better understand the formation mechanisms,depositional environments and geological significance of these carbonate grains,studies relating to paleontology,sedimentology,and petrography were carried out on the ooid-bearing limestones in the upper Du'an Formation of the Lower Carboniferous in Dujie section of Long'an area,Guangxi. Five ooid types have been identified: Radial ooids(O1),regular concentric-radial ooids(O2),irregular concentric-radial ooids(O3),micrite ooids(O4-A,O4-B),and composite ooids(O5). The microscopic fabric and depositional environments of these ooids uncovered different forming processes,indicating that the formation and distribution of the ooids were influenced by the hydrodynamic conditions. These ooids formed in the late Visean to Serpukhovian,corresponding to the onset of the ice age in the late Mississippian. Influenced by changes of ice volume,sea level changed frequently,resulting in shallow shoal and tidal flat environment in the study area. Dujie area was located in the low latitude,providing shallow,warm,and agitated waters for the ooid formation. Furthermore,calcimicrobes and microbial mats were abundant in the ooid-bearing limestone,indicating that the microbial activities were common and might be related to the formation of these ooids.