Sedimentological Analysis Of The Lacustrine Carbonate Successions From The Paleocene Second Member,Funing Formation,Jinhu Depression Of The Subei Basin

Fossil tubes are common in Paleogene lacustrine carbonate successions across East China.These tubular fossils in lake basins have been interpreted as “marine polychaete tubes” induced by marine incursions since the late 1970 s.However,this is problematic because they are closely associated with a large number of non-marine fauna and algae.The carbonate succession from the Palaeocene second member,Funing Formation of the Jinhu Depression,Subei Basin was selected for this research because this formation has the most abundant fossils tubes in East China.The aim of this research is to identify the fossil tubes from the Paleocene second member,Funing Formation,and to ascertain their paleoecological significances,and to characterize the petrology,mineralogy and sedimentology of the carbonate successions.The carbonate rocks from the Paleocene second member,Funing Formation in the Jinhu Depression can be grouped into 3 categories and 7 types based on their original composition(whether they are organically bound at deposition)and major controlling factors(depositional/biological/diagenetic): 1)depositional carbonate(lime mudstone,wackestone,floatstone,packstone and grainstone),2)microbial carbonate(boundstone),and 3)diagenetic carbonate(cementstone).The boundstone is dominated by microbially mediated microfabrics,such as clotted,dendritic,laminated and filamentous micrite.The cementstone comprise abundant spherulitic,fan-shaped,and dendritic crystalline calcite microfabrics.The carbonate from the Paleocene second member,Funing Formation is in absence of evaporite minerals.It comprises major minerals of calcite and dolomite,and minor minerals of feldspar,quartz,apatite,pyrite,and various clay minerals.The barite and analcime cements are rare and locally present.The abundant fossils tubes within the Paleocene second member,Funing Formation are not marine polychaete tubes,but belong to the tubular cases of terrestrial trichopteran larvae(caddisfly larvae),which are trace fossils built by the larvae of caddisflies.Most caddisfly cases are curved and slightly tapered,and have one to multi nested walls,which are agglutinated with similarly sized and neatly arranged sediment particles.Fully silken caddisfly cases are rare,and they are not agglutinated.Caddisfly cases were commonly accumulated on nearshore stable substrate and encrusted by carbonate laminae of caddisfly case-dominated spring mounds or microbial-caddisfly bioherms,which are commonly distributed parallel to the lake shoreline.The lacustrine carbonate of the Paleocene second member,Funing Formation was mainly formed within transgressive systems tract and deposited on the lake flooding surfaces above the deltaic deposits and basalt platform.The western Jinhu Depression is typified by extensive waved-dominated high-energy carbonate platform and slope deposits,which consist of 6 facies: carbonate shoals,longshore sand bars,caddisfly casedominated spring mounds,microbial-caddisfly bioherms,carbonate gravity flow deposits,as well as deep lacustrine shale and mudstone.The eastern Jinhu Depression is characterized by localized low-energy carbonate platform and slope deposits overlying the basalt platform.The eastern carbonate facies are similar to that in the west,but it lacks the caddisfly case-dominated spring mounds,and the micrite content is much higher.In addition,the thickness and distribution of eastern carbonate facies are also much smaller than the western carbonate facies.The carbonate deposits in the Paleocene second member,Funing Formation were formed in the marginal area of a mildly alkaline,freshwater to brackish-water lake.The formation,depositional characteristics and distributions of the lacustrine carbonate from the Paleocene second member,Funing Formation are mainly controlled by the following factors: the extensional tectonic setting,interfault slope belt and basalt platform geomorphology,humid and rainy paleoclimatic condition,shallow-water photic environment,caddisfly biology(univoltine life cycle,live within wavedominated photic shallow water,congregate together for pupation,preferentially gather at spring vent),microbial mediation(calcified microbes,microbial metabolism,EPS function),calciumrich spring water/groundwater mixing with alkaline lake water and wave agitation.