The Changes Of Depositional Environments And Its’ Influence Of The Early Triassic Nanlinghu Formation In Zuoshan Area, Susong County, Anhui Province

The lower Triassic is complete and well outcropped in Zuoshan area, SusongCounty, Anhui Province. This area is located in the western part of theLower-Yangtze region and an ideal area for the Early Triassic researches in thearea. The early Triassic Nanlinghu Formation with a thickness of280m hasdistinct sedimentary characteristics. It is conformably contacted with themedium–beded dolomite limestones of the underlying Early Triassic HelongshanFormation and the thick-bedded dolomite limestones of the overling MiddleTriassic Dong Ma’anshan Formation.The Nanlinghu Formation can be sudivided into five parts according toactual measurement of the Zuoshan profile and distributional features ofdepositional types, body fossils, trace fossils and microbialites. Part1is mainlycomposed of thin-bedded argillaceous limestones, with a few trace fossils andbody fossils; Part2is also composed of thin to medium bedded argillaceouslimestones, but with few fossils and bioturbation. Part3mainly consists of themiddle-thin-bedded limestones, with abundant trace fossils and a few gastropods,bivalves and brachiopods. Part4is microbialites layer, stromatolites andthromblites formed lenticular reefs obviously, several ostracodas andforaminiferas can be found in the basement and cap layers of the reefs. Part5iscomposed of banding limestones with few fossils.According to field profile researches and microscopic analysis， fivedepositional types of the Nanlinghu Formation can be identified as mud limestones,banding limestones, calcirudites, microbialites and bioclastic limestones.Sedimentary structures such as horizontal beddings, cross beddings, flushingsurfaces, biological sedimentary structures, bioturbating structures, biologicalcondensation layers and Storm sedimentary structures are observed also. Accordingto Flügel’s carbonate microfacies classification, eight mirofacies types have beendivided as lime mud limestones (MF1), calcirudites (MF2), lamina limemudlimestones (MF3), bioturbate wackstones (MF4), bioclastic wackstones(MF5), Laminated stromatolitic bindstone (MF6), packaged bioclastic packstones(MF7),fecal peloidal wackstones(MF8). Four facies zones were summarized as platformmargin, open carbonate platform, restricted carbonate platform and evaporationplatform. Sedimentary characteristics and microfacies indicate that the NanlinghuFormation was deposited in shallow marine and seawater became shallowergradually.Lenticular reefs built by forked-columnar stromatolites are the main characterof the part4of the Formation. Top surfaces and forked-columnar features of thereefs indicate that stromatolite reefs were formed in subtidal zone to storm wavebase or lower intertidal zone with strong hydrodynamic. Many foraminifers havebeen found in basement and overlying strata of stromatolite reefs, might indicate astrong water power. Combined with macro-fossils,trace fossils and previous studies,microbialites in the Nanlinghu Formation might indicate a good environmentalcondition for the recovery, the microbial contribution for the early Triassicrecovery was provided also.