Characters And Importances Of Earliest Triassic Anachronistic Limestones In Northwestern Sichuan Area

Earliest Triassic anachronistic carbonate rocks were widely deposited on thecarbonate platforms and ramps of Tethys Ocean after the latest Permian massextinction. The anomalous carbonate rocks represented as anachronistic rocks werethe common sedimentary rocks in the Early Triassic Oceans (Shi et al.,2011). Theanachronistic rocks, such as flat-pebble conglomerates, microbiolites, vermiculatelimestones, ribbon limestones, etc. were common seen in Precambrian–EarlyPaleozoic sediments and were developed in the extreme and aberrant environmentsfrom the Late Paleozoic to Cenozoic period. While they were commonly found in theEarly Triassic marine deposits, it was thought that they were misplaced for time andenvironment, that’s why they were named as anachronistic facies (Zhao etal.,2008;Shi et al.,2011).Earliest Triassic anachronistic limestones, e.g. breccia limestones, flat-pebbleconglomerates, thin-bedded limestones, vermiculate limestones, subtidal wrinklestructures and carbonate seafloor fans etc. were widely found in Nothwestern Sichuanarea. The typical Griesbachian sections of Early Triassic1stmember of Feixianguanformation located on Shangsi, Guangyuan and situated in Dagouli, Qingchuan wereobserved, described and sampled in detail. The lithological characters andsedimentary structures of the two sections were researched and such measurements ascarbon-oxygen isotope, microscopic chip analysis, X-ray diffraction for clay mineral,etc. were carried. The following conclusions were drawn:（1） Breccia limestones, flat-pebble conglomerates, thin-bedded limestoneswere most developed and the vermiculate limestones, subtidal wrinkle structures andcarbonate seafloor fans were locally found in Nothwestern Sichuan area. Theseanachronistic limestones represented the aberrant oceanic and paleoclimaticconditions in the Tethys Ocean of northwestern Sichuan area after the P-T massextiction.（2）The Shangsi Briccia bed (SBB) was a special layer for the1st member of Feixianguan formation due to the changes of lithological characters, sedimentarystructures, carbon isotopes and content of clay minerals, etc. for the above and belowlayers. An erosion face developed between the bottom of SBB and the underneaththin-bedded limestones, and bedded flat-pebble conglomerates occurred above theSBB. The carbon isotopic values (δ13C) were negative for SBB and the below layersof Feixianguan formation, while the values were positive for the rocks above the SBB.（3）Based on the analysis of the GR energy spectrum collected from the laterPermian to Early Triassic carbonate rocks in Shangsi Section, it was indicated that thesedimentary layers are controlled by Milankovitch cycles. During the earliest Triassicinterval, weathering rate increased sharp and mass terrigenous material fluxed intoshallow sea, which resulted in a rapid deposition rate of25cm/ka in Shangsi Section.The rapid rate of deposition probably restrained the diversity of marine fauna anddelayed the resuscitating of marine ecosystems．（4）The sedimentary structures of Lowest Triassic tempestites e.g. storm-generated flat-pebbles, storm erosion faces, storm tear structures and moundcross-bedding were significant, indicating the storm origin of breccia limestones,flat-pebble conglomerates in Northwestern Sichuan area．（5） Content of clay minerals was hardly measured for SBB and the belowlayers of Feixianguan formatio, while illites and chlorites were found in theflat-pebble conglomerates above the SBB. The similar content character of minorelements e.g. Fe, Mn, Ni, Sr was revealed, indicating a rapid weathering rate startingfrom the SBB in Northwestern Sichuan area.Forthermore, carbonate seafloor fans and subtidal crinkle structures were firstlyreported in China for the1st member of Feixianguan formation, and the carbonateseafloor fans was considered to grow in a high basiciticy ocean that restrained thediversity of marine fauna. The subtidal crinkle structures, which was similar to theanachronistic subtidal wrinkle structures, were found on the surface of thin-beddedlimestones interlayered by flat-pebble conglomerates, indicating the sameenvironment origin for subtidal crinkle structures and flat-pebble conglomerates in theearly Triassic oceans.