Middle Ordovician Bioturbation Structures from Southeastern Margin of Ordos Basin and Their Environmentary Interpretation

Trace fossils were discovered in the Middle Ordovician Badou Member ofFengfeng Formation and the Member 1 of Jinsushan Formation in Mt. Jinsu of the Fuping region whichlies at the southeastern margin of the Ordos basin. The rocks of the fore-mentioned parts contain aconsiderable amount of bioturbation structures, in which ichnogenus and ichnospecies can not beidentified, and abundant stromatolites. The distributions and characteristics of the bioturbationstructures are analyzed and summarized in this paper. The paleoenvironments of related formationsand members were interpreted in the method of semiquantitative analysis of bioturbation structureswith researches on stromatolites and sedi-mentology. It is suggested that Fuping zone, located inthe southeastern margin of the Ordos basin, was a carbonate platform where stromatolite-developingtidal flat and low-energy subtidal open platform environments dominated. Also that this zoneexperienced three paleogeographic evolutions from tidal flat to low-energy subtidal open platformenvironments, that is, three larger-scale eustatisms occurred during the sedimentary period ofMiddle Ordovician Badou Member of Fengfeng Formation to the Member 1 of Jinsushan Formation.

Two episodic changes of trace fossils through the Permian-Triassic transition in the Meishan cores,Zhejiang Province

Six ichnogenera,Chondrites,Palaeophycus,Planolites,Rhizocorallium,Skolithos,and Thalassinoides,were identified from the drilling cores near the Global Stratotype Section and Point(GSSP) of Permian-Triassic boundary at Meishan,Zhejiang Province.The ichnological indices,including abundance,bioturbation index,and disturbed depth of trace fossils,show two episodes of intense changes near the Permian-Triassic Boundary(PTB).Episode I occurred in Beds 25-27b when the ecologically complicate forms such as Chondrites,Skolithos,Rhizocorallium,and Thalassinoides disappeared hereafter,the bioturbation index reduced from 1-5 to 1-3,and the disturbed depth declined from 5-66 to 2-5 cm.Episode II took place at the base of Bed 33 with the disappearance of Palaeophycus and Planolites,and subsequent absence of trace fossils and bioturbation till the middle-upper part of Bed 41 when the disturbed structures reoccurred,but they are only tiny Planolites and the bioturbation index was never higher than 3 and the disturbed depth less than 4 mm.Episode I shows an intense change,corresponding to the main stage of the end-Permian mass extinction,whereas Episode II is relatively weak,corresponding to the epilogue of the mass extinction of trace makers in the Early Triassic.Subsequently,ichnofossils were dominated by surface tracks in simple ecological habit and structures.This phenomenon indicates that the Early Triassic benthonic fauna is changed from sessile benthic system to mobile benthic system after the end-Permian mass extinction.In other words,the evolution of the trace fossils across the Permian-Triassic transition had an episodic process similar to the body fossils.In addition,the change of ichnofabrics is well coincided with the negative excursion of carbon isotopes and the expansion of cyanobacteria.As the results of physical and biogenic processes,trace fossils provided unique materials for the study of the biotic and environmental events,as well as their coupling evolution through the great Permian-Triassic transition.