Trace Fossils And Their Paleoenvironmental Significance Of The Early Triassic Anshun Formation In Guiyang Area

The end-Permian mass extinction was the largest biotic catastrophe in the Phanerozoic, and also maked the end of the history of Paleozoic marine ecosystems.Followed by the slow environment reconstruction in the Early Triassic, a new marine ecosystem which the benthic activity and carnivorous biologic could prosper together had been built in the Middle Triassic. All sorts of palaeontological and sedimentological records became the important clues and evidence that the geologists know the geological history, the trace fossils as a common carrier for biological activity and sedimentary environment information has become a new hot spot in the study of the environmental evolution in the Early Mesozoic. The trace fossils from the Anshun formation in Guiyang area has become the research material in this paper. The benthic ecosystem recovery and sedimentary environment in the Late Early Triassic were analyzed by analyzing the different ichnology indicators.The ichnofossil evolution model during the period of the Induan in the Early Triassic reflected the process that the marine benthic ecological environment improved gradually during this period, but the benthic ecosystem which built in this period is fragile, and it was destroyed quickly by the regional environmental change in late Induan. In the next Olenekian period, benthic trace-making organisms got a development again, its capacity to transform sediments increased gradually, the Ichnofossil diversity also increased gradually.The trace-making organisms had a development from the early single polychaetes worms to the species-rich polychaete, and until the crustaceans created a larger ecological space in sediments. Trace fossils in this stage not only had a more complex structure behavior, and had a development toward to large-scale and three-dimensional space. Combined with the characteristic that the body fossils increased observably in Late Olenekian, we could get that the perfect marine benthic ecosystem had been built in this period. The evolution of trace fossils in this region showed the characteristic that the process of biological recovery-radiation in the Early Triassic had multiple intrusions, which also illustrated the recovery of benthic ecosystem was the earliest recovery link in the ecological chain after the end-Permian mass extinction. The recovery of benthic ecosystem had a certain function in playing an ecological basic role in the marine ecosystem and "Qingyan Biota" in this area.In addition, the carbon and oxygen isotope samples from the Lower Triassic Anshun formation were analyzed, and discussed the effects of environmental change in the late Induan and Olenekian on the recovery of the benthic ecosystem. Results of carbon analysis showed that the carbon of the Lower Triassic frequently fluctuate in the Induan and Early Olenekian, and became stable until Late Olenekian, which indicated that the marine environment in the Early Triassic changed frequently by the way of“deterioration-improvement- deterioration”. The evolution of delta 13 C and delta 18 O value reflected that the marine environment has experienced rapid deterioration and frequent turbulent changes in the early stage of Induan. Trace-making organisms got a development in the middle of the Induan. Its habits structure experienced from simple to complex, and individual variation changed from small to large. These characteristic showed the ability of trace makers to displace sediments were enhanced, the “elementary benthic ecosystem”was preliminarily established, and the benthic marine environment had improved greatly.Carbon isotopic curves display pronounced negative excursions, followed by slow rise during the early stages of Olenekian. During this period, the trace fossils experienced that its abundance got an evolution model with reduction-increase-rich, and it also had a development trend from the two-dimensional to the three-dimensional. There was a good corresponding relationship between the evolution model of benthic trace-making organisms with the carbon and oxygen isotopic stratigraphic profile, and it was a sign that the environmental factors which induced the extinction events had greatly improved, it also reflected the benthic trace-making community had the function to transform the ecological environment at the bottom of the ocean.