Paleoenvironmental analysis of the ichnogenus Zoophycos in the Lower Devonian tempestite sediments of the Longmenshan area, Sichuan, China

Abstract

The trace fossil Zoophycos is characteristic of the Lower Devonian in the Longmenshan area of southeastern China. Three distinct storm-generated or induced types of sedimentary sequences bearing Zoophycos have been identified in shallow marine deposits of the Bailiuping Formation and the Yangmaba Formation. The type A deposit is composed of siltstones with shell concentrations and wave ripples and small Zoophycos at the top. The type B deposit is characterized by fine-graded sandstone with long wavelength hummocky cross-stratification and black gray mudstone as well as the largest and densest Zoophycos at the top. The type C deposit has graded bioclastic limestone with short wavelength hummocky cross-stratification, muddy wackestone and large Zoophycos at the top. Based on the features of the types of Zoophycos, the distribution zone of the Zoophycos ichnogenus that preserved in tempestite sequence was built between the fair weather wave base and storm wave base in the Lower Devonian. This research suggests that the distribution patterns of Zoophycos formed in storm-induced deposits may not provide significant clues for the estimation of relative water depths.The hydrodynamic conditions and sedimentation rate are two important environmental factors of the development of the Zoophycos in the Lower Devonian shallow marine deposits where it was mainly affected by storms between the fair weather and storm wave bases. The abundance of Zoophycos at the top of locally frequent storm deposits could indicate a fairly opportunistic behavior and they were formed in the early HST deposit, which had a relatively stable environment with low energy and higher sedimentation rate.