Thalassinoides Ichnofabrics of Lower Cambrian Longwangmiao Formation (Stage 4, Toyonian) on the Yangtze Platform, South China: Improving Paleoenvironmental Interpretations

Abstract

AbstractAn analysis of Thalassinoides ichnofabrics in the Cambrian Stage 4 Longwangmiao Formation from six sections near Chonqing in the Middle and Upper Yangtze Block of South China was conducted to promote the understanding of the paleoenvironment of this period. Thalassinoides ichnofabrics are divided into three types according to their morphology, bioturbation index, abundance and related parameters: banded, mottled and grid. The completeness of the ichnofabrics gradually increases from banded to grid, the scale of the burrows expands, and the structure transitions from two‐dimensional to three‐dimensional. Malacostracans, phyllocarids and enteropneusts are the most likely burrowers. The findings reveal that frequent changes in paleoenvironmental conditions are the dominant factors for generating the various Thalassinoides ichnofabrics. The changes in depositional rate, substrate properties and nutrient levels affect the behavior and building patterns of the burrowers, thus leading to the diversity of Thalassinoides ichnofabrics. In the lower Longwangmiao Formation beds, many incomplete Thalassinoides burrow systems were built in the substrates with terrigenous detrital minerals, revealing significant environmental pressure on the burrowers. Later, the Thalassinoides burrows went into complete three‐dimensional boxworks. At the end of the stage, bioturbation almost disappeared, which indicates that burrowers were finding it difficult to survive in the harsh environment.