Early Devonian Coastal-Plain Sedimentology and Ichnology on the Northeastern Gondwanan Margin of the Palaeotethys Ocean: Insights from the Guijiatun Formation of Qujing, Yunnan

The Silurian–Devonian transition was a critical interval in the coupled development of Palaeozoic plant terrestrialization (PPT) and coastal sedimentary systems. The Lower Devonian succession of Qujing, Yunnan, preserves a continuous record of fluvial to coastal marine deposits, recording the opening of the Palaeotethys Ocean. The Lochkovian Guijiatun Formation represents a probable coastal-plain system characterized by abundant paleosols with root traces and diverse animal trace fossils, providing an important archive for examining early interactions among terrestrial plants, benthic animals, and sedimentary substrates. Based on sedimentological and ichnological evidence from five sections, 10 lithofacies, 12 ichnogenera and 22 ichnospecies are recognized in the Guijiatun Formation, and these can be grouped into three facies associations and three ichnoassemblages. The Chondrites-Diplocraterion, Rhizocorallium-Phycodes, and Cruziana-Diplichnites-Treptichnus ichnoassemblages indicate estuary–tidal-flat, interdistributary-bay, and levee subenvironments, respectively. The sedimentological and ichnological data further point to two related processes: bank strengthening by rooted early vascular plants, and above-ground baffling that promoted the retention of fine-grained sediment, thereby influenced the formation of paleosols and coastal-plain landscapes, whereas benthic animals had already begun to rework the substrate extensively in different coastal settings, increasing the structural complexity of the sediment. These alternating plant- and animal-mediated effects on sedimentation suggest that the Guijiatun Formation preserves one of the earliest known records of biogenic modification of coastal depositional processes of PPT during the opening of the Palaeotethys Ocean. During the Early Devonian, the Qujing area was situated on the northeastern margin of Gondwana along the Palaeotethys Ocean, thus the Guijiatun Formation not only provides a key case study for the evolution of coastal environments and ecosystems in South China, but also offers evidence for organism-substrate interactions along the marginal Palaeotethys Ocean under hothouse climate conditions and during the PPT event.