Statistical Evidence for the Colonization of Gravelly Coastal Channels by Early Cambrian Burrowers

The coastal realm contains various architectural elements that differ from both typical marine and terrestrial deposits. These architectural elements represent novel habitats for marine benthos in terms of hydrodynamic regime, and some may even serve as a gateway for colonization into aquatic environments on land. Here, I present vertical burrows from the Wulongqing Formation (Cambrian Series 2) in the Longbaoshan area, Yunnan, China, suggesting that early Cambrian burrowers colonized gravelly coastal channels within a fluvio-tidal system. The succession is dominated by mudstone (>80% of total thickness), with conglomerate beds present at its basal part. The characteristics of the mudstone (lenticular sands, mud drapes, bioturbations) demonstrate tidal influence and benthic activity. The conglomerates are interpreted as coastal channel deposits, characterized by sheet-like geometry (width to thickness ratio > 500), trough cross-stratifications (CXt), and inclined gravel-sand stratifications (CXgs). Furthermore, statistical analysis reveals that the occurrence frequency of vertical burrows at the contact between mudstones and overlying conglomerates (4 in 7) is significantly higher (p < 0.05, OR = 12.27, 95%CI: 2.01–64.32) than within mudstones (5 in 51), indicating that burrowers colonized these coastal channels. Given the similarity between these sheet-like conglomerates and pre-Devonian sheet-braided conglomerates, this colonization implies colonization at the marine end of fluvial systems during the Cambrian explosion.