The Chengjiang Biota inhabited a deltaic environment

The Chengjiang Biota (Cambrian Stage 3, China) preserves an exceptionally diverse soft-bodied fauna, providing a key window into the Cambrian Explosion. Despite extensive taxonomic and taphonomic work, the depositional environment of the Chengjiang Biota has remained contentious, with interpretations ranging from estuarine and embayment settings to shoreface–offshore complexes and shelf settings influenced by turbidity currents. Prior interpretations relied largely on weathered outcrops that obscure fine-scale sedimentary structures critical for reconstructing flow dynamics. Concurrently, modern advances have transformed understanding of fine-grained sedimentation, recognizing roles for hyperpycnal flows, fluid muds, ocean floods, and bottom currents. This study analyzes fresh, non-weathered core through the Yu’anshan Formation to determine the precise sedimentary environment of the Chengjiang Biota and to clarify ecological and taphonomic controls, with implications for comparing Burgess Shale-type Lagerstätten.

Previous models for the Chengjiang Biota span tidally influenced estuaries/embayments, wave-dominated shoreface–offshore systems, and shelf settings with turbidity currents. However, tidally generated structures are absent in the new core analysis. Wave-generated structures are common but occur alongside abundant evidence of unidirectional flows (hyperpycnal, turbidity), inconsistent with a simple shoreface. Earlier studies were based on weathered outcrops, limiting recognition of diagnostic structures. Broader literature on fine-grained systems demonstrates that mudstones may form via diverse mechanisms (hyperpycnal flows, wave-enhanced fluid muds, plug flows), a perspective seldom applied systematically to Cambrian Burgess Shale-type deposits. Comparisons with modern and post-Paleozoic storm-flood–dominated deltas (e.g., Baram Delta, Trent River) inform interpretation, with adjustments for Cambrian non-actualistic conditions (braided, vegetation-free catchments and episodic sediment delivery).

A 130 m-thick core was drilled in Jinning County, Eastern Yunnan Province, China (24°42′59″N, 102°31′09″E), spanning the entire Yu’anshan Formation, including the Maotianshan Shale Member. Half of the core was sampled for geochemical analyses; the archived half was used for non-destructive sedimentary facies analysis. A total of 161 thin sections (typically 5.5 × 3.0 cm; 30 µm thick) were prepared. Sedimentary facies were described using the fine-grained rock classification of Lazar et al. Core observations were supplemented by thin-section petrography and outcrop observations at the Xiaolantian and Kunyang Phosphate Mine sections. Biogenic structures were documented and bioturbation intensity quantified using the Bioturbation Index (BI) scale from 0 to 6 (0 = no bioturbation; 6 = fully bioturbated), with ichnotaxa identified following standard ichnotaxonomic practices. Palaeoenvironmental terminology was standardized (shoreface: low tide line to fair-weather wave base; offshore: fair-weather to storm wave base; shelf: storm wave base to slope break; delta subdivided into delta plain, delta front, prodelta). The study integrates recent insights from sediment-flow experiments, depositional modeling, and sedimentologic/ichnologic core analysis to infer flow processes and depositional environments responsible for the Chengjiang succession.

• The Yu’anshan Formation in core includes four members (top to bottom): Upper Siltstone Member, Maotianshan Shale Member, Black Carbonaceous Member, and Black Siltstone Member, with an overall coarsening- and thickening-upward trend.
• Five principal deposit types were recognized:
  1. Oscillatory-flow deposits: parallel-laminated and hummocky cross-stratified very fine–fine sandstone with soft-sediment deformation, locally BI up to 3. Interpreted as proximal delta front storm-wave reworked sands above fair-weather wave base.
  2. Hyperpycnal flow deposits: thin–medium bedded combined- to oscillatory-ripple cross-laminated, climbing oscillation-ripple laminated and hummocky cross-stratified very fine sandstone and mudstone, with gutter casts, waning-flow structures, soft-sediment deformation, BI 0–4. Interpreted as flood-derived sustained hyperpycnal flows active during storms; deposited in distal delta front to proximal prodelta.
  3. Wave-enhanced fluid mud deposits: erosion-based dark gray massive mudstones that may pass upward to faintly laminated intervals (BI 0–3), commonly capping wave-generated sandstones. Interpreted as wave-enhanced fluid muds from storm wave resuspension or storm-enhanced fluvial runoff; distal delta front to proximal prodelta.
  4. Plug flow and low-density turbidity current deposits: thin, sharp- to erosive-based coarse mudstones alternating with fine mudstones; massive to graded, locally laminated, with wavy lamination and carbonate caps; BI 0–3 with small Planolites and Palaeophycus. Interpreted as deposition from unstable plug flows/quasi-laminar clay-rich suspensions and occasional low-density turbidites (including surge- and hyperpycnal-derived), near storm wave base in distal prodelta to mudbelt. These beds host most exceptionally preserved fossils.
  5. Hemipelagic deposits: dark fine mudstones, locally organic-rich, largely massive with rare cross-lamination, BI 0–2. Represent most distal shelf/offshore deposition under low-oxygen conditions.
• The succession records a storm-flood–dominated delta (hyperpycnal littoral delta) where river floods (triggered by storms) and storm waves acted together to transport and rework fine-grained sediment far from distributary mouths.
• Ichnologic patterns (low diversity, patchy and sparse bioturbation, small tracemakers, monospecific suites, scarcity of suspension-feeding burrows) support a delta-influenced setting with stressors including freshwater discharge, high sedimentation rates, and turbidity.
• Geochemical and sedimentologic evidence indicate normal salinity during fair weather and reduced salinity during wet seasons, consistent with distributary discharge; fluctuating salinity likely caused mortality events and may explain absence of stenohaline echinoderms at Chengjiang.
• Taphonomy: Most animals lived in well-oxygenated, nutrient-rich delta-front waters but were transported basinward by event flows to prodelta settings where rapid burial in fine-grained plug-flow/turbidite beds under dysoxic–anoxic bottom waters favored exceptional preservation; carbonate-cemented bed tops indicate low-oxygen conditions.
• Findings necessitate revising the long-held view that Burgess Shale-type faunas exclusively represent stable distal shelf/slope communities.

The analysis of fresh core material reveals a mixed wave- and river-influenced depositional system consistent with a storm-flood–dominated delta, resolving long-standing debates on the Chengjiang Biota’s paleoenvironment. Diagnostic structures (hummocky cross-stratification, combined-flow ripples, gutter casts, climbing oscillation ripples, erosion-based fluid muds, plug-flow mudstones, and low-density turbidites) collectively indicate coupled storm-wave and hyperpycnal processes operating from proximal delta front to distal prodelta and shelf. This framework explains ecological and taphonomic patterns: well-oxygenated, nutrient-rich but physically unstable delta-front habitats produced communities exhibiting low, patchy bioturbation and few suspension feeders; event-driven transport delivered carcasses to dysoxic–anoxic prodelta beds where rapid burial enhanced soft-tissue preservation. Adjustments for Cambrian non-actualistic boundary conditions (vegetation-free, sheet-braided rivers, episodic sediment delivery) underscore how storms likely amplified flood frequency and hyperpycnal activity. The results have broad implications for interpreting Burgess Shale-type deposits, advocating integration of storm–river coupling and fine-grained event stratigraphy into paleoenvironmental reconstructions and ecological comparisons across Cambrian Lagerstätten.

This study establishes that the Chengjiang Biota inhabited a storm-flood–dominated deltaic system. Core-based facies analysis documents multiple event-driven deposit types (including hyperpycnal sands, wave-enhanced fluid muds, and plug-flow/turbidite mudstones) spanning delta front to prodelta and shelf. The ecological picture is of organisms living predominantly in oxygenated, nutrient-rich but physically unstable delta-front settings, with exceptional preservation occurring after transport and rapid burial in low-oxygen prodelta muds. These findings revise the prevailing model of Burgess Shale-type faunas as strictly distal shelf/slope communities and provide a sedimentologically grounded framework for comparing Cambrian Lagerstätten. Future work could apply similar core-focused, process-based analyses to other Burgess Shale-type deposits, integrate high-resolution geochemical proxies for oxygenation and salinity variability, and conduct experimental/field studies to further quantify transport and burial pathways of soft-bodied organisms in delta-influenced systems.