The Chengjiang Biota inhabited a deltaic environment

Abstract

The Chengjiang Biota is the earliest Phanerozoic soft-bodied fossil assemblage offering the most complete snapshot of Earth's initial diversification, the Cambrian Explosion. Although palaeobiologic aspects of this biota are well understood, the precise sedimentary environment inhabited by this biota remains debated. Herein, we examine a non-weathered core from the Yu'anshan Formation including the interval preserving the Chengjiang Biota. Our data indicate that the succession was deposited as part of a delta influenced by storm floods (i.e., produced by upstream river floods resulting from ocean storms). Most Chengjiang animals lived in an oxygen and nutrient-rich delta front environment in which unstable salinity and high sedimentation rates were the main stressors. This unexpected finding allows for sophisticated ecological comparisons with other Burgess Shale-type deposits and emphasizes that the long-held view of Burgess Shale-type faunas as snapshots of stable distal shelf and slope communities needs to be revised based on recent sedimentologic advances.

Fig. 1. Stratigraphic succession of the Yu’anshan Formation, showing lithologic log, bioturbation index, and the distribution of the dominant deposits.

OF oscillatory-flow deposits, HF hyperpycnal flow deposits, WEFM wave-enhanced fluid mud deposits, PF-TC plug flow, low-density turbidity current deposits, and H hemipelagic deposits. VF very fine, F fine, M medium, C coarse, VC very coarse. Note the clear large-scale coarsening- and thickening-upward trend that is common in wave-influenced shallow-marine depositional systems, including most notable deltas. The so-called “Upper Siltstone Member” is actually sandstone-dominated.

Fig. 2. Core photographs of the different deposits observed in the Yu’anshan Formation. All scale bars represent 10 mm.

a, b Hummocky cross-stratified and climbing oscillation-ripple cross-laminated (cor), fine-grained sandstone of the oscillatory-flow deposits. Note strongly aggradational component of oscillatory climbing ripples in b. c, d Current-ripple (cr) and climbing-ripple (clr) cross-laminated, very fine-grained sandstone, interbedded with bioturbated intervals (biot), representing hyperpycnal or turbidity flow deposits. Massive fine mudstone (Mm) and coarse mudstone laminae (M_sh) are interbedded. e Normal-graded (white triangles) and current-ripple cross-laminated (cr), fine-grained sandstone with minor soft-sediment deformation structures (ssd) of the hyperpycnal flow deposits, interbedded with massive mudstone (Mm) with erosive bases (eb) typical of the wave-enhanced fluid mud deposits. Burrows (bu) occur on top of the mudstone beds. f Rippled fine-grained sandstone (cr) with sharp-lined firmground burrows (bu) penetrating from the base of a hyperpycnal flow layer into an intensely bioturbated mudstone initially colonized under softground conditions. Fluid mud deposits with normal-graded mudstone (white triangles), coarse mudstone laminae (M_sh), and erosive bases (eb) are observed, interbedded with bioturbated intervals (biot). g–i Low-angle cross-laminated (la), current-ripple cross-laminated (cr), and hummocky cross-stratified (hcs), very fine-grained sandstone of the hyperpycnal flow deposits, interbedded with fluid mud intervals with erosive bases (eb). These beds alternate with successions of plug flow and low-density turbidity current deposits consisting of massive mudstone (Mm) with some parallel coarse mudstone laminae (M_sh), current-ripple cross-laminated coarse mudstone (M_sr), and normal and inverse grading (white triangles), and with dark, organic-rich mudstone (M_or) of the hemipelagic deposits. Note bioturbation structures (bu) and bioturbated intervals (biot) at the top of beds. j Massive mudstone (Mm) of the plug flow and low-density turbidity current deposits, with local normal grading (white triangles) and faint lamination (fl), locally intercalated with bioturbated intervals (biot), the latter representing the hemipelagic deposits. Burrows (bu) can be observed towards the top of mudstone beds.

Fig. 3. Block diagram showing the storm-flood-dominated delta with its characteristic deposits.

Idealized core intervals are shown for each type of deposit. Animals inhabited an oxygen-rich delta front and were transported by different types of flows to a more distal setting where preservation occurred under oxygen-depleted conditions. HCS hummocky cross-stratification.