Ichnology of the Ediacaran–Cambrian Chapel island formation of Newfoundland, Canada : unraveling bioturbation at the onset of the Cambrian explosion

Omtale

The Chapel Island Formation is a 1000+ m-thick, mainly siliciclastic succession that is well exposed in coastal cliffs of Burin Peninsula, southeastern Newfoundland, eastern Canada. This unit contains an outstanding record of late Ediacaran-early Cambrian trace fossils with some intervals rich in small shelly fossils, and in 1992 the Fortune Head section was ratified as Global Stratotype Section and Point for the Cambrian System. This contribution represents the first study integrating sedimentologic and ichnologic information for the whole formation and the first systematic monographic work, involving documentation of the trace fossils in classic sections at Fortune Head, Grand Bank Head, and Little Dantzic Cove, as well as in less-explored sections at Fortune North, Lewin's Cove, and Point May. More than 1700 m of strata were logged, and fourteen sedimentary facies composing five facies associations were described and interpreted to be deposited in: (1) mud-flat, mixed-flat, sand-flat, and tide-dominated or -influenced embayments (Facies Association A); (2) middle and lower shoreface (Facies Association B); (3) offshore transition, upper offshore, and lower offshore (Facies Association C); (4) shelf (stricto sensu) (Facies Association D); and (5) carbonate subtidal and intertidal environments (Facies Association E). An extensive trace-fossil dataset was gathered and provides records of bioturbation intensity (1596 data points on vertical bioturbation, 1481 data points on bedding plane bioturbation) and of trace-fossil metrics (3162 data points on burrow width, 1473 data points on burrow depth). A comprehensive revision of the trace-fossil composition (3508 trace fossil specimens recorded) allowed the description of twenty-eight ichnogenera and fifty-two ichnospecies, which consist of cf. Allocotichnus dyeri, Archaeonassa fossulata, Arenicolites aff. A. carbonarius, Arenicolites isp., Bergaueria perata, B. cf. B. radiata, Circulichnis ligusticus, C. montanus, Cochlichnus anguineus, C. luguanensis, Conichnus conicus, Cruziana problematica, Curvolithus multiplex, C. simplex, Curvolithus isp., Dendroidichnites aff. D. irregulare, Didymaulichnus miettensis, Dimorphichnus isp. A, Dimorphichnus isp. B, cf. Dimorphichnus isp., ?Diplocraterion isp., Gordia marina, Gyrolithes gyratus, G. scintillus, Halopoa imbricata, Helminthoidichnites tenuis, Helminthopsis abeli, H. hieroglyphica, H. tenuis, Monomorphichnus bilinearis, M. lineatus, M. needleiunm, Monomorphichnus isp., Palaeophycus annulatus, P. tubularis, Palaeophycus isp., Psammichnites gigas circularis, P. cf. P. saltensis, Rosselia erecta, Rosselia isp., Rusophycus avalonensis, Rusophycus dabardae isp. nov., Rusophycus isp., Saerichnites kutscheri comb. nov., Teichichnus rectus, Torrowangea rosei, Treptichnus bifurcus, T. coronatum, T. pedum, T. pollardi, Trichichnus linearis, and Trichichnus isp. The trace-fossil distribution and other ichnologic information allow the identification of three palaeoecologic events in the succession: (1) an Ediacaran matground ecology, dominated by simple horizontal trails associated with microbially stabilized surfaces and limited vertical bioturbation; (2) a Fortunian matground/firmground ecology, with evidence of penetrative shallow-tier bioturbation and a burst in trace-fossil diversity; and (3) a late Fortunian/Cambrian Age 2 mixground ecology, with high bioturbation intensities and the development of a shallow mixed layer and deeper transition layer with a structure similar to that of modern seafloors. Overall, this study reinforces the status of the current Cambrian Global Stratotype Section and Point and advocates for the need of comprehensive and multi-disciplinary approaches to fully decipher the scale, tempo, and loci of the early evolution of animal life on Earth.