The Chapel Island Formation of Newfoundland (Canada) revisited: integrating ichnologic and sedimentologic datasets to unravel early metazoan evolution

he Chapel Island Formation (CIF) is a 1000+ m-thick, mainly siliciclastic succession that is well exposed along the coastline of Burin Peninsula, southeastern Newfoundland, eastern Canada. The CIF contains an outstanding record of latest Ediacaran-early Cambrian trace fossils with some intervals rich in small shelly fossils, and in 1992 the Fortune Head section was ratified by the International Commission on Stratigraphy (ICS) and the International Union of Geological Sciences (IUGS) as the Global Stratotype Section and Point (GSSP) for the Cambrian System. This was the first system-level GSSP defined primarily on the basis of trace fossils, a decision that evoked considerable discussion among the geological community. This thesis represents the first modern study of the sedimentology and the first taxonomic appraisal of the trace fossils since the original studies that proposed the GSSP in the 1980’s. More than 1700 m of CIF strata were logged in the sea cliffs of Burin Peninsula at Fortune Head, Fortune North, Grand Bank Head, Lewin’s Cove, Little Dantzic Cove, and Point May. A revision of the sedimentology permitted the description and interpretation of fourteen sedimentary facies composing five facies association (FA), which are interpreted to be deposited in: (1) mud-flat, mixed-flat, sand-flat, and tide-dominated or -influenced embayments (FA-A); (2) middle and lower shoreface (FA-B); (3) offshore transition, upper offshore, and lower offshore (FA-C); (4) shelf (stricto sensu) (FA-D); and (5) carbonate subtidal and intertidal environments (FA-E). An extensive trace-fossil dataset was built from careful field observations and provided a comprehensive record 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). In addition, a comprehensive revision of the trace-fossil composition (3508 trace fossils identified) allowed the description of twenty-eight ichnogenera and fifty-one ichnospecies, which correspond to cf. Allocotichnus dyeri, Archaeonassa fossulata, Arenicolites aff. carbonaria, Arenicolites isp., Bergaueria perata, B. cf. radiata, Circulichnis ligusticus, C. montanus, Cochlichnus anguineus, C. luguanensis, Conichnus conicus, Cruziana problematica, Curvolithus multiplex, C. simplex, Curvolithus isp., Dendroidichnites aff. 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. saltensis, Rosselia socialis, Rusophycus avalonensis, Rusophycus isp. A, Rusophycus isp. B, Saerichnites kutscheri comb. nov., Teichichnus rectus, Torrowangea rosei, Treptichnus bifurcus, T. coronatum, T. pedum, T. pollardi, Trichichnus linearis, and Trichichnus isp. Sectioning and polishing of 47 lithic samples from throughout the CIF showed that the sediment mixed layer that characterizes modern oceans developed through a series of steps that took place in the early Cambrian rather than in the Silurian as previously advocated. The main evolutionary innovations took place in the offshore environment with three paleoecologic stages that comprised: (1) an Ediacaran matground ecology, with surficial and very shallow infaunal grazing organisms living on and within microbially bound seafloors; (2) a Fortunian matground/firmground ecology, with a burst in behavioural and anatomical innovations and the first evidence of colonization of the shallow-tier; and (3) a late Fortunian/Cambrian Age 2 mixground ecology, with the development of a shallow mixed layer and deeper discrete burrows of the transition layer. Evaluation of outcrop quality based on accessibility, lateral and vertical continuity of beds, stratigraphic completeness, and type of exposure, demonstrated that Fortune Head, Fortune North, Grand Bank Head, and Little Dantzic Cove represented the best suited section to perform trace-fossil analyses, whereas Lewin’s Cove and Point May suffered from exposure biases affecting their trace-fossil records. The Ediacaran-Cambrian boundary interval was also studied in detail at Fortune Head, Grand Bank Head, Lewin’s Cove, and Point May, and the base of the Cambrian was placed confidently at the first appearance of trace fossils of the Treptichnus pedum Ichno-Assemblage Zone. This study demonstrates that only through detailed, comprehensive, and integrative approaches, can research provide new empirical evidence that further unfold our understanding of the history of animal life on Earth.