EDIACARAN BIOTA ON BONAVISTA PENINSULA, NEWFOUNDLAND, CANADA

Journal of Paleontology, Jan 2008 by Hofmann, H J, O'Brien, S J, King, A F

The Ediacaran strata of eastern Bonavista Peninsula, like their correlatives on the Avalon Peninsula, represent a depositional transition from deepwater basin and slope (Conception Group) to shallowing-upwards basinal, pro-delta and delta front (St. John's Group), and ultimately alluvial conditions (Signal Hill Group). Large quantities of volcanic ash were brought into this arc-adjacent marine basin throughout its depositional history and were important in the burial and preservation of the Ediacaran biota.

LITHOSTRATIGRAPHY AND PALEOENVIRONMENTS

CONCEPTION GROUP

Drook Formation.-The oldest exposed unit in the map area is the Shepherd Point Member of O'Brien and King (2005), part of the Drook Formation in the Conception Group. This member forms the core of the Catalina Dome (Fig. 1), and is characterized by evenly laminated gray-green siltstone interspersed with numerous thin laminae to thin beds of fine-grained, light gray weathering sandstone, and dark gray-green to black mudstone. Some black mudstones show minute irregular or wavy laminations several millimeters thick; preliminary studies suggest they may represent microbial mats on the sediment surface. Individual centimeter-scale sets of fine sand, silt, and mud are of constant thickness and laterally continuous for several meters, where they pinch out or are terminated by slightly irregular, very low-angle erosional discontinuities. Small scours, small-scale synsedimentary slump folds and faults, and mud rip-up clasts are common and indicate that erosive bottom currents flowed over an unstable slope. Interspersed throughout the member are medium beds of parallel-laminated, poorly sorted, structureless sandstone and rare beds of cross laminated sandstone. This unit has not yet yielded fossils in the study area, but is fossiliferous on the Avalon Peninsula (Narbonne and Gehling, 2003; Clapham et al., 2004).

Mistaken Point Formation.-The overlying Mistaken Point Formation was subdivided by O'Brien and King (2005) into a lower, siliceous Goodland Point Member and an upper, argillaceous Murphy's Cove Member; both are fossiliferous throughout. The Goodland Point Member is a medium- to thick-bedded siliceous sequence composed of parallel- and cross-laminated sandstone, parallel-laminated siltstone, structureless mudstone and tuff, interpreted as representing Bouma turbidite subdivisions BCDE. The silt, mud, and ash may have been deposited as DE beds or as hemipelagic sediment associated with ash-fall material that was slowly deposited from suspension within the deep sea. Large-scale slump folds (up to several meters in height) are locally preserved and result from gravitational instability of sediment deposited on a slope. This member resembles, and may correlate with, the Middle Cove Member of the Mistaken Point Formation, eastern Avalon Peninsula (King, 1990). Like the soft-bodied Ediacaran fossils on the Avalon Peninsula, the impressions of the organisms on the Bonavista Peninsula are confined to turbiditic siliciclastics and are typically preserved on upper bedding surfaces of interturbidite mudstones, underneath thin water-laid tuffs.