EDIACARAN BIOTA ON BONAVISTA PENINSULA, NEWFOUNDLAND, CANADA

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

Large-scale slumps and disrupted beds are present throughout the Trepassey Formation, representing periodic rapid burial of the habitat that sustained the organisms in this deep-marine setting. The slumps are of the same composition as their respective members and indicate the presence of a slope on which original sediment deposits became unstable, either because of sediment load or seismic shock; the mass movement was probably slight as the slumped material is near slump scars from which they were derived. The increasing influx up-section of laminated, fine- to medium-grained sand indicates high-energy downslope processes and may reflect shoaling of the basin or sea-level changes.

Fermeuse Formation.-The black-shale-dominated Fermeuse Formation lies in sharp and conformable stratigraphic contact above the Trepassey Formation. The succession studied to date consists primarily of three principal, interbedded lithofacies. Following the terminology of O'Brien et al. (2006), the prominent facies (A-f), seen in the lowermost part of the formation, is dark gray to black shale and mudstone with laminae, and thin to medium interbeds of gray siltstone, fine grained, brown-weathering gray sandstone, and minor tuff. Impoverished current ripples and cross-lamination are locally present but are usually indistinct; rhythmically alternating sand-mud graded units are common. A second, characteristically remobilized facies (B-f) consists of slumped folds of sandstone resedimented in a mud matrix that occur with debris flows. The latter consist of a mixture of sand, mud, and coarse angular to rounded cobble-size fragments of siltstone and sandstone. A third facies (C-f), developed in the upper part of the succession, includes black shales with rare or widely spaced laminae or thin beds of silty sandstone. In general, the proportion of sand increases upwards through the formation. Tuff beds are most common in the lower 300 m of the section and are associated with Ediacaran fossils.

The remobilized facies (B-f) occurs throughout several hundred meters of stratigraphic section and contains repetitive and spectacularly preserved tabular units of synsedimentary folds and disrupted beds of sandstone, each unit commonly several meters thick, interbedded with black shale and silty sandstone. The slumped units are locally capped by sand-rich sedimentary breccias overlain by shale. Thin beds of ash within the shales within facies A-f and B-f preserve a variety of Ediacaran fossils at several stratigraphic levels.

The slumped and disrupted units are attributed to gravitational sliding of poorly consolidated beds of sand and mud on a sloping paleosurface during normal pelagic sedimentation. Huge masses of mixed sand, mud, and coarse clastic debris were transported as debris and other mass flows into deeper parts of a submarine slope and basin. The presence of sharp bedding planes directly above truncated folds of sandstone and coarse breccias at the top of the disrupted unit indicate periods of erosion by intense bottom currents.