New taphonomic and sedimentological insights into the preservation of high-relief Ediacaran fossils at Upper Island Cove, Newfoundland

Abstract

Southeastern Newfoundland, Canada, is home to the oldest Ediacaran fossils in the Avalonian Assemblage mostly being preserved beneath tuffites in association with microbial matgrounds. A unique fossiliferous surface at Upper Island Cove, known as the Allison Surface, exhibits three-dimensional preservation of Ediacaran fronds without an associated tuffite or microbial matground. Previous models have invoked entrainment of fronds within turbidity currents, or by their felling into erosive scours. Our work demonstrates that the fossils were preserved within beds before being partially exhumed and cast by a subsequent sandy turbidite. Many of the structures previously identified as rangeomorph stems are longitudinal erosional features within obstacle sours. Additional observations of stemmed/erect organisms coinciding with reclined taxa suggest crosscutting/palimpsesting of frondose taxa with deeply emplaced holdfasts. Many of the fossil organisms are found within the top of a Td unit, with early pyritization probably aiding in their three-dimensional preservation. Most fronds are incomplete and oblique to the axis of the surrounding scours, indicating they were partly exhumed after burial. The scours likely formed in the lee of erect stemmed rangeomorph and arboreomorph taxa, which are commonly preserved as holdfasts on the surface, and occasionally crosscut buried fronds. The crosscutting of frondose taxa by holdfasts and the presence of pyritic tubes in the overlying sedimentary units suggests the preservation of three separate communities: 1) the initial Td entombed mainly reclined organisms; 2) a pre-turbidity current community of erect taxa with bulbous holdfasts that were the loci of obstacle scours; and 3) a later community of erect organisms preserved as holdfasts/stems.

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