FOSSILIFEROUS LAMINITES OF THE PROTEROZOIC CAMBRIDGE ARGILLITE, BOSTON BASIN

Ediacaran fossils occur in Boston Bay Group laminites of the Cambridge Argillite (CA), but interpreting these body fossils has been difficult. Aspidella terranovica (“ring fossils”) from the area were first described as pseudofossils in 1923. Aspidella specimens were discovered in outcrop at Hewitt's Cove, Hingham, Massachusetts, in the early 1990s. The Boston Bay Group Aspidellas are well preserved, showing internal laminations that may be either convex or concave downward, sometimes both in the same specimen. Similar internal fills have been observed in other Ediacaran fossils, such as Beltanelliformis brunsae from Yukon, Canada. Such laminations may be absent at other Aspidella localities, where the fossils are often preserved only as a basal impression.

The fossils at Hewitt's Cove and Slate and Grape Islands, Massachusetts, occur within silty to fine sandy laminites. The conspicuously laminated CA bears many graded layers that are commonly interpreted as fine-grained turbidites. However, rhythmically laminated facies also occur that do not support a sediment-gravity flow interpretation. These rhythmites raise the possibility of sedimentation influenced by tides or waves, and suggest that the CA consists of a subtle, but definite, mosaic of laminite facies that record various depositional processes, and perhaps various water depths. Preliminary analysis of another “Avalonian” terrane, the Carolina Slate belt, indicates the presence of similar rhythmically laminated facies associated with Ediacaran fossil-bearing strata.