THE PEAKS ISLAND MEMBER OF THE CUSHING FORMATION INTERPRETED AS A SUB-AQUEOUS VOLCANIC FLOW SUCCESSION IN SOUTHEASTERN MAINE

The Peaks Island member of the Cushing Formation in southern Maine is a metamorphosed, Ordovician sub-aqueous pyroclastic and/or epiclastic intermediate volcanic flow succession. This interpretation is based on the following observations of the rocks: 1) feldspar rich composition; 2) abundant layers which display moderate variation in their color, texture, and composition; 3) crystal tuff textures; 4) fine to coarse volcanic breccia textures.

Overlying the Cushing Formation is the Cape Elizabeth Formation which is a metamorphosed turbidity current sediment near the contact between the two Formations. Within the lower 10-20 meters of the Cape Elizabeth Formation material from the Cushing Formation is observed as lenticular beds up to 1 m wide and extending 3 m+ along strike. This observation suggests that some of the Peaks Island volcanic flow succession is un-welded material that was transported via epiclastic flows. However, this interpretation does not rule out the possibility that some of the Peaks Island member may have been transported by hotter pyroclastic flows. Furthermore, these lenticular beds of the Cushing Formation observed in the lower extent of the Cape Elizabeth Formation suggest that the flow was sub-aqueous not sub-aerial. The interpreted sub-aqueous nature of the flow is further supported by the observation that the Peaks Island member of the Cushing Formation interfingers with the Wilson Cove member of the same Formation.

The interpretation that the Peaks Island member of the Cushing Formation was emplaced as a sub-aqueous pyroclastic and/or epiclastic volcanic flow succession and the observation that it is, to some degree, inter-layered within the basal extent of the Cape Elizabeth Formation suggest that the contact between the Cushing Formation and the Cape Elizabeth Formation is conformable, not a fault contact or an angular unconformity both of which have been advocated in previous studies. This conclusion, the observations which support it, and additional information describing the interfingering of different members of the Cushing Formation also suggest that the various members of the Cushing Formation represent facies variations within the formation as a whole.