Abstract

Differentiated volcanic rocks and co-magmatic plutons in southeast coastal Maine have been mapped as Silurian to Devonian in age. They are flanked by volcaniclastic and pelitic rocks considered Upper Cambrian to Lower Ordovician based on isotopic and paleontologic dating, and because of fragmental rocks, interpreted as representing erosional unconformities between the lavas and pelites. However, metal deposits contained within both lithologies have trends in form and element assemblages consistent with progression from proximal, through distal, to basinal volcanic environment. Therefore, we have re-evaluated the stratigraphy and the criteria on which relative ages were assigned previously.

Remapping of critical exposures and measurements of structural data were undertaken at type localities to check for differing deformational histories in major rock units, the main basis for previous age distinctions. The structural elements in the major formations display a consistent orientation, and the geometry of minor structures in rocks with mechanically equivalent properties is comparable. It is concluded from orientation data, and from consideration of volcanic processes, that there is no basis for assigning different tectonic histories, and therefore different ages, to formations above and below inferred unconformities, which are herein viewed as volcanic breccias. The Castine and Ellsworth Formations are, at least in part, simply proximal and distal manifestations of the same volcanic episode.

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