CONTACT RELATIONSHIPS BETWEEN THE LIBERTY-ORRINGTON AND FREDERICTON BELTS IN THE BOOTHBAY REGION, MID-COASTAL MAINE

In the Boothbay region of Maine, the contact between Ordovician peri-Gondwanan rocks of the Liberty-Orrington belt and Silurian syn-orogenic strata of the Fredericton belt is well exposed, though complexly folded and metamorphosed to amphibolite facies in the Devonian. Adding to the complexity, two additional rock sequences are present, the East Harpswell Group and Cross River Fm., both of uncertain age and affinity. New detailed geologic mapping, along with U-Pb geochronology (in progress), provides protolith details for all of these rock units, and insight into pre-Devonian contact relationships.

The Ordovician Cape Elizabeth Fm. in the Liberty-Orrington belt is exposed in an ~40 km across strike width. It is dominated by metamorphosed feldspathic siliciclastic rocks, but locally abundant amphibolite is present. In the northern Casco Bay area, rocks of the East Harpswell Group (EHG) are exposed in a 3-4 km wide antiformal belt interpreted as a thrust window. Essential to the thrust interpretation is an unpublished U-Pb age from a metavolcanic rock in the EHG that suggests they are ~25 m.y. younger than the structurally overlying Cape Elizabeth Fm.

The Silurian Bucksport Fm. in the Fredericton belt is dominated by a monotonous sequence of metamorphosed calcareous turbidites. Exposed in two antiformal 2-5 km wide belts within the Bucksport Fm. are strongly migmatitic pelitic gneisses of the Cross River Fm. The protolith ages of these rocks are unknown, as are their contact relationships with surrounding rocks of the Bucksport Fm.

Finally, the contact between the Cape Elizabeth and Bucksport formations (the Dog Bay Line of Reusch and van Staal, 2012) has long been interpreted to be an early thrust in this region (Boothbay thrust). Interestingly, as noted by Hussey (1986) and confirmed in this study, at multiple locations rocks of the two formations are interlayered at the contact, which could be explained by either a stratigraphic or structural origin. Central to the thrust hypothesis has always been an interpreted difference in protolith age between the two units. Currently these differences are based on along strike correlations with units 10s to 100s of kms away. U-Pb analysis are pending from the Cape Elizabeth Fm., Bucksport Fm., Cross River Fm., and EHG and should shed light on contact relationships between all units.