A STRUCTURAL COMPLEX RELATED TO THE SENNEBEC POND FAULT, MID-COAST MAINE

At the edge of the St. Croix belt in Appleton and Searsmont, Maine, is a set of rocks mapped as the Jam Brook Formation (Bickel, 1971), correlated with the Megunticook Formation on the 1985 state map. Our new mapping partly supports this correlation, but shows other rocks interleaved with Megunticook types in a structural complex. This complex is in sharp contact to the NW across the Sennebec Pond fault with the Appleton Ridge Formation and Ghent Phyllite of the Fredericton belt. Those formations appear virtually unaffected by the fault. In contrast, rocks immediately SE of the fault are strongly foliated, interleaved at scales from 100 to less than 5 meters, cut by thin zones of ribbony mylonite, and retrograded to greenschist facies, features also not present in the Penobscot Formation to the southeast.

Rock types in the complex include marble, calc-silicate rocks, quartzite, polymictic pebble conglomerate, gneiss with blue quartz grains, cummingtonite-garnet gneiss, light gray quartz-mica schistose granofels, and quartz-mica-garnet schist. Some units, notably the conglomerate and some quartzites, can be traced for several kilometers, though in detail the sequence of rock units changes along strike. Most rocks can be found in the Megunticook Fm or underlying rocks in Rockport Harbor, indicating that only St. Croix belt rocks are involved. The Muzzy volcanics, previously assigned to the Appleton Ridge, we place SE of the Sennebec Pond fault, primarily because of its metamorphic contrast with the Ghent, and also because of similarity to some Penobscot Fm volcanics.

This structural complex is jammed against the southeast-dipping Sennebec Pond fault near a 45° bend in the fault trace, suggesting it is related to the fault. Previous work has shown the fault truncates inferred Devonian (ca. 386 Ma?) isograds in the Appleton Ridge, and is intruded by the 371 ± 2 Ma Mt. Waldo pluton. A thrust is not likely because a) Devonian metamorphism is absent from the St. Croix belt so it must have been at higher structural levels, and b) footwall rocks would be expected in the fault complex. We conclude that it is a late-Acadian down-to-the-southeast normal fault that postdates the late-metamorphic dextral shear in the Appleton Ridge, and might reflect local extension accommodating voluminous intrusion of granite in the coastal region.