A STRUCTURAL TRANSECT OF THE SEBOOMOOK FORMATION, CAUCOMOGOMOC LAKE, NORTHWESTERN MAINE

A detailed structural transect of approximately one kilometer was measured along a length of road in the Devonian Seboomook Formation near Caucomagomoc Lake, Northwestern Maine. Here, the Seboomook Formation displays fine- to coarse-grained metasandstone with variable amounts of graded beds. Within this section five lithostratigraphic units were identified and topping directions were determined from graded beds. The strike of bedding is nearly perpendicular to the transect and outcrops occur roughly every eight to ten meters. Bedding, two generations of cleavage development (early continuous and a later spaced), bedding-cleavage intersection lineations, and fold hinges were observed and are assumed to be Acadian structures. Bedding is deformed by open, similar to chevron style folds with nearly horizontal hinges that trend NE and SW and limbs that dipping shallowly to the west and steeply to the east. Continuous cleavage strikes NE and dips steeply to the west and spaced cleavage, where observed is sub-parallel to the older continuous cleavage. Young, slickensided, brittle normal faults are also present but do not show significant offset. Pi-plots show that the observed fold hinges lie roughly parallel to rare measurable intersection lineations between bedding and continuous cleavage.

A retro-deformation of the rocks along this transect has implications for nearby pre-Acadian structures. To the west is the late Cambrian (?) to early Ordovician-aged Caucomgomoc inlier, a mélange of deep marine sedimentary and volcanic rocks, inferred to have been part of an intra-Iapetus accretionary prism that dips steeply to the west and is cut by reverse (west-over-east) faults. Retro-deforming the Acadian structure may allow a better understanding of the early Ordovician subduction zone orientation and facing.