ASPECTS OF THE TACONIAN ACCRETIONARY PRISM AND POST – TACONIAN SEDIMENTATION, NORTHERN MAINE

A polydeformed slate and metasandstone unit which crops out in northern most Maine and adjacent Quebec is interpreted as remnants of the Taconian accretionary prism. The dimensions of this outcrop belt, as mapped, are approximately 110 km long by approximately 3 to 4 km wide. This sequence has not been assigned a formal name to date. In northern Maine, this belt is unconformably overlain by a shallow water molasse sequence of late Ashgill age. Rocks of the interpreted accretionary prism are variably lithostratigraphic and tectonostratigraphic in nature. Stratiform rocks consist of thickly bedded quartz metaarenite and slate. On the western edge of the outcrop belt, the stratiform sequence (Groupe de Rosaire) exhibits a transition to non-stratiform mélange – like rocks. Also, the composition of metasandstones between the moderately well bedded portion and mélange shifts from one which is highly quartz – rich to one which is richer in feldspar and lithic grains. Non-stratiform rocks cannot be characterized as true block -in -matrix mélange because intraclasts of "exotic" lithologies are not incorporated within the poly deformed slate matrix. These non-stratiform rocks are complex assemblages of olistostrome and broken formation together with the more common structurally transposed and sheared metasandstone, metasiltstone and slate. Small to large scale shear bands, together with small scale thrust stacks and/or duplex structures are common. Discontinuous cataclastic fault zones consisting of a mixture of slate and broken quartz veins are uncommon. A complex event of folding is associated with the shearing. This first fold event produced non-cylindroidal folds whose geometries and orientations are variable. These earlier folds and cleavages are overprinted by upright folds with northeast trending and plunging fold axes. This younger event is consistent with fold styles observed in the Devonian Seboomook Group of the area. A spectrum of Late Ordovician (Ashgill) neritic near shore, beach and tidal flat environments unconformably overlie the accretionary prism rocks. The beach and neritic nearshore environments contain a variety of fossil invertebrates which universally confirm the age of the Little East Lake Formation as Ashgill.