USING STRUCTURAL ANALYSIS TO MAKE STRATIGRAPHIC CORRELATIONS, BERWICK, ME TO RAYMOND, NH

The Phyllonite at Church Road (Scr; formally the Gonic formation) crops out west of the Berwick Fm (Sb) near Berwick Maine. The extension of this pelitic unit to the southwest is uncertain as it is cut by the Barrington Pluton. Correlation with the pelitic Gove Member (Sbg) of the Sb in SE NH is reasonable based on stratigraphic location and similar bt-grt-str assemblage. This work proposes an alternative, correlating it with the Watson Hill (Sbw) member of the Sb based on structural analysis.

In the Somersworth quad, metasedimentary rocks on both sides of the Scr contain remnant bedding with shallow- to moderate-dips that steepen into concordance with the vertical phyllonitic fabric in the Scr near their contacts. Bedding is unrecognizable in the Scr and at most locations the NE- to ENE-striking phyllonitic fabric is all that is present. In several locations, however, outcrop and microstructural analysis identified two planar fabrics, a NNE- to NE-striking penetrative biotite foliation (S1) and a more easterly striking axial planar cleavage (S2) where S1 is deformed into S-folds that plunge moderately to the NE.

Along strike to the southwest, near Raymond, NH, two NE-striking pelitic units crop out east and west of the Flint Hill Fault Zone (FHFZ), the bt-grt-str grade Sbg and the bt-grt-sill grade Sbw, respectively. The Sbg contains only one planar fabric that is parallel to compositional layering, which may represent bedding. Local folding of this fabric is about horizontal axes interpreted as drag folds due to east-side down, dip-slip movement along the adjacent FHFZ. The Sbw, however contains 2 foliations with an intersection lineation plunging moderately NE. This lineation is recognized at nearly all locations yet is not seen in the Sbg. Furthermore, NE-plunging S-folds with well developed crenulation cleavage are recognized in several places. Where strain is strongest in the Sbw it crops out as a phyllonite with deformed quarts stringers, very similar in appearance to the Scr.

The similarity of structural styles in the Scr and Sbw support the argument that these units are correlative. Additionally, rocks adjacent and east of the Sbw are interpreted as Sb, in agreement with the stratigraphic order in Berwick ME. The difference in mineral assemblage can be explained by late uplift of the Sbw during movement along the FHFZ.