MICROSTRUCTURAL EVALUATION OF WATERVILLE FORMATION IN CENTRAL MAINE, USA

Outcrop-scale structures in interbedded Silurian phyllite and limestone of the Waterville Fm. in central Maine record Silurian to early Devonian NW–SE-directed shortening and Devonian dextral transpression. Here we evaluate the style and timing of fabric formation relative to metamorphic mineral growth through microstructural analyses and field observations at the type locality in central Maine.

Thin sections were cut parallel with the strike of layering and perpendicular to dip. The pelitic matrix contains quartz + muscovite + dolomite + chlorite + albite ± siderite with accessory apatite, tourmaline, rutile, iron sulfides, monazite, and zircon. The carbonate matrix has a similar composition except it contains more dolomite and fewer sheet silicate and accessory grains. Mn- and Mg-zoned 0.03–0.4-mm-wide siderite porphyroblasts with inclusion-rich cores and inclusion-free rims are abundant in pelitic layers, and they often make the rocks look like wackes in outcrop. Late chlorite caps often overgrow both porphyroblast phases. Muscovite ± quartz pressure shadows with straight fabrics also commonly overgrow porphyroblasts.

The sections exhibit three foliations. S₁ is defined by inclusion trails in the porphyroblasts. It is oblique to layering and its orientation varies up to 90° in a single section. S₂ and S₃ are defined by muscovite ± chlorite. S₂ is a penetrative layer-parallel foliation in the matrix that corresponds to a phyllitic axial-planar foliation associated with outcrop-scale, NE-striking, upright isoclinal folds. S₃ is a discontinuous pressure-solution cleavage with concentrated sheet silicates that is oblique to S₁, S₂, and layering. It also truncates porphyroblasts, chlorite caps, and pressure shadow overgrowths.

Sections also contain multiple generations of carbonate-quartz veins. Many veins show fibrous dolomite on their edges, and some exhibit equidimensional calcite in their centers. Most veins at high angles to layering are folded while veins at low angles to layering are boudinaged and show flanking folds indicating dextral rotation, locally exceeding 90°. Dolomite vein fibers curving into S₂ in the matrix and S₃ wrapping around veins and overprinting flaking folds suggest that vein deformation is coeval with S₂, which therefore is linked to regional Devonian transpression.