PRECISON DIGITAL MAPPING TO ILLUSTRATE GEOLOGIC EVENTS ON RICHMOND ISLAND, CASCO BAY, MAINE

The geology of Richmond Is. in Casco Bay, ME consists of four major events: Dev. NW-SE continental convergence in the Acadian Orogeny, L. Dev. to Perm. regional Norumbega shearing, Triassic dike emplacement, and Pleistocene glaciation. Field data was collected by digitizing of dikes and faults with RTK GPS and Total Stations, and using handheld GPS to spatially referenced and store structural measurements of beds, foliation, lineation, quartz veins, shearbands, kinkbands, faults, dikes, and glacial striations. LiDAR-based DEMs and high-resolution orthoimagery were used to digitize visible dikes, faults, and glacial features not mapped in the field. Acadian convergence formed meter- to 10 meter-scale F1 recumbent fold clusters in the NE and S regions of the island with an average fold axis and axial plane of 12⁰S52W and N42E19NW, respectively. Bedding strikes NE-SW, gently dipping to the NW and SE throughout most of the island with locally steep dipping beds within the F1 fold noses. A near-horizontal hinge-parallel stretching lineation contributes to the woody texture in the phyllitic quartzites of the host Kittery Fm. Quartz veins, cm to ~20 cm in width, were emplaced orthogonal to lineation, have rotated CW about an axis at 88⁰S88W striking near parallel to mineral elongation due to regional dextral strike-slip shear associated with the Norumbega. Previously constructed fault maps were georeferenced to new orthoimagery and were combined with new lineaments in the LiDAR-based hillshade DEM to complete the E-W striking island-wide strike-slip fault system. Conjugate dextral shearbands and cm-wide sinstral kinkbands show a sigma-1 direction of 10⁰N89E, determined through stereonets. The NE-striking Triassic basalt dike array on the island consists of numerous dike exposures with an average exposed length of 23.6m, an intensity of 11 dikes per km, and range in width from .05 to 3.2 m with an average width of 1.42 m. Average opening direction for dike emplacement is N43W-S43E accommodating 3.7% regional extension in the host rock. LiDAR-based DEMs show some microtopography aligned with SE glacial flow, as well as, nested paleo-berms due to fluctuating sea-level. Top of the most dominant berm is ~9 above mean sea-level and can be dated at ~11,700 years BP using Maine’s post glacial sea-level curve (Barnhardt et. al., 1995).