ALL-DIGITAL OUTCROP MAPPING AT HIRAM FALLS, SACO RIVER, MAINE

Detailed mapping of outcrop-scale structures in a granite exposure on the Saco River at Hiram Falls, western Maine was conducted using an all-digital integrated surveying system. This system employs dual frequency GPS receivers and electronic total stations to collect digital field data and ArcView GIS to compile and coordinate field data with digital outcrop photos and orientation analysis. A sub-cm global datum was established using post processed geodetic GPS data to position a GPS broadcasting field base receiver that worked with real-time kinematic (RTK) GPS rover receivers capable of 2-cm precisions. Conventional total station surveying was tied to the GPS global coordinates with sub-cm precisions. These digital instruments were used to map basalt dike contacts, xenolith outlines and traces of normal faults that cut the exposed granite as well as record position and orientation during structural data collection and elevations for topographic mapping of the outcrop surface. ASCII data files in Excel were converted to point and shape files for use in ArcView projects creating map arrays that served as a base for structural analysis. Digital mapping has revealed 100+ xenolith blocks of metamorphic host rock ranging from 1-20 meters in diameter. Crosscutting the xenolithic granite are ~20 basalt dike intrusions up to ~2 m in width with a more dominant early NE phase and a later EW phase. The NE phase dikes show intricate geometries due to the interaction of initially en echelon fractures upon intrusion and dilation with an average dike width of 0.72 m, a dike intensity equivalent of 125 dikes per km and a dike extension of 9.2-10.1%. Over 65 small-scale NNE normal faults, 1-35m in length crosscut the exposure contemporaneous with dike intrusion as some dikes are crosscut by the faults and some faults are used by the dikes during intrusion. The normal faults have cm-displacements and preserve faint slickenlines with slip surfaces that dip dominantly to the SE and less frequently to the NW. Orientation analysis suggests that the small-scale normal faults form a conjugate set where the NE dikes preferentially intruded the NW dipping segments under a gently NE plunging sigma 2 direction and a subhorizontal NW-SE trending sigma 3 regional Mesozoic extension direction.