AN EVALUATION OF LINEAMENTS, FAULTS, AND DIKES USING LIDAR HILLSHADE AND DETAILED GEOLOGIC MAPPING DATA IN CHATHAM COUNTY, NORTH CAROLINA - A POTENTIAL TOOL FOR COUNTY STAFF

Diabase dikes, fracture zones, and faults zone are areas of increased groundwater permeability and flow and have important ramifications to groundwater resources and pollutant transport. Few counties in North Carolina utilize detailed geologic data on a regular basis to assist them in their decision-making processes and various permitting activities. Detailed geologic mapping in Chatham County, North Carolina by the NC Geological Survey, with partial support from STATEMAP, has identified multiple diabase dikes, faults, and fracture zones. Many of these features are present as prominent lineaments on hillshade LiDAR images and, in some cases, extend for multiple miles.

Geologic maps are inherently difficult to interpret for non-geologists and are often not utilized by county staff in the decision-making process. To be useful to more stakeholders, geologic data needs to be displayed and/or explained in a simpler manner. To that end, we are developing categories that rank the degree of confidence for dikes, faults and lineaments. These linear features are being compiled in a GIS. Each line element within the GIS will include a ranking. It is the goal, that when the county staff are reviewing permitted activities (e.g. new well permits, building permits, etc), that the GIS layer of dikes, faults and lineaments can be used to determine the presence/absence of linear geologic features and be incorporated in the permit process.

Draft categories are as follows: Type 1 features indicate a high confidence that the geologic feature exists. Type 1 lineaments are well defined and easy to identify from Hillshade LiDAR. A Type 1 diabase dike has multiple boulders and or outcrop locations identified along the trace of the dike. A Type 1 fault has locations of brecciated rock, quartz breccia and/or truncated geologic units identified along its trace.

Type 2 features indicate a moderate degree of confidence that the geologic feature exists. Type 2 features may warrant additional site-specific investigation to confirm presence. The existence of a Type 2 feature is based on the experience and judgement of the geologist and not on direct observations in the field.