NEW METHODS FOR ANALYZING FRACTURE AND STRATIGRAPHY EFFECTS ON SURFACE AND GROUND WATER FLOW IN THE SACRAMENTO MOUNTAINS, SOUTH CENTRAL NEW MEXICO

Fractures are typically assumed to influence water movement, but the relationships of stream orientation or spring discharge locations with fractures are rarely quantified. By combining fracture measurements in the field with GIS analysis, fracture control on stream orientations and spring discharge locations are assessed. Results from a new method, which selects stream segments that are parallel to field-measured fractures, correlate with spring locations significantly better than results from manual lineament analysis. This method provides a means for analyzing the effects of fractures on both surface and ground water flow in heavily vegetated areas where bedrock is typically covered by alluvium and soil. The study area in the southern Sacramento Mountains, south central New Mexico fits in this category, making direct stratigraphic and structural observations difficult. Another new method uses elevations of a mapped geologic contact to produce data for a three-dimensional stratigraphic surface, which subsequently enables calculations of the stratigraphic levels of the springs. A histogram of spring stratigraphic depth indicates zones of preferential ground water flow.

Topographic relief and east dipping strata, consisting of San Andres Formation limestone and Yeso Formation limestone and mudstone at the ground surface, cause most streams in the study area to flow generally eastward towards the Pecos River. However, stream segments that are parallel to field-measured joints form prominent peaks on a plot of total stream length vs. orientation. Joint-parallel stream segments selected from streams mapped with a DEM are up to 900 meters in length and are parallel to fractures observed in the field. The abundance of surveyed springs throughout the study area decreases logarithmically with distance from joint-parallel stream segments. These findings indicate that joints strongly affect surface and groundwater flow, and most likely recharge. Future field observations will quantify the effects of fractures on the interaction between surface and ground water by measuring stream flow rates upstream and downstream of joint-parallel stream segments.