FRACTURE DENSITY COMPARISON OF THE LOWER CRETACEOUS KAINER FORMATION OF THE EDWARDS GROUP AND UPPER GLEN ROSE FORMATION, COMAL COUNTY, TEXAS

Fracture orientations and distributions were measured at two localities in Comal County, Texas: Bear Creek Fault (displacement is highly debated) and the Canyon Dam Spillway (adjacent to a fault with greater than 30 m displacement).

The lithologies studied: The Kainer Formation of the Edwards Group is 110-130 ft of massively bedded cyclic subtidal to tidal-flat mudstone to grainstone dolomitic limestone. Chert occurs throughout the unit varying in amounts and honeycomb porosity is common (Collins 2000). The Edwards Group is the main aquifer-bearing unit in the San Antonio area. The Upper Glenn Rose Formation is 350-500 ft of thinly bedded limestone and marl located stratigraphically below the Edwards Group and is considered the lower confining unit.

At the Bear Creek Fault both Edwards and Glen Rose crop out due to movement along the Bear Creek Fault. Thirty meters of a four hundred-meter roadcut of the Dolomitic Member (Edwards) was studied on the down-thrown side of the Bear Creek Fault. Dilational fractures were recorded throughout the given interval in the Edwards outcrop. The Edwards fractured extensively throughout the measured section. A sixty-meter length of roadcut was studied in the Glen Rose outcrop on the up-thrown side of the Bear Creek Fault. Fracture densities within the Glen Rose outcrop were concentrated near the fault and fracture density decreased further away from the fault. Fractures match regional strike and dips.

The Canyon Dam Spillway is cut through the Upper Glen Rose Formation. A sixty-five meter interval was studied at the spillway. A scanline traverse was taken perpendicular to the principal spillway fault. Fracture density decreased further away from the fault. Fractures match regional strike and dips.

The data suggests that the Glen Rose lithologies dissipate fault-related stresses more readily than the Edwards. The significance of this data suggests different flow paths for the two units. Water flow in a carbonate aquifer is dominated by fracture porosity. The Glen Rose shows evidence for a more restricted flow due to less abundant paths that are concentrated at and near the fault. While the Edwards would have a higher percentage of flow paths due to extensive fracturing throughout the unit.