FAULT RELATED FRACTURES IN THE POTSDAM FORMATION, ALTONA FLAT ROCK, NORTHEAST NEW YORK

Altona Flat Rock (AFL) is a spectacular pavement outcrop (~30 km²) of C-O Potsdam sandstone, NE NYS, formed during the rapid drainage of glacial Lake Iroquois. The SE region of AFL is underlain by the Ausable member, while the Keesville member is exposed in the NW. The bedding dips gently NE, and is apparently cut by a NW striking splay associated with the Champlain Valley fault system. The surface expression of the fault consists of straight cliffs and valleys (500-1000 m long) that interrupt the gentle topography, forming sub-rectangular drainage. There is no apparent offset of the stratigraphy, so a study of fractures was completed in an attempt to understand the fault kinematics and to better constrain the location of the fault.

During the summer 2010, the SUNY Oswego geology field program completed field mapping of the AFL region. Most exposed surface of AFL were examined and detailed fracture data was collected including attitude, abutting relationships, density (counted perpendicular to strike for each fracture set: frac / meter), and kinematic information. AFL is host to two mutually intersecting steeply dipping fracture sets. One set strikes NE in the western AFL, but the strike gradually varies to E-W moving to the eastern region. This fracture set has a density of 1-3 fractures/m across the entire AFL. The other fracture set strikes NNW and N-S (difference of 25-30^o) on the SW and NE sides of the fault respectively. The systematic difference in strike occurs across the fault region. There is no vertical displacement on individual fractures for either set, but the N-S/NNW striking set frequently displays minor (cm’s) dextral offset of the E-W striking set. The N-S/NNW set has a varied density of 2-17 fractures/m. In some instances, fracture density profiles have good correlation with the location of the apparent fault, however, density contour maps revealed that most regions of highest fracture density do not correlate. Considering that the region immediately adjacent to the fault scarps are concealed by wetlands in most places, the lack of correlation is the result of the data distribution. At least two of the highest fracture density regions form linear trends (N-S & E-W), and they are probably the surface expressions of basement faults that did not completely penetrate the overlying strata.