ORIGIN OF COLUMNS IN WEST ROCK RIDGE DIABASE: COOLING CRACKS OR TECTONIC JOINTS

West Rock Ridge is an ~ 11km long sill-like intrusion of diabase, with a maximum exposure thickness of 215 m, located along the western margin of the Mesozoic Rift Valley, north of New Haven, CT. The sill is contemporaneous to the Talcott Basalt, the first flow in the basin. The diabase forms an impressive ridge along its western face, with steeply dipping columns several meters wide and over 100 m long. Previous authors, Dana (1891), Longwell and Dana (1932), Skinner and Rogers (1985), and deBoer and Wareham (2013) have noted the large columnar joints and interpreted them to represent cooling columns that formed during crystallization. Also, they propose the curved-nature of the map view of the ridge is controlled by changes in column orientation. Furthermore, they suggest the variation in column width is a product of cooling rates and represents multiple intrusions. However, field observations and measurements along the length of the ridge more strongly support the assertion that the columns are the result of intersecting tectonic joints. Relative to their length and width, the columns exhibit a low hexagonality index value, inconsistent with thermal contraction. Viewed from the surface, the joints do not terminate when intersecting each other, as in a typical cooling columns do. Instead, they continue for meters to tens of meters before terminating, often forming a parallel set. The joints lack typical striae or ‘chisel marks” which form as a cooling fracture propagates in vertical increments, following the isotherm. The ridge face was divided into 10 segments where the strike and dip of joints were measured. The prominent joint directions are 185°, 82°W, 055°, 86° NW, 190°, 78° NW and 145°, 90°. There is not a significant difference between the orientation and number of joints among ridge segments. We propose West Rock Ridge columns are tectonic joints and the shape of the exposed ridge is not controlled by a change in joint orientation.