PETROGRAPHIC ANALYSIS OF WEST ROCK SILL: IMPLICATIONS FOR EMPLACEMENT HISTORY AND SILL THICKNESS

West Rock sill is a diabase intrusion along the western edge of the Mesozoic Hartford Basin. It intrudes the New Haven arkose and based on geochemical data (Steinen and Charney, 2019) is thought to be contemporaneous with the Talcott basalt. It’s assumed the magma intruded horizontally. Later fault movements tilted it and bounding sedimentary strata ~25° East. West Rock’s strike is nearly North-South. Samples were taken along the western face at different elevations, each representing a different height within the intrusion. Additionally, cores from the CT DOT Repository drilled from the eastern face, originally interpreted as the near-top of the sill, are described. All samples are composed of plagioclase, clinopyroxene, FeMgTi oxides, and minor orthopyroxene. The sample at 150 ft elevation is the finest grained and is thought to represent the near-base of the intrusion. Plagioclase and pyroxene have average sizes of 0.25 mm and 0.5 mm, respectively. A pyroxene phenocryst gives a maximum size of 1.5 mm. The sample from 425 ft elevation is composed of ophitic clinopyroxene enclosing euhedral plagioclase laths. The average size of plagioclase and pyroxene is 0.5 mm and 1.5 mm, respectively. The largest plagioclase and pyroxene phenocrysts are 1.5 mm and 2 mm. The sample from 620 ft elevation was previously assumed to represent the near-top of the intrusion. It is composed of hydrothermally altered plagioclase lath clusters and glomerocrystic clinopyroxene. Subophitic pyroxene encloses plagioclase laths. Plagioclase laths and pyroxenes have average sizes of 0.5 mm and 1 mm, respectively; the largest clinopyroxene is 2 mm. The core consists of medium grained plagioclase and pyroxene at the surface. Grain size increases with depth, reaching its coarsest ~75 ft below the surface. Minerals stay coarse until ~150 ft below the surface. Below 150 ft, grain size systematically decreases from medium to very fine at a depth of ~350 ft. The true thickness of West Rock can be calculated as ~ 775 ft based on structural data. However, the coarse grain sizes in the near-surface of the eastern face core and the high elevation outcrops don’t reflect a quickly-cooled upper contact. Instead, they are more consistent with a slowly cooled center. Using the coarse grained sample locations, we calculate the sill may be 1100 ft thick with a dip closer to 40°.