IGNEOUS AND SEDIMENTARY CLASTS FOUND WITHIN THE CONGLOMERATE STRATA OF THE JURASSIC PORTLAND FORMATION: IMPLICATIONS FOR SOURCE REGIONS

The Portland Formation (Early Jurassic) is the uppermost strata in the Newark Supergroup located within the Hartford Basin. It is composed of both lacustrine and fluvial deposits of poorly sorted, micaceous arkose and siltstone with occurrences of conglomerate strata along the basin margin, adjacent the Eastern Border Fault. In Durham, CT, an outcrop of conglomerate strata, ~600 m long with ~25 m of relief, is exposed. The conglomerate is clast supported with a matrix consisting of angular, medium- to coarse- grained sand. Crude layering is illustrated by graded beds and alternating layers of sandstone and conglomerate beds. Clasts are imbricated, range in size from 4 mm to ~250 mm, and range in shape from subrounded to angular. The imbrication and rounded nature of the clasts is indicative of deposition in a braided river system.

The clasts consist primarily of moderate grade metamorphic rocks indicative of an Eastern Highlands source area. However, the outcrop also contains clasts of basalt and sandstone, which form outcrops only within the Basin. Therefore, the presence of these clasts do not fit with the standard alluvial fan source model, implying that either 1) the basin was previously larger, extending over the Eastern Border Fault or 2) there was a separate source area, within the basin, possibly towards the north.

The basalt clasts are composed of a range of textures including cryptocrystalline, hypocrystalline, and aphantic. In all cases, altered glass and pyroxene is intersertal to plagioclase laths. Amygdules are abundant and filled with calcite and vermiculite. XRF analysis show the basalt is a high-Ti tholeiite, consistent with the composition of the Hampden Basalt, the uppermost lava flow in the basin. Sandstone clasts are composed of white, well-washed, subrounded quartz sand grains with minor feldspar. A single sample contains plant fossils. There is no current analogue for the sandstone clasts within the Basin or Eastern Highlands. We conclude the Durham conglomerate was deposited in a braided fluvial environment with variable source areas.