LITHOLOGICAL STUDY OF THE CONGLOMERATE STRATA OF THE PORTLAND FORMATION: IMPLICATIONS FOR PROVENANCE AND DEPOSITIONAL MODEL

The alluvial fan conglomerate at Guida Farm, an outcropping of the Portland Formation, is located in Durham, CT. 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. At Guida Farm, an outcrop of conglomerate strata, ~120 m long with ~13 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 with sparse distribution of sand lenses. 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 some clasts is indicative of braided river system deposits. The clasts consist primarily of moderate-grade metamorphic rocks consistent with an Eastern Highlands source area. However, the outcrop also contains clasts of fine-grained quartz sandstone and outsized (~250 mm) meta-sedimentary clasts composed mainly of mildly sutured quartz grains with the presence of epidote and hematite. The presence of these trace minerals in the meta-sedimentary clast are indicative of alteration of clay, further confirming the slight metamorphism of the clast found. However, upon conducting research into possible candidates for a source within the area, no formation currently present matched the mineralogical composition of the sample found. Thus the clast of meta-sedimentary quartz sandstone can possibly be sourced to strata that have been eroded from the Eastern Highlands following the formation of the alluvial fan. The presence of these clasts are not noted in previous studies, therefore the purpose of this project is to identify a possible source for the clasts found within the conglomerate, if a source can be found. From the data obtained from a clast count and determination in a lithological study of the sandstone and meta-sedimentary clasts, a percentage of the clasts with and without a present-day source can be found, which will lend a new perspective to the scale of material eroded since the formation of the alluvial fan conglomerate.