RELATIONSHIP BETWEEN FAULT GEOMETRY AND COARSE SEDIMENTARY FACIES ALONG THE JURASSIC EASTERN BORDER FAULT, CONNECTICUT AND MASSACHUSETTS

Sedimentary facies distribution in the Portland Formation (Lower Jurassic) of the Hartford Basin of Connecticut and Massachusetts records tectonic controls associated with a system of normal faults located on the eastern margin of the basin. These faults, collectively known as the Eastern Border Fault (EBF), were active during deposition of the Portland Formation. Coarse sedimentary facies deposited at various locations adjacent to the EBF vary in clast size, reflecting lateral changes in fault geometry and fault-generated topographic relief.

Facies data were collected from the upper Portland Formation at 8 locations along the eastern margin of the Hartford Basin, all within 1.5 km of the EBF. The abundances of several clastic facies types were recorded and compared to structural data that reflect the steepness of the EBF. At locations where the EBF dips between 55° and 80° (based on direct measurement of the fault plane and slickensided slip planes), coarse alluvial facies are predominant, and 44% of the stratigraphic section is composed of cobble conglomerate. At one location, where slickensided slip planes indicate that the fault dips between 30° and 40°, coarse conglomeratic facies is less abundant, and cobble conglomerate makes up only 12% of the section. Cobble conglomerate is absent in a number of other locations that are greater than 1.5 km from the EBF.

These observations suggest that topographic relief was a major influence on sedimentary facies development. High relief associated with steep fault planes created conditions favorable for the deposition of coarse alluvial fan sediments. Coarse alluvial facies are uncommon in regions where fault-generated topography is interpreted to have been subdued because it was associated with a low angle fault. Thus, the presence or absence of cobble conglomerate can possibly be used to interpret the geometry of the EBF where it is not exposed. Furthermore, since cobble conglomerate only occurs within 1.5 km of the EBF at all locations studied, its presence may be used to locate the depositional edge of the basin.