3-D STRATIGRAPHIC CORRELATION OF SHALLOW, DENSELY SPACED CORES FROM THE JURASSIC LOWER PORTLAND FORMATION IN THE HARTFORD BASIN (HARTFORD, CONNECTICUT)

The lower Portland Formation (Jurassic) of the Hartford Basin is composed of mudstone-rich lacustrine and playa facies with lesser amounts of alluvial sandstone deposited in a rift basin associated with the break-up of Pangea. These facies weather easily in CT, and are typically best exposed along 2-D road cuts and core transects drilled during major construction projects. In 1999-2000, 71 closely spaced bedrock cores from the lower Portland Fm were drilled in the Adriaen’s Landing region of downtown Hartford, CT. These cores are typically short (1.5-3m, with a few up to 14m) but densely spaced (30-50m from neighboring boreholes). Thus, they provide a unique opportunity to examine 3-D stratal relationships within the lower Portland Fm.

To establish the utility of correlating these cores, we first produced a 520m long, N-S strike-parallel transect of 18 cores located along Columbus Blvd in Hartford. All of the cores in this cross-section contain a bed with light-colored carbonate nodules, an uncommon feature in these strata, making the correlation robust. Thus, it is possible to characterize along-strike variations in the dimensions of stratigraphic elements, such as the width of channels. Regionally the lower Portland Fm strata dip about 15° to the east. Even though the wells are closely spaced, the short interval drilled for most wells means that there is little to no stratigraphic overlap between adjacent wells in the dip direction, making 3-D correlation challenging. Fortunately, two distinct trends of black shale lake beds prove useful for developing a 3-D framework. However, the lake beds appear to be only 2-3m apart stratigraphically, far less than the typical c. 15m between lake beds in regional cores and outcrops, suggesting a fault occurs between the two lake trends. Finally, the Adriaen’s Landing cores were correlated with the older, better studied transect of Park River Auxiliary Conduit cores located 330m south of the Columbus Blvd section, in order to place the cores within a long-term stratigraphic framework. Sedimentary structures, notably carbonate nodules within gray shale, suggest the two lacustrine intervals identified with the Adriaen’s Landing cores correlate to the 3^rd and 4^th lake beds down from the top of the 500+ meter Park River composite stratigraphic column.