STRATIGRAPHIC ARCHITECTURE OF COARSE-GRAINED BRAIDED FLUVIAL DEPOSITS IN THE JURASSIC PORTLAND FORMATION, HARTFORD BASIN, CENTRAL CONNECTICUT

The upper Portland Formation (Lower Jurassic, central Connecticut) is the youngest stratigraphic unit preserved in the extensional Hartford Basin. It is composed of braided fluvial sand/gravel bars and channel fill deposits, and records the waning stages of rift basin development when sedimentation rates exceeded rates of accommodation increase.

Fluvial deposits of the Portland Formation are arranged into 5-15 m thick multistory channel belts that are separated by thin (5-30 cm) mudstone layers. The base of each channel belt is erosive, but the relief on the erosional surface does not exceed a few centimeters. Individual channel surfaces within channel belt packages are difficult to recognize unless they are overlain by conglomeratic lags. Instead, each channel belt is composed of several fining-upward beds that are coarse conglomerate/conglomeratic sandstone at the base and finer sandstone at the top. Some of the sandstone is trough cross-bedded, and is interpreted to represent channel bedforms such as dunes. However, most of the sandstone and finer gravel is planar bedded, and is interpreted to have been deposited under upper flow regime conditions in shallow, rapidly flowing streams. Planar bedded sandstone is most abundant near the top of each fining-upward bed, suggesting decreased stream depth with time. Each multistory channel belt contains 3-7 such fining-upward beds, and is capped by mudstone interpreted to be deposited in a floodplain environment. Mudstone layers are laterally continuous over the extent of the outcrop (< 100 m), and contain evidence of exposure including Scoyenia burrows, gleying, and occasional rhizoliths.

Each channel belt forms a laterally extensive multistory fluvial sandstone/conglomerate sheet. Evidence for degradation is minimal. Sedimentary geometries and structures imply that vertical accommodation was low while the stream was active. For the most part, channels remained relatively shallow and were forced to shift laterally. This implies that the fluvial system was isolated from base level shifts associated with sea level change. Instead, the fining-upward beds, and the shifts from one channel belt to another, reflect climatically driven changes in sediment and water supply superimposed on the slow increase in accommodation associated with waning basin subsidence.