SEDIMENTOLOGY OF THE 31-T DRILL CORE FROM THE LOWER JURASSIC PORTLAND FORMATION, HARTFORD BASIN, CONNECTICUT

Deposition in the Hartford Basin during Triassic-Jurassic rifting of North America resulted in more than 4 km of alluvial fan, fluvial and lacustrine sediments and basalt flows. The Portland Formation, the youngest unit in the basin, records a transition from dominantly lacustrine to primarily fluvial deposition. The lower Portland Formation, and thus the transition, is poorly exposed in outcrop. Core 31-T, one of a series of cores across the center of the basin drilled during the Park River tunnel project and located just south of the city of Hartford, samples part of the transition.

Detailed study of the 38 meter-long core revealed six sedimentary facies: 1) laminated mudstone, which contains thin siltstone beds with ripple cross-laminations, mud cracks and rare bioturbation; 2) trough cross-bedded sandstone, erosively based medium- to coarse-grained sandstone with beds up to 1 m thick; 3) rippled siltstone, muddy siltstone with beds up to 0.75 m thick; 4) massive mudstone, featureless and poorly sorted silty to sandy mudstone; 5) planar cross-bedded sandstone, medium- to coarse-grained sandstone with 0.1 m thick cross beds; and 6) ripple laminated sandstone, fine- to medium- grained sandstone with mud rip-ups and beds up to 1.2 m thick. Mudstone facies comprise 80% of the core. Most sandstone is contained within two 3-4 meter intervals, one each near the base and the top of the core, The sandstone intervals are separated by nearly 25 m of mudstone with only a few thin sandstone beds.

The thick sequence of mudstone indicates an overall low-energy depositional environment, either a floodplain or playa mudflat environment. Mudcracks indicate prolonged subaerial exposure of the mudstones; the massive mudstone is interpreted as laminated mudstone that was highly disrupted by the formation of mudcracks and bioturbation and by other soil-forming processes. The nature of the sandstone units does not support deposition by a well-developed channeled fluvial system. Instead, they were likely deposited in an ephemeral system of shallow, interlacing, poorly defined channels.

This interpretation implies that the transition to primarily fluvial deposition occurs in younger strata. Alternatively, fluvial deposition may have been confined to the margins of the basin, perhaps throughout the deposition of the Portland Formation.