FACIES ARCHITECTURE AND CORRELATION OF JURASSIC LAKE CYCLES FROM THE LOWER PORTLAND FORMATION, HARTFORD RIFT BASIN, CONNECTICUT

The Jurassic lower Portland Formation was deposited in the Hartford Rift Basin when accommodation was high, coarse clastic material was confined to the basin margins, and the central part of the basin was dominated by mudstone and siltstone. These fine-grained strata are packaged into repetitive playa and perennial lake cycles that were influenced primarily by climatic fluctuations. A proper understanding of lake evolution under changing climatic conditions in the Jurassic hinges on establishing an accurate temporal framework for the lake beds. The limited exposures of the lower Portland Formation are widely scattered, and placing them into a stratigraphic framework is difficult. However, a series of cores taken in Hartford, CT, preserve a nearly complete section of the lower 500m of the Portland Formation.

The base of lower Portland cycles is placed below intervals that contain a cluster of beds composed of white, planar bedded and cross-bedded, fine to medium sandstone. These beds represent sheet flood deposits forming on an arid alluvial plain. The sandstone beds decrease in abundance upward and transition into reddish-brown rippled or structureless (homogenized) siltstone strata that contain abundant mudcracks and are interpreted as playa deposits. Finally, most cycles contain gray and black shale deposits, representing perennial lake deposits, near their tops. These cycles can be related to climate fluctuations that result in variations in sediment supply to the basin, sediment composition, surface flow, and groundwater hydrology. Playa deposits are typical of the drier periods, while perennial lakes form under more humid conditions.

Correlation of the lake beds among the cores has been accomplished by combining traditional litho- and cyclostratigraphy with spectral gamma-ray and elemental abundance data. Through these techniques, we were able to recognize all or parts of 8 distinct depositional cycles (ranging from 6 to 22 meters in thickness for complete cycles) in 3 cores that penetrate the upper part of the lower Portland Formation. Trends in gamma ray and elemental abundance data were instrumental in distinguishing cycles that are lithologically similar.