FACIES AND FACIES STACKING PATTERNS IN LACUSTRINE STRATA OF THE LOWER JURASSIC PORTLAND FORMATION, HARTFORD BASIN, CONNECTICUT

The lower Portland Formation (Lower Jurassic) from the Hartford Rift Basin (Connecticut and Massachusetts) is characterized by a suite of sedimentary facies that reflect deposition under fluctuating climatic conditions in a continental setting with a closed basin hydrology. This study examines facies types and stacking patterns preserved in the 136 ft (41 m) Park River Tunnel – 30 T core taken from Hartford, CT. The lower Portland Formation at this location contains seven facies which, ranked from deepest (wettest) to shallowest (driest), include: (1) laminated black shale, (2) thin-bedded gray shale with rare mudcracks, (3) red siltstone with crinkly laminae and abundant mudcracks, (4) cross-bedded white sandstone, (5) current-rippled red siltstone and fine sandstone, (6) massive red mudstone, and (7) pedogenically-altered red mudstone and siltstone. The black and gray shale are interpreted to represent perennial lacustrine environments. All other facies are interpreted to represent playa lakes, playa-related mud flats, and unconfined sheet flood deposits.

Facies are stacked into repeating vertical patterns that reflect two orders of cyclicity between drier and wetter climatic conditions. High frequency cyclicity, on the order of about 3 m (10 ft) is recognized as “drying-upward” facies successions bounded by an abrupt change back to wetter conditions. In playa-related facies, this is typically expressed as a gradual change from facies that exhibit evidence of standing or flowing water to pedogenically-altered mudstone/siltstone. In perennial lacustrine facies, it is preserved as a change from deeper lake facies to shallower lake or playa-related facies. The high frequency cycles stack into lower frequency cycles about 13 m (40 ft) thick that record more gradual changes between drier playa-related mudflat and sheet flood environments and wetter perennial or playa lake environments.

Spectral gamma-ray data collected from similar facies in the underlying East Berlin Formation show distinct spikes in K/Th and U/Th ratios that are associated with black and gray shale facies. There are three additional spikes in the U/Th ratio that are not related to black and gray shale, but may instead be associated with climate-related lithologic changes contained within red playa-related facies.