A RECORD OF LOWER JURASSIC CLIMATE CYCLES FROM THE LACUSTRINE EAST BERLIN AND LOWER PORTLAND FORMATIONS, HARTFORD BASIN, CT

The Jurassic East Berlin and lower Portland Formations (Hartford Basin, CT) contain a series of lacustrine cycles that reflect changes in accommodation, sediment type and sediment supply controlled largely by climatic fluctuations. Each cycle (5-20 meters thick) represents a depositional sequence. The base of each sequence contains cross-bedded sandstone and rippled reddish-brown siltstone with mudcracks that are interpreted to represent deposition in a playa environment. These facies become progressively “wetter” until they transition into gray mudstone and black laminated shale that represent deposition in a perennial lake environment. Each sequence is capped by a thinner “drying-upward” phase characterized by a return to reddish-brown rippled playa siltstone containing dolomite nodules and other evidence of pedogenic alteration.

More than 20 lacustrine sequences were deposited in the active Hartford rift basin while tectonically-driven accommodation was high and the basin hydrology was closed. These cycles exhibit regular periodicity and stacking patterns that are interpreted to result from orbitally-controlled (Milankovitch-scale) changes in precipitation rates in an overall arid climate setting. Coarse clastic sediments accumulating at the basin margins were washed into the basin at the start of each sequence, signifying an increase in precipitation and sediment transport. Facies transition upwards into wetter environments, eventually culminating in extensive perennial lakes. The distribution of evaporite minerals, fish fossils, and geochemical species show that these lakes varied between being freshwater and saline/alkaline. A return to drier playa conditions at the top of each sequence is characterized by development of vertisols, and represents a decrease in precipitation and sediment accumulation rates.

Higher order sequence sets are defined by cycles of 4-5 sequences that become progressively drier, and are interpreted to be controlled by orbital parameters with lower frequencies than the individual sequences. These observations suggest that several orders of Milankovitch frequency climate cycles, typically observed in environments affected by glacio-eustatic sea level fluctuations, can also be preserved in arid continental deposits isolated from sea level.