PALEOENVIRONMENTAL AND SEQUENCE STRATIGRAPHIC INTERPRETATIONS OF PALEOSOLS IN CONTINENTAL STRATA OF THE HARFORD RIFT BASIN (TRIASSIC/JURASSIC), CONNECTICUT

Hartford Basin sedimentary rocks are Triassic to Jurassic in age, and were deposited in continental environments during the breakup of Pangaea. Paleosols preserved in these strata vary by stratigraphic position, and are interpreted to reflect changes in climate, sedimentary environment, and duration of subaerial exposure. These paleosols are used for both paleoenvironmental reconstruction and for sequence stratigraphic interpretation.

Paleosols are best developed in the fluvial New Haven Arkose (Late Triassic), the lowest stratigraphic interval in the Hartford Basin. They are easily recognized as distinct horizons of amalgamated carbonate nodules and rhizoliths. They form continuous beds in floodplain deposits associated with single-story braided streams. Paleosols in the lacustrine- and playa-dominated East Berlin Formation (Early Jurassic) are characterized by beds of intense mudcracking. They also contain isolated carbonate nodules, rhizoliths, and rare evaporite molds. The continual wetting and drying of playa mudflat and ephemeral lake sediments resulted in much different pedogenic features than in the fluvial floodplain deposits of the New Haven Arkose. The Upper Portland Formation (Early to Middle Jurassic) records a return to fluvial deposition dominated by multistory braided streams with little preserved floodplain sediment. Paleosols are poorly preserved, and evidence of exposure is limited to burrows and mottled textures (possibly carbonate) in the limited floodplain exposures. The high sand-to-mud ratio in the Portland Formation suggests that the time available for paleosol development was limited compared to that of the New Haven Arkose.

Paleosols in the East Berlin Formation are useful for ranking bounding surfaces within a sequence stratigraphic framework. Sequences are interpreted to be approximately 20,000 years in duration based on correlation to orbitally-controlled Van Houten cycles. Pedogenic features associated with sequence boundaries include carbonate nodules, rhizoliths, and zones of deep, dense mudcracks, reflecting periods of significant exposure. Parasequences that reflect cyclicity on the order of 2,000 to 5,000 years typically contain only mudcracks at their upper boundaries, reflecting less time for paleosol development.