STRATIGRAPHY AND AGE OF THE EARLY JURASSIC PORTLAND FORMATION OF CONNECTICUT AND MASSACHUSETTS: A CONTRIBUTION TO THE TIME SCALE OF THE EARLY JURASSIC

The stratigraphy of the more than 4 km thick Portland Formation of the Hartford basin has remained relatively poorly known despite nearly two centuries of study. Within the last three years, however, we have been able to piece together a fairly complete outline of the stratigraphy of the lower 2 km of the formation which, together with that of the underlying East Berlin and Shuttle Meadow formations and coeval strata in the Newark basin, allow an astronomical calibration of the time scale of the Hettangian Age.

Cyclical lacustrine strata, similar in to that seen in the East Berlin Formation, characterize the lower 2 km of the Portland. Most of the hierarchy of climatic precession-related cycles are present including the 20 ky Van Houten cycle, the ~100 ky short modulating cycle, the 406 ky McLaughlin cycle, and the ~1.75 my long modulating cycle. Van Houten cycles reflect the climatic precession cycle of ~20 ky, and are about 20 m thick over much of the Hartford basin, thickening towards the eastern border fault. Varve, cyclostratigraphic and paleomagnetic correlations show that individual Van Houten cycles can be traced over most, if not all, of the basin, allowing the scattered outcrops to be compiled to a common cyclostratigraphic scale. Four full McLaughlin cycles are present in the lower Portland, with one continuing from the underlying East Berlin Formation; we propose to recognize these mappable units as members as follows (from the bottom up): "Northampton", "East Granby", "South Hadley Falls", "Mittinegue", and "Stony Brook" members. Correlation to the paleomagnetic reversal stratigraphy of the Paris basin marine section suggests that the zones of reversed polarity in the "Mittinegue" and "Stony Brook" members in the upper part of the lower Portland are of Sinemurian age. Thus, the Hettangian Age, encompassing in the Hartford basin the duration of the uppermost New Haven through lower Portland formations, is about 2 million years long, consistent with estimates from marine cyclostratigraphy. Simple extrapolation of the accumulation rate into the upper, largely fluvial, Portland (~2 km) suggests that there is an additional 2 million years represented, probably all of Sinemurian age.