MILANKOVITCH FORCING IN JURASSIC LACUSTRINE STRATIGRAPHY IN THE LOWER PORTION OF THE PORTLAND FORMATION OF THE HARTFORD BASIN

Novel research on Solar System dynamics, climate change, limnology, and similar fields have allowed for the interpretation of tropical paleoclimates that date back to the Jurassic period based on lacustrine stratigraphic records (Kent and Olsen, 2008; Laskar, 2020). This study shows the initial results of analyses done on an 800 ft (~250 m) lacustrine record in the lower portion of the Portland Formation in South Hadley, MA. This record presents post-CAMP, early Jurassic stratigraphy.

The stratigraphic record reveals red-gray-black cyclicity that shows strength in natural gamma ray and elemental XRF measurements, as well as in the optical televiewer record. This study proposed that cyclicity in the record was caused by eccentricity and precession modulated signals. This was proven by statistically testing the gamma ray signal with the TimeOpt solution embedded in the Acycle program, which tests the hypothesis of Milankovitch forcing on a stratigraphic record (p<0.000). Moreover, the different elemental variations throughout the record suggest changing climatic conditions in response to Milankovitch forces. Changing climatic conditions were determined by observing changes in relative concentrations of pyrite-bound S, gypsum-bound S, Fe, and U. Relative concentrations revealed shifts from oxic to near anoxic conditions in the basin throughout the record. Further research into the elemental signatures in the record might reveal more accurate interpretations of the lacustrine paleoenvironmental processes.

These findings reinforce the Milankovitch frequencies that have been proposed in the statistical models used in this study, which predict astronomical frequencies from 200 million years ago, and adds on to the existing literature on the stratigraphy of the Hartford basin.

Kent, D.V., and Olsen, P.E. 2008. Early Jurassic magnetostratigraphy and paleoaltitudes from the Hartford continental rift basin (eastern North America): Testing for polarity bias and abrupt wander in association with the central Atlantic magmatic province, J. Geophys. Res., 113

Laskar, J. 2020. Chapter 4 - Astrochronology, in: Ogg, J.G., Schmitz, M.D., Ogg, G.M. (Eds.), Geologic Time Scale 2020. Elsevier, Amsterdam, pp. 139-158.