MESOZOIC STRATA PRESERVED IN THE MIAMI RIVER GRABEN, CENTRAL ADIRONDACKS, NEW YORK

AFT thermochronology of earlier workers demonstrated unroofing of the Adirondacks during the Middle Jurassic to Late Cretaceous, and it was responsible for the reactivation of NE striking faults and graben. Ordovician strata occur within several of the Adirondack graben, but there have been no reports of Mesozoic sedimentary rocks until now. The Miami River and Lewey Lake occur in a steep-walled, flat-bottom, bow-shaped northeast trending valley (6 km long, 1 km) within the rugged terrain of the central Adirondacks. Lewey Lake occurs in the north half of the valley, and lies adjacent to a vast wetland to the south that is drained by the Miami River. A magnetic anomaly survey was completed for the north half of the valley. A pronounced negative magnetic anomaly under the lake cannot be explained with magnetic susceptibility (MS) measurements of the local metamorphic rocks exposed along the lake shore. Recently, outcrops of sedimentary rocks were discovered along the Miami River. They are dolomite cemented angular to well-rounded granule conglomerate and sandstone derived from granitic gneiss, varied metamorphic minerals, limestone and dolostone. A few rounded lithic fragments of limestone also contain fragments of unidentified fossils. There are sand grains of quartz, garnet and feldspar. The MS range (0.026-0.048 x 10^-3 SI units), is higher than measurements for the regional Ordovician carbonates, that have often displayed negative MS readings (-0.078-0.0026 x 10^-3 SI units). Over 300 detrital zircon analyses show a population of Ottawan and Shawinigan ages and lesser amounts of older Proterozoic zircon up to 2000 Ma. The detrital zircon ages, taken with the clast and sand grain compositions indicate a mostly local source, while zircon with ages between 2540-2800 Ma are likely derived from the Superior Province. We propose that the Miami River-Lewey Lake valley is a graben associated with the unroofing and extension of the Adirondack basement during the Middle Jurassic to Late Cretaceous. As well, the newly discovered clastic rock formation in the graben was derived from the local metaplutonic rocks and Paleozoic strata that once covered the region. Structural analysis of a nearby valley with the same geomorphology may lead to further identification of possible Mesozoic rocks in the central Adirondacks.