INSIGHT FROM CORES IN THE POTSDAM GROUP, NORTHERN NEW YORK

Drill core within the Potsdam Group near Ellenburg, New York provides further insight into the depositional environments that existed prior to blanketing of the area by subarkosic to quartz arenites of the Ausable Formation. Three of the four cores taken within a few hundred meters of one another show variable lithologies ranging from thin (1-2 cm or less) hematitic, clay-rich mudstones, and siltstones, sandstones, pebbly granulestones, and upward fining conglomerates. A fourth core is dominated by a prominent 10 m thick layer(s) of hematitic, mud-supported conglomerate interpreted as the distal portion of debris flow(s). This core has abundant and dispersed, vertical water-escape pathways as shown by differences in color and texture defining discrete zones where fines were washed out. Major and trace element and petrographic analysis provide insight into the source of the sedimentary deposits. Silica ranges from 42.29-69.38%, whereas most other major elements, aside from MgO and CaO (carbonate alteration), fall within tight ranges (e.g. K2O = 5.3 – 8.94%; Na2O = 0.41-0.78%). Most trace elements also fall into narrow ranges, except those dominated by resistant heavy minerals like zircon (Zr = 319.7-1265.0 ppm and Hf = 8.1-33.1 ppm). Rare earth elements yield consistent patterns generally 2-3x UCC abundances but depleted slightly in the LREEs. These patterns match nearby Precambrian basement rocks (Lyon Mountain Gneiss - LMG). Numerous, large grains of perthitic feldspar and large zircon grains, with cores and metamorphic rims characteristic of the LMG, document the likely source of the sequence. Samples with the highest amount of silica and largest grain-size closely match the geochemistry of the LMG, suggesting little dilution by other sources, minimal alteration, and rapid burial. These observations expand the inventory of depositional environments found at the base of the Potsdam sandstone to include debris flows, and strengthen paleogeographic models for an Adirondack source shedding sediment derived from fault scarps to the south (present coordinates) into the Ottawa Embayment. Stratigraphically these rocks may be similar to coarse clastics sporadically exposed at the base of the Hannawa Falls Member of the Ausable Formation, or proximal equivalents to the newly defined basal Jericho Member (B. V. Sanford, dissertation, Ottawa University, 2006).