Bedrock Geology of the Adirondack Region

Precambrian rocks of Adirondack Region were part of a global system of mountains whose formation approximately one billion years ago led to the assembly of a supercontinent called Rodinia. In New York State, the eroded remnants of these enormous mountains extend beneath the Paleozoic cover rocks on the edge of the Adirondack topographic dome to form the basement rocks of New York State and connect, through exposures in the Thousand Islands Region, to the bulk of the contiguous Grenville Province of the Canadian Shield. Similar rocks are exposed in basement windows along the spine of the much younger Appalachian Mountains and can be traced into Mexico and beyond. Like other areas in the Grenville Province, the High Peaks region of New York is underlain by a large intrusive body of massif anorthosite, a rock composed of exceptionally large crystals of plagioclase feldspar. Rocks in the Adirondacks range in age from approximately 1350 to 1000 million years old and record as many as three or four tectonic events which were part of the Grenville Orogenic Cycle. The net results of these events were high-grade metamorphism, strong deformation, and the widespread overprinting of original relationships and primary textural features. Younger Paleozoic rocks include Cambrian and Ordovician sandstones, limestones, and shales deposited on the eroded metamorphic and igneous basement. These sedimentary rocks are found in fault-bounded outliers within the Adirondack massif and around the Adirondack margins. The current topography of the Adirondacks is related to doming which began about 180 million years ago, when the Atlantic Ocean opened; although the reason(s) for this doming remain to be fully elucidated. Doming has stripped away the younger Paleozoic rocks and exposed the roots of the mountains, which at one time were deformed and metamorphosed deep in the crust

End-member river water composition in the acidified Adirondack Region, Northern New York, USA

Study region: From its headwaters in the Adirondacks to its confluence with the St. Lawrence River, the Raquette flows across acidic crystalline rock, a marble dominated metasedimentary sequence, and Paleozoic sedimentary rocks with increasing capacity to neutralize acidity. Although its drainage basin is largely forested and has a limited population, seventeen hydroelectric reservoirs occur along its mid to lower reaches. Study focus: The goal of the study was to document the geochemistry of Raquette River waters during discharge events. The river was sampled for 69 elements and 7 anions, along its length during stormflow associated with Tropical Storm Irene. One year later the same sites were sampled during a drought with a flow-duration percentage was 98.65. New hydrological insights for the region: Samples collected during average discharge volumes documented chemical gradients corresponding to bedrock spatial distribution. These trends were muted during both stormflow and baseflow, and imply that other factors influence water chemistry during high and low-flow events. Our study documents an example of event river chemistry responding less to extremes of flow or variation in underlying geology than anticipated. During the stormflow sampling one sample had elevated specific conductance (160.4 μS cm−1) and pH (8.21). This data, anomalous geochemistry, and images from Google Earth suggest that the river chemistry is sporadically impacted by discharge from a dolostone quarry located 6 km upstream during runoff events

Editorial for Special Issue “Minerals of the Southern Grenville Province”

The southern Grenville Province is famous for both the large number of mineral localities and the diversity of the mineral species found [...

Age and Origin of the Mesoproterozoic Iron Oxide-Apatite Mineralization, Cheever Mine, Eastern Adirondacks, NY

At the Cheever Mine, located in the eastern Adirondack Mountains of the Mesoproterozoic Grenville Province, iron oxide-apatite ore forms a narrow (<3 m) sheet cross-cutting metasomatically altered, magnetite-bearing, albite-rich leucogranitic host rocks of the Lyon Mountain Granite suite. Zircon from the ore and five samples of country rock were dated by Laser Ablation-Multi-Collector-Inductively Coupled Plasma-Mass Spectrometry. The ore yielded a Concordia age of 1033.6 ± 2.9 Ma while three samples of host rock yielded ages of 1036.3 ± 2.9, 1040 ± 11, and 1043.9 ± 4.1 Ma. Two additional samples of host rock yielded older ages of 1059.6 ± 3.4 and 1066.0 ± 6.3 Ma and contain zircon xenocrystic cores with 207Pb/206Pb ages up to 1242 Ma. The zircons analyzed, including those separated from the ore, have characteristics typically associated with an igneous origin including size, shape, inclusions, oscillatory zoning, typical chondrite-normalized REE patterns, U contents, and U/Th ratios. This data establishes the age of the ore and alteration and a temporal, and likely genetic, connection between the ore and members of the Lyon Mountain Granite suite. A model invoking melting of Shawinigan country rocks, magmatic differentiation, and long-lived magmatic and metasomatic input along extensional fault conduits is proposed for the ore’s genesis. At the Cheever Mine, magmatic hydrothermal fluids and/or post-intrusion alteration appears not to have had a major impact on zircon, which preserves original U-Pb systematics