PRELIMINARY BEDROCK GEOLOGIC MAP OF THE SOUTHERN HALF OF THE STAMFORD 7.5-MINUTE QUADRANGLE IN SOUTHWESTERN CONNECTICUT

We present a preliminary EDMAP-supported, GeMS-conforming, 1:24,000 scale bedrock geologic map of the Stamford quadrangle in southwestern CT based on newly collected field data and integrating whole-rock geochemistry, U-Pb zircon crystallization ages, and amphibole and muscovite Ar/Ar plateau ages. Field data were collected between 2019 and 2022 using the FieldMove software on an Apple iPad paired with a BadElf GPS tracker. The bedrock map was produced using ESRI’s ArcMap 10.8.1 and ArcGIS PRO 3.2 software; final edits were made in Adobe Illustrator 28+.

The Stamford quadrangle is dominated by foliated dioritic gneiss and amphibolite facies mafic schist, mapped as the Harrison Gneiss (Oh) and Schistose Harrison Gneiss (Ohs), respectively. Medium-grained Oh with plagioclase phenocrysts and relic orthopyroxene has a mean U-Pb zircon crystallization age of 447.8±3.0 Ma. In Ohs, composed of hbl+bt+spn+pl+qtz, hbl yielded an Ar/Ar plateau age of 372±2.4 Ma. Oh and Ohs are intruded by undeformed medium-grained granite (D?g), pegmatite (D?p), and the Siscowit granite (O?sg) along the axial traces of plunging S3 syn- and antiforms in the south and west of the quadrangle. O?sg did not yield a unique U-Pb zircon crystallization age but rather an array of rim ages between 460-388 Ma; zircon cores range in age between 1.5-1.0 Ga. Muscovite from O?sg gave an Ar/Ar plateau age of 322±2.1 Ma. Both Oh and the suite of granitic rocks intrude rusty weathering schist and granofels interpreted as pelitic and psammitic members, respectively, of the Carringtons Pond member of the Trap Falls formation (Otfc), correlative with metasediments mapped in the adjacent Glenville, Norwalk North, and Norwalk South quads, and Crowley’s (1968) Trap Falls in the Bridgeport and Long Hill quads where McDaniel et al. (1997) found detrital zircon ages ranging from 1113 to 992 Ma.

The crystallization age of Oh, combined with trace and REE tectonic discrimination, suggest an intrusive arc-related origin of the Oh and Ohs during the Taconic orogeny followed by Acadian deformation and metamorphism, which produced the penetrative foliation in Ohs. The age and origin of the Siscowit granite is enigmatic, but the muscovite cooling age from it suggests a long period of deep burial and is in accord with the timing of regional exhumation (Hillenbrand et al., 2023).