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Bedrock geologic mapping at a scale of 1:24,000 in the northern part of the Stamford quadrangle and the Connecticut part of the Pound Ridge quadrangle shows that three map units dominate the map area: the Harrison Gneiss, the Siscowit Granite, and the Ratlum Mountain Schist. I mapped these three dominate map units and two less prevalent units using standard field techniques enhanced by digital technology and integrated with whole-rock geochemistry and thin section petrography. The Harrison Gneiss (map unit label Oh) is a meta-gabbro and meta-diorite, that correlates with mapped Harrison Gneiss to the southwest in New York and with other mafic orthogneiss to the northeast in the Danbury quadrangle near Bethel. The Siscowit Granite (Osg) is a medium-grained, light gray to pink colored biotite-muscovite granite that intrudes the Harrison and Ratlum Mountain Schist (it is well exposed adjacent to Mead Farm in Greenwich). The Siscowit Granite was previously mapped as Ordovician? granite (Og). The Ratlum Mountain Schist (Or) is largely a rusty weathering fine-grained, well-foliated bt-ms-pl-qtz-kfs schist and amphibolite interpreted as layers of strongly deformed metasedimentary and metavolcanic rocks. The Ratlum Mountain Schist was previously mapped as multiple members of the Trap Falls Formation (Otf and Otfc). The absolute ages of the Siscowit Granite and the Ratlum Mountain Schist are uncertain. The two less prevalent, unnamed map units include a newly mapped pegmatite (Devonian/Ordovician pegmatite, DOp), and a white, garnet-bearing granitic gneiss (Ordovician? Granite (Ogbr). Whole-rock geochemistry and cross-cutting relationships demonstrate that the pegmatite (DOp) and granitic gneiss (Ogbr) are distinct and are not part of the Siscowit Granite (Osg). These two map unit (DOp and Ogbr) are mappable at a 1:24,000 scale, but only occupy a small portion of the mapped region. Both map units (DOp and Ogbr) will require further mapping to determine the true extent of their coverage in this region. |
