Prepared in cooperation with the State of Connecticut, Department of Environmental Protection, Geological and Natural History Survey Bedrock Geologic Map of the Old Lyme Quadrangle, New London and Middlesex Counties, Connecticut By Gregory J. Walsh, Robert B. Scott, John N. Aleinikoff, and Thomas R. Armstrong Pamphlet to accompany Scientific Investigations Map 3052 Supersedes USGS Open-File Report 2006–1296 2009 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Suzette M. Kimball, Acting Director U.S. Geological Survey, Reston, Virginia: 2009 For product and ordering information: World Wide Web: http://www.usgs.gov/pubprod Telephone: 1–888–ASK–USGS For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment: World Wide Web: http://www.usgs.gov Telephone: 1–888–ASK–USGS Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Cover: View across the Connecticut River looking southwest toward Saybrook Point from Great Island. Outcrop is biotite gneiss of the Old Lyme Gneiss. Photograph by Janet R. Stone. Suggested citation: Walsh, G.J., Scott, R.B., Aleinikoff, J.N., and Armstrong, T.R., 2009, Bedrock geologic map of the Old Lyme quad- rangle, New London and Middlesex Counties, Connecticut: U.S. Geological Survey Scientific Investigations Map 3052, 2 sheets, scale 1:24,000, 25-p. text. (Supersedes USGS Open-File Report 2006–1296.) iii Contents Contents..........................................................................................................................................................iii Introduction.....................................................................................................................................................1 Stratigraphy ....................................................................................................................................................1 Igneous Rocks of Permian Age ..........................................................................................................1 Westerly Granite ..........................................................................................................................1 Granite Pegmatite ........................................................................................................................2 Alaskite and Granite Gneiss .......................................................................................................5 Granite Gneiss ..............................................................................................................................6 Migmatite Gneiss .........................................................................................................................6 Silurian to Devonian Metasedimentary Rocks ................................................................................6 Schist and Gneiss at Obed Heights ..........................................................................................7 Old Saybrook Schist ....................................................................................................................7 Silurian Metasedimentary Rocks of the Merrimack Terrane ........................................................7 Neoproterozoic Metaigneous Rocks of the Avalon Terrane .........................................................7 Rope Ferry Gneiss ........................................................................................................................7 Neoproterozoic Metaigneous Rocks of the Gander Terrane ........................................................8 Granodiorite Gneiss at Lord Hill ................................................................................................8 Granite Gneiss at Becket Hill .....................................................................................................8 Granodiorite Gneiss at Johnnycake Hill ..................................................................................8 Neoproterozoic Metasedimentary and Metavolcanic Rocks of the Gander Terrane ...............9 Old Lyme Gneiss ...........................................................................................................................9 Structural Geology .......................................................................................................................................12 Ductile Structures ...............................................................................................................................12 D1 ...................................................................................................................................................12 D2 ...................................................................................................................................................14 D3 ...................................................................................................................................................15 Ductile Faults .......................................................................................................................................15 Brittle Structures.................................................................................................................................17 Faults ............................................................................................................................................17 Joints............................................................................................................................................17 Parting..........................................................................................................................................20 Fracture Trend Analysis ............................................................................................................22 Metamorphism .............................................................................................................................................22 References Cited..........................................................................................................................................22 Figures 1. Photographs showing granite dikes in the Old Lyme Gneiss ...............................................3 2. Geochemical discrimination diagrams for the rocks from the Old Lyme quadrangle ..............................................................................................5 3. Azimuth-frequency diagrams for dikes of Westerly Granite and granite pegmatite ..................................................................................................................................6 iv 4. Photographs showing the granodiorite gneiss at Johnnycake Hill ..................................10 5. Photographs showing rock types in the Old Lyme Gneiss ..................................................11 6. Photographs showing D1 and D2 structural features in the Old Lyme quadrangle .........13 7. Lower hemisphere equal area projections of three generations of Alleghanian structures in the Old Lyme quadrangle ..............................................................................14 8. Photographs showing D3 structural features in the Old Lyme Gneiss biotite gneiss and Permian granite pegmatite from the Old Lyme quadrangle .......................16 9. Photograph showing the Deep Creek fault ...........................................................................17 10. Photograph showing a steeply dipping brittle normal fault in the granodiorite gneiss at Johnnycake Hill ....................................................................................................17 11. Photographs showing fracture characteristics in the Old Lyme quadrangle .................18 12. Lower hemisphere equal area projection (stereonets) and normalized azimuth-frequency (rose) diagram of joints and brittle faults in the Old Lyme quadrangle..............................................................................................................................19 13. Summary histograms of parting by map unit and rock type ...............................................20 14. Station location map for U-Pb geochronology samples and photographs referred to in the report .......................................................................................................................21 Tables 1. Summary of stratigraphic nomenclature used in this and previous maps ........................2 2. Major and trace element geochemistry of rocks from the 7.5-minute Old Lyme quadrangle, Connecticut ........................................................................................................4 3. Summary of U-Pb zircon ages from the Old Lyme quadrangle. ...........................................8 4. Locations of U-Pb geochronology samples and photographs referred to in the text ...........................................................................................................................................21 Conversion Factors Multiply By To obtain Length inch (in.) 2.54 centimeter (cm) inch (in.) 25.4 millimeter (mm) foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer