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Prepared in cooperation with the Commonwealth of Massachusetts, Massachusetts Geological Survey

Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

By Gregory J. Walsh,¹ John N. Aleinikoff,² and Michael J. Dorais³

¹U.S. Geological Survey, Montpelier, VT 05601.
²U.S. Geological Survey, Denver, CO 80225.
³Brigham Young University, Provo, UT 84602.

Introduction The bedrock geology of the 7.5-minute Grafton, Massachusetts, quadrangle consists of deformed Neoproterozoic to early Paleozoic crystalline metamorphic and intrusive igneous rocks. Neoproterozoic intrusive, metasedimentary, and metavolcanic rocks crop out in the Avalon zone, and Cambrian to Silurian intrusive, metasedimentary, and metavolcanic rocks crop out in the Nashoba zone. Rocks of the Avalon and Nashoba zones, or terranes, are separated by the Bloody Bluff fault.  The bedrock geology was mapped to study the tectonic history of the area and to provide a framework for ongoing hydrogeologic characterization of the fractured bedrock of Massachusetts. This report presents mapping by G.J. Walsh, geochronology by J.N. Aleinikoff, geochemistry by M.J. Dorais, and consists of a map, text pamphlet, and GIS database.  The map and text pamphlet are available in paper format or as downloadable files (see frame at right). The GIS database is available for download. The database includes contacts of bedrock geologic units, faults, outcrops, structural geologic information, and photographs.

Posted February 24, 2012 SIM3171.PDF (146 MB - Map) SIM3171P.PDF (10.7 MB - Text pamphlet) Downloads Directory The Downloads directory contains the following files and folders: readme.txt, metadata.html, metadata.faq.html, metadata.txt, metadata.xml, SIM3171B.ZIP (compressed database files, 116 MB), and SIM3171B_without_photos.ZIP (compressed database files without photographs, 7.16 MB). See the Readme Text file for a full description of the database files. Order a paper copy from the USGS Store For additional information contact: Gregory J. Walsh U.S. Geological Survey P.O. Box 628 87 State Street, Room 324 Montpelier, VT 05602 Telephone: (802) 828–4528 Fax: (802) 828–4465 E-mail: gwalsh@usgs.gov Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge.

Suggested citation:

Walsh, G.J., Aleinikoff, J.N., and Dorais, M.J., 2011, Bedrock geologic map of the Grafton quadrangle, Worcester County, Massachusetts: U.S. Geological Survey Scientific Investigations Map 3171, 1 sheet, scale 1:24,000, 39-p. text.

Contents

Introduction

Stratigraphy

U-Pb Geochronology

Methods

Results

Ponaganset Gneiss (Zpg)

Northbridge Granite Gneiss (Zncg)

Hope Valley Alaskite Gneiss (Zhvb and Zhv)

Grafton Gneiss (Єgg)

Marlboro Formation—Amphibolite (Єm)

Geochemistry

Methods

Results

Structural Geology

Ductile Structures

First-Generation Foliation—D₁ Deformation

Second-Generation Foliation—D₂ Deformation

Third-Generation Foliation—D₃ Deformation

Ductile Faults

Spencer Brook and Assabet River Faults

Bloody Bluff Fault

Burlington Mylonite Zone

Fisherville Pond Fault

Comment on the Hope Valley Shear Zone

Brittle Structures

Brittle Faults

Metamorphism

Acknowledgments

References Cited