### Bedrock geologic map of the Worcester South quadrangle, Worcester County, Massachusetts

By: Gregory J. Walsh**Arthur J. Merschat**

                            https://doi.org/10.3133/sim3345

#### Links

- Plate:  **SIM 3345 Map**                            (pdf)
- Metadata:                               - **SIM 3345 - Metadata FAQ**                            (html)
  - **SIM 3345 - Metadata**                            (html)
  - **SIM 3345 - Metadata Txt**                            (txt)
  - **SIM 3345 - Metadata (XML)** 
- Read Me:  **SIM 3345 - Read Me File**                            (txt)
- Spatial Data:                               - **SIM 3345 - With Photographs**                            (zip)
  - **SIM 3345 - Without Photographs**                            (zip) 
- NGMDB Index Page:  **National Geologic Map Database Index Page**
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#### Abstract

First posted September 29, 2015

**For additional information, contact:**
Eastern Geology and Paleoclimate Science Center U.S. Geological Survey 926A National Center 12201 Sunrise Valley Drive Reston, VA 20192http://geology.er.usgs.gov/egpsc/

Gregory J. Walsh U.S. Geological Survey P.O. Box 628 87 State Street, Room 228 Montpelier, VT 05602 Email: gwalsh@usgs.gov

The bedrock geology of the 7.5-minute Worcester South quadrangle, Massachusetts, consists of deformed Neoproterozoic to Paleozoic crystalline metamorphic and intrusive igneous rocks in three fault-bounded terranes (zones), including the Avalon, Nashoba, and Merrimack zones (Zen and others, 1983). This quadrangle spans the easternmost occurrence of Ganderian margin arc-related rocks (Nashoba zone) in the southern New England part of the northern Appalachians, and coincides with the trailing edge of Ganderia (Merrimack and Nashoba zones) where it structurally overlies Avalonia (Hibbard and others, 2006; Pollock and others, 2012; van Staal and others, 2009, 2012).

Neoproterozoic intrusive rocks and minor metasedimentary rocks crop out in the Avalon zone and structurally underlie the rocks of the Nashoba zone along the Bluddy Bluff fault. Due to poor exposure, the position of the Bloody Bluff fault is not well-constrained and its location is partly extrapolated from mapping in adjacent areas (Barosh, 2005; Walsh and others, 2011a). Cambrian intrusive rocks and Cambrian to Silurian metasedimentary and metavolcanic rocks crop out in the Nashoba zone, and are overlain by largely Silurian metasedimentary rocks of the Merrimack zone along the Clinton-Newbury fault. Ordovician to Permian(?) plutonic rocks intrude the Merrimack and Nashoba zone rocks. Paleozoic metamorphism in the Merrimack and Nashoba zones peaked during Salinic, Acadian, and Neoacadian orogenesis from the Silurian to Mississippian (Wintsch and others, 2007; Stroud and others, 2009; Walsh and others, 2011a; Hepburn and others, 2014). Metamorphism in the Avalon zone peaked during Alleghanian orogenesis in the Mississippian to Permian (Wintsch and others, 1992, 1993, 2001; Attenoukon, 2008). Evidence for garnet-grade extensional Alleghanian mylonitization showing normal motion along the Clinton-Newbury fault occurred after presumed original terrane juxtaposition by left-lateral Acadian thrusting (Goldstein, 1994). Subsequent post-peak metamorphic deformation produced outcrop-scale open folds and weak cleavage, local faults, veins, shear bands, and pegmatite dikes. Locally, along re-activated ductile faults such as the Bloody Bluff fault and along the Wekepeke fault, late Paleozoic to Mesozoic mainly brittle normal fault motion led to the current configuration of fault-bounded lithotectonic terranes (Goldstein, 1982, 1994, 1998; Goldstein and Hepburn, 1999; Goldsmith, 1991; Attenoukon, 2008; Wintsch and others, 2012). The youngest deformation includes kink bands, brittle faults, and joints.

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 Gregory J. Walsh and Arthur J. Merschat from 2008 to 2010. The report consists of a map and GIS database, both of which are available for download at http://dx.doi.org/ 10.3133/sim3345. The database includes contacts of bedrock geologic units, faults, outcrop locations, structural information, and photographs.

#### Suggested Citation

Walsh, G.J., and Merschat, A.J., 2015, Bedrock geologic map of the Worcester South quadrangle, Worcester County, Massachusetts: U.S. Geological Survey Scientific Investigations Map 3345, 1 sheet, scale 1:24,000, http://dx.doi.org/10.3133/sim3345.

ISSN: 2329-132X (online)

#### Study Area

#### Table of Contents

- Correlation of Map Units
- Description of Map Units
- Explanation of Map Symbols
- Discussion
- References Cited

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