COnu

SDag

COtb

Sshd

OSas

OSq

OSp

COas

COgn

OShc

COna

COtb

OSaygp

COna

OSq

SDagf

OSayg

OScs

COna

SDgdCOgn

COna

COna

SOhc

COna

Dacg

CObs

COna

CObs

COrs

OShc

COna

COsgCOrs

COms

OSq

Dacg

COnm

COnm

SDag

OShc

COsg

Dacg

Dacg

COnm

COnm

Dacg

COnm

COnm

COnm

Dacg

COnu

COnu

COnu

COnu

OSq

CLIN

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ASSABET RIVER - SPENCER BROOK FAULT ZONE

OFFICE OF THE MASSACHUSETTS STATE GEOLOGISTUNIVERSITY OF MASSACHUSETTS

Hydrography from MassGIS 1:5000 scale orthophotography.Topography, cultural features and roads taken from scanned images of USGS mylar separates, 1969.Projection is Universal Transverse Mercator, Zone 19, North American Datum, 1927.

Geology mapped by Joe Kopera. Fracture mapping by Steve Mabee, Tamee Albrecht and Dylan Malynn, 2004, compilation by Jonathan Felis and Steve Mabee, 2005.

Prepared in cooperation with the COMMONWEALTH OF MASSACHUSETTS

STATE GEOLOGIST

FRACTURE CHARACTERIZATION MAPHUDSON QUADRANGLE, MASSACHUSETTS

SHEET 1 OF 5

This manuscript is submitted for publication with the understanding that the United States Government is authorized to reproduce and distribute reprints for government use.

This research was supported by the U.S. Geological Survey, National Cooperative Geologic Mapping Program, under assistance Award No. 04HQAG0028. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.

Citation:Mabee, S.B. Fracture characterization map of theHudson quadrangle, Massachusetts.

Massachusetts Geological Survey (4rth), (in review, June 2005).

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QUADRANGLE LOCATION

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FRACTURE CHARACTERIZATION MAP OF THE HUDSON QUADRANGLE, MASSACHUSETTS:HYDRO-STRUCTURAL DOMAIN MAP - SHEET 1 OF 5

ByStephen B. Mabee

2005

Weakly foliated felsic gneiss in the Nashoba formation showing moderately well-developed sheeting. Note the northwest-trending, throughoing and open cross joint to the right of the person in the picture.

Example of Sheeting Joints

Photograph of Fault Surface

Large fault plane in the Nashoba formation exposed in a large quarry. Fault trends 34° and dips southeast 43°. Fault surface is iron stained and contains hematite. Fault is subparallel with foliation but cuts foliation at a high angle.

Example of Steeply-Dipping Layered RocksSteeply dipping, foliated gneiss in the Nashoba formation. Foliationstrikes left to right across the picture and dips steeply into the hillaway from the face of the wall. Despite the strong foliation devel-opment the rock exhibits very little to no tendency to open parallelto foliation. The foliation planes exposed here are due to blasting.

Schist in the Nashoba formation showing the throughgoingsteeply dipping, 117°-trending fractures. These fractures are important for vertical recharge.

Foliation

Example of a Fracture Zone

Sheetling

Foliation Fault Surface

Fracture zone developed in a gneiss unit of the Nashoba formation. Fracture zone trends 117°. Individual fractures within the zone show a spacing of approximately 0.3 m. Width of the zone is approximagely 2 m. Fracture zone provides excellent vertical recharge and may appear as a fracture trace or lineament on aerial photography.

Example of Northwest Trending Fractures

DISCUSSION

INTRODUCTION

The Hydro-Structural Domain Map is a summary map that represents the synthesis of 2179 brittle fracture measurements collected at 58 outcrops distributed across the quadrangle. The units and overlay zones shown on the map define regions that contain attributes thought to be important in influencing groundwater availability and flow in the bedrock. These attributes include bedrock type, the presence or absence of layering (foliation) in the rocks, the degree of development of transmissive partings parallel to the layering, the intensity of sub-horizontal sheeting development, the number and distribution of regional joint systems and outcrop-scale faults, and the distribution of permeable surface materials. The raw data from which this summary map was assembled are summarized in Appendices A-D.

GENERAL GEOLOGY

The rocks of the Hudson quadrangle have been described previously by Bell and Alvord (1976), Castle et al. (1975), Hansen (1956), Kopera (2004) and Barosh, who conducted additional field work in 2004. Cambrian to Devonian intrusive and metamorphic rocks underlie this area. Volcanoclastic rocks and pelites of the Nashoba Formation and metasedimentary rocks of the Merrimack Belt underlie the northwestern four-fifths of the area. These rocks are intruded by the Straw Hollow diorite, Acton granite, Andover granite, Ayer granite, an unnamed granodiorite and numerous pegmatites. The southeastern one-fifth of the area is underlain by the Andover granite and Straw Hollow diorite. Two major structures traverse the area: the Clinton-Newbury fault zone, which forms the boundary between the Nashoba terrane to the souteast and the Merrimack belt to the northwest and the Assabet River-Spencer Brook fault zone in the southeast. Both faults exhibit highly sheared and altered rocks.

FRACTURE CHARACTERIZATION

Joints and Outcrop-Scale Faults

The steeply-dipping (dips >60°), 217°±16°, 117°±13° and 142°±11°-trending joint sets are the dominant joint sets in the area. They occur predominantly northwest of the Assabet River-Spencer Brook fault zone due to a lack of outcrop in the extreme southeast corner of the quadrangle. The 217°±16° set is parallel to foliation but does not exhibit a strong tendency to part (open). In general, partings (openings) parallel to foliation are very poorly developed within the migmatitic gneisses of the Nashoba formation, which comprises approximately 60% of the area. The cross fractures (117°±13° and 142°±11°) are throughgoing, where observed, and tend to exhibit greater transmissive characteristics than the foliation-parallel fractures. Approximately 40% of all fracture zones (closely-spaced fractures that occur in a zone with finite width) parallel the 117°±13° joint set. Thus, these northwest trending features are important avenues for vertical recharge.

Two minor joint sets include the 90°±13° and 180°±10° sets. The 90° joint set occurs sporadically but is found most commonly in the vicinity of the Clinton-Newbury fault zone. The 180° joint set is weakly developed and sparsely distributed.

Median spacing of all fractures is 0.38 m (n=1373) and median trace length is 1.5 m (n=1740). The 217° joint set has slighty closer spacing with a median spacing of 0.26 m. Outcrop-scale faults (n=77) occur in three groups: 206°±10°, which corresponds with the foliation-parallel 217° regional joint set, 94°±11°, which occur most commonly in a band in the south central portion of the area, and 240°±11°.

Foliation and Sheeting

Foliation development occurs in two general zones. Proximal to the Clinton-Newbury fault zone and within 4 to 5 km on either side, the foliation trends strongly north northeast (215°). Approaching the Assabet River-Spencer Brook fault zone to the southeast, the trend of the foliation becomes more east northeast (230°). The changes in azimuths of the foliation reflect the influence of the Clinton-Newbury and Assabet River-Spencer Brook fault zones. The Clinton-Newbury fault trends 210° whereas the Assabet River-Spencer Brook fault trends on average 235°. Rocks in close association with these faults exhibit considerable shearing and alteration. Occasional southeast trending folation is observed in smaller scale folds. Foliation dips steeply (67° to 85°) to the northwest. The potential for water transmission is enhanced in those zones where foliation exhibits a moderate to strong tendency to part parallel to foliation. These areas occur most commonly in the amphibolites, foliated portions of the Andover granite and in altered and sheared rocks associated with the Assabet River-Spencer Brook fault.

Sub-horizontal sheeting is weakly developed throughout the area but may be best developed in the massive igneous rocks and foliated granites. Trends and dips of sheeting tend to follow local topography and show no consistent patterns. Median spacing of sheeting joints, where observed, is 0.4 m and median trace length is 2.0 m.

Summary of Hydrologically Significant Features

The dominant water-bearing features observed in the area are: 1) the 117° and 142° regional joint system, 2) northwest trending fracture zones, 3) cataclasite-cored brittle faults, 4) areas exhibiting strong to moderate partings parallel to foliation, and 5) locally important sheeting joints. Partings parallel to foliation, while present, are not well developed in the area except in the foliated granites, altered and sheared rocks associated with major faults and the amphibolites. Foliated granites provide the most consistent, orthogonal networks of interconnected fractures, however, their exposure in the area is minimal.

REFERENCES CITEDBell, K.G. and D.C. Alvord, 1976, Pre-Silurian stratigraphy of northeastern

Massachusetts: Geological Society of America Memoir 148, pp. 179-216.Castle, R.O., H.R. Dixon, E.S. Grew, A. Griscom, and I. Zietz, 1975, Structural

dislocations in eastern Massachusetts: U.S. Geological Survey Bulletin 1410, 39p.

Hansen, W.R., 1956, Geology and mineral resources of the Hudson and Maynard quadrangles, Massachusetts: U.S. Geological Survey Bulletin 1038, 104p.

Kopera, J., 2005, Preliminary bedrock geologic map of the Hudson quadrangle, Massachusetts: Massachusetts Geological Survey (4rth), 1 pl., scale 1:24,000.

Massive Rocks - Rocks exhibiting no foliation or occasional weak foliation. This unit includes non-foliated Andover granite (SDag), Ayer granite (OSayg and OSaygp (porphyritic)) and larger bodies of the Acton granite (Dacg). In the Ayer granite, the unit is cut by a prominent steeply-dipping, through-going joint set trending 117°. In the Andover granite the unit contains two steeply dipping joints sets trending 217° and 190°. Northwest trending fractures are poorly developed. Sheeting was not observed in the Andover granite due to a lack of exposures but is expected to be present.

Steeply-Dipping Layered Rocks - Foliated rocks exhibiting dips greater than 60°. Includes migmatitic gneisses (COnu), amphibolites (COna), calc-silicate rocks (COnm), the Tadmuck Brook schist (COtb), various unnamed schists, granulite gneisses and altered and sheared rocks of the Nashoba terrane, foliated Andover granite (SDagf), unnamed foliated granodiorite (SDgd) and stratified rocks of the Merrimack belt. Unit is cut by three steeply-dipping, through-going joint sets trending 117°, 142° and 217°. Sheeting is generally not well developed except in a few islolated locations.

Partings Parallel to Layering - Overlay pattern shows areas where outcrops have a moderate to strong tendency to part (open) parallel to dominant foliation. Partings are pervasive, generally through-going and generally extend across the outcrops. Spacing between partings are often less than 0.5 m and commonly less than 1.0 m.

Permeable Surface Materials - Overlay shading shows areas where permeable and conductive overburden is observed at the surface and may lie above the bedrock.

Massive Rocks - The primary avenue for vertical recharge in this hydro-structural domain is through the steeply-dipping, 117°- and 142°-trending fracture sets in the Ayer granite (OSayg) northwest of the Clinton-Newbury fault zone. Although exposures of the Andover granite southeast of the Assabet River-Spencer Brook fault zone are generally lacking, the dominant fractures in this unit are expected to be 217° and 190° based on previous work in the Marlborough quadrangle to the south. Lateral connectivity in this unit is provided by sub-horizontal sheeting joints, when present.

Steeply-Dipping Layered Rocks - The steep dips (>60°) in the layering provide good vertical connection with surface waters and overburden aquifers. Vertical recharge is enhanced by steeply dipping, 117° and 142°-trending fractures, 115°-trending fracture zones, associated north northeast and east-west outcrop-scale faults and, to a lesser extent, foliation-parallel partings (217°). Lateral connectivity is provided by sheeting joints which are generally very poorly developed in this unit.

Partings Parallel to Layering - Rocks showing a moderate to strong tendency to part (open) parallel to dominant foliation have excellent potential for vertical recharge. These areas are expected to demonstrate a strong flow anisotropy parallel to foliation under pumping conditions and are vulnerable potentially to surface contamination. Overburden wells and water bodies overlying these areas may experience water level reductions under pumping conditions.

Permeable Surface Materials - Areas of permeable overburden may provide a source of recharge to the bedrock particularly in areas with steeply-dipping layering. This also includes swamps and other water bodies located within permeable overburden. Overburden wells located in these areas may experience water level reductions under bedrock pumping conditions. Water bodies, swamps and streams may also be affected by bedrock pumping with a potential impact to wetland and riparian ecosystems.

EXPLANATION OF HYDRO-STRUCTURAL DOMAIN UNITS

HYDROLOGIC SIGNIFICANCE OFHYDRO-STRUCTURAL DOMAINS

COna - Amphibolite

COnm - Marble and calc-silicate rocks

COnu - Nashoba Formation migmatitic gneisses, undifferentiated

COtb - Tadmuck Brook Schist, Cambrian-Ordovician (?)

Dacg - Acton granite, Devonian(?)

SDag - Andover granite, pegmatitic phase, Silurian-Devonian

SDagf - Andover granite, foliated binary phase, Silurian-Devonian(?)

SDgd - Unnamed foliated coarse-grained granodiorite, Silurian-Devonian(?)

Dp - Pegmatite, Devonian (?)

Sshd - Straw Hollow Diorite, Silurian (?)

OSas - Altered and sheared gneisses and schists, Ordovician-Silurian (?)

OSayg - Ayer granite, Ordovician-Silurian (?)

OSaygp - Ayer granite, porphyritic phase, Ordovician-Silurian (?)

CObs - Unnamed biotite-silliminite schist and altered/sheared rock, Cambrian-Ordovician (?)

OScs - Unnamed chlorite schist and altered/sheared rock, Ordovician- Silurian (?)

COgn - Unnamed amphibole gneiss, Cambrian-Ordovician (?)

OShc - Unnamed metaconglomerate at Harvard, Ordovician-Silurian (?)

COms - Unnamed muscovite schist, Cambrian-Ordovician (?)

OSp - Unnamed phyllite, Ordovician-Silurian (?)

OSq - Unnamed quartzite at Vaughn Hills, Ordovician-Silurian (?)

COrs - Unnamed rusty-weathering schists, Cambrian-Ordovician (?)

COsg - Unnamed rusty-weathering schist and granulite-gneiss, Cambrian-Ordovician (?)

EXPLANATION OF LITHOLOGIC UNITS

NASHOBA TERRANE (LATE PROTEROZOIC -EARLY PALEOZOIC (?))

Nashoba Formation (Cambrian - Ordovician?)

Stratified Rocks

Intrusive Rocks

MERRIMACK BELT (PALEOZOIC)

Intrusive Rocks

Stratified Rocks

COas - Unnamed amphibole and biotite schists, Cambrian-Ordovician (?)

Please note that due to active development in area, some exposure mayhave been removed and new exposure may have been added since this map was drafted.

Individual OutcropAreas of abundant outcrops

or shallow bedrock (within 10 ft (3 m) of surface)

EXPLANATION OF MAP SYMBOLS

Lithologic Contacts

Zone of sheared and cataclastic rocks

Location inferred

Location approximate (within 200 m)

Faults

Contact (dotted where concealed by water) locationgenerally approximate

Contact, gradational, of plutonic rocksWhere contact consists of dikes or foliation-parallel sillsinterfingered with country rock at scale too small todelineate on map. (dotted where concealed by water)

Only faults traceable at map-scale and on surface are shown.

Bedrock Exposure