BEDROCK GEOLOGIC MAP OF THE MARLBOROUGH QUADRANGLE, MASSACHUSETTS

Office of the Massachusetts State Geologist (OMSG) Geologic Map 06-01

1:24,0000 scale

Last Revised 3/13/08

This file outlines the digital contents of the Bedrock Geologic Map of the Marlborough Quadrangle, includes Comments to the Map user, and explains outstanding Geologic problems that will be addressed in future editions of the map.

The GIS data is intended to be used alongside the print copy of the bedrock geologic map.  This is available for download in Adobe PDF format from our website (http://www.geo.umass.edu/stategeologist).  It is strongly advised that you consult the paper or PDF version of the map before using the GIS files.

Files associated with this map:

- marlborough_bedr3-08.pdf - Adobe PDF of Bedrock Geologic Map
- marlborough_bedr3-08.jpg - Low resolution Browse graphic of PDF file
- marlborough_bedr3-08_GIS.zip - GIS files used to compile the Bedrock Geologic Map


Contents of ma_bedr_2008_GIS.zip

	.\metadata - FDGC metadata in ASCII text format for each shapefile

	.\shapefiles - GIS data in ESRI shapefile format used to construct geologic map.  Please see appended metadata for a more detailed description of each file

	.\topographic_base - 8-color GIF files of the 1969 1:24K topographic map used as the base for the geologic map.  For more information, please consult the metadata associated with this file.

	.\wells_used_for_map_compilation - Selected wells from the Well Inventory of the Marlborough Quadrangle (Duncan and Mabee, 2004) used for determining lithology in areas of poor exposure.

For questions regarding this map, please contact Joe Kopera at the Office of the Massachusetts State Geologist (jkopera@geo.umass.edu)

Other OMSG Products Associated with the Marlborough quadrangle:

 - Fracture Characterization Map of the Marlborough Quadrangle (Mabee and Salamoff, 2006)
 - Well Inventory of Marlborough Quadrangle (Duncan and Mabee, 2004)
 - Surficial Geologic Map of the Marlborough Quadrangle (Hildreth and Stone, 2004)
 - Surficial Materials Map of the Marlborough Quadrangle (Stone and Hildreth, 2004)
 - Prototype 3D Geologic Map of the Marlborough Quadrangle (Nathan and Mabee, 2006)

COMMENTS TO THE MAP USER:

A geologic map displays information on the distribution, nature, orientation and age relationships of rock and deposits and the occurrence of structural features. Geologic and fault contacts are irregular surfaces that form boundaries between different types or ages of units. Data depicted on this geologic quadrangle map are based on reconnaissance field geologic mapping, compilation of published and unpublished work, and photogeologic interpretation. Locations of contacts are not surveyed, but are plotted by interpretation of the position of a given contact onto a topographic base map; therefore, the accuracy of contact locations depends on the scale of mapping and the interpretation of the geologist(s). Any enlargement of this map could cause misunderstanding in the detail of mapping and may result in erroneous interpretations. Site-specific
conditions should be verified by detailed surface mapping or subsurface exploration. Topographic and cultural changes associated with recent development may not be shown.

We recommend reading Reading Maps with a Critical Eye: Becoming an Informed Map Reader by the Maine Geologic Survey (http://www.maine.gov/doc/nrimc/mgs/mapuse/informed/informed.htm), to make the best use of a geologic map.

Cross sections are constructed based upon the interpretations of the authors made from geologic mapping, and available geophysical and subsurface (drillhole) data. Cross sections should be used as an aid to understanding the general geologic framework of the map area, and not be the sole source of information for use in locating or designing wells, buildings, roads, or other man-made structures.


OUTSTANDING GEOLOGIC PROBLEMS:

As with every geologic map, a number of geolgic problems still exist at the time of publication.  The map will be periodically updated as these problems are resolved.

- Remapping of the Proterozoic plutonic rocks south of the Bloody Bluff Fault zone is underway to better correlate with plutonic rocks mapped in the Milford quadrangle by Kopera and Shaw(OMSG Open File Report - 2007)and in the Grafton and Uxbridge quadrangles (Walsh, in progress; USGS).  These changes will be incorporated into future versions of the map.

- A number of subsidiary brittle faults associated with the Bloody Bluff / Lake Char- faults system that crop out in the Metrowest Water Supply Tunnel are not shown on the map.  The authors felt that at the time of publication, the relative lack of exposure at the surface and the high density of intersecting brittle faults and joint systems in the vicinity of the fault zone made projection of such faults to the surface arbitrary at best.  A more detailed fracture analysis and lineament study, coupled with a more thorough study of Metrowest Tunnel Data (Hager Geoscience, unpublished data) may occur in the future, and a map may be produced at larger scale to better delineate these faults in the vicinity of the fault zone.

- Several semi-ductile through brittle faults exist in the Marlborough formation and elsewhere in the Nashoba terrane.  These faults have been displayed on the map where they could be traced using available exposure and evidence from nearby outcrops and subsurface data(fractures, mineralization, veining, etc...).  Many more of these faults exist in the quadrangle than are displayed on the map.  Again, the authors felt that the relative lack of exposure at the surface would make the delineation of these faults arbitrary without more extensive fieldwork and lineament analysis.  Updates to the map in the future may incorporate this.

- Mapping currently underway in the Nashoba formation to the north and east (Westford, Ayer, and Hudson quadrangles) is subdividing lithologies and structures within the  formation in greater detail than is shown on this map.  This new mapping will be incorporated on future versions of this map.

- Ample field evidence demonstrates that the Marlborough formation is internally folded, and that several large folds exist at map scale. The axes of these folds are not shown on the map. The lack of a mappable consistent internal stratigraphy or marker units in the Marlborough formation, and lack of exposure, has made delineating the axes of these folds on the map arbitrary at best.  These folds are shown in the cross-section, but are not shown on the map.  The changes in orientation of foliation throughout the Marlborough formation is most likely due to this folding.

- Recent analysis of zircons in the Westborough Formation exposed along I-495 show that the Westborough Formation at that outcrop may be younger than the NeoProterozoic granites in the quadrangle (Hepburn et al., 2008 - NEGSA Annual Meeting Abstract).  Other quartzites and schists elsewhere in the quadrangle that are intruded by the granites may be of a different age, and may be mapped as a separate formation on future maps.