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Subsurface identification of Merrimack, Putnam-Nashoba, and metavolcanic Avalon terrane rocks, and ductile to brittle fault rocks in the 1.45-kilometer-deep research hole, Moodus, Connecticut

By
C. P. Ambers
C. P. Ambers
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;
R. P. Wintsch
R. P. Wintsch
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Published:
January 01, 1990

All rock units, many fabrics, and two major faults mapped in the Hadlyme and Willimantic areas, Connecticut, are identified in a 1.45-km-deep research hole drilled at Moodus, Connecticut. Correlation of surface information and existing shallow drill-core data with the Moodus data was partially based on the description of core and cuttings from the deep hole, with supplemental chemical analyses, petrography, and a geochemical log. Rocks to a depth of 709.9 m are a mixture of quartz-diopside granofels and biotite-quartz-plagioclase schist identical to outcrops of Hebron Formation and are assigned to the Merrimack terrane. A 46.1-m section of biotite granitic gneiss within the Hebron Formation at Moodus correlates with the Canterbury gneiss. Below the Hebron Formation is 26.2 m of mylonitic muscovite-biotite-plagioclase-quartz schist, and 79.2 m of mylonitic, strongly layered garnet-sillimanite-muscovite-biotite-plagioclase-quartz schist and biotite-hornblende-plagioclase-quartz gneiss. These units correlate with the Yantic and Lower members, respectively, of the Tatnic Hill Formation, included in the Putnam-Nashoba terrane in eastern Connecticut.

The rocks below 815 m in the Moodus hole are dominated by biotite- and hornblende-bearing plagioclase gneisses, but also contain thin layers of granitic gneiss, amphibolite, and pegmatite. The total lithologic assemblage is very similar to and correlated with the sequence of rocks of the Avalon terrane south of the Honey Hill fault. Subdivision of these rocks into the Hadlyme and Mamacoke Formations of the Waterford Complex is proposed. Chemical variability identified in the geochemical log is used to divide these gneisses into 44 layers 12 to 30 m thick, and is interpreted to reflect primary compositional fluctuations of volcanic and intrusive protoliths. Fabrics in the core and chips from the research hole indicate that mylonitic fault zones occur above (125 m thick) and below (145 m thick) the Canterbury gneiss. Phyllonitic fabrics overprint these mylonitic rocks and lesser deformed rock higher in the Hebron Formation, at the base of the Canterbury gneiss, and throughout the Avalon section. Brittle faults indicated by the presence of breccia, microbreccia, and slickensided chlorite surfaces with associated chlorite-zeolite-calcite-quartz–filled fractures occur throughout the well and are relatively common in the Avalon section. The range of temperatures and deformation mechanisms reflected by these rocks is similar to those identified in the Honey Hill fault zone. This suggests that the long history of reactivation of the Honey Hill fault zone extends as far northwest as Moodus and is represented there by a wider zone of deformed rock.

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GSA Special Papers

Geology of the Composite Avalon Terrane of Southern New England

Anthony D. Socci
Anthony D. Socci
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;
James W. Skehan
James W. Skehan
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;
Geoffrey W. Smith
Geoffrey W. Smith
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Geological Society of America
Volume
245
ISBN print:
9780813722450
Publication date:
January 01, 1990

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