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Devonian Hobbstown Formation

Devonian Hobbstown Formation
StateMaine
NameDevonian Hobbstown Formation
Geologic ageDevonian
Original map labelDhb
CommentsNamed as a formation for Hobbstown Township in northeast part of Spencer Lake 15-min quad, northern Somerset Co., ME. Type area is Hobbstown Township between Hardwood and Spencer Mountains. Main part consists of interbedded coarse-grained arkose and conglomerate with poorly rounded pebbles and cobbles, all blue gray, gray, dark green, and white weathering. Arkose is fine to coarse quartz grains with 10 to 30 percent feldspar. Conglomerate is felsite, platy calcareous siltstone, and limestone cobbles. Thickness is up to 1,500 ft. Overlies Hardwood Mountain formation in some places and basement complex in others. Lower conglomerate member consists of felsite and granitic pebbles ranging from fraction of an inch to several feet in diameter. Matrix is medium-grained, quartzitic, dark sandstone. Thickness is up to 200 ft. Unconformably overlies Hardwood Mountain formation. Member is absent outside of Hobbstown type area. Report includes geologic map and correlation chart. Overall age is Late Silurian to Early Devonian. Late Silurian fossils occur in a thin bed above base of lower member, but they may have been reworked from Hardwood Mountain formation. Member is at least of Tonoloway (Late Silurian) age but may be as young as lower part of Seboomook formation of New Scotland to Oriskany (Early Devonian) age. Ostracodes in lower member correlate with those in Hardwood Mountain formation. Brachiopods similar to Henryhouse and Brownsport formations (TN and OK). (ME003). Unit description from USGS GEOLEX website (ME078). GS - Greenschist facies; Protolith H - Lithic sandstone and conglomerate
Primary rock typearkose
Secondary rock typeconglomerate
Other rock types
Lithologic constituents
Major
Sedimentary > Clastic > Sandstone > ArkoseGS - Greenschist facies; Protolith H - Lithic sandstone and conglomerate
Sedimentary > Clastic > ConglomerateGS - Greenschist facies; Protolith H - Lithic sandstone and conglomerate
Map references
Digital bedrock data downloaded from Maine Office of GIS in September 2005 from http://apollo.ogis.state.me.us/catalog/catalog.asp?state=2&extent=cover#bedrock Bedrock units table (publication date 11/15/2002) downloaded from same site. Permission to redistribute these data granted by Dr. Robert Marvinney, State Geologist of Maine, in September 2006.
Unit references
Osberg, P. H., Hussey, A.M., and Boone, G. M., 1985, Bedrock geologic map of Maine: Maine Geological Survey, Dept. of Conserv., scale 1:500,000.
Boucot, A.J., 1961, Stratigraphy of the Moose River synclinorium, IN Contributions to general geology, 1959: U.S. Geological Survey Bulletin, 1111-E, p. E153-E188.
Moench, R.H., Boone, G.M., Bothner, W.A., Boudette, E.L., Hatch, N.L., Jr., Hussey, A.M., III, Marvinney, R.G. and Aleinikoff, J.N., 1995, Geologic map of the Sherbrooke-Lewiston area, Maine, New Hampshire, and Vermont, United States, and Quebec, Canada: U.S. Geological Survey Miscellaneous Investigations Series Map, I-1898-D, 56 p., 2 sheets, scale 1:250,000, Prepared in cooperation with the Maine Geological Survey and the New Hampshire Geological Survey.
Boucot, A.J., 1969, Geology of the Moose River and Roach River synclinoria, northwestern Maine: Maine Geol. Surv., Bull. 21, 123 p.
USGS Geologic Names Lexicon (GEOLEX)
Geographic coverageSomerset

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