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The Roxbury Conglomerate, also informally known as Roxbury puddingstone, is a name for a rock formation that forms the bedrock underlying most of Roxbury, Massachusetts, now part of the city of Boston. The bedrock formation extends well beyond the limits of Roxbury, underlying part or all of Quincy, Canton, Milton, Dorchester, Dedham, Jamaica Plain, Brighton, Brookline, Newton, Needham, and Dover. It is named for exposures in Roxbury, Boston area.〔Zen, E., R. Goldsmith, Richard N.M. Ratcliffe, P. Robinson, R.S. Stanley, N.L. Hatch, Jr., A.F. Shride, E.G.A. Weed, and D.R. Jones (1983) (''Bedrock geologic map of Massachusetts.'' ) Special Map, 3 sheets, scale 1:125,000. U.S. Geological Survey, Reston, Virginia.〕〔Goldsmith, R. (1991) (''Stratigraphy of the Milford-Dedham zone, eastern Massachusetts; an Avalonian terrane.'' ) In N.L. Hatch, ed., pp. E1-E62, Chapter E, The bedrock geology of Massachusetts. Professional Paper. no. 1366-E. U.S. Geological Survey, Reston, Virginia.〕 ==Description== The Roxbury Conglomerate comprises the lower part of the Boston Bay Group, which is a 5,000-meter-thick (3 miles) sequence of sedimentary rocks that fill the Neoproterozoic Boston Basin in eastern Massachusetts. The upper part of the Boston Bay Group consists of the Cambridge Argillite, which overlies the Roxbury Conglomerate. The Roxbury Conglomerate traditionally has been subdivided into three subdivisions; (1.) basal Brookline Member (conglomerate and sandstone), (2.) medial Dorchester Member (mostly sandstone with minor conglomerate) and (3.) upper Squantum Member (largely diamictite). However, these three subdivisions of the Roxbury Conglomerate complexly interfinger with each other and lack the simple layer-cake distribution that past studies have described.〔Emerson, B.K. (1917) ''Geology of Massachusetts and Rhode Island. United States.'' Bulletin no. 597, U.S. Geological Survey, Reston, Virginia.〕〔Dott, R.H. (1961) ''Squantum ‘Tillite’, Massachusetts, evidence of glaciation or subaqueous mass movement?'' Geological Society of America Bulletin. 72(9)1289–1306.〕〔Billings, M.P. (1976) ''Geology of the Boston Basin.'' In: J.W. Skehan and D.P. Murray, eds., pp. 5-30, Studies in New England Geology. Memoir no. 46. Geological Society of America, Boudler, Colorado.〕〔Socci, A.D., and G.W. Smith (1990) ''Stratigraphic implications of facies within the Boston Basin.'' In: A.D. Socci, J.W. Skehan, and G.W. Smith, eds, pp. 55-74, Geology of the Composite Avalon Terrane of Southern New England. Special Paper no. 245. Geological Society of America, Boudler, Colorado.〕〔Carto, S.L., and N. Eyles (2011) ''Chapter 43 The Squantum Member of the Boston Basin, Massachusetts, USA'' In: E. Arnaud, G.P. Halverson, and G. Shields-Zhou, eds. pp. 475-480, The Geological Record of Neoproterozoic Glaciations. Memoirs no. 36. Geological Society, London, England.〕 The Brookline Member of the Roxbury Conglomerate is the classic ‘puddingstone’ that is typically discussed and illustrated in popular web pages, articles, and other publications. It is about 150–1,300 m (490–4,300 ft) thick and consists of massive clast-supported pebble and cobble conglomerate beds interbedded with beds of argillite and sandstone. The conglomerates consist of grey feldspathic sand and well-rounded pebbles and cobbles of quartzite, granite, felsite, and quartz monzonite. The ‘puddingstone’ of the Brookline Member is complexly interbedded with layers of laminated and graded argillite and sandstone and massive diamictite.〔〔〔〔Bailey, R. H. (1987) ''Stratigraphy of the Boston Bay Group, Boston Area, Massachusetts.'' In: D.C. Roy, ed., pp. 209-212, Northeastern Section of the Geological Society of America. Centennial Field Guide no. 5. Geological Society of America, Boudler, Colorado.〕〔Smith, G.W., and A.D. Socci (1990) ''Late Precambrian sedimentary geology of the Boston Basin.'' In: A.D. Socci, J.W. Skehan, and G.W. Smith, eds., pp. 75-84, Geology of the Composite Avalon Terrane of Southern New England. SpecialPapers no. 245. Geological Society of America, Boudler, Colorado.〕 The Dorchester Member consists of purplish, greenish and grey siltstone, sandstone, and medium-to fine-grained argillite. As traditionally defined, it is 180–500 m (590–1,600 ft) thick and dominated by medium-to fine-grained argillite. It contains lesser amounts of sandstone and conglomerate than the Brookline Member. The sandstone beds within this member commonly exhibit full or partial Bouma sequences. The beds within this member commonly exhibit evidence of penecontemporaneous deformation due to downslope slumping.〔〔〔〔〔 The well-known Squantum Member is also known as the Squantum diamictite, Squantum ‘Tillite’, Squantum Tillite, or the Squantum Tilloid. It consists largely of diamictites that are a heterogeneous and poorly sorted admixture of rare boulders up to 1.2 m in diameter, pebbles, cobbles, and sand in a silty-clay matrix. These diamictites occur as beds, which range in thickness from 18 to 215 m (60 to 705 ft) and are typically interbedded with purplish, greenish and grey siltstone, sandstone, and medium-to fine-grained argillite. Typically, the diamictites are mostly massive and lenticular in form; some are crude to moderately well sorted; and some exhibit normal grading. Most of the diamictite outcrops exhibit chaotic bedding in the form of contorted and folded patches of sand, local clusters of gravel, and coherent slump blocks of mudstone. In outcrops, lapilli tuff beds have also been identified in close association with the diamictite of the Squantum Member. At Squantum Head, the diamictite is interbedded with laminated mudstone beds that are 2 to 10 cm (0.8 to 4 in) thick. Some of laminated mudstone beds contain a few outsized pebbles or cobbles that have depressed the underlying laminae and giving them the appearance of dropstones.〔〔〔〔〔〔〔Thompson, M.D (1993) ''Late Proterozoic stratigraphy and structure in the Avalonian magmatic arc southwest of Boston, Massachusetts.'' American Journal of Science. 293(8):725–743.〕〔Rehmer, J. A. (1981) ''The Squantum tilloid member of the Roxbury Conglomerate of Boston, Massachusetts.'' In: M.J. Hambrey and W.B. Harland, eds., pp. 756-759, Earth’s Pre-Pleistocene Glacial Record. Cambridge University Press, London, England.〕 The gravel-size portion of the Squantum Member diamictites consists of range from sub-rounded to angular clasts, 5–60 cm (2–24 in) in diameter, to well-rounded clasts 3–8 cm (1.1–3 in) in diameter. They are composed of multicoloured, locally derived felsic and mafic volcanic rocks, granodiorite, quartzite and massive, graded and laminated sandstone and siltstone. The sand- and gravel-sized fraction of the diamictites consist of volcanic, granitic and metasedimentary lithic fragments that have the same composition as the sediments of the Brookline and Dorchester members. Glacially striated pebbles, chattermarked quartz grains and dropstones have been reported from these diamictites. However, none of these reports have been substantiated by later research. For example, previously identified dropstones have been re-interpreted as having been emplaced by lateral sediment-gravity or current processes.〔〔〔〔Bailey, R.H., and B.H. Bland (2001) ''Late Proterozoic stratigraphy and structure in the Avalonian magmatic arc southwest of Boston, Massachusetts Recent developments in the study of the Boston Bay Group.'' In: D.P. West, Jr., and R.H. Bailey, eds. pp. U1-U23, Guidebook for Geological Field Trips in New England, 2001 Annual Meeting of the Geological Society of America, Boston, Massachusetts.〕〔Wolfe, W. (1976) ''Geology of Squaw Head, Squantum, Massachusetts.'' In: B. Cameron, ed., pp. 107-116, Geology of Southeastern New England: Princeton, New Jersey, New England Intercollegiate Geologic Conference, 68th Annual Meeting, Guidebook, Boston, Massachusetts.〕 The Roxbury Conglomerate has been significantly altered by metamorphism. Metamorphism has altered its sedimentary rocks to subgreenschist facies and created a slaty, well-developed, spaced cleavage that oriented approximately perpendicular to bedding within it. Typically, tectonism has flattened, stretched, indented, and fractured the pebbles and associated matrix of the Roxbury Conglomerate to the point that it often has the appearance of flow structure.〔〔Mansfield, G. R., 1906, ''The Origin and Structure of the Roxbury Conglomerate.'' Bulletin of the Museum of Comparative Zoology at Harvard College. no. XLIX, p. 180〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Roxbury Conglomerate」の詳細全文を読む スポンサード リンク
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