Stop #5 - Stook Hill

History:

Stook Hill is located in the village of Saunderstown (the ancestral home of Steve Saunders), which straddles the towns of North Kingstown and Narragansett. Gilbert Stuart, whose father ran the first snuff mill in America, is this area’s most famous resident. A painter who was trained mostly in Europe, Stuart is best known for his long series of portraits of George Washington; examples of his work can be seen in the collections of the Boston Museum of Fine Arts, RI School of Design Museum, and on the one dollar bill. The family’s 1751 homestead and snuff mill still stand on Gilbert Stuart Road between Routes 1 and 1A.

Description:

This stop indicates Pennsylvanian sediments underwent upper amphibolite facies metamorphism synchronous with polyphase deformation that produced large-scale, recumbent, nappe-like folds. This deep-seated, high grade collisional deformation occurred between 190 and 275 Ma, somewhat later than the high grade deformation further west in the basement. Despite multiple deformations and high-grade metamorphism, these rocks still preserve primary structures characteristic of a fluvial depositional environment. This stop is also described in Grew and Day (1972), Murray and Skehan (1979), and Burks et al. (1981).

(source: Burks, Mosher, and Murray)

These roadcuts are composed of upper amphibolite facies metasedimentary rocks with unusually well preserved primary features. In order of decreasing abundance, the lithologies present are metaconglomerate, metasandstone, pelitic schist, garnet amphibolite, and pegmatitic granite. The first four of these comprise the Rhode Island Formation, whereas the last is considered to be related to the leucocratic facies of the Narragansett Pier Granite. Primary structures are best seen on the northern side of the road, but diagnostic metamorphic assemblages are most abundant on the southern side. Parageneses within pelitic schists (various combinations of garnet, staurolite, biotite, kyanite, rare fibrolite, muscovite, quartz, graphite, and ilmenite) indicate metamorphic conditions of 600°C and 5 or more kb (Grew and Day, 1972; Murray, unpublished data). Although the shales have completely recrystallized to schists, primary fluvatile structures are well preserved in coarser grained rocks. Cross-bedding, erosional surfaces and graded bedding all indicate that the section is predominantly overturned.

Bedding and cleavage (S1) are roughly parallel, and in thin section one can distinguish two schistosities. S1 strikes ENE (65°-70°), and dips to the north gently (25° - 30° on the northern side of the road) to moderately (35° on the southern side). Tight macroscopic folds of S1 are limited to close proximity to small shear zones. Pegmatitic granite truncates S1 and occurs both as massive granite and boudinaged dikes, with boudin axes roughly horizontal and trending E-W. An analysis of pebble geometry from this outcrop by several methods (D. Jones, personal communication, 1985) indicates that they have been deformed into "tongue" shapes, with their plane of flattening coincident with S1. Their direction of elongation is 30°NE, approximately coincident with the downdip direction of S1, fold axes, and pebble elongation directions seen elsewhere in the southern Narragansett Basin.

 

The sequence of structural events seen here is considered to be:

  1. Isoclinal folding about NE trending axis, associated with the development of S1 and regional metamorphism
  2. Development of shear zones and local folding of S1
  3. Emplacement of granite along fractures
  4. Boudinage of granitic dikes, possibly contemporaneous with open folding of earlier structures about horizontal E-W axes.

(source: D.P. Murray. The Alleghanian Orogeny in the Narragansett Basin area, southern Rhode Island. GSA Centennial Field Guide - Northeast Section, 1987)