Stop #4 - Jamestown Bridge


Here the Rhode Island Formation consists of interlayered carbonaceous schists, metasandstone and metaconglomerate. Pennsylvanian plant fossils have been found in the schists 0.3 mi (0.5 km) to the south, while approximately 1.2 mi (2 km) to the north there are calc-silicate horizons and the remains (now nearly inaccessible) of a small graphite mine. The schist contains twinned staurolite, garnet and biotite porphyroblasts within a matrix of biotite, muscovite, quartz ilmenite, and graphite (Figure 1 (picture taken on scouting trip)). Garnet-biotite mineral equilibria yield temperatures of 525° C (Murray, unpublished data). Porphyroblasts have overgrown the earliest fabric in the rock (S1), but have been deformed by a well-developed crenulation cleavage (S2). Chlorite grade retrograde metamorphism increases in intensity northward along the coast. S1 is approximately parallel to bedding, at N20°E 30°SE, and younging inferred from bedding/cleavage relationships suggests that the section is largely overturned. Local F1 fold hinges are also present, although the evidence is subtle and not entirely convincing. A NNE-striking crenulation cleavage (S2) plus minor folds (F2 or F3) are also present, and are progressively better developed to the north. Pebbles are oblate within the plane of S1. This stop is also described by Grew and Day (1972), Murray and Skehan (1979), Burks et al. (1981), Hepburn and Rehmer (1981), and Murray et al. (1981).

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

Figure 1: Garnet-staurolite schist at Jamestown Bridge.