Mélanges interpreted as olistostromes are present in the late Precambrian Westboro Formation, Blackstone Group, and Newport Neck Formation of the Avalonian terrane of southeastern New England. All of these metasedimentary sequences were intruded by plutons dating 600 to 640 Ma and were variably deformed during late Precambrian(?) to late Paleozoic orogenic events. These Avalonian olistostromes contain quartzarenite and carbonate olistoliths, from 10 to 1,000 m in length, and are interstratified with turbidites and laminated and deformed mudstones. The Blackstone and Westboro sequences are associated with mafic volcanics, whereas the Newport Neck Formation contains interstratified felsic tuffs. Olistostromes and interstratified rocks are interpreted as submarine-slide, debris-flow, and sediment gravity flow deposits of midslope to proximal-fan environments. These deposits provide evidence for an episode of significant Late Proterozoic (650 to 800 Ma) rifting and marginal collapse of continental crust.

Location This guide describes the rocks of southernmost Aquidneckand Conanicut Islands, Rhode Island. Follow Rhode Island 114south into Newport, until it meets Thames Street. Follow Thames Street to Bellevue Avenue and turn right (south) on Bellevue Avenue at the Tennis Hall of Fame (0.0 mi; 0.0 km) toward the Newport Mansions. Follow Bellevue Avenue to Ocean Avenue;watch for the sharp left turn (Fig. 1; 2.2 mi; 3.5 km). Continue on Ocean Avenue to the northwest end of the Brenton Point State Park and stop in the parking lot (5.2 mi; 8.3 km). Walk a fewhundred ft north along Ocean Avenue to Stop 1 (Fig. 2; 5.3 mi;8.5 km). To get to Stop 2 in Jamestown on Conanicut Island, retracethe route into Newport. Follow the signs for Rhode Island 138-Newport Bridge and Jamestown (Fig. 1), and take Rhode Island 138 west across Newport Bridge. From the toll booth (0.0 mi; 0.0 km), continue west for 1.6 mi (2.6 km) and turnsouth on North Main Road (Fig. 1). Cross Narragansett Avenue (Fig. 1) in Jamestown and proceed south on Southwest Avenue. At 2.6 mi (4.2 km), bear right and continue across the sandbar at Mackeral Cove. Follow Beavertail Road on the western islandinto Beavertail State Park. Park as close to the Beavertail Lighthouseas possible;