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.