The Traprock Landscapes of New England. Peter M. LeTourneau

GREAT RIFT VALLEYS OF PANGAEA

      Along with the eruption of the CAMP basalts, the geologic forces that were splitting the crust of Pangaea along the future Atlantic margin also created a series of elongate troughs, or “rift” basins, very similar in shape and size to the present-day great rift valleys of East Africa. The East African valleys containing Lake Malawi, Lake Tanganyika, and Lake Rukwa, among others, are good modern analogues for the size, shape, and regional configuration of the Triassic- and Jurassic-age rift basins of eastern North America. The interesting rocks, useful minerals, valuable coal deposits, and unique fossils found in the North American rift valleys have attracted the attention of geologists from North America and Europe since the 1800s.

      As the regional-scale blocks of continental crust subsided like trap doors along active faults, large quantities of sediment washed down from the surrounding igneous and metamorphic uplands and filled the rift valleys; in the Connecticut Valley, more than three miles of sediment and interlayered basalt accumulated while the rift was active during the Late Triassic and Early Jurassic. The sedimentary rocks in the Connecticut Valley consist mainly of red-brown sandstone, siltstone, and shale, deposited in rivers, floodplains, and shallow lakes. Layers of black shale, some containing well-preserved fossil fish, plants, and other remains of early Mesozoic aquatic life, reveal the presence of deep lakes that filled the valley during humid climatic intervals. During dry periods, ephemeral streams, sand dunes, saline lakes, and mud flats occupied the valley floor.

      Distribution of exposed and subsurface Triassic-Jurassic–age continental rift basins in eastern North America.