The eastern Laurentian margin in northeastern North America is marked by promontories and embayments that are defined by northeast-striking rift zones offset by northwest-striking transform faults. The complete history of the northeastern margin, from the initiation of continental rifting to the onset of passive-margin thermal subsidence, is preserved in a dynamic stratigraphic succession and in anorogenic magmatic suites. Late Neoproterozoic–Early Cambrian clastic and volcanic deposits overlie ca. 1.0 Ga and older Laurentian basement and define multiphase continental extension that rifted Laurentia out of Rodinia, opening the Iapetus Ocean as well as the more marginal Humber Seaway. Continental extension is also expressed in a set of basement fault systems that extend into the craton perpendicular to the northeastern Laurentian margin. Lower Cambrian sandstones at the base of a transgressive passive-margin succession overlie synrift rocks and basement, defining the time of transition for the eastern Laurentian margin from an active rift to a passive-margin environment. The passive margin is expressed as a broad late Early Cambrian through early Middle Ordovician carbonate bank and associated offshelf facies. Synthesis of the available data reveals significant along-strike variations in the thickness, composition, age, and facies of important synrift and postrift stratigraphic successions between the northern Appalachian rift zones. These variations are consistent with models for low-angle detachment rift systems and allow for the resolution of the underlying basement architecture of the eastern Laurentian margin specific to low-angle detachments, including upper-plate margins, lower-plate margins, and transform faults that bound zones of oppositely dipping low-angle detachments.

Relict deltas of well-sorted and well-drained sands are among numerous strand-line deposits that mark the former shoreline positions of glacial Lake Iroquois in northern New York. In this study, ground penetrating radar (GPR) was used to image the subsurface architecture of four Late Pleistocene lacustrine deltas to provide information about their depositional environment. The surveyed deltas indicate two distinct glacial Lake Iroquois water levels, the Frontenac and Trenton water phases. A pulseEKKO 100 GPR unit and a 400 Vtransmitter, combined with 50 and 100 MHz antennas are used to provide a better understanding of the internal structures, delta thickness, and distinct facies units. Delta thickness varies generally from 10 to >20 m. High-resolution GPR profiles exhibit variable reflection continuity. Depositional patterns of four distinct radarfacies are described as being characteristic of foreset bed, braided channel, channel cut and fill, and lacustrine clay, in addition to fine-grained till deposits. Facies units reflect an environment of a braided delta in which high sediment volumes and unstable directions of deposition dominated. Larger boulders of nonriverine origin that are located within deltaic sediments are interpreted as dropstones. Waterlevels of glacial Lake Iroquois appear to be stable during the relatively short periods of delta formation. Deltas of glacial Lake Iroquois exhibit lobate morphology typical of a constructive environment.