LATE PALEOZOIC OF THE WESTERN U.S.: AN ACTIVE CONTINENTAL MARGIN

The scenario for periodic tectonic disruption of the Late Paleozoic continental margin of the western U.S. has been evolving since the late 1980's and early 1990's (e.g., Calvin Stevens and Paul Stone, east-central California; James Trexler, for the Mississippian of central Nevada, and current authors for the Pennsylvanian-Permian).

Within the late Paleozoic continental margin, the unconformities that separate genetically related stratigraphic successions reflect discrete tectonic events of varying magnitude. These stratigraphic intervals represent regional depositional packages created by a combination of eustatic and tectonic influences. The result is regional, and relatively long-lived, depositional systems composed of widely varied lithofacies that encompass time intervals from 5 to 15 million years. Evidence for the tectonic origin of these unconformities varies widely - even for the same unconformity. Where subjacent strata are deformed by folds and thrust faults, the tectonic origin is obvious. More subtle evidence may come from superjacent strata that reflect changes is provenance of clastic detritus or paleocurrent directions. Classic lithostratigraphy and low resolution biostratigraphy are insufficient to identify these discrete successions, rather detailed measured sections coupled with equally detailed biostratigraphy are essential. The result these detailed studies is a tectonostratigraphic event chart that is being extended into the Triassic. Furthermore, there is emerging evidence that these events are recorded: 1) to the north in the Canadian continental margin, and 2) in outboard terranes. The latter point is important because it underscores the notion that the continental margin and offshore terranes collectively reflect a tectonically active continental margin sensu lato, and that the Late Paleozoic was never a “passive margin” as is commonly stated. The most likely scenario is to invoke translational tectonics affect the margin at least periodically during the Late Paleozoic.