INVESTIGATING THE ACADIAN-NEOACADIAN OROGENIES: MULTIPLE FORELAND LESSONS

Frank Ettensohn’s 1985 “The Catskill Delta Complex and the Acadian Orogeny” provided a new model to interpret the orogeny from foreland basin data. Since then, added research in the foreland basin system and orogen brings further insights.

Various lines of sedimentary-based evidence include sediment type, petrography; detrital zircon dates; sediment geometries reflecting foreland basin flexure & migration; altered airfall tephra beds, their distribution, & comparison with dated igneous rocks in the orogen. Also, a broader view of Devonian orogenesis of eastern Laurentia is obtained by looking beyond the Appalachian Basin, a subbasin of the greater Acadian- Neoacadian foreland basin system.

Bradley et al. (2000) noted Acadian orogenesis began in coastal Maine close to the Silurian-Devonian boundary. The foredeep & wedge-top/deformational front subsequently migrated cratonward through the early Late Devonian. Through the Late Silurian-Early Devonian, the bulk of the Acadian foreland basin system was in New England. As the orogen migrated cratonward over time, the foreland basin system also migrated cratonward. Latest Silurian-Early Devonian carbonates in eastern New York (~420-414 Ma) were deposited in a back-bulge basin. By the late Early Devonian (~408 Ma), the cratonward margin of the foredeep migrated into the New York outcrop belt. By the early Late Devonian (~380 Ma), the bulk of the foredeep lay within New York.

As the orogen progressed, some early Acadian synorogenic sediments were buried to at least greenschist metamorphic grade, then thrust to the surface and transported into the Catskill Mountains. Common greenschist rock fragments and chlorite floating in vein quartz pebbles are mixed with other grains in Middle to lower Upper Devonian synorogenic sediments. Combined with detrital zircon dates, a history of Acadian sources and unroofing becomes evident.

Another major aspect of the Acadian Orogen was igneous activity, preserved in New England and the foreland. Over 100 altered Devonian volcanic “tephras” are known in the eastern U.S. Appalachian Basin data points to silicic volcanism down the central to southern Appalachians. Comparison of known foreland basin tephras and dated silicic igneous rocks in New England indicate the former are a ~poor proxy for explosive volcanism in the Acadian Orogen.