INVESTIGATING THE ACADIAN-NEOACADIAN OROGENIES: MULTIPLE FORELAND LESSONS

A broader view of Devonian orogenesis of eastern Laurentia is obtained by looking beyond the Appalachian Basin, a subbasin of the greater Acadian and Neoacadian foreland basin system. Various lines of sedimentary evidence (e.g., sediment type, petrography; detrital zircon dates; sediment geometry reflecting foreland basin flexure & migration; altered airfall tephra beds, their distribution, & comparison with dated igneous rocks in the orogen; & vertical intervals of deformation/a lack there of) record orogenic history.

Bradley et al. 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 (Frasnian). In New York, Silurian deposition previously restricted to central to western areas rapidly shifted orogenward in the latest Silurian. Latest Silurian to early Early Devonian carbonates (Lochkovian, ~418 Ma) in eastern to central New York were deposited in a back-bulge basin. By late Early Devonian (Emsian, ~410 Ma), the cratonward margin of the foredeep migrated into eastern New York. The bulk of the foredeep entered New York in the early Late Devonian (Frasnian, ~380 Ma).

Devonian synorogenic sand & gravel in New York were initially quartz rich (Emsian & Eifelian); during the lower Givetian, composition shifted through milky quartz & chert rich to increasing % chlorite-rich metamorphic rocks fragments. By the mid Frasnian (~377 Ma), % quartz increased.

Over 100 altered Acadian-Neoacadian airfall volcanic tephras are known in the eastern U.S. Reanalysis of phenocryst size--distribution maps of Eifelian Tioga airfall tephras (Dennison & Textoris) indicate Tioga tephras originated from 6 or more volcanic centers, from ~southeastern Pennsylvania to South Carolina. Comparison of sedimentary tephras & dated plutonic & volcanic rocks in New England show sedimentary tephras are a poor proxy for Plinian Devonian volcanism.

Lochkovian to lower Givetian marine strata in eastern New York are structurally deformed. However, overlying Givetian to Frasnian strata are mostly undisturbed; younger strata, estimated at <4 km, are missing. Could this indicate the wedge-top of the Acadian-Neoacadian foreland did not reach the Hudson Valley region until at least middle Frasnian?