LOOKING ELSEWHERE: INSIGHTS INTO ACADIAN OROGENESIS FROM THE APPALACHIAN FORELAND (SEDIMENTATION, BASIN FLEXURE, AND K-BENTONITES)

Unroofing an orogenic belt destroys large portions of the history of tectonism. The redistribution of the eroded hinterland as layered sediments in an adjacent foreland, however, provides a time-significant record of orogenesis. High resolution stratigraphic subdivision and analysis of the foreland basin fill can better constrain the timing and character of tectonic events and processes. New study of Devonian sedimentation, foreland basin flexure, and K-bentonites in the Appalachian foreland provides new insights into the Acadian Orogen.

Large scale alternations of hinterland-derived extrabasinal clastics and intra-basinal carbonates-quartz arenites have been interpreted to reflect four major cycles (tectophases) of active tectonism to relative quiescence in the Acadian Orogen. Recent age dating of K-bentonites at the beginning of two tectonic cycles (Tectophases I and II, upper Lower and lower Middle Devonian) indicates a duration of approximately 18 million years for the first Acadian tectophase.

Changes in the geometry of intra- versus extrabasinal sediments across the basin reflect not only sediment source, but also flexure of the basin associated with tectonic loading, relaxation, infilling and erosional beveling. Widespread, relatively shallow marine intrabasinal carbonates alternate with thick, basinal clastics in the proximal basin foredeep and thinning or unconformities over a distal forebulge. Patterns also indicate the timing of cratonward versus hinterlandward migration of a forebulge.

In recent years, the number of reported Devonian K-bentonites (altered volcanic ashes) has greatly increased. Four major clusters of beds are known; additional K-bentonites occur through the foreland basin succession. These provide geochemical data on the source magmas and U-Pb age dates (zircon), which are globally applicable. In addition, K-bentonites provide a window into the timing and source regions of silicic volcanism in the orogen. Combined with the history of changes in sediment type and foreland basin flexure, they better constrain the character of major events in the orogenic belt. For the future, a new synthesis of foreland and hinterland data sets will provide a more detailed history of the timing and nature of events and processes active during the Acadian orogeny.