GEOLOGIC IMPLICATIONS OF LATE QUATERNARY ISOSTATIC ADJUSTMENT IN THE PENOBSCOT/KENNEBEC RIVER SYSTEM, MAINE

Late Quaternary isostatic adjustment affected central and coastal Maine in several ways. Initial isostatic depression caused extensive marine invasion of the lowlands, followed by sea-level fall resulting from isostatic rebound. Subsequent, localized, isostatic adjustment resulted in the shift of the outlet of Moosehead Lake, Maine’s largest lake, from the Penobscot drainage into that of the Kennebec River. Modern stream gaging is used to approximate the results of the profound change in discharge and alteration in sediment transport and deposition. This analysis suggests a 51% decrease in the discharge of the Penobscot and a 74% increase in the Kennebec at gages on each river, closest to the lake. The linkage between terrestrial and marine portions of the river system is illustrated by correlation of the abandonment of the Moosehead/Penobscot outlet with the cessation of deposition in a newly discovered Penobscot paleodelta. The Penobscot paleodelta is at approximately –30 m depth and buried by a variable thickness of Holocene mud up to 10m in upper Penobscot Bay. Both the outlet change and paleodelta termination are constrained by radiocarbon dating. Expansion of the Kennebec River paleodelta is also correlated with increased discharge and sediment load related to the change in lake outlets. Terrestrial change associated with this isostatic adjustment has the potential to affect settlement, resource procurement, and travel patterns of early native occupants of the region, and should be included in interpretation of the archeological record. Although local in scale, this study suggests the presence of similar processes and deposits in other glaciated coastal regions.