EVOLUTION OF HOLOCENE TIDAL CHANNELS AND THE TIDAL RAVINEMENT SURFACE IN PENOBSCOT BAY, MAINE

Recent high-resolution seismic reflection surveys reveal a series of prograding clinoforms preserved in paleo-fluvial and estuarine channels in upper Penobscot Bay, Maine. The geometry of these channels, their infill, and the lower and upper bounding surfaces constrain models of evolution of the bay in relation to changing rates of sea-level rise within the complex bedrock-framed glaciated coast. A well-preserved, buried channel with 20 m relief occurs in the eastern arm of Penobscot Bay, overlying glaciomarine sediments. The channel is filled with steeply dipping, westward prograding clinoforms that onlap and fill the channel. The eastern base of the channel is concentric with the underlying draped glaciomarine beds, while a chaotic reflector (channel lag?) underlies the thalweg. This succession is interpreted as a point-bar or prograding spit facies. A second channel to the east is smaller, less than 5 m deep. Both channels are cut by the tidal ravinement unconformity and abruptly overlain by Holocene embayment mud. The paleochannel may represent the primary channel of the Penobscot River following deglacial rebound and relative sea-level fall to lowstand 50-60 m below present, approximately 10.8 ka. The channel backfilled during early stages of transgression, and then migrated west. Abandonment of the channel may have occurred during the rapid rise in sea level between 10 and 8.5 ka. Alternatively, the major estuarine channel may have switched to the west of Islesboro Island. The thick Holocene muds and a large natural gas and pockmark field in that location, however, argue for a quieter depositional system, perhaps protected by a subsequently breached morainal sill. This newly recognized, well preserved channel infill facies may be strong evidence for rapid local relative sea-level rise in the early Holocene, that increased its preservation potential. Tidal ravinement surfaces remove early estuarine facies in analogous settings in several narrower estuaries on the Maine coast. Coring and further seismic reflection profiling are planned for this year.