PALEOGEOGRAPHIC RECONSTRUCTIONS OF THE BLACK LEDGES POCKMARK FIELD, PENOBSCOT BAY, MAINE FROM 13 KA TO PRESENT

An extensive geophysical and coring program was undertaken in the Black Ledges Pockmark Field, Penobscot Bay, Maine to determine the sources and potential implications of shallow free gas in this estuarine setting. We are reconstructing the changes in sea level and sedimentary environments of the region since deglaciation (ca. 13 ka). Over three field seasons we collected several hundred kilometers of seismic reflection profiles and approximately 75 km² of sidescan sonar imagery. The geophysical data served as a baseline for collection of 46 cores 3 to 5 m in length. The cores were recovered from a variety of water depths shallower than 40 m and diverse surficial environments. Several penetrated through the Holocene and into the uppermost Pleistocene section. All cores were analyzed for general stratigraphy, grain size, magnetic susceptibility, shear strength and macrofossil content. In addition, one core, PBVC0210, was analyzed for organic geochemical biomarkers, bulk sediment isotopic carbon and nitrogen composition, C/N values, and foraminifera content and speciation. Samples have been submitted for radiocarbon dates.

Generalized core and seismic stratigraphy coupled with biological indicators suggest proximal grading to distal glaciomarine sedimentation in an open marine basin transitioning to subaerial exposure as sea level fell to the lowstand, creating isolation basins and restricted marginal marine environments. As post-lowstand sea level rose over the area, a ravinement unconformity developed, consisting of sandy-pebble lags. Deepening water gradually produced the modern muddy estuarine environment. Various nearshore, tidal and potentially terrestrial environments have been recognized and reconstructed based on depth below present sea level. Chronology will be added as data becomes available.

Detailed basin analysis conducted with seismic and core data allow the placement of this region into the existing sequence stratigraphic framework for the greater Penobscot Bay area. This combined approach provides insights into understanding the depositional environments and preservation potential of organic-rich sediments that could produce the methane required to source the pockmark fields.