Date of Award

5-2002

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Geological Sciences

Advisor

Daniel F. Belknap

Second Committee Member

Joseph T. Kelley

Third Committee Member

Stephen M. Dickson

Abstract

Seafloor depressions, called pockmarks, have been known to exist in Penobscot Bay, Maine since the mid 1980's (Knebel and Scanlon, 1985). Earlier workers (Ostericher, 1965) recognized "channels" on sonoprobe records that are in the sanle area as the pockmarks recognized by Knebel and Scanlon (1985). Their origins and pathways of evolution are unknown. Much speculation about the sources of pore fluids, levels of activity, and evolutionary pathways has occurred since their discovery. Two surveys of Belfast Bay, in 1998 and 1989, have shown differences in the pockmark field population. Over the course of a decade, 36% of the field's 1998 population has either been created or destroyed by erosion and infilling. Creation of new pockmarks has outnumbered destruction of older pockmarks by 342 to 287 or by about 16%. This is definitive proof that the field is active. If the field were senescent, the destructions would outnumber the creations, creations would not be present, or the field population would be reserved from year to year. New, high-resolution geophysical equipment was used to track the changes to a small area of the seafloor in Belfast Bay from 1998 to 2000. Over the 2-year period, significant changes to the seafloor topography occurred. The bottom was covered with drag marks from both anchoring and fishing in 1998. The 2000 survey also showed a bottom covered with drag marks, but when the two surveys were compared, it was apparent that the drag marks on the 2000 survey were not the same as those on the 1998 survey. In fact, none of the marks from 1998 correlated with the marks from 2000. In addition to the changes in drag marks, two new types of small-scale pockmarks were identified. Tadpole pockmarks are pockmarks that are located at the terminal end of a drag mark. Beaded pockmarks are pockmarks that are arranged along a drag mark, resembling a string of pearls. The identification of these features has indicated the actions of society, specifically anchoring of large vessels and drag fishing, as possible mechanisms for pockmark initiation. Recent detailed mapping of other locations in Penobscot Bay have revealed the presence of another significant pockmark field located close to the Black Ledges in East Penobscot Bay. Unlike the Belfast Bay field, the Black Ledges field is actually a conglomeration of six smaller, discrete pockmark fields that occur in isolated Holocene sedimentary basins Penobscot Bay holds a wealth of information about biogenic-methane sourced pockmarks. Areas of the bay hold massive fields, small fields, isolated pockmarks, and extremely gas-rich sediments. The combination of all of these environments leads to a possible evolutionary model for pockmarks and pockmark fields. All of the locations examined in detail within the bay appear to be in infant to mature stages of evolution. The old age, death and birth stages do not appear to be represented. Addition of sites from other locations in Maine could help to further constrain the model.

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