Passamaquoddy Bay Pockmarks

This page is a work in progress.

Introduction.
The floor of Passamaquoddy Bay, New Brunswick is densely populated with pockmarks of uncertain origin. The regional distribution has been described initially by Fader et al. (1977) based on reconnaissance sidescan. Subsequently denser multibeam coverage of nearly the entire bay floor has now confirmed their distribution and provided much more detail on their specific geometry.

The highest densities occur between Navy and Deer Islands where the pockmarks frequently occur as linear chains. The trend of almost all the linear chains is predominantly NW-SE matching the alignment of large scale glacial fluting apparent in the provincial topographic relief.

A grid of sub-bottom profiler transects was constructed to investigate the relationship between the surface expression of the pockmarks and the shallow sedimentary structure. Interpretation of these profiles show that the pockmarks are all developed in an upper acoustically-transparent unit, with deepest pockmark penetration extending just to the base of this unit. No fault offsets, associated with the linear chains of pockmarks, were observed at the base of the unit. This surface unit overlies an erosional unconformity. Below the unconformity two contrasting acoustic facies define a strong structural relief of order 10-30m high (although some of the highs appear to be truncated at the unconformity). The predominant structural orientation of the highs is NW-SE. The chains of pockmark preferentially appear along the boundaries of these structural highs.

Geology and Geomorphology of the Passamaquoddy Bay Region.
	The Passamaquoddy Bay is an estuary in the northwest of the Bay of Fundy along the New Brunswick - Maine border. Bay topography is dominantly a gently rolling landscape with a drowned shoreline. Geology. The erosively resistant bedrock consists mainly of Silurian and Devonian sedimentary and volcanic facies which occurs between a Devonian granite to the north and a PreCambrian granite to the south. Ordovician sedimentary and intrusive rocks occur in the north of the area and PreCambrian volcanic rocks are present in the southern part of the Bay region (Cummings. 1967).

	Glaciation. In the Pleistocene Passamaquoddy Bay was traversed by glaciers moving to the south east. Subsequent to the loading by the ice sheets the area has experienced isostatic depression, rebound, submergence and emergence. Surficial material now consists entirely of glacial and fluvioglacial deposits which being unconsolidated deposits erode easily and provide a rich and mobile source of sedimentary material.The classic stoss and lee forms, and striated bedrock surfaces commonly occur. Amongst the larger scale glacial deposits are a terminal moraine across the southern part of Deer Island, outwash deposits near Pennefield Ridge and local amounts of glacial and fluvioglacial sands and gravels. The regions present drainage system is characterized by a southeasterly flowing river system which cuts across the northeasterly trend of the bedrock. The Didgequash and Magaguadavic Rivers may have had their courses changed by glaciation (Cummings. 1967).
Structural Geology and Seismicity. During the Mesozoic the Acadian Orogeny produced a gently folded asymmetric syncline with a northeast trend, this major structure lies just to the south of Passamaquoddy Bay. Within the region there are northeast trending anticlines and minor synclines. The Passamaquoddy Bay region has been identified by Barosh (1981) as a seismically active area with some fifty seismic events since 1870. There are two major faults in the region both strike slip. The primary fault is the Lubec Fault which trends northeast. This fault is offset by the complementary north to northwest trending Oak Bay Fault which follows the international boundary and the St. Croix River (Cummings, 1967).
	Plots of the epicentres of seismic events on a geological map of the region suggested that earthquake activity might be related to movement on the Oak Bay Fault (Rast et al.,1979). Other workers have related the earthquake activity to a general subsidence of Bay, with accompanying minor movements on the faults in the area (Barosh, 1981). The Oak Bay Fault offsets Silurian and Devonian rock units. The fault is transected by a Triassic dyke but there is no apparent offset, which would tend to indicate that there had been no recent lateral movement along this fault (Burke and Stringer, 1993). An examination of glacial striations at twenty-four locations in the vicinity of the fault indicated no postglacial displacement. Quaternary sediments found adjacent to the fault were not displaced.
A marine geophysical survey in 1988 partially mapped the bay seafloor making the discovery of an abundance of pockmarks and plumose structures. In a report by Pecore and Fader (1990), it was noted that there was a north western alignment of some of these pock marks which they associated with northwest trending faults. It was suggested that recent movement along the faults in Passamaquoddy Bay may have allowed the release of gas or fluid creating the pock marks in the overlying muddy unconsolidated sediments. This does not look to be the case.

Topography, Bathymetry, BackScatter and Sub-bottom Profile Data sets.
A Digital Terrrain Model (DTM) of the topography was created from Service New Brunswick and United States Geological Survey data. This detailed digital terrain model (DTM) of the topography revealed the obvious and dominant northwest-southeast trending fabric. This fabric is not related to the tectonics of the region but to glacial morphology and glacial deposits. The southeasterly movements of the ice over the bedrock and the glacial detritus in the form of roche moutonee, drumlins, moraines, eskers and outwash has produced a very pronounced north west- southeast fabric. The bathymetry and backscatter were gathered over a 10 year period in four separate surveys. The 1992 and 1995 surveys, by the Canadian Hydrographic Service onboard the CSS Frederick G. Creed with an EM1000, 100kHz multibeam sonar system. In 2002 the OMG/CHS launch CSL Heron with an EM3000S 300kHz multibeam sonar system surveyed the environs of St. Andrews and Navy Island. Also in 2002, the CHS launch, the CSL Plover filled in a data gap using the EM3000S system. The sub bottom profile data was gathered in 2002, using a 3.5kHz transducer and a Knudsen 320 M system.	Digital Terrain Model produced from Service New Brunswick and United States Geological Survey point elevations.
Further when the bathymetry of Passamaquoddy Bay is added to this DTM we see that the seafloor bathymetry, structure, sediments and the pockmarks have this trend as well. The bedrock, the islands, formed of glacial till, the overall geomorphology and the glacial deposits are northwest-southeast trending and appear to structurally control the location and trend of the linear pockmarks and the pockmark fields. There are obvious drumlin like features on the bay floor, all of which have a northwest-southeast trend, outlined by a string of pockmarks. The sub-surface trend of the bedrock and glacial deposits draped with reworked glacial, fluvial and marine sediments is also northwest - southeast.

The topography/bathymetry of a drumlin on
Passamaquoddy Bay seafloor.

A cross section through the drumlin.

Sub-bottom examination of the Pockmarks in Passamaquoddy Bay.
A survey grid was constructed and 3.5kHz sub-bottom data was gathered using Knudsen 320 controller. The lines were processed in a similar fashion to a seismic line. Each line is georeferenced and 'projected' as a profile.

Multibeam bathymetry over the area selected for the sub-bottom survey.

The sub-bottom profiles presented as fence diagrams with the multibeam backscatter of the area for reference.

The sediments in the bay are predominately unconsolidated Holocene muds with the eroded local bedrock surface gently dipping beneath. Occasional outcrops and features on the bay floor appear to be glacial deposits, such as moraines, eskers, drumlins and outwash.

The depth in the surveyed area of Passamaquoddy Bay ranges from less than 2 metres to over 90 metres. The deepest pockmark in the area is in 27 metres of water and it reaches a depth of 72 metres. The deeper pockmarks seem to be associated with the structural highs rather than just deeper water or thicker unconsolidated sediments.

The presence or absence of the pockmarks is firstly controlled by the bedrock. Pockmark development seems to require a sediment thickness in the region of 5 - 10 metres of the unconsolidated muds to be draped over the bedrock.

The linear pockmark features form as an outline to structural highs which are structured glacial deposits such as eskers, moraines and drumlins. These structural highs can outcrop, be buried by the acoustically transparent horizon, or be eroded and buried by the second distinct acoustic horizon, glacial and marine deposits.

Although the 'eyed' pockmarks occur in all locations, both in the linear pockmarks and in the fields. The high acoustic signature seems to be more prevalent in the pockmark fields and in the smaller (younger??) pockmarks.

Other workers have mentioned that seismic and sub-bottom profiles taken through pockmarked areas have been adversely effected by acoustic masking. This masking is taken to be gas in the sediments. Although, there is some masking observable in the sub-bottom profiles in Passamaquoddy Bay, it is not pervasive. This poses the question about the level of activity and gas escape in the Passamaquoddy Bay Pockmark Field.