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