Shallow-Water Pockmark Formation in Temperate Estuaries: a Consideration of Origins in the Western Gulf of Maine with Special Focus on Belfast Bay
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Marine Geology 225 (2006) 45–62 www.elsevier.com/locate/margeo Shallow-water pockmark formation in temperate estuaries: A consideration of origins in the western gulf of Maine with special focus on Belfast Bay Jeffrey N. Rogers a,*, Joseph T. Kelley b, Daniel F. Belknap b, Allen Gontz b, Walter A. Barnhardt c a GeoSyntec Consultants, 289 Great Rd Suite 105, Acton, MA 01720, USA b Department of Earth Sciences, University of Maine, Orono, ME 04469-5711 USA c U.S. Geological Survey, Woods Hole, MA, USA Received 29 September 2004; received in revised form 15 July 2005; accepted 19 July 2005 Abstract A systematic mapping program incorporating more than 5000 km of side scan sonar and seismic reflection tracklines in the western Gulf of Maine has identified more than 70 biogenic natural gas deposits, occupying 311 km2 in nearshore muddy embayments. Many of these embayments also contain pockmark fields, with some exhibiting geologically active characteristics including the observance of plumes of escaping fluids and sediment. Pockmarks, hemispherically shaped depressions of various size and depths, formed through fluid escape of gas and/or pore water, are sometimes found within or outside gas fields, although many gas fields lack pockmarks altogether. Although the origin of the natural gas remains unclear, if coastal environments at times of lower sea level were similar to the present, numerous lake, wetland, valley fill and estuarine sources of organic-rich material may have formed on the inner shelf. If these deposits survived transgression and remain buried, they are potential gas sources. Intensive mapping of the Belfast Bay pockmark field in 1998 produced the first nearly continuous side scan sonar mosaic of a Gulf of Maine pockmark field with a corresponding 3-dimensional geological model generated from seismic data. Statistical analysis of pockmark geometry, gas deposit loci, and subsurface evidence for gas-enhanced reflectors suggest that gas migration from deeper lateral sources along permeable subsurface strata may be the mechanism for pockmark formation in areas lacking gas- curtain seismic reflections. The coarse-grained transgressive ravinement unconformity between Pleistocene glacial-marine mud and Holocene mud may act as a conduit for distributing methane to the field’s margins. D 2005 Elsevier B.V. All rights reserved. Keywords: methane; Belfast Bay; Maine; Quaternary; marine mud; sea-level change 1. Introduction Pockmarks were first described as bconcave, crater- like depressions that occur in profusion on mud bot- toms across the Scotian ShelfQ (King and MacLean, * Corresponding author. Tel.: +1 978 263 9588; fax: +1 978 263 1970, p. 3141). They reach diameters of hundreds of 9594. meters and depths of tens of meters. Pockmarks are E-mail address: [email protected] (J.N. Rogers). recognized in a variety of continental margin settings 0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2005.07.011 46 J.N. Rogers et al. / Marine Geology 225 (2006) 45–62 all over the world (Hovland and Judd, 1988). Pockmark Fader, 1991; Kelley et al., 1994; Wever et al., 1998), fields are commonly found near deltas (Nelson et al., freshwater escape (Albert et al., 1998; Bussman and 1979) and areas of petroleum production (Hovland et Suess, 1998; Whiticar, 2002); and ice rafting (Paull et al., 1987, Uchupi et al., 1996), or tectonic activity al., 1999) were each invoked to explain the origin of (Field et al., 1982; Hasiotis et al., 1996; Vogt et al., pockmarks within geologically young sediments. In 1994). Shelf basins (Fader, 1991; Josenhans et al., some locations in both shallow and deep water, pock- 1978; McClennen, 1989), continental slopes and rises mark fields apparently lacking measurable quantities of (Orange et al., 1999; Paull et al., 2002) also host fields natural gas or escaping freshwater, were deemed of pockmarks. In all these locations, fluid escape is binactiveQ by Paull et al. (2002) and Ussler et al. (2003). invoked as the forcing mechanism for pockmark for- In discussions of pockmark origins it is essential to mation. Thermogenic and biogenic natural gas from distinguish between the origin of the fluid(s) and the petroleum occurrences, organic-rich sedimentary process by which the fluid(s) form and maintain a deposits and methane hydrates are probable sources pockmark. In coastal Maine (Fig. 1) neither of these for the fluids and buoyancy required to form pock- origins is established, and both thermogenic and bio- marks, although escaping groundwater (Whiticar, genic gas have been postulated to exist (Fleischer et al., 2002) and other more exotic mechanisms (ice rafting, 2001). Acoustic wipeout zones, which indicate the Paull et al., 1999; meteorites, Nelson et al., 1979) have presence of gas, are common in seismic reflection been suggested as causative agents. profiles of the northwestern Gulf of Maine, as are Pockmarks are also widespread in mid-latitude estu- pockmarks (Belknap et al., 1986, 2002; Kelley et al., aries (Fleischer et al., 2001; Missiaen et al., 2002; Gar- 1989b). However, a one-to-one correspondence be- cia-Gil et al., 2002; Ussler et al., 2003), especially in tween pockmarks and gas does not exist in coastal formerly glaciated regions (Scanlon and Knebel, 1989; Maine because biogenic gas fields often lack pock- Barnhardt and Kelley, 1995; Kelley et al., 1994; Fader, marks, and pockmarks without associated gas are also 1991; Whiticar, 2002). Biogenic gas (Albert et al., 1998; observed (Kelley et al., 1994; Gontz et al., 2002). Fig. 1. The location of the study area in the western Gulf of Maine. Boxes locate other figures. (a) The area inside the box is enlarged in b; (b) the shoreline of the State of Maine with natural gas fields shaded green (gas only) and red (gas and pockmarks). J.N. Rogers et al. / Marine Geology 225 (2006) 45–62 47 Based on one recent set of very shallow cores and till across the landscape and present seafloor (Kelley passive sensing, Ussler et al. (2003) concluded that and Belknap, 1991; Shipp et al., 1989, 1991; Kelley et neither groundwater nor natural gas escape was detect- al., 1998). Moraines stretch across many embayments able in the Belfast Bay, Maine field that was earlier (Shipp et al., 1987, Knebel, 1986; Knebel and Scanlon, considered to have an active pockmark formation (Kel- 1985; Miller, 1998) and probably acted as temporary ley et al., 1994). barriers to rising sea level in the Holocene (Barnhardt The mechanism of pockmark formation is compli- and Kelley, 1995; Shipp, 1989). Numerous lakes and cated and requires more study. In addition to ques- wetlands are framed by bedrock and glacial material tions of pockmark origins, Kelley et al. (1994) along the present coast. described other distinctive attributes of the Belfast The Presumpscot Formation glacial-marine sediment Bay field, such as acoustically reflective beyesQ in (Bloom, 1963) covers rock and till outcrops in most the center of some pockmarks, reflective circular coastal areas seaward of the late Pleistocene marine limit structures (bdark spotsQ) lacking significant bathymet- (Thompson and Borns, 1985; Dorion et al., 2001). ric relief, and linear chains including dozens of pock- Glacial-marine sediment often interfingers with coarse- marks and extending for more than a kilometer that grained, ice-proximal outwash deposits near its base were unexplained. (Thompson, 1982; Belknap and Shipp, 1991). The ma- In this paper we describe the geological setting in jority of the Presumpscot Fm. is a slightly sandy mud which natural gas and pockmarks exist along more than with ice-rafted dropstones. The upper part of the Pre- 400 km of inner continental shelf and estuaries in the sumpscot Fm. sometimes contains marine fossil mol- northwestern Gulf of Maine. Although we focus on lusks and barnacles, macroalgae and abundant organic Belfast Bay, Maine, we present geophysical, strati- matter, and shows little direct glacial influence (Fig. 2a) graphic, and sedimentological data from all along the (Stuiver and Borns, 1975; Retelle and Weddle, 2001). Maine coast to develop a model of pockmark formation Stratified glacial-marine muddy sediment is the most applicable to pockmarks in mid-latitude, formerly gla- common deposit along the inner shelf, and completely ciated settings. fills some bedrock basins with more than 50 m of sediment (Barnhardt et al., 1997). On land, thick, rela- 2. Geological Setting tively impermeable deposits of till and glacial-marine sediment fill many former river valleys, deranging con- The study area is in the northwestern Gulf of Maine, temporary drainage and ponding water into thousands of on the inner shelf and littoral of Maine, between the lakes and wetlands (Thompson and Borns, 1985; Tol- shoreline and the 100 m isobath (Fig. 1). This area is man et al., 1986). framed by Precambrian through Mesozoic bedrock, but The isostatic effects of glaciation have strongly in- primarily by Paleozoic igneous and metamorphic rocks fluenced sea-level during the late Quaternary (Belknap (Osberg et al., 1985). Differential erosion of the regional et al., 1987a,b; Belknap and Shipp, 1991; Kelley et al., bedrock, most recently by glaciation, shaped the coast 1992; Barnhardt et al., 1995, 1997). Marine incursion and inner shelf regions (Johnson, 1925; Barnhardt et al., accompanied deglaciation and glacial-marine sediment 1996a,b,c,d,e,f,g; Kelley, 1987; Kelley et al., 1998; crops out as eroding bluffs along the coast (Fig. 2; Uchupi, 2004). Thus, prominent headlands are sup- Thompson and Borns, 1985; Kelley and Dickson, ported by erosion-resistant rocks, while embayments 2000; Kelley, 2004). Retreat of the ice led to isostatic are carved from more easily eroded material. Peninsulas uplift, and sea level fell to approximately À60 m by of resistant rock continue offshore as chains of islands 10.8 ka (uncalibrated; Barnhardt et al., 1995, 1997; and shoals; and associated estuaries project seaward as Shipp et al., 1991; Shaw et al., 2002).