Geo-Mar Lett (2007) 27:279–288 DOI 10.1007/s00367-007-0077-z
ORIGINAL
Gas hydrate disturbance fabrics of southern Hydrate Ridge sediments (ODP Leg 204): Relationship with texture and physical properties Elena Piñero & Eulàlia Gràcia & Francisca Martínez-Ruiz & Juan Cruz Larrasoaña & Alexis Vizcaino & Gemma Ercilla
Received: 31 January 2006 / Accepted: 12 December 2006 / Published online: 5 April 2007 # Springer-Verlag 2007
Abstract Soupy and mousse-like fabrics are disturbance sedimentary features that result from the dissociation of gas hydrate, a process that releases water. During the core retrieval process, soupy and mousse-like fabrics are produced in the gas hydrate-bearing sediments due to changes in pressure and temperature conditions. Therefore, the identification of soupy and mousse-like fabrics can be
E. Piñero (*) : E. Gràcia : A. Vizcaino Unitat de Tecnologia Marina (CSIC), Centre Mediterrani d’Investigacions Marines i Ambientals, Pg. Marítim de la Barceloneta 37-49, 08003 Barcelona, Spain e-mail:
[email protected] F. Martínez-Ruiz Facultad de Ciencias, Instituto Andaluz de Ciencias de la Tierra (CSIC)/Universidad de Granada, Campus Fuentenueva, 18002 Granada, Spain J. C. Larrasoaña Laboratori de Paleomagnetisme, Institut de Ciències de la Terra “Jaume Almera” (CSIC)/Universitat de Barcelona, c/ Lluís Solé i Sabarís s/n, 08028 Barcelona, Spain J. C. Larrasoaña Departamento de Ciencias de la Tierra, Universidad de Zaragoza, c/ Pedro Cerbuna 12, 50009 Zaragoza, Spain G. Ercilla Institut de Ciències del Mar (CSIC), Centre Mediterrani d’Investigacions Marines i Ambientals, Pg. Marítim de la Barceloneta 37-49, 08003 Barcelona, Spain
used as a proxy for the presence of gas hydrate in addition to other evidence, such as pore water freshening or anomalously cool temperature. We present here grain-size results, mineralogical composition and magnetic susceptibility data of soupy and mousse-like samples from the southern Hydrate Ridge (Cascadia accretionary complex) acquired during Leg 204 of the Ocean Drilling Program. In order to study the relationship between sedimentary texture and the presence of gas hydrates, we have compared these results with the main textural and compositional data available from the same area. Most of the disturbed analyzed samples from the summit and the western flank of southern Hydrate Ridge show a mean grain size coarser than the average mean grain size of the hemipelagic samples from the same area. The depositional features of the sediments are not recognised due to disturbance. However, their granulometric statistical parameters and distribution curves, and magnetic susceptibility logs indicate that they correspond to a turbidite facies. These results suggest that gas hydrates in the southern Hydrate Ridge could form preferentially in coarser grain-size layers that could act as conduits feeding gas from below the BSR. Two samples from the uppermost metres near the seafloor at the summit of the southern Hydrate Ridge show a finer mean grain-size value than the average of hemipelagic samples. They were located where the highest amount of gas hydrates was detected, suggesting that in this area the availability of methane gas was high enough to generate gas hydrates, even within low-permeability layers. The mineralogical composition of the soupy and mousse-like sediments does not show any specific characteristic with respect to the other samples from the southern Hydrate Ridge.
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Introduction Gas hydrates are ice-like compounds formed by low weight gas molecules that fill cages of water molecules (e.g. Kvenvolden 1988). These clathrate structures are stable only under high-pressure and low-temperature conditions, and the presence of enough gas molecules seems to be one of the main constraining factors for their occurrence (Ergov et al. 1999; Milkov et al. 2005). Therefore, gas hydrates occur naturally only in continental margin areas at more than 300 m water depth, as well as in high-latitude lakes and permafrost areas (e.g. Kvenvolden 1988). In marine sediments, gas hydrates have been recognised with different morphologies, such as nodular, massive, lens, layers, veins, veinlets and disseminated. They are usually found filling sediment pore spaces conformably to bedding or filling fractures transecting sedimentary structures (e.g. Westbrook et al. 1994; Tréhu et al. 2003). Due to the amount of water contained in clathrate structures, the dissociation of gas hydrate molecules as a result of changes in pressure and/or temperature conditions (natural or during core retrieval) disturbs the sedimentary features of the host sediments (Fig. 1). ‘Soupy’ and ‘mousse-like’ are the two main fabrics interpreted to result from the process of gas hydrate dissociation (e.g. Kastner et al. 1995). Soupy sediments are always watery, homogeneous and fluidized. Because of this high water content, they have been related to the dissociation of nodular to massive hydrates (Tréhu et al. 2003). Soupy sediments retain no original sedimentary structures and are able to flow from their original position during core recovery, resulting in void spaces in cores (Fig. 1b). Sediments with mousse-like fabrics can be divided into two different types, depending on water content: (1) wet, watery mousse-like sediment, which is soft and deforms plastically under slight pressure from a finger (Fig. 1c); and (2) dry mousse-like sediments, which are stiffer and tend to form brittle flakes that break off under the pressure of a finger. These drier, stiffer sediments often appear foliated when split by the core cutter wire. Both types of mousseFig. 1 a Gas hydrate sample and gas hydrate dissociation disturbance fabrics, b soupy sediment and c mousse-like sediment, from southern Hydrate Ridge ODP Leg 204 (images modified from Tréhu et al. 2003)
Geo-Mar Lett (2007) 27:279–288
like texture contain numerous gas vesicles and obscure primary sedimentary structures. They are thought to result from the dissociation of disseminated gas hydrates in finegrained sediments (Tréhu et al. 2003). During Ocean Drilling Program (ODP) Leg 146, the recognition of sediments from Hydrate Ridge (Cascadia subduction zone) showing soupy and mousse-like disturbance fabrics related to gas hydrate dissociation (Westbrook et al. 1994), and the subsequent confirmation of this relationship during the visual core description process soon after sediment retrieval in ODP Leg 204 (Tréhu et al. 2003), led to the use of soupy and mousse-like fabrics as a proxy for gas hydrate presence. Soupy and mousse-like fabrics have also been identified in gas hydrate-bearing sediments recovered from several active and passive continental margins, such as the Amazon Fan (e.g. Soh1997), Blake Ridge (e.g. Paull et al. 1996; Egeberg and Dickens 1999), Costa Rica accretionary wedge (e.g. Kimura et al. 1997), Nankai accretionary prism (e.g. Moore et al. 2001), Congo-Angola Basin (e.g. Charlou et al. 2004) and in the Gulf of Mexico (e.g. Francisca et al. 2005). In other cases, soupy and mousse-like fabrics have been related to the presence of gas hydrates in mud volcano areas such as the Black Sea (e.g. Blinova et al. 2003; Aloisi et al. 2004a), Eastern Mediterranean (e.g. Cita et al. 1996; Robertson et al. 1998; Karisiddaiah 2000; Aloisi et al. 2004b) and the Gulf of Cadiz (e.g. Pinheiro et al. 2003; Somoza et al. 2003). As soupy and mousse-like fabrics can be used as proxies for the presence of gas hydrate and free gas, the definition of the textural and chemical composition of these disturbed sediments is of crucial importance in gas hydrate-rich settings. With this aim, we have carried out analyses of grain size, mineralogical composition, magnetic susceptibility and carbonate content of selected samples from the summit and western flank of the southern Hydrate Ridge. These new data allow us to characterize the lithofacies and composition of southern Hydrate Ridge disturbed sediments, and to study the relationship between grain size and gas hydrate formation.
Geo-Mar Lett (2007) 27:279–288
Geological setting Hydrate Ridge is a structural high located on the OregonWashington continental margin within the accretionary wedge of the Cascadia subduction zone where active vents and gas hydrates have been detected on the seafloor, and a ubiquitous bottom simulating reflector (BSR) in seismic profiles suggests widespread distribution of gas hydrates (Tréhu et al. 1999). Much of the Oregon slope consists of a series of structural hills and ridges that enclose small basins partially filled with hemipelagic and turbidite deposits (Carlson and Nelson 1987). The Cascadia accretionary complex builds as the Juan de Fuca Plate subducts obliquely beneath the North American Plate at a 4.5 cm/year convergence rate (Tréhu and Flueh 2001; Fig. 2a). This is an area of active fluid flow and high pore pressure, as inferred from the strong venting detected on the seafloor (e.g. MacKay 1995; Moore and Vrolijk 1992; Suess et al. 2001). The seafloor at the southern Hydrate Ridge is within the gas hydrate stability zone; therefore, the presence of methane bubbles at the seabed suggests a rapid transport of methane through the sediments (Tréhu et al. 2003; Torres et al. 2004). Hydrate Ridge is a 25-km-long and 15-km-wide crest located in the slope basin (up to 1,500 m depth) and has two highs: the northern (600 m depth) and the southern (800 m depth) summits. According to multi-channel seismic data, it is formed by an upper well-stratified seismic facies of folded and uplifted Pleistocene and Holocene sediments overlying a low-frequency incoherent zone interpreted as highly deformed accretionary complex material (Tréhu et al. 2002). Since cold seeps were first discovered near Hydrate Ridge about 20 years ago (e.g. Kulm et al. 1986), this part of the Oregon margin has been the location of numerous geological and geophysical research cruises. During ODP Leg 204, nine sites (1244–1252) were drilled in the summit, on both flanks, and in the western slope basin of the Fig. 2 a Plate tectonic setting of the Cascadia accretionary margin. Black outlined box shows the location of Hydrate Ridge. b Detailed bathymetric map (20 m contour intervals) showing the location of the ODP Leg 204 drilled sites in southern Hydrate Ridge. Samples presented in this study are from the sites depicted by a large white dot (modified after Tréhu et al. 2003)
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southern Hydrate Ridge (Fig. 2b). Abundant gas hydrates were sampled during this cruise, and soupy and mousselike fabrics were identified during visual core description of sediments in almost all the sites drilled during ODP Leg 204.
Gas hydrate distribution at southern Hydrate Ridge During ODP Leg 204, nodules, veins, lenses and disseminated patches of gas hydrates were detected down to the base of the gas hydrate stability zone (GHSZ) at almost all drilled sites. By integrating results obtained onboard by different methods, such as pressure core samplers, measurements of chloride concentrations in pore waters, infrared (IR) thermal scan images and resistivity at the bit (RAB) analysis, Tréhu et al. (2004a) obtained a quantitative estimate of the spatial distribution of gas hydrates in the southern Hydrate Ridge. A very high concentration (30– 40% of pore space) of gas hydrates was calculated for the upper 20-40 m near the summit of the southern Hydrate Ridge (sites 1248-1250; Fig. 2b). These massive gas hydrates were related to the vigorous fluid venting in this area (Tréhu et al. 2004a), as indicated by the presence of a 50-m-high carbonate spire, known as the Pinnacle, which has been interpreted to result from authigenic precipitation (Tréhu et al. 2003). On both flanks of the southern Hydrate Ridge, the average gas hydrate content in the GHSZ is
