INTRODUCTION

Hydrate Ridge is a structural high located in the accretionary complex of the Cascadia subduction zone, offshore Oregon (USA) (Fig. F1A), where gas hydrates were recovered from the seafloor (e.g., Suess et al., 2001) and a ubiquitous bottom-simulating reflector (BSR) suggests widespread distribution of gas hydrates (Tréhu et al., 1999). During Ocean Drilling Program (ODP) Leg 204, nine sites were drilled on southern Hydrate Ridge (Tréhu, Bohrmann, Rack, Torres, et al., 2003; Tréhu et al., 2004b) (Fig. F1B). High-resolution three-dimensional multichannel seismic (MCS) reflection data from southern Hydrate Ridge (Tréhu et al., 2002) were available during Leg 204, illustrating the stratigraphic and structural complexity of the ridge as well as the relationship between subsurface reflectors and summit vents (Fig. F2).

The BSR, which is commonly interpreted to result from free gas underlying gas hydrate at the base of the gas hydrate stability zone, is observed in almost all seismic profiles. Other seismic reflectors with high amplitude, reflectivity, and good lateral continuation are referred to as Horizons A, B, B´, X, and Y (Tréhu, Bohrmann, Rack, Torres, et al., 2003) (Fig. F2). They were drilled and sampled during Leg 204 in order to investigate their nature and role in methane migration and gas hydrate formation. Horizon A corresponds to a 2- to 4-m-thick interval composed of multiple volcanic glass-bearing sediments and ash sequences (Shipboard Scientific Party, 2003a) and has been suggested as a conduit feeding gas from deep accretionary complex sediments to the surface vents and gas hydrate deposits (Torres et al., 2004; Tréhu et al., 2004a). Horizons B and B´ are pervasively faulted and located at the east flank of southern Hydrate Ridge (Fig. F2A). Horizon B is composed of multiple turbidites clustered into two 2-m-thick intervals. Horizon B´ is characterized by silt layers containing volcanic glass and ash (Shipboard Scientific Party, 2003a). Horizons X and Y do not seem to correlate with any apparent lithological change. Five lithostratigraphic units (Units I–V), ranging in age from early Pleistocene to Holocene (Tréhu, Bohrmann, Rack, Torres, et al., 2003), were defined on board and are also depicted on Figure F2.

Here we present results of grain size and carbonate content from seven Hydrate Ridge sites grouped into three environments: (1) summit, where active seafloor venting occurs (Sites 1249 and 1250); (2) east flank (Sites 1244 and 1246), characterized by the presence of Horizons B and B´; and (3) west flank (Sites 1245, 1247, and 1248), crossed by Horizons A and Y (Figs. F1, F2). Bulk and clay mineralogy were also obtained in these three environments at Sites 1244, 1247, and 1250. These new data allow us to define sediment texture and characterize the lithofacies and composition of southern Hydrate Ridge sediments in order to reconstruct sediment provenance and depositional environments. Furthermore, we will try to define a possible relationship between sedimentary texture and composition with the presence of gas hydrates. Analytical data presented here are depicted in Figures F3, F4, and F5 and compiled in Tables T1 and T2. Further data interpretation and discussions will be presented elsewhere (Pińero et al., 2004).

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