A simplified analysis of parameters controlling dewatering in accretionary prisms

Bekins, Barbara A.; Dreiss, Shirley J.

doi:10.1016/0012-821x(92)90092-a

Cited by 112 publications

(143 citation statements)

References 7 publications

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“…This inferred upward pore water migration is a critical component of the dewatering process in the Cascadia accretionary prism, where the incoming ascending sediment section is deformed and squeezed by the tectonic processes that form the accretionary wedge. Modeling conducted by Malinverno et al (2008) assumed relatively low advection rates (a maximum of 0.017 cm/y was inferred), which is about an order of magnitude lower than rates used in previous studies (Bekins and Dreiss, 1992;Wang et al, 1993;Hyndman and Davis, 1992). Fluid advection modeling is also strongly dependent on accurate sedimentation rates.…”

Section: Inorganic Pore Water Constituents and Upward Fluid Migrationmentioning

confidence: 97%

Expedition 311 Synthesis: scientific findings

Riedel¹,

Collett²,

Malone³

2010

Proceedings of the IODP

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Integrated Ocean Drilling Program Expedition 311 was conducted to study gas hydrate occurrences and their evolution along a transect spanning the entire northern Cascadia accretionary margin. A transect of four research sites (U1325, U1326, U1327, and U1329) was established over a distance of 32 km, extending from Site U1326 near the deformation front to Site U1329 at the eastern limit of the inferred gas hydrate occurrence zone. In addition to the transect, a fifth site (U1328) was established at a cold vent setting with active fluid and gas expulsion, which provided an opportunity to compare regional pervasive fluid-flow regimes to a site of focused fluid advection. In this synthesis, a revised gas hydrate formation model is proposed based on a combination of geophysical, geochemical, and sedimentological data acquired during and after Expedition 311 and from previous studies. The main elements of this revised model are as follows:1. Fluid expulsion by tectonic compression of accreted sediments at nonuniform expulsion rates along the transect results in the evolution of variable pore water regimes across the margin. Sites closer to the deformation front are characterized by pore fluids enriched in dissolved salts at depth, where zeolite formation from ash diagenesis is dominant. In contrast, the landward portion of the margin shows a freshening of pore fluids with depth as a result of the progressive overprinting of diagenetic salt generation with freshwater generation from the smectite-to-illite transition at greater depth. 2. In situ methane produced by microbial CO 2 reduction within the gas hydrate stability zone is the prevalent gas source for gas hydrate formation. 3. Some minor methane advection from depth is required overall to explain the occurrence of gas hydrate (and the associated downhole isotopic signatures of CH 4 and CO 2 ) within the sediments of the accretionary prism and the absence of gas hydrate within the abyssal plain sediments. In contrast, methane migrating from depth is a dominant source for gas hydrate formation at the cold vent Site U1328 (Bullseye vent). 4. Gas hydrate preferentially forms in coarser grained sandy/silt turbidites, resulting in very high local gas hydrate concentrations. Typically, gas hydrate occupies <5% of the pore space throughout the gas hydrate stability zone. Higher gas hydrate saturations were observed in intervals with abundant coarse-

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Section: Inorganic Pore Water Constituents and Upward Fluid Migrationmentioning

confidence: 97%

Expedition 311 Synthesis: scientific findings

Riedel¹,

Collett²,

Malone³

2010

Proceedings of the IODP

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“…The absolute fluid velocity v is the vector sum of Darcian fluid velocity vw and sediment velocity vs, determined by the method described by Bekins and Dreiss [1992]. We used the computer code SUTRA [Voss, 1984] (with modifications made by Bekins et al, [1995]) to solve the two-dimensional forms of (1) and (2).…”

Section: Model Equationsmentioning

confidence: 99%

Episodic fluid flow in the Nankai accretionary complex: Timescale, geochemistry, flow rates, and fluid budget

Saffer

Bekins²

1998

J. Geophys. Res.

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“…It is likewise important to make accurate predictions of how those mineral assemblages will change with depth, structural position, temperature, and time. Knowledge of thermal structure, as well as the velocities of sed-iment movement within zones of frontal offscraping and underplating, present challenges of their own (Bekins and Dreiss, 1992;Ferguson et al, 1993). To complicate matters further, smectite-to-illite diagenesis may be occurring in situ near the Barbados décollement, but at unusually low temperatures Buatier et al, 1992;Tribble and Yeh, 1994).…”

Section: Thematic Backgroundmentioning

confidence: 99%

Clay mineralogy and clay geochemistry in the vicinity of the décollement zone, northern Barbados Ridge

Underwood¹,

Deng²

1997

Proceedings of the Ocean Drilling Program, 156 Scientific Results

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Fine-grained sediments recovered during Leg 156 of the Ocean Drilling Program contain assemblages of clay minerals that change considerably with depth. Key structural and stratigraphic features within the toe of the northern Barbados accretionary prism include the basal décollement and a sharp boundary between lithologic Units II and III; this lithologic boundary occurs within the décollement zone. We analyzed three size fractions (4−8 µm, 1−4 µm, and <1 µm) by X-ray diffraction and obtained chemical data from the <1-µm size fractions by scanning electron microscopy coupled with energy dispersive spectroscopy. Relative proportions of expandable clay minerals increase significantly, but erratically, near the top of the décollement zone. Proportions of discrete smectite and smectite-rich illite/smectite (I/S) mixed-layer clays decrease steadily down the décolle-ment zone. The base of the décollement zone is characterized by unusually high concentrations of illite. These changes in clay mineralogy influence the shear strength of claystones at the deformation front and help control the position of the décollement.Relative abundances of illite within all lithofacies units increase in coarser size fractions, and this trend is consistent with a detrital source. Kaolinite does not display any systematic partitioning within Unit II, but its proportions increase in the finer size fractions within Unit III. The smectite content of Unit II increases with decreasing grain size; Unit III shows the opposite trend between particle size and % smectite. Bentonite-like samples probably originated through in situ alteration of disseminated volcanic ash, but we believe that the pyroclastic material was mixed into a background sediment that already contained abundant detrital smectite. Illitic mixed-layer clays within the underthrust domain (Unit III) also appear to be detrital in origin. Correlations among both % smectite and % kaolinite and most major-element oxides are statistically significant, but correlations for illite are not. Expandable clays vary considerably in their chemical make-up; the most common type appears to be a Mg-poor, K-rich, Fe-Al-beidellite. We rely upon actualistic provenance connections for late Quaternary clays to guide our interpretations of Miocene and Oligocene sediment dispersal. If these analogues are valid, then the proto-Orinoco and protoAmazon Rivers were the most important detrital sources for the older hemipelagic and turbidite deposits of Barbados Ridge.

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A simplified analysis of parameters controlling dewatering in accretionary prisms

Cited by 112 publications

References 7 publications

Expedition 311 Synthesis: scientific findings

Expedition 311 Synthesis: scientific findings

Episodic fluid flow in the Nankai accretionary complex: Timescale, geochemistry, flow rates, and fluid budget

Clay mineralogy and clay geochemistry in the vicinity of the décollement zone, northern Barbados Ridge

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