The Toms River and Silver Bay embayments are located in the northwest part of Barnegat Bay, New Jersey. Previous studies of the Barnegat Bay environment include the ecology, salinity, water quality and sedimentology. Estimated contaminant loading rates to such water bodies are usually made from land based wells or simulation models. This paper focuses on the direct measurement of groundwater flow rate to the water body. The instrument can be adapted to take groundwater samples for calculating contaminant loading rates to the surface water body. The pore pressures within shallow, relatively fine-grained sediments were measured along three north-south transects in both the Toms River and Silver Bay embayments. Measurement was conducted using a Portable, In-Situ Pore Pressure Instrument (PISPPI) developed at Lehigh University. Permeability and pressure head are determined as the pressure pulse produced by a hand-held pump hydraulically connected to the probe decays to excess pressure head. Darcy's law can then be solved for discharge using the values for permeability and pressure head. Preliminary analysis of PISPPI-derived hydraulic conductivity values range from 3.54·10-8 cm/s - 2.36·10-6 cm/s in Toms River and from 2.19·10-8 cm/s -1.31 10-6 cm/s in Silver Bay. Sediment core samples were collected at selected PISPPI deployment stations for subsequent laboratory analysis. About 0.6 liters/sec are estimated to flow through the silt and clay layer and into the Silver Bay embayment (approximately 4 million square meters). In Toms River where there is more topographic relief, 1.7 liters/sec are estimated to flow through the silt and clay cap into the embayment (approximately 8 million square meters). The flow was measured through the surficial silt and clay layer than envelopes all but the immediate perimeter of 534the embayments. This contribution to flow is diffuse recharge (recharge spread over a large area). Greater recharge rates were measured in sand, which line the edges of the embayments. Additional recharge may occur from concentrated flow from springs or seeps which will be located and measured in the future. The total flow rate can then be compared to predictions of a groundwater flow model by refining an existing numerical model. Determination of in-situ pressure gradients and permeabilities provide critical constraints to the flow model. Furthermore, the measured groundwater flow rates increase our understanding of salinity and nutrient levels within the bay that are essential to ecosystem analysis and management.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

A stratigraphic synthesis of dozens of deep-sea cores, most of them overlooked in recent decades, provides new insights into deep-sea turbidites as guides to earthquake and tsunami hazards along the Cascadia subduction zone, which extends 1100 km along the Pacific coast of North America. The synthesis shows greater variability in Holocene stratigraphy and facies off the Washington coast than was recognized a quarter century ago in a confluence test for seismic triggering of sediment gravity flows. That test compared counts of Holocene turbidites upstream and downstream of a deep-sea channel junction. Similarity in the turbidite counts among seven core sites provided evidence that turbidity currents from different submarine canyons usually reached the junction around the same time, as expected of widespread seismic triggering. The fuller synthesis, however, shows distinct differences between tributaries, and these differences suggest sediment routing for which the confluence test was not designed. The synthesis also bears on recent estimates of Cascadia earthquake magnitudes and recurrence intervals. The magnitude estimates hinge on stratigraphic correlations that discount variability in turbidite facies. The recurrence estimates require turbidites to represent megathrust earthquakes more dependably than they do along a flow path where turbidite frequency appears limited less by seismic shaking than by sediment supply. These concerns underscore the complexity of extracting earthquake history from deep-sea turbidites at Cascadia.

Diagenetic carbonate cements and gas hydrates occur at or near the seafloor where pore fluids seep from continental margins. Because these deposits have acoustic impedances that differ significantly from those of hemipelagic deposits, they can be mapped with sidescan sonar if topographic effects (that affect backscattering angle and amplitude) are removed. We have made this topographic correction, using registered GLORIA and SeaBeam data, for a portion of the Oregon continental slope. Comparison of the processed images with local structure indicates that focused fluid expulsion is controlled by faults, but not all faults are active flow paths. Incipient thrust faults in Cascadia Basin just seaward of the base of the slope and an out-of-sequence thrust fault in the Pliocene section of the lower slope apparently channel fluids rapidly to the surface. Near-surface gas hydrates are inferred in the former location and known to precipitate very near the seafloor in the latter. In contrast, the frontal thrust fault that dips landward to the décollement at the toe of the slope shows no evidence of diagenesis associated with focused fluid flow. Instead, pore waters at the décollement may laterally migrate in this region to transverse strike-slip faults that define the northern and southern boundaries of the frontal thrust sheet. Preliminary flow measurements at vent sites, hydrogeologic tests, and thermal anomalies at Ocean Drilling Program (ODP) Site 892 suggest that near-surface formation of gas hydrates is associated with active, rapid fluid discharge. Carbonate cements may reflect slower seepage or relict deposits.

Two hydrogeological techniques are used to determine the subaqueous fluid discharge rate front sediments with excess pore water pressure into the overlying water column. The first is based on determining diffuse flow (Q) as the product of vertical gradient (i), hydraulic conductivity (K), and area (A) over which the near-vertical discharge occurs. Results are presented from Barnegat Bay, New Jersey, USA in shallow water depths ranging from about 1 in to 10 m, and deep water on the order of 1000 m on an active accretionary prism thrust Vault accretionary prism off the coast of Oregon, USA. the portable instrument probe developed by the authors is used to obtain the excess pore pressure approximately I in into the sediment, from which the gradient (i) is obtained. The decay rate of an induced pressure pulse matched to adapted slug test curves to determine the hydraulic conductivity (K). The second technique was used on Ocean Drilling Program (ODP) Leg 146, Hole 892 at a water depth of 674 m, on the Oregon accretionary prism. The open interval, just over 100 in below the seafloor, intersects a fault identified by seismic imaging, as contributing to concentrated flows at subaqueous vent sites. A constant-rate flow test using a pump aboard the Alvin submersible and subsequent recovery allowed estimation of hydraulic conductivity (K) and transmissivity (T), the latter a product of K and thickness of the fractured interval. By determining the pressure in excess of hydrostatic over the 10 month period, the flow rate contributed by this fault to the seafloor vent site is estimated. These hydrogeologic tests provided the first in situ estimates of fault-zone transmissivities within a modern. hydrogeologically active accretionary prism fault zone, and indicated that fracture dilation is necessary for this fault zone to act as a significant fluid conduit.

A 403-day in situ field experiment at Ocean Drilling Program Site 892B sought to quantify the flux of methane along a fluid-active fault and to experimentally determine rates of methane hydrate and authigenic carbonate deposition associated with fluid expulsion from the borehole. An instrument package was deployed that osmotically sampled fluid, measured borehole pressure and flow rates, and contained reaction chambers in which deposition of gas hydrates and carbonates was anticipated, and from which microbial communities might be extracted. Flow is highly variable in the three-phase water–methane system that exists at Site 892B. Flow rates fluctuate over two orders of magnitude in response to tidally induced pressure variations and gas hydrate formation and dissociation. Hydrate formation began 45 days into the experiment and reduced the initial flow (∼2 l/day) to 20 ml/day. Unexpectedly, the hydrate destabilized after about 125 days. Tidally induced flow reversals are common (∼25% of time) in this setting characterized by ‘overpressured’ pore waters. These reversals pump sulfate-rich bottom water into near-surface sediments where Archaea anaerobically oxidize CH4 and induce carbonate precipitation. At the sediment–water interface, authigenic carbonates are undergoing dissolution. Methanotrophs dominated the microbial community where fluid is discharged to ambient seawater. All expelled methane is apparently oxidized in the water column.

A bstract : A one year in situ experiment to quantify the flux of carbon, primarily as methane, from an overpressured thrust fault at the Cascadia convergent margin (ODP Site 892B) is described. Most of the expelled carbon is sequestered in two solid phases, methane hydrate and authigenic carbonates and an unknown portion is lost in the water column and/or atmosphere. 14 ages of clam shells from the vicinity of the site suggest active methane venting for at least 21–24 kyrs. The water column chemistry provides information on the potential effects of global warming on rapid massive gas hydrate dissociation and on the effects of microbial oxidation of the released methane on the local oceanic oxygen and CO 2 contents. The local and global implications of these processes for the oceanic carbon cycle are being assessed.

Fault zones are hypothesized to play a major role in focusing fluid expulsion from accretionary complexes. A small number of previous investigations have examined the hydrogeologic properties of these fault zones using borehole packer or submersible‐based slug and constant rate flow tests. While these single‐well experiments yielded the first data on the permeability of active fault zones, they could only investigate a small area around the borehole and could not ensure reliable determination of formation storage properties. Recent data from an unplanned experiment in the décollement zone of the Barbados accretionary prism demonstrate the potential for multiple‐well hydrogeologic tests. Pressure data from a sealed borehole show a clear signal of drilling activity ∼45 m away. Analysis of the pressure response suggests that decollement zone permeability lies between 1.0 and 1.2×10−14m2. This permeability is ∼2–4 orders of magnitude greater than overlying and underlying sediment and thus appears sufficient to focus fluid flow along the decollement zone. This inadvertent test evaluated fault zone hydrogeologic parameters over a larger radius and longer timescale and at pressures closer to in situ than previous single‐well tests.

Accretionary prisms are wedges of saturated sediment that are subject to intense deformation as a result of lithosphere convergence. Compressive stress and rapid burial of the accreted deposits result in sediment compaction and mineral dehydration. These latter processes, in conjunction with fermentation or thermal maturation of entrained organic matter, yield hydrocarbon‐bearing pore fluids that are expelled from the prism. Regional fluxes of heat and a number of dissolved chemical species, most notably carbon, are controlled by the advective expulsion of the pore waters. Numerical modeling, observation and monitoring of flow patterns and rates, and recent in situ hydrogeological tests quantify the conditions that control rates of fluid flow. Dispersed, intergranular flow (10−8 to 10−11 m/s), controlled by the vertical permeability of the prism (10−14 to 10−20 m²), is limited by low‐permeability lithologies and seems not to vary much from margin to margin. Focused flow (10−1 to 10−8 m/s) above the décollement is controlled by fault zones or sedimentary intrusions (diapiric structures). At low‐fluid pressures, fault zone permeability may be similar to that of adjacent wall rock, but as fluid pressure increases from hydrostatic (λ* = 0) to near lithostatic levels (λ* ≈ 1.0), fault zones dilate, and (fracture) permeability increases by 2–4 orders of magnitude (10−10 to 10−16 m²). Similarly, mud volcanoes and diapirs provide high‐permeability fluid conduits to the sediment‐water interface. As a result, faults and intrusions become primary flow paths and support surface vents at which syntectonic deposits (carbonate and gas hydrates) accumulate and chemosynthetic organisms cluster. Models of thermal and chemical anomalies and epigenetic deposits indicate that flow is temporally variable. That conclusion has been quantified by extended (1–10 months) seafloor and borehole experiments that measured temperature anomalies associated with flow events. On the Cascadia prism, flow (estimated velocity ∼3 × 10−5 m/s; 950 m/yr), confined to a thrust fault that cuts upward through the prism, has brought thermogenic hydrocarbons from depths >1.5 km to the surface within the last 400 years.

The Kettle Creek embayment is located in the northwest part of Barnegat Bay, New Jersey and is typical of shallow bays behind North America's east coast barrier island system. Understanding the bay ecology as well as the physical and chemical environment have been the focus of many recent studies. However, the distribution, direction and flow rate of water movement between the bay and the underlying aquifer have not been adequately determined. This flow rate is required to properly understand bay circulation patterns and distributions of salinity, nutrients, and contaminants. The pore pressure within shallow, relatively fine-grained sediments was measured in a small bay at the mouth of Kettle Creek in the northwest part of Barnegat Bay using a Portable, In-Situ Pore Pressure Instrument (PISPPI) developed by the authors. The flux rate is the product of hydraulic conductivity and gradient. Numerous sediment cores were obtained to determine sediment properties. About 10 gallons per minute are estimated to flow up into the embayment (approximately 20 million square feet) through the silt and clay cap that was found at all stations. This contribution to flow is called diffuse recharge e.g. recharge spread over a large area. Additional recharge may occur from concentrated flux from springs or seeps which will be located and measured in the future to obtain a total flow rate, which can then be compared to groundwater flow model predictions. Predicted groundwater flow rates will allow an increased understanding of conditions such as salinity and nutrient levels within the bay that are essential to ecosystem analysis and management.

Hydrogeologic tests were conducted at a sealed borehole penetrating the decollement of the Barbados Ridge accretionary complex, At low excess pore pressures [lambda* = (P-p-P-h)/(P-l-P-h) = 0.0 to 0.36, where P-p = pore pressure, P-h = hydrostatic pressure, and P-l = lithostatic pressure], estimated permeabilities were comparable to those of similar, unfractured sediment, These tests complement shipboard packer tests completed at higher fluid pressures (lambda* = 0.5 to 1.0) during Ocean Drilling Program (ODP) Leg 156, Together, the test results suggest a 4- to 5-order-of-magnitude permeability increase as fluid pressure varied from hydrostatic (lambda* = 0) to lithostatic (lambda* = 1), However, unlike the results of the shipboard packer tests, the test results presented here exhibit no evidence of a relationship between permeability and pore pressure, The combined findings from the two sets of hydrogeologic tests indicate that significant permeability increases can occur within the decollement at pore pressures below lithostatic pressure.

In situ transmissivity of a hydrogeologically active fault zone within the Oregon accretionary prism was tested during Ocean Drilling Program (ODP) Leg 146.This experiment used an inflatable drill-string packer to conduct pressurized slug tests and constant-rate injection tests.Pressure responses during testing indicate dilation of fractures as a result of fluid injection.Consequently, test data allow examination of the transmissivity of an open fracture network.Analysis of pressurized slug-test data yields an average transmissivity of 1.0 × I0" 5 m 2 s" 1 , while recovery data from the constant-rate injection tests indicate transmissivities ranging from 4.7 to 9.2 × I0" 5 m 2 s~1.Test data indicate background borehole fluid pressure was 0.25 to 0.30 MPa greater than hydrostatic (approximately one-half lithostatic excess pressure) and indicate that fractures within the fault zone remain open at pressures approximately 0.315 to 0.325 MPa above hydrostatic.

Two holes drilled into the Cascadia accretionary prism during Ocean Drilling Program (ODP) Leg 146 were instrumented with borehole seals (CORKs) for long-term monitoring of temperatures and pressures at in situ conditions. We report the results obtained during submersible data recovery operations at the sites in September, 1993, 9.5 months after the CORK instruments were emplaced. The installation at Hole 889C off Vancouver Island was severely damaged during a deployment made very difficult by poor weather and unstable hole conditions; no useful data were recovered there. In contrast, the installation at Hole 892B in the accretionary prism off Oregon produced excellent thermal and pressure data that provide constraints on the hydrogeology at that site. Site 892 is located over the hanging wall of a hydrologically active thrust fault that is penetrated at a depth of about 100 m in the 146-m-deep Hole 892B. In addition, there is a well-defined regional bottom-simulating reflector (BSR) whose depth shoals about 8 m to 72 mbsf at the site, presumably because of the thermal effects of fluid flow in the fault zone. Results of numerical modeling demonstrate that the local shoaling of the BSR is consistent with the effects of recent up-dip fluid flow that initiated roughly 400 yr ago at an average flux per meter along strike of 1 × 10~6 m3s'; steady-state flow is precluded. Hole 892B was sealed with a pressure gauge and a 10-thermistor chain extending to a depth of 122 mbsf. Temperatures in the CORKed hole define a generally uniform gradient of about 68 mK m1. At the depth of the regional BSR, this gradient gives a temperature identical to that on seawater-methane-hydrate phase boundary at the equivalent pressure. The gradient is significantly greater than that defined by shipboard temperature measurements made in exploratory holes about 200 m to the southwest. The disagreement can be explained if the exploratory holes intersected fault-controlled zones of fluid upflow at shallower depths than the CORKed hole. The gradient defined by the shipboard measurements may reflect locally diminished heat flow in the footwall of the fault. CORK temperatures also define a distinct thermal anomaly at the depth of the fault zone, which is consistent with results of numerical simulations of a transient fluid flow event. The up-dip fluid flux is constrained to be approximately 6 × I05 m3s~1, nearly two orders of magnitude greater than the average rate inferred from the shoaling of the BSR. Pressures in the sealed hole decayed from an initial (shut-in) superhydrostatic value of 70 kPa to a low, relatively stable value of 13 kPa within a few months after drilling (lithostatic pressure at 100 mbsf in this hole is about 630 kPa). The initial superhydrostatic value may have been caused by charging of the formation during drilling, although it is more likely that high pressures were present in the fault zone initially and drained after the fault was penetrated, probably to the surrounding formation spanned by the open section of hole where lower fluid pressure may be present. This conclusion is reached by considering the hydraulic transmissivity required to support the high rates of flow inferred from the CORK and BSR data in light of the transmissivity determinations made at both elevated and reduced pressures by Screaton et al. (this volume). Attenuation and phase of the seafloor tidal loading signal recorded in the sealed hole remained constant throughout the 9month recording period at 0.5 and 0.3 hrs (-9° phase lead at 12 hr period), respectively. These characteristics are consistent with the presence of approximately 2% free gas in the pore volume of the sediments below the BSR and above the perforated interval, and high fault-zone transmissivity connecting the perforated interval to the zone above containing free gas.

In situ transmissivity of a hydrogeologically active fault zone within the Oregon accretionary prism was tested during Ocean Drilling Program (ODP) Leg 146. This experiment used an inflatable drill-string packer to conduct pressurized slug tests and constant-rate injection tests. Pressure responses during testing indicate dilation of fractures as a result of fluid injection. Consequently, test data allow examination of the transmissivity of an open fracture network. Analysis of pressurized slug-test data yields an average transmissivity of 1.0 × I0"5 m2s"1, while recovery data from the constant-rate injection tests indicate transmissivities ranging from 4.7 to 9.2 × I0" 5 m2s~1. Test data indicate background borehole fluid pressure was 0.25 to 0.30 MPa greater than hydrostatic (approximately one-half lithostatic excess pressure) and indicate that fractures within the fault zone remain open at pressures approximately 0.315 to 0.325 MPa above hydrostatic.

Two sets of hydrogeologic tests conducted at Ocean Drilling Program (ODP) Hole 892 on the Oregon Accretionary Prism provided the opportunity to determine hydrogeologic properties of an active accretionary prism fault zone. The first set of tests consisted of shipboard packer tests conducted during ODP Leg 146 (fall 1992), while the second set of tests were constant‐drawdown and constant‐discharge tests conducted in fall 1993 using the submersible Alvin. Pressure response during the first set of tests suggests that fractures remained open until excess fluid pressure (relative to hydrostatic) dropped below 0.315 to 0.325 MPa (λ* ∼ 0.53 to 0.54, where λ* = (pore pressure ‐ hydrostatic)/(lithostatic‐hydrostatic)). Analysis of the packer test data suggested an apparent background pressure of 0.25 MPa (λ* ∼ 0.42 to 0.50). Because the borehole had been open for 12 hours prior to the packer tests, formation pore pressures may have exceeded this value prior to drilling of the borehole. These overpressures dissipated by the time the second set of tests were conducted. One possible explanation for this decay is that the borehole may provide a vertical conduit between the overpressured zone and overlying or underlying sediments that had previously been hydraulically separated from the overpressured zone. The second set of tests were conducted at pressures (≤0.019 MPa or λ* ∼ 0.03) below that estimated to maintain open fractures and yielded transmissivities 1 to 2 orders of magnitude less than estimated for the packer tests (when fractures were open). Constraints on fluid flow rate along the fault are provided by observed displacement in a bottom‐simulating reflector (BSR) at its intersection with the fault zone. The closed‐fracture transmissivities are insufficient to produce flow rates capable of displacing the BSR; therefore open‐fracture transmissivities under conditions of elevated pore pressure are inferred to be necessary for the observed BSR displacement. In addition, calculated rates of specific discharge through the fault zone are 2 to 3 orders of magnitude lower than discharge measured at an associated seafloor vent site; fluid flow must become spatially or temporally focused as it moves up the fault zone toward the seafloor.

The Cascadia Margin is characterized by rapid pore fluid expulsion from a thick sedimentary section during the accretion process.Ocean Drilling Program (ODP) Leg 146 focused particularly on the nature and movement of these fluids.The results, summarized below, define the first comprehensive characterization of flow within a modern accretionary prism.They reflect post-cruise analysis of seismic and logging data, returned samples, packer test results, data collected over 9.5 months at the instrumented borehole seal deployed at Hole 892B, and pump tests conducted at the same site from the submersible Alvin.Fluid brought to the margin is originally incorporated in sediments deposited in Cascadia Basin.Initial deformation of these turbidite/hemipelagic deposits off Oregon and Vancouver Island occurs ~6 km seaward of the topographic toe of the prism and is manifested by a series of blind thrust faults that exhibit both positive and negative polarity seismic reflections.Velocity data and sidescan sonar suggest that significant dewatering occurs in this proto-thrust zone.Further landward, deformation is manifested by anticlinal folding and imbricate thrust faulting.Based on long-term (>2 × I0 5 yr) total pore volume loss off Oregon, the seawardmost 18 km of the prism is estimated to discharge fluid at rates of 3.2 × I0" 12 to 2.9 × lO^πAirV~1 (0.1-0.9 mm/yr).Although the relative importance of dispersed versus focused flow to the total discharge is unknown, faults are clearly important pathways along which pore fluids are lost to the accreted section.Detailed fluid flow studies on Leg 146 concentrated on focused flow along fault zones, particularly a fluidactive thrust fault penetrated by Site 892.Thermal anomalies define episodic flow events presumably driven by variation in fluid pressure.The instrumented borehole seal at Site 892 recorded a temperature excursion of 4°C at 100 meters below seafloor (mbsf) over 5.5 months, a decline in pore pressure from 70 kPa above hydrostatic at hole shut-in to 2 kPa 5 months later, and an attenuated tidal signal in the pressure record that exhibits short-period fluctuations.Flow may respond to pressure variations on a variety of temporal scales, but shoaling of the bottom-simulating reflector (BSR) and calculated darcian flow based on observed pressures and measured transmissivities indicate an average flow within the fault zone at Site 892 of ~l × I0" 6 m 3 m~V (-3.2 × I0 4 mm/yr).The seafloor heat flow anomaly, however, requires that present flow rates along the fault zone cannot have continued for more than about 400 yr.Flow episodicity in the fault zone at Site 892 is inferred to be a direct function of variations in the pressure field, and an indirect result of the pressure dependence of the hydraulic conductivity (permeability).Both laboratory tests on sediment from the fault zone and in situ hydrogeologic tests indicate that fractures within the fault zone control and localize flow.Pore pressures in excess of 50%-70% lithostatic stress apparently dilate fractures and increase hydraulic transmissivity by at least two orders of magnitude relative to closed fractures under near-hydrostatic conditions or to unfractured wall-rock.The marked dependence of fracture permeability on excess pore pressure and thus the effective confining stress yields a non-linear flow response in the fracture zone to pressure fluctuations.

Point‐discharge fluid expulsion on accretionary prisms is commonly indicated by diagenetic deposition of calcium carbonate cements and gas hydrates in near‐surface (<10 m below seafloor; mbsf) hemipelagic sediment. The contrasting clastic and diagenetic lithologies should be apparent in side scan images. However, sonar also responds to variations in bottom slope, so unprocessed images mix topographic and lithologic information. We have processed GLORIA imagery from the Oregon continental margin to remove topographic effects. A synthetic side scan image was created initially from Sea Beam bathymetric data and then was subtracted iteratively from the original GLORIA data until topographic features disappeared. The residual image contains high‐amplitude backscattering that we attribute to diagenetic deposits associated with fluid discharge, based on submersible mapping, Ocean Drilling Program drilling, and collected samples. Diagenetic deposits are concentrated (1) near an out‐of‐sequence thrust fault on the second ridge landward of the base of the continental slope, (2) along zones characterized by deep‐seated strikeslip faults that cut transversely across the margin, and (3) in undeformed Cascadia Basin deposits which overlie incipient thrust faults seaward of the toe of the prism. There is no evidence of diagenetic deposition associated with the frontal thrust that rises from the dècollement. If the dècollement is an important aquifer, apparently the fluids are passed either to the strike‐slip faults which intersect the dècollement or to the incipient faults in Cascadia Basin for expulsion. Diagenetic deposits seaward of the prism toe probably consist dominantly of gas hydrates.