Physical property evolution in sedimentary sequences, including accretionary prisms, cannot be comprehensively evaluated with recovered cores and must be studied in situ. Elastic rebound and microcracking of coherent sedimentary samples degrade laboratory physical property measurements. Fault gouge and other incoherent lithologies are either not recovered or cannot be measured after recovery. Transient properties (e.g., excess pore pressures) must be measured in situ (Fisher et al., 1996; Screaton et al., in press). Logging while drilling (LWD) is the best available tool for measuring physical properties of the typically underconsolidated prism sediments; LWD results can be used as input to time transient models of prism evolution.
Large strains at subduction zones accelerate the rate of change in the physical properties of sedimentary sequences that are accumulating and deforming there. Accretionary prisms allow us to study these changes because deformational features are shallowly buried and shallowly dipping, and therefore can be cored and imaged seismically. Studies of fault geology, sedimentary consolidation, and seismic imaging at subduction zones aid in understanding hydrocarbon migration, groundwater flow, and seismicity in other less active sedimentary environments. Logging while drilling was conducted in a transect across the toe of the northern Barbados accretionary prism to better understand the interrelationships of deformation, fluid flow, seismic imaging, and changes in physical properties (Figs. 2, 3, 4).
Tectonic Setting of the Northern Barbados Accretionary Prism
The northern Barbados accretionary prism is the leading edge of the Caribbean Plate that is being underthrust by Atlantic Ocean floor at rates estimated between 20 and 40 km/m.y. (Dorel, 1981; Jordan, 1975; Sykes et al., 1982; DeMets et al., 1990). On the west, the Lesser Antilles defines the volcanic arc, whereas east of the arc the island of Barbados is an outcrop of the forearc accretionary prism. Frontal structures south of the Tiburon Rise include long wavelength folds, widely spaced ramping thrust faults, and extensive décollement reflections (e.g., Bangs and Westbrook, 1991; Westbrook and Smith, 1983). North of the Tiburon Rise, trench sediment thickness is much thinner, and prism thrusts are more closely spaced (Biju-Duval et al., 1982; Westbrook et al., 1984). North of the Tiburon Rise, the Barbados accretionary prism reaches at least 10 km thick and 120 km wide in addition to a 50-km-wide forearc basin to the west (Bangs et al., 1990; Westbrook et al., 1988). Thus, the accretionary prism forms a wide low-taper wedge (Figs. 2, 3, 4).
Deep Sea Drilling Project (DSDP) Leg 78A and Ocean Drilling Program (ODP) Legs 110, 156, and 171A all focused on the northern flank of the Tiburon Rise. Here the décollement is relatively shallow, and the dominantly hemipelagic/pelagic sedimentary section offers good drilling conditions and good biostratigraphic resolution. In this area, previous drilling documented numerous biostratigraphically defined faults of mostly thrust displacement (Brown and Behrmann, 1990). The décollement zone becomes better defined in a landward direction and is a shear zone up to 40 m thick at Sites 671/948 (Mascle, Moore, Taylor, et al., 1988; Shipley, Ogawa, Blum, et al., 1995). Anomalies in pore-water chemistry (Gieskes et al., 1990; Kastner, in press) and temperature (Fisher and Hounslow, 1990) indicate focused fluid flow along fault zones and in sand layers. Models simulating this fluid expulsion from the prism suggest that the flow is transient (Bekins et al., 1995). The faults are characterized by suprahydrostatic, and locally near lithostatic, fluid pressures (Brückmann et al., in press; Labaume and Kastner, in press; Screaton et al., in press; Zwart et al., in press).
A 3-D seismic reflection survey (Figs. 1-4; Shipley et al., 1994; Moore et al., 1995b) has greatly improved the interpretation of drilling results from the northern Barbados accretionary prism. In addition to better defining the stratigraphy and structure, the seismic survey also has outlined patches of positive and negative seismic polarity on the décollement (Figs. 2B, 3). These polarity signatures may signify differing fluid regimes and stress states along the décollement zone (Shipley et al., 1994; Bangs et al., 1996; Tobin and Moore, in press; Shipley et al., in press). Determining the physical properties that define these polarity signatures was a major goal of Leg 171A.
Logging While Drilling
LWD is the most effective tool for measurement of physical properties in poorly consolidated sediments where standard wireline systems previously acquired either no data or poor quality data. LWD provides measurements from the seafloor to the bottom of the deepest level of bottom-hole assembly penetration. It acquires a continuous log of physical properties directly above the drill bit where hole conditions are optimal for logging, and it measures properties of the formation minutes after cutting the hole, thereby closely approximating in situ conditions.
Objectives of the Logging Program
1.Overall Prism Consolidation. Porosity distribution is the foundation for a variety of studies of the large-scale, long-term fluid budget of accretionary prisms. We can use logs to determine a continuous record of density and porosity as a function of depth as was done at Sites 947 and 948 during Leg 156. Between-site variation in the porosity-depth relationship provides an estimate of the amount of fluid expulsion (and therefore volumetric strain). Unfortunately in accretionary prisms, these measurements of volume change are usually impossible with standard logs, as they frequently fail due to the typically unstable hole conditions. Even under ideal conditions wireline logs do not obtain data from the top 60-120 m (because the drill pipe extends below the seafloor) nor the bottom 60-120 m of the hole (because of fill). The shallowest 100 m is of particular interest in volumetric studies because this is where porosity reduction is the greatest. Only LWD can obtain reliable porosity logs from the entire depth range, including the critical top 100 m.
4.Physical Characteristics of Negative Polarity Seismic Reflections From Fault Zones. Seismic reflections are created by changes in physical properties that can be measured in boreholes. In principle the seismic data provide a proxy for changes in physical properties on a tens of meters scale. Models reproduce the polarity and shape of seismic waveforms from the décollement zone beneath accretionary prisms (Bangs and Westbrook, 1991; Moore and Shipley, 1993). Accordingly, negative polarity reflections are interpreted as either due to overthrusting of higher over lower impedance sediment (Shipley et al., 1990) or due to the reduction of fault zone impedance through dilation (Bangs and Westbrook, 1991; Shipley et al., 1994; Bangs et al., 1996). The modeling, however, is incomplete without documenting the in situ physical properties across fault zones in areas with high quality 3-D seismic data.
Previous drilling at Barbados has provided high quality structural, pore-water chemistry, heat flow, and shipboard physical property studies. This information provides independent determination of fault locations, of fluid flow activity, and of correlative physical properties. A wealth of shipboard information and subsequent scientific results provide a rich framework to enhance log interpretation.
The décollement zone occurs at depths that can be reached by drilling.
Barbados is one of only two convergent margins with 3-D seismic reflection surveys. These extraordinary data sets vastly expand the opportunity for core-log-seismic integration and the consequent 3-D analysis of faulting, fluid flow, and consolidation.
Finally, the northern Barbados Ridge is an end-member accretionary prism characterized by moderately thin pelagic and hemipelagic sediment, in contrast to, for example, the Cascadia accretionary prism, which is forming from a voluminous coarse terrigenous clastic influx.
LWD investigations of the northern Barbados prism build on existing LWD measurements at Site 948 (ODP Leg 156) that penetrated the décollement where it is of positive polarity (Fig. 2). Although LWD was conducted at Site 947 (ODP Leg 156), this locality was never cored nor reached the décollement because of the unstable hole conditions encountered during the LWD penetration. The Leg 171A sites (Figs. 2, 3, 4) begin with logging of the incoming sedimentary section at Site 1044, which was previously cored as Site 672 (ODP Leg 110). The remaining sites extend westward across the deformation front and sample various stages of development of the accretionary prism, décollement, and the reflections along the décollement.
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