Northern Barbados Research Articles

Influence of kinetics on the smectite to illite transition in the Barbados accretionary prism

Low chloride pore fluids observed along faults in clay‐rich accretionary complexes are commonly attributed to the release of interlayer water during the smectite to illite transformation. However, to date, there has been no thorough analysis of the location and quantity of fluids that may be generated by this mechanism. To address this problem, a temperature and time dependent rate expression describing the dehydration reaction was coupled to a kinematic model of the northern Barbados accretionary complex. Temperatures in the complex were estimated by modeling heat flow through the prism sediments as they thicken arcward. The sediments' temperature‐time histories were computed using a model for the velocities of sediment motion through the complex. In this model the prism sediments follow uniformly diverging paths from the toe, while the underthrust sediments undergo uniaxial strain. The model predictions are validated against clay mineralogy data from Barbados Island mudstones. Our results show that the location of the peak dehydration rate is 20 km farther arcward in the underthrust sediments than in the prism complex. The fresh water produced by the reaction results in pore waters that are 10–30% fresher at a distance of 50–70 km from the prism toe. This constraint indicates a probable fluid migration path of more than 50 km from the reaction zone to the sites where freshened pore fluids have been observed. The peak rate of fluid production when expressed as the volume of fluid per volume of sediment per second is 2×l0−15.

Seismically inferred dilatancy distribution, northern Barbados Ridge decollement: Implications for fluid migration and fault strength

A 5 x 25 km, three-dimensional seismic survey of the lower part of the northern Barbados Ridge accretionary prism creates a three-dimensional image of a major active decollement fault. The fault is usually a compound negative-polarity reflection modeled as a low-velocity, high-porosity zone less than ∼14 m thick. This thickness is significantly less than that defined by drilling of a >40 m zone of deformation at Ocean Drilling Program (ODP) Site 671B, located within the surveyed area. We infer that the seismically defined fault is a thin, high-porosity zone and is thus an undercompacted, high-fluid-pressure dilatant section. If these inferences are correct, then map-view variations in seismic-reflection waveform and amplitude illustrate complex patterns of fault-zone fluid content and fluid migration paths. The amplitude map suggests kilometre-wide channels of locally high porosity and thus focused fluid flow. These paths are only subparallel to the expected minimum head, as inferred from the shape of the overlying sediment wedge; other factors must modify fluid concentrations and ultimately migration. Several areas of positive-polarity fault reflections define square-kilometre-sized regions inferred to be lower porosity sections producing strong asperities in an otherwise weak fault. One, coincident with Site 671B, may explain the success of drilling through the fault here. All other holes drilled in the area were within the negative-polarity regions and were unsuccessful in penetrating through the entire fault zone, possibly because of instability associated with high fluid pressures and a weak fault. ODP Leg 156 planned for 1994 will test inferences related to fault permeability and fluid pressures.

A simplified analysis of parameters controlling dewatering in accretionary prisms

One of the many dynamic geologic processes taking place at subduction zones is the compaction-driven dewatering of seafloor sediments as they are accreted to the overriding plate. The rate of dewatering is equal to the divergence of the sediment matrix velocity field. This rate can be estimated analytically if simplifying assumptions are made about the geometry of the prism and motion of the sediments. The analytical expression depends only on the sediment accretion velocity, thickness of the accreted section, prism taper angle, and sediment porosity distribution. A sensitivity analysis of the solution shows that the fluid production distribution is relatively insensitive to the sediment porosity distribution. However, the solution is very sensitive to the taper angle of the wedge. High-angle wedges expel almost all of the incoming water within 20 km, while low-angle wedges may retain a significant fraction of the incoming water for 50 or more kilometers. Thickness of the incoming section is also important. Dewatering of thin accreted sections is more concentrated near the toe of the wedge. Analyses of transects through the Northern Barbados, Makran, Vancouver, and Nankai accretionary prisms illustrate a range of dewatering rates and spatial distributions of dewatering.

Plumbing accretionary prisms: effects of permeability variations

Fault zones focus fluid expulsion in the muddy northern Barbados Ridge accretionary prism with fault-parallel permeabilities about 1000 times greater than intergranular permeabilities in the adjacent sediment. In the Oregon prism the low bedding-perpendicular permeability (due to mudstones) inhibits intergranular dewatering; however, intergranular flow is concentrated where submarine erosion breaches high permeability sandy layers. Even so, faults can capture fluid flow from these exposed sandy layers suggesting the faults have a still higher permeability. Such observations coupled with laboratory measurements permeabilities suggest that faults off Oregon may have fault-parallel permeabilities at least 10-10000 times greater than the adjacent sediments. Results from Barbados and Oregon suggest fluid flow is concentrated along the most active faults. At the toe of prisms the fault zones are being progressively loaded by the thickening wedge and are undergoing compaction. Preliminary experiments show that permeability decreases relative to the surrounding wall rocks along faults within this compactive deformation regime; we believe that these faults must undergo dilation, perhaps linked to transient increases in pore pressure if they are to be preferential fluid conduits. Farther upslope erosion exposes rocks that are more consolidated, commonly more cemented, and generally of lower intergranular permeability than rocks of equivalent burial further seaward. Because of their lithification and overconsolidation these rocks dilate during faulting, locally enhancing fracture permeability. In such dilative regimes, faults become evermore focused zones of fluid expulsion relative to occluded intergranular pathways.

Geochemical and geothermal evidence for fluid migration in the Barbados Accretionary Prism (ODP leg 110)

Measured geochemical and geothermal effects of fluid migration in the northern Barbados accretionary prism indicate that: (1) fluid flows laterally along low‐angle active faults; (2) the entire length of the Leg 110 transect is a fluid discharge zone; (3) migration results in heating of sediments across the transect but most strongly at the deformation front; and (4) migration results in dilution of pore water‐Cl and 18O enrichment more in the arcward than the seaward side of the prism, suggesting that the Cl‐poor, 18O‐rich fluids that intrude the prism are derived from smectiteillite reactions occurring arcward. These conclusions have strengthened those derived from the initial appraisal of shipboard data, modified ideas about where fluids are heated most and where the geochemistry is strongly altered, and provided new ideas about the sources of fluids and their relative contributions.

Seismic modeling of the decollement zone at the base of the Barbados Ridge Accretionary Complex

Wide‐aperture seismic reflection profiling of the northern Barbados Ridge accretionary complex at 16°12′N has imaged the decollement horizon across more than 100 km of the accretionary complex revealing prominent changes in the amplitude of the decollement reflection. Synthetic seismograms from models of the variation of velocity and density with depth were compared with selected seismic traces between 2.5 km seaward of the toe of the accretionary complex and 20 km landward of the toe to determine the changes in velocity‐density properties along the decollement thrust zone. Modeling requires a thin, low‐velocity layer to develop landward along the decollement beneath the complex, while no such layer exists at the corresponding stratigraphic boundary in the undeformed sediments seaward of the complex. Two‐dimensional modeling that incorporated lateral variations in topography, velocity, and density was performed on a 7‐km section of the decollement 20 km landward of the deformation front. The best fitting model includes a 20‐m‐thick low‐velocity layer at the decollement horizon with a relative velocity decrease in the layer of 0.2 km/s. Changes in the reflection amplitude along this 7‐km section could be produced by either a change in the velocity anomaly within the layer or a thinning of the decollement layer to less than 4 m. The shape of the reflected wavelet from the decollement indicates that the principal cause of the amplitude variation is a change in the velocity contrast rather than a change in thickness. The low‐velocity layer can be explained by a decollement shear zone that has high porosity maintained by elevated pore fluid pressure. High fracture permeability within the decollement permits dewatering of sediment attached to the oceanic crust and thrust beneath the accretionary complex.

Hydrogeochemistry of the Northern Barbados Accretionary Complex Transect: Ocean Drilling Project Leg 110

Detailed studies of the major element geochemistry, oxygen and hydrogen isotope ratios of pore fluids, and the 87Sr/86Sr ratio of dissolved strontium have made it possible to unravel physical and chemical processes that affect the pore fluid chemistry in a transect of drill holes across the northern Barbados accretionary complex. These processes include (1) alteration of volcanic ash buried in the Pleistocene‐Pliocene sediment column; (2) alteration of underlying basalts of layer 2 of the oceanic crust; (3) movement of fluids from deep in the accretionary complex along fault zones (particularly the décollement) and minor permeable layers; these fluids from deeper in the complex are characterized by low chloride concentrations and increased δ18O(H2O) values, presumably as a result of dehydration of smectite interlayers; and (4) mixing processes involving the migrating fluids cause incongruities in the geochemical anomalies of these fluids.

Tectonics and hydrogeology of the northern Barbados Ridge: Results from Ocean Drilling Program Leg 110

Drilling near the deformation front of the northern Barbados Ridge cored an accretionary prism consisting of imbricately thrusted Neogene hemipelagic sediments detached from little-deformed Oligocene to Campanian underthrust deposits by a decollement zone composed of lower Miocene to upper Oligocene, scaly radiolarian claystone. Biostrati-graphically defined age inversions define thrust faults in the accretionary prism that correlate between sites and are apparent on the seismic reflection sections. Two sites located 12 and 17 km west of the deformation front document continuing deformation of the accreted sediments during their uplift. Deformational features include both large- and small-scale folding and continued thrust faulting with the development of stratal disruption, cataclastic shear zones, and the proliferation of scaly fabrics. These features, resembling structures of accretionary complexes exposed on land, have developed in sediments never buried more than 400 m and retaining 40% to 50% porosity. A single oceanic reference site, located 6 km east of the deformation front, shows incipient deformation at the stratigraphic level of the decollement and pore-water chemistry anomalies both at the decollement level and in a subjacent permeable sand interval. Pore-water chemistry data from all sites define two fluid realms: one characterized by methane and chloride anomalies and located within and below the decollement zone and a second marked solely by chloride anomalies and occurring within the accretionary prism. The thermogenic methane in the decollement zone requires fluid transport many tens of kilometers arcward of the deformation front along the shallowly inclined decollement surface, with minimal leakage into the overlying accretionary prism. Chloride anomalies along faults and a permeable sand layer in the underthrust sequence may be caused by membrane filtration or smectite dewatering at depth. Low matrix permeability requires that fluid flow along faults occurs through fracture permeability. Temperature and geochemical data suggest that episodic fluid flow occurs along faults, probably as a result of deformational pumping.

Evolution of structures and fabrics in the Barbados Accretionary Prism. Insights from leg 110 of the Ocean Drilling Program

Detailed analysis of ODP Leg 110 cores provides evidence for a complex structural evolution of the frontal thrust system in the northern Barbados Accretionary Prism. Initial east-directed in-sequence thrusts are biostratigraphically documented, and are overprinted towards the west by out-of-sequence overthrusts, largescale folds and penetrative deformation of the accreted rocks. This structural progression is interpreted to reflect the need for secondary shape adjustments of the accretionary prism in order to maintain a critical taper. At present the basal décollement is located within Lower Miocene strata. Accreted Eocene sediments suggest either fossil frontal off-scraping at a lower stratigraphic level or duplex formation and underplating. The present localization of the basal detachment is provoked by a zone of initial high porosity in the Lower Miocene radiolarian-rich mudstones. Shearing is accompanied by partial porosity breakdown in a 30–40 m wide zone of intense scaly fabrics and stratal disruption that constitutes the Atlantic-Caribbean plate boundary at present. Large displacements are taken up by a relatively narrow zone of contact strain. Initially the preferred orientation of clay minerals (incipient scaly fabrics) may be a flattening fabric, but with progressive rotational flow the scaly planes may act as discrete microshears and provide small-scale flow partitioning. Alternatively, scaly fabrics may be interpreted as brittle microshears. Mud-filled veins are present in the accreted rocks as well as at the oceanic reference site east of the deformation front. Carbonate-filled veins are restricted to the accreted rocks. This indicates persistence of high fluid pressures, but a fundamental change in the type of fluid-rock interaction during frontal accretion tectonics.

Holocene and Pleistocene Calcareous Crust (Caliche) Profiles: Criteria for Subaerial Exposure

ABSTRACT Surficial, calcareous crust profiles, hard, irregular, subhorizontal, calcareous laminae, surrounded by crumbly, chalky carbonate, form at the surface of many limestone successions in semi-arid areas. Detailed study of Holocene profiles, developed in late Pleistocene reef limestones on northern Barbados, reveals that diagenesis of limestone in such profiles creates a diagnostic, repeatable set of textures and fabrics. The original limestone is altered by brecciation, recrystallization (to microspar), micritization and boring. The CaCO3 thus put into solution and augmented by CaCO3 from fallout and salt spray is reprecipitated as calcite (less than 4 mole percent MgCO3), in the form of crystals often, quite different from those in the vadose zone below. Calcite crystals are precipitated as flower spar, micrite, random needle fibres, and tangential needle fibres. Several of these crystal morphologies are similar to those precipitated from highly supersaturated solutions or solutions that contain appreciable amounts of other dissolved ions. These 4 basic crystal types are not only precipitated as void-filling cement, but, more important, combine to form characteristic structures such as oolith-like coated particles, pelletoids, and the laminar crusts themselves. The diagenetic textures and fabrics described above are also the basic units of many Florida and Middle East calcareous crust (caliche) profiles, suggesting that the above features may be characteristic of calcareous crust profiles in general. Fossil calcareous crusts and associated features, buried in the Pleistocene limestone succession on northern Barbados without alteration, indicate that these features may well be preserved as indicators of subaerial exposure in the fossil record.

Scleractinian Coral Alteration in Subaerial Vadose Diagenetic Environment: ABSTRACT

Exoskeletons of scleractinian corals in the elevated late Pleistocene reefs on northern Barbados illustrate a series of diagenetic textures that document aragonite coral alteration under subaerial vadose conditions. Two major solution-precipitation processes are recognized: (1) concomitant solution-precipitation on a fine scale leading to preservation of coral microstructure; and (2) total leaching and destruction of the microstructure followed by later precipitation of void-filling spar. The pathway of aragonite solution in both processes is similar and controlled by the original coral microstructure. Solution is initiated in the fine equant crystallites forming the axis (center of calcification) of each trabecula and moves outward into the surrounding zone of closely packed aragonite needles by preferential solution along the linear intercrystalline contacts between needles. This results in the friable aragonite chalk commonly observed in Pleistocene corals. Aragonite chalk is also observed as a zone between aragonite and calcite in corals undergoing solution-precipitation on a fine scale. The coral microstructure is preserved by concomitant precipitation of calcite between separated aragonite needles adjacent to the calcite alteration front, incorporating minor impurities and irregularities of the original structure into the calcite crystals. This results in a new calcite texture that mimics the original aragonite one. Corals completely altered to calcite by concomitant solution-precipitation on a fine scale exhibit a coarse mosaic of blocky calcite with a relict fibrous texture. The once dark trabecular axes are preserved as a clear central canal. A similar calcite trabecular texture is observed in certain Devonian tabulate corals and other fossil calcareous organisms, suggesting that their skeletons may have been aragonite and altered in a manner similar to that described. End_of_Article - Last_Page 630------------

Holocene and Pleistocene Subaerial Crusts and Pisolites, Northern Barbados, West Indies: ABSTRACT

Laminated calcareous crusts are present on most surface outcrops of Pleistocene limestone on northern Barbados and are similar to the caliche so common in areas with semiarid climate and a source of CaCO3. Crust development is most intense at the surface of relatively young, poorly lithified, commonly little diagenetically altered carbonates. The complete zone of alteration (irregular, hard calcareous bands alternating with soft, chalky carbonate) may extend to depths of 6 ft or more. Large amounts of salt spray, either wind blown or direct splash, also contribute CaCO3 to the system, resulting in the formation of thick crusts. When developed on poorly indurated carbonates, crusts are usually interbedded with pisolites. These pisolites are either laminated microcrystalline grains, formed in the process of limestone alteration or, where salt spray is heavy, coated skeletal grains. The Pleistocene section on northern Barbados includes a succession of transgressive reef complexes, unlike the generally regressive sequence on the rest of the island. Each episode of reef building is considered to represent a separate high stand of sea level during the late Pleistocene. Occurrence of fossil calcareous crusts and pisolites (similar to those forming on the surface today) between overlapping reef complexes in the transgressive sequence suggests a period of subaerial exposure and diagenesis between the formation of each successive reef complex. The presence of comparable calcareous crusts and pisolites, both Holocene and fossil, in other Pleistocene and some Paleozoic limestone successions indicates they are useful criteria for subaerial exposure. End_of_Article - Last_Page 345------------