Jane Schoonmaker

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.

COMPARISONS BETWEEN THE DIAGENESIS OF DIOCTAHEDRAL AND TRIOCTAHEDRAL SMECTITE, BRAZILIAN OFFSHORE BASINS

Burial diagenetic reactions of di- and trioctahedral clay minerals were compared in Brazilian offshore, basinal sediment sequences of Cretaceous age. Originally dioctahedral smectite-rich shales of three basins—Potiguar,Ceara, and Ilha de Santana—exhibited the classical smectite-to-illite burial pattern. Trioctahedral clay-rich shales and trioctahedral clay-mineral cements in sandstones, however, showed a burial sequence of saponite to mixed-layer chlorite/saponite with progressive increase in the percentage of chlorite layers with increasing burial depth.The change from disordered to ordered interstratifications of chlorite/saponite occurred in the temperature range 60°-70°C at a vitrinite reflectance value of about 0.65. These values are lower than those found for the ordering of illite/smectite clays. Increasing substitution of Al for Si in tetrahedral sites, followed by fixation of interlayer hydroxide sheets was found to be the major chemical change promoting transformation of saponite to chlorite via corrensite.

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

The microstructures and crystal fabrics associated with the development of an amphibolite facies quartzo-feldspathic mylonitic shear zone (Torridon, NW Scotland) have been investigated using SEM electron channelling. Our results illustrate a variety of microstructures and fabrics which attest to a complex shear zone deformation history. Microstructural variation is particularly pronounced at low shear strains: significant intragranular deformation occurs via a domino-faulting style process, whilst mechanical incompatibilities between individual grains result in characteristic grain boundary deformation accommodation microstructures. A sudden reduction in grain size defines the transition to medium shear strains, but many of the boundaries inherited from the original and low shear strain regions can still be recognized and define distinctive bands oriented at low angles to the shear zone margin. Grains within these bands have somewhat steeper preferred dimensional orientations. These domains persist into the high shear strain mylonitic region, where they are oriented subparallel to the shear zone margin and consist of sub-20 μm grains. The microstructures suggest that the principal deformation mechanism was intracrystalline plasticity (with contributions from grain size reduction via dynamic recrystallization, grain boundary migration and grain boundary sliding). Crystal fabrics measured from the shear zone vary with position depending on the shear strain involved, and are consistent with the operation of several crystal slip systems (e.g. prism, basal, rhomb and acute rhomb planes) in a consistent direction (probably parallel to a and/or m). They also reveal the presence of Dauphine twinning and suggest that this may be a significant process in quartz deformation. A single crystal fabric evolution path linking the shear zone margin fabric with the mylonitic fabric was not observed. Rather, the mylonitic fabric reflects the instantaneous fabric which developed at a particular location for a particular shear strain and original parental grain orientation. The mature shear zone therefore consists of a series of deformed original grains stacked on top of each other in a manner which preserves original grain boundaries and intragranular features which develop during shear zone evolution. The stability of some microstructures to higher shear strains, the exploitation of others at lower shear strains, and a continuously evolving crystal fabric, mean that the strain gradient observed across many shear zones is unlikely to be equivalent to a time gradient.

Hydrogeochemistry in the barbados accretionary complex: Leg 110 ODP

Abstract Detailed studies of the chemical composition of interstitial waters in the sediments obtained along a drill hole transect across the accretionary prism of the Northern Barbados Ridge have revealed a complex set of processes: (1) In Plio-Pleistocene volcanic ash-bearing sediments increased concentrations of dissolved calcium and decreases in dissolved magnesium are the result of volcanic ash alteration; (2) below the decollement large concentration increases in dissolved calcium, accompanied by large decreases in sodium as well as a granual decrease in dissolved magnesium, suggest exchange of these constituents with the deeper seated rocks of layer 2 of the oceanic crust; (3) low chloride concentrations, particularly associated with the decollement zone separating the converging plates and underthrusted sandstones, and also with recent faults in the accretionary complex, suggest advective transport of low chlorinity waters from deeper within the accretionary complex. In and below the decollement zone elevated concentrations of methane of thermogenic origin have been measured.

TECTONIC IMPLICATIONS OF ILLITE/SMECTITE DIAGENESIS, BARBADOS ACCRETIONARY PRISM

The depth distribution of illite/smectite (I/S) compositions was investigated for a well drilled to a depth of 3462 m on Barbados Island, the only subaerial exposure of the Barbados accretionary complex. The classical pattern of increasing percentage of illite interlayers in the mixed-layer clay with increasing burial depth was not observed. Rather, the data describe an irregular, zig-zag trend with depth. This trend is probably the result of reverse faulting in the section. I/S data were also used to infer several kilometers of uplift and subsequent erosion of the section. This study shows that irregular patterns of clay diagenesis with depth can be anticipated for sequences that have undergone complicated tectonism and deformation. Combined with other geologic information, these patterns can help to determine the tectonic history of the sedimentary sequence.

Phanerozoic Carbonate Diagenesis--A New Model: ABSTRACT

Observations from petrography, paleontology, and geochemistry indicate the varied carbonate mineralogy of Holocene sediments may not be representative of pre-Carboniferous rocks. Petrographic examination of Phanerozoic oolites reveals that ooids with calcitic relict textures are characteristic of pre-Carboniferous carbonate rocks, whereas ooids with aragonitic relict textures are dominant in younger rocks. Marine invertebrates that secreted magnesian calcite or aragonite hard parts characterize End_Page 764------------------------------ post-Permian fossil assemblages, whereas calcitic forms decline or become extinct. These observations imply a post-Carboniferous change in the chemistry of the earths surface environment. To determine the relative importance of factors controlling magnesian calcite solubilities in seawater, saturometry experiments were performed. It is important to recognize from these experiments that the composition of a magnesian calcite precipitating at saturation from seawater can be treated as a function of atmospheric CO2 pressure, at an essentially constant seawater Mg/Ca ratio. Thus, observed trends in the textures of ooids and their interpreted mineralogy through the Phanerozoic may be due to changes in atmospheric CO2 levels, not necessarily to a major change in the Mg/Ca ratio of seawater. It is possible that pre-Carboniferous CO2 levels were high favoring precipitation of calcitic ooids and skeletal parts; after the Carboniferous, CO2 levels fell and aragonite and high magnesian calcites increased in abundance as precipitates. Diagenetic implications of the time variance in CO2 pressure are many. Diagenetic patterns based on Holocene models may not be valid for the Paleozoic; solution-reprecipitation and inversion would be less common than recrystallization prior to the Carboniferous. Reservoir targets of intraparticle porosity in relict aragonite oolitic sands would be less important in pre-Carboniferous rocks. Higher atmospheric CO2 pressure favors dolomite formation. Finally, the CO2 content of an environment rather than the Mg/Ca ratio may be the important parameter to consider in many carbonate diagenesis studies. Figure End_of_Article - Last_Page 765------------

Diagenesis and Mass Transfer in Sandstone-Shale Sequences: ABSTRACT

An analysis of diagenesis and mass transfer is made drawing on the literature and our work from the Brazilian shelf and Barbados. It is shown (although not unanticipated) that the initial sedimentary mineral composition is a major control of diagenetic products. For example, dioctahedral clay minerals, chlorite, and quartz characterize arkoses, whereas trioctahedral clays and zeolites are most commonly found in lithic sandstone. Dioctahedral smectite-rich shales exhibit the classical smectite/illite to illite burial pattern. However, mafic, trioctahedral clay-rich shales show a burial sequence of saponite to chlorite-rich phases with increasing burial depth. Other compositionally dependent reaction paths are also discussed. To assess mass transfer between shale and sandstone during burial, all major diagenetic pathways must be known for both rock types. A model for the Brazilian shelf sandstone-shale sequence is used as an example of quantification of mass transfer. Both sands and shales act as nearly isochemical systems; sandstones lose less than 2% K+ to shales, and gain less than 3% H2O, H+, and CO2 during burial diagenesis. It is shown using data from Barbados and the literature that burial diagenetic reactions are essentially irreversible, at least until the stage of weathering. Thus, these reactions can be used to assess the amount of overburden removed. Comparison of the diagenetically produced trend of illite/smectite compositions with depth in Barbados to trends produced in areas which have undergone only subsidence (e.g., Gulf Coast) suggests that about 3,300 to 9,800 ft (1,000 to 3,000 m) of overburden has been removed in Barbados. The irreversible and nearly isochemical nature of burial diagenetic reactions places constraints on the role of diagenesis in the sedimentary rock cycle. An attempt is made to quantify the global important of these reactions in the rock cycle. End_of_Article - Last_Page 508------------

Diagenesis and Mass Transfer in Sandstone-Shale Sequences and the Sedimentary Cycle: ABSTRACT

An analysis of diagenesis and mass transfer is made drawing on the literature and our work from the Brazilian shelf and Barbados. It is shown (although not unanticipated) that the initial sedimentary mineral composition is a major control of diagenetic products. For example, dioctahedral clay minerals, chlorite, and quartz characterize arkoses, whereas trioctahedral clays and zeolites are most commonly found in lithic sandstone. Dioctahedral smectite-rich shales exhibit the classical smectite/illite to illite burial pattern. However, mafic, trioctahedral clay-rich shales show a burial sequence of saponite to chlorite/saponite mixed layer, a progressive increase of chlorite-rich phases with increasing burial depth. Other compositionally dependent reaction paths are also di cussed. To assess mass transfer between shale and sandstone during burial, all major diagenetic pathways must be known for both rock types. A model for the Brazilian shelf sandstone-shale sequence is used as an example of quantification of mass transfer. Both sands and shales act as nearly isochemical systems; sandstones lose less than 2% K+ to shales, and gain less than 3% H2O, H+, and CO2 during burial diagenesis. It is shown using data from Barbados and the literature that burial diagenetic reactions are essentially irreversible, at least until the stage of weathering. Thus, these reactions can be used to assess the amount of overburden removed. Comparison of the diagenetically produced trend of illite/smectite compositions with depth in Barbados to trends produced in areas which have undergone only subsidence (e.g., Gulf Coast) suggests that about 3,300 to 9,800 ft (2 to 3 km) of overburden have been removed in Barbados. The irreversible and nearly isochemical nature of burial diagenetic reactions places constraints on the role of diagenesis in the sedimentary rock cycle. An attempt is made to quantify the global importance of these reactions in the rock cycle. End_of_Article - Last_Page 1445------------