Formation MicroScanner Research Articles

IODP Expedition 356: Drilling to reveal a 5 million year carbonate and subsidence history on the Northwest Shelf of Australia

The late Cenozoic carbonates of the Northwest Shelf are important subsidence history archives that also cause significant sonic velocity problems affecting seismic imaging of underlying strata. Despite their substantial thickness and areal extent, these carbonates have been sampled only in engineering foundation boreholes and as cuttings and rare sidewall cores in petroleum wells. In August-September 2015 the International Ocean Discovery Program (IODP) will drill a transect of shelf to shelf margin cores in this region. Six sites will be drilled over 10° latitude from the Perth Basin to the Bedout sub-basin by RV JOIDES Resolution with continuously cored penetrations of 300 m to 1.1 km. An array of shipboard and post-cruise biostratigraphic, sedimentological and geochemical analyses will be carried out on these cores to achieve three primary aims: 1. Provide empirical input into the spatiotemporal patterns of subsidence along Northwest Australia that can be used to place fundamental constraints on the interaction between Australian plate motion and mantle convection and to ground truth geodynamic models. 2. Determine the timing and variability of regional oceanographic features in order to understand the controls on Neogene carbonate stratigraphy and reef development. 3. Obtain a tropical to subtropical climate and ocean archive, directly comparable to deep-ocean oxygen isotope and ice-core archives, to chart the variability of the Australian monsoon and the onset of aridity in northwestern Australia. Each site will be triple cored using a combination of Advanced Piston Coring (APC), Extended Core Barrel (XCB) and Rotary Core Barrel (RCB). An array of downhole measurements will be taken using three standard IODP tool string configurations: the triple combination (triple combo), Formation MicroScanner (FMS)-sonic, and Versatile Seismic Imager (VSI). These will be used to correlate the cores to regional multichannel seismic profiles in order to gain a better understanding of Northwest Shelf stratigraphy and neotectonics.

Volcanic evolution of the submarine super volcano, Tamu Massif of Shatsky Rise: New insights from Formation MicroScanner logging imagery

Tamu Massif, the southernmost plateau of Shatsky Rise, is recently reported as the largest single volcano known on Earth. This work seeks to understand the type of volcanism necessary to form such an anomalously large single volcano by integrating core and high-resolution wireline logging data. In particular, resistivity imagery obtained by the Formation MicroScanner, in Integrated Ocean Drilling Program Hole U1347A, located on the eastern flank of Tamu Massif, was used to construct a logging-based volcanostratigraphy. This model revealed two different volcanic stages formed Tamu Massif: (i) the core part of the massif's basaltic basement was formed by a “construction phase” of volcanism with cyclic eruption events from a steady state magma supply and (ii) the very topmost basaltic section was formed by a “depositional phase” of volcanism during which long-traveling lava flows were deposited from a distant eruption center.

Magma flow directions in the sheeted dike complex at superfast spreading mid-ocean ridges: Insights from IODP Hole 1256D, Eastern Pacific

Integrated Ocean Drilling Program (IODP) Hole 1256D successfully sampled a complete section of an intact oceanic crustal sheeted dike complex (SDC) (from 1061 to 1320 meters below seafloor; mbsf) on a 15 Ma old Cocos Plate. A series of rock magnetic measurements were carried out to understand the magmatic processes that accreted this end-member, superfast-spread (200 mm/yr full rate) oceanic crust. Results indicate that main ferromagnetic minerals are predominantly pseudo single-domain (titano)magnetite crystals, responsible for both anisotropy of magnetic susceptibility (AMS) and magnetic remanence signals. AMS fabrics were reoriented into a geographic reference frame using magnetic remanence data, and corrected for a counterclockwise rotation of the Cocos Plate relative to the East Pacific Rise (EPR) ca. 15 Ma. Corrected AMS fabrics were then compared with the orientations of chilled margins previously obtained from Formation MicroScanner (FMS) images of the SDC at Hole 1256D. For some samples taken from close to dike margins, a dike-normal orientation of the minimum AMS axes (Kmin) of prolate AMS ellipsoids mean that the long axis (Kmax) can be used to infer magma flow directions. Subvertical Kmin orientations in the interior of the dikes, however, may have required settling or compaction of the magma shortly after intrusion, thus rearranging the AMS fabric. Despite this orientation of Kmin axes, orientation of Kmax axes indicate a rather constant subhorizontal paleo-flow direction, suggesting that magmas most probably traveled to the surface considerable distances from source regions within the EPR system.

Simulation of paleotectonic stress fields within Paleogene shale reservoirs and prediction of favorable zones for fracture development within the Zhanhua Depression, Bohai Bay Basin, east China

Abstract Objective Tectonic fractures are the most important reservoir spaces within shale reservoirs and can significantly improve the permeability of a reservoir, the development and distribution of these fractures are controlled by paleotectonic stress fields. An important hydrocarbon reservoir is hosted by a Paleogene shale unit within the lower section of the third member of the Shahejie Formation (Es3) within the Zhanhua Depression of the Bohai Bay Basin, east China. Industrial-level oil and gas production has been obtained from over 30 wells with the highest single well production of 93 t/d, indicating the large oil and gas potential of these reservoirs in the Zhanhua Depression. Methods The data obtained from cores, logs, and drilling in the Zhanhua Depression can be used to identify the processes involved in the development of such tectonic fractures. In this study, these data were combined with additional acoustic emission and rock mechanics data to identify the effects of faulting and lithological variations on the development of fractures using a finite element method (FEM) stress analysis approach that simulated paleotectonic stress fields during the late Dongying stage, the period of time when the majority of the fractures developed. Estimations of rock failure criteria and comprehensive indexing of rupture rates for tectonic fractures were undertaken to determine the quantitative development of fractures and to predict favorable zones for fracture development. Results Tectonic fractures within the shale reservoir in the lower part of the Es3 unit include both tensional and shear fractures, these fractures are generally unfilled or half-filled. The NE–SW strike of these fractures is consistent with the orientation of the present stress field, meaning that these fractures were high-priority targets during initial well targeting. Tectonic fractures can be identified during logging by increased resistivity (R2.5), increased acoustic time difference (AC) values combined with cycle skip of the peaks, and highly variable but generally elevated gamma ray (GR) values. Fractures can also be directly identified, and fracture parameters can be determined using Formation MicroScanner Image (FMI). The magnitude of the maximum principal stress during the major period of fracture development within the Zhanhua Depression was 53.2 MPa, and the paleotectonic stress field was controlled by the location of fault zones. In contrast, areas without fractures have stress fields that were influenced by lithological variations, leading to the development of high stress fields in areas with rocks containing high concentrations of carbonate. Conclusion Fracture development is controlled by tectonic stress fields and fractured areas are generally located between fault zones, at the intersection of faults, in areas when fault orientations change, and at high stress areas near fault tips. The present results regarding predictions of the locations of fractured areas are consistent with the location of producing oil and gas wells.

Fractures of lacustrine shale reservoirs, the Zhanhua Depression in the Bohai Bay Basin, eastern China

Fractures play a vital role in the exploration and development of shale oil and gas by providing effective space for shale reservoirs and significantly improving the fluid flow capability. Core observations, microscopic analyses of thin sections, scanning electron microscopy, and Formation MicroScanner Imaging (FMI) were used to determine the types, causes of formation, and development characteristics of the fractures in lacustrine shale reservoirs in the lower part of the Paleogene Shahejie Formation (Es3) in the Zhanhua Depression, Bohai Bay Basin, eastern China. X-ray diffraction (XRD) analysis, total organic carbon (TOC) measurements, and porosity and permeability measurements were used to study the controlling factors of the fractures in the shale reservoirs, and to analyze the impact of the fractures on the shale reservoirs properties and subsequent exploration and development. The studied shale reservoir mainly displays tectonic fractures as well as various types of non-tectonic fractures. The non-tectonic fractures mainly include over-pressure fractures, diagenetic fractures, inter-layer bedding fractures, and fractures of mixed origins. In the study area, the tectonic fractures which were formed under the combined action of tensile and shear stress display the following characteristics. The dip angle of the tectonic fractures varies significantly. Unfilled or half-filled effective fractures have a high proportion. These fractures are mainly oriented in the NE–SW, NNE–SSW, and WNW–ESE directions, with fractures in the NE–SW direction accounting for the highest proportion. The tectonic and non-tectonic fracture development is affected by multiple types of factors such as the presence of faults, mineral composition, lithology, abnormal pressure and organic matter content. Abnormally high pore pressure is a very important factor in the development of non-tectonic fracture. It is inferred that the over-pressure is mainly related to hydrocarbon generation during thermal evolution. Fractures effectively improve the porosity and permeability of the shale reservoirs, and the enhancement of permeability is particularly significant. The current stress field affects the fluid flow capability of the fracture reservoirs, and the present maximum principal stress in Zhanhua Depression is oriented in the NEE–SWW direction, which has a small angle with fractures in NE–SW direction. We propose that the fractures in this direction have the greatest connectivity and thus are a high-priority target for petroleum exploration and development.

“Imaging” the cross section of oceanic lithosphere: The development and future of electrical microresistivity logging through scientific ocean drilling

Abstract A detailed understanding of the architecture of volcanic and magmatic lithologies present within the oceanic lithosphere is essential to advance our knowledge of the geodynamics of spreading ridges and subduction zones. Undertaking sub-meter scale observations of oceanic lithosphere is challenging, primarily because of the difficulty in direct continuous sampling (e.g., by scientific ocean drilling) and the limited resolution of the majority of geophysical remote sensing methods. Downhole logging data from drillholes through basement formations, when integrated with recovered core and geophysical remote sensing data, can provide new insights into crustal accretion processes, lithosphere hydrogeology and associated alteration processes, and variations in the physical properties of the oceanic lithosphere over time. Here, we introduce an alternative approach to determine the formation architecture and lithofacies of the oceanic sub-basement by using logging data, particularly utilizing downhole microresistivity imagery (e.g. Formation MicroScanner (FMS) imagery), which has the potential to become a key tool in deciphering the high-resolution internal architecture of the intact upper ocean crust. A novel ocean crust lithostratigraphy model based on meticulously deciphered lava morphology determined by in situ FMS electrofacies analysis of holes drilled during Ocean Drilling Program legs (1) advances our understanding of ocean crust formation and accretionary processes over both time and space; and (2) allows the linking of local igneous histories deciphered from the drillholes to the regional magmatic and tectonic histories. Furthermore, microresistivity imagery can potentially allow the investigation of (i) magmatic lithology and architecture in the lower ocean crust and upper mantle; and, (ii) void space abundances in crustal material and the determination of complex lithology-dependent void geometries.

Performance evaluation of active wireline heave compensation systems in marine well logging environments

The basic functionality and performance of a new Schlumberger active wireline heave compensation system on the JOIDES Resolution was evaluated during the sea trial and a 3-year period of the IODP Phase II operations. A suite of software programs was developed to enable real-time monitoring of the dynamics of logging tools, and assess the efficiency of wireline heave compensation during downhole operations. The evaluation of the system effectiveness was performed under normal logging conditions as well as during stationary tests. Logging data were analyzed for their overall quality and repeatability, and to assess the reliability of high-resolution data such as formation microscanner (FMS) electrical images. This revealed that the system reduces 65–80 % of displacement or 88–98 % variance of downhole tool motion in stationary mode under heave conditions of ±0.2–1.5 m and water depths of 300–4,500 m in open holes. Under similar water/heave conditions, the compensator system reduces tool displacement by 50–60 %, or 75–84 % variance in downhole tool motion during normal logging operations. Such compensation efficiency (CE) is comparable to previous compensation systems, but using advanced and upgradeable technologies, and provides 50–85 % heave motion and heave variance attenuation. Moreover, logging down/up at low speeds (300–600 m/h) reduces the system’s CE values by 15–20 %, and logging down at higher speeds (1,000–1,200 m/h) eliminates CE values by 55–65 %. Considering the high quality of the logging data collected, it is concluded that the new system can provide an improved level of compensation over previous systems. Also, if practically feasible, future integration of downhole cable dynamics as an input feedback into the current system could further improve its compensation efficiency during logging operations.

The internal structure of an oceanic core complex: An integrated analysis of oriented borehole imagery from IODP Hole U1309D (Atlantis Massif)

Oceanic core complexes at slow‐spreading ridges represent the uplifted footwalls of large‐offset ‘detachment’ faults that initiate at steep dips, and rotate to flatten via a ‘rolling hinge’ mechanism in response to flexural unloading. A key question is whether oceanic core complex development is accommodated entirely by displacement on the detachment fault zone, or if significant internal deformation of the footwall occurs during flexure and rotation. We investigate this by constraining the internal architecture of the Atlantis Massif oceanic core complex (Mid‐Atlantic Ridge, 30°N) using Formation MicroScanner borehole wall images and cores from the 1416 m‐deep Integrated Ocean Drilling Program Hole U1309D. Two distinct sets of structures are observed. N‐S‐striking, E‐dipping structures dominating the upper 385 m are interpreted as a brittle to semi‐brittle zone of fracturing in the footwall. Structures with this geometry occur down to 750 m below seafloor, suggesting that the detachment damage zone extends deep into the footwall. The nature of this deformation is, however, enigmatic: several cataclastic shear zones with reverse geometry in their current orientations may be rotated extensional faults or relate to shortening at the base of the flexing beam of a very weak footwall. By contrast, E‐W‐striking, N‐ and S‐dipping structures dominate the lowermost kilometer of the borehole. They likely represent conjugate fractures formed in the hanging wall of a late, E‐W normal fault zone, separating gabbroic rocks of the central dome of Atlantis Massif from serpentinized peridotite to the south, responsible for post‐detachment uplift of the southern margin of the massif.

Comparison of electrical image log with core in a fractured carbonate reservoir

Borehole image logs are kind of the modern logs which are frequently used to detect small-scale fractures intersecting well in fractured reservoirs. Electrical imaging tools like FMS (Formation MicroScanner) and FMI (Fullbore Formation MicroImager), electrically scan the borehole wall and deliver a pseudo-picture of the formation. Characteristics of fractures such as type, aperture and orientation can be determined by interpreting these logs.In this paper, we attempt to evaluate the ability of electrical image logs in detecting fractures and other geological features of a carbonate reservoir (Asmari) and explain the possible reasons of mismatch between the image log and core. To achieve this goal, we have compared the FMS of a well with the core. The results are verified with FMI and core data from seven nearby wells.Comparison of the FMS with the core revealed that FMS, as representative of electrical image logs, is robust in detecting open fracture and anhydrite; it is average in detecting bedding plane and flexural slip and is weak in detecting filled fracture, fault and compactional feature in carbonate reservoirs.

FMS Processing of IODP Expedition 324 and Its Application to Restoration of Structural Elements at Shatsky Rise, NW Pacific

Formation Microscanner Service ( FMS) is an advanced logging technology. Because of its advantages of high resolution and intuition,FMS measurement has been used in the recovery of original dip direction of structural elements,and that of sedimentary beddings. The Shatsky Rise is a key study area during the IODP Expedition 324. After FMS processing for Holes U1347A,Hole U1348A and Hole U1349A,interpretations of geological structures and their dip directions are discussed in this paper. We conclude that the formation of the TAMU Massif is consistent with the seafloor spreading theory while the formation of the Ori Massif has no obvious preferred stress field,being deduced to be related to a mantle plume head.

Lava deposition history in ODP Hole 1256D: Insights from log‐based volcanostratigraphy

A log‐based volcanic stratigraphy of Ocean Drilling Program Hole 1256D provides a vertical cross‐section view of in situ upper crust formed at the East Pacific Rise (EPR) with unprecedented resolution. This stratigraphy model comprises ten electrofacies, principally identified from formation microscanner images. In this study, we build a lava flow stratigraphy model for the extrusive section in Hole 1256D by correlating these electrofacies with observations of flow types from the modern EPR, such as sheet flows and breccias, and pillow lavas and their distribution. The resulting flow stratigraphy model for the Hole 1256D extrusive section represents the first realization of detailed in situ EPR upper oceanic crust construction processes that have been detected only indirectly from remote geophysical data. We correlated the flow stratigraphy model with surface geology observed from the southern EPR (14°S) by Shinkai 6500 dives in order to obtain the relationship between lava flow types and ridge axis‐ridge slope morphology. This dive information was also used to give a spatial‐time reference frame for modeling lava deposition history in Hole 1256D. In reconstructing the lava deposition history, we interpreted that the origins of the ∼100 m thick intervals with abundant pillow lavas in Hole 1256D are within the axial slope where pillow lavas were observed during the Shinkai 6500 dives and previous EPR surveys. This correlation could constrain the lava deposition history in Hole 1256D crust. Using the lateral scale of ridge axis–ridge slope topography from the Shinkai 6500 observations and assuming the paleospreading rate was constant, 50% of the extrusive rocks in Hole 1256D crust were formed within ∼2000 m of the ridge axis, whereas nearly all of the remaining extrusive section was formed within ∼3000 m of the ridge axis. These results are consistent with the upper crustal construction model previously suggested by seismic studies.

Astrochronology of the late Jurassic Kimmeridge Clay (Dorset, England) and implications for Earth system processes

The Late Jurassic Kimmeridge Clay Formation (KCF) is an economically important, organic-rich source rock of Kimmeridgian–Early Tithonian age. The main rock types of the KCF in Dorset, UK, include grey to black laminated shale, marl, coccolithic limestone, and dolostone, which occur with an obvious cyclicity at astronomical timescales. In this study, we examine two high-resolution borehole records (Swanworth Quarry 1 and Metherhills 1) obtained as part of a Rapid Global Geological Events (RGGE) sediment drilling project. Datasets examined were total organic carbon (TOC), and borehole wall microconductivity by Formation Microscanner (FMS). Our intent is to assess the rhythmicity of the KCF with respect to the astronomical timescale, and to discuss the results with respect to other key Late Jurassic geological processes. Power spectra of the untuned data reveal a hierarchy of cycles throughout the KCF with ∼ 167 m, ∼ 40 m, 9.1 m, 3.8 m and 1.6 m wavelengths. Tuning the ∼ 40 m cycles to the 405-kyr eccentricity cycle shows the presence of all the astronomical parameters: eccentricity, obliquity, and precession index. In particular, ∼ 100-kyr and 405-kyr eccentricity cycles are strongly expressed in both records. The 405-kyr eccentricity cycle corresponds to relative sea-level changes inferred from sequence stratigraphy. Intervals with elevated TOC are associated with strong obliquity forcing. The 405-kyr-tuned duration of the lower KCF (Kimmeridgian Stage) is 3.47 Myr, and the upper KCF (early part of the Tithonian Stage, elegans to fittoni ammonite zones) is 3.32 Myr. Two other chronologies test the consistency of this age model by tuning ∼ 8–10 m cycles to 100-kyr (short eccentricity), and ∼ 3–5 m cycles to 36-kyr (Jurassic obliquity). The ‘obliquity-tuned’ chronology resolves an accumulation history for the KCF with a variation that strongly resembles that of Earth's orbital eccentricity predicted for 147.2 Ma to 153.8 Ma. There is evidence for significant non-deposition (up to 1 million years) in the lowermost KCF ( baylei– mutabilis zones), which would indicate a Kimmeridgian/Oxfordian boundary age of 154.8 Ma. This absolute calibration allows assignment of precise numerical ages to zonal boundaries, sequence surfaces, and polarity chrons of the lower M-sequence.

Determination of the volcanostratigraphy of oceanic crust formed at superfast spreading ridge: Electrofacies analyses of ODP/IODP Hole 1256D

The objective of this study is to construct a representative volcanostratigraphy of Ocean Drilling Program Hole 1256D, the first complete penetration of intact upper oceanic crust formed at a superfast spreading rate. An accurate knowledge of the volcanostratigraphy is vital to understand processes of crustal construction and submarine magmatism and to estimate chemical exchange with seawater, but this is rarely achieved due to very low recovery rates in most basement holes. We used two approaches to determine the rock types that form the wall rocks in the basement sections of Hole 1256D: (1) user guided interpretations of electrofacies acquired by imaging tools combined with other wireline tools; and (2) the use of an artificial neutral network to objectively classify the responses of all available logging information. Great availability of formation microscanner (FMS) images provided superior coverage of the borehole wall compared to previous attempts at core‐log integration. This has resulted in more confident and detailed lithologic classifications, such as with the distinction between pillows and different styles of breciation. Ten lithology types are suggested for a volcanostratigraphy model: massive flows, ponded lava, fractured massive flows, fragmented flows, thin flows or thick pillows, pillows, breccias, dikes in dike complex, isolated dikes, and gabbros. Three major lithology types in the extrusive section are massive flows (both massive and fragmented massive flow, 22%), fragmented flows (32%), and breccias (19%). Pillow lavas make up only 1.9% of the volcanic section and are confined to a 100 m interval. Below the extrusive section, subvertical contacts interpreted to be dike margins are typically observed every 1 to 2 m with brecciated zones along the contacts. The dikes dip steeply to the northeast indicating slight rotation away from the ridge axis. We used an artificial neural network (ANN) approach to determine a quantitative lithostratigraphy. The ANN is most strongly influenced by porosity and alteration degrees and the resulting stratigraphy most closely resembles the above classifications when clustered by FMS texture as opposed to lithologic interpretation. The ANN thus provides a porosity‐based stratigraphy of the basement rather than the traditional lithology‐based stratigraphy.

Facies reconstruction from resistivity-at-the-bit images recorded in a submarine felsic volcanic succession

For the first time in history of the Ocean Drilling Program (ODP), Leg 193 drilled into an active hydrothermal vent field hosted in felsic submarine volcanic rocks using Logging-While-Drilling (LWD) techniques and specifically, the Resistivity-at-the-Bit (RAB) tool. In this manuscript, we present the results obtained from penetrating into the uppermost portion of hydrothermally altered dacite volcanic rocks hosting the subsurface of the PACMANUS active black smoker field along the Pual Ridge (Papua New Guinea). ODP Hole 1189C was drilled to a total depth of 166 m below seafloor (mbsf) and continuous electrical images of the borehole wall provide a full 360° coverage of the surrounding formation. For comparison, higher resolution borehole images were also recorded over an interval of approximately 50 m using a wireline Formation Micro Scanner (FMS). These data, together with geological observations obtained from drill cores provide the background for the interpretation of the electrical formation properties. The interpretation of the RAB images shows that coherent dacite facies and two types of breccias with distinctive clast size populations can be distinguished. These have been classified as breccia type I with predominance of coarse clasts containing maximum diameters of 95 cm and breccia type II with predominace of fine clasts containing maximum diameters of 55 cm. Coherent and breccia facies are intercalated in variable proportions with coherent facies becoming gradually dominant in the lower sections of the hole. The RAB data include the upper 10–30 m below surface where coring and traditional wireline logging data were not obtained. The interpretation of the RAB data confirms the principle observation from drill cores that the Roman Ruins site is located in a proximal to medial position relative to the eruptive vent and show that strongly altered fractured rocks extend to the seafloor enabling mixing of hot hydrothermal fluids and ocean bottom water. The continuous coverage of the RAB data is the only means of interpreting sub-bottom variations in the volcanic stratigraphy and the proportions of different volcanic facies.

Anomalous reflection amplitudes from fractured reservoirs—Failure of AVOA?

A fractured reservoir embedded in an isotropic background is equivalent to an anisotropic medium whose symmetry depends on fracture orientation. For this configuration, a variation of PP-reflection amplitude as a function of azimuth, φ at a given incidence angle is often observed. Here we report on an observation of unusual amplitude behavior for reflections from the top Cadotte and Falher formations in the Lynx Field, Western Canadian Sedimentary Basin, Alberta, Canada. Formation microscanner (FMS) images suggest considerable fracturing in the shallower Cadotte interval and resistivity measurements show fracturing in the Falher zone. Interestingly, reflection amplitudes from the Cadotte do not show the expected azimuthal variation consistent with a vertically fractured medium. Reflections from the deeper Falher interval, however, do exhibit large, unexpected azimuthal variation.

The role of intermediate-depth currents in continental shelf-slope accretion: Canterbury Drifts, SW Pacific Ocean

Abstract The Late Oligocene to Recent Canterbury Drifts were deposited in water depths between c . 400 and c . 1500 m by northward-flowing, cold, intermediate-depth water masses: Sub-antarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW) and their predecessor current flows. Drift accumulation started at c . 24 Ma, fed by terrigenous sediment derived from the newly rising Alpine Fault plate boundary in the west, which has built a progradational shelf-slope sediment prism up to 130 km wide at rates of eastward advance of up to 5.4 km Ma -1 . Gentle uplift associated with the nearby plate boundary has exposed older Late Oligocene and Miocene drifts onland (Bluecliffs Formation). Ocean Drilling Program Site 1119, located 100 km offshore at a water depth of 394 m, penetrated a 428 m thickness of mid-Pliocene to Pleistocene (0–3.9 Ma) drift located just seaward of the eastern South Island shelf edge. Uniquely, these large (>60 000 km 3 ), regionally extensive, intermediate-depth sediment drifts can be examined in outcrop, in marine drill-core and at the modern sea bed. The drifts comprise planar-bedded units up to several metres thick. Some sand intervals have sharp, erosive bases and normally graded tops into overlying siltstone; others are symmetrically graded with reverse-graded bases and normally graded tops. Bioturbation is moderate and rarely destroys the pervasive background, centimetre-scale, planar or wispy alternation of muddy and sandy silts displayed by Formation Micro-Scanner imagery. These features are consistent with deposition from rhythmically fluctuating bottom currents. Texturally, the drifts are poly modal quartzofeldspathic silty sands, sandy silts, silts and silty clays, with varying admixtures of benthic and biopelagic carbonate and silica. Miocene samples are mostly dominated by coarse silt (45–60 µm) and very fine sand (70–105 µm) grain-size modes, whereas strong fine silt (11–13 µm) and very fine silt-clay (<5 µm) modes become dominant after c . 3.1 Ma in the Late Pliocene, consistent with an increasing input of glacially ground material. Over the Plio-Pleistocene part of the succession, the sand-silt lithological rhythmicity occurs in synchroneity with Milankovitch-scale climate cycling, with periods of inferred faster current flow (sand intervals) mostly corresponding to warm, interglacial periods. Northward drift dispersal has helped cause the seaward growth of the eastern South Island shelf-slope system since the Late Oligocene probably by clinoform progradation and by episodic welding of mounded slope drifts onto the pre-existing sediment prism. Such along-slope, contourite drift accumulation occurs even in the absence of mounded drifts on seismic profiles, and represents a previously underemphasized mechanism for the progradation of shelf-slope clinoforms, worldwide. The Canterbury Drifts vary in thickness from c . 300 m near the early Miocene shoreline, where they were accumulating in limited shallow-water accommodation, to c . 2000 m under the modern shelf edge. Mounded drifts first occur in the Middle Miocene, at c . 15 Ma, their appearance perhaps reflecting more vigorous intermediate water flow consequent upon the worldwide climatic deterioration between 15 and 13 Ma. At Site 1119, a further change from large (>10 km wide) to smaller (1–3 km wide) mounded slope drifts occurs at c . 3.1 Ma, marking further cooling and perhaps the inception of discrete SAMW flows and initiation of the Subantarctic Front.

Downhole magnetic measurements of ODP Hole 801C: Implications for Pacific oceanic crust and magnetic field behavior in the Middle Jurassic

Downhole horizontal and vertical magnetic field measurements within the 474 m thick Jurassic crustal section drilled at ODP Hole 801C in the western Pacific show anomalies that are “in phase,” indicating that this site formed in the southern hemisphere and moved across the paleoequator to its present location at 18.6°N. The inclination computed from the horizontal and vertical anomaly logging data varies significantly downhole and can be explained by progressive rotation of the lava sequence as it is buried during accretion. This is compatible with the observed dips in the Formation Micro Scanner (FMS) data, which show up to 42° of rotation toward the fossil spreading axis. We restore the magnetic inclination to prerotated values and obtain an estimated pretilt inclination of 39.9° ± 6.6°, corresponding to a paleolatitude of 22.7° ± 5°S. The magnetic logging data document six polarity units within the drilled volcanic extrusive crust. The upper 132 m thick logged section of lavas contains four polarity units, which apparently formed 7 Myr after the formation of the underlying basement. The lower 212 m logged section of the hole is reversely magnetized with an intervening normal polarity zone. More than one reversal downhole would indicate a rapid reversal rate given the time it takes to form the extrusive section of oceanic crust: ∼45,000 years at a fast spreading rate of 66 km/Myr. Previous results from a contemporaneous section in Spain suggest rapid reversals throughout the late Bajocian–early Bathonian Stage (170–165 Ma). Nevertheless, the complicated formation history prevents quantification of the geomagnetic reversal rate for the 801C crustal section.

Integration of electrical and optical images for structural analysis: a case study from ODP Hole 1105A

Abstract A significant problem with basement core data acquired by the Ocean Drilling Program (ODP) is that it is unorientated with respect to grid north and in those cores with < 100% core recovery the data may also be inaccurately located within the borehole. In order for structural data from basement cores to be of any real value, the recovered core must firstly be accurately located and orientated. Here, we develop methods for core reorientation and accurate location within a borehole that integrates wireline borehole electrical images with whole-core digital images. Azimuthal orientation and location of individual core pieces allows for the detailed interpretation of structural data, including fabrics and fractures and reorientation of other spatially anisotropic properties of core, such as palaeomagnetism. ODP Hole 1105A is one of a pair of holes drilled on the Atlantis Bank in the SW Indian Ocean, which penetrate sections of the lower ocean crust. Hole 1105A was drilled to a depth of 158 metres below sea-floor (mbsf), with core recovery of 83% and the vast majority of the recovered material being represented by gabbroic rocks. Zones of crystal-plastic deformation occur throughout the core, but overall their frequency increases downhole. In the deformed zones, the gabbros are transformed into equigranular or porphyroclastic gneissose textured rocks, characterized by very prominent planar fabrics. These localized deformation zones relate to the early exhumation history of the lower crust in an environment similar to core-complex formation in continental regions. Using a combination of digital core and formation microscanner (FMS) images, it is now possible to fully orientate structural features in a borehole and so map the oxide gabbro and crystal-plastic deformation zones in Hole 1105A. From the analysis of the crystal-plastic fabrics, the majority appear to dip southwards, with a few ( c .5%) dipping to the north. This would suggest that the major exhumation surfaces in this hole are represented by southward-dipping structures, formed at high temperatures during unroofing of the Atlantis Bank.

Rheological implications of the strong seismic reflector in the Kakkonda geothermal field, Japan

We re-processed the seismic reflection survey data of the Kakkonda geothermal field. The pre-stack migration delineates a strong and continuous reflector between 1800- and 2800-m depth, below which formations are not reflective. Earthquake data exhibit seismicity in the upper crust. The lower boundary of seismogenic layer is interpreted as the brittle–ductile transition. The thermal structure is thought to be the major factor controlling its depth. We compared the strong reflector with the thermal and rheological structure from drillholes. The depth of the reflector corresponds to the top of the highly–very highly fractured zone observed from formation microscanner imagery (FMI) logging in the Miocene formations. The density of fracture in the Kakkonda granite is very low, suggesting that granite corresponds to the nonreflective zone. The temperature–depth profile of well WD-1a shows that the temperature at the highly–very highly fractured zone is about 350 °C. This corresponds to a hydrothermal convection zone filled with two-phase geothermal fluid. The cut-off depth of seismicity that indicates the brittle–ductile transition lies at the isotherm of 300–350 °C near the reflector. We conclude that the strong seismic reflector is a strong contrast in acoustic impedance at the top of the fractured layer. The fractured layer could be a decoupling plane caused by different tectonic behaviors between the upper brittle and the lower ductile layers or a dehydration front by thermal diffusion. The similarity between the strong reflector and K-horizon, the strong reflector, found in southern Tuscany, Italy suggests that the P-wave reflector at the top of highly fractured zone at the brittle–ductile transition be common in areas with magmatic activity.

Implications for crustal accretion at fast spreading ridges from observations in Jurassic oceanic crust in the western Pacific

Downhole logging data and basement stratigraphy interpretations are used to determine the spreading environment and crustal accretion history of the ocean basement cored at ODP Hole 801C located in the Jurassic Magnetic Quiet Zone of the western Pacific. High‐resolution microresistivity data obtained with the Formation MicroScanner are used to measure the dip of the extrusive layers and indicate a 10°–30° increase in dip down the drill hole with lava flow contacts dipping back toward the original ridge axis. This structural pattern and the high proportion of massive flows relative to pillow units are consistent with prevailing crustal accretion models proposed for faster spreading ridges (e.g., >60 km/m.y., full‐rate). A detailed analysis of the age data, basement lithology, related geochemistry, and structural attitudes suggest the shallowest 100 m of the drilled section (e.g., Sequences I–III) were emplaced just off the ridge (Sequence III) or significantly farther off‐axis up to 5–15 m.y. later (Sequences I and II). The remainder of the drilled section (e.g., Sequences IV–VIII) has geochemical, lithological and physical trends that are assumed to be representative of crust created at faster spreading ridges. The pattern of dipping lava flow contacts from this deeper section of the drill hole suggests lava flows emanating from the ridge axis are limited to 1–2 km off‐axis. Our results suggest ocean crust drilled at Hole 801C was created at faster spreading rates; however, caution should be used when incorporating Sequences I–III into geochemical reference sections for faster spreading ridges.