Downhole Logging Summary
During Leg 207, five sites were cored (four logged) along the northern margin of Demerara Rise (Figure 1). The sites are located in a depth transect (present water depths are 1900-3192 m) along a grid of high-resolution multichannel seismic reflection lines, supplemented by existing industry lines (Figure 1).
Figure 1. a) sites cored and logged (red) and cored only (white), bathymetry and pre-cruise and cruise-acquired seismic lines; b) Location map showing the operational area on Demerara Rise.
The transect of Cretaceous and Paleogene cores will be used to evaluate the following:
Further details regarding the scientific rationale, objectives, geologic setting etc. can be found in the Leg 207 Scientific Prospectus and the Leg 207 Preliminary Report.
Four of the five sites cored on Leg 207 were logged (Figure 1). Logging operations utilised the standard ODP toolstrings: the Triple Combo (TC) with the addition of the TAP and MGT tools, the FMS-sonic and the WST:
The TC was run first in all of the holes and subsequently powered down for acquisition of MGT data. This toolstring was followed downhole by the FMS-sonic running the LSS in place of the DSI (dipole sonic imager). Reduced telemetry requirements of the LSS provided the opportunity to run the toolstring at the maximum logging speed of 1800 ft/hr, double the normal speed when running the DSI. Comparative passes run at 900 ft/hr (pass 1) and 1800 ft/hr (pass 2) were undertaken in Holes 1257A and 1258C. The results showed a marginal improvement in data quality running at the higher logging speed. All passes in the remaining holes were completed at 1800 ft/hr. Table 1 provides a brief summary of the logging operations. Heave conditions during all of the logging operations were <2 m i.e., within range of the WHC (wireline heave compensator), except during the first two passes of the TC at Hole 1261B when heave conditions (~3 m) caused the WHC to stroke out on a few occasions.
Heave motion of the ship and borehole diameter are the two major factors influencing the quality of wireline data in ODP operations. The WHC was fully operational on all passes, except the first two in Hole 1261B. Borehole diameters in all holes were good to excellent (just beyond bit size in many instances) except in Hole 1261B from the bottom of the pipe down to 274 mbsf. Comparison of data from successive passes shows excellent repeatability, reflecting good heave compensation and borehole conditions. In softer sediments, found higher up in the formation at a number of the sites, minor borehole enlargements are found in association with core barrel recovery and reload, due to incomplete heave compensation of the drill string. Time-series analysis on data through such sections must correct for these borehole diameter induced artefacts. All passes in one hole are depth shifted to sea floor using the gamma ray signal associated with the seafloor and depth matched to the master log (usually the HNGS run on the TC) using gamma ray logs (MGT and SGT) that are recorded on all passes, except the WST.
Pattern matching of log data with core physical property data provides a rapid, visual, method of quality control. In all of the holes where the log data areconsidered to be good there is excellent correlation (when account is taken of depth miss-matches) between the core and log data.
The four logged sites provide a transect from contrasting, present-day, water depths and geographic locations over the Rise (Figure 1). The formation physical properties (density, porosity and velocity) from the logged sites are shown in Figure 3. Generally the sediments follow a normal consolidated line (which has not been overprinted by diagenesis) from the surface to the depth of the K/T boundary. Significant perturbations in these profiles are found coincident with the P/E and K/T boundaries, as well as various hiatuses. At Sites 1257 (from physical property data), 1258 and 1260 acoustic velocity increases linearly with depth until the P/E boundary (Figure 3). In the interval between the P/E and K/T boundaries, velocity fluctuates about a fixed baseline, varying with cyclical changes in sediment composition and degree of lithification. Across the K/T and into the Cretaceous, highly lithified Maastrichtian sediments gradually give way to lower velocity, higher porosity, calcareous claystones with glauconite-rich horizons at their base. Transition into the black shales (Santonian through Cenomanian in age) sees a decrease in the baseline density and velocity with a co-varying increase in porosity. These are however punctuated by cyclic, high amplitude fluctuations (Figure 3). Beneath the black shales are found the Albian syn-rift sediments composed variously of silty claystones, through, silty limestones to quartz sandstones.
One of the main objectives of the leg was the recovery of continuous expanded black shale sequences and in total ~650 m of black shale was recovered. OAEs result from major shifts in ocean circulation patterns and are hypothesized to have played a major role in the evolution of Earth's climatic and biotic history. On Demerara Rise the sequence of black shale sediments has a cyclical overprint of organic matter-rich black shale alternating with laminated foraminiferal packstone and occasional glauconitic bioturbated intervals. These alternations reflect varying levels of bottom water dysoxia and surface water productivity and may show Milankovitch forcing periodicities. As with any paleoceanographic study the collection of continuous sequences is of prime importance. Downhole logging has provided excellent data through the black shale sequences at all of the logged sites (Figure 4a and 4b). High gamma ray and porosity with concomitant low resistivity, density and velocity characterize the black shales. Data from spectral tools (HNGS and MGT) reveals the gamma ray source to be potassium (clay) and uranium (organic matter). As noted above there is a cyclic depositional sequence through the black shale, seen as large amplitude fluctuations in all of the logs. The packstones and glauconitic horizons appear as peaks in density, resistivity and velocity, and troughs in gamma ray and porosity logs and are readily visible in FMS resistivity images (Figures 4a and 4b). The PEF log indicates that they are calcite cemented (PEF calcite 5.08).
Figure 4. b) Geophysical logs and FMS imagery through the black shale interval in Hole 1260B. Note expression of the cyclic deposition, and shorter period cycles in the porosity and MGT logs.
Pore waters at this site are characterized by the presence of a brine with maximum chlorinity of 823 mM (~50% > seawater average). The maximum chlorinity is centered ~200 mbsf in the black shales, decreasing above and below the unit. The borehole temperature profile recorded on the first logging-down pass of the TAP tool (it is located on the bottom of the TC tool string) is shown in Figure 5. Pipe effects are clearly obvious down to 90 mbsf. From 90 mbsf to 128 mbsf there is a steep temperature gradient, indicating mixing between the cold pipe and warmer borehole fluids. Data below 138 mbsf is thought to be more representative of the true formation temperature. A clear perturbation (inflow of cooler fluid) in the temperature profile occurs between 162 mbsf and 212 mbsf. The temperature perturbation, high-porosity of some intervals within the shale and the porewater geochemistry suggest that the brines are sourced externally inflowing through the black shales.
Figure 5. Borehole temperature recorded by the TAP tool on the first logging-down pass of the TC. Note the location of the temperature perturbation.
As noted above, gamma ray levels through the shales are characteristically high and correlate exceptionally well with the core MST profiles. All the sites show a distinct two level division within the sequence (high Vs low counts), low shifting to high downhole in the west (1258 and 1260, Figure 4b) and the reverse in the east (1261 and 1257, Figure 4a). Spectral information from the HNGS and MGT suggest this is mainly a function of uranium content. It is at this stage unknown if this reflects primary depositional differences, or, in light of the fluid flow suggested above, post depositional leaching and/or deposition.
TOC (total organic carbon) can be estimated from the log data providing a continuous profile of organic carbon deposition. The result is only approximate because the shale porosity is assumed to equate to that of the sediments above and values for some densities (e.g., organic matter) that are not well constrained, are also assumed (Rider, 1996). The results are plotted along with values measured from core samples in Figure 6. Despite the facts that core and log data have not been depth matched, the log data are from Hole 1257A only and the measured values are taken from Holes 1257A, 1257B and 1257C, the results are very satisfactory.
A checkshot survey was undertaken at three sites (1257, 1260 and 1261) in order to best integrate the core, log and seismic data. The checkshot survey in Hole 1257A collected 14 stations at ~30 m intervals, stacking multiple signals at each station. These were used to calibrate the velocity logs. Figure 7 shows the impedance profile, calculated from core (0-74 mbsf) and log (74-288 mbsf) density x velocity, the synthetic seismogram and an interpreted section of seismic line GeoB220, for Site 1257. This allowed the regional B, B', and C reflectors to be re-interpreted. The C reflector represents the base of the black shales, unconformably overlying the middle to late Albian syn-rift sediments. The B' reflector is the top of the black shales (173 mbsf) and the B reflector is the density and velocity step at 138 mbsf representing the middle Paleocene/early Maastrichtian unconformity.
Key Cretaceous/Paleogene events of biotic turnover and/or inferred climate extremes, particularly across the Cretaceous/Paleogene (K/T) and the Paleocene/Eocene (P/E) boundaries were recovered at a number of sites. An expanded and apparently complete section from the middle Eocene to below the P/E boundary was also recovered at Sites 1258 and 1260 (Figure 3). At both of these sites cyclicity is well developed in the high-resolution porosity (APS) and gamma ray (MGT) logs and will provide an excellent basis for postcruise cyclostratigraphic research (Figure 8).
Figure 8. Geophysical logs through Paleocene nannofossil chalk in Hole 1260B (a) and clayey nannofossil chalk in Hole 1258C (b). Note the cyclic fluctuations in the gamma ray (MGT) and porosity logs. FMS images also show well developed cycles exemplified in (b).
The K/T boundary was recovered at three sites of which two, 1258 and 1260, were logged. At Site 1260 perturbations are observed in the gamma ray, resistivity, porosity, density, velocity and PEF logs and also in the FMS images (Figure 9), coincident with the K/T boundary.
Four of the five sites on Leg 207 were logged, with a checkshot survey undertaken in three of these sites. High quality data was acquired in all of the logged holes due to a combination of good heave compensations and excellent borehole conditions. The logging data will be used for a range of research topics including, cyclostratigraphic analyses (e.g. Figures 8 and 9), core-log correlation for refining core composite depth splices, characterising complete black shale sequences (e.g. Figure 4a and 4b), and interpreting the complex seismic stratigraphy of Demerara Rise (e.g. Figure 7).
Brice Rea: Logging Staff Scientist, Department of Geology, University of Leicester, University Road, Leicester, UK