Leg 166 Logging Summary
Shipboard Scientific Party
In this document we present a summary of the wireline logging results during ODP Leg 166. Details of the overall results of the leg can be found in the Leg 166 preliminary report located on the ODP/TAMU web site.
Background & Objectives:
Specific objectives of the Bahamas Drilling Transect/ODP Leg 166 were to:
1) determine the timing of the sequence boundaries and relative sea-level fluctuations during the Neogene;
The transect of holes drilled along the western margin of the Great Bahama Bank during Leg 166, together with two holes drilled on the bank top, Unda and Clino, complete the Bahamas Drilling Transect (Figure 1) and Figure 2). Holes Unda and Clino continuously cored sediments dated from the Late Miocene to the present, and an extensive program of analyses has been completed, including bio-, magneto- and strontium-isotope stratigraphy, as well as of petrophysical analyses including wireline logs and core sample measurements. Results from the bank top drilling are reported in McNeill et al. (SEPM Contributions to Sedimentology volume edited by GN Ginsburg) and Eberli et al. (Proc. ODP, Initial Repts., volume 166).
Leg 166 was quite successful in achieving its objectives right at sea or obtaining sufficient material for shore-based analyses that will enable their achievement post-cruise. However, core recovery averaged 53%, and several intervals had little or no recovery (see tables below). Therefore a detailed characterization of the facies and dating of sequence boundaries (objectives 1-4) could be compromised. Fortunately a suite of wireline logs were obtained at all deep penetration sites of the transect that will enable a reconstruction of the facies and bedding patterns within the sequences. Thus, logging data was of critical importance for the success of the leg, in particular those relating to the characterization of sequences and their bounding unconformities. The major highlights are outlined below:
a) Determination of precise depth to seismic traveltime conversion using check shot surveys: Because of the rapid changes in velocity downhole due to complex diagenetic alteration of the carbonates, sonic logs alone did not result in accurate depth-time conversion. The accurate determination of the depth of sequence boundaries seen in seismic data was of crucial importance to relate the stratal characteristics of the various sequences to the facies seen in the cores and their respective ages.
b) Characterization of lithology from logs: Core recovery averaged 53%, but was often much lower at depth. FMS images, together with geophysical and geochemical logs integrated to core data will provide an important complement to characterize the facies of sequences and the nature of the sediments across bounding unconformities.
c) Site to site correlations: Log signatures of many sequences appear to have unique aspects that will help with precise correlation across the transect. Pronounced changes in log characteristics across many sequence boundaries also will help to narrow sampling for biostratigraphic purposes, thus helping with precise dating of these boundaries.
d) In-situ physical properties:
e) Depositional cycles: Depositional sequences and their bounding unconformities are clearly expressed in the logs across the Bahamas transect. In addition, the logs display the fine-scale detail of decimeter to meter scale variations in sediment type and diagenetic alteration that will be important in characterizing the stratigraphic response of the carbonate platform to sealevel and climate changes.
Tables 1 and 2 below contain a summary of all the downhole logs obtained during Leg 166, including tools logged in each hole, respective depth intervals, and totals of logs obtained at each site:
TABLE 1 - Leg 166 Logging Operations
TABLE 2 - Leg 166 summary statistics
* Recovery within open-hole logged interval
Summary plots of the geophysical and gamma-ray logs for each site, including the core-recovery columns, lithologic units, major age boundaries, as well as core physical properties can be viewed by clicking on the respective link below:
Some technical highlights
Hole 1005A was the first Hole in ODP to be logged with the Schlumberger Integrated Porosity-Lithology tool string (IPLT). This tool string included a spectral gamma-ray tool (HNGS), a porosity tool (APS) and a litho-density tool (HLDS):
Figure 3 is an example of logging data obtained from the IPLT at Site 1005. Neutron porosity data in ODP often is of poor quality. Besides the fact that neutron tool does not have good resolution at high porosity, the tool could not be placed against the borehole wall since the early eccentralizers had too large a diameter. A new "in-line" centralizer is a significant improvement, since it ensures that the tool remains against the borehole wall. The array of neutron sensors also enables an estimate of tool standoff, which provides for quality control on the neutron data, analogously to the density correction factor (neutron data is considered poor when standoff is > ~1 inch).
A comparison of the spectral gamma-ray logs obtained with the new HNGS tool and with the NGT is shown in Figure 4. In these carbonates, the gamma-ray signal resulted primarily from uranium, which in places reached up to 15 ppm. In the high uranium content intervals the tools compare well, although the HNGS log is clearly smoother because it has more counts and thus less affected by noise and statistical variation. The advantage of the tool is perhaps best displayed in sections where the total signal is low, due to only small amounts of clay (thorium and potassium signals) and uranium (probably associated to precipitation of uranium during diagenesis). In such cases, the high signal to noise ratio of the HNGS data allows for a much better resolution of the spectral components in the gamma-ray signal.
Leg 166 was first experimental deployment of a Ku-band communications sattellite, developed by OMNES (a division of Schlumberger).
Major advantages experienced during Leg 166:
Cyclicity in the sedimentary sucession:
Large scale changes in the physical properties of the GBB sediments, within units tens of meters in thickness, reflect major changes in the types of sediment and their diagenetic characteristics, which compose the major sedimentary packages or stratigraphic sequences bounded by unconformities (sequence boundaries). In particular pelagic oozes and chalks display a rather monotonous signature in the resisitivity logs, with a typical example being the Early Pliocene section recognizable at all sites (see summary Geophysical plots above). Changes in the degree of cementation are well expressed by increases in resistivity and velocity. The petrophysical properties reflect the alternations in the flux of neritic (aragonite rich) versus pelagic (low MG calcite rich) sediment. Gamma-ray logs appear to contain a more complex signature, including the effects of uranium precipitation as a result of diagenesis, as well as increases in the flux of clays. Hardgrounds and firmgrounds, probably formed during periods of low sealevel when sedimentation rates are reduced, have pronounced peaks in all petrophysical and gamma-ray logs. The influx of clay minerals, which probably come from Hispaniola and Cuba, are interpreted to increase during relatively low sealevel stands. The near surface sediments on the GBB margin (upper 20 meters) are composed primarily of aragonite, but both logging and core measurements show that they are very weakly radioactive. This suggests that the precipitation of uranium in these sediments occurs during diagenesis, although it is possibly controlled by the original composition of the sediment.
Packages containing calcareous turbidites and current-reworked deposits typically display high velocity and resistivity in the logs. The gamma-ray signature is variable and probably reflects the nature of the redeposited sediment. Asymmetric cycles with a "funel-shaped" resistivity signature are often present in association with intervals where cores suggest changes in grain size and sedimentary strucutures possibly caused by reworking by bottom currents (e.g., Site 1007 near 350 mbsf and Site 1003 near 400 mbsf).
Because the diagenetic potential of aragonite- and organic-rich platform derived sediment is high, the various forcing functions that affect the flux of platform derived sediments (such as sealevel and climate changes), are recorded in the petrophysical properties.
The relation between changes in the type of sediments accumulated along the margin and their diagenetic potential, leads to changes in the petrophysical properties which ultimately cause seismic reflections. Increase in the flux of metastable aragonite during relatively high sealevel changes, when the productivity in the bank increases, contrasts with the pelagic sedimentation which predominates during periods of low neritic input. Degradation of organic matter drives the diagenetic reactions and formation of cements and development of of porosity, which lead to pronounced log signatures. Because the sediment types change primarily as a result of sealevel changes, petrophysical properties and seismic reflections are accurately portraying the stratigraphic architecture of the margin and the changes in stratigraphy associated with sealevel changes. Changes in bottom-current intensity, perhaps associated with changes in the physiography of the Straits of Florida due to sealevel oscillations and carbonate platform growth, result in changes in the influx of siliciclastic sediments to the Bahamas region. The bulk of the sediments drilled were carbonates, but beds with secondary amounts of clay, likely brought by bottom currents from Cuba and Hispaniola during relatively low stands of sealevel, are clearly depicted in the logs.
Superimposed on the large scale changes associated with the pulses of platform progradation are decimeter to meter scale changes in physical properties that probably result from high-frequency sea-level changes. These are particularly striking in the Miocene intervals. Cores reveal the alternation between darker bio-wackestones, with flattened burrows, and rich in organic matter and clays; and lighter bio-wackestones, that are well cemented and display open burrows. The well cemented bio-wackestones result from higher influx of neritic sediment, while a higher proportion of material of pelagic/hemipelagic origin (including forams and nannos, and clays) occurs in the darker intervals. These decimeter to meter scale cycles are well expressed in the sonic, density, porosity, gamma-ray and resistivity logs, including FMS images. An example of the expression of these cycles in the logs can be seen in Figure 5.
Spectral analysis (with contributions from Nathan Clark)
Preliminary spectral analyses on the wireline logs, using the shipboard biostratigraphic age models, reveal the presence of both precession, obliquity and eccentricity cycles in the Miocene. Example spectra from different sequences at Site 1003 illustrate (Figure 6): a) the consistent occurrence of Milankovitch periods at several age intervals, suggesting that the preliminary age models are probably of high quality; b) the consistent appearance of peaks at 30 ka and 54 ka, with a strong dominance of the 30 ka period.
The availability of a well dated transect of closely spaced holes provides a unique opportunity to examine the nature of the stratigraphic response across the margin. For instance Sites 1005 and 1003 are close to the "carbonate factory" whereas Sites 1007 and 1006 are in a more distal position in the basin. The distal sites are presumably more affected by bottom currents, while still containing a record of the changes in the carbonate production on the bank. In contrast, the slope sites should be more influenced by changes in the flux of carbonates derived from the bank top. A comparative analysis of the spectrum for all sites within the same age interval should provide insights into the nature of the cyclicity along the margin, and whether these different processes are acting at similar frequencies. The analysis for a late Miocene interval (Figure 7) suggests that the spectrum is very similar at all sites, but while some of the peaks occur at the same frequency at all sites (e.g, 30 ka peak), others are shifted in frequency from site to site. This may be due to the fact that the log expression probably reflects the stratigraphic response of the platform to the various processes that build the margin. For instance, turbidites and current controlled bedding may be more prevalent in the lower portion of the platform, while the effects of changing the flux of platform derived material probably exert stronger influence in the stratigraphy of the upper part of the margin.
Integration with seismic reflection data:
Check-shot surveys were performed at all deep sites of the transect. This proved to be a crucial step for the objectives of the leg because it enabled precise correlation of the seismic stratigraphic interpretation with the core and log data (Figure 8). The conversion from two-way traveltime to depth was of foremost importance to accurately place bio-stratigraphic datums with respect to the seismic boundaries, and therefore to achieve one of the key results of the leg, the demonstration that seismic sequence boundaries are time lines, and thus have chronostratigraphic significance.
Errors in time to depth conversion of up to 50 meters would have resulted if we had relied solely on the integration of the sonic logs (Figure 9). This is probably a result of the presence of large changes in sonic velocity at very small spatial scales, due to the common presence of cyclic alternations of well cemented and less cemented beds (e.g., Figure 8). In addition, it proved important to obtain VSPs at all margin sites because of the pronounced lateral variability in diagenetic characteristics of the bank margin, and thus of the velocity profiles. VSP acquisition was very time-effective, with an average of about 5 to 10 minutes per station, where we stacked 5-8 air-gun shots per station to obtain a travel time.
Leg 166 Logging Scientists:
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