In this chapter, we have assembled information that will help the reader understand the basis for our preliminary conclusions and help the interested investigator select samples for further analysis. This information concerns only shipboard operations and analyses described in the site reports in the Initial Reports volume of the Leg 176 Proceedings of the Ocean Drilling Program. Methods used by various investigators for shore-based analyses of Leg 176 data will be described in the individual scientific contributions published in the Scientific Results volume and in publications in various professional journals.
The separate sections of the site chapter were written by the following shipboard scientists (authors are listed in alphabetical order, no seniority is implied):
Leg Summary: Dick, Natland
Drilling sites are numbered consecutively from the first site drilled by the Glomar Challenger in 1968. A site refers to one or more holes drilled while the ship was positioned over a single acoustic beacon. Multiple holes are often drilled at a single site by pulling the drill pipe above the seafloor (out of the hole), offsetting the ship some distance from the previous hole (without deploying a new acoustic beacon), and drilling another hole.
For all Ocean Drilling Program (ODP) drill sites, a letter suffix distinguishes each hole drilled at a single site. The first hole at a given site is assigned the suffix A, the second hole is designated with the same site number and assigned suffix B, and so on. Note that this procedure differs slightly from that used by the Deep Sea Drilling Project (DSDP; Sites 1-624) but prevents ambiguity between site- and hole-number designations. These suffixes are assigned regardless of recovery, as long as penetration takes place. Distinguishing among holes drilled at a site is important, because recovered rocks from different holes, particularly when recovery is less than 100%, are likely to represent different intervals in the cored section.
The cored interval is measured in meters below seafloor (mbsf); sub-bottom depths assigned to individual cores are determined by subtracting the drill-pipe measurement (DPM) water depth (the length of pipe from the rig floor to the seafloor) from the total DPM (from the rig floor to the bottom of the hole; see Fig. F1). Water depths below sea level are determined by subtracting the height of the rig floor above sea level from the DPM water depth. The depth interval assigned to an individual core begins with the depth below the seafloor at which the coring operation began and extends to the depth that the coring operation ended for that core (see Fig. F1). Each coring interval is equal to the length of the joint of drill pipe added for that interval (~9.4 to 10.0 m). The pipe is measured as it is added to the drill string, and the cored interval is usually recorded as the length of the pipe joint to the nearest 0.1 m. However, coring intervals may be shorter and may not be adjacent if separated by intervals drilled but not cored or washed intervals.
Cores taken from a hole are numbered serially from the top of the hole downward. Core numbers and their associated cored intervals (in mbsf) are usually unique in a given hole; however, this may not be true if an interval must be cored twice, because of caving of cuttings or other hole problems. The maximum full recovery for a single hard-rock core is nominally 9.5 m of rock contained in a core barrel (5.6-5.8 cm diameter; Fig. F2). Recovery >100% may occur, however, where core drilled was not fully recovered from the previous interval. Only rotary coring barrel bits were used during Leg 176.
Because of core-jamming in liners, nearly all cores recovered during Leg 176 were collected without core liners. The pieces of the core were pulled from the bottom of the core barrel. The bottoms of oriented pieces (i.e., pieces that clearly could not have rotated about a horizontal axis in the core barrel) were marked with a red wax pencil to preserve orientation during the splitting and labeling process. The pieces were transferred to a 1.5-m-long split core liner for handling. Contiguous fragments with obvious features allowing realignment were considered to be a single piece. Plastic spacers were used to separate the pieces. Each piece was numbered sequentially from the top of each section, beginning with number 1; reconstructed groups of pieces were lettered consecutively (e.g., 1A, 1B, 1C, etc.; see Fig. F3). Pieces were labeled on external surfaces, and, if oriented, a this-way-up arrow was added to the label.
Recovery rates were calculated based on the total length of a core recovered divided by the length of the cored interval (see Fig. F1). As most hard-rock coring operations are characterized by <100% recovery, the spacers between pieces can represent intervals of no recovery up to the difference in length between a cored interval and the total core recovered. Of the more than 120 cores recovered during Leg 176, 18 had a curated length in excess of the cored interval. While this is not unique when sampling sediments (gas expansion in the core liners causes anomalously high curated core lengths), it is problematic when dealing with curation of hard-rock cores (which do not expand). In 15 of the 18 circumstances, the error is <5% and is almost certainly caused by misestimation of the depth of penetration at the rig floor. During most of the Leg 176 operations, rig floor heave was >2 m; error of 20-30 cm in depth of penetration yields 2-3% error in recovery calculations. In all other cases, the incidence of high recovery was immediately preceded by a lower recovery core. Amount of unrecovered core in the previous cored interval was invariably greater than the amount of material oversampled and was likely caused by a piston of rock that was left behind as a core barrel was retrieved, only to be picked up on the subsequent coring run. Since the database was not designed to process cores starting at a level above the depth of penetration of the previous core, we could not make this correction to archived data. The interested reader should be aware of these anomalies, however, and account for these discrepancies when using curated length or recovery data. All cores recovered during Leg 176 were designated "R" (rotary drilled) for curatorial purposes. For detailed descriptions of each core sampled, including photographs of each core, and for thin-section descriptions, see the "Core Descriptions" contents list. Also included on this CD-ROM are logs that list details of Leg 176 photomicrographs (see "Photomicrographs" contents list).
As an aid to the interested investigator, we have compiled summary information of core descriptions on a section-by-section basis and presented these on hard-rock visual core description (HRVCD) forms (see the "Core Descriptions" contents list). These forms summarize the igneous, metamorphic, and structural character of the core, and present graphical representations of the pieces recovered and the lithologic units identified (see Fig. F4). The far right side of these forms presents an image of the archive half of the core captured shortly after splitting. To the right side of the image, several columns contain information about the core. In left-to-right sequence, these columns include archived piece numbers and a graphic representation of piece shape with additional details (veins, fractures, etc.) added to help distinguish features in the image. Next to these is a column indicating pieces that could be oriented relative to the core top and a vein number column for correlation with a vein log (see "Metamorphic Petrology"). The next column indicates the location of shipboard samples. For reference, the samples noted conform to the sampling code in the JANUS database (XRF = X-ray fluorescence analysis; TSB = polished thin-section billet; PMAG = paleomagnetic samples, most of which were passed to the physical properties laboratory; and XRD = X-ray diffraction analysis). A graphic lithology column illustrates changes in lithologic units (see "Igneous Petrology"); lithologies recovered are represented by the patterns illustrated in Figure F5. A unit number corresponds to each lithologic unit and is recorded in the next column. Metamorphic intensity intervals are represented by uppercase letters (see "Metamorphic Petrology"). We also present a graphic representation of structural features in the core and a column noting particular structural features (see "Structural Geology"). On the right side of these forms is a text summary of observations from each section. The upper and lower contacts of each lithologic interval are noted, as are primary lithology and other comments summarized from igneous descriptions. Text summaries of metamorphic and structural description for each section are also compiled on these forms.
1Examples
of how to reference the whole or part of this volume can be found
under "Citations" in the preliminary pages of the
volume.
2Shipboard
Scientific Party addresses can be found under "Leg 176
Participants" in the preliminary pages of the volume.
Ms 176IR-102