Eight holes (1135A-1142A) were drilled on the Kerguelen Plateau and Broken Ridge during Leg 183. Hole 1135A did not reach basaltic basement, Hole 1140A encountered pillow basalts, and the lavas in Holes 1141A and 1142A were exceptionally altered. Thus, this chapter only considers Holes 1136A-1139A. Table T1 briefly summarizes the basement units encountered in those holes. In addition to mafic lava flows, sedimentary, volcaniclastic, and evolved lava flows were encountered. This chapter only examines the mafic (basaltic-trachybasaltic) lava flows.
The drill core recovered during Leg 183 is 6-7 cm in diameter and cut in half longitudinally on board the JOIDES Resolution. Both the exterior and the cut surfaces of the core were used for making the observations, but most quantitative measurements were made on the flat, sawed surfaces. Usually, the bit advanced 9.6 m between cores. Recovery was generally good (>50%) in the lavas but was highly dependent on the structure of the rock. Only in Hole 1137A were downhole logging measurements available to compare with recovered mafic subaerial lava flows, providing a direct measure of core recovery (Coffin, Frey, Wallace, et al., 1999). Massive portions of the flows often had 92%-100% recovery, but vesicular lavas had 27%-91% recovery. The worst recovery was in sections where the vesicle diameters approached the core diameter. Breccias had 62%-87% recovery but were sometimes significantly disturbed by the drilling and sawing processes.
The difficulty in recovering the vesicular and brecciated portions of the lava flows was exacerbated by alteration and weathering processes. The more permeable vesicular portions of the flows were often significantly more altered than the dense interiors. Breccias were even more intensely altered, and there was evidence for postemplacement mechanical weathering of some of the breccias. In the most severe examples, the alteration and weathering could completely mask the original shapes of the clasts. Both alteration and weathering preferentially attacked the most angular protrusions on the breccia clasts, making them seem rounder than they originally were. Interestingly, vesicle shapes were often still recognizable because of resistant secondary mineral fillings. There was also significant sediment fill within a number of the breccias (e.g., Fig. F2A). Whereas it is possible that some of the sediment-breccia mixtures are peperites, the preferred interpretation is that these are sediments that were deposited onto (and into) the breccia after lava flow emplacement.
Before the detailed examination of the lava could begin, the core had to be divided into units. Although a strong effort was made to have unit boundaries reflect individual lava packages, the term "unit" cannot be considered synonymous with "lava flow" for a number of reasons. The first problem was that the unit boundaries were usually fixed before the complete investigation of the core. Only rarely were changes made to the unit boundary locations, even when the initial justification for placing the unit boundaries was lost as the understanding of the site improved. Also, some unit boundaries reflect major physical changes in the core (e.g., brecciated vs. massive) that were visible in the physical properties and downhole measurements. These units may be parts of the same lava flow. Finally, when the precise location of the boundary could not be determined in the initial examination, an arbitrary decision was required for the core description to proceed. This was a common problem in areas with extreme alteration.
The observations recorded during Leg 183 are some of the most detailed and systematic macroscopic data ever collected from lava flows. Specific observations were made of (1) lava surface morphology, (2) vertical vesicle distribution, and (3) sedimentological characteristics of the breccias.
Observation of the surface morphology of the lava was often not possible because flow surfaces are rarely recovered and are most affected by alteration, weathering, and erosion. When visible, the key attributes of the flow surfaces that were recorded were (a) smooth pahoehoe vs. autobreccia, (b) glassy vs. microcrystalline, and (c) evidence for time between successive lava flows (e.g., weathering and sediments).
Vesicle distribution has been shown to be a key indicator of the style of emplacement of lava flows (e.g., Aubele et al., 1988; Cashman and Kauahikaua, 1997; Self et al., 1998). The systematic description of vertical vesicle distribution was made by documenting (1) volume percentage of vesicles, (2) size range (maximum, minimum, and mean diameters), (3) number density, (4) shape (sphericity and angularity), and (5) grading (fining up or coarsening up). The measurements were made over intervals appropriate for the variability shown in the core (typically every 1-30 cm). During these measurements, notes were also taken on the presence of mesostasis blebs, orientation of elongated vesicles, changes in groundmass texture, and other macroscopically visible petrographic features.
The detailed description of the volcanic breccias relied on the techniques used to describe sedimentary breccias. Specifically, clast sizes, sorting, grading, roundness, and lithology were documented. The porosity of the breccia was measured, and evidence for cementing and welding was noted. Of special interest were breccia clasts that showed evidence for deformation while hot and plastic (i.e., that the breccia was an "autobreccia" formed during flow emplacement). The clearest evidence that a breccia formed while the lava was hot came from clasts that had enveloped earlier clasts. The nature of the interface between the breccia and the coherent interior of the flow was also of special interest.