The structures identified in the core from Hole 1191A were primary volcanic layering, elongation of vesicles, and silica-pyrite veins. These were described on the structural visual core descriptions (see "Site 1191 Visual Core Descriptions") and recorded in the structural log (see "Site 1191 Structural Log").
In the volcanic rocks found throughout the recovered core, the original layering was identified from the orientation of elongate flattened and stretched vesicles in some of the massive lavas, and in other parts of the core, from millimeter-scale color banding attributed to flow. The individual flow bands consist of differences in the amount and size of the microlites formed during crystallization and by the different degrees of vesicularity. The vesicles were found commonly to display a stretching lineation. The stretching and flattening of vesicles most probably formed during flow of viscous lava, and thereby they define the direction of flow.
The lineations show shallow plunges, between 4° and 21°, except for one measurement of 38°, whereas the layering defined by the arrays of vesicles has dips of between 7° and 88° (Fig. F8A). In the upper meter of the hole, the dip of the layering changes from ~45° at 0.12 mbsf to subhorizontal at 0.52 mbsf. Most of the measurements are from Sections 193-1191A-2R-1 and 2, between 9.51 and 12.03 mbsf. In this interval, the dip of layering has a trend from subvertical at 9.51 mbsf, gradually decreasing downward to a dip of ~40° at 12 mbsf. In Section 193-1191A-3R-1, the measurements indicate a steepening trend of the dips of layering from ~40° at 15.21 mbsf to ~60° at 15.56 mbsf. The plunges of the lineations, however, as mentioned above, are generally <20°. If the variations in the dips of layering were caused by the intersections of different rotated lava blocks, a change in orientation of the plunges of lineations would also be expected. Because this is not the case, the most likely explanation for the variations in dips of layering is folding. The folding would have happened during the flow of the viscous lava.
In six cases in the hole between 9.51 and 15.21 mbsf, it was possible to measure both the direction of stretching and layering in the same pieces (Fig. F8B). These measurements show that although the layering as defined by the trails of the flattened vesicles is generally steep, the flow direction of the lava, as defined by the direction of vesicle elongation lineation, is close to horizontal. This could mean that Hole 1191A intersected the outer parts of a lava flow in this interval, as depicted in Figure F8C.
Only 16 veins, commonly forming the broken ends and surfaces of the core pieces, were intersected in Sections 193-1191A-2R-1 and 2R-2, between 10 and 12 mbsf. With but one exception, these veins are dominated by fine-grained pyrite and/or marcasite with minor silica. The exception was a <0.2-mm-thick vein consisting solely of silica. The veins are all very thin, mostly <1 mm in thickness, only one vein being 1-1.5 mm thick.
With respect to dips of the veins, one is nearly horizontal and one is vertical, whereas the others dip between 19° and 59° (Figure F9).
The thickest vein, with a dip of 52°, was the only one that was examined under the microscope (Sample 193-1191A-2R-2 [Piece 16, 103-106 cm]). This vein is 1-1.5 mm thick and consists of framboidal pyrite or greigite(?) overgrown by marcasite (see "Sulfide Petrology"). Fine-grained silica, probably cristobalite, is present in trace amounts between the sulfide grains. A faint, <0.3-mm-thick halo of silica-clay is present beside the vein. Disseminated marcasite, partly with cores of framboids, similar to the assemblage in the vein, extends for 2-3 mm out from both sides of the vein, posing questions regarding conventional models for the origin of framboids.