PHYSICAL VOLCANOLOGY

Hole 1140A recovered 43.4 m of interbedded pillow lavas and sediments in the basement. These have been divided into 6 units (Table T5), which were divided on the basis of inferred time breaks. This includes 5 distinct packages of pillow and massive basalts and a dolomite interbed. In this section, we provide detailed descriptions of the volcanic features in each unit followed by their interpretation. There is a very small amount of disseminated volcanic detritus, including alkali feldspar crystals, lithic fragments, and glass shards, throughout the overlying sediments (see "Lithostratigraphy"), but only one discrete, basaltic glass shard-rich tephra horizon (interval 183-1140A-18R-6, 102-107 cm) was identified at Site 1140.

Unit 1 (Interval 183-1140A-25R-6, 0 cm, to 31R-1, 0 cm)

Recovered rock from Unit 1 consists of 12.46 m of ~0.5-m diameter pillows of aphyric to sparsely plagioclase-phyric basalt with metamorphosed calcareous sediments between many of the pillow margins. However, in the middle of the pillow pile (interval 183-1140A-27R-1, 70 cm, to 27R-4, 51 cm), 3.9 m of massive lava was recovered. Elsewhere, glassy pillow lobe margins are present at 5- to 50-cm intervals. Metamorphosed sediments in contact with the pillow margins commonly contain 5-mm angular glass fragments and well-preserved foraminifers (see Fig. F20) (see "Igneous Petrology and Geochemistry"). Where recovered, the glassy rinds are consistently very close to 1 cm thick (Fig. F6). Rinds that appear wider than 1 cm on the open face of the core are cut obliquely; they are not thicker than other rinds. Inward from the glassy rind, there is an ~1-cm-thick zone with a very fine grained to glassy groundmass and sparse vesicles. In Unit 1, these vesicles make up ~3 vol% of the lava and have irregular angular shapes elongated normal to the margin of the pillow. The remainder of the interior of each pillow contains 0.03-0.07 spherical vesicles/cm² with 0.5- to 0.8-mm diameters (~0.01 vol%). The vesicle distribution in the 3.9-m-thick lobe is shown in Figure F7.

Unit 2 (Interval 183-1140A-31R-1, 0-134 cm)

Recovered rock from Unit 2 consists of three pillows of highly plagioclase-phyric basalt. The first lobe (interval 183-1140A-31R-1, 0-30 cm) has radial and concentric joints that suggest a 40-cm-diameter pillow. The second lobe (interval 183-1140A-31R-1, 34-67 cm) has a margin with a curvature that would indicate a 50- to 70-cm-diameter pillow, if it has a near cylindrical shape. The third lobe (interval 183-1140A-31R-1, 67-134 cm) has no recovered basal chill, indicating that it is >67 cm in diameter. Unlike Unit 1, the margins and pillow interiors of Unit 2 have a similar vesicularity. Unit 2 contains 2%-3% spherical vesicles 3.5-0.3 mm in diameter, with ~1 mm being the most common vesicle size. Vesicle number density varies from 2.5 to 4.5 vesicles/cm², with higher number densities where vesicles are smaller.

Unit 3 (Interval 183-1140A-31R-1, 134 cm, to 32R-3, 43 cm)

Recovered rocks from Unit 3 consist of four lobes of aphyric to sparsely-plagioclase-phyric basalt topped by a piece of green calcareous chalk (see "Lithostratigraphy"). The recovery of the upper part of Unit 3 is poor, with only 1- to 11-cm loose pieces. The pieces midway through Section 183-1140A-31R-2 contain a pillow margin, suggesting a lower part of one lobe and the uppermost part of a second in this poorly recovered portion. Core 183-1140A-32R has better recovery and starts with the interior of a large lobe with a well-preserved basal chill in interval 183-1140A-32R-1, 143-144 cm. Neither the upper nor lower chill on the third lobe were recovered, but the top of the fourth lobe is well preserved in interval 183-1140A-32R-2, 73-77 cm. Unit 3 ends with the basal chill of this lobe. The vesicularity within Unit 3 is distinct from both Units 1 and 2; long, irregular pipe vesicles are the most striking feature (Fig. F8). Figure F9 shows the distribution of vesicles in the central part of Unit 3. In addition, the last lobe has an unusually thick upper chill zone, with 2 cm of fine-grained groundmass followed by >7 cm of a vesicular top. This zone has 2 vol% vesicularity with a mix of both spherical and irregular angular shapes ranging in size from 0.15 to 4 mm. Below this vesicular top, the lava is massive with only the long irregular pipe vesicles, except for a short interval (interval 183-1140A-31R-2, 107-113 cm), where the vesicularity is identical to the vesicular zone at the top of the lobe. The massive lava contains ~1.5% irregular angular vesicles 0.1-5 mm in size in addition to the larger pipe vesicles.

Unit 4 (Interval 183-1140A-32R-3, 43 cm, to 32R-4, 0 cm)

Unit 4 is a dolomite to dolomitic-nannofossil-chalk that is more highly dolomitized toward the top and bottom. This unit is described in more detail in the "Lithostratigraphy" and "Alteration and Weathering".

Unit 5 (Interval 183-1140A-32R-4, 0 cm, to 34R-4, 0 cm)

Recovered rock from Unit 5 consists of thick lobes of moderately plagioclase-phyric basalt. The top of the unit is altered to grayish orange (10YR 7/4) and was relatively poorly recovered. Only two pillow margins are present in intervals 183-1140A-33R-1, 19-20 cm and 37-39 cm, suggesting that the recovered portion of Unit 5 is composed of only three lobes. The second margin has a very fine grained orange (palagonitized) rim followed by a zone with irregular, ill-formed pipe vesicles up to 3 cm long (Fig. F10). Downhole measurements indicate the presence of a sedimentary interbed in the upper part of Unit 5 (see "Downhole Measurements"). This probably overlies the orange pillow rim, but there is insufficient change in the recovered rocks to justify the subdivision of Unit 5. The lava is significantly more vesicular than Units 1 and 2; point counting within the "massive" lava yields 3.9 vol% vesicles. These vesicles are generally 0.2-2.2 mm in diameter, irregular subangular in shape, and at a density of ~20 vesicles/cm². Figure F11 shows the details of the vesicle distribution within the upper part of the third lobe of Unit 5. Note in particular the occurrence of pods with as much as 70 vol% vesicles and the thin sheet-like vesicular regions.

Unit 6 (Interval 183-1140A-34R-4, 0 cm, to 37R-4, 34 cm)

The recovered rock from Unit 6 is a sequence of pillows of aphyric to moderately-plagioclase-phyric basalts overlain by a 6-cm piece of orange dolomite (see "Lithostratigraphy"). The morphology of the pillows is similar to those in Unit 1, with a ~1-cm glassy rim, followed by a vesicular zone with some irregular ~1-mm-diameter pipe vesicles elongated perpendicular to the chill margin and a generally nonvesicular interior. Unlike Unit 1, the vesicular zone is up to 5 cm thick with ~2 vol% vesicles. Figure F12 shows the vesicle distribution through one pillow in Unit 6. Another feature that was present in Unit 1 but better preserved in Unit 6 is pieces of the chill margin that have been pushed 2-3 cm back into the pillow lobe with metamorphosed sediments filling the resulting space (Fig. F13). Also, similar to Unit 1, Unit 6 contains one >4-m-thick massive lobe. This thick lobe has a coarser groundmass texture. The upper 30 cm of the lobe has ~0.5% round vesicles ~0.5 mm in diameter. Glassy mesostasis blebs are wispy in interval 183-1140A-36R-2, 65-105 cm and distinct in interval 183-1140A-36R-4, 43-77 cm. The zone with distinct mesostasis appears completely devoid of vesicles, but the remainder of the lobe has 0.2 round vesicles/cm² with diameters 0.5 mm. These vesicles may be distributed in ~10-cm-thick more-vesicular layers and 10-cm-denser layers, but the vesicles are too rare for confident statistics across a 5- to 6-cm-diameter core. There is also a region of extreme alteration and clay-filling in Section 183-1140A-35R-1 at 116-123 cm.

Unit Distinctions

The placement of basement unit boundaries in Hole 1140A was relatively straightforward. The dramatic increase in phenocryst content after >20 m of no recovery (Cores 183-1140A-29R and 30R) required separating Units 1 and 2. Logging data indicates the presence of an unrecovered 2.2-m-thick sediment package at the top of Unit 2, and a magnetic reversal confirms a significant time gap between the emplacement of these two lava packages (see "Downhole Measurements" and "Paleomagnetism"). A sedimentary interbed and decrease in phenocryst content marks the change from Unit 2 to Unit 3. Unit 4 was a well-recovered >1-m-thick sediment package that was designated as a separate unit, and a package of dolomite marks the Unit 5/6 boundary.

Lobe Sizes

Two effects make lobe-size determinations from the recovered rocks of Site 1140 imprecise. First, the core recovery is incomplete, and glassy margins are the most likely material to be lost. Second, the core produces a random vertical cut through the cylindrical pillows. However, plots of the estimated average thickness of the lobes in the recovered rocks show some interesting differences in the units (Fig. F14). In particular, Unit 5 has an average lobe thickness three to five times greater than the other units. Downhole logging shows that Unit 5 includes an ~10-m-thick massive lobe. Units 1 and 6 each contain one lobe 3 m thick that is also visible in the downhole measurements (see "Downhole Measurements"). The size of these lobes suggests that they were produced differently than the stacks of 0.5-1-m-thick pillows.