We drilled Hole 1141A 72.1 m into basement. The igneous rocks recovered were divided into six units interpreted to represent five mafic lava flows overlain by a coarse-grained sedimentary deposit made up of gabbroic pebbles (Table T5). Hole 1142A penetrated 50.9 m into basement and the rocks recovered were divided into six units (Table T6). These units include a very diverse range of lithologies, including possible pillow basalts, subaerial deeply weathered (felsic?) lavas and volcanic sediments, rhyolite lava flows, and olivine-phyric basalt lava flows. Despite the close proximity of Sites 1141 and 1142 (~1 km), the recovered basement sections show very large differences.
We describe each unit first, then provide unit-by-unit interpretations in the following section. The rationale for placing the unit boundaries in the locations we chose is also explained in the interpretive section.
The sedimentary sequence overlying basement contains a very small amount of disseminated volcanic detritus, including dispersed black sand-sized lithic grains in Cores 183-1141A-4R and 5R and rare, altered sand- to pebble-sized basalt clasts in interval 183-1141A-11R-CC, 0-6 cm (see "Lithostratigraphy").
The recovered material from Unit 1 consists of three angular pebbles of gabbro (see "Igneous Petrology" for description).
The 7.36 m of material recovered from Unit 2 consists of highly altered and locally intensely oxidized lava (see "Alteration and Weathering"). Only the lowermost 19 cm of the unit (the top of Section 183-1141A-16R-2) is relatively unaltered. Table T7 describes the few patches with recognizable vesicles. In the better preserved part of Unit 2, vesicularity generally decreases with depth but vesicle size increases. The least-altered rocks in the lowermost part of Unit 2 range from moderately to highly vesicular with near-spherical vesicles (Fig. F7).
Recovered rocks from Unit 3 consist of 2.29 m of altered mafic lava. The alteration is complete in most of the unit, but the bottom of Unit 3 (the top 30 cm of Core 183-1141A-17R) consists of relatively unaltered loose pieces. The top of Unit 3 is also strongly oxidized (Fig. F7; see "Alteration and Weathering").
Despite the generally complete alteration, the >1-mm vesicle fillings are commonly well preserved, allowing the distribution of larger vesicles to be determined in several locations (Fig. F8). Recognizable vesicularity is ~7% until interval 183-1141A-16R-3, 40-53 cm, where a zone of 1- to 2-cm-diameter vesicles appears (Fig. F9). Below this, vesicularity grades down to ~3% and into a zone of 3- to 10-mm horizontally elongate blebs (Fig. F10). Below the blebs the vesicularity becomes too low to detect in these altered rocks except for thin horizontal sheets of vesicles in the core catcher from Core 183-1141A-16R at 36 and 42.5 mm. The loose pieces at the top of Core 1831-1141A-17R consist of a mix of dense (~2% vesicles) and highly vesicular (~30% vesicles) lava.
The recovered rocks from Unit 4 consist of 5.92 m of variably altered mafic lava. The vesicularity pattern (Figure F11) generally changes from a vesicular flow top to a massive interior and back to a thinner vesicular base. Domains of vesicle-rich mesostasis material are found in the dense interior, including a 3-mm-wide inclined sheet (Fig. F12), 8-mm-wide vertical pipes (Fig. F13), and ~20-cm-thick irregular pods. The groundmass texture of the massive interior in the vicinity of the inclined vesicle sheet is finer grained than in the vesicular flow top (Fig. F12). The base of the unit is highly vesicular and the lowermost 6 cm is variably altered (Fig. F14). This alteration makes the primary features of the base difficult to discern, especially because the alteration includes millimeter-scale rounded blebs that are both subhorizontal and spherical.
The 6.37 m of recovered rocks from Unit 5 are variably altered coherent basalt. The vesicle distribution is shown in Figure F15, and the familiar pattern of a thicker vesicular upper part, massive interior, and vesicular base is repeated. The uppermost centimeter of Unit 5 is a somewhat altered, smooth, undulating surface that contains abundant <1-mm rounded, horizontally elongated vesicles. There is a pair of identical parallel but slightly inclined surfaces 10-12 cm down into Unit 5 (Fig. F14). Within the vesicular upper portion of Unit 5, brecciation in the form of collapsed larger vesicles is common. Small vesicle trains are common in the lower part of the upper vesicular zone (Section 183-1141A-20R-2). The transition from vesicular to massive lava is marked by a megavesicle horizon with a horizontal sheet of vesicular material (Fig. F16). The bottom 2-3 cm of Unit 5 (Fig. F17) is more heavily altered and appears sheared (see "Alteration and Weathering"). Few vesicles are visible.
The recovered rocks from Unit 6 include an upper breccia, a thick massive interior, and a small amount of breccia at the base. The breccias are pervasively altered to clays and are intensely disturbed, making it difficult to discern primary textures in most places. However, local features can be described (Fig. F17). Figure F18 plots vesicularity vs. depth for Unit 6 and shows data for several intervals. Interval 183-1141A-21R-4, 3-30 cm, appears to be a single coherent lobe with abundant large vesicles in the interior and a denser exterior. Interval 183-1141A-22R-1, 65-142 cm, contains the best preserved portion of the breccia (Fig. F19). In this interval, 80% of the breccia consists of clasts 1-7 cm in size. The remainder consists of subequal proportions of 1- to 10-mm rounded to subangular fragments and the clay matrix. The smaller fragments are in ~10-cm pods and as halos around larger clasts. The vesicularities and vesicle morphologies of the clasts are variable (Fig. F18) but generally rather dense (3%-10%). The remainder of the disturbed and altered breccia appears to be broadly similar to this interval but includes 5%-10% clay-filled voids in the uppermost part of the breccia.
The coherent interior starts in Section 183-1141A-22R-2, 72 cm, with no recovered transition. The vesicle patterns are patchy across 4- to 7-cm spheroidal and subplanar domains in interval 183-1141A-22R-2, 72-136 cm. Below this, vesicularity gradually decreases with depth until just above the base of the flow (Fig. F19). The largest step in vesicularity is marked by a horizon of megavesicles (Fig. F20). There are two unusual types of features within the interior of Unit 6. The first is an interweaving of lava with different groundmass textures. Figures F21 and F22 show two examples of this texture. The other unusual class of features is sheets of nonvesicular segregated material (Fig. F23).
The core catcher from the last core contains 17 cm of altered, disturbed breccia broadly similar to that at the top of Unit 6.
These loose pieces are presumed to be eroded intrusive rocks that have been transported to this site. Their relationship to the lava flows underneath is not known. The angular shapes suggest limited transport but could also be a result of fracturing during drilling.
No contact between the gabbroic pebbles and the altered lava was recovered.
The patches of recognizable vesicles in Unit 2 are not diagnostic (or even strongly suggestive) of any particular type of lava flow.
The change from relatively unoxidized to intensely oxidized lava marks the boundary between Units 2 and 3. Given the patchy nature of the oxidation in the overlying recovered rocks, this does not necessarily represent a significant time break or even a physical break between two flows. However, we have interpreted this as the relatively less altered base of the flow in Unit 2 over the weathered and altered vesicular top of Unit 3.
The vesicle distribution, a megavesicle horizon grading into a massive interior, and the horizontal vesicle sheets with associated mesostasis blebs are all characteristics of an inflated pahoehoe lava flow. However, some transitional lava flows seen at previous sites on Leg 183 also have similar interiors. The loose pieces of relatively unaltered rock at the base of Unit 3 show no evidence of brecciation and are likely to come from a smooth, coherent, lower vesicular crust. Such a basal surface strongly suggests a pahoehoe flow.
The change from relatively unaltered to completely altered lava marks the boundary between Units 3 and 4. We have interpreted this as the relatively less altered base of the flow in Unit 3 over the weathered and altered vesicular top of Unit 4.
The distribution and morphology of vesicle features in Unit 4 are diagnostic of an inflated flow. The consistently round vesicle shapes and smooth base are strong indicators of a pahoehoe flow. The fine-grained massive interior of the flow has been hypothesized to be a dike (see "Igneous Petrology"), but the vesicle features are completely consistent with being part of the interior of a thick, inflated pahoehoe flow. The fine-grained nature of the interior is difficult to explain, but variations in groundmass textures in thick flows are not documented.
The surfaces with small, stretched vesicles near the boundary between Units 4 and 5 are interpreted to be altered glassy chill margins. However, the precise location of the basal chill of Unit 4 is difficult to pinpoint because of the alteration, possible tectonic shearing, and the topography on the top of Unit 5. The 10-cm-thick lobe between the two thicker flows could belong to either unit. However, based on the identical oxidation of the thin lobe and the rest of Unit 5, it is placed with Unit 5. The deep oxidation of the upper part of Unit 5 suggests a substantial time break, but there is little evidence for extensive erosion or weathering of the top of Unit 5.
Multiple lobes of pahoehoe
define a compound pahoehoe flow. The features within the thick lobe are
diagnostic of inflation. This indicates that the underlying ~7-m-thick lobe
started as a lobe similar to the 
10-cm-thick
lobe on top. The zone of irregular vesicle trains and relatively low
vesicularity record a time during which the flow began to stagnate. The
underlying horizontal vesicular zone indicates that the flow was reactivated
with injections of fresh, bubble-laden lava. The lower vesicular crust and the
morphology of the basal contact are also typical of inflated pahoehoe flows.
The contact between the pahoehoe base of Unit 5 and the brecciated top of Unit 6 is well preserved and easy to locate. However, the alteration and disturbance is sufficient to make it impossible to determine the amount of weathering and reworking the breccia was subjected to before being covered by Unit 5.
The patchy vesicular
domains in the upper part of the coherent lava are interpreted to be remelted
entrained breccia clasts. This and the irregular, angular morphology of many of
the vesicles are strong evidence that the lava flow in Unit 6 was emplaced with
a disrupted, brecciated flow top. A vesicular 30-cm-thick
coherent lobe in the upper part of the breccia indicates that this was not an aa
flow.
The intermixing of lavas with grossly different groundmass textures seen in Unit 6 is a feature we have not noted in any other lava flows. It is possible that this is actually quite common in lava flows that have remelted entrained material but is not visible without a style of alteration that enhances these features. The relationship between the different groundmass textures and the nonvesicular sheets of segregated material is also unclear. The breccia at the base of Unit 6 is equally likely to be the basal breccia of Unit 6 or the flow-top breccia of the underlying lava.
The recovered rocks from Unit 1 consist of 1.91 m of massive basalt. The uppermost pieces are pebbles and cobbles with a 0.5-cm oxidation rim and have calcareous sediments cemented to their margins. Although the smallest pebbles are well rounded, the original shapes of the larger cobbles cannot be determined because of drilling disturbance. There is no visible difference between the larger cobbles and the underlying coherent massive lava. The massive lava contains 0.3%-0.5% round vesicles and is remarkably featureless. The few pieces of calcareous sediments above Unit 1 are described in "Lithostratigraphy".
The 1.03 m of rock recovered from Unit 2 consist of a highly altered breccia overlying a mixture of different volcanic rocks. The uppermost pieces in Core 183-1142A-3R are pieces of calcareous sediment identical to those found above Unit 1. The breccia of Unit 2 contains ~25 vol% clasts in the 1- to 5-cm size range and 40%-60% clay matrix (Fig. F24). The remainder is sand- to silt-sized fragments. Clasts are subangular to well rounded and consist of variably altered and oxidized lava. Most lava appears dense. However, the largest clast is a loose 1 cm × 1.5-cm spheroidal piece of secondary vesicle fill.
The rocks below the
breccia are poorly recovered with 
10-cm-long
pieces, of which only one can be oriented. Lithologies are varied, including
pieces that include large olivine phenocrysts and others with quartz and
possible pumice fragments (see "Igneous
Petrology"). Interval 183-1142A-3R-1, 73-93 cm, consists of 1- to
7-cm pieces of an altered breccia. The clasts within the breccia are subrounded
pieces of sparsely vesicular and moderately olivine? phyric lava that have a
halo of smaller fragments embedded in a clay matrix (Fig. F25).
Unit 3A contains 1.31 m of completely altered and highly disturbed broken rubble and clay-sized material. Zones within the disturbed material preserve angular broken rock fragments. These are either massive, nonvesicular lava fragments with devitrification textures (e.g., interval 183-1142A-4R-1, 0-3 cm), or red, altered, massive, clay-rich material (e.g., interval 183-1142A-4R-1, 9.5-12 cm).
In the upper part of Unit 3B, pieces of altered, relatively massive, dark reddish gray, moderately indurated (basalt?) lava were recovered (interval 183-1142A-5R-1 [Pieces 1-4, 0-23 cm]). Below this interval, the altered volcanic rock is dark brown and massive (interval 183-1142A-5R-1 [Pieces 5-7, 23-47 cm]) to brecciated (interval 183-1142A-5R-1 [Pieces 7 and 8, 47-57 cm]). Brecciation in this interval appears to be caused by brittle deformation. Angular pebble-sized clasts sit in a matrix of dark red (oxidized) clay-sized material and coarse sand to small granule-sized, variably oxidized, angular, lava fragments (Fig. F26). Lower in Section 183-11442A-5R-1 there is another interval of dark reddish gray, relatively massive, altered basalt? lava (interval 183-1142A-5R-1 [Pieces 9-17, 57-66 cm]). In most pieces an internal texture ~30° from horizontal is defined by a coalescence of thin (<1 mm), orange, elongate domains of altered material in the mesostasis (e.g., interval 183-1142A-5R-1 [Piece 12, 70-80 cm]) (Fig. F27). In one interval the orange altered domains are up to 0.6 cm × 2 cm (interval 183-1142A-5R-1 [Pieces 13-14, 103-118 cm]). In some places the altered domains have coalesced into long narrow trains that define a pseudobreccia texture in the rock (interval 183-1142A-5R-1 [Pieces 15-16, 118-131 cm]) (Fig. F28).
At the top of interval 183-1142A-5R-2, 0-12 cm, and in interval 183-1142A-5R-2, 24-28 cm, there is some dark reddish gray pebble-sized rubble. This rubble has the same texture as material at the base of Section 183-1142A-5R-1, and lower in Section 183-1142A-5R-2, and probably fell downhole during drilling. Within the same lithology, a breccia with dark red, flattened clasts in a reddish brown clay matrix is found in interval 183-1142A-5R-2 (Piece 2, 12-24 cm) (Fig. F29). Olivine was recognized in three cobble-sized pieces of dark reddish gray, altered, (basalt?) lava (interval 183-1142A-5R-1 [Pieces 4-6, 28-48 cm]) below the breccia interval (see "Igneous Petrology"). Similar dark reddish gray altered (basalt?) lava continues to the bottom of the section (interval 183-1142A-5R-2 [Piece 7, 48-75 cm]). These rocks have an internal fabric ~30° from horizontal defined by an in situ brecciation texture with alignment of slightly flattened subrounded clasts. This may be an alteration texture, after a primary flow fabric.
The upper 28 cm of Core 183-1142A-6R-1 contains mixed volcanic rubble reworked from higher in the hole during drilling. Lithologies present include altered, resinous, clay-rich, porphyritic volcanic rock (rhyolite?); dark reddish gray lava (Unit 3B); greenish gray altered basalt pebbles; and red brown indurated clay pebbles. Below this, a pale brown to brown, indurated, altered, relatively massive lava with an internal fabric ~30° from horizontal defined by thin (<1 mm), pale brown, elongate domains of altered material in the mesostasis (Sample 183-1142A-6R-1 [Pieces 5A-5D, 28-52 cm]). This seems to be an alteration texture in otherwise massive lava. Interval 183-1142A-6R-1, 52-67 cm, includes one dark reddish gray pebble similar to Subunit 3B and an interval of broken rock that was highly disturbed by drilling.
A brownish gray, altered, relatively massive (basalt?) lava is located in interval 183-1142A-6R-1 (Pieces 8 and 9, 67-114 cm). This lava also has an internal fabric ~30° from horizontal, but it is defined by thin (<1 mm), pale orange, elongate domains of altered material, separated by whiter devitrified zones in the mesostasis. Although the texture in this rock is similar to the textures of rocks in interval 183-1142A-5R-1 (Piece 4, 28-52 cm), less olivine was identified in this rock. Another interval of highly disturbed volcanic rock (drilling fragmentation) is interval 183-1142A-6R-1, 114-146 cm. Two dark gray, massive pebbles with aligned, elongate domains of orange altered material, forming a 1-cm-wide zone around their perimeter, were preserved at the base of the disturbed material.
Unit 4 is a normally graded (intervals 183-1142A-6R-2, 27-40 cm, and 40-46 cm; Fig. F30) to matrix-supported (intervals 183-1142A-6R-2, 0-27 cm, and 46-49 cm; interval 183-1142A-7R-1, 9-92 cm; Fig. F31), red (oxidized), very coarse sand to granule-bearing clay. Coarse to very coarse sand-sized clasts are dominantly angular, and very coarse sand to small granule-sized clasts are dominantly rounded, indicating the incorporation of more than one clast population. The clasts include quartz, kaolinitized feldspar, variably altered volcanic lithic fragments, and feldspar crystals. The red clay matrix is homogeneous and massive and does not preserve pedogenic features. In interval 183-1142A-7R-1, 0-9 cm, there is some brownish gray, altered, pebble-sized rubble with a subparallel internal texture that is similar to material in interval 183-1142A-6R-1 (Piece 8, 67-104 cm) higher in the sequence.
A relatively coherent brownish gray lava is preserved in interval 183-1142A-7R-1, 100-137 cm. It has an arcuate alteration front along one side that reflects a round lobe shape in the core. Within the more altered zone, there is alignment of small, elongate, altered zones, possibly altered phenocrysts or variolites, in the mesostasis. Within the less altered zone, the internal texture is massive. Pieces above and below this interval are brecciated (intervals 183-1142A-7R-1 [Piece 8, 92-100 cm] and 183-1142A-7R-1, 137-151 cm). The breccia has dark reddish gray clasts in a red clay matrix, with incorporated coarse sand- to granule-sized, angular to subrounded and partially flattened, spalled lava fragments (Fig. F32). A few of the larger lava clasts show irregular to fluidal margins, and there is some evidence of flattening. All of the pieces in Core 183-1142A-7R-2 are brecciated. Piece 1 is massive, dark-red, altered material with a 1 cm × 1.5 cm included clast, and Piece 2 contains clasts with either chilled or oxidized margins that are possibly joined along a suture. Pebbles in interval 183-1142A-7R-2, 12-22 cm, are subangular to subrounded dark red and dark gray breccia fragments. The lower part of interval 183-1142A-7R-2, 22-48 cm, is a dark reddish gray breccia with a subhorizontal texture and possibly some clast flattening (Fig. F33). This breccia texture may have been enhanced during alteration. Section 183-1142A-8R-1 is almost totally disturbed by drilling, but domains of less broken rock are similar to the breccia in interval 183-1142A-7R-2 (Piece 4, 22-48 cm).
The 8.27 m of recovered rock from Unit 6 consists of less altered, generally massive basalt. The lava also contains abundant veins and oxidation halos (Fig. F34; see "Alteration and Weathering") and small faults (Fig. F35). The veins in intervals 183-1142A-9R-2, 58-61 cm, and 9R-4, 0-5 cm, have a thick, altered margin with a distinctly finer grained groundmass (Figs. F36, F37). The lava is remarkably nonvesicular; we were unable to find a single macroscopic vesicle in the recovered rocks from Unit 6.
The massive featureless lava from Unit 1 is interpreted to be from the interior of a thick lava flow. The sparse vesicularity and oxidized top tentatively suggest a subaerial flow rather than a submarine flow.
The change from a relatively unaltered massive lava to a highly altered breccia marks the change from Unit 1 to 2. The breccia appears to be extensively reworked and suggests a major time break between these two units.
The wide range of lithologies within Core 183-1142A-3R cannot be all in place. However, the lack of a visible matrix and the presence of a clast that contains a breccia are difficult to explain in a debris flow or sedimentary breccia. We tentatively suggest that Unit 2 is a reworked talus pile that was covered by the flow of Unit 1.
Several observations suggest that breccia at the top of Unit 2 was extensively reworked. The scarcity of larger clasts and the wide mixture of different types of fine-grained lava fragments are distinctly different from the primary flow-top breccias seen earlier during Leg 183 (see "Physical Volcanology," in the "Site 1136" chapter, "Physical Volcanology," in the "Site 1137" chapter, and "Physical Volcanology," in the "Site 1138" chapter). The loose piece of vesicle fill is also evidence that this breccia at the top of Unit 2 is composed of lava that was completely disaggregated.
A transition from relatively unaltered rocks to a completely altered and disturbed rock and the top of a new core mark the change from Unit 2 to Unit 3.
Extensive drilling disturbance in these highly altered rocks make it difficult to interpret the volcanological aspects of this unit.
Subunit 3B is composed of intervals of altered coherent (basaltic?) lava and at least two associated breccias. The upper breccia (interval 183-1142A-5R-1 [Pieces 7-8, 47-57 cm]) (Fig. F26) has angular clasts and formed because of brittle deformation of volcanic rock. There is no evidence of welding, and a partially pulverized matrix indicates clast/clast contact during breccia formation. This is a tectonic breccia, probably formed after the lava was emplaced and had cooled. The lava is aphanitic, has low vesicularity, and is internally massive (Figs. F27, F28). A secondary fabric of coalesced, elongate, altered domains may follow a primary flow alignment. These domains may originate as altered phenocrysts or variolites in the rock that are later modified during alteration. It is not clear whether the lava has been emplaced in a subaerial or subaqueous environment.
The lower breccia (interval 183-1142A-5R-2 [Piece 2, 12-24 cm]) (Fig. F29) has a subhorizontal fabric that could be the result of either shearing or flattening. The clasts are similar to the lava material above and below. It is difficult to flatten solid lava; hence, this is either a shear zone or a zone where clasts were flattened while hot. There are no chilled margins on clasts or welding textures preserved and no other evidence of heat in this breccia, implying that the breccia probably has elongated clasts because it has been sheared.
This unit is dominated by two intervals of altered brown to brownish gray, sparsely olivine-phyric basaltic? lava (intervals 183-1142A-6R-1, 28-52 cm, and 6R-1, 67-114 cm). Both are relatively massive although they have secondary alteration textures that highlight a fabric ~30° from horizontal. This fabric is delineated by aligned and coalesced, elongate alteration/devitrification domains within the lavas that may relate to primary flow in the lavas. It is not clear whether the lava was emplaced in a subaerial or subaqueous environment. The balance of the unit is composed of material that has probably fallen down the hole during drilling (e.g., interval 183-1142A-6R-1, 0-28 cm) and highly disturbed intervals, broken and mixed during drilling (e.g., interval 183-1142A-6R-1, 114-146 cm).
The transition from highly disturbed material at the base of Subunit 3C to a (baked?) clay with disseminated granules marks the transition into Unit 4. The Unit 4 sediment is red (oxidized) and pervasively weathered, implying a significant time break at this boundary.
The mixture of incorporated clast lithologies and morphologies as well as the normal grading in the very coarse sand to granule-bearing clay indicates that this is a reworked interval. The high concentration of fine-grained material and the disseminated crystal feldspar? content in this unit suggest that it is a resedimented volcanic ash that has been pervasively altered. The transition from normal-graded to matrix-supported textures is consistent with mass-flow resedimentation of the clay-rich sediment, and it is probably a mudflow deposit. The uniform red oxidation color throughout this unit more likely results from deep weathering in a subaerial environment rather than baking in either a subaerial or subaqueous environment. However, no other regolith features are preserved in this interval to confirm a subaerial weathering history. Alternatively, the red matrix could have been derived by erosion of intensely weathered clay-rich material, with no post emplacement weathering.
The boundary between Units 5 and 6 is located at the change from indurated granule-bearing clay into a massive aphyric volcanic breccia.
Unit 5 is largely composed of brecciated rocks with one >35-cm-diameter lobe of coherent lava near the top. The breccia materials vary from tightly juxtaposed clasts, which may show contacts along sutures (interval 183-1142A-7R-1, 92-100 cm), to a breccia with sheared and/or flattened clasts in a granule-bearing red clay matrix (interval 183-1142A-7R-1, 137-151 cm) to an in situ breccia with polygonal fracture networks separating clasts (interval 183-1142A-8R-1, 44-48 cm). These breccias may show transitional states in an autobreccia carapace, in which there is both brittle deformation of earlier-formed cooled rock and interaction of clasts and lobes that may have been hot. Examples of the latter include the suture textures in the upper breccia and enhanced alteration around perimeter of the intrusive lobe. The matrix in the breccia with more sheared and flattened clasts is similar in composition to the overlying granule-bearing red clay, implying some interaction between intrusive lobes or lava material and sediment. The flattening of clasts may be caused by hot emplacement and welding, variable alteration and load flattening, or shearing through the breccia zone. Lack of evidence of heating of the breccia materials and the observation that larger clasts seem altered to a similar degree suggest some shearing of this breccia after emplacement. The lava lobe in this interval is massive and aphyric and has zoned alteration. The secondary fabric is defined by coalescence of elongate, millimeter-scale, altered domains in the lava.
The change from highly disturbed altered rock at the base of Unit 5 to a relatively coherent massive lava (and the top of a new core) mark the change from Unit 5 to Unit 6. It is possible that parts of Unit 5 are the deeply weathered and highly altered upper part of Unit 6.
The most distinguishing feature of Unit 6 is its complete lack of vesiculation. The patterns delineated by the veins are reminiscent of the pattern of chill margins seen in pillow basalts (cf. Figs. F36 and F37 to Figs. F6 and F13 in the "Site 1140" chapter), but the identification of chilled margins remains equivocal (see "Alteration and Weathering"). Chilled margins are diagnostic of emplacement in a subaqueous environment, and the lack of vesicles would suggest eruption in relatively deep water. The two possible chill margins could be simply altered veins.
All the lavas seen in Hole 1141A are consistent with being subaerially emplaced, and there is no evidence for interaction with water during emplacement. Without downhole logging data, we are cautious about estimating flow thicknesses. However, these flows are probably all between 5 and 20 m thick. These thicknesses are greater than are typical for historical Hawaiian flows but thinner than typical continental flood basalt flows. Instead, they are most similar to the largest recent Icelandic lava flows.
Unit 6 in Hole 1142A is possibly submarine and the features in the other lavas are inconclusive with regard to the environment into which they were emplaced. However, the alteration and weathering suggests that the rocks were exposed for some time in a subaerial environment (see "Alteration and Weathering").
The differences between Sites 1141 and 1142 are rather striking, considering that they are only 800 m apart. These differences demonstrate the complex geologic history of this part of Broken Ridge.
