To supplement the brief summary in the "Site 1277" chapter of the Leg 210 Initial Reports volume (Shipboard Scientific Party, 2004b), a detailed description and local interpretation is given for lithologic Unit 1 together with a summary of lithologic Unit 2. To aid interpretation of the temporal evolution of Site 1277 the lithologies are discussed in stratigraphic order from the base upward (rather than as summarized collectively from the top downwards as in the Initial Reports volume).
The lithologic boundary between Unit 2 and Unit 1 (Fig. F3) was placed by the Shipboard Scientific Party above the highest serpentinite that shows a pervasive nonsedimentary fabric (i.e., the lower part of Section 210-1277A-6R-1) and beneath the lowest sedimentary breccia-conglomerate formed by sedimentary processes (Sample 210-1277A-5R-3, 75.5 cm). However, the contact itself was not recovered.
In general, the top of the basement unit is interpreted as the eroded surface of a presently low-angle fault, possibly a detachment fault that exposed serpentinized mantle lithosphere on the seafloor, where it was covered by basalt and polymict sediments (Shipboard Scientific Party, 2004b).
Most of Unit 2 is dominated by serpentinized harzburgite, which ranges from homogeneous serpentinite, through a tectonic breccia composed of interlocking jigsaw-type clasts, to a well-foliated porphyroclastic serpentinized peridotite. Subordinate dunite is also present. Many of the harzburgite pieces that were recovered exhibit well-developed spinel foliation that is typical of serpentinized upper mantle that has undergone ductile deformation as a result of asthenosphere flow prior to exhumation. This foliation is steeply inclined (>80°) in many intact pieces of core recovered. The more massive serpentinized harzburgite lower in the section is locally cut by gabbroic segregations. A network of calcite- and talc-filled veins is also present. These observations indicate that the peridotite experienced high-temperature deformation, injection of small volumes of gabbroic melt, and later low-temperature tectonic fracturing and veining. The serpentinized peridotite includes several intervals of serpentinite mylonite and foliated mylonite, which are interpreted to be the result of ductile (high-temperature) shearing related to exhumation of the mantle to the seafloor.
Postcruise geochemical studies of the plutonic rocks (Müntener and Manatschal, 2006) show that the sepentinized spinel peridotites from Unit 2 occurring as clasts in Unit 1 are strongly depleted, similar to the most depleted abyssal peridotites and to the tectonite (depleted mantle) of typical "suprasubduction zone ophiolites."
The gabbro breccia recovered from just beneath the contact between Units 2 and 1 was determined to be a cataclasite because the clasts are angular and exhibit an interlocking jigsaw-type fabric without any evidence of a sedimentary matrix (Fig. F4A). Tectonic clasts within the cataclasite range from angular to subrounded and vary from medium-grained to pegmatitic. One piece is foliated with asymmetric porphyroblasts, whereas another is a cataclasite with angular interlocking clasts. Calcite-filled veins and fractures are common. The directly underlying harzburgite shows similar tectonic brecciation (Fig. F4B). It should be noted that all of the plutonic rocks of Unit 2 form parts of a coherent "basement" and as such they differ in setting from basalts that were erupted as flows within the overlying clastic sediments of Unit 1, as discussed below.
Six main sedimentary or volcanic intervals are recognized, specifically consisting of three basaltic layers and three main occurrences of polymict sediments dominated by detrital serpentinite and gabbro (Fig. F3A).
The lowermost recovery above Unit 2 consists of pieces of brecciated gabbro, polymict serpentinite-rich breccia, and basalt (Fig. F3A). The gabbro pieces range from medium grained to coarse grained and include gabbro pegmatite.
A single sandstone piece (interval 210-1277A-5R-2, 33–42 cm) was described on the ship as finely laminated pink, purple, and gray graded ferruginous sandstone (Fig. F5A). The well-sorted texture and fine grain size of this sedimentary rock differ from the other clastic lithologies recovered at Site 1277. In hand specimen, the lower part of this interval is pale orange and passes upward through a purple-red interval into a bright red, strongly oxidized uppermost interval. A thin section studied after the cruise shows that this sediment is strongly altered, although primary grading and a primary planar lamination are clearly visible. This sediment appears to have been eroded from a lava flow, as it includes highly altered basalt clasts and volcanic glass. In common with the other sediments, it shows evidence of fracturing and calcite veining after cementation.
The recovered basalt above this interval (210-1277A-5R-1, 5–135 cm) shows evidence of strong tectonic brecciation. High-angle fractures are partly infilled with volcaniclastic silt and were later partially cemented by calcite spar. Some unfilled fractures are lined with prismatic calcite spar (Fig. F5B).
The nature of the lowermost interval suggests that deposition began with sedimentary breccias rich in clasts of brecciated gabbro. These clasts are likely to have been eroded from fault gouge, and it is therefore inferred that an extensional detachment fault is present between Unit 2 peridotites and the base of the overlying clastic section. The intense fracturing of sedimentary rock clasts indicates that they were deformed after lithification, similar to other sedimentary lithologies recovered from higher in the succession (see below). The graded volcaniclastic sandstone is likely to represent a sand turbidite, derived from erosion of basalt in an upslope area. This was distinct from the more local setting that is inferred for the clastic debris and basaltic volcanics. The different provenances are particularly suggested by the strong contrast between the well-sorted, graded, laminated sediment and the typically much coarser, poorly sorted mass flows that were cored at Site 1277. Assuming that the basaltic sands are turbidites, a supply of basaltic material must have been available some distance from Site 1277 (i.e., hundreds of meters to kilometers away). This would further imply that basalts were erupted more widely than the lava flows locally cored at Site 1277. The basalt above the recovered clastic sediment could be as thick as several meters, and it is interpreted as the lowermost lava flow that was cored (marked as Flow 1 on Fig. F3A). This flow was strongly fractured and veined.
The lowermost heterogeneous interval is overlain by a distinctive interval that is composed of reddish brown, matrix-supported, poorly sorted breccia-conglomerate, estimated to be as thick as 30 m (intervals 210-1277A-4R-1, 1 cm, through 5R-1, 25 cm). This interval (Fig. F3A) includes angular clasts and is termed "breccia-conglomerate," in contrast to polymict breccia higher in the succession, in which the clasts are generally more rounded ("conglomerate-breccia"). Clasts of serpentinite and gabbro, mostly <5 cm in size, comprise ~30% of this interval (Fig. F5C, F5D–F5F). Occasional large clasts of gabbro (e.g., Fig. F5E) and serpentinite (e.g., Fig. F5F) are present within a finer grained matrix.
Thin sections of the polymict breccia-conglomerate confirm the predominance of serpentinite clasts set in a fine-grained calcareous matrix. Where little altered, the serpentinite clasts range from massive to foliated to mylonitic. Several clasts of massive serpentinite retain a primary magmatic foliation as defined by preferential orientation of spinel grains (e.g., interval 210-1277A-4R-1, 7–15 cm). Remnants of a ductile foliation are also preserved in several mylonite clasts. In the most highly altered clasts, serpentinite is mainly replaced by coarsely crystalline calcite. Gabbro clasts are locally present; these clasts range from little-altered gabbro to occasional clasts in which the gabbro is largely altered to blue chlorite. In addition, several clasts of recrystallized biotite-rich metabasic rocks were observed in thin section and these appear to have metamorphosed basic igneous rocks as their protoliths (e.g., interval 210-1277A-4R-1R, 39–43 cm).
The matrix, ~70% of the rock, is reddish brown silty sandstone cemented by calcite spar (e.g., Fig. F5E) and locally shows a vague sub-horizontal lamination defined by coarser grains. Sand-sized grains are mainly subangular to subrounded serpentinite and gabbro of the same composition as the larger clasts.
The polymict breccia-conglomerate is cut by numerous calcite veins (e.g., Figs. F5E, F5F, F6A). Steeply inclined fractures (Fig. F5E) are as wide as 2 cm and in some cases show evidence of several generations of carbonate precipitation. Some of the widest veins cut even the largest clasts and their enclosing matrix, showing that the entire unit was relatively well cemented before fracturing and cementation took place. Many of the larger clasts are rimmed by carbonate cement (e.g., Fig. F5F), possibly because cracks preferentially opened along the interfaces between clasts and matrix. In some samples several phases of crosscutting veins are visible (Fig. F6A).
Above the breccia-conglomerate is a distinctive interval of graded granulestone (Fig. F5G) cemented by calcite spar (Fig. F6A, F6B). This clastic sediment retains a primary contact with overlying lava, described in the next section.
The matrix-supported breccia-conglomerate, characterized by mostly angular clasts, was mainly derived from serpentinized peridotite and, to a lesser extent, from gabbro. The material includes serpentinite mylonite and gabbro cataclasite, which were probably derived from an underlying inferred extensional detachment fault zone, as outlined earlier. The serpentinite mylonite was veined by carbonate spar before being incorporated into the breccia. The matrix of finer grained material was derived from the same lithologies. The matrix-supported fabric and angular clasts are suggestive of an origin as cohesive mass flows in which little abrasion and rounding of clasts took place during local transport. This material is unlikely to have experienced more than one cycle of gravity emplacement because there are no signs of mixing of clasts of differing roundness or of reworking of previously deposited sediment. This texture contrasts with more texturally mature conglomerate-breccias higher in the succession that are interpreted as less cohesive pebbly debris flows that were reworked before final emplacement. The breccia-conglomerate shows evidence of high-angle fracturing that created a network of cracks that were infilled with calcareous sediment and cemented by calcite spar. This suggests that the entire recovered interval experienced horizontal axis extension that took place after emplacement and substantial lithification. The well-sorted granulestone above this polymict interval possibly represents material from the underlying, relatively cohesive serpentinite-rich debris flow that was reworked as a noncohesive pebbly debris flow before being overridden by a basaltic sheet flow.
An interval of internally brecciated massive basalt (Fig. F3A) is inferred to represent two lava flows, with a total thickness estimated at as much as 15 m (interval 210-1277A-2R-2, 69 cm, through 3R-4, 65 cm) (Fig. F5H, F5I, F5J, F5K, F5L, F5M), although the recovery was incomplete.
The base of the thicker, lower flow (marked as Flow 2 on Fig. F3A) exhibits a primary eruptive contact with clastic sediment, as noted above (Fig. F5G). Also, the top of the lower flow (interval 210-1277A-3R-2, 35 cm) is marked by a well-preserved chilled margin (Fig. F3A). The lower flow begins with an interval of massive basalt interspersed with minor amounts of concentrically laminated, angular to elliptical hyaloclastite cemented by coarse calcite spar. Localized reworking of basaltic material is present, but nonvolcanic clasts (e.g., gabbro or serpentinite) do not occur. The grain size of the lower flow gradually decreases upward from medium grained to very fine grained. The flow finally passes upward through an interval of vesicular basalt (1 cm thick) and green, crustose, laminar hyaloclastite (as thick as 0.9 cm). A devitrified glass texture was observed locally near the top of this flow (Fig. F6D).
The upper flow (interval 210-1277A-2R-2, 69 cm, through 3R-2, 37 cm) (Flow 3 on Fig. F3A) is uniformly fine grained. Its upper surface is brecciated, and spaces between clasts contain subrounded pebbles of calcite-cemented hyaloclastite (<1 cm) and angular basalt clasts (Fig. F5M). This basalt includes several thin intervals (several centimeters) of well-cemented medium-grained volcaniclastic sandstone containing small (centimeter sized) pebbles of green hyaloclastite (Figs. F5L, F6E, F6F).
Both basalt flows are cut by irregular fractures, as wide as ~1 cm, and are mainly filled with calcite spar. Partly cemented veins are lined with dogtooth spar. Larger fractures, as much as 3–4 cm wide, are infilled with poorly sorted silt-sized carbonate sediment and basaltic detritus (Fig. F5H, F5I, F5J, F5K). Weak planar lamination is defined by the input of fine sand-sized to granule-sized basalt and hyaloclastite (Fig. F5H). Small pebbles of concentrically laminated hyaloclastite show little sign of fracturing or abrasion and were cemented in place after percolating downward into open fractures (Fig. F5K, F5L). In one interval the basalt clasts and the matrix are stained a reddish color, especially along the margins of clasts. This possibly reflects the flow of oxidizing seawater after deposition (Fig. F5J).
Several samples of the sediment-filled fractures were studied in thin section. The "internal sediment" shows weakly developed parallel lamination highlighted by small basalt and hyaloclastite particles. In one clear example, primary sedimentary laminae are deflected over small projections in the surface of the cavity and ponded into small (millimeter sized) depressions (Fig. F6C). This internal lamination is subhorizontal, indicating that little tilting took place after the sediment was deposited.
The two lava flows (2 and 3) were perhaps emplaced quite rapidly as they are internally massive and show no evidence of pillow structure. This in turn suggests that the local seafloor was gently inclined, at least locally, and possibly resulted in ponding of lava. The breccia at the top of the upper flow resulted from a primary eruptive process to create hyaloclastite, coupled with some reworking of cooler basalt and hyaloclastite. Minor hyaloclastite is also present within Flow 2, suggesting that eruption was spasmodic. Both of these lava flows have been tectonically brecciated and contain subvertical neptunian fissures. This brecciation occurred after eruption, as many of the clasts are angular to subrounded, retain a jigsaw-type fabric, and lack glassy selvages. Exotic material (e.g., serpentinite) and evidence of sedimentary reworking are absent from within, or between, the two lava flows. This suggests that all of the breccia material was derived locally from the two lava flows. The relatively rounded nature of some basalt clasts probably reflects abrasion that resulted from faulting or gravity collapse.
After cooling and initial cementation, the tectonic fracturing of the lava flows allowed calcareous sediment, together with basalt and hyaloclastite grains, to filter downward through open cracks and fractures (Fig. F5H, F5I). This sediment can be compared with well-known geopetal fabrics that have formed as infills within cavities in lithologies such as carbonate rocks (e.g., Bathurst, 1971). Such "internal sediment" is a good indicator of the paleohorizontal (e.g., in carbonate reef complexes). The neptunian fissures are subvertical, whereas the lamination within the internal sediment is gently inclined (e.g., Fig. F5H; 104–107.5 cm). In places, the lamination is concave downward because of differential compaction (Fig. F6C); however, it is subhorizontal within the largest fissures. The basalt flows, therefore, cannot have been tilted significantly (less than ~10°) after the fissures opened on the seafloor. The fact that the volcanic-sedimentary succession was not significantly tilted at a later stage places constraints on the tectonic history of the site after volcanism and sedimentation ended, as discussed later in this paper.
At the base of this distinctive interval is a conglomerate-breccia that was termed "serpentinite breccia" in the Leg 210 Initial Reports volume (Shipboard Scientific Party, 2004b). It is composed of granule- to pebble-sized material (interval 210-1277A-2R-2, 40 cm, through 2R-2, 66 cm) (Fig. F3A). Clasts of serpentinite and gabbro (as long as 1.5 cm) are set in a poorly sorted fine-grained carbonate matrix of siltstone to sandstone (Fig. F6I). These clasts range from subangular to subrounded and are cemented by sparry calcite.
A notable interval, >1.6 m thick, of greenish cataclastic gabbro was recovered above this interval (210-1277A-2R-1, 12 cm, through 2R-2, 40 cm) (Fig. F5N). Crude foliation is defined by the preferred orientation of a chloritic matrix and by crude subparallel alignment of small (<2 cm) elongate gabbro clasts. Many individual gabbroic clasts are typically strongly altered. By contrast, larger, more equidimensional gabbroic clasts retain a primary magmatic fabric and show little or no preferred orientation. Thin sections show that variably altered, highly angular plagioclase and pyroxene crystals, and also gabbro fragments, are aligned within a foliated chloritic matrix (Fig. F6G, F6H). This entire rock was formed by tectonic fragmentation of gabbro (i.e., as porphyroclastic gabbro) and is not considered to have a sedimentary origin.
Short intervals of basalt recovered above this interval (210-1277A-2R-1, 0–10 cm) were interpreted by the Shipboard Scientific Party as isolated basalt pebbles (Shipboard Scientific Party, 2004b) (Fig. F3A). However, it is also possible that they record an additional thin basalt flow that was mainly not recovered.
A piece of intact calcite-cemented polymict conglomerate-breccia was recovered from interval 210-1277A-1W-2, 91–105 cm (Fig. F5O). The clasts within this breccia as large as 2 cm and range from angular to subrounded to occasionally well rounded. This sediment is normally graded with the largest clasts at the base, passing upward from pebble to granule to coarse sand–sized material. Several unusually large, relatively well rounded clasts are present toward the top of this depositional unit. In thin section, mainly sand-sized lithoclasts, including gabbro and serpentinite, are set in a fine-grained carbonate matrix. The matrix includes small detrital grains of serpentinite and iron oxide together with later stage coarser calcite spar; however, basalt and hyaloclastite were not observed within the matrix.
Above the conglomerate-breccia, several small well-rounded pebbles of cemented coarse clastic sediment were recovered (interval 210-1277A-1W-2, 81–90 cm), followed upsection by several isolated angular fragments of strongly altered coarse-grained gabbro (5 cm in size) (interval 210-1277A-1W-2, 75–80 cm). The different pebbles show either primary sedimentary fabric (i.e., sedimentary lamination) or tectonic fabric (e.g., foliation), respectively. One pebble of conglomerate-breccia is dominated by subrounded to subangular detrital grains of altered serpentinite and gabbro set in a calcareous matrix. The calc-siltite matrix is partially recrystallized to microspar-sized carbonate and cut by calcite veins (Fig. F6J).
The polymict conglomerate-breccia is interpreted as one, or several, subaqueous mass flows that are dominated by clasts of serpentinite and gabbro within a calcite-cemented matrix. The well-rounded nature of some of the clasts implies that extensive reworking occurred, in turn suggesting a significant distance of transport prior to final deposition (perhaps more than hundreds of meters). In the absence of evidence of rounding in a high-energy shallow-marine or nonmarine setting (i.e., neritic fossils are absent), it is probable that the rounding resulted from abrasion during subaqueous downslope sedimentary transport. The co-existence of both rounded clasts and angular clasts suggests sediment recycling, possibly involving reworking of preexisting debris flows. The source of the clastic material was mainly serpentinite and gabbro that experienced intense shearing beneath an inferred extensional detachment before being incorporated into the mass flows.
The angular, tectonically brecciated gabbro interval (>1.4 m thick) represents a block (i.e., an unusually large clast) that was derived from the inferred underlying extensional detachment that was originally exposed at the seafloor around Site 1277. The relatively large size of the block suggests a proximal source. The foliated cataclastic breccia originated as plagioclase-pyroxene gabbro that experienced subseafloor brecciation, shearing, and metamorphism prior to incorporation within this mass flow interval. The ductile matrix may have formed by a combination of cataclasis of gabbro and fluid-assisted alteration of the host gabbro to form chlorite and related hydrothermal and diagenetic minerals. The presence of a block of this size suggests that the exposed detachment was at least locally fractured to form more than meter-scale relief, thus providing an appropriate source.
The isolated pebbles of brecciated gabbro, serpentinite, and sedimentary conglomerate-breccia near the top of this interval probably represent clasts derived from one or more poorly cemented pebbly debris flows. These clasts may have been selectively recovered by drilling, whereas the enclosing matrix was lost. Some of these well-rounded pebbles are preserved within a matrix of carbonate cement, confirming that the rounding, where present, cannot be an artifact of drilling.
The polymict breccia and its enclosed gabbro cataclasite block are overlain by a lava flow, marked as Flow 4 in Figure F3A, that is made up of medium-grained gray aphyric basalt. This is the uppermost of the three volcanic intervals recovered. Massive basalt at the base contains chilled selvages composed of variably altered green hyaloclastite (Fig. F5P) together with pockets of calcite-cemented hyaloclastite and pink fine-grained calcareous internal sediment (calc-siltite) (Fig. F6L). The carbonate sediment is locally recrystallized or partially dissolved to form cavities (vugs) that were later lined with calcite spar (Fig. F6K, F6M).
The basalt is cut by anatomizing veins, as long as tens of centimeters long and several centimeters wide. Two generations of millimeter-sized calcite veins are visible, commonly intersecting at high angles. Numerous hairline cracks are filled with calcite spar. Some of these cracks later widened, allowing hyaloclastite mixed with fine basaltic grains to fill cavities and be cemented by sparry calcite (e.g., interval 210-1277A-1W-2, 32–40 cm) (Fig. F5P). There are also numerous small irregular calcite-filled veins, as long as several millimeters, of uncertain relative time relations.
The massive basalt is overlain by an interval of altered basalt, lava breccia, brecciated basalt, and hyaloclastite, which contain subvertical sediment-filled fissures (Fig. F5Q). This volcanic interval is dominated by greenish breccia composed of poorly sorted, well-indurated breccia and hyaloclastite (altered volcanic glass). Primary glass is commonly recrystallized or replaced by carbonate. The breccia includes angular fragments of aphyric basalt as large as tens of centimeters. The larger basalt clasts are randomly strewn through a matrix of greenish hyaloclastite sand to fine breccia. Clasts of variably altered basalt range in color from green to pale green through gray to black. Clasts of all sizes are cemented by calcite spar.
The brecciated basalt and lava breccias are cut by a prominent subvertical fissure (e.g., interval 210-1277A-1W-1, 40–60 cm) (Fig. F5Q). Part of this fill is shown on the left side of this core photograph. Two generations of sediment fill must have existed within the fissure. The first was soft, brownish, massive, fine-grained, apparently ferromanganiferous sediment. This is now mainly brecciated, but locally it remains in direct contact with coarse clastic host breccia (e.g., green reworked hyaloclastite in interval 210-1277A-1W-1, 56–58 cm). This shows that the host was consolidated and/or cemented at a relatively early stage and filled by the fine-grained sediment. After cementation, this first generation of sediment fill was brecciated and the preexisting fracture was reopened. A second generation of sediment (paler in color) then filtered in, composed of greenish gray calcite-cemented calcareous silt (calc-siltite) together with sand- to granule-sized grains of basalt and hyaloclastite. Some of these small detrital grains show evidence of abrasion and rounding.
Other fissures, in these cases cutting altered basalt, show evidence of filling with calc-siltite of similar composition to the pale second-stage clastic fill described above. These fills show a subhorizontal lamination defined by coarser and finer grains. One small, near-vertical, straight-sided vein, ~4 mm wide, is filled with fine-grained calc-siltite including hyaloclastite particles (Fig. F6N). Close inspection of the fine-grained sediment reveals gently tilted lamination (~20°) that could reflect tilting after deposition. Any pore space remaining after sediment filling, especially between hyaloclastite fragments, was filled with coarse carbonate spar.
Numerous millimeter-sized carbonate veins are continuous between the greenish volcaniclastic matrix and the basaltic clasts (i.e., they run from the one lithology into the other without a break and also cut the sediment infills). These veins indicate late-stage brittle fracturing that postdated the episodic filling of neptunian fissures.
The highest recovered interval of volcanic breccia consists of angular to subangular fragments of calcified basalt set within subrounded grains of green chloritic hyaloclastite (e.g., interval 210-1277A-1W-1, 10–28 cm; uppermost interval 5) (Fig. F5R).
These uppermost basalts (Flow 4) probably erupted episodically. The presence of inclined or curved glassy selvages might suggest the presence of pillow lavas; however, these features could simply represent cooling cracks. The presence of hyaloclastite reflects the interaction of hot lava with cold seawater. Hyaloclastite typically forms along the fronts of advancing lava flows that may then be overridden by lava or incorporated into lava flows. Hyaloclastite is, for example, abundant where lava is erupted on a sloping seafloor, increasing the rate and amount of interaction with cold seawater. In addition, lava breccias can form in a range of settings, which include gravity collapse of flow fronts and breakup of the solidified crusts of lava flows (Fisher and Schmincke, 1984).
After initial accumulation of lava breccia and its cementation by hydrothermal calcite, fracturing took place, opening subvertical fissures that allowed mainly silt-sized carbonate sediment to percolate downward, creating neptunian dykes. The first generation of calcareous sediment infill includes an apparently Fe-Mn rich hydrothermal metaliferous component. Following infill and cementation, a second phase of fracturing widened the early fissures. These cracks were infilled by a more homogeneous, pale-colored calc-siltite that includes detrital basalt and hyaloclastite grains.
Four possible origins of the calc-siltite are considered. The first is from background pelagic calcareous sediment that filtered down into open cracks. This is unlikely because microfossils (e.g., foraminifers) were not observed. The sediments show little sign of recrystallization and so it is unlikely that microfossils were dissolved during diagenesis. The second alternative is that the carbonate sediment was derived from the Newfoundland continental margin. This is most unlikely, as the fissure-filling sediment lacks terrigenous components. A third possibility is that the fine carbonate sediment is a primary hydrothermal precipitate. However, this is clearly not the case because the sediment is finely laminated and shows other sedimentary features (e.g., compaction into small depressions). A fourth alternative, favored here, is that the carbonate sediment was derived by reworking of early calcite cement and calcite-cemented basalt associated with faulting. This carbonate material was derived from within the lava flows because there is no evidence of exotic material derived from above the lava flows. The initial carbonate could have been precipitated as veins and/or as calcite-cemented hyaloclastite. When faulting took place the early carbonate cement was comminuted and filtered down to deeper levels within neptunian fissures as carbonate silt. In one scenario, brittle fault-controlled fissures opened and were lined with carbonate cement. With further faulting the cement and the host volcanics were fragmented and abraded, creating fine-grained sediment composed of silt- to sand-derived carbonate, basalt, and hyaloclastite that filtered downward to create the observed neptunian infill.
A single piece of well-indurated brown ferromanganiferous clastic sediment at the top of the cored section is composed of poorly sorted medium- to coarse-grained sandstone and includes scattered granule-sized lithic grains (Sample 210-1277A-1W-1, 5–9 cm) (Fig. F5R). The clastic sediment includes variably altered basalt, gabbro, and carbonate, together with feldspar and rare spinel. In common with the underlying sediments at Site 1277, terrigenous sediment (e.g., detrital quartz and mica) is conspicuously absent. The clastic sediment forms a colloform-textured microlaminated ferromanganese crust, comprising centimeter-scale laterally linked domes. An orange botryoidal lamination appears to have formed initially (iron oxide rich), followed by darker and more regularly laminated material (manganese oxide rich). The crust contains agglutinated benthic foraminifers, encrusting wormlike tubes, and several tiny planktonic foraminifers, none of which were age diagnostic (Shipboard Scientific Party, 2004b).
The top of Core 210-1277A-1W (interval 1W-1, 1–3 cm) is a micaceous sandstone that may be lithified sediment from the overlying sedimentary section, or it may be a glacial dropstone (Fig. F5R).
This coarse, clastic, metal-rich sediment records a period of submarine erosion or nondeposition on the igneous basement. The clasts are similar to the composition of underlying coarser polymict sediments, including material derived from both mafic and ultramafic rocks. The poorly sorted nature of the ferromanganiferous sediment is consistent with a relatively local provenance from the crest of Mauzy Ridge. The presence of ferromanganese oxides suggests accumulation in a strongly oxidizing setting. The planktonic foraminifers indicate an open-marine setting that was possibly near the calcite compensation depth, in view of the paucity and poor preservation of the calcareous microfossils. The absence of terrigenous sediment suggests that the area was isolated from a supply of terrigenous turbidites of the type drilled closer to the Newfoundland margin at Site 1276.