As at other Ocean Drilling Program (ODP) drill sites within the Costa Rica forearc wedge, the sediment recovered at Site 1255 is dominated by fine-grained clastic facies. Dark greenish gray claystones with variable proportions of biogenic pelagic sediments and coarse siltstones and sandstones were recorded. Coring at Site 1255 resampled the interval above and below the décollement seen at Site 1043 (Fig. F5) (Shipboard Scientific Party, 1997) and so penetrated the lowermost lithostratigraphic division of the forearc wedge defined as Unit T1 at Site 1043. Unit T1 represents a lateral equivalent of lithostratigraphic Subunit P1B at Sites 1254 and 1040 (see "Lithostratigraphy" in the "Site 1254" chapter) but was distinguished as the toe of the forearc wedge on the basis of the slightly different sediment facies (i.e., intervals of clay and silty clay interbedded with thin intervals of matrix-supported breccia) (Shipboard Scientific Party, 1997).
As at Sites 1254 and 1040, the transition from the forearc wedge unit into the underthrust section (Subunit U1A) is best charted by the variability in diatom abundance seen in microscopic smear slide analysis. This is because in the cut core surface the two units both appear to be dominated by structureless greenish gray claystones with minor variations in the proportion of clay that cause slight changes in color (Fig. F6). Certainly the color scanner data show no strong shift in either lightness or chromaticity downsection across the décollement (Fig. F5). Clastic units are noted both above and below the unit boundary, making a ready distinction difficult.
Microscopic analysis shows a downcore increase in the abundance of diatoms (Fig. F7). These represent 1% or less of the total sediment toward the top of Section 205-1255A-2R-1 (132.70 mbsf), then increase to 5%-8% of the total sediment volume in Section 205-1255A-2R-3 (134.42 mbsf) (Fig. F8B), and finally rise to >20% below Section 205-1255A-3R-2, 18 cm (144.08 mbsf; Fig. F9). The full smear slide analyses are presented in "Site 1255 Smear Slides."
Figure F8D shows a typical example of the sediment with >20% diatoms from within the underthrust lithostratigraphic Subunit U1A. This sediment contains whole well-preserved diatoms and silicoflagellates, as well as rare radiolarians within the otherwise homogeneous clay-rich sediment. There is a clear transition between largely clastic diatom-free claystone at shallow depths in the forearc wedge through a zone of low (5%-8% in Section 205-1255A-2R-3) diatom concentration into a more diatom-rich hemipelagic sequence below. We choose to place our unit boundary in Section 205-1255A-3R-2, 18 cm (144.08 mbsf), the level below which diatoms are very abundant, following the scheme followed at Site 1043 (Shipboard Scientific Party, 1997). The top of lithostratigraphic Subunit U1A is 144.08 mbsf at Site 1255, compared to 150.57 mbsf in Hole 1043A. We do not consider the difference between these two depths to be significant because the proportion of recovery in each core is rather low. The recovered material in Core 205-1255A-3R is necessarily curated to the top of the drilled interval, whereas in reality it may have been recovered from anywhere within the upper 5.31 m of that interval. Given that the real uncertainty in depth is greater than the apparent mismatch, there is no reason to require a difference in décollement depths between these neighboring sites.
Real differences between Sites 1043 and 1255 are apparent in the sedimentary facies of the underthrust sediments. The sediments under the décollement at Site 1255 are more clastic than the purely hemipelagic diatomaceous oozes found at Site 1043. At the unit boundary itself, the underlying sediments comprise a graded sand unit with a microconglomeratic base. The lower part of Core 205-1255A-3R shows several dark-colored muddy sand/silt units, either normally graded or massive with sharp tops and bases suggestive of a redeposited origin. Of particular note is the presence of a 4-cm-thick conglomerate layer in interval 205-1255A-3R-3, 97-101 cm (146.20 mbsf), with clasts up to 1 cm in diameter suspended in a coarse sand matrix and grading up into a 15-cm-thick sandstone interval overlain in turn by a 1-cm-thick claystone (Fig. F10).
We interpret the sandstone unit as the product of deposition from a turbidity current, likely within the trench axis given the stratigraphic location at the top of the underthrust section. The presence of common glauconite, mica, quartz, and feldspar grains within the sand supports an origin of these sediments by erosion of the Costa Rica arc and redeposition downslope toward the trench. Other redeposited clastic intervals include intervals 205-1255A-3R-3, 35-60 cm (145.58 mbsf), and 3R-3, 66-84 cm (145.89 mbsf). Because the total thickness recovered below the décollement at Site 1255 is so small (2.3 m), it is not clear how thick these clastic units are. However, the fact that they were not recognized at Site 1043 indicates that this may be a thin interval. Possibly, these sediments were not recovered at Site 1043. Alternatively, they may be tectonically excised by underthrusting at Site 1043 or they may never have accumulated there because of lateral facies changes over short distances in this trench environment. The current data set does not allow a definitive answer to these possibilities. Nonetheless, we define the underthrust section at Site 1255 as lithostratigraphic Subunit U1A, contrasting with the purely hemipelagic Subunit U1B underlying the décollement at Site 1043.
Further evidence for redeposition in this sequence is seen in interval 205-1255A-3R-1, 144-148 cm (143.84 mbsf), just above the lithostratigraphic unit boundary, where a couple of angular light-colored carbonate blocks are identified (Fig. F11A). These blocks are up to 18 mm in diameter and are suspended in a matrix of claystone with silt. Significant recrystallization of the limestone is observed under microscopic analysis (Fig. F11B), with needlelike crystals of calcite composing much of the matrix. A minor proportion of the rock is composed of microcrystalline carbonate grains, up to 500 µm in diameter, together with trace amounts of silt-sized altered volcanic material (palagonite). The cobbles may be defined as wackestone fragments. The lithology appears to be exotic to the background sediment and by comparison with similar clasts found in the overthrust forearc wedge sequences at Site 1254 (see "Lithostratigraphy" in the "Site 1254" chapter), we interpret these blocks to be redeposited fragments of shallow-water carbonate emplaced on the lower trench slope within a muddy debris flow.
The proportion of carbonate material in the sediment recovered at Site 1255 is low (<5% according to smear slide analysis and shipboard chemical analysis) (see "Organic Geochemistry"), but in one location in interval 205-1255A-3R-2, 141-146 cm (145.31 mbsf), there is a unit of nannofossil chalk with clay, deformed by drilling and located immediately below the sandy unit at the top of lithostratigraphic Sublayer U1A (Fig. F12). The chalk itself is structureless but is underlain by a thin interval of planar laminated clay. In microscopic analysis the sediment is revealed to be largely composed of nannofossils and especially Discoasters (Fig. F8C) with minor volumes of clastic material (palagonitized volcanic glass, quartz, opaque minerals, and mica). The deposit is apparently a pelagic interval within the dominantly redeposited facies of lithostratigraphic Subunit U1A.
Because the bulk of the biogenic input into this sequence is siliceous and there is no evidence for redeposition of this bed, the presence of the chalk interval must reflect a brief deepening of the carbonate compensation depth (CCD). Van Andel (1975) placed the recent Pacific CCD close to 4.5 km depth, although it may be as much as 1 km shallower in regions of upwelling and close to continental margins, as is the case here (Balsam, 1982). The presence of a pelagic carbonate bed of this type thus represents a major, albeit brief, excursion in the chemistry and productivity of the Costa Rican margin.
Unlike other sites drilled during Leg 205, the sediments at Site 1255 contain no primary volcanic tephra beds. However, volcanic material does comprise a significant proportion of the clastic sediments, which are largely derived from the erosion of the Costa Rica volcanic arc. Smear slide analysis shows 10%-15% to be the typical range for the proportion of either fresh volcanic glass shards, palagonitized glass, or zeolites formed from the alteration of volcanic glass. Locally, that proportion can increase to >25%, with specific intervals being found in Sections 205-1255A-2R-1, 10 cm (132.80 mbsf); 2R-3, 6 cm (134.48 mbsf); and 3R-1, 0 cm (142.40 mbsf). A compilation of the smear slide information from Sites 1043 and 1255 shows that although volcanic glass is present in both wedge and underthrust sediments, it is most abundant in the forearc wedge sediments (Fig. F8).
Diagenesis at Site 1255 seems to be developed to a similar degree in both forearc wedge and underthrust sediments. The sediment is firm and weakly indurated but not yet fully lithified, equivalent to a chalk grade for carbonate sediments. An exception to this lies in interval 205-1255A-2R-1, 0-15 cm (132.70 mbsf), where a cobble of well-lithified massive dark brownish gray siltstone is recognized (Fig. F13). This block is carbonate cemented and may not be in place, coming at the top of the recovered interval.
Major and trace element analyses of three sediment samples were conducted aboard ship following the methods described in "Igneous Petrology" in the "Explanatory Notes" chapter. Major, minor, and trace element analyses are presented in Table T2. Analyses were made on freeze-dried bulk samples that are a mixture of sediment particles and sea salts (see "Igneous Petrology" in the "Explanatory Notes" chapter). The data have not yet been corrected for porosity and pore water contribution to the bulk sediment composition. Major element concentrations show no discernable downsection trend and lie within the range of values determined from across the décollement at Sites 1040 and 1254. There is no sediment chemistry available from the closest borehole at Site 1043.
In terms of the trace element composition, a pattern is discernable across the lithostratigraphic unit boundary at 144.08 mbsf. Barium, and to a lesser extent Ni, increase downsection into lithostratigraphic Subunit U1A. This pattern was observed at Sites 1040 and 1254, where Ba concentrations average 426 ppm above the décollement but shift to an average value of 1575 ppm for samples below that level. This is consistent with the proposed location of the unit boundary between intervals 205-1255A-2R-2, 0-3 cm, and 3R-2, 87-127 cm. High concentrations of Ba can be driven by the presence of volcanic materials in the sediment. However, the lack of any change in the SiO2 and K2O concentrations, together with the smear slide analyses, suggest that volcanic material content is not a key control on the Ba chemistry in this case. We interpret the increased Ba concentration in the sediment below the unit boundary to reflect higher authigenic Ba within lithostratigraphic Subunit U1A.
The results of X-ray diffraction (XRD) analyses of randomly oriented bulk sediment powders from Leg 170 Site 1043 and Leg 205 Site 1255 are presented. Shipboard Leg 170 XRD data were not evaluated previously, nor were mineral peaks identified. Data from Leg 170 diffractograms were analyzed for the first time during Leg 205 and are thus considered here. Site 1043 peak intensity and peak area data are listed in Table T3, and peak area ratios are listed in Table T4. Site 1255 peak intensity and peak area data are listed in Table T5, and relative abundance data are listed in Table T6.
The section recovered from Hole 1255A can be readily split into lithostratigraphic Unit T1 and Subunit U1A at the level of 144.08 mbsf. The division marks the sharp junction between a series of claystones with silts and low or no diatoms above and an underlying series of diatom-rich claystones with interbedded silts, sands, and even occasional fine-grained conglomerates. The clastic sediments in the underthrust section (Subunit U1A) differ from the purely hemipelagic diatom ooze recorded at Site 1043 (Subunit U1B), although the degree of recovery and uncertainties in the precise depth within the cored interval from which these sediments were taken are sufficient to suggest that these sediments may have been developed at Site 1039 but simply not recovered. Blocks of reworked carbonate in the overlying lithostratigraphic Unit T1 indicate that these sediments are mostly debris flows, derived originally from shallow nearshore environments. The underthrust section represents a trench depositional setting with turbidite silts and sands interbedded with biogenic hemipelagic mudstones.