Leg 206 is the first leg of a two-leg program to sample a complete section of upper oceanic crust through the extrusive lavas, sheeted dike complex, and into the upper oceanic gabbros. Site 1256 was drilled into ~15-m.y.-old crust formed during an episode of superfast accretion at the East Pacific Rise. Seismic observations (e.g., Purdy et al., 1992) suggest an inverse relationship between spreading rate and the depth to axial low-velocity zones at mid-ocean ridges. These low-velocity zones are hypothesized to be partially molten magma chambers that, when frozen, will form the uppermost gabbros. If this is the case, gabbroic rocks should be at their shallowest in superfast spreading crust, and drilling crust formed at this rate will provide the best opportunity to sample a complete section of upper oceanic crust. At least in terms of seismic structure, ocean crust formed at fast spreading rates is relatively simple and probably best conforms to the ideal stratigraphy envisioned for Penrose-type ocean crust. Although perhaps only 20% of the present-day mid-ocean ridges are spreading at fast rates (>80 mm/yr), ~50% of the present ocean basins formed at this style of ridge axis. Hence, one deep drill hole through a complete upper crustal section can be reasonably extrapolated to describe a significant portion of the Earth's surface.
Before we embarked on our major aims of drilling operations at Site 1256 it was first necessary to thoroughly characterize the sedimentary overburden above the oceanic crust and determine the stratigraphy, sedimentation and mass accumulation rates, and the role of fluid processes in the sedimentary blanket. Following the sampling of the sedimentary section, operations then concentrated on the underlying basement, and a reentry cone and casing string to basement could be installed to enable deep drilling of the upper oceanic crust.
Four holes were drilled at Site 1256 (Table T4). Hole 1256A was a single piston core, shot to establish the mudline at Site 1256. In Hole 1256B, the complete sedimentary sequence was drilled by APC and XCB coring, and the very uppermost basement was tagged at 250.7 mbsf. Further samples of the sedimentary sequence were recovered by RCB drilling from 220.1 mbsf in Hole 1256C, and this pilot hole was drilled into the upper basement to a depth of 340.3 mbsf (89.6 m subbasement). After the successful installation of the reentry cone and casing strings to basement in Hole 1256D, coring of the lavas resumed and continued until the end of the leg, when the hole was prepared for wireline logging. Hole 1256D was opened to a total depth of 752 mbsf, or 502 m into basement. Hole 1256D was left clear of junk and in excellent condition for deepening during a future scientific drilling expedition.
Drilling in Holes 1256A, 1256B, and 1256C recovered a complete section of the sedimentary stratigraphy at Site 1256 (Fig. F19). The sediments are subdivided into two principal lithologic subdivisions, although more complex structure is revealed by the paleomagnetism, geochemistry, and physical property investigations. Unit I is clay rich with a few carbonate-rich intervals, whereas Unit II is predominantly biogenic carbonate.
Lithologic Unit I (0–40.6 mbsf) comprises clay-rich sediments with Pleistocene dark brown to yellow-brown silty clays (Subunit 1A) (Table T6) with calcareous nannofossils, overlying light olive-gray to yellowish brown Pliocene to upper Miocene sandy clays to silts with calcareous nannofossil–rich intervals (Subunit IB). The Subunit IA/IB transition is taken as the Pleistocene/Pliocene boundary with a change in the relative contribution of clastic and biogenic components apparent in the color reflectance and magnetic susceptibility. Bioturbation is moderate to abundant throughout lithologic Unit I with Planolites, Chondrites, Zoophycos, and Skolithos the most common trace fossils identified. Rare volcanic ash is present in Unit I, and a quartz- and feldspar-rich ash layer was recovered in interval 206-1256B-3H-2, 34–36 cm.
Lithologic Unit II (40.6–250.7 mbsf) (Table T6) comprises upper Miocene to middle Miocene light greenish gray to dark gray calcareous nannofossil ooze with varying amounts of clay and other microfossil groups. Diatoms are a significant minor component that increase in abundance with depth, forming a siliceous biogenic ooze at 85 mbsf. With the exception of two intervals, the remainder of the sedimentary section is dominated by light greeenish gray to white, nearly pure calcareous nannofossil ooze. Bioturbation is common throughout Unit II, and trace fossils include calcified solid and rind burrows as well as Planolites and Skolithos.
The interval from 111 to 115 mbsf consists of a laminated diatom mat comprising very abundant tubular diatom tests with minor nannofossils. From 140 to 195 mbsf the nannofossil ooze has significant but variable clay and diatom components, which are most clearly recognized by changes in the color reflectance and density.
Chert nodules are a common feature in the sediments from ~111 mbsf down to the basement. Distinct chert layers at 111 and 158 mbsf were identified by increased resistivity and low porosity in the downhole measurements, and very poor recovery in the lowermost sediment cores indicates that further chert layers are present between 230 mbsf and the basement. Red-brown iron oxide–rich silicified sediments directly overlie the basement (within 1 m), and these may be recrystallized metalliferous sediments.
Rare 5-cm-wide dark greenish gray granular glauconite bands were identified at 76–82 and 217–225 mbsf. At the base of the sedimentary sequence the nannofossil ooze has a distinct pale bluish hue, and X-ray diffraction determined a significant mica component, possibly celadonite or glauconite, in the sediment, perhaps derived from the recrystallization of biogenic opal and hydrothermally derived iron oxides.
A biostratigraphic framework for Site 1256 has been established by the inspection of calcareous microfossil assemblages in core catcher and additional samples from the sedimentary sequence. Calcareous nannofossils are generally abundant and moderately to well preserved, albeit with some fragmentation. Assemblages above ~25 mbsf are affected by etching and those below ~118 mbsf show the effects of dissolution and overgrowth. There was no evidence for significant reworking of fossils, and more than a dozen nannofossil datums were determined, providing a modest biostratigraphic resolution for the Pleistocene through middle Miocene (Table T7).
The biostratigraphic sequence at Site 1256 is generally similar to those recorded from nearby sites in the Guatemala Basin, Sites 844 and 845 (Mayer, Pisias, Janecek, et al., 1992). Sedimentation rates are discussed in greater detail below, but the drastic decrease from high (>35 m/m.y.) sedimentation rates in the eastern equatorial Pacific in the middle Miocene to much lower (8–14 m/m.y.) rates in the late Miocene, referred to as the "carbonate crash" (Farrell et al., 1995), is also observed at Site 1256.
Alternating-field demagnetization of sedimentary split core sections and discrete samples was used to construct a magnetostratigraphic record of the sedimentary overburden at Site 1256. Nearly all chrons and subchrons are recorded from Chron C1n (Brunhes Chron; 0.0–0.780 Ma) through most of Subchron C5n.2n (9.920–10.949 Ma) (Fig. F20, F21; Table T8). The termination of Subchron C5n.2n (9.920 Ma) is clearly identified at 92.53 mbsf, but the onset of Subchron C5n.2n (10.949 Ma) could not be identified, either due to a coring gap or poor resolution of the characteristic remanent magnetization below 110 mbsf. From 110 mbsf to basement, the sediments have extremely weak intensities, resulting in a paleomagnetic signal dominated by noise. Thus, no polarity stratigraphy could be determined for this part of the sedimentary section.
The age assignments from Site 1256 magnetostratigraphy are in very good agreement with the biostratigraphic constraints from calcareous nannofossil datums (Fig. F22). The magnetostratigraphy provides higher resolution in the late Miocene up to the Brunhes/Matuyama reversal at 0.78 Ma, whereas the biostratigraphy provides constraints within the late Pleistocene, where there is an absence of polarity reversals, and in the middle Miocene, where the paleomagnetic signal was poorly resolved.
Sedimentation rates vary from ~6 to 36 m/m.y., with the rate being about four times faster in the middle Miocene than the average rate during the late Miocene to present (Fig. F23). Linear sedimentation rates were estimated for five intervals (Fig. F22) within which the slope of the age vs. depth data from the magnetostratigraphy and biostratigraphy was constant or nearly so (Fig. F22). The lowermost linear sedimentation rate is 36.4 m/m.y., calculated for interval 9.92–13.6 Ma (95–212.65 mbsf). When this rate is extrapolated to the basement at 250.7 mbsf, the mean age obtained for the basement is 14.6 Ma, consistent with the ~15-Ma age of the oceanic crust estimated from marine magnetic anomalies (Fig. F9).
The high sedimentation rate in the middle Miocene can be attributed to the productivity being very high while the site was near the paleoequator and to complete preservation on young, shallow seafloor above the lysocline. The more recent slower rates can be attributed to lower productivity away from the equator and to partial dissolution after the seafloor subsided through the carbonate compensation depth (CCD). The rapid decrease in sedimentation rate in the late Miocene, however, requires an influence additional to the northward drift and subsidence of the Site 1256 ocean crust. A similar pattern of sedimentation rate variations was observed at Sites 844 and 845, with the event that occurred at the end of middle Miocene being referred to as a carbonate crash (Farrell et al., 1995; Lyle et al., 1995). The carbonate crash extended from ~11.2 to 7.5 Ma, with the crash nadir at ~9.5 Ma in the equatorial Pacific (Farrell et al., 1995). Presumably, the beginning of the carbonate crash at Site 1256 corresponds to the base of the diatom mat (115 mbsf) that has an age of ~10.9 Ma. The carbonate content in this interval is ~12 wt%, whereas it averages ~79 wt% below. Although the density of carbonate analyses for Site 1256 is relatively low, the crash nadir would presumably occur in the vicinity of the extreme carbonate low (0.25 wt%) at 89.55 mbsf, which has an age of ~9.6 Ma (within Subchron 4Ar.2n). Late Miocene sedimentation rates in the tropical eastern Pacific are complex in detail and must reflect changes in either productivity or the CCD (Farrell et al., 1995; Lyle et al., 1995). Similar carbonate crash events are also observed in the Caribbean, and it has been suggested that the PCO2 of ocean waters in this region may have been influenced by either the onset of North Atlantic Deep Water formation or the partial closing of the Panama Gateway (Lyle et al., 1995; Shipboard Scientific Party, 1997; Roth et al., 2000).
The sediment deposited at Site 1256 is composed of calcium carbonate, terrigenous grains, and biogenic silica with no discernible metalliferous component (Fig. F24). Total organic carbon concentrations are low throughout the sedimentary sequence. Calcium carbonate is dominant below 115 mbsf, although at 187 mbsf the terrigenous component increases to 20%. At ~115 mbsf, biogenic silica is the most abundant phase. Above 115 mbsf, the proportion of terrigenous material increases toward the surface and is the most important component in lithologic Unit I (above 40.6 mbsf).
Ba/Ti ratios are commonly used as a chemical proxy of the biogenic productivity in the overlying waters because sedimentary calcium carbonate, opal, and organic carbon are commonly altered during diagenesis. Ba/Ti ratios are between 2 to 10 in the upper 112 m of Site 1256 and values below 112 mbsf are within the range of 20 to 30, typical of the eastern equatorial Pacific Ocean (Fig. F25). There was a significant decrease in biogenic production following an extended episode of high productivity between 14.6 and 10.8 Ma, with the exception of a brief interval of low productivity at 12.9 Ma. More significantly, productivity decreased significantly at 10.8 Ma and has remained low to modern day. This change in productivity could be a response to the northward movement of the Cocos plate away from the equator, where productivity and, hence, Ba/Ti are commonly higher.
The primary influence on the interstitial water chemistry at Site 1256 is diffusion between seawater and basement fluids (Fig. F26). A chert bed located at 159 mbsf and observed in the downhole logs is continuous enough to form a low-diffusivity barrier that causes abrupt jumps in cation (magnesium and calcium) concentrations in many of the depth profiles. The concentrations of lithium and potassium mimic those of magnesium in the interstitial waters and are strongly influenced by diffusion between seawater and basement.
The low organic carbon content limits the extent of pore water sulfate reduction (SO42– > 17.5 mM), as seen in decreases in alkalinity in the pore waters with depth. Alkalinity, conversely, is reflecting the dissolution and reprecipitation of biogenic calcite, and this process is also shown by the increases in the Sr/Ca ratio of the pore waters with depth. Silica concentrations clearly illustrate the dissolution of biogenic organisms during diagenesis and the precipitation of chert.
Changes in physical properties are mostly gradual downhole, with the exception of those properties that are sensitive to the major lithologic and compositional changes that occur at the lithologic Unit I/Unit II boundary (40.6 mbsf), at the diatom mat (111–115 mbsf), and ~40–45 m above basement (at ~205–210 mbsf), below which the occurrence of chert and glauconite increase. Density and porosity are dominated by gradual compaction, with lesser effects of grain composition (Fig. F27). P-wave velocity is nearly uniform and low at ~1540 m/s, with a slight increase below 205 mbsf. Magnetic susceptibility is moderate in the clay-rich parts of lithologic Unit I and drops to very low in the nannofossil oozes.
Downhole temperatures were measured during APC coring of Hole 1256B at depths of 53.6–158.1 mbsf, plus in the bottom water. Heat flow values average 113 mW/m2, with a slight but possibly significant decrease downhole (Fig. F28). This value is close to the predictions for conductive cooling of oceanic lithosphere, implying that hydrothermal circulation is no longer a major mechanism of heat transport at Site 1256.