Sediment composition is now considered on a site-by-site basis, working from north to south across the northern rift margin (Sites 1115 and 1109), followed by the rift basin (Site 1118), and then the southern rift margin (Site 1108). The sediments from the Moresby Seamount are considered last (Sites 1114 and 1116).
Site 1115 is located on the northern rift margin (Fig. F2). This site spans the longest time interval of any drilled during Leg 180, from middle Miocene-Holocene time (Fig. F5). The oldest sediments form the uppermost part of a middle Miocene inferred forearc succession that shallows upwards and passes unconformably into a late Miocene nonmarine, fluvial to lagoonal succession. Latest Miocene-Pleistocene sediments record a shallow-marine succession that deepened progressively upwards into a dominantly clastic and hemipelagic succession that was deposited during subsidence of the Woodlark rift. Forty-seven samples were analyzed from this site (Table T2). Fine-grained hemipelagic sediments are present within most of the lithostratigraphic units recognized by the Shipboard Scientific Party, as outlined below:
Unit I of Pleistocene age (0-4.4 meters below seafloor [mbsf] in Hole 1115A and 0-35.7 mbsf in Hole 1115B) contains abundant nannofossil ooze, nannofossil silty clay, calcareous clay, and volcanic ash. The background pelagic sediment is dominated by nannofossils, planktonic foraminifers, radiolarians, and shell fragments. Mineralogy, as determined by shipboard XRD, is mainly calcite, quartz, and aragonite. Mainly siliceous volcanic glass is interpreted as primary air fall tuff. Shore-based XRD revealed abundant calcite, moderate amounts of quartz, and minor amounts of albite, aragonite, kaolinite, and pyrite (Table T1).
Three samples from Unit I were chemically analyzed (Table T2). The sample from the shallowest depth (6.89 mbsf) is very calcareous (42.41 wt%), whereas another, from 17.03 mbsf, is only weakly calcareous (1.94 wt% CaO). The major elements Al2O3 (<13.48 wt%), Na2O (<5.24 wt%), and K2O (<3.97 wt%) exhibit relatively high values. Of the trace elements, the following show relatively elevated values: Th (11 ppm), Ba (538 ppm), and Zr (<546 ppm). In addition, Nb (<15 ppm), and REEs exhibit high values (i.e., Y [<58 ppm], Ce [<92 ppm], and Nd [<42 ppm]). Values include the highest recorded for these elements in the Woodlark Basin. By contrast, Cu, Ni, and Cr are relatively depleted.
A Ca-poor sample (from 17.05 mbsf) is interpreted as a volcanic ash. This sample is enriched in Al2O3, Na2O, K2O, Th, Ba, Zr, and REEs compared to the interbedded clayey sediments. By contrast, trace metals (i.e., Cu, Ni, and Cr) are again relatively depleted. Unit I is inferred to contain volcanic ash of a distinctive high-K composition, correlated on the basis of electron microprobe studies, with high-K calc-alkaline volcanic rocks located in the Dawson Strait, Moresby Strait, or Dobu Seamount (Smith et al., 1977; Smith and Johnson, 1981; Stolz et al., 1993; Lackschewitz et al., 2001).
Unit II of late Pliocene-Pleistocene age (35-149 mbsf) exhibits a transition from mainly ooze to clay with less ash but relatively more volcaniclastic layers. Near the top, clay-rich sediments contain common bioclasts, foraminifers, nannofossils, pteropods, and scattered shell fragments. Smear slides exhibit quartz, feldspar, volcanic glass, nannofossils, foraminifers, and sponge spicules. Shipboard XRD revealed mainly calcite, quartz, and plagioclase. In addition, shore-based XRD revealed abundant calcite, moderate amounts of quartz, and minor amounts of aragonite, albite, pyrite, and kaolinite. One sample contains abundant albite (Table T1).
Eight samples were analyzed from Unit II. All are strongly calcareous (CaO 27.40-35.48 wt%; Sr <1688 ppm). SiO2 values are variable (5.50-10.67 wt%). MnO values are high but only relative to the low values in Unit I (to 2.68 wt%). On the other hand, K2O and Th are low relative to Unit I. Values of Ni (<111 ppm), Cr (<144 ppm), and Zr (<265 ppm) are relatively high.
The above data are consistent with the presence of a calcareous volcanogenic mud composed of calc-alkaline volcanic rocks, in contrast to the high-K volcanic ash of Unit I.
Unit III, of early? to middle Pliocene age, extends from 149.7 to 293.1 mbsf in Hole 1115B and from 283.2 to 388.5 mbsf in Hole 1115C. This is the thickest single lithostratigraphic unit at Site 1115 (240 m) and is marked by the appearance of discrete thin graded beds of volcaniclastic sands within calcareous silty clay. CaCO3 values generally decrease from >30 wt% to ~20 wt% below 250 mbsf (Taylor, Huchon, Klaus, et al., 1999). The physical composition of the clayey sediments is similar to Unit II. Smear slides revealed quartz, feldspar, mica, calcite, nannofossils, planktonic foraminifers, sponge spicules, and volcanic glass. Thin sections of impregnated clay-rich sediments confirmed the above composition, with the addition of plagioclase, hornblende, biotite, lithic fragments, and minor pyrite. Shipboard XRD revealed large amounts of calcite; also minor quartz, plagioclase, chlorite, and illite; rare ankerite, amphibole, and occasional dolomite. Carbonaceous debris appears lower in the unit. Shore-based XRD revealed abundant quartz, albite, and calcite, together with moderate amounts of phlogopite, tremolite, and pyrite.
Nineteen analyses were carried out on clayey sediments from Unit III. Compared with Unit II (see "Lithostratigraphic Unit II"), these sediments are less calcareous, whereas SiO2, Al2O3, and TiO2 are higher. Values of most metals and REEs are little changed from Unit III but, on average, slightly higher, commensurate with lower biogenic carbonate contents. Thus, the thick Unit II shows a similar, mainly volcanogenic, provenance to Unit III (i.e., basic to intermediate composition source volcanics) but with a reduced biogenic input.
Unit IV is dominated by calcareous sandy silty claystone of early to middle? Pliocene age (388.5-417.30 mbsf). Observations of the cores suggested that the clayey sediments are poorly sorted and quite variable in grain size. Smear slides and shipboard XRD revealed similar compositions to Unit IV above. CaCO3 values are similar to those from the base of Unit III (Taylor, Huchon, Klaus, et al., 1999).
Two samples that were chemically analyzed from this unit show little significant difference from Unit I (see "Lithostratigraphic Unit I"). One of the main reasons for recognizing Unit V on the ship was its slightly coarser grain size with increased silt and sand compared to Unit IV. However, the fine-grained sediment composition remained unchanged between Units IV and V.
Unit V (417.3-474.9 mbsf) is mainly sandstone of early to middle Pliocene age, in which only minor siltstone/claystone occurs, mainly in the upper parts of graded turbiditic sandstone. Smear slides exhibit quartz, plagioclase, biotite, rock fragments, volcanic glass, amphibole, bioclasts, and pyrite. Shipboard XRD revealed plagioclase and quartz, plus minor chlorite, illite, pyrite, amphibole, and aragonite. Samples were not chemically analyzed from this unit.
Unit VI (479.9-513.4 mbsf) is mainly sandy siltstone to silty sandstone of latest Miocene to early Pliocene age. There is a decrease in grain size compared to Unit V. Little claystone is present. The finest grained sediment present is calcareous and clayey siltstone. Four samples analyzed from Unit VI are compositionally similar to the clayey sediments of Unit IV. However, several samples exhibit unusually high values of Cr (<304 ppm) and Zr (<229 ppm). These data suggest sediment input from mafic or ultramafic igneous sources (see "Discussion").
Unit VII (513.4-551.8 mbsf) is mainly siltstone of late Miocene age. Silty claystone makes up only a very minor component of the unit, which is mainly siltstone and sandstone. Smear slides revealed the presence of quartz, plagioclase, rare volcanic glass, amphibole, calcite, pyrite, rare nannofossils, and rare foraminifers. Shipboard XRD indicated the presence of plagioclase, calcite, quartz, illite, chlorite, aragonite, and pyrite. The abundance of shelly material points to a relatively shallow-water setting in common with Unit VI. Common plagioclase indicates locally abundant volcaniclastic input (see Sharp and Robertson, this volume). No samples were analyzed from Unit VII, as claystone is effectively absent.
Unit VIII (551.8-565.7 mbsf) comprises organic-rich silty claystone and bioclastic limestone of late Miocene age. The claystone is restricted to massive and finely laminated material, scattered particles, and discrete laminae of organic-rich sediment. In addition, greenish gray, less organic-rich claystone is present near the base of the unit. A single sample of the above greenish gray claystone was chemically analyzed (Table T2). This is relatively depleted in CaO (2.03 wt%), P2O5 (0.032 wt%), and Pb but is relatively enriched in TiO2 (1.59 wt%), Cr (924 ppm), V (284 ppm), and Nd, and Y. The laminae are thus interpreted as fine-grained volcanogenic sediment.
Unit IX (567.7-571.9 mbsf) is composed of sandstone, siltstone, and conglomerate of late Miocene age. Claystone is absent, and thus, no chemical analysis of this unit was carried out. The conglomerate and sandstone were mainly derived from basalt, now weathered. A littoral, beach, or fluvial setting was inferred (Taylor, Huchon, Klaus, et al., 1999; Robertson et al., 2001).
Unit X (579.9-603.85 mbsf) is sandstone, siltstone, claystone, and conglomerate of middle Miocene age. Shipboard XRD analysis of the finest grained sediment present, mainly siltstone, revealed the presence of pyroxene, amphibole, hematite, and illite. In addition, shore-based XRD analysis revealed abundant smectite and albite together with subordinate quartz, hematite, and calcite. In addition, minor abundances of the zeolite mineral, heulandite, probably reflects an input of volcanogenic material (Table T1).
Two samples analyzed from Unit X reveal relatively high values of MgO (to 10.3 wt%), K2O (<4.71 wt%), Cu (<110 ppm), Ni (<207 ppm), and Cr (<505 ppm). These sediments are interpreted as mainly igneous-derived but differ in composition from the volcanogenic muds of the overlying units. The relatively high K2O content could correlate with the occurrence of relatively K-rich volcanic glass in this interval (Lackschewitz et al., 2001), although the clay mineral, illite, is an additional potassium-rich source.
Unit XI (603.85-657.8 mbsf) is mainly sandstone, carbonate packstone, siltstone, and silty claystone. This unit contains rare interbeds of massive and parallel-laminated silty claystone with sharp basal and upper contacts. The clay is locally organic rich. Associated siltstones are reddish colored and contain scattered planktonic foraminifers. One sample analyzed from this unit (619.96 mbsf) (Table T2) is very MgO rich (15.18 wt%) but is otherwise similar to the claystones of Unit XI above, and is also of inferred mafic volcanogenic origin (i.e., Ni = 423 ppm and Cr = 458 ppm).
Unit XII (657.8-802.5 mbsf) is dominated by calcareous sandy siltstone and silty sandstone of middle Miocene age. The lower part of the unit includes silty claystones, first as laminae then as thin discrete beds. Smear slides reveal quartz, feldspar, sporadic biotite, pyroxene, rock fragments, calcite, and pyrite together with nannofossils, planktonic foraminifers, and rare sponge spicules. Shipboard XRD analysis revealed abundant plagioclase with subordinate calcite, quartz, pyroxene, smectite, and zeolite. In addition, shore-based XRD revealed abundant smectite, moderate amounts of quartz and albite, and minor calcite, kaolinite, and pyrite (Table T1). Petrographic study of the sandstones suggests a relatively basic calc-alkaline volcanic origin, probably from the now-submerged Trobriand forearc (see Sharp and Robertson, this volume). Four samples were chemically analyzed from Unit XII. These are compositionally quite constant and very similar to those of the overlying Units XI and X.
Plots of CaO vs. Al2O3, CaO vs. Fe2O3, and CaO vs. MgO indicate inverse relations between CaO and the other three constituents (Fig. F6) but only for higher values of CaO (>20 wt%). At <20 wt% CaO, little correlation exists. High values of K2O (>2.5 wt%) occur in the uppermost sediments (<30 mbsf) and correspond to input of distinctive high-K calc-alkaline volcanic ash. Al2O3, correlates positively with both TiO2 and K2O. The high MgO and Cr values correspond to intervals (e.g., ~600 mbsf) rich in material of calc-alkaline volcanic origin. In general, the more calcareous sediments have a high pelagic content, whereas those rich in Al2O3, TiO2, K2O, Fe2O3, MgO, Na2O, Cr, and other heavy metals are mainly turbidites. The marked positive correlation of Cr and Ni indicates these elements are related, probably within minerals associated with ultramafic rocks or their alteration products. These could include Cr- or Ni-rich montmorillonite, Cr-rich pyroxene, serpentine minerals (e.g., crysotile), spinel, or magnetite (Deer et al., 1975).
Site 1115 differs from all the others analyzed in showing pronounced vertical trends (Fig. F7) that are correlated with a general upward increase in CaO values related to relatively increased biogenic pelagic carbonate content (Taylor, Huchon, Klaus, et al., 1999). Al2O3, SiO2, Na2O, Cr, and other constituents show inverse trends but with local variations (Fig. F7).
Site 1109 on the northern margin of the Woodlark rift (Fig. F2) penetrated a 773-m-thick succession floored by massive dolerite. Samples were chemically analyzed from 1.33 to 375.03 mbsf (Table T3).
The uppermost unit of middle-late Pliocene-Pleistocene age (0-9.50 mbsf in Hole 1109A; 0-14.80 mbsf in Hole 1109B; and 0-83.4 mbsf in Hole 1109C) is composed of calcareous silt, sand, and clay with volcaniclastic sand and volcanic ash. The calcareous clay is a major constituent of the lower part of the unit (Unit 1B), where it forms repeated intercalations up to several tens of meters thick. This lower interval contains several thin layers of volcanic ash (each ~2 cm thick) together with greenish silt and rare volcaniclastic sands. The ash is interpreted as a primary pyroclastic deposit, possibly derived from volcanoes of the Trobriand arc or the Dawson Strait area (Taylor, Huchon, Klaus, et al., 1999; Lackschewitz et al., 2001). Only fine-grained nannofossil clays were analyzed in this study. However, the chemical composition of Unit I (six samples) is influenced by the presence of ash, as shown by smear slide analysis. Samples analyzed from Unit 1 are relatively rich in CaO (<25.66 wt%) and Sr (<1552 ppm). Within this uppermost unit individual elements show no systematic downhole variation.
Unit II (84.40-169.70 mbsf) is defined by a marked change to greenish gray silt interbedded with abundant volcaniclastic sand. The presence of repeated normal-graded beds and other sedimentary structures is indicative of a turbiditic origin (Taylor, Huchon, Klaus, et al., 1999). Magnetic susceptibility is relatively high in this interval. Most of the silt and silty clay occurs in the upper part of graded beds, but some intervals of massive clay are also present, each up to several tens of centimeters thick. Shipboard XRD indicated the presence of calcite, chlorite, quartz, plagioclase, illite/muscovite, and amphibole. Ferromagnesian minerals (augite and amphibole) appear at the top of Unit II, together with clays of mixed-layer type. In addition, shore-based XRD revealed the presence in two samples of abundant albite and quartz and moderate amounts of talc, clinocrysotile (serpentinite), chlorite (Cr rich?), tremolite, and kaolinite (Table T1).
The major elements show a very marked drop in CaO, from 26.2 wt% in the lowest sample of Unit II analyzed (80.83 mbsf), to 4.30 wt% in the uppermost sample of Unit II analyzed. There is a corresponding marked increase in SiO2, Al2O3, Fe2O3, MgO, Na2O, and K2O. However, TiO2 shows no corresponding drop, which suggests that the clayey sediments of Unit I contain a relatively Ti-rich phase. Of the trace elements, there is a general increase in abundance up-section of Ni (to 741 ppm), Cr (to 1108 ppm), and Zr (to 382 ppm). Also, Th, Pb, Nb, Rb, and La increase slightly, Zn, Cu, Ce, and Y show little change, whereas Sr decreases.
Unit III (169.70-246.70 mbsf) is defined as clayey silts, and silty clay interbedded with clayey silt to coarse sand of middle to late Pliocene age. Greenish gray silty clay is the dominant sediment type in this unit. Smear slides reveal quartz, feldspar, volcanic glass, carbonate grains, planktonic foraminifers, nannofossils, radiolarians, and sponge spicules. The smear slides reveal a mixed lithogenic and biogenic composition. Shipboard XRD indicated the presence of mixed-layer clays. Shore-based XRD analysis of one sample confirmed the presence of abundant poorly crystalline material, possibly altered volcanic glass, together with moderate amounts of calcite, albite, and quartz but very little clay (Table T1).
Two samples of silty clay were chemically analyzed from Unit III. The chemical composition is similar to Unit II above; however, one sample exhibits higher values of trace elements and REEs (Table T3).
Unit IV of middle to late Pliocene age (246.7-362.2 mbsf) is composed of relatively uniform greenish clayey siltstone and silty claystone with scattered planktic foraminifers and rare volcanic ash layers. Shipboard XRD indicated the presence of common calcite, plagioclase, and quartz with variable amounts of chlorite, illite, augite and amphibole. Smear slides reveal mixed lithogenous and biogenic material as in Unit III, however, volcanic glass is relatively sparse. CaCO3 values range from 8.7-39 wt% (Taylor, Huchon, Klaus, et al.; 1999).
Three samples were analyzed from Unit IV. These show a marked increase in CaO (<16.80 wt%) and Sr (<1793 ppm) but a decrease in Ni, Cr, and Zr relative to samples analyzed from Unit III.
Unit V (352.8-387.6 mbsf in Hole 1109D) is made up of silty claystone interbedded with volcaniclastic layers and is of middle Pliocene age. Shipboard XRD revealed calcite, plagioclase, quartz, illite, chlorite, amphibole, augite, smectite, and rare alkali feldspar. Three samples analyzed exhibit near-average values for the site as whole with no unusual abundances.
Unit VI (387.6-570.4 mbsf) is clayey siltstone and silty claystone interlayered with silty claystone to coarse sandstone and is of early Pliocene-middle Pliocene age. Shipboard XRD of the claystone revealed calcite, plagioclase, and quartz together with subordinate amphibole and rare pyroxene. Illite/muscovite is the dominant clay mineral with variable amounts of chlorite and rare smectite. One sample contains potassium feldspar, and aragonite is locally present. Shore-based study revealed abundant calcite, albite, and quartz, together with moderate amounts of smectite and pyrite (Table T1). Of two samples analyzed chemically, one of these is unusual as it exhibits high values of Cr (241 ppm), Ce (56 ppm), Zr (255 ppm), La (30 ppm), Ce (66 ppm), and Nd (32 ppm). Shore-based XRD study of this sample revealed albite, quartz, smectite, and kaolinite.
Unit VII (570.4-671.8 mbsf) is heterogeneous sandstone, packstone, and grainstone. No samples were chemically analyzed, as fine-grained argillites are effectively absent. Beneath this, Unit VIII (671.8-705.3 mbsf) comprises poorly dated silty claystone and clayey siltstone. Claystones are restricted to thin (<3 mm) local anastomosing laminae rich in dark carbonaceous material. These were not chemically analyzed. Below this, no samples from Unit VIII were analyzed, as claystones are absent.
Unit IX (705.3-737.2 mbsf), near the base of the sedimentary succession, is dominated by clayey siltstone and sandstone above a basement of dolerite. Vaguely mottled blue-green silty claystone is common near the base of the unit. Associated clay-rich siltstone contains quartz, feldspar, biotite, clay, volcanic rock fragments, accessory minerals, and minor inorganic calcite, as indicated by shipboard smear slide analysis. XRD analysis further indicates that smectite is abundant. Also, small red goethite concretions and rare carbonate concretions were observed in the cores and thin sections Sharp and Robertson (this volume). This unit is interpreted to have accumulated in a coastal swamplike setting under tropical weathering conditions (Taylor, Huchon, Klaus, et al., 1999; Robertson et al., 2001).
Unit X (737.2-772.9 mbsf) comprises an igneous-derived conglomerate mainly composed of clasts of variolitic basalt and dolerite, the latter containing altered plagioclase, olivine, or augite. There is also minor interstitial claystone and siltstone. Shore-based XRD study of one sample revealed abundant smectite and moderate amounts of albite and quartz (Table T1). Two samples from Unit X are unusually rich in Al2O3 (18.22 wt% and 18.83 wt%), Fe2O3 (15.64 wt% and 16.47 wt%), and TiO2 (1.80 wt% and 2.17 wt%) but are low in CaO (1.78 wt% and 1.92 wt%). K2O (0.340 wt% and 0.325 wt%), and P2O5 (0.105 wt% and 0.094 wt%). Of the trace elements and REEs, Zn (108-113 ppm), V (to 425 ppm), and Y (31-34 ppm) are relatively high, whereas Sr is low (239-229 ppm) as is Rb (5 ppm). Beneath, the well terminated in massive dolerite.
Site 1109 shows similar correlations to Site 1115 further north. Al2O3 vs. CaO, and Al2O3 vs. K2O show inverse correlations (Fig. F8). A positive correlation of Cr and Ni points to an association of specific mineral phases (e.g., pyroxene, or amphibole) in volcanogenic turbidites.
Site 1118 is located towards the northern margin of the Woodlark rift basin, 9 km south of Site 1109, and was positioned to determine the nature of the rift succession and underlying "basement" (Fig. F2). Owing to a shortage of time at the end of the leg, the site was washed down to 205 mbsf and then drilled into the "basement." The upper part of the cored succession, of late Pliocene-Pleistocene age, comprises volcaniclastic sandstones and siltstones of turbiditic origin interbedded with claystones and minor volcanic ash. Beneath this, the lower part of the succession contains abundant terrestrial material including woody debris. The succession passes downwards into early Pliocene limestones, calcareous paraconglomerates, and volcaniclastic sandstone deposited in a marine lagoon rich in calcareous algae. The well bottomed in nonmarine conglomerate mainly composed of diabase and minor basalt with paleosol intercalations (Taylor, Huchon, Klaus, et al., 1999).
The uppermost unit cored, from 205.0 to 377.8 mbsf, is composed of siltstones, claystones, and graded sandstones of late Pliocene-Pleistocene age. The background sediment is greenish gray silty claystone and clayey siltstone. A number of distinctive horizons of reddish silty claystone exist. Smear slides of the claystone exhibit common nannofossils and planktonic foraminifers, feldspar, biotite, volcanic glass, hornblende, and inorganic calcite. Shipboard XRD identified the presence of plagioclase, calcite, pyroxene, minor amphibole, illite, and smectite and possible mixed-layer clays. Dolomite and chorite occur occasionally. Below 290 mbsf, calcite, plagioclase, and quartz predominate with minor clay minerals (e.g., chlorite). Unit I is comparable to sediments of the same age at Site 1109, and for this reason samples were not chemically analyzed.
Unit II extends from 377.8 to 492.35 mbsf and comprises reddish claystones, siltstones greenish gray siltstones, and sandstones of middle Pliocene to late Pliocene age. Both smear slide and XRD studies reveal very similar compositions to Unit I. Chemical analyses of this unit were not made either.
Unit III, from 492.35 to 679.27 mbsf, consists of sandstones, siltstones, and fine-grained sediments of middle Pliocene age. The background sediment comprises silty claystone and clayey siltstone. Smear slides indicate little change in composition from above. There are also sporadic thin layers of pale volcanic ash. Shipboard XRD indicated a very constant composition dominated by calcite, plagioclase, and quartz with minor amphibole, pyroxene, chlorite, illite, smectite, and pyrite. In addition, shore-based XRD analysis revealed abundant calcite and quartz, moderate amounts of albite and smectite, and minor amounts of kaolinite (Table T1).
Eleven samples of fine-grained sediment were chemically analyzed from Unit III (Table T4). The content of major elements, trace elements, and REEs is remarkably constant, more than in any other unit of equivalent thickness in the Woodlark Basin. Of the major elements, only CaO shows significant variation (from 5.90 to 18.03 wt%). Trace metals exhibit moderately high values (Ni = <113; Cr = <262; and Zr = <62 ppm). Some of the interbedded sandstones include abundant grains derived from mafic igneous rock. Volcanic ash layers in Unit III were not analyzed.
Unit IV, from 679.27 to 810.84 mbsf, is made up of silty claystone, clayey siltstone, graded sandstone, and siltstone turbidites of middle Pliocene age. Overall, the unit is coarser grained than the overlying sedimentary succession. The fine-grained sediments differ from those in the units above, as they commonly contain mixtures of detrital volcanic grains and volcanic ash. There are also numerous thin pale layers rich in silicic volcanic glass. In addition to pyroclastic ash, detrital volcanic grains present are plagioclase, quartz, biotite, hornblende, and lithoclasts of basic and acid extrusive rocks.
Six samples that were chemically analyzed (Table T4) do not differ in any perceptible way from the fine-grained sediments of Unit III. These samples did not include discrete volcanic ash layers, although the background sediments are clearly rich in dispersed volcanic ash.
Unit V, from 810 to 857.1 mbsf, is made up of mixed sandstone, siltstone, and volcaniclastic sandstone of middle Pliocene-latest early Pliocene age at the base. Intercalated claystones are admixed with scattered volcaniclastic grains and organic matter. Pyrite is occasionally present. Shore-based XRD revealed abundant quartz and smectite, moderate amounts of albite and calcite, minor amounts of kaolinite, tremolite, muscovite or illite and pyrite (Table T1). Four samples that were chemically analyzed are similar to the finer grained sediments higher in the succession. Several samples are relatively rich in trace metals (Ni = 106; Cr = 227 ppm; and Zr = 147 ppm).
Unit VI, from 857.1 to 859.0 mbsf, is a packstone and grainstone (limestone) of early Pliocene or older. Clays are absent, and thus, no samples were chemically analyzed from this interval.
Unit VII, from 859.00 to 873.1 mbsf, consists of undated paraconglomerate composed of basalt and diabase clasts in a matrix of bioclastic debris. One clay-rich sample from near the base of the unit contains 3.84 wt% K2O. Of the trace elements and REEs, V and Y are relatively enriched, whereas Ba, Ce, Pb, and Rb, are depleted compared to the overlying fine-grained sediments, above.
Finally, Unit VIII, from 873.1 to 929.6 mbsf, of unknown age, is dominated by weathered conglomerate with clasts composed of dolerite and diabase with interbedded mainly reddish paleosols. A single sample of palaeosol was analyzed from 900.75 mbsf. This exhibits high Fe2O3 and TiO2 but low K2O.
The expanded rapidly deposited Pliocene interval cored at Site 1118 (Fig. F4) ideally illustrates interelement relationships (Fig. F9). Al2O3 shows a positive correlation with Na2O, K2O, and SiO2 and a less distinct correlation with TiO2. The Al/Na relationship is attributed to the abundance of plagioclase feldspar, mainly albite. The Al/K correlation probably relates to the presence of clay minerals (illite and smectite). The two relatively Ti-rich, but K2O-poor samples were derived from low-K tholeite of probable ophiolitic origin (see "Discussion"). The Al2O3/SiO2 relationship reflects the input of aluminosilicates (e.g., feldspar and clay minerals). The sole markedly anomalous sample with >60 wt% SiO2 is attributed to the presence of siliceous volcanic glass, as detected in smear slides from this interval (i.e., 767.03 mbsf) and confirmed by electron microprobe analysis (Lackschewitz et al., 2001). Al2O3 shows strong correlation with Zr, possibly corresponding to the occurrence of zircon of continental origin. Al2O3 shows a slight positive correlation with Y but none with MgO or Cr (not shown). MgO shows a weak positive correlation with Cr, Ni, Cu, and Zn, reflecting the common occurrence of minerals inferred to have been derived from mainly ultramafic rocks. Fe2O3 remains relatively constant compared to MnO. Relatively high values appear sporadically within the succession but are mainly within the range of composition of hemipelagic clay in general (Turekian and Wedepohl, 1961; Gromet et al., 1984). The unusually high Fe2O3 values are from altered material associated with basic igneous rock (conglomerate) near the base of the succession drilled. Cr and Ni are again positively correlated as at other sites. Several other typical correlations are apparent, notably Fe2O3 with Y and CaO with Sr. Ba also shows a negative correlation with MnO.
Site 1108 is located near the base of the northern slope of the Moresby Seamount (Fig. F2) and was positioned with a view to penetrating the extensional detachment fault defining the southern margin of the Woodlark rift. The upper part of the succession (Units I-III) comprises superficial calcareous hemipelagic sediments underlain by a breccia composed of heterogeneous clasts (Fig. F5), inferred to have come from upslope on the Moresby Seamount (Robertson et al., 2001). During this study, samples of fine-grained sediments were chemically analyzed only from Unit IV, a succession of middle-late Pliocene hemipelagic clayey sediments and gravity-deposited clastic sediments, interpreted mainly as classical turbidites and high-density turbidites. Details of the depth ranges are given in Taylor, Huchon, Klaus, et al. (1999).
Subunit IVA is dark greenish gray claystones interbedded with dark gray graded coarse- to medium-grained sandstones and siltstones interbedded with minor pebblestones. Shore-based XRD analysis revealed abundant quartz, and albite, moderate amounts of smectite, kaolinite, and calcite, and minor amounts of muscovite, or illite and pyrite (Table T1). A sample from low in the succession contains abundant smectite. Subunit IVB is a mainly finer grained interval of greenish gray to dark greenish gray foraminifer-bearing clayey siltstones interbedded with fine-grained sandstone. Bioturbation occurs throughout. Occasional coarser grained pebblestones were defined as Subunit IVC.
Seven claystone samples were analyzed from Unit IV (Subunits A and B) (Table T5). Of the major elements, SiO2 varies over a narrow range (42.8-53.71 wt%), as do Al2O3 (12.31-17.3 wt%), MgO, CaO, Na2O, K2O, TiO2, MnO, and P2O5. One sample is unusually calcareous (14.81 wt%). This sample is also relatively rich in MnO (0.25 wt%) and P2O5 (1.28 wt%). Three of the other samples contain >10 wt% Fe2O3. Of the trace elements, the following elements are relatively abundant (Zr = 97-232; Cu = 53-130; Ni = 9-297; and Cr = 197-309 ppm). None of the elements exhibit any systematic downhole variation, which is not surprising given the small number of samples analyzed.
Al2O3 shows a clear positive correlation with K2O and to a lesser extent with Na2O, presumably mainly related to clay mineral and feldpsar content (Fig. F10). By contrast, no correlation is evident between Al2O3, and either TiO2 or MgO (not shown). However, MgO correlates with both Ni and Cu; also Ni and Cu correlate positively. The probable source of these elements is from ultramafic rock or their erosion products (see "Discussion").
Site 1114 is located close to the northeastern edge of the Moresby Seamount (Fig. F2). Beneath thin superficial unconsolidated Pleistocene hemipelagic sediments, a several-hundred-m-thick succession of more consolidated hemipelagic sediments (Unit II and Subunit IIIA), subordinate sandstones (Unit IV), and conglomerates (Subunit IIIB) were recovered, all of Pliocene age (Fig. F11). At the base of the sedimentary succession, a tectonic breccia 5 m thick was penetrated before the well bottomed in metadiabase. The sediments recovered are dominated by volcaniclastic turbidites, interbedded with mudstones and siltstones. Coarse-grained sandstones predominate in the lower part of the succession, whereas finer grained sandstones, siltstones, and claystones dominate higher up. These sediments accumulated from suspension or by gravity deposition and include intervals interpreted as high-density turbidity current deposits (Taylor, Huchon, Klaus, et al., 1999; Robertson et al., 2001).
Six samples from Units III (55.0-237.6 mbsf) and IV (237.6-276.1 mbsf) were chemically analyzed, ranging from 65.14 to 237.76 mbsf (Table T6). Within Unit III, subordinate weakly lithified dark gray claystone is interbedded with sandstones and siltstones. Smear slides of claystone reveal quartz, plagioclase, biotite, clay, rare small volcanic rock fragments, volcanic glass, pyrite, inorganic calcite, accessory minerals, poorly preserved nannofossils, and rare planktonic foraminifers. Shipboard XRD analysis revealed quartz and plagioclase with variable amounts of calcite, amphibole, pyroxene, and illite. Shore-based XRD indicated the presence of abundant quartz, and albite; also moderate amounts of calcite, smectite, and kaolinite; and locally minor pyrite (Table T1).
Of the samples analyzed, one in the upper part of Unit IIIA is strongly calcareous (39.73 wt%). The remaining samples are relatively rich in TiO2 (up to 1.02 wt%), Cu (to 161 ppm), Ni (to 125 ppm), and Cr (to 251 ppm), V (to 273 ppm), and Ba (469 ppm).
The few samples analyzed show little signs of positive correlations of Al2O3 (e.g., with K2O, and Na2O), in contrast to the sites discussed earlier. However, positive correlations are still apparent between MgO and Ni, Cu and Cr, suggesting the presence of an ultramafic source, as discussed further below.
Site 1116 is located near the southern edge of the Moresby Seamount (Fig. F2). The sedimentary succession (Fig. F11) is mainly Pliocene claystone, siltstone, and sandstone (Units I and III) with a conglomerate intercalation (Unit II). As at Site 1114, the succession is interpreted as mainly hemipelagic claystones and mudstones intercalated with turbidity current deposits, including some that accumulated from high-density turbidity currents (Taylor, Huchon, Klaus, et al., 1999; Robertson et al., 2001). Shipboard XRD study of Units I (0.0-33.8 mbsf) and III (62.5-158.9 mbsf) confirmed the presence of quartz, plagioclase, and subordinate calcite, chlorite, illite, smectite, amphibole, and pyroxene. Shore-based study indicated the presence of abundant albite and smectite, moderate amounts of calcite and quartz, and minor pyrite (Table T1).
Six samples of hemipelagic sedimentary rocks were chemically analyzed (Table T7), two from Unit I and four from Unit III. Major element oxides show little variation other than for Fe2O3 (6.97-12.13 wt%). Some trace elements and REEs exhibit relatively high values (Zr = <103; Cu = <172; Ni = <126; Cr = <224; Ba = <369; Zn = <247; and Ce = <68 ppm). These fine-grained sediments are again mainly volcanogenic muds rich in ferromagnesian minerals from calc-alkaline or ultramafic sources. Few interelement correlations are apparent on scatter plots.