67; Cr/Cr + Al 33) represent evidence of eroded ultramafic mantle rocks (Pl. P2, fig. 2).
In the thin sections studied, arenite matrix varies from calcareous-argillaceous matrix by the common presence of volcaniclastic glass to essentially glassy texture. The fundamental constituents identified by matrix analysis are listed in Table T6. Alteration of the glassy component produced clay minerals, such as saponite, talc, and chlorite (Table T7). Clay minerals from terrigenous detrital components are probably also present in the matrix; planktonic foraminifers, benthic foraminifers, and small shell fragments are also locally abundant.
As a general feature, the abundance of volcaniclastic supplies tends to decrease the carbonate rate (Pl. P1, fig. 4).
Planktonic and benthic foraminifers that show different degrees of test preservation were observed at all sites. Subangular to subrounded bioclasts appear regularly and are generally easily recognizable. In particular, as revealed in the thin sections studied, mollusc fragments are common in the northern margin sites (Holes 1109C, 1109D, 1115C, and 1118A). A few bryzoan fragments were found in Hole 1108B between 14.5 and 226.79 meters below seafloor (mbsf). Bryozoans occur in calcareous fine-grained sediments or packstones at Site 1109 (intervals 180-1109C-21X-CC, 40-42 cm; 180-1109D-32R-2, 101-103 cm; and 180-1109D-36R-6, 51-54 cm), Site 1115 (intervals 180-1115C-23R-1, 93-94 cm, and 33R-3, 68-71 cm), and in Hole 1118A between 853.87 and 868.7 mbsf. In the same interval in Hole 1118A, coral and calcareous red algal fragments are also present. Red algal fragments were found also in Hole 1108B (interval 180-1108B-31R-2, 20.5-23.5 cm), in Hole 1115C between 566.83 and 648.11 mbsf, and in conglomerate in Hole 1116A (interval 180-1116A-6R-1, 9-11 cm). A large coral fragment is also present in Hole 1109C (interval 180-1109C-24X-CC, 18-20 cm). Ostracode shells are very rare and are found only in Hole 1108A (intervals 180-1108B-22R-4, 61-64 cm, and 28R-3, 28-31 cm).
The organic carbon content is generally very low to absent in most sandstones, but concentrations are present in fine-grained siltstones and claystones. Some samples were analyzed for organic carbon on the grounds of its presence as scant patches at all sites, as ribbons (Site 1108 at 448.16 mbsf, Site 1112 between 1.7 and 20.52 mbsf, Site 1114 at 36.04 mbsf, Site 1116 at 8 mbsf, and Hole 1109D between 697.1 and 708.68 mbsf), or localized within foraminifer tests (Samples 180-1115C-15R-3, 79-83 cm, and 180-1118A-69R-3, 58-61 cm). The N content is negligible or below the method detection limits, whereas the C content is as much as 15.01 wt% (Table T8).
The total carbonate data shown in Figure F2 was integrated with XRD determination of mineral phases (Table T6). The predominant phase is calcite followed by aragonite, restricted to some samples at Sites 1109 and 1118, and dolomite (Samples 180-1109C-27X-4, 67-71 cm; 180-1109D-4R-5, 59-62 cm; 180-1109D-36R-6, 51-54 cm; and 180-1118A-39R-3, 100-103 cm). In two samples, ankerite was also found (Samples 180-1109D-34R-1, 10-13 cm, and 180-1115B-10H-4, 55-61.5 cm). The carbonates largely relate to shells and skeletal structures of organisms (also fragmentary bioclasts) and to the finest portion of the rock (matrix) because of either an organic or biochemical origin. The abundance of volcaniclastic supplies tends to decrease the carbonate rate.
Middle Miocene sediments in Hole 1115 usually have low carbonate contents. Average values increase significantly in the upper Miocene sequence. Pliocene turbiditic sedimentation is characterized by sharp variations in carbonate contents clearly linked to the nature and provenance of the turbidity currents.
Pelagic and hemipelagic Pleistocene sedimentation is characterized by highest carbonate values, consistent with the occurrence of nannofossil-rich silty clay (Sites 1108, 1114, and 1115). This can be related to the construction of a carbonate platform northwest of the Woodlark Rift (Robertson et al., in press).
Clasts of quartz-mica schists, gneisses, and amphibolites are present in the rift basin from the middle Pliocene (Site 1108) to the Pleistocene (Sites 1112). The fine grain size of mica schists and gneisses allows only a broad pertinence to amphibolite facies. The occurrence of muscovite in these sequences likely has the same significance. At the footwall, rare retrogressed amphibolites are found; amphibolite clasts are medium to fine grained with green hornblende and minor saussuritic plagioclase. The alteration of hornblende to actinolite is common and consistent with retrograde overprint to greenschist facies (Shipboard Scientific Party, 1999).
Pebbles and cobbles of mica schists, gneisses, and amphibolites, together with granites, were recovered from the talus at the foot of Moresby Seamount (Sites 1110 and 1113) (Shipboard Scientific Party, 1999).
Epidote ± chlorite clasts lacking schistosity at Site 1116 (0-20 mbsf), Site 1114 (200 mbsf), and Site 1108 (between 14 and 302 mbsf) likely derive from reworking of fracture-filling materials formed under greenschist/subgreenschist conditions. A fault gauge origin is likely also for talc schist, chlorite schist, and serpentine schist, which occur diffusely in the rift and footwall sites but are exceptionally rare in the northern margin sites.
Granites from footwall and rift-basin sites are microgranular with hypidiomorphic to granophyric texture (Pl. P2, fig. 3). They include plagioclase, K-feldspar, quartz, and biotite. Their nature and texture point to an origin from intrusions in the upper continental crust.
Fragments (1-5 mm in size) of serpentinite showing mesh and ribbon textures, sometimes partially altered to talc and chlorite, and grains of Cr spinel (Mg# 67; Cr/Cr + Al 33) represent evidence of eroded ultramafic mantle rocks (Pl. P2, fig. 2).
The spinels are very similar in color and composition (except for rare secondary ferric brown). The composition is consistent with spinels in poorly depleted lherzolites (Hoogerduijn Strating et al., 1990). Serpentinites and spinel grains are present at Site 1108 between the middle Pliocene-Pleistocene down to 332 mbsf and are diffuse throughout Hole 1116A. In the northern margin at Site 1115, rare spinel grains and rock fragments occur in upper Miocene sandstone (Sample 180-1115C-29R-1, 72.0-73.0 cm). The diffuse occurrence of serpentinite clasts throughout the sequences, as suggested by Shipboard Scientific Party (1999), was not confirmed.
Gabbroic clasts are present at the foot of Moresby Seamount at Site 1116 between 0.58 and 42.77 mbsf. In the rift basin, gabbros are rare (Sample 180-1108B-47R-1, 33.0-37.5 cm) and are doubtfully represented by sheared metagabbros at Site 1112. In the northern margin, clasts are present at Site 1115 in upper Miocene conglomerates and sandstones directly overlying the unconformity (Sample 180-1115C-30R-CC, 16.0-19.0 cm), and in Hole 1118A, gabbro occurs in middle Miocene conglomerate with prevalent dolerite clasts (Samples 180-1118A-71R-1, 13.0-15.0 cm, and 71R-1, 29.0-31.0 cm).
The grain size varies from medium to medium fine. One lithoclast (Sample 180-1108B-47R-1, 33-37.5 cm), showing mesh-textured serpentine and plagioclase (An75), likely represents the only record of a cumulate olivine gabbro. More commonly, the clasts are Fe-Ti oxide gabbros (plagioclase An 70, clinopyroxene, and ilmenite ± rare olivine), Fe-Ti oxide diorites, and quartz diorites with clinopyroxene and hornblende as mafic silicates. On the whole, the mineral composition is consistent with the more evolved members of intrusive tholeiitic sequences.
In gabbroic clasts, actinolite diffusely replaces igneous mafic phases and chlorite and albite overgrow plagioclase. In Samples 180-1116A-6R-1, 9-11 cm, and 6R-1, 37-39 cm, gabbroic clasts show diffuse development of prehnite replacing plagioclase and filling veins (Pl. P3, fig. 1). Prehnite is also present as reworked grains at the same site.
Clasts of mafic effusives and subintrusives occur at Site 1116 between 0.58 and 48.58 mbsf in Pliocene sandstones and conglomerates. Dolerites and basalts occur at Site 1108 between 245.19 and 437.82 mbsf in middle Pliocene sediments. In the northern margin at Site 1118, they occur in upper Miocene grainstones and sandstones and in the underlying conglomerates (between 861.18 and 878.69 mbsf). At Site 1115, they occur in upper Miocene conglomerates (between 566.83 and 566.89 mbsf), whereas at Site 1109 (between 763.66 and 765.79 mbsf), a dolerite basement is reworked as clasts in upper Miocene conglomerate.
Dolerite clasts show intersertal to rarer ophitic texture; basalts show a textural range from holocrystalline to hypocrystalline with skeletal to spherulitic plagioclase. Olivine is present diffusely as phenocrysts or microphenocrysts (Pls. P2, fig. 2, P3, figs. 2, 4). Ilmenite is a diffuse accessory phase; in some clasts it exceeds 3% in volume, suggesting ferro-basalt compositions. The petrographic features of investigated clasts are consistent with tholeiitic magma series and are comparable with dolerites analyzed at Sites 1109, 1111, 1114, 1117, and 1118 that show enriched mid-ocean-ridge basalt similarities (Shipboard Scientific Party, 1999). On modal and textural grounds as a whole, the clasts represent the hypabyssal sheeted dikes (dolerite) and effusive parts of a tholeiitic sequence. The development of fast-cooling textures, comparable to those in pillow lavas, and evidence of spilitization are consistent with emplacement as submarine effusions.
Basalts and dolerites are pervasively altered; the albitization of plagioclase (± chlorite ± calcite) and the replacement of olivine, glass, and, in part, pyroxene by phyllosilicates suggests spilitization as an alteration process. In Hole 1116A, prehnite veins cut dolerite clasts and plagioclase is altered to prehnite. More rarely, dolerite clasts are pervasively replaced by pumpellyite with minor prehnite. The marked green-yellow pleochroism of pumpellyite suggests Fe-rich composition.
The middle(?) Miocene conglomerates at Sites 1118 and 1109 include well-rounded dolerite and basalt clasts that show oxidative alteration produced in a tropical continental environment (Shipboard Scientific Party, 1999). Fragments with goethite concretions also support this hypothesis. The clasts reworked at Sites 1115 (middle Miocene packstones and sandstones) and 1116 (Pliocene sandstones and conglomerates) only in minor part show evidence of subaerial weathering, suggesting a provenance from deeper erosional levels.
In Hole 1115C (628.81 mbsf), alkalic volcanic clasts from middle Miocene fine-grained sandstone (Sample 180-1115C-37R-CC, 1.0-3.0 cm) are defined as tephrites; phenocrysts of zoned pyroxene with aegirine-augite rims, feldpathoid (sodalite/nosean), and rare biotite are present in feldspar microlith-rich glassy groundmass (Pl. P2, fig. 1). More rarely, phenocrysts of altered olivine constrain a basanite composition. This is the first recorded regional occurrence of alkaline volcanism of presumable early to middle Miocene age.
Volcanics are present as (1) reworked lava fragments (up to 1 cm in size); (2) pumiceous clasts (up to 1 cm in size); (3) vitric shards; and (4) single-grain, more or less fragmented, phenocrysts (phenoclasts). The lava fragments are abundant, occurring in middle Pliocene to Pleistocene sediments in the footwall (Holes 1114A and 1116A) and rift basin (Site 1108). In the northern margin sequences, lava clasts are well represented since middle to late Miocene (Site 1115) and from early to middle Pliocene (Sites 1118 and 1109). Phenoclasts associated with shards and pumices, suggesting a significant explosive activity, are ubiquitous at all sites but are rare or lacking in the upper Pliocene sequence of the northern margin (Sites 1109, 1115, and 1118) (Tables T3, T4, T5).
The lava clasts show porphyric texture with glassy to microcrystalline groundmass, sometimes with fluidal texture. The mesostasis can be glassy to vitrophyric, rarely microcrystalline, with plagioclase microliths or sometimes alkali feldspar. Smith (1982), Smith and Milsom (1984), and Lackschewitz et al. (in press) evidenced the medium- to high-K character of the island-arc Miocene-Pliocene volcanism in the area. These volcanic types are mirrored by different phenocryst assemblages found in the glassy groundmass of basalt andesite composition (Fig. F3; Table T9); plagioclase + clinopyroxene ± orthopyroxene; plagioclase + red hornblende ± clinopyroxene, and, more rarely, plagioclase + red hornblende + biotite ± clinopyroxene. Plagioclase + clinopyroxene, plagioclase + biotite ± clinopyroxene, plagioclase + clinopyroxene + green hornblende ± biotite, and rarer quartz occur in a glassy groundmass of andesite to rhyolite composition. Sanidine sometimes included in biotite is rarely present among phenocrysts (Samples 180-1118A-66R-3, 24-28 cm, and 180-1108B-3R-1, 41-44.5 cm). The appearance or lack of hornblende (rarely biotite) in relatively primitive basalt andesite and andesite points to differing hydration conditions of the magmas. In spite of local prevalence of basalt andesite and andesite, a regular distribution over the sites and through time of the calc-alkaline terms is not recorded. However, quartz phenoclasts and quartz-bearing pumices are mostly present at the forearc sites.
The glassy groundmass of volcanic clasts from the northern margin, footwall, and rift sites was analyzed by electron microprobe. Many analyses have a low total oxide sum, likely due to incipient hydration processes and/or microvesicularity, and the compositional data are only approximate. Therefore, the reported analyses were selected (total oxide sum > 90 wt%) and recast to 100%. The glass shards range in composition from basalts to rhyolites (Fig. F3; Table T9) of calc-alkaline medium- and high-K series.
Trachyte clasts at Site 1116 represent the record of high-K to transitional (shoshonitic) volcanism in middle to upper Pliocene sandstones. Also at Sites 1110-1112, the talus sediments include acid-intermediate volcanics, quartz trachyte, and lamprophyre (Taylor et al., 1999). They have been related to intrusions within the Moresby Seamount and were eventually compared with the high-K volcanic equivalents of the Papuan Peninsula and D'Entrecasteaux Islands and to the Pliocene-Pleistocene comendites belonging to the Miocene Trobrian arc (Ashley and Flood, 1981).
Generally, the explosive products are present in epiclastic deposits from turbidity currents or mass flows and are associated with minor bioclastic and terrigenous materials. However, millimeter- to centimeter-thick layers composed of glass shards with pockets of phenoclasts and sparse pumices are relatively common mostly in the northern margin, interbedded with turbidites, and likely represent the deposition by fallout from onland eruptions (Samples 180-1109C-28X-1, 4-8 cm; 180-1109D-4R-7, 0-2 cm; 180-1115C-6R-2, 17-19 cm; and 180-1115C-10R-1, 5-7 cm). The explosive activity is mostly represented by felsic materials in accordance with the dacitic-rhyolitic nature of the glassy shards (Lackschewitz et al., in press) and by the presence of localized quartz phenoclasts, whereas quartz is rare in the lava clasts.
The volcanic clasts display moderate or no alteration at all. Alteration to phyllosilicate minerals (mostly celadonite) under low-temperature hydrothermal conditions can affect glass, biotite, and, more rarely, amphibole. Clasts in calcareous strata sometimes show partial syn-diagenetic impregnation by carbonates. More rarely, andesite clasts affected by cortical carbonate replacement are subsequently rounded (Sample 180-1116A-16R-1, 103-107 cm).
Sedimentary rock fragments are scant at all sites. In particular, silty claystone fragments were recovered in the rift basin in fine- to medium-grained sandstone (Sample 180-1108B-31R-2, 20.5-23.5 cm). In the footwall, siltstone fragments were recovered in the same lithology (Sample 180-1114A-14R-1, 106-108 cm) and limestone fragments in conglomerate (Sample 180-1116A-5R-1, 79-81 cm). In the same hole, limestone and siliceous sedimentary rock fragments are also present in conglomerate (Sample 180-1116A-6R-1, 37-39 cm).
A conglomerate sample (180-1116A-5R-1, 79-81 cm) contains a well-rounded chert clast with poorly preserved shells. The clast shows an evident oxidized rim, likely originating from prolonged subaerial exposition.
Clinopyroxenes in microgabbros vary in composition of clinopyroxenes from diopside with high wollastonite component to augite (Fig. F4).
In the calc-alkaline volcanics, clinopyroxene phenocrysts coexist with plagioclase (An50-43) in basalt andesite to dacite glassy groundmass. Their compositions fall within the augite field, evolving from higher to lower Wo/En ratios in parallel with evolving glass composition. However, the complete evolution can be observed within the same zoned grain. Coexisting clinopyroxene (Wo37En44) and orthopyroxene (En70) were found in Sample 180-1114A-28R-CC, 3-5 cm. Representative analyses are reported in Table T10 and Figure F4.
Analyzed biotites are optically homogeneous phenoclasts with constant composition; they are characterized by relatively high Mg content (Table T11) that points to phlogopite-rich compositions. The Altot-Mg correlation (Nachit et al., 1985) is consistent with pertinence to the calc-alkaline series.
Igneous calcic amphiboles referable to the calc-alkaline volcanics were distinguished on the grounds of optical and compositional features (Table T12; Fig. F5): (1) red hornblende (high AlIV-AlVI ratios) present in the most primitive compositions (basalt andesites and andesites) and (2) green hornblendes (lower AlIV-AlVI ratios) that occur in relatively evolved andesites and dacites.
Colorless metamorphic hornblendes from chlorite-bearing schist show a low AlIV-AlVI ratio.
The composition of plagioclase in the gabbroic clasts varies between An 65 and 75 mol% (Table T13; Fig. F6). In calc-alkaline volcanic clasts, phenocrysts from basalt andesites have An42-55 mol%, whereas the corresponding microliths range between An 25 and 43 mol%. Anorthite <15 mol% is observed in the microliths of more felsic terms. Alkali feldspars with a wide Ab/Or compositional range were analyzed in high-K andesites; Or >90 mol% corresponds to K-feldspar in a rhyolite.
Red spinel associated with serpentinite clasts shows homogeneous composition with Mg = 67 and 100 x Cr/Cr + Al = 33. Such composition is comparable to that observed in Cr-spinel of poorly depleted mantle lherzolites (Hoogerdujin Strating et al., 1990).
