LITHOSTRATIGRAPHY
Introduction
Nearly 286 m of sediments
and sedimentary rocks above a contact with a basement of metamorphic rocks was
cored in Hole 1114A. Seven lithostratigraphic units are recognized on the basis
of sediment or rock type, grain size, sedimentary structure, color, compositions
determined in smear slides and thin sections, bulk mineralogy (X-ray diffraction
[XRD]), and carbonate determinations. The recovery was poor, and geophysical
logs (especially FMS) were useful to help reconstruct the succession.
The lithostratigraphic
units recognized are shown in Figure F1,
and a bed-by-bed log is shown in Figure F2.
Unit I is a surficial, soft pelagic and hemipelagic ooze and clay. By contrast,
all the underlying units are at least partly lithified. Unit II consists mainly
of siltstone to claystone. Unit III, by far the thickest, is composed of
intercalated sandstone, siltstone, and claystone. Two subunits are recognized
within Unit III: (1) Subunit IIIA is a thin interval of calcite-cemented
siltstone and sandstone and very minor limestone (packstone) and (2) Subunit
IIIB is distinguished by the presence of minor granule conglomerates. In Unit
IV, the sandstones change to an unusual dusky red color. Below this is a thin
interval of finer grained silty claystone forming Unit V underlain by a tectonic
breccia (Unit VI) that overlies metadolerite (see "Igneous
and Metamorphic Petrology").
- Description:
clay-bearing nannofossil ooze
- Interval:
180-1114A-1R-1, 0-37 cm
- Depth: 0.0-0.37 mbsf
- Age: Pleistocene
Lithostratigraphic Unit I
is represented by only 37 cm in Section 1R-1 in which the following sediment
types are present.
Foraminifer-Bearing
Nannofossil Ooze and Clay
This is pale, homogeneous,
fine-grained ooze and clay, with scattered foraminifers and rare laminations
rich in planktonic foraminifers (e.g., interval 180-1114A-1R-1, 7-32 cm). A
calcium carbonate analysis of Sample 180-1114A-1R, 15-16 cm, indicates a value
of 65 wt%. In smear slides, the calcareous clay contains rare quartz, volcanic
glass (colorless and brown), opaque minerals, planktonic foraminifers,
calcareous nannofossils, radiolarians, and sponge spicules (see "Site
1114 Smear Slides"). The XRD analyses reveal the presence of
only calcite and minor plagioclase within one sample of foraminifer-bearing
nannofossil ooze (Table T3).
Silt
Silts observed in smear
slides are made up of quartz, plagioclase, volcanic rock fragments, volcanic
glass (partly devitrified), and carbonate grains together with rare accessory
minerals (pyroxene and amphibole), pyrite, and sponge spicules (see "Site
1114 Smear Slides"). Common planktonic foraminifers,
nannofossils, and rare sponge spicules are also present. An impregnated thin
section contains quartz, plagioclase, basaltic lithoclasts, altered acidic
grains, devitrified glass, hornblende, pyroxene, organic-rich (plant?) material,
and rare planktonic foraminifers set in a clay-rich matrix (see "Site
1114 Thin Sections"). In addition, occasional thin interbeds
of silt contain small angular intraclasts (several millimeters) of indurated
quartz-rich siltstone (e.g., interval 180-1114A-1R-1, 32-37 cm).
Sand
There are occasional thin
beds of foraminifer sand with rare detrital material. In smear slides, this
sediment contains, in addition to planktonic foraminifers and calcareous
nannofossils, rare quartz, volcanic glass, opaque grains, calcite grains
(undifferentiated), radiolarians, and sponge spicules (see "Site
1114 Smear Slides").
Pebbles
Small (several
centimeters), angular pebbles of quartz-rich siltstone are found just above the
contact with the underlying lithified sediment (Unit II; e.g., interval
180-1114A-1R-1, 32-37 cm).
Interpretation
Lithostratigraphic Unit I
is interpreted as calcareous pelagic and hemipelagic sediment that accumulated
during the Pleistocene at a middle bathyal water depth (500-2000 m) (see "Biostratigraphy").
There is an abrupt change from soft pelagic and hemipelagic ooze and clay of
Unit I to well-lithified sedimentary rocks of Unit II, which is interpreted as
an unconformity (see "Lithostratigraphic
Unit II"). Nannofossil Subzones NN19E and NN19F are missing
(see "Biostratigraphy");
possible reasons are discussed in the concluding depositional history
discussion. The planktonic foraminifer-rich sands may record episodes of
relatively high productivity or be caused by winnowing of the clay fraction. The
silts contain fine-grained volcaniclastic sediment, including altered volcanic
lithoclasts and glass. Similar volcaniclastic material is strewn through the
background pelagic and hemipelagic sediments. Recovery was inadequate to more
fully document depositional processes.
- Description:
siltstone to claystone
- Interval: Cores
180-1114A-2R through 6R
- Depth 6.6-55.0 mbsf
- Age: late
Pliocene-Pleistocene
Lithostratigraphic Unit II
is marked by an abrupt change from soft sediment to much less calcareous
lithified claystone to siltstone.
Claystone
The claystone is dark
green and structureless with Chondrites burrows imparting a slight
color mottling. The claystone is locally silty or sandy and both silt- and
sand-filled burrows are present. Determinations of calcium carbonate content
indicate values ranging from 3.1 to 4.4 wt% (see "Organic
Geochemistry"). Smear slides show that the claystone is rich
in nannofossils with rare quartz, feldspar, biotite, clay, colorless volcanic
glass, accessory minerals, calcite, and planktonic foraminifers (see "Site
1114 Smear Slides"). Silty claystones contain rare foraminifer
tests and detrital sand grains. The XRD analysis of a silty claystone (Sample
180-1114A-5R-1, 17-18 cm; see Table T3),
indicates the presence of major plagioclase and quartz, and minor pyroxene,
chlorite, amphibole, and illite. The XRD analysis of a claystone (Sample
180-1114A-6R-2, 4-4.5 cm; see Table T3)
indicates the presence of major plagioclase and minor quartz, pyroxene, and
illite.
Siltstone
Siltstones occur either as
homogenous intervals (several tens of centimeters thick) or as thin beds or
partings. The siltstone is slightly calcareous, based upon reaction with dilute
HCl. The unit is characterized by repetitive intercalations of siltstone and
claystone. For example, nine such transitions were noted in Section 3R-2.
Several of the silts grade into silty clay, but the base of the beds can rarely
be observed owing to drilling disturbance. Where preserved, the bases of the
beds are sharp. Partings exhibit diffuse tops and bases (e.g., interval
180-1114A-3R-1, 49-52 cm). These partings are mainly structureless and poorly
sorted with angular fine-sand grains, including quartz, feldspar, rock
fragments, and accessory minerals (pyroxene and amphibole). The rock is locally
tectonically deformed (see "Structural
Geology").
In smear slides, the
clayey silts were observed to contain common quartz, clay, and nannofossils, and
rare feldspar, biotite, volcanic rock fragments, volcanic glass (mainly
colorless), accessory minerals, calcite, planktonic foraminifers, and sponge
spicules (see "Site
1114 Smear Slides").
Sandstone
Very thin beds of
fine-grained sandstone exhibit sharp, scoured bases and gradational tops (e.g.,
interval 180-1114-3R-1, 57-58 cm). Several medium beds (<20 cm) of
fine-grained sandstone grade up into siltstone, then silty claystone (e.g.,
interval 180-1114A-2R-2, 42-60 cm). Sharp bases are rarely preserved (e.g.,
Section 6R-1). A possible reason is that these intervals were more sand rich and
were commonly not recovered by drilling. In one instance, fine-planar lamination
was noted (i.e., interval 180-1114A-5R-1, 68-93 cm; see Fig. F3).
The relative abundance of fine- to medium-grained sandstone increases toward the
base of the unit (i.e., Section 5R-1).
In smear slides, the
sandstones contain feldspar, quartz, and reddish altered rock fragments (see "Site
1114 Smear Slides"). In addition, two thin sections contain
unaltered plagioclase, quartz, planktonic foraminifers, muscovite, basaltic and
acidic rock fragments, and phosphatic grains (Fig. F4).
A small number of grains of polycrystalline quartz were observed (see "Site
1114 Thin Sections"). XRD analysis additionally reveals the
presence of plagioclase, quartz, hornblende, pyroxene, calcite, rare chloritic
grains, vesicular glass, and opaque grains set in a carbonate cement (Table T3).
Interpretation
Lithostratigraphic Unit II
records argillaceous hemipelagic sediment interspersed with siltstone and fine
to rare medium-grained sandstone, interpreted as turbidites. The benthic
foraminifer assemblage indicates middle bathyal water depths (see "Biostratigraphy").
The silty claystone, siltstone, and sandstone all contain similar volcaniclastic
sediment assumed to have been derived from a calc-alkaline source area, in view
of the nature of the mineral grains (mainly quartz, plagioclase, biotite, and
amphibole). Compared to Unit I, there were marked increased detrital
constituents, as opposed to calcareous pelagic sediment, including plankton,
either because of a decrease in primary productivity or because of dilution by
increased volcaniclastic input. Also, the calcium carbonate content is greatly
diminished (see "Organic
Geochemistry"), reflecting the decreased abundance of
nannofossils in Unit II compared to Unit I. However, bottom conditions remained
oxidizing in view of the presence of Chondrites burrows.
- Description:
sandstone, siltstone and claystone
- Interval: Cores
180-1114A-7R through 25R, except for Subunit IIIA (14R-1, 80 cm, through
15R-CC) and Subunit IIIB (20R-1, 0-60 cm)
- Depths: 55.0-237.6
mbsf, except for 123.2-141.6 mbsf (Subunit IIIA) and 180.1-180.7 mbsf
(Subunit IIIB)
- Age: late Pliocene to
middle Pliocene
Lithostratigraphic Unit
III makes up the greater part of the succession in Hole 1114A, although the
recovery was very poor (Fig. F1).
However, the missing intervals were reconstructed by using geophysical log and
FMS data. In addition, two thin distinctive intervals were recovered. Subunit
IIIA is composed of calcite-cemented siltstone and sandstone and minor limestone
(packstone), mainly composed of bioclasts. Subunit IIIB, lower in the section,
is composed of conglomerate and sandstone.
The following lithologies
are present in decreasing order of abundance: sandstone, siltstone, and
claystone.
Sandstone
Abundant sandstone varies
from weakly to strongly calcareous. The sandstones are mainly fine to medium
grained, but locally coarse grained (e.g., interval 180-1114A-24R-1, 7-11 cm).
Colors are predominantly dark gray to locally dark greenish gray in the upper
part, but then change to brown to reddish brown in the lower part of the
succession (i.e., below Core 15R).
Many of the sandstones are
thin to medium bedded and exhibit normal grading, although inverse to normal
grading is seen rarely (Fig. F5).
For example, in interval 180-1114A-7R-2, 0-53 cm, four beds of sandstone 10-25
cm thick grade upward from medium- to fine-grained sandstone, then to siltstone,
and then to claystone at the top. Where visible, the bases of individual beds
are sharp (e.g., 180-1114A-12R-1, 54 cm).
Most of the sandstones are
structureless, but planar lamination is occasionally present, especially in the
lower part of individual graded sandstones (e.g., intervals 180-1114A-9R-2,
20-76 cm; 12R-1, 0-5.5 cm; and 12R-1, 56-75.5 cm; see Fig. F6).
Exceptionally, convolute lamination was noted, followed abruptly by low-angle
planar lamination (intervals 180-1114A-11R-CC, 15-24 cm, and 23R-1, 49-69 cm;
see Fig. F7). The base
of this particular bed is sharp. In addition, small (3 cm × 1 cm) shale rip-up
clasts were locally noted in the upper part of the succession (interval
180-1114A-13R-1, 61-102 cm).
The sandstone is weakly
(e.g., interval 180-1114A-7R-1, 8-11 cm) to strongly (e.g., interval
180-1114A-8R-1, 15-34 cm) bioturbated in places. Individual burrows are commonly
sand filled. Fragments or laminae rich in woody organic matter are locally
abundant within sand beds in the upper part of the unit (intervals
180-1114A-7R-2, 0-33 cm; 8R-1, 0-11 cm; 9R-1, 53.5 cm; and 11R-1, 26.5-27.5 cm).
Lower in the unit organic-rich matter was rarely observed (e.g., intervals
180-1114A-15R, 100-110 cm, and 21R-1, 33-49 cm).
Scattered benthic
foraminifers were rarely observed (intervals 180-1114A-12R-1, 65-75.5 cm, and
15R-2, 0-35 cm). Within a few intervals, relatively coarse grained
volcaniclastic material, especially red altered grains (e.g., intervals
180-1114A-9R-2, 0-34 cm and 71-76.5 cm), was evident in the sandstone when
viewed with a hand lens.
Smear slides of the
sandstone allowed us to identify quartz, plagioclase, biotite, clay, subrounded
chloritic basic volcanic rock fragments, volcanic glass, rare accessory minerals
(i.e., hornblende and biotite), inorganic calcite, and foraminifers (see "Site
1114 Smear Slides"). Within single samples the volcanic glass
ranges from nondevitrified glass (green and colorless; e.g., Sample
180-1114A-15R-1, 20 cm), to both fresh and devitrified glass (colorless; e.g.,
Sample 180-1114A-23R-1, 84 cm) to mainly devitrified glass (colorless; e.g.,
Sample 180-1114A-24R-3, 1 cm).
XRD analysis of the
sandstone (see Table T3)
reveals major amounts of plagioclase and quartz, and minor amounts of calcite,
pyroxene, amphibole, chlorite, and possibly smectite. However, most of the
available information on sandstone composition was provided by the study of
eight thin sections from throughout Unit III (see "Site
1114 Thin Sections").
The typical sandstones are
poorly sorted and contain mainly angular to subangular grains (see "Site
1114 Thin Sections"). Most of the sandstones contain a silty
matrix but a few are cemented by calcite spar (e.g., interval 180-1114A-11R-CC,
18-20 cm). Sandstones in the upper part of the succession (above Core 13R)
contain plagioclase, quartz, basalt, biotite, pyroxene, hornblende, rare
planktonic foraminifers, calcite, opaque grains, pyrite framboids, intraclasts
of micrite, plant debris, shell fragments, phosphatic grains (collophane), rare
limestone lithoclasts, and rare spherulitic zeolite.
The basalt-derived
material in different samples ranges from crystalline basalt with
microphenocrysts of olivine (commonly flow banded) to pyroxene-phyric basalt to
black glassy basalt to basalt with variolitic texture to red partly devitrified
basic glass (hyaloclastite) to green chloritized basic extrusive rock and to
very rare blue chlorite.
In addition, there are
rare occurrences of large laths of muscovite, locally abundant polycrystalline
quartz (of probable metamorphic origin), calc-schist, muscovite schist, and rare
green chloritic schist (Figs. F8,
F9). The white mica is
commonly pleochroic (colorless to bluish). In addition, several grains of
serpentinite were noted in Sample 180-1114A-12R-1, 3-4 cm, and rare grains of
chromite are also present.
Based on the study of six
thin sections, there is a change in the composition of the sandstones below ~110
mbsf, marked by an increased input of texturally immature sandstones with fresh
basaltic lithoclasts and related phenocrysts (pyroxene and olivine). However,
metamorphic-derived lithoclasts (e.g., calc-schist and serpentinite) were not
observed in the underlying sandstones. Otherwise, the composition is generally
similar to the sandstone described above (e.g., interval 180-1114A-26R-1, 96-97
cm). Also, unlike the upper part of the succession, bioclasts are locally
abundant and include coral, shell fragments, calcareous algae, bryozoans, and
benthic foraminifers (e.g., Sample 180-1114A-22R-1, 74-76 cm). Small rounded
lithoclasts of partly recrystallized limestone are also present (in the
last-mentioned sample). Several sandstones from low in the succession are very
poorly sorted with angular, broken mineral grains. Also, relatively rare acidic
grains (Figs. F8, F9)
are present in addition to mainly basaltic ones.
Siltstone
Siltstone is present as
interbeds between sandstone and claystone and forms the upper part of graded
sand beds. Siltstone is structureless or rarely planar laminated (e.g., interval
180-1114A-11R-2, 44-49 cm). In a few intervals, the siltstone is present as very
thin (<3 cm) normal-graded couplets of thickly to thinly laminated siltstone
passing into claystone (Fig. F10).
Each of the graded couplets exhibits a sharp base with traces of scouring. Rare
bioturbation is present in the overlying claystone.
In smear slides, the
following constituents are seen: quartz, plagioclase, biotite, clay, volcanic
rock fragments (basic and acidic), volcanic glass, opaque mineral grains,
pyrite, and common inorganic carbonate grains, together with rare foraminifers
and nannofossils (see "Site
1114 Smear Slides"), and occasional sponge spicules. Quartz
grains are rarely observed to be undulose. Foraminifer tests from lower in the
succession are pyrite filled. Several samples are sufficiently rich in
plagioclase to be termed (fine grained) arkose. A thin section of a relatively
well sorted siltstone (Sample 180-1114A-22R-1, 74-76 cm) contains subangular
grains of quartz, plagioclase, common small muscovite laths, biotite (kinked),
green chlorite, polycrystalline quartz, pyroxene, green chloritic grains, rare
fragments of calcareous algae, and minute volcanic-derived grains (see "Site
1114 Thin Sections").
XRD analysis of siltstone
reveals the presence of plagioclase, quartz, calcite, chlorite, and possibly
smectite (see Table T3).
In addition, one thin section of cross-bedded siltstone was studied (Sample
180-1104A-22R-1, 74-76 cm). This is quite well sorted and includes mainly
angular grains of quartz, plagioclase, and muscovite (as minute laths), as well
as rare grains of volcanic rock, biotite (as kinked laths), chloritic material,
pyroxene, polycrystalline quartz, and rare calcareous algae.
Claystone
Subordinate dark gray
claystone is found between the sandstones and siltstones. The claystone is
mainly weakly burrowed to well burrowed (intervals 180-1114A-23R-1, 26-45 cm;
and 9R-CC, 0-4.5 cm). The clay is only weakly lithified, especially where
present between graded sandstones (e.g., interval 180-1114A-11R-1, 36.5-52.5
cm). Relative to sandstone, clay is very subordinate, especially in the lower
part of the succession and was commonly recovered only as angular fragments
(i.e., drilling breccia; e.g., Section 21R-CC).
Smear slides of the
claystone ("Site
1114 Smear Slides") reveal quartz, plagioclase, biotite, clay,
rare minute volcanic rock fragments, volcanic glass, pyrite, inorganic calcite,
accessory minerals, poorly preserved nannofossils, and rare planktonic
foraminifers. XRD analysis indicates the presence of major amounts of quartz and
plagioclase and variable, mainly minor, amounts of calcite, amphibole, pyroxene,
and illite (Table T3).
- Description:
calcite-cemented siltstone and sandstone, packstone mainly composed of
bioclasts
- Interval: Sections
180-1114A-14R-1, 80 cm, through 15R-CC
- Depth: 123.2-141.6
mbsf
Subunit IIIA is restricted
to minor recovery in only Cores 14R and 15R. It is distinguished by the
occurrence of texturally mature relatively coarse grained, calcite-cemented
sandstone, siltstone, and rare bioclastic limestone.
Calcite-Cemented
Sandstone
This sandstone is poorly
sorted with scattered bioclasts and intra-clasts of dark siltstone. In the
middle part of Subunit IIIA inverse to normal grading was occasionally observed
(e.g., interval 180-1114A-15R-1, 39-48 cm). The sediments are fine to medium
grained at the base and then change to coarse and very coarse grained, and then
back to medium grain size at the top of the bed. The sand is rich in shell
fragments, quartz, feldspar, and rock fragments (Fig. F11).
The composition is one of mixed carbonate and volcaniclastic grains, including
coral, calcareous algae, and shell fragments together with detrital grains.
Subangular lithoclasts (<2 mm) of altered volcanic rocks are also present.
The intraclasts and bedding are nearly vertical as a result of tectonic
deformation (see "Structural
Geology").
Two thin sections revealed
a composition that is very similar to that of the adjacent sandstones of Unit
III above and below (see Fig. F12),
but the texture of the sandstones in Subunit IIIA differs markedly. It is
characterized by well-rounded clasts and abundant carbonate bioclasts derived
from a shallow-water setting. Sample 180-1114A-14R-1, 106-108 cm, is dominated
by well-rounded grains of basalt together with quartz, zoned plagioclase,
amphibole, acidic extrusive grains, and rare brown glass (see "Site
1114 Thin Sections"). The bioclasts are calcareous algae,
echinoderm plates, shell fragments, coral, and benthic foraminifers. The
sediment is well cemented by calcite spar.
Sandy
Limestone (Packstone)
Two small pieces of
well-cemented bioclastic sandy limestone were recovered in the core catcher of
Section 14R-CC. The bioclasts are very poorly sorted and comprise calcareous
algae, coral fragments, echinoderm plates and spines, and foraminifers. Numerous
grains (<2 mm in size) of very altered volcanic rock and several (similar
sized) elongate siltstone rip-up clasts are present.
- Description:
conglomerate and sandstone
Interval: 180-1114R-20R-1, 0-60 cm
Depth: 180.1-180.7 mbsf
Subunit IIIB consists of
minor granule conglomerate and sandstone that are present only in the interval
180-1114A-20R-1, 0-60 cm.
Granule
Conglomerate
The conglomerate
consists of angular clasts up to 0.9 cm long set in a matrix of coarse-grained
sandstone. Texturally, the sediment is poorly sorted. The clasts show a very
vague subhorizontal alignment. Clasts are mainly volcanic (dark basalt) with a
few white carbonate bioclasts. The sandstones in the cores above and below
interval 180-1114A-20R-1, 0-60 cm, are of similar composition to those of the
present subunit. This subunit is thus recognized only on the basis of the
presence of gravel-sized clasts.
Interpretation
Unit III, as well as
Subunit IIIA, records variable deposition from turbidity currents in a middle
bathyal setting during middle-late Pliocene time (see "Biostratigraphy").
The normal grading is suggestive of deposition from decelerating currents.
Burrowing is present but rarely well developed, suggesting that bottom
conditions were possibly suboxic but not reducing. However, beneath the
seafloor fully anoxic conditions were probably developed during deposition of
the whole of Unit III, as suggested by the subdued colors and common abundance
of organic matter of mainly terrestrial origin (i.e., woody material), locally
elevated contents of organic carbon (up to 1.4%; see "Organic
Geochemistry"), and common pyrite. Subunit IIIA records
deposition of sediment ultimately derived from a shallow-water depositional
setting (see "Depositional
History" for further discussion). In addition, the granule
conglomerates are interpreted as deposits from high-density turbidity currents
(Pickering et al., 1989).
The provenance of Unit
III was from a volcanic arc-type terrane, including a range of basic,
andesitic, and vitric volcanics. It is notable that the provenance of the
detrital component of the fine-, medium-, and coarse-grained sediments is very
similar. This suggests that the sediment was derived relatively locally from
within a single basin and was not derived from different source areas. The
presence of variolitic textured basalt suggests that some of the basalts were
extruded subaqueously. On the other hand, volcanic glass is relatively minor
and typically altered, suggesting that explosive subaerial (or near subaerial)
volcanism is not greatly represented. Many of the volcaniclastic-rich
sandstones are texturally immature. Samples from near the base include
unaltered glassy basalt and olivine, suggesting that the volcanic material was
eroded rapidly and redeposited without significant subaerial weathering.
In addition, the rare
muscovite laths, strongly recrystallized (polycrystalline) quartz, rare
calc-schist, and very fine grains of mica schist are inferred to have been
derived from a low-grade metamorphosed volcanic terrane.
The geophysical and FMS
logs support the above interpretation (see "Downhole
Measurements"). Only features interpreted as being of
primary stratigraphic significance are mentioned here; details of the
structural features including the evidence for the dip of bedding, location,
and orientation of fractures are presented in "Structural
Geology". From the top of the logged interval (~96 mbsf) to
125 mbsf (near the top of Subunit IIIA), the response on most logs is similar
and is interpreted to reflect relatively uniform deposition of claystone,
siltstone, and sandstone. The FMS images indicate well-developed layering with
thin sandstones ~10 cm thick standing out as resistive layers. Around 120-140
mbsf, the gamma-ray and resistivity logs peak, suggesting that more sandy beds
may be present, possibly correlating with Subunit IIIA. From 113 to 118 mbsf,
possible cross bedding is present. At 125 mbsf, the hole conditions
deteriorated and the FMS images are more difficult to interpret.
The photoelectric effect
and the corrected limestone porosity log show a highly variable response
between 142.5 and 200 mbsf (with 11 major peaks ~2 m thick and 12 smaller
peaks ~1 m thick). This overall interval includes Subunit IIIA composed of
calcite-cemented siltstone, sandstone, and packstone in which calcium
carbonate content is elevated (up to 4.6 wt% and locally higher, based on
other smear-slide observations). However, the amplitude of these peaks is
considered to be too great to be explicable by increased calcium carbonate
content alone, even if limestone was present but not recovered (see "Downhole
Measurements"). Instead, the variable photoelectric effect
could correlate with the presence of reduced iron minerals (e.g., pyrite,
marcasite, or siderite). XRD analysis indicated a single occurrence of
marcasite but at a greater depth (~240 mbsf; see Table T3).
In addition, pyrite was commonly observed in the cores and smear slides.
A small peak in
resistivity at 177.5 mbsf, equivalent to a position near the top of Subunit
IIIA, may indicate the presence of a thin sandstone interval. Variably sandy
intervals appear to be present from 128 to 177 mbsf. From 179 to 181.5 mbsf,
there is a distinctly resistive interval that may correlate with relatively
coarse grained sandstone making up Subunit IIIB. The Th/U ratio reaches a
maximum at this depth and may indicate high input of relatively radiogenic
volcaniclastic sandstone. This interval could also contain U-rich organic
matter derived from a shallow-water setting by mass flow processes. There is
then an interval of low resistivity from 223 to 228 mbsf that correlates with
an enlarged caliper located within the lower part of Unit III (beneath Subunit
IIIB). One other feature is the presence of a tight-limbed fold observed in
the cores from ~208 mbsf that could possibly represent a slump fold, although
a tectonic origin (i.e., deformation of sediment while still soft) cannot be
ruled out (see "Structural
Geology").
- Description: fine-
to coarse-grained sandstone
- Interval: Cores
180-1114A-26R through 29R
- Depth: 237.6-276.1
mbsf
- Age: middle to late
Pliocene
The unit is
predominantly sandstone that is distinguished from Unit III mainly by a change
from a grayish to a dusky red brown color. Individual sandstone intervals vary
from well lithified to less well lithified.
Sandstone
The sandstone is fine to
coarse grained and ranges from weakly lithified to well lithified. It is
mainly fine to medium grained, but is rarely coarse grained with scattered
granules up to 4 mm in size (e.g. interval 180-1114A-29R-CC, 0-7.5 cm).
Unfortunately, the recovery of these sandstones was minimal, and it is not
possible to specify information including bed thickness and the nature of
contacts.
Smear slides of the less
lithified material (see "Site
1114 Smear Slides") revealed quartz and plagioclase,
together with rare biotite, clay, volcanic rock fragments (see Fig. F13),
devitrified volcanic glass, accessory minerals, calcite rhombs, and
foraminifers (see "Site
1114 Smear Slides"). XRD analysis confirmed the presence of
major amounts of plagioclase and minor quartz, pyroxene, and amphibole. In
addition, very minor claystone was seen in XRD to contain quartz, plagioclase,
calcite, and minor pyrite, marcasite, chlorite, possible smectite, illite, and
amphibole (see Table T3).
Two thin sections of
sandstone from Unit IV were studied (see Fig. F13).
One was present only a few centimeters above the tectonic breccia forming Unit
VI (see "Lithostratigraphic
Unit VI"). This is similar to the sandstones higher in the
succession and includes plagioclase, clinopyroxene, lithoclasts of
clinopyroxene-phyric basalt, variolitic basalt, devitrified basic glass (hyaloclastite),
minor acidic volcanics, and rare polycrystalline quartz (altered acidic
volcanic rock?), chloritic grains, and organic material.
Interpretation
Unfortunately, there is
inadequate information on sedimentary characteristics and paleobathymetry to
specify the mode of deposition. It is assumed, however, that the sandstones
record very similar deposition by turbidity currents as in Unit III, and a
similar provenance from a calc-alkaline type volcanic arc terrane is inferred,
including unaltered volcanic rocks.
Below 233 mbsf, the hole
conditions were generally poor and FMS images are difficult to interpret (see "Downhole
Measurements"). Unit IV includes a zone of faulting (see "Structural
Geology"). Locally, at ~249 mbsf, an interval of low
resistivity may correlate with the presence of strongly altered dusky red
sandstones in the cores.
- Description: silty
claystone
- Interval: Core
180-1114A-30R through Section 31R-1, 25 cm
- Depths:
276.1-286.05 mbsf
- Age: middle to late
Pliocene
This unit is restricted
to silty claystone with minor siltstone and sandstone. The fine grain size
contrasts strongly with the overlying coarse-grained sandstone of Unit IV.
This unit is only weakly lithified in places. It is also strongly deformed
(see "Structural
Geology"), which inhibits sedimentological analysis.
Silty
Claystone, Calcareous Silt, and Clay
The silty claystone and
fine-grained sandstone are highly deformed, and few primary sedimentary
structures can be discerned other than several thin, sharp-based interbeds of
fine-grained sandstone (<10 cm thick; see Fig. F14).
In smear slide, the
calcareous silt contains quartz, plagioclase, biotite, clay, accessory
minerals (hornblende), inorganic calcite, rare nannofossils, and sponge
spicules (see "Site
1114 Smear Slides"). A sample of calcareous clay is similar
in composition and includes numerous inorganic calcite crystals. Associated
clay contains major amounts of plagioclase, quartz, and minor smectite?,
chlorite, and pyroxene.
One sandstone bed
(interval 180-1114A-30R-CC, 0-22 cm) is composed of angular and rounded
fragments of claystone and siltstone (2-3 mm); these are very strongly
deformed and brecciated, and are set in a fine- to medium-grained, poorly
sorted matrix. A smear slide of the sandstone revealed an arkosic composition
marked by common plagioclase, altered volcanic rock fragments (including
basalt; Fig. F15),
accessory minerals (pyroxene, hornblende), rare quartz, and inorganic calcite.
A thin section of the sandstone indicates it contains grains of quartz,
feldspar, pyroxene, epidote, biotite, hornblende, variolitic basalt, acidic
volcanics, dolerite, and carbonaceous material (see "Site
1114 Thin Sections"). XRD analysis of the sandstone revealed
major amounts of plagioclase and quartz, and minor chlorite, smectite?, and
pyroxene (Table T3).
The base of Unit V is
composed of only weakly indurated brown clay (interval 180-1114A-31R-1, 0-15
cm) within thin (several millimeters), weakly indurated silt laminae and
secondary color mottling.
Interpretation
Unit V records a period
of mainly fine-grained deposition with only very subordinate siltstones and
sandstones deposited by turbidity currents. The composition of rare sandstones
remains similar to that of the sandstones of Unit IV without a trace of
lithologies derived from the local metamorphic basement (see "Igneous
and Metamorphic Petrology").
- Description:
tectonic breccia
- Interval: Sections
180-1114A-31R-1, 25 cm, through 31R-CC
- Depth:
286.05-287.35 mbsf
- Age: middle to late
Pliocene
Unit VI is a thin but
distinctive interval of tectonic breccia that separates the sediments of Unit
V above from metamorphic rocks beneath (Unit VII). The unit is subdivided into
two parts.
The upper part of the
unit is a breccia (interval 180-1114A-31R-1, 25-74 cm) composed of angular to
subangular clasts of greenschist, <0.6 cm in size, set in a greenish matrix
that also includes scattered, well- rounded grains (red, brown, and
colorless). A crude layering is present but without any recognizable
sedimentary structures or bedding planes.
A smear slide of the
matrix contains quartz, plagioclase, clay, accessory minerals, opaque grains,
and inorganic calcite. A thin section revealed an unusual texture and
composition. The greenish silty matrix contains subrounded grains of chlorite/actinolite
schist and altered dolerite.
The lower part of the
unit is a contrasting type of breccia (intervals 180-1114A-31R-1, 74-130 cm,
and 31R-CC, 0-18 cm) that is composed of weakly indurated angular clasts
(<1 cm in size) of finely crystalline, very altered greenschist set in a
greenish silty matrix. Careful inspection did not reveal the presence of any
sedimentary grains. In places, relict altered veins are present that can be
traced through the body of the rock. Patchy yellow/brown alteration is also
present (on a scale of several centimeters).
A smear slide was found
to contain quartz, plagioclase, biotite, clay, accessory minerals, and
inorganic calcite. A thin section exhibits brecciated actinolite schist, with
angular clasts and evidence of veining and partial silicification. This
material is similar in lithology to the underlying "basement" (see "Igneous
and Metamorphic Petrology"). In addition, a vein infill
contains major amounts of plagioclase, calcite, kaolinite, smectite?, and
minor quartz (Table T3).
Interpretation
The breccia is tectonic
in origin, and no sedimentary constituents associated with the overlying
succession were identified. The upper unit (interval 180-1114A-31R-1, 25-74
cm) contains numerous angular, to subangular, to subrounded clasts of altered
metamorphic rock. The more rounded grains are not interpreted as sedimentary
grains. More probably they are fragments of tectonic breccia that were
subrounded by shearing associated with faulting. On the other hand, the lower
unit (intervals 180-1114A-31R-1, 74-130 cm, and 31R-CC, 0-18 cm) includes a
vein fabric that although altered and deformed, is seen as being inherited
from the underlying metamorphic basement. In addition, an unusual occurrence
of kaolinite within the veins of the tectonic breccia may relate to
low-temperature hydrothermal alteration of the intrusive basement rocks. The
presence of chloritized vein fills also suggests that hydrothermal alteration
played an important role.
An interval of
~287.7-292 mbsf corresponding to the tectonic breccia is imaged on the FMS as
a dark, conductive interval (see "Downhole
Measurements"). The image is similar to the shear zones
located at the Unit IV/Unit V boundary and suggests that only part of this
fault breccia was recovered. Little textural or other structural information
could be gained because of the insufficient quality of the data, most likely
owing to enlarged hole diameters. However, there is a big increase in
resistivity at 292 mbsf, which corresponds to the contact between Unit VI and
the metamorphic basement beneath. This contact is sharp and dips at ~40º to
the southwest subparallel to the inferred orientation of the fault zone on
seismic profiles (see "Downhole
Measurements").
- Description:
metadolerite (greenschist facies)
- Interval: Cores
180-1114A-32R through 37R
- Depth: 295.4-352.8
mbsf
Lithostratigraphic Unit
VII is composed of metadolerite that is brecciated and contains quartz,
calcite, and epidote veins. This unit is described in detail in "Igneous
and Metamorphic Petrology".
The geophysical and FMS
log data help to fill major gaps in the recovery and are interpreted to
suggest that the succession as a whole consists of alternating sandy and silty/clayey
sediments with a maximum of sand ~180 mbsf corresponding to Subunit IIIB, then
becoming generally more silty downward.
Sedimentation at Site
1114 records rapid rift-related volcaniclastic deposition that can only be
dated as middle-late Pliocene in age. Almost the entire succession was
deposited by redepositional processes including turbidity currents of variable
concentrations in an upper bathyal setting (150-500 m; see "Biostratigraphy").
The recorded succession began with an interval of relatively fine grain size
(Unit V). The origin of the overlying dusky red sandstones of Unit V is
unclear, but could relate to (oxidative) diagenesis (i.e., related to fluid
flow within the sediment), and may thus not reflect primary depositional
characteristics. Unit III preserves relatively monotonous turbidity current
deposits that rarely reached conglomerate size (i.e., granule conglomerate of
Subunit IIIB).
In addition, there was a
single interval of sandstone and rare packstone, Subunit IIIA, that was
deposited by turbidity currents. These include abundant shallow-water derived
fossils and well-rounded clasts of basalt and other volcanic rocks. These
sediments record an unusual supply of sand from a high-energy littoral source
(i.e., coastal or shoal).
The accumulation of
coarse-grained sediments is probably related to turbidity currents that
changed to deposition of fine-grained sediments. There was then a dramatic
change from the well-lithified clastic sedimentary rocks of Units II through
IV to superficial Quaternary clay-bearing nannofossil ooze. The probable
explanation of the strong difference in lithification state is that at least
several hundred meters of the sediments was deposited and then removed,
presumably by current activity (i.e., winnowing), slumping, or tectonic
processes resulting from the uplift of the seamount. The hiatus in the
sedimentary record is reflected by the absence of micropaleontologic Subzones
NN19E-F (see "Biostratigraphy").
Removal of at least 165 m is inferred from the porosity vs. depth relationship
of the sediment of Site 1114 (see "Physical
Properties"). This was followed by pelagic and hemipelagic
deposition in a setting removed from clastic sediment input, which probably
occurred after uplift of the Moresby Seamount to near its present position.
Throughout the
deposition of sand and sandstone at Site 1114 derivation was from a
calc-alkaline-type volcanic terrane. Volcaniclastic sediments were probably
eroded from the Miocene Trobriand Arc (including Amphetts Island and Egum
Atoll) (Lock et al., 1987; Davies et al., 1987). This arc was uplifted during
rifting of the Woodlark Basin in the Pliocene or earlier (Goodliffe et al.,
1993). The volcanic arc was not deeply eroded at this stage because plutonic
rock detritus is rare or absent. On the other hand, the unusual grains of
muscovite, calc-schist, locally abundant polycrystalline quartz and rare
serpentinite, and zircon are inferred to have been derived from an ultramafic
ophiolitic terrane and metamorphic basement, as exposed in the D'Entrecasteaux
Islands and southeastern Papua New Guinea (Davies, 1980; Davies and Warren,
1988). The metamorphic debris and ophiolite-derived grains are present mainly
above 125 mbsf and may thus be related to rift faulting and uplift of
metamorphic basement in the vicinity of the Moresby Seamount.
After deposition, the
entire sedimentary succession, except the thin, surficial sediments of
Quaternary age, experienced faulting concentrated within discrete zones (see "Structural
Geology"). It is assumed that this deformation relates to
uplift of the Moresby Seamount. If this is correct, then the uplift probably
took place between late Pliocene and Pleistocene deposition of Unit II and
Pleistocene accumulation of Unit I. In detail, the absence of nannofossil
Subzones NN19E and NN19F (see "Biostratigraphy"),
in contrast to a continuous stratigraphic record from Subzone NN19B downward
within the talus derived from the Moresby Seamount (see "Biostratigraphy"
in the "Sites
1110-1113" chapter), suggests that uplift of the seamount took
place between 0.46 Ma (top of Subzone NN19F) and ~1.67 Ma (base of Subzone
NN19E).
