Sediments and sedimentary rock recovered from the 377-m sedimentary column at Site 1179 are clayey siliceous ooze, clay, and chert. We recovered one core from Hole 1179A, six cores from Hole 1179B, 27 cores from Hole 1179C, and 11 cores from Hole 1179D. Recovery averaged 98.8% in the ooze and clay above 283 mbsf, with lowest recoveries (90%) in the stiff brown clay (246-283 mbsf). Recovery ranged from 2% to 11% in the cherty section below. Sediments above 283 mbsf are soft, mainly massive, and varied in color and composition. The sediments below 283 mbsf, mostly cherts and porcellanite, are more compact and show less variation. Cores of softer sediments drilled using the APC are largely undisturbed to slightly disturbed by drilling. The cherty rocks, drilled with the RCB, are highly fragmented by drilling. Lithology and recovery are similar to those of pelagic northwest Pacific sites drilled during earlier DSDP and ODP legs (see Fig. F4). Based on composition and color, we divide the section at Site 1179 into four lithostratigraphic units (Fig. F11) and describe their sedimentary features below.
Cores 191-1179A-1H through Section 191-1179C-20H-2, 22 cm, contain predominantly diatoms, radiolarians, and clay. The proportion of these three major components varies from core to core and even from section to section. Generally diatoms dominate, but clay and radiolarians are also common. According to the classification shown in Figure F3 in the "Explanatory Notes" chapter, this lithostratigraphic unit is typically composed of clay-rich to clay-bearing radiolarian-bearing diatom ooze (Fig. F12A) with intervals of diatom-rich clay (Fig. F12B).
Sediments show a moderate range of coloration, mainly in the green part of the spectrum. Olive gray is the dominant background color through this unit. Other colors include greenish gray, purplish gray, brown, pale olive, and grayish green. Furthermore, the unit is ~35% mottled in terms of variegated coloration. Colors had faded markedly in some cores that were reexamined a few days after initial splitting.
Texturally, the sediments are classified as immature. Silt-sized particles commonly dominate over sand- and clay-sized particles in this lithostratigraphic unit. Silt-sized particles include small whole and fragmented frustules of diatoms and tests of radiolarians and less numerous grains of silicic light-colored glass. Trace components include quartz, feldspar, and silicoflagellate tests. Authigenic zeolite, where present, is silt sized. Sand-sized skeletal remains of diatoms and radiolarians or clay-sized clay dominate locally. The maximum particle size ranges from 100 to 300 µm; the minimum particle size is <2 µm. Opaline particles have organic shapes; diatoms are mainly discs and flakes, radiolarians are irregular, and the spines of radiolarians and sponge spicules are largely rod and horn shaped. Glass and quartz grains are subangular to angular in shape.
Overall, sediments of this lithostratigraphic unit are massive in appearance (Fig. F13), although thin to thick (i.e., a few millimeters to 35 cm) laminations and beds (Fig. F14) are observed across Unit I. These layered features are discriminated largely in terms of color contrast and changes in texture, which apparently are related to local composition (i.e., changes in the proportion of siliceous ooze to clay). In this sense, they are considered to be primary bedding features and are mentioned as thus in the visual core description (VCD) forms. If the siliceous ooze content in one of these beds is >75%, the sediments are largely fine sand to coarse silt; whereas if the ooze content is lower than ~65%, the sediments are medium silt to clay. Internal sedimentary structures within the interbedded laminae and beds include planar laminae (Fig. F15), silt laminae, flaserlike laminae, and isolated laminae, as well as wavy parallel bedding, cross laminae, bioturbation, concretions, and nodules. The first four types of laminae mentioned above are found randomly throughout the oozes. Vertical continuity of >4 cm is rare. Oscillatory ripples, wavy parallel bedding, and cross laminae are very rare. Overall, Unit I is ~35% bioturbated, as evidenced by colored mottles, which can be subtle in places. The degree of bioturbation ranges from common to rare and generally decreases with depth. The marks are largely 1-3 cm in size. Marks are commonly laterally oriented, although a few vertical escape burrows are present. The marks are mostly subrounded, tubular, and cylindrical in shape, but a few branching ones were also found. Some groups of marks represent an ichnocoenosis. Vertical distribution patterns of bioturbation resemble tiering in a few places (Fig. F16). Ichnogenera include Planolites, Zoophycos, Granularia, Phycosiphon, Helminthopsis, Chondrites, and possibly Teichichnus. Isolated single or multiple nodules and hard concretion-like structures were found. The nodules streak sooty black across cut cores and presumably are manganese oxide. The indurated masses are clay-poor siliceous but dominantly dolomitic fillings of burrows, including an Ophiomorphia-like filling with a patterned burrow lining (Fig. F17).
In petrographic detail, biogenic constituents of Unit I include the opaline-silica remains of diatoms (40%-85%), radiolarians (10%-35%), sponges (3%-9%), silicoflagellates (trace to 4%), and, less commonly, the phosphatic remains of fish (trace to 1%). Whole and large pieces of frustules of discoidal centric diatoms are especially common, but many other shapes of whole and broken frustules are common throughout the unit. Countless broken pieces range in size from medium sand down to fine silt. Radiolarians are present as whole tests, broken tests, and broken spines. Sponge spicules are remarkable in size (up to 400 µm long × 100 µm across) and appearance in terms of their central tube and, where present, surface ornamentation. No trace of diagenetic alteration of the siliceous tests is found in the smear slides high in this unit, but near its base and continuing in the radiolarian ooze below, phillipsite commonly fills and coats radiolarian tests. Inorganic constituents include clay, commonly with zeolites (10%-40%), volcanic glass (1%-7%), quartz (trace to 1%), and opaque minerals (trace). X-ray diffraction (XRD) results show the prevalence of quartz and that illite is the principal clay, with some mixed-layer chlorite (corrensite). The diatomaceous siliceous ooze (Fig. F18A, F18B) shows several of these constituents.
Accessory lithologies include at least 12 gray to light gray volcanic ash layers (Fig. F11) and numerous green and brown zeolitic and ashy clay layers. The ash layers, which are 2-10 cm thick, show thin beddinglike features with sharp basal contacts and diffuse tops (Fig. F19). Ash beds are largely unbioturbated. The proportion of glass in these ash layers is as much as 90% (Fig. F18C, F18D). The glass is mainly in the fine sand to coarse silt range and is predominantly transparent and light (rather than brown); tubular vesicles are common. Green zeolitic and ashy firm clay layers have been previously noted as characteristic of the Neogene north and west of this area at Sites 579, 580, and 581 (Heath, Burkle, et al., 1985). Here, they are 1 mm to 5 cm thick but are mainly ~1 cm in thickness. Typically, they are firmer than the adjacent ooze, perhaps from the zeolite and clay alteration products of ash. Brown zeolitic clay layers show thin bedding, planar laminae, flaserlike laminae, isolated laminae, and bioturbation. The bedding contacts are largely gradational or bioturbated. In these green and brown zeolitic clay layers, the proportion of zeolitic clay can be as large as 85%.
In the upper part of Core 191-1179C-20H, a gradual change in color from the top of Section 191-1179C-20H-1 to the base of Section 20H-3 indicates a gradual downcore increase in the proportion of radiolarian to diatom remains. Smear slides show the gradation. The lower contact of Unit I is placed at the base of Chron C4n.1n, which is approximately the level below which radiolarians predominate. The sedimentation rate (see "Sedimentation Rates") changes at this depth; the diatom ooze of Unit I accumulated at ~29.29 m/m.y., and the radiolarian ooze of Unit II accumulated at ~7.56 m/m.y.
Sections 191-1179C-20H-2, 22 cm, through 22H-5, 125 cm, contain radiolarians as the dominant component. Clay or zeolitic clay and diatoms are significant additional components. The proportion of these three major components varies from core to core and from section to section, so that the lithologic names of this unit are clay-rich and diatom-bearing radiolarian ooze, clay-bearing radiolarian ooze, or clay-rich radiolarian ooze.
Unit I is green and Unit II is brown. A gradational color change from pale olive to yellowish brown in Core 191-1179C-20H indicates a gradual change in lithology from diatomaceous to radiolarian ooze. The rest of Unit II is light yellowish brown, light brown, and brownish gray, with mottles in some places.
Silt-sized particles dominate over sand- and clay-sized particles through much of this lithostratigraphic unit, but locally, sand or clay size may predominate. The maximum particle size ranges from 100 to 400 µm. The minimum particle size is <2 µm, although the clay is largely in aggregates held together by zeolite. The characteristic particle shape is rodlike, owing to the many fragments of spines.
Sediments of Unit II are massive in appearance (Fig. F20). In terms of sedimentary structures, the unit is featureless except for rare planar laminae, flaserlike laminae, silt laminae, and isolated lamination. The degree of bioturbation is rare to moderate, as evidenced by subtle mottles (Fig. F21). Marks are either vertically or laterally oriented; the vertical ones resemble escape burrows. They are 1 mm to 2.5 cm in size and rounded to subrounded and cylindrical in shape. The ichnogenera include Planolites, Granularia, Chondrites, and Techichnus.
Petrographically, the unit is a radiolarian siliceous ooze (Fig. F12C). Biogenic constituents include the tests, broken tests, and broken spines of radiolarians (40%-60%) with lesser amounts of whole and broken diatom frustules (13%-17%), sponge spicules (trace to 2%), silicoflagellates (1%-4%), and fish remains (trace). Inorganic constituents include illite and some corrensite as clays, phillipsite (20%-35%), volcanic glass (trace to 2%), feldspar (trace to 1%), and opaque minerals (trace). Phillipsite is present as masses in clay but more characteristically as fillings of radiolarian tests and coatings on radiolarians, large sponge spicules, and, in some instances, on grains of glass.
Accessory lithology is almost absent in this unit. A layer of light gray-colored vitric ash in Section 191-1179C-22H-3, 40 cm, contains volcanic glass (90%), feldspar (5%), and clay (5%). The contact of Unit II with Unit III is gradational both in color and composition within Section 191-1179C-22H-5 and is placed at 127 cm, where a smear slide has only a few corroded radiolarians in clay. The entire gradation is within, but near, the base of Chron C5n.2n.
Sections 191-1179C-22H-5, 125 cm, through 26X-CC contain pelagic brown clay, which by composition is zeolitic clay or ferruginous zeolitic clay. The clay is sticky to waxy on its cut surface, and it is compact.
As was noted for the top contact of Unit II, the top contact of Unit III also represents a change in the rate of sediment accumulation. Paleomagnetic results (see "Sedimentation Rates") show the top part of the clay (246.0-256.6 mbsf) to have accumulated at 1.56 m/m.y. Downhole increases in gamma radiation, with peak gamma radiation logged at ~246 mbsf, witness the slow accumulation of clay (See "Downhole Measurements").
Sediments recovered from this unit show a wide range of coloration, although it is characteristically brown, including medium brownish gray, light brownish gray, yellowish brown, medium dark to dark gray, dusky yellow brown, and chocolate colors. There are numerous mottles of contrasting colors, including orange pink.
Clay-sized (75%-99%) particles dominate over silt- (1%-21%) and sand- (0%-6%) sized particles. Sand- or silt-sized particles are chiefly zeolite. Clays are largely in aggregates, cemented with zeolite (Fig. F12D).
Recovered sediments are completely massive, and apparently they are structureless, except for a rare to intense degree of bioturbation in the form of contrastingly colored mottles (Fig. F22) in some parts of each section. The bioturbation marks are very subtle in a few sections where they are outlined with ferruginous globules or micronodules. The marks are 1 mm to 2 cm in diameter and are largely subrounded in shape. They are laterally or vertically oriented. These ichnofossils provide an ichnocoenosis worthy of further study. The ichnogenera include Planolites, Granularia, Chondrites, and Phycosiphon. Besides these bioturbation marks, rare planar laminae, flaserlike laminae, isolated laminae, and silt laminae were noted in the Unit III clay.
Petrographically, the sediments are composed of clay (75%-99%), phillipsite (3%-20%), iron oxide (0%-20%), opaque minerals (trace to 2%), and volcanic glass (0%-10%). Phillipsite, at the top of the pelagic clay, is largely of coarse silt to fine sand size, but the deeper parts of the clay have no phillipsite. Ferruginous materials (e.g., iron oxide) are found as globules, and some chert is present as an aggregate of tiny leptispheres, ball-like masses of ~0.2 mm that may have filled a radiolarian test. Unit III is apparently barren except for rare fragments of fish bone.
Core 191-1179C-27X and Core 191-1179D-2R through Section 191-1179D-11R-1, 14 cm, contain pieces of chert with a few pieces of porcellanite. The top of the cherty interval was within Core 191-1179C-27X, but recovery was poor in these cores. Cores 191-1179D-4R, 7R, and 10R had better recovery, 10.3%, 11.5%, and 11.4%, respectively. Note that the variously colored and moderately compact zeolitic clay layers in Section 191-1179D-1R-1 were recovered from anywhere across several meters of section; the core should have been designated a wash core.
Drilling disturbance was intense in this brittle rock. By analogy with other Pacific sites containing Cretaceous chert where recovery was marginally better or that were logged, perhaps Unit IV has an alternation of several centimeters of chert and several decimeters of such softer lithologies as chalk, siliceous limestone, and porcellanite that were not recovered. Indeed, the finest pieces broken during drilling must have been circulated out of the hole rather than recovered. Most of the recovery was of pieces ranging from equant ones 5 cm in diameter or less down to 1-cm chips. A few larger pieces that are cylindrical to ovoid are the only ones certain to have their tops still in the proper stratigraphic position.
Recovered pieces of chert display a remarkable range of coloration, which includes brown, very light olive gray, yellowish brown, reddish yellow, dark yellowish orange, moderate reddish brown, pink, dark brown, very pale brown, dark reddish gray, moderate reddish orange, dusky yellowish brown, pale blue, light olive brown, dark to light greenish gray, olive brown, strong brown, gray, light gray, bluish gray, dark red, dusky red, very dusky red, greenish white, dark to moderate yellowish brown, and dusky blue green. Several pieces show more than one color (Fig. F23).
Chert pieces are fractured along their surfaces with subconchoidal to conchoidal patterns. Chert pieces show subvitreous or waxy to dull lusters. The dullest pieces, porcellanites, have a microporous surface and are less dense. Porcellanite pieces are less common. They contain both cristobalite and quartz. Some porcellanite pieces contain replacements or internal molds of radiolarians by chalcedonic quartz. Some chert pieces show conspicuous mottles, which are 1 mm to 4 cm in size and are subrounded and tubular in shape. In a few pieces, large (>2 cm) mottles are composed of lighter colored (gray, light gray, or brown) porcellanite. These mottles represent ichnofossils. The ichnogenera include Planolites, Granularia, and Phycosiphon?. At least two episodes of postchertification brecciation and resilicification or healing can be seen in the subtle color contrasts and distinct fracture-filling veins of some of the larger pieces that were recovered. Some of the 1- to 2-mm-thick veins retain vugs lined with drusy quartz (see "Site 1179 Core Descriptions" for the drawing of Sample 191-1179D-9R-1, 0-4 cm).
Basaltic crust was encountered below 375 mbsf. A detailed description of the basalt rocks from Hole 1179D can be found in "Igneous Petrology".
Deep-sea sedimentary columns, like those elsewhere, represent changes in the source, deposition, preservation, and diagenesis of sediments through a period of time. All the siliceous sediments, including cherts, represent one element of the sediment, and clay and ash are the two other elements recovered at Site 1179. Presumably, biogenous production of carbonate was an additional element but our information about carbonate is skimpy here.
Most of the silica formed as organic skeletons high in the water column. The first-order variation results from the early Miocene increase in silica preservation in the Pacific, here of radiolarians, and the middle Miocene increase in diatom production in the North Pacific. Little can be learned of the Cretaceous and Paleogene history of silica at this site.
Textural terms of grain size and sorting that are used to indicate sediment maturity have little significance in this realm of mixed deposition of fine detrital clay and coarser biogenous silica that settled through the water column, with reworking by bioturbation and gentle traction as the main early postdepositional physical processes. The siliceous remains and the other elements were deposited largely by settling, grain by grain, or more likely they accumulated in fecal pellets, sedimentation processes that provided a sediment relatively homogenous in composition and texture. Modest, ephemeral deep-sea currents apparently were the source of the subtle thin layerlike pattern of fine sand- and silt-textured siliceous ooze overlain by coarse silt- to clay-textured siliceous ooze found in lithostratigraphic Units I and II. Such currents may have sorted silica-rich and -poor layers in Unit IV before its chertification. Local ephemeral currents are also indicated by intervals with a vertical distribution of planar laminae, flaserlike laminae, silt laminae, and isolated laminae.
Deposition of Units II, III, and IV occurred at slow rates in an oxic environment, as evidenced by their ichnofossils and brown colors. The conclusion about rates is confirmed for Units II and III by paleomagnetic dating (see "Sedimentation Rates"). Unit I has green colors as well as bioturbation, indicating dysoxic conditions from relatively more rapid deposition, which also is confirmed by paleomagnetism. In all four lithostratigraphic units, the oxic or dysoxic ecological state was at times interrupted by anoxia, as is observed from discontinuities of the bioturbated intervals. Nevertheless, environments lasted long enough for the trace-making organisms to forage or dwell within the deposited sediments for a while, as the bioturbation marks are largely of living rather than of escaping burrow patterns. Absence of significant carbonate deposits indicates that the ocean floor was below the CCD through the deposition of the brown pelagic clay of Unit III and the siliceous oozes of Units II and I. Poor recovery prevents a definitive answer to whether or not the cherts are interbedded with any carbonates.
The clays may have had any or all of the three sources that have been proposed for pelagic clay. Some are likely wind-blown detrital grains from a continental source. Eolian components generally increase in the upper Cenozoic pelagic sediments of the Pacific. The Pacific plate moved ever closer to Asia, and the interior of Asia became dryer, providing a source of "hot" loess. The finest grained clay, suspended in seawater, may have moved in ocean currents far from its fluvial sources. These detrital particles of eolian and fluvial origin were deposited and subjected to the local postdepositional processes mentioned in the previous paragraphs. Some of the clay, and in particular that with zeolites or in ashy intervals, is presumably of authigenic origin. XRD results may clarify clay sources.
Ash layers and grains of glass high in the section indicate the approach of the Pacific plate to the circum-Pacific volcanic source, although there also have been proposals that the latest Cenozoic was a time of globally increased rates of volcanism.
In summary, all the sedimentary data indicate that Site 1179 sediments were deposited in a deep-water, quiet pelagic environmental setting. Investigations here make a modest contribution to the analysis of ichnofossils and to the histories of the silica budget and volcanism.