Coring at Site 1204 began beneath the Meiji drift sequence (Rea, Basov, Scholl, and Allan, 1995) at a depth of ~762 mbsf, recovering a relatively condensed sequence of nannofossil chalk with volcanic ash layers of middle Eocene to late Paleocene age. Two holes, 1204A and 1204B, were drilled at the site, located ~500 m southeast of Site 883 (Leg 145) (Rea, Basov, Janecek, Palmer-Julson, et al., 1993). Time constraints limited the coring of sediment to the last 54.5 m (Hole 1204A) and 3.3 m (Hole 1204B) above basement encountered at ~817 and ~810 mbsf, respectively. RCB recovery of the sedimentary sequence was 90.7% (Core 197-1204A-1R) to 48.1% (Core 197-1204A-2R) in Hole 1204A and 48.4% in Hole 1204B (Core 197-1204B-1R).
Core descriptions in Hole 1204A are based on visual core observations, analysis of smear slides, inspection of magnetic susceptibility data, and chemical analyses of carbonate content, which together provided information for constraining the unit boundaries. Additional observations have been made on selected cores (e.g., Cores 197-1204A-3R, 5R, and 6R) for better understanding of instrumental (magnetic susceptibility) and lithostratigraphic (i.e., color, texture, and composition) correlations. Magnetic susceptibility was measured at 5-cm resolution (see "Physical Properties").
In general, bedding in the sediment is not disturbed by the drilling. Moderate bioturbation, horizontal to subhorizontal Zoophycos trace fossils, and vertical burrows are sparse throughout. Thin laminations and discordant and undulating bedding planes are common and indicate deposition by bottom currents.
In Hole 1204A, four lithostratigraphic units (Units I, II, III, and IV) of Late Cretaceous (Campanian) to middle Eocene age as constrained by nannofossil analyses (see "Biostratigraphy") are present. The units are characterized by finely laminated chalk alternating with clayey nannofossil chalk and clay, which are interbedded with massive to laminated and strongly altered to unaltered volcaniclastic material. The lowermost unit has been divided into two subunits, IVA and IVB, on the basis of compositional (Table T2) and lithologic variability (Figs. F2, F3, F4). The two subunits are recognized in both Holes 1204A and 1204B but differ in thickness between sites. In Hole 1204B, Unit IV was recovered in the single core (197-1204B-1R) (Fig. F3).
The upper part of Hole 1204A (Unit I) consists almost exclusively of calcareous pelagic chalk with a high nannofossil content (CaCO3 = 50.6-92.0 wt%; average = 77.4 wt%; N = 6). In Unit II, nannofossil content drops to 3%-30%, although the measured carbonate content varies widely (0.77-88.75 wt%; average = 46.1 wt%; N = 18). The Unit I/II boundary is also associated with an abrupt upward decrease in volcanic components (i.e., opaque minerals, Fe oxides, ash particles, and related alteration products such as palagonite and clay).
In Unit II, clay-sized carbonate particles (transparent to translucent calcareous clay) dilute the nannofossil content and represent the main source of carbonate in the sediment. This sequence contains a zone of sedimentary slumping (Fig. F4A), also preserved at Site 883 and at Site 884 (at the base of the eastern flank of Detroit Seamount) in the form of blocks of well-bedded ash layers (Fig. F4A). These ash layers also produce elevated peaks in magnetic susceptibility (Fig. F2).
In Unit III, consisting of chalk of early Eocene age (see "Biostratigraphy"), carbonate accumulation and preservation reach optimum values and CaCO3 never drops below 82 wt% (Fig. F2A; Table T2). Unbroken and fragmented foraminifers reach maximum abundance in interval 197-1204A-5R-2, 85-112 cm (802.6-802.9 mbsf). Also, calcite and dolomite peaks are in phase with altered feldspars. Volcanic ash and palagonite fragments are virtually absent in this unit but increase in abundance (to maximum values) in Unit III and Subunit IVA, which are Late Cretaceous to late Paleocene in age. At the Subunit IVA/IVB boundary, altered volcaniclastic clay and silt give way to a redeposited low-carbonate diamictite (Fig. F3) that was more completely recovered in Hole 1204B.
Unit I (761.9-774.7 mbsf) is characterized by light colors and consistently high carbonate values (51-92 wt%) (Table T2) that are directly related to the high content of nannofossils (80%-100%) and foraminifer fragments (up to 6%). Contacts are mainly sharp and often penetrated by horizontal and vertical burrows.
The unit is also characterized by low magnetic susceptibility values (20 x 10-6 to 100 x 10-6 SI) (Fig. F2). Differences in color in calcareous and nannofossil chalk (i.e., pinkish white and light gray and light brownish gray) are due to varying amounts of clay, Fe oxide, carbonate, and organic debris. In this unit the magnetic susceptibility values (
120 x 10-6 SI) appear to be correlated with several factors, including slight changes in grain size (i.e., clay to silt) and composition (e.g., Fe oxide and carbonate). Specifically, magnetic susceptibility seems to be sensitive to the presence of low but variable abundances of Fe oxide in the sediment (see "Site 1204 Smear Slides").
The Unit I/II boundary at Section 197-1204A-2R-3, 17 cm (774.7 mbsf), is at the upward termination of the cyclic deposition of ash-rich yellowish brown silt and nannofossil beds. This boundary is a sharp contact between very pale brown (10YR 8/3) chalk and the underlying brown beds. At the same depth nannofossil content decreases abruptly downward, from ~98% to 27%, along with the carbonate content (Fig. F2A; Table T2). Magnetic susceptibility peaks seem to correlate with composition, thickness, intensity, and structure of each single brown bed. For example, in Section 197-1204A-3R-1, the presence of 5- to 7-cm-thick beds is correlated with magnetic susceptibility values of 500 x 10-6 and 720 x 10-6 SI, whereas values of 450 x 10-6 to 500 x 10-6 SI are present in intervals containing thinner brown laminae (<1 cm) of silt-sized volcanic material.
Unit II contains nannofossil chalk and calcareous to clayey nannofossil chalk that exhibit pervasive submillimeter lenticular and undulating laminations. They are interbedded with brown to very dark gray beds of unaltered vitric ash with coccoliths as a secondary component. Centimeter- to decimeter-thick beds of dark yellowish brown (10YR 4/4) and black (5YR 2.5/1) volcaniclastic sandy silt contain unaltered and altered (palagonite) glass fragments. Sometimes, the brown beds are distinctly laminated, whereas on other occasions they are structureless with vertical burrows at the top. The brown beds generally contain more altered glass (palagonite) than the black beds.
The thickest bed is 57 cm and is sharply laminated with unaltered glass fragments at the bottom (intervals 197-1204A-3R-4, 120-125 cm, and 3R-5, 15-27 cm) and glass fragments altered to palagonite at the top (intervals 197-1204A-3R-4, 130-135 cm, and 3R-5, 5-13 cm) (e.g., Fig. F4B). The occurrence of such a thick volcanic interval explains the highest magnetic susceptibility values (1100 x 10-6 to 1300 x 10-6 SI) obtained for sediment in this hole.
A highly tilted "rotated block" of laminated volcaniclastic sediment (brown Fe oxide and palagonite-rich silt to silty clay), which is scoured and burrowed at its top, is present in interval 197-1204A-3R-2, 17-45 cm. This block is associated with high magnetic susceptibility values (up to 700 x 10-6 SI) and lies on folded and laminated calcareous chalk (see Fig. F4A). This suggests tectonically induced postdepositional slumping or sliding, although this interpretation should be taken with caution because it is based only on a preliminary examination of this core section.
The Unit II/III boundary is at Section 197-1204A-4R-3, 105 cm (794.8 mbsf). At this level nannofossil and carbonate abundance shows an abrupt downward increase (Fig. F2A). This boundary is associated with a downcore decrease in the magnetic susceptibility amplitude from maxima of 370 x 10-6 to 180 x 10-6 SI rather than with an abrupt shift in the absolute values at the lithologic boundary.
Unit III is white nannofossil chalk interbedded with clayey nannofossil chalk and nannofossil-foraminifer chalk. Magnetic susceptibility values rarely exceed 300 x 10-6 SI. These dominant lithofacies and the highest carbonate measurements (82.3-92.7 wt%; average = 87.4%; N = 6) indicate stable conditions for the deposition and/or preservation of biogenic carbonate.
The Unit III/IV boundary at Section 197-1204A-6R-1, ~60 cm (810.5 mbsf), is characterized by a sharp and bioturbated contact between light reddish brown (5YR 6/4) nannofossil chalk and pale red (10YR 6/3) clay-rich calcareous chalk (Fig. F4C).
This unit has been divided into two subunits, IVA and IVB.
In Hole 1204A, Subunit IVA is thicker (i.e., 511 cm thick) than in Hole 1204B (i.e., 197 cm thick) (Fig. F3).
A sharp drop occurs in nannofossil and carbonate content in Subunit IVA (late Paleocene to Late Cretaceous). Nannofossil chalks (CaCO3 = 57.5 wt%; N = 1) are frequently interbedded with Fe oxides and palagonite-rich clay (CaCO3 = 0.11-0.23 wt%; N = 2) and/or nannofossil clay that is more depleted in nannofossil and carbonates (CaCO3 = 7.2 wt%; N = 1).
In Subunit IVA bioturbated yellow and brown beds of silty clay and olive clay are interbedded with coarse- to fine-grained sediment containing volcanic ash. Yellow-olive-brown to olive clay includes fine black specks of Fe/Mn oxides (confirmed by inductively coupled plasma-atomic emission spectroscopy [ICP-AES] analysis) exhibiting spotty to laminated or vertical dendritic patterns produced by diagenetic growth. Similar centimeter-thick beds of this material alternate with more bioturbated ones, which are brown to yellow layered clay (after volcanic ash?) and finely laminated clayey Fe oxide-rich nannofossil chalk (interval 197-1204A-6R-3, 78-105 cm).
The basal section of Subunit IVA (i.e., interval 197-1204A-6R-3, 105 cm, to 6R-5, 12 cm) contains a thick (1.64 m) olive palagonite clay interval that exhibits consistently low magnetic susceptibility values (i.e., 50 x 10-6 to 100 x 10-6 SI). This is a typical signature for this lithology and is the same for thinner centimeter- to decimeter-thick beds. These are present throughout the Fe oxide-bearing chalky beds showing higher values (i.e., 150 x 10-6 to 450 x 10-6 SI) for the subunit.
Subunit IVA brackets the Cretaceous/Tertiary (K/T) boundary (upper Paleocene to upper Campanian), but its location and recovery remains uncertain (see "Biostratigraphy").
The Subunit IVA/IVB boundary (Fig. F3) corresponds to a sharp contrast between olive palagonite clay (Subunit IVA) and diamictite (Subunit IVB). The sharp change in the magnetic susceptibility corresponds to the marked compositional contrast across the contact between the firm olive clay and mud conglomerates (diamictite). This boundary is characterized by the same magnetic susceptibility signature in the two holes. Specifically, magnetic susceptibility values are 70 x 10-6 to 350 x 10-6 SI in Hole 1204A and 150 x 10-6 to 450 x 10-6 SI in Hole 1204B for Subunits IVA and IVB, respectively.
In Hole 1204A Subunit IVB is condensed (19 cm thick), whereas in Hole 1204B Subunit IVB is ~144 cm thick (Fig. F3). In Hole 1204B, Subunit IVB consists of poorly lithified and generally unsorted conglomerate (diamictite). Pebble-sized clasts are densely packed in a carbonate-poor silt-clay matrix (CaCO3 = 1.2-4.6 wt%; N = 2). The clasts consist of (1) silty clay and clay grains (mud grains) and (2) rounded to subangular pebbles of basalt and red silty sandstone. Sand to gravel-sized grains have a range of colors, reflecting differences in their make up, such as Fe oxide, palagonite, and gypsum-rich clay (see "Site 1204 Smear Slides"; Fig. F3A). Sand-sized crystals of tabular gypsum were observed (63-2000 µm) in sieved samples in intervals 197-1204B-1R-2, 30-31 cm, and 1R-2, 111-112 cm. Because of their mixed Maastrichtian to Campanian age (see
"Biostratigraphy") and reworked materials, Subunit IVB can be defined as a resedimented diamictite, and the lithologic logs in Figure F3B reflect this variation in the bulk compositions of matrix and
clasts.
Integration of the work done by the sedimentology and physical properties groups provided better synopsis and constraints on the origin of the volcaniclastic and biogenic sediment recovered at Site 1204. Magnetic susceptibility data commonly correlate with lithology in Holes 1204A and 1204B and aided interhole correlation. Because of low drilling disturbance in Holes 1204A and 1204B, magnetic susceptibility data were more useful in detecting lithologic boundaries and compositional/textural changes than in Hole 1203A.
High-resolution shipboard observations have raised questions related to the Upper Cretaceous/Tertiary sedimentation before deposition of the Meiji drift sequence. First, the Hole 1204B lowermost Subunit IVA has a different thickness than Subunit IVA in Hole 1204A (spaced at ~100 m). This observation and the evidence of heterogeneous, unsorted detrital material (i.e., nannofossil chalk, palagonite clay, gypsiferous clay, red sandstone, and basalt pebbles) and reworked debris suggest erosion from various source areas (e.g., shallow-water marine sediment, subaerial and submarine lava flows, and subaerial sedimentary sequences). Second, the exact position of the K/T boundary in Subunit IVA remains uncertain.
In the upper units of Hole 1204A, two periods of persistent calcareous sedimentation (Units I and III) are associated with current-related structures (e.g., undulating microlaminations) and intense bioturbation. This indicates good ventilation at the ocean floor and stable conditions for biogenic calcareous deposition and carbonate preservation during early to middle Eocene time.
However, more intense volcanic activity and occasional periods of regional tectonic instability occurred during the deposition of Unit II (early to middle Eocene time) on the slopes of Detroit Seamount. This is suggested by intraformational sediment folding and slumped blocks, also preserved at Sites 883 and 884 (Rea, Basov, Janecek, Palmer-Julson, et al., 1993) that might be responsible for regional downslope sediment redistribution (Schlanger and Premoli-Silva, 1981; Rea and Thiede, 1981). The strong reduction in preservation or deposition of carbonate observed for the lower Eocene pelagic sediment seems to be associated with frequent episodes of nearby volcanic (subaerial) activity, as recognized both at scattered locations in the central North Pacific Ocean (Vallier et al., 1983) and during Leg 145.
At Site 883 the carbonate content was suggested to be entirely regulated by the sedimentation of calcareous nannofossils rather than other detrital calcareous components (Rea, Basov, Janecek, Palmer-Julson, et al., 1993). However, at the top of the lower Eocene sequence (Unit II) the total carbonate (CaCO3) does not appear to be correlated with the relative abundance of calcareous nannofossils. Further work is needed to confirm this observation and examine its potential implication.
