Drilling at Site 1220 recovered a 200.3-m section of pelagic sediments overlying seafloor basalt. The upper 25 m is lower Miocene clay and radiolarian ooze, underlain by 45.6 m of Oligocene nannofossil and radiolarian ooze and 129 m of Eocene radiolarian ooze with alternations of nannofossil ooze, clay, and chert. A 25-cm-thick upper Paleocene chalk section was recovered above basement.
The sedimentary sequence at Site 1220 has been divided into six major lithologic units and subunits (Fig. F3). Lithologic descriptions for 0 to 114.0 mbsf are primarily based on sediments from Hole 1220A. Below 114.0 mbsf, descriptions are based on the sequence recovered in Holes 1220B and 1220C.
Unit I consists of very dark grayish brown (10YR 4/2) clay with zeolites grading to a dark brown (10YR 3/3) clay. Zeolites are a significant component (10%-15%) in the upper 14.9 m (Sections 199-1220A-2H-1 through 2H-4, 93 cm). Mottles are common in the uppermost 9.5 m of Unit I and rare between 9.5 and 19.4 mbsf (Sections 199-1220A-1H-CC through 2H-CC). Iron oxides and manganese nodules are present in minor to trace amounts below 18.2 mbsf (Section 199-1220A-2H-6, 130 cm).
The dominant lithology in Unit II is very dark brown (7.5YR 2.5/3) to dark yellowish brown (10YR 3/4) radiolarian ooze. Thin alternations of yellowish brown (10YR 6/4) nannofossil radiolarian ooze are present between 24 and 27 mbsf (interval 199-1220A-3H-4, 55 cm, through 3H-6, 85 cm). Contacts between the lithologies are gradational and mottled. Brownish yellow (10YR 6/6) mottles are common to abundant in both lithologies.
Unit III consists of pale brown (10YR 8/3) to yellowish brown (10YR 5/4) nannofossil ooze. Between 37.2 and 56.9 mbsf (interval 199-1220A-4H-6, 123 cm, through 6H-7, 40 cm), the nannofossil ooze alternates with decimeter-scale intervals of dark brown (10YR 3/3) radiolarian ooze and light yellowish brown (10YR 6/4) nannofossil diatom ooze. The thickness of the nannofossil ooze layers increases downcore, and color grades to very pale brown (10YR 8/2). Contacts between lithologies are gradational and are obscured by bioturbation.
Clay content (based on smear slide descriptions) ranges from <5% in nannofossil ooze to 15%-20% in radiolarian ooze. Diatoms are absent in the upper portion of Unit III but increase from 15% to 45% downcore in the lowermost 6 m of the unit.
Unit IV is divided into two subunits, based on the presence or absence of chert. Subunit IVA consists of radiolarian ooze with varying clay content, and Subunit IVB is composed of chert and clayey radiolarian ooze.
The dominant lithology in Subunit IVA is radiolarian ooze with clay. Color ranges from dark yellowish brown (10YR 3/4) to dark brown (10YR 3/3). Yellow (10YR 7/6) and pale brown (10YR 6/3) mottles are common. Diatom abundance is generally very low but occasionally increases to 35% in minor lithologies from smear slide estimates.
An interval of clay with radiolarians is present near the base of Subunit IVA (Hole 1220B: 144.2-152.0 mbsf). The thickness of this clay is probably exaggerated in Hole 1220B by sediment flow-in associated with coring a 5-cm-thick claystone interval (intervals 199-1220B-13H-1, 75-80 cm, and 199-1220C-11X-2, 89-94 cm).
Subunit IVB consists of chert and clayey radiolarian ooze. The chert and surrounding sediment were poorly recovered. Chert occurs as black (N1) fragments broken by the drilling process. Clayey radiolarian ooze ranges from strong brown (7.5YR 4/6) to yellowish brown (10YR 5/6). Decimeter-scale alternations of clay and radiolarian ooze with iron oxides are present between 173.9 and 183.5 mbsf (Sections 199-1220B-16X-4, 25 cm, through 16X-CC and 199-1220C-14X-1, 0 cm, through 14X-CC). Yellowish brown (10YR 5/4) mottles are common throughout this subunit. A layer of volcanic ash is present near 171.5 mbsf (interval 199-1220B-16X-2, 88-92 cm).
Unit V primarily consists of light yellowish brown (10YR 6/4) to very pale brown (10YR 7/4) nannofossil ooze and radiolarian nannofossil ooze. Dark yellowish brown (10YR 4/6, 10YR 3/6) radiolarian ooze is present between 190.42 and 190.8 mbsf (interval 199-1220B-18X-3, 20-58 cm). Very pale brown (10YR 7/2) mottles are common. Authigenic dolomite crystals constitute a significant proportion of the nannofossil ooze (10%-35%).
Between 198.9 and 199.7 mbsf (interval 199-1220B-20X-2, 4-85 cm), there is an interval of calcareous chalk and clay. Based on biostratigraphic data (see "Biostratigraphy"), this sequence represents the P/E boundary. The P/E boundary was only recovered in Hole 1220B. Layers of calcareous chalk and clay display a large range in composition and color downcore (Figs. F4, F5). Alternating intervals of white and very pale brown (10YR 8/3) calcareous chalk are present between 198.9 and 199.4 mbsf (interval 199-1220B-20X-2, 10-48 cm) and contain a minor amount (5%-10%) of poorly preserved nannofossils. White layers are present between 199.15 and 199.23 mbsf (interval 199-1220B-20X-2, 25-33 cm) and contain 15%-30% planktonic foraminifers. The sediments below this interval consist of faintly banded, dark yellowish brown (10YR 3/4) calcareous chalk but are barren of microfossils (interval 199-1220B-20X-2, 48-60 cm). Dolomite (up to 10%) and clay (10%-35%) also are present in this lithology.
Below 199.50 mbsf (Section 199-1220B-20X-2, 60 cm), is a 4-cm-thick layer of very dark brown (7.5YR 2.5/3) calcareous chalk underlain by a black (10YR 2/1) clay. Major components of both sediments are dolomite, iron manganese oxides, clay, and possibly sulfides. Volcanic ash is present as a minor component. Dolomite content decreases from 199.52 to 199.54 mbsf, and no calcareous or siliceous fossils were observed in smear slides.
Directly beneath the black clay, there is a 4.5-cm-thick interval of yellowish red (5YR 5/6) calcareous chalk (interval 199-1220B-20X-2, 64-69 cm; 199.54-199.59 mbsf). Within this interval is a 1-cm-thick brownish yellow (10YR 6/6) layer of calcareous chalk. Sediments within the yellowish red layers are weakly laminated and contain clay, dolomite, calcite, and a minor percentage of nannofossils. Iron manganese oxides and opaque minerals are minor components. The brownish yellow chalk layer contains clay, dolomite, iron manganese oxides, volcanic glass, and abundant small mottles. Nannofossils are rare in these layers.
The base of the sedimentary section contains a 9-cm-thick interval of brownish yellow (10YR 6/6) clayey calcareous chalk (interval 199-1220B-20X-2, 69-78 cm; 199.59-199.68 mbsf) with thin black (10YR 2/1) laminations. Dolomite comprises up to 10% of the sediment. Zeolites and nannofossils also are present as minor components. Iron manganese oxides are rare. Laminations contain clay, iron manganese oxides, dolomite, zeolites, and small amounts of volcanic ash and are barren of calcareous fossils.
Basaltic fragments containing aphanitic to fine-grained phaneritic texture were recovered at the base of Holes 1220B and 1220C. Thin glassy rinds are present on some of the pieces. Exterior surfaces are moderately altered from weathering processes. Interior surfaces show common oxidation of some of the minerals. Numerous calcite veins ranging from 1 to 5 mm in thickness are found in several of the pieces from Hole 1220C.
At Site 1220, Paleocene seafloor basalt is overlain by 200.3 m of pelagic sediments that are divided into five lithologic units. Although sediments here are dominated by siliceous ooze, the sequence can be correlated roughly with the more calcareous Units I-V at Sites 1218 and 1219.
The Paleocene-lower Eocene sequence is carbonate rich. The P/E boundary section (Hole 1220B: 199.37-199.68 mbsf) consists of calcareous chalk and black clay and differs in lithology and color from the dark brown nannofossil clay of similar age at Site 1215.
The Eocene section in Hole 1220C is 129 m thick and contains brown to dark brown radiolarian ooze with varying amounts of clay and nannofossils. The E/O boundary is marked by a transition from dark brown radiolarian ooze below to pale brown nannofossil ooze above. This contact is not sharp, probably because of mixing by bioturbation. A transition from siliceous sedimentation during the Eocene to carbonate deposition during the Oligocene is also observed in sediments from several other sites in the region (e.g., Sites 1218 and 1219, and Deep Sea Drilling Project Sites 161 and 162) and probably reflects a deepening of the CCD (van Andel et al., 1975).
Lower Oligocene sediments consist mainly of pale brown nannofossil ooze. The upper Oligocene section contains alternations of dark yellowish brown radiolarian ooze, and the O/M boundary is represented in Core 199-1220A-3H. Sediments from the lower Miocene are dominated by dark brown radiolarian ooze and show a relatively sharp transition to clay and clay with zeolites in the lower Miocene just above the O/M boundary. Sediments younger than the early Miocene were not recovered at this site.