At Site 1227, we recovered a 151-m-thick sequence of alternating diatomaceous and siliciclastic sedimentary packets. Lithologic, textural, and compositional variations observed in these sediments justify subdivision into four lithostratigraphic units (Fig. F2). The proposed lithostratigraphy framework is mostly based on Hole 1227A, as it was the only hole at Site 1227 that was continuously cored from the sediment surface to a depth of 151.1 mbsf. The lithostratigraphic description of the sedimentary sequence is based on visual observation of sediment color and sedimentary structures (visual core description), smear slide analysis, and color reflectance. XRD and laboratory measurements of magnetic susceptibility, density, and natural gamma ray (see "Physical Properties") were also used to detect lithologic changes (Fig. F3). As Site 1227 is located in close proximity (within 100 m) of Site 684, which had previously been drilled during Leg 112, the age framework presented in this chapter was obtained by applying the timescale of Berggren et al. (1995a, 1995b) to the magnetostratigraphic and biostratigraphic observations of the Leg 112 Shipboard Scientific Party (1988).
The main lithology of Unit I consists of well-laminated olive silt- and clay-rich diatom ooze. In the lower part of the unit, two coarse-grained green to dark olive diatom-bearing, silt-rich foraminifer ooze layers alternate with laminated clay-bearing diatom ooze. The laminated interval between the two foraminifer ooze layers ranges between 2.0 and 2.6 m thick. These differences in lithology and texture between the upper and the lower part of Unit I account for its subdivision into Subunits, IA and IB (Fig. F2).
Subunit IA consists of mostly olive to green clay-rich diatom ooze characterized by very fine scale lamination (laminae range from <1 mm to a few millimeters in thickness). Most of the laminae show parallel bedding; however, the presence of low-angle laminations and pinch-out terminations of sedimentary layers seems to indicate some reworking by bottom currents (Fig. F4A). Coarser and lighter-colored laminae are usually richer in opal-A diatom frustules, whereas darker foraminifer-bearing laminae are composed of calcite, quartz, and feldspars. Fish scales and vertebra as well as crab debris are locally present in these sediments. Subunit IA corresponds to a low magnetic susceptibility zone (Fig. F3).
The diatom ooze of the overlying Subunit IA grades into a dark olive to green silt-rich foraminifer ooze containing scattered shell debris as well as small thick-walled taxodont bivalves (Fig. F4B). The foraminifers are mostly benthic. A bone fragment was found in Section 201-1227A-2H-1 at 39 cm (Fig. F4C). Sediments of Subunit IB are characterized by a pronounced excursion in magnetic susceptibility, natural gamma radiation, and density (Fig. F3).
Near the base of Section 201-1227D-2H-2, a ~15-cm-thick phosphatic hardground defines the base of Subunit IB (see "Mineralogy"; XRD results for Sample 201-1227D-2H-2, 139-140 cm). The phosphate is black and hard (D-phosphate, according to the Leg 112 classification; Shipboard Scientific Party, 1988) and replaces a burrowed interval. Phosphate nodules composed of carbonate fluorapatite are also present in Section 201-1227A-2H-4 (Fig. F5A). The transition to Unit II is marked by a pronounced shift in the chromaticity (b*) data toward lower values (Fig. F2).
Unit II at Site 1227 consists of dark olive-brown to black bioturbated silty sediments. Glauconite and phosphate grains are common. Based on variations in diatom content, two subunits were established within Unit II. Subunit IIA is characterized by low diatom concentrations (<5%), whereas the sediments of Subunit IIB show diatom contents of up to 25% as well as small amounts of biogenic carbonate (mostly foraminifers) (Fig. F2). The boundary between the two subunits is not sharp but rather is a gradual transition from an almost pure siliciclastic interval to sediments containing greater amounts of biogenic silica.
The dominant lithology of Subunit IIA is a dark olive-brown bioturbated diatom-bearing clay- and pyrite-rich silt. In Section 201-1227A-2H-4, the contact between the overlying coarser-grained foraminifer ooze of Subunit IA and the dark brown diatom-bearing silt of Subunit IIA is sharp, and two black phosphate nodules (Figs. F3, F4D; XRD results for Sample 201-1227-2H-4, 92-93 cm) were observed. Variable amounts of quartz and glauconitic components were detected in sediment-filled burrows.
Natural gamma radiation and magnetic susceptibility data for this subunit (Fig. F3; see also "Physical Properties") show a prominent peak. These high values probably reflect a relatively high content of terrigenous components as well as a concentration of an unknown magnetic mineral in Subunit IIA.
Subunit IIB was distinguished from overlying Subunit IIA based on higher amounts of diatom frustules, fewer clastic components, and darker color. The main lithology consists of homogeneous to slightly bioturbated dark brown to black clay-, pyrite-, and diatom-bearing to diatom-rich silt with scattered sand-sized benthic foraminifers. At Section 201-1227A-4H-5, 62 cm, a lighter-colored calcite-cemented layer is present (XRD Sample 201-1227A-2H-4, 92-93 cm). Pyritized radiolarian and foraminiferal tests are common. In interval 201-1227D-4H-5, 90-110 cm, two vertical veinlets of a very fine grained barite are present (Fig. F4E) (see also "Mineralogy" and XRD results for Samples 201-1227D-4H-5, 100-101 cm, and 5H-4, 94-95 cm). The uppermost boundary of Subunit IIB is marked by a sudden decrease in both natural gamma radiation and magnetic susceptibility (Fig. F3) (see also "Physical Properties"), which probably reflects a decrease of terrigenous input and lower concentrations of magnetic minerals.
A reduction of the diatom component (Fig. F2) and a parallel increase of clay, quartz, feldspar, silt-sized pyrite, and glauconite characterize this unit. Two main lithologies, dolomite-bearing diatom- and nannofossil-rich clayey silt and dark brown dolomite-, quartz-, feldspar-, and pyrite-rich sandy silt with scattered foraminifers and variable amounts of glauconite, characterize the uppermost and the lowermost part of this subunit, respectively. The concentration of glauconitic clasts as well as the grain size of the sediment varies throughout the unit. Both fining- and coarsening-upward sequences are common (e.g., the top of Section 201-1127A-6H-2 consists of silt-sized pyrite and glauconitic grains, whereas the bottom is a diatom-rich clay). However, diatom-rich clay is mostly present in the upper part of the unit, and clasts tend to be coarser toward the bottom of the unit, where both sand grains and millimeter- to centimeter-sized shell fragments are common. Evidence of bioturbation also appears to be more intensive in the lower part of the unit (i.e., below 45 mbsf in Hole 1227A). Usually, bioturbated intervals were found near the boundaries between layers of different grain size, where they are manifested by mottles and burrows of silty/sandy sediment in clayey/silty layers and vice versa. Section 201-1227A-6H-3 contains a reddish brown diatom-rich layer that includes a pale pink lens composed of calcite and apatite (Fig. F5C) (see also "Mineralogy").
The fining-upward trend observed in Unit III is also reflected by a significant decrease in both magnetic susceptibility and natural gamma radiation (Fig. F3) (see also "Physical Properties"). In particular, both parameters show a peak between 40 and 51 mbsf, where most of the silt and sand layers are also concentrated. A similar parallelism between positive excursions in grain size, magnetic susceptibility, and natural gamma radiation was already observed in Subunit IB.
The main lithology of Unit IV is a dark green clay- and nannofossil-bearing diatom ooze (Fig. F2). According to the biostratigraphic determinations from Site 684, the boundary between Units III and IV corresponds to a major stratigraphic hiatus (Shipboard Scientific Party, 1988). In particular, nannoplankton data indicate that this stratigraphic gap ranges between 5.7 and 8.7 Ma (Shipboard Scientific Party, 1988, modified after Berggren et al., 1995b). The sediment is generally homogenous and contains only a few scattered laminated or bioturbated layers. Volcanic ash layers and dark volcanic glass shards are present throughout the unit. Section 201-1227A-12H-1 contains a laminated dolomite nodule at a depth of 85 cm in the section (Fig. F5D). The core catcher of Core 201-1227A-8H contained gravel-sized dark green, olive (probably phosphate), and white (probably dolomite) clasts, mixed with shell hash and olive-green nannofossil- and clay-bearing diatom ooze. Gravel of the same composition as the gravel recovered in Core 201-2117A-8H was found in the first 30-40 cm of every subsequent core, which strongly indicates that the presence of gravel at the tops of Cores 201-1227A-9H through 18H is, in fact, a drilling artifact. The sediments in Cores 201-1227A-12H and 13H are diatom poor and consist mostly of dark olive-green to brown clay and silt (Fig. F2). Below Core 201-1227A-14H, both recovery and preservation of the cores were very poor (Fig. F2).
X-ray diffraction analyses were performed on 19 samples from Site 1227. XRD spectra of the main sedimentary components confirm the results of visual core descriptions and smear slide analyses. Biogenic opal-A and calcite dominate the composition of the foraminifer-bearing diatom-rich sediments of Subunits IB and IIA and Unit III. Pyrite is common throughout the sedimentary section (e.g., XRD results for Sample 201-1227A-6H-3, 90-91 cm) (Fig. F5B).
XRD analyses confirmed the presence of various diagenetic minerals in Site 1227 sediments. As already noted by the Leg 112 Shipboard Scientific Party (1988), there appears to be a close association between depositional environment and the formation of certain diagenetic mineral assemblages at Site 1227. Authigenic carbonate nodules are closely associated with diatom-rich upwelling-derived sediments, whereas massive phosphates and glauconite appear to be associated with sediments that were deposited in a shallower, current-dominated environment.
Black, hard nodules of apatite and carbonate fluorapatite (D-phosphate, according to Leg 112 classification; Shipboard Scientific Party, 1988) define the base of Subunit IB at Site 1227 (Fig. F5A; XRD results for Samples 201-1227A-2H-4, 92-93 cm, and 201-1227D-2H-2, 139-140 cm).
Phosphate nodules composed of apatite and carbonate fluorapatite are also present in Unit III (XRD results for Sample 201-1227A-6H-3, 92-93 cm). The formation of these D-phosphate nodules has been attributed to variable and high-energy conditions during past sea level lowstands (Garrison and Kastner, 1990).
Carbonate nodules are more common in sediments dominated by biogenic components. XRD analysis of a laminated carbonate nodule from Unit IV (XRD results for Sample 201-1227A-12H-1, 85-86 cm) revealed the presence of dolomite (Fig. F5D). Authigenic calcite was also observed (e.g., XRD results for Sample 201-1227A-6H-3, 135-136 cm) (Fig. F5C). For a detailed discussion of the diagenetic environment at Site 1227, see Shipboard Scientific Party (1988) and Garrison and Kastner (1990).
Thin white veinlets and nodules of authigenic barite were observed in Hole 1227D at depths between 33.5 and 45.6 mbsf (XRD results for Samples 201-1227D-4H-5, 100-101 cm; 5H-4, 94-95 cm; and 6H-1, 15-16 cm) (Fig. F5E). XRD spectra point to strontian barite as the main barite mineral. The presence of barite is closely tied to the interstitial water sulfate, barium, and methane cycle at Site 1227. Barite is inferred to form in Site 1227 sediments when interstitial water sulfate levels approach concentrations around ~2 mM, due to anaerobic oxidation of methane (see "Biogeochemistry"). This in turn triggers barite precipitation and the formation of a "barite front" near the base of the sulfate reduction zone.
The sedimentary sequence at Site 1227 consists of large-scale alternations of predominantly biogenic sediments, formed in an upwelling environment analogous to today's conditions along the Peruvian continental margin (Unit I, lower part of Unit II [Subunit IIB], and Unit IV), and sediments that are dominated by siliciclastic components, indicating deposition in sedimentary environments different from modern conditions (Unit II [Subunit IIA] and Unit III). Lithostratigraphic Unit I consists of laminated olive silt- and clay-rich sediments dominated by diatoms. In the lower part of the unit, two coarse-grained green to dark olive diatom-bearing silt-rich foraminifer ooze layers alternate with laminated clay-bearing diatom ooze. Unit II is dominated by dark olive-brown to black bioturbated silty sediments. Based on variations in diatom content, two subunits were established. Subunit IIA is characterized by low diatom concentrations, whereas the sediments of Subunit IIB show, on average, higher diatom content as well as small amounts of biogenic carbonate. The boundary between the two subunits is not sharp but rather is a gradual transition from an almost pure siliciclastic interval to sediments containing greater amounts of biogenic silica. The upper part of Unit III is characterized by dolomite-bearing diatom- and nannofossil-rich clayey silt, whereas the lower part is mostly a dark brown dolomite-, quartz-, feldspar-, and pyrite-rich sandy silt with scattered foraminifers and variable amounts of glauconite. Dark green clay- and nannofossil-bearing diatomaceous sediments characterize Unit IV. The sediment is generally homogeneous and contains only a few scattered laminated or bioturbated layers. Volcanic ash layers and dark volcanic glass shards are present throughout the unit. Material of the same composition as the gravel layer recovered in Section 201-2117A-8H-CC was found in the top of every subsequent core, which strongly indicates that the presence of gravel in core tops below ~65 mbsf is due to drilling artifacts. Below Core 201-1227A-14H, both recovery and preservation of the cores were very poor.
Diagenetic processes in Site 1227 sediments caused the formation of authigenic phosphates (apatite and carbonate fluorapatite), carbonates (calcite and dolomite), and barite. The formation of different diagenetic minerals can be correlated to distinct geochemical and/or sedimentary environments, such as ongoing anaerobic oxidation of methane or changes in oceanographic conditions (sea level fluctuations) during the last 11 m.y.