At Site 1228, we recovered a 200.9-m-thick sequence of biogenic and siliciclastic Pleistocene and Pliocene sediments that were deposited on the Peruvian shelf. Based on textural, color, and compositional variations, sediments from Site 1228 were divided into two lithostratigraphic units (Fig. F1). The proposed lithostratigraphy is mostly based on Hole 1228A, as it was the only hole at Site 1228 that was continuously cored from the sediment surface to a depth of 200.9 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. X-ray diffraction (XRD) and laboratory measurements of magnetic susceptibility, density, and natural gamma radiation (NGR) (see "Physical Properties") were also used to detect lithologic and sedimentologic changes. As Site 1228 is located in close proximity (within 100 m) of Leg 112 Site 680, the age framework presented in this chapter follows the chronostratigraphic observations of the Leg 112 Shipboard Scientific Party (1988b).
Lithostratigraphic Unit I is characterized by alternating predominantly biogenic sediments (mostly laminated diatom ooze) and silt- and clay-sized predominantly siliciclastic sediments. These alternations presumably reflect shifts in the relative dominance of the two main sources for sediments at the Peruvian continental margin: hemipelagic upwelling-derived diatomaceous oozes and reworked terrigenous clay, silt, and sand. The further subdivision of this unit into three subunits is based on smaller-scale lithologic and compositional changes and variations of other parameters including color reflectance and magnetic susceptibility (Fig. F1).
The uppermost 3 m of this unit is dominated by dark olive, weakly laminated nannofossil- and diatom-bearing silt. The base of this interval is marked by a sedimentary layer of increased terrigenous (quartz and lithic fragments) and glauconitic components, which correlates with a small peak in magnetic susceptibility (Fig. F2). A few centimeter-thick light gray volcanic ash layers are present in Sections 201-1228A-1H-1 and 1H-2 (Fig. F3A). Section 201-1228B-1H-2 contains a pink F-phosphate nodule (XRD Sample 201-1228A-1H-2, 121-122 cm; friable phosphate, according to Leg 112 classification; Shipboard Scientific Party, 1988b).
Below 3 mbsf, the sediment grades into a greener and more strongly laminated nannofossil-bearing silt-rich diatom ooze. Several fish remains including vertebrae are present through Core 201-1228A-1H. The lamination is caused by an alternation of pale green to yellow and darker layers. These layers range in thickness from a few millimeters to a few centimeters. The lighter-colored laminae are rich in diatom frustules. Although most laminae show horizontal orientation, cross-, low-, and high-angle laminations were also observed (e.g., in Sections 201-1228A-2H-7 and 2H-6). In Section 201-1228A-2H-6 between 142 and 146 cm, laminae are present at a very high angle and are probably the result of rotational slumping (Fig. F3B).
Below ~5 mbsf, the dominant laminated diatom oozes alternate with darker, more homogeneous and siliciclastic-rich sediments that are similar to the sediments of the uppermost 3 m. Pale gray ash layers containing rare opaque clasts and abundant clay are also present throughout this subunit. In particular, volcanogenic material was observed in Cores 201-1228A-1H and 3H, where three ash layers varying in thickness from 4 to 6 cm show graded bedding and sharp basal contacts, indicating a redistribution of terrigenous and volcanic material on the seafloor. Small cream or pink nodules of F-phosphate were observed throughout the unit and are particularly abundant in Core 201-1228A-3H.
This subunit is characterized by an increase in terrigenous components, which include both clay and silt fractions. The boundary between Subunits IA and IB is also highlighted by a positive excursion in magnetic susceptibility (Fig. F2) (see also "Physical Properties").
The dominant lithology is dark brown to black diatom-rich clay and diatom-rich clayey silt with a few scattered foraminifers. Yellow diatomaceous laminae are composed of opal-A and dolomite (Fig. F4A) (see also "Mineralogy"). Clay layers are poorly laminated and are interbedded with brown to green well-laminated silt layers composed of calcite, quartz, feldspar, and variable amounts of dolomite (XRD Sample 201-1228A-4H-5, 125-126 cm). Several of the silt layers show graded bedding and erosive basal contacts. At Section 201-1228A-4H-5, 62 cm, a carbonate-cemented layer (presumably dolomite) is present within a lighter-colored interval. Pyritized radiolarians and foraminifers are common. Three volcanic ash layers are present in Section 201-1228A-4H-6. In Core 201-1228A-5H, several centimeter-thick silty sand layers with sand-sized foraminifers alternate with dark brown to yellowish brown clay-rich diatom ooze layers. Normal grading, erosional contacts, bioturbated intervals, and sigmoidal fluid escape structures (particularly abundant in Section 201-1228A-5H-4) are common sedimentary features of these silty sand layers.
Subunit IC is composed of several lithologies. The dominant lithology is a brown to olive dolomite-bearing diatom ooze that shows varying degrees of lamination. The diatom ooze alternates with light brown clay-, quartz-, and feldspar-rich silt layers and diatom-bearing clay layers. Two ash layers with both parallel and cross lamination were observed at Sections 201-1228A-6H-4, 60 cm (Fig. F3C), and 6H-6, 60 cm. Yellow diatom-rich laminae are common, and several pink phosphate nodules were observed throughout the subunit (e.g., XRD Sample 201-1228A-6H-6, 14-15 cm). Fluorapatite is the main component of black, hard phosphate concretions (e.g., XRD Samples 201-1228A-6H-4, 17-18 cm; 6H-4, 137-138; and 6H-6, 14-15 cm) (Fig. F3D) (D-phosphates, according to Shipboard Scientific Party, 1988b), some of which also contain dolomite (Fig. F4D) (see also "Mineralogy"). In interval 201-1228A-6H-2, 70-100 cm, and at Section 7H-2, 70 cm, two dolomite layers are present (Fig. F3E). A black vitreous opal-CT porcelanite lens was found at 52.9 mbsf in Hole 1228A (XRD Sample 201-1228A-7H-1, 53-54 cm; Fig. F4B) (see also "Mineralogy"). In Section 201-1228A-7H-4, a black phosphatic layer, probably a hardground, has a basal erosional surface indicative of a hiatus at this depth.
The top of Unit II corresponds to the end of a sedimentary regime in which sedimentation was strongly dominated by the terrigenous input of clay, silt, and sand. Hemipelagic biogenic input was only secondary. The top of Unit II at 56.7 mbsf coincides with a shift of color reflectance, a sharp peak and an overall increase of magnetic susceptibility, and a decrease of porosity (Figs. F1, F2). Three subunits were recognized in Unit II. These subunits differ mainly in the proportions of terrigenous and biogenic components.
The main lithology of Subunit IIA is gray diatom- and quartz-bearing clay (XRD Sample 201-1228A-7H-6, 100-101 cm; Fig. F4C). This portion of the sedimentary record is mostly homogeneous and grades into green laminated diatom- and quartz-bearing clay and silt toward the base of the subunit. A sharp peak in magnetic susceptibility indicates a higher concentration of magnetic minerals at this level (Fig. F2). The upper boundary of this subunit is well defined by prominent variation of color reflectance (Fig. F1).
Subunit IIB is characterized by alternations of gray diatom- and quartz-bearing clay and silt with coarser green-olive glauconite-bearing quartz-rich lithic sand. Sediments dominated by biogenic components such as diatoms and nannofossils are rare and are mostly present in clay-rich layers in the uppermost part of the subunit. Cross lamination and erosional contacts at the base of coarser-grained layers are common features. Scattered greenish laminated layers of diatom-rich silt, clay, and volcanic glass shards are also common in this subunit (Fig. F3F, F3G).
Drilling disturbance strongly affected this portion of the cored interval. The top of most cores is marked by as much as 40 cm of pebbles and/or angular clasts of different sizes and lithologies (including dolostones and phosphates as well as well-rounded rock pebbles of terrigenous origin). These core-top layers probably originated from the collapse of higher stratigraphic levels within the hole. Drilling disturbance is also suggested by the homogenized and "soupy" appearance of many of the sections recovered from this interval.
Cores retrieved from Subunit IIC were commonly poorly preserved and soupy. For this reason, detailed lithologic and sedimentologic observation could only be performed in a few cores.
The main lithology of this subunit consists of greenish gray diatom- and quartz-rich feldspar silt and sand. Pale green and yellow to dark olive clay- and nannofossil-bearing diatom ooze with few scattered diatom laminae, pink F-phosphate, and dolomite nodules are also present in this subunit (e.g., Samples 201-1228A-16H-1, 83-84 cm, and 16H-2, 97-98 cm; Fig. F4E) (see also "Mineralogy"). Between ~110 and 120 mbsf (Core 201-1228A-13H), sediments show higher silt and clay content.
X-ray diffraction analyses were performed on 15 samples from Site 1228. The purpose of mineralogic sampling was to determine both small-scale compositional changes (e.g., differences between laminae of different colors) and the mineralogy of authigenic nodules. Overall, the mineralogic composition of the samples collected from Unit I reflects the high biogenic content of these upwelling-related hemipelagic sediments. Lamination, which is a prominent characteristic of this unit, is caused by episodic and possibly cyclic deposition of very pure opal-A diatomaceous ooze layers (Fig. F4A). Smear slide and XRD analyses also show that some of these diatomaceous laminae contain scattered dolomite rhombohedra (Fig. F4A). A high concentration of biogenic silica is perhaps the cause for the shallow depth of the opal-A to opal-CT phase change expressed by the presence of a black vitreous opal-CT porcelanite lens at 53 mbsf (Fig. F4B). Deposition of Unit II was mainly influenced by terrigenous input, especially in Subunit IIA, which consists of very fine grained gray silt composed of quartz, feldspar, and clay minerals (Fig. F4C).
At Site 1228, two kinds of authigenic minerals were observed, apatite and dolomite. Both minerals are present throughout the recovered interval but are concentrated in Subunits IC and IIB. Apatite is either present as small (a few millimeters) white to pink friable nodules (F-phosphate) (Fig. F4E) or as dark, hard nodules that often replace burrows and may have been formed during depositional hiatuses (D-phosphate) (Fig. F4D). Dolomite nodules are usually white to yellow and, in a few cases, green. The XRD spectrum for a green dolomite nodule in Subunit IIC is shown in Figure F4F.
The upper 56.7 mbsf of the sedimentary sequence at Site 1228 is characterized by predominantly biogenic hemipelagic sediments (Unit I), formed in an upwelling environment analogous to today's setting along the Peruvian continental margin. In contrast, the lower part of the sediment column is dominated by fine-grained siliciclastic components that indicate the local sedimentary environment was influenced by terrigenous input and, thus, was different from its modern condition (Unit II). Lithostratigraphic Unit I consists of alternating intervals of predominantly biogenic (mostly laminated diatom ooze) and silt- and clay-sized predominantly siliciclastic sediments. Subunit IA grades from dark olive, weakly laminated nannofossil- and diatom-bearing silty sediments into a greener and more strongly laminated nannofossil-bearing silt-rich diatom ooze. Deeper in this subunit, diatom ooze alternates with darker, more homogeneous and again siliciclastic-rich sediments. Pale gray layers of volcanogenic origin and siliciclastic-rich sediments showing graded bedding and erosional basal contacts are present throughout the subunit, indicating a redistribution of terrigenous and volcanic material by bottom currents. The dominant lithology of Subunit IB is dark brown to black diatom-rich clay and diatom-rich clayey silt. Yellow diatomaceous laminae composed of opal-A and dolomite are frequently present. Clayey intervals are poorly laminated and alternate with brown to green well-laminated silt layers composed of calcite, quartz, feldspar, and variable amounts of dolomite. Diatom ooze of Subunit IC alternates with light brown clay- and feldspar-rich silt layers and diatom-bearing clay layers. Ash layers, diatom-rich laminae, and pink phosphate nodules are common throughout the subunit. Fluorapatite is the main component of black, hard phosphate concretions. A black phosphatic layer with an erosional base probably reflects the formation of a hiatus and marks the base of Unit I. The onset of Unit II corresponds to the beginning of a sedimentary regime strongly dominated by the terrigenous input of clay, silt, and sand. Subunit IIA is characterized by mostly homogeneous gray diatom- and quartz-bearing clay that grades into green laminated diatom- and quartz-bearing clay and silt toward the base of the interval. Subunit IIB shows an alternation of gray diatom- and quartz-bearing clay and silt with coarser olive-green glauconite-bearing quartz-rich lithic sand. Cross lamination and erosional contacts are common features of this subunit. Biogenic components such as diatoms and nannofossils are rare and are mostly present in clay-rich layers in the uppermost part of the subunit. Drilling disturbance strongly affected this portion of the sedimentary record. The main lithology of Subunit IIC consists of greenish gray diatom- and quartz-rich feldspar silt and sand; however, drilling disturbance and poor recovery precluded a more detailed description of the lowermost part of the cores from Site 1228.
Diagenetic processes in Site 1228 sediments are dominated by the formation of authigenic dolomite and phosphate nodules and concretions. Dolomite was especially common throughout the recovered interval, particularly in Subunits IC and IIB. Phosphate is present both as pink friable nodules (F-phosphate) and as dark, hard concretions (D-phosphate), which often replace burrows and form hardgrounds. Phosphate composition is usually fluorapatite, but smaller amounts of carbonate fluorapatite and dolomite were also detected. The presence of porcelanite at relatively shallow depth (~53 mbsf) and the presence of authigenic phosphate and dolomite minerals throughout the sediment column emphasize the wide range of diagenetic processes at Site 1228.