At Site 1229 we recovered a 194.4-m-thick sequence of mixed diatomaceous and siliciclastic sediments. According to the biostratigraphic and magnetostratigraphic chronostratigraphy established for Site 681, which is located in close proximity of Site 1229 (Shipboard Scientific Party, 1988), updated according to Berggren et al. (1995), the entire sedimentary sequence is late Pliocene to Pleistocene age. In order to establish unit and subunit boundaries, information from Holes 1229A and 1229D is combined to fill gaps caused by poor recovery. Based on lithologic changes detected by means of visual core description, smear slide analysis, color reflectance, X-ray diffraction (XRD), and laboratory measurements of magnetic susceptibility, bulk density, and NGR (see "Physical Properties"), two lithostratigraphic units were established. Unit I was further subdivided into Subunits IA-IC (Figs. F1, F2).
The lithology of Unit I is characterized by alternations of olive-green well-laminated diatom- and clay-rich silt and clay-and silt-rich diatom ooze. Gray, partly green, more homogeneous silty clay is more abundant in the central part of the unit (Fig. F1). The lower boundary is defined by a transition from green diatom-rich sediments above and clayey silt and sand below. The lithostratigraphic boundary is present below the lowermost layers of diatom-rich sediment in Section 201-1229A-16H-2.
The major lithology of Subunit IA consists of olive-green diatom-and clay-rich silt. Millimeter-scale laminated zones alternate with slightly grayer, almost homogeneous zones with higher clay and silt content. The yellow laminae are enriched in diatoms, some of them showing an almost pure pinnate diatom assemblage. A few calcite-rich nannofossil ooze laminae are present (e.g., XRD Sample 201-1229A-2H-1, 105-106 cm). The uppermost 3 m of Cores 201-1229A-1H and 201-1229D-1H consist of brown, bioturbated, and poorly laminated clay-rich diatom ooze. Bioturbation is poorly developed throughout the rest of the unit. Gray silty clay layers are common sedimentary features. Cross-lamination is present in Sections 201-1229A-5H-4 and 5H-6 and also in interval 201-1229D-1H-2, 90-120 cm (Fig. F3A). Two erosional surfaces covered with coarser silty and fine sandy layers are present in Sections 201-1229A-2H-6 and 6H-2. The first authigenic dolomite nodule is present at 3.8 mbsf (XRD Sample 201-1229A-1H-3, 84-85 cm). X-ray diffractions of the uppermost dolostone nodules revealed the presence of a small calcitic component together with dolomite (Sample 201-1229A-2H-1, 56-57 cm) (see "Mineralogy"), which might be due to nannofossils. Authigenic dolomite becomes more common below 30 mbsf. Disseminated rhombs of dolomite are concentrated around dolomite nodules or in yellow laminae (Fig. F3B). The uppermost occurrence of phosphate is at 1.2 mbsf (XRD Sample 201-1229E-1H-1, 122-124 cm). Sections 201-1229E-1H-3 and 1H-4 contain several centimeter-sized phosphate nodules. They are commonly present together with dolomites, but a clear relationship between these two diagenetic minerals could not be established.
The lower boundary of Subunit IA is marked by an erosional surface overlain by phosphatic sand. A gravel zone at the top of Core 201-1229A-6H, which also contains phosphatic nodules, is probably an artifact of the drilling process.
Changes in color reflectance were most obvious in the yellow-blue color reflectance spectrum (chromaticity variable b*; Fig. F1). Decreasing b* values coincide with changes in some of the physical properties, such as magnetic susceptibility and bulk density (see "Physical Properties"). A sharp spike of natural gamma radiation at 40.7 mbsf coincides exactly with the erosional surface at the lower boundary of Subunit IB.
An alternation of gray homogeneous quartz- and feldspar-rich clay layers and olive-green laminated diatom-rich silty clay characterizes the lithology of Subunit IB. Three ~10-m-thick gray siliciclastic layers present in the subunit are spread roughly 25 m apart. Slumping, cross-lamination, sandy graded beds, and lenses of shell debris are common (Fig. F3D). Quartz and plagioclase are present in variable amounts throughout the subunit. Dolomite forms pale yellow and gray nodules and semilithified yellow laminae, which are especially abundant in the lower part of the subunit (Fig. F1). Dolomite can also be found as white precipitates along millimeter-scale quartz sand-filled dikes that represent fluid escape structures (Section 201-1229A-10H-1; XRD results for Sample 201-1229A-10H-1, 111-114 cm) (Fig. F3C).
The upper boundary of Subunit IB is sharp and is present below phosphatic sediments overlying an erosional surface (see Subunit IA). The lithologic change that marks the lower boundary of the subunit was observed only in Section 201-1229A-11H-6 because the recovery in Hole 1229D at this depth was poor. This boundary is defined by a change from gray homogeneous quartz and feldspar-rich clay above to olive-green diatom-rich silty clay below, and it is characterized by a silty layer and bivalve shell debris overlying an erosional surface (Fig. F3D). A similar boundary between "dark gray sandy mud" above and "olive-gray diatomaceous mud" below an erosional surface with black phosphate clasts was observed at Site 681 (Section 112-681A-11H-4; 99.2 mbsf) (Shipboard Scientific Party, 1988). Both the upper and lower boundaries of Subunit IB match sharp peaks in natural gamma radiation (Fig. F2). The color reflectance data for Subunit IB show low average values for the chromaticity variable b*, except for two diatom-rich layers at ~60 to ~70 and ~80 to ~90 mbsf (Fig. F1). These shifts are directly coupled with gray and olive-green observed during visual core description.
The dominant component of Subunit IC is diatom ooze, which is present mixed with variable amounts of sand, silt, and clay. Based on the relative amount of diatom ooze in the sediment, the subunit can be further subdivided into three parts (Fig. F1).
The uppermost part of Subunit IC (between 96.9 and 108.0 mbsf) is characterized by diatom-rich silty clay with cross-bedding, and a few shell debris lenses (Fig. F3D), erosional surfaces overlain by phosphatic hardgrounds (Fig. F3E), and dolomite and phosphate nodules. Fine-grained disseminated dolomite rhombohedra are present in dolomite-cemented semilithified yellow laminae and in the sediments that surround hard dolomite nodules.
The middle part of Subunit IC (between 108.0 and 128.0 mbsf) is a well-laminated clay- and silt-rich diatom ooze. High chromaticity correlates well with the olive-green diatom ooze layers (Fig. F1).
The lowermost part of Subunit IC (between 128.0 and 138.6 mbsf) consists of gray pyrite- and diatom-rich clay. It represents a transitional facies from the sandy siliciclastic sediments of Unit II below to laminated diatom ooze above. The color reflectance variable b* is low in this interval and has similar values as in Unit II. Variable concentrations of scattered dolomite rhombohedra were observed.
The boundary between Units I and II is assumed to exist somewhere between 138.6 and 155.4 mbsf in the poorly recovered interval (Fig. F1). A major lithologic change occurs from diatom-rich silt and clay of Unit I to silty and sandy sediments of Unit II in this interval. Downhole logging NGR and electric resistivity data show increased values in two steps at 128.0 and 138.6 mbsf, where this change occurs (Fig. F2) (see also "Downhole Logging"). Each step is additionally marked by a sharp spike in electric resistivity. On the basis of these observations, the bottom boundary of Subunit IC was located below the lowermost occurrence of gray diatom-rich sediment, which lies within drilling rubble at 138.6 mbsf (bottom of Section 201-1229A-16H-2).
Unit II consists of an almost pure siliciclastic packet of Pleistocene sediments. The main lithology is dark gray hornblende-bearing feldspar-quartz sand with variable amounts of clay. Feldspar and quartz grains are well rounded, and their relative amounts vary strongly. The sediments are slightly laminated and show few burrows. In Core 201-1229A-18H, sedimentary structures, such as flat and subangular gray mud clasts, possibly related to synsedimentary slumping, were observed. Authigenic minerals, such as phosphate and dolomite, are present in Unit II but are less abundant compared to Unit I. They are present as disseminated grains in sand layers (see "Mineralogy"), and nodules were no longer observed.
The upper boundary of Unit II is marked by the onset of diatomaceous ooze layers, which indicates the end of a time period with major siliciclastic input. This transition is poorly documented in Core 201-1229A-16H (see above). However, the transition is well defined in wireline logging data (NGR and electrical resistivity) (see "Downhole Logging"). Because of both the gray color and the low diatom content, Unit II usually shows low and less variable chromaticity values.
A total of 21 samples were collected at Site 1229 for XRD analysis. In general, XRD data confirm lithologic observations made during visual core description.
For instance, calcite was found in Sample 201-1229A-2H-1, 105-106 cm, which had been collected from a nannofossil-rich layer in Subunit IA. The presence of quartz and feldspar in all measured samples from Site 1229 suggests a continual supply of terrigenous input, even during the deposition of the biogenic subunits (e.g., Subunit IC). Quartz is especially common in some of the gray layers observed in Subunit IA (Sample 201-1229A-1H-2, 70-71 cm). In all XRD samples from Unit I, feldspar is only a minor mineralogic component, whereas in Unit II it can be more abundant than quartz (Samples 201-1229A-19H-2, 40-41 cm, and 19H-3, 60-61 cm). Pure quartz sand was observed as filling of a fluid escape structure (Sample 201-1229A-10H-1, 111-114 cm). Hornblende was mainly found in the silts and sands of Unit II (Samples 201-1229A-19H-2, 40-41 cm, and 19H-3, 60-61 cm).
The main diagenetic minerals at Site 1229 are dolomite and phosphate. Dolomite is mostly present as pale yellow and gray (3-5 cm thick) hard nodules (Samples 201-1229A-1H-3, 84-85 cm, and 8H-1, 62-64 cm). Only dolomite nodules from Subunit IA contained trace amounts of calcite, possibly due to calcareous nannofossils (Sample 201-1229A-2H-1, 105-106 cm). Disseminated dolomite was detected both by smear slide and XRD analyses (Sample 201-1229A-3H-1, 56-57 cm). It was observed in pale yellow altered zones, which are preferentially present around hard lithified dolomite nodules and within diatom-rich laminae. These scattered dolomite rhombohedra might represent an early stage of the formation and growth of a hard nodule. Dolomite peaks are commonly offset by ~0.2°2
relative to the standard peak for ordered dolomite, indicating calcium-rich dolomite (Fig. F4) (Samples 201-1229A-1H-3, 84-85 cm; 3H-1, 56-57 cm; 2H-1, 105-106 cm; and 8H-1, 62-64 cm). Phosphate grains found in black sands (Unit II) were identified by XRD as fluorapatite (Sample 201-1229A-21H-1, 30-31 cm); however, dark D-phosphate nodules and several orange spots of F-phosphate were found (Samples 201-1229A-13H-2, 103-104 cm, and 21H-1, 30-31 cm).
The 194.4-m-thick late Pliocene to Pleistocene sequence drilled at Site 1229 was divided into two units. Unit I consists of 138 m of mixed diatom-rich hemipelagic and siliciclastic sediment. Unit II is mostly composed of siliciclastic sands. Based on the relative amount of clay and silt in the diatom ooze-dominated sediments of Unit I, three subunits were recognized: Subunit IA consists of olive-green laminated clay-rich diatom and nannofossil ooze and diatom-rich silt and clay. The uppermost 3 m of the subunit consists of poorly laminated brown clay-rich diatom ooze. The lower boundary is marked by an erosional surface overlain by phosphatic sand. In Subunit IB, both diatom ooze and terrigenous silt and clay are present. Cross-lamination, erosional surfaces, and burrowing characterize the diatom-poor layers. Subunit IC consists of a 42-m-thick interval of diatom-rich silty clay and diatom ooze. Planar lamination is present throughout the unit. Cross-lamination and erosional surfaces are present at the top of the subunit.
Unit II consists of homogeneous slightly burrowed hornblende-bearing quartz- and feldspar-rich sand. The most prominent diagenetic feature of this site is the presence of dolomitic and phosphatic minerals, which were observed also at very shallow burial depth. Nodules of dolomite and phosphate are abundant throughout Unit I, and disseminated dolomite is present in Unit II. Disseminated dolomite rhombohedra are abundant in pale yellow zones around hard lithified dolomite nodules and in yellow diatom-rich laminae. Dolomite nodules are often present together with phosphatic nodules.
Variations between diatom-rich and predominantly siliciclastic lithologies observed in the different units and subunits are also identifiable by several physical properties, including color reflectance, magnetic susceptibility, and bulk density. Erosional surfaces observed during visual core description appear to match some of the spikes in natural gamma radiation.
