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doi:10.2204/iodp.proc.307.103.2006 LithostratigraphyThe lithology at Site U1316 can be divided into three main lithostratigraphic units based on visual descriptions of Hole U1316A (Fig. F3). Unit 1 is Pleistocene, Unit 2 is late Pliocene–Pleistocene, and Unit 3 is early–middle Miocene in age. Each unit is further divided into two subunits. Typical lithology of each subunit is shown in Figure F4. The major boundaries and lithostratigraphic units defined in Hole U1316A can also be applied to Holes U1316B and U1316C. The differentiation into units is based on sediment composition and age, as well as color reflectance and natural gamma radiation (Fig. F3). A major unconformity located between Units 2 and 3 is recognized at 55.0, 58.3, and 51.5 mbsf in Holes U1316A, U1316B, and U1316C, respectively, and separates the upper Pliocene–Pleistocene from the underlying lower–middle Miocene sediments. Unit 1
Unit 1 is divided into two subunits by a distinct erosive boundary (Fig. F5). The following core description is based on Hole U1316A unless noted otherwise. Subunit 1A
Subunit 1A is dominated by mottled grayish brown to dark grayish green silty clay to clayey silt. The sediments are partly interbedded with centimeter-thick fine-sand layers that occasionally grade upward. Bioturbation rarely occurs in this subunit and appears as round fine-sand lenses. Dropstones occur frequently. Fossils are generally rare and quite dispersed. At 5.90 mbsf, a very thin shell debris layer occurs. The base of Subunit 1A is defined by an erosive boundary overlain by a 12 cm thick, graded medium- to fine-sand layer with up to centimeter-sized lithoclasts at the base. The same sedimentary package corresponds to a soft-sediment disturbance structure overlain by a medium-sand layer grading into clayey silt in Hole U1316B at 26.30 mbsf (Sample 307-U1316B-4H-2, 28 cm) (Fig. F5). Subunit 1B
Subunit 1B is characterized by laminated very dark olive-gray to dark gray silty clay interbedded with fine-sand layers (Fig. F4). Lamination thickness varies from millimeter to centimeter scale. Mottled intervals are generally rare. Dropstones occur frequently and are limited to discrete intervals. At 41.00 mbsf, a mixture of silty clay layers with a calcareous fine to medium sand forms a 20 cm thick soft-sediment deformation structure possibly indicating slumping (Fig. F6). The deformation feature also appears in Hole U1316B as a 10 cm thick bed of poorly sorted medium sand that grades upward into clayey silt (Sample 307-U1316B-5H-6, 90 cm; 42.90 mbsf). Just above this layer, a medium-sand to fine-gravel layer is observed consisting of a mixture of dropstones, coral fragments of Lophelia pertusa, and other bioclasts (interval 307-U1316A-5H-4, 55–63 cm) (Fig. F6). This distinctive bed is correlated with a lithologically similar layer in interval 307-U1316B-5H-6, 55–64 cm. The base of Subunit 1B is defined by an erosive unconformity overlain by a fining-upward sequence of graded fine to very fine sand beds about 1 m and 70 cm thick in Holes U1316A and U1316B, respectively (Fig. F7). Unit 2
Unit 2 is very heterogeneous and consists of coral-bearing layers interbedded with sands, silty sands, silt, and silty clays (Fig. F4). It is divided in two subunits by a distinct erosional unconformity. Subunit 2A is dominated by siliciclastic fine to medium sands, whereas corals predominate in Subunit 2B (Figs. F8, F9). Subunit 2A
Subunit 2A is dominated by fine to medium sand and rarely coarse sands that grade upward into silty clay interbedded with fine-sand and silt layers. In Hole U1316A, Subunit 2A is further divided into six, or possibly seven, fining-upward sequences with erosional contacts at their bases (at depths of 44.90, 45.20, 46.00, 46.90, 48.10, 49.20, and 50.90 mbsf). These fining-upward sequences can be correlated to measured cyclic changes in the natural gamma ray log (Fig. F9). Dropstones are common within these intervals and range up to 3 cm in diameter. Subunit 2B
Corals occur in distinct layers interbedded with siliciclastic sediments in Subunit 2B. Thicknesses of the coral-bearing layers vary from 20 to 170 cm. The coral assemblage in these intervals is dominated by L. pertusa. Preservation of the corals is normally poor, but in some cases the primary aragonitic mineralogy of the corals is still preserved. The coral layers commonly display a floatstone to rudstone texture. Other coral species, such as Madrepora oculata, Desmophyllum crsitagalli, Dendrophyllia cornigera, and caryophyllid coral, are often associated with L. pertusa in present-day environments; however, they are not likely observed because of poor preservation. In the upper parts of the subunit, from 50.90 to 51.40 mbsf in Hole U1316A, a dense rudstone rich in highly fragmented corals occurs. The top of the rudstone defines the boundary between Subunit 2A and 2B. A likely corresponding layer in Hole U1316B was identified at 53.40–53.60 mbsf (Section 307-U1316B-7H-1); however, the very coarse reworked layer could also represent fallen drilling breccia commonly found at the top of Section 1 in some cores. Unit 3
Unit 3 is divided into two subunits mostly based on the carbonate content of the sediments rather than on visual differences (Fig. F25). The upper section, Subunit 3A, generally consists of greenish gray sandy silt and clay with carbonate content at ~30 wt%, whereas the lower section of the core, Subunit 3B, consists of similar color and grain-size material but with carbonate content >50 wt%. The top of Unit 3 consists of lower–middle Miocene sediments. The overlying sediments of Unit 2 are late Pliocene–early Pleistocene age; therefore, this boundary represents a major hiatus. Subunit 3A
The boundary contact with overlying Unit 2 is erosive. The uppermost 10 cm shows gradual change from light gray to greenish gray (Fig. F10). The erosional surface shows no evidence of submarine lithification or Fe/Mn impregnation. The sediments of Subunit 3A are mostly composed of greenish gray, well sorted, poorly to moderately lithified clayey and sandy silts that are frequently mottled (Fig. F4). The greenish gray color is likely due to the presence of glauconite. At several intervals, thin centimeter-thick layers of fine bioclastic sand with echinoid spines, bivalves, and gastropods are found. Otherwise, the subunit is characterized by generally homogeneous sediments. These sandy layers are recorded as distinct decreases in natural gamma radiation (NGR) (Fig. F30). Aragonitic mollusks are partly preserved or seen as molds. Sporadic lithoclasts, shell fragments, and other bioclasts are found disseminated throughout the subunit. Apart from quartz, nannofossils are the primary sediment component. X-ray diffraction data indicate that dolomite occurs in low concentrations throughout the subunit but is >20% in distinct layers (e.g., in Hole U1316A at 90 and 120 mbsf). Core disturbance of Subunit 3A below 66.30 mbsf is high because of a change from APC to XCB coring. This is expressed in the formation of sediment “biscuits,” where coherent and in situ sediment biscuits are separated by a mixture of ground-up sediment and drilling mud (Fig. F11). Subunit 3B
Subunit 3B appears similar in color, texture, and grain size to the overlying Subunit 3A but has a significantly higher carbonate content (>50 wt%), thus placing it within the carbonate sediment classification regime outlined in “Lithostratigraphy” in the “Methods” chapter. The sediments consist of moderately lithified, bioturbated, and mottled clayey to sandy silt with infrequently dispersed shells and shell fragments (Fig. F4). The sediment matrix composition is comparable to Subunit 3A but contains more nannofossils. Coring disturbance in Subunit 3B is high. Frequent distinct layers of highly lithified sediment occur throughout the subunit. DiscussionSediments recovered from Site U1316, located basinward of Challenger Mound, contain a sedimentary suite of pre-, syn-, and postmound growth phases. A central objective is to correlate Site U1316 with the on-mound Site U1317 in order to understand the sedimentary processes related to mound growth and burial history. The top of Unit 3 is believed to represent the major unconformity observed in many seismic sections of Porcupine Seabight (e.g., Van Rooij et al., 2003) on which the cold-water coral mounds began to grow in this region. This distinct boundary represents a major hiatus between early–middle Miocene and late Pliocene or early Pleistocene (see “Biostratigraphy”). Immediately above this boundary, the base of Unit 2 is characterized by a coral layer. Because of the poor preservation and complex texture of the coral rudstone, it has not been possible to distinguish whether the corals were autochthonous or allochthonous. This would be an important distinction as in situ buried corals would imply a wide initial mud-layer mound base in contrast to corals transported from the adjacent Challenger Mound. For the uppermost coral horizon in Hole U1316A (50.90–51.60 mbsf), a debris flow transport process is suggested by (1) the high fragmentation of corals compared to the coral-bearing horizons below and (2) the fine mudstone matrix sediment between the corals. The abrupt change in sediment composition from a coral-dominated to a siliciclastic-dominated sediment between Subunits 2B and 2A forms the basis for implying major change in oceanographic conditions and subsequent mound growth. However, this hypothesis must be tested in forthcoming analyses, in particular with an improved age model and paleoclimatic reconstruction of Sites U1316 and U1317. The general decrease in grain sizes in Unit 1, compared to those in Unit 2, possibly reflect a decrease in energy of the bottom current regime. The well-developed laminations in Subunit 1B mostly exibit thin fine-sand layers grading upward into silty clay. These layers are interpreted as turbidites. Restriction of dropstones to specific intervals in Unit 1 may indicate deglaciation events. |
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