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doi:10.2204/iodp.proc.308.103.2006 LithostratigraphyThe 156 m thick sedimentary section at Site U1319 was divided into six lithostratigraphic units (Table T4; Fig. F9) based on visual observation, analysis of smear slides, and spectrophotometric data. The high recovery at this site provided a good record of the sedimentary history of this basin margin location. The section is amenable to fine division in the upper part of the hole but is relatively uniform below 31 mbsf. Subdivisions of this condensed basin margin sequence were made to facilitate the expected correlations to the thicker basin fill section at Site U1320. When splitting sedimentary units between cores we chose the depth of the unit boundary as depth of the top of the next core, as this is closest to the true drilling depth below seafloor (see “Introduction to Expedition 308 shipboard methods” in the “Methods” chapter). This resulted in some inconsistencies, typically <0.5 m, between core depth recorded in the visual core descriptions (VCDs) and unit boundary depths. Description of lithostratigraphic unitsUnit I
Lithostratigraphic Unit I consists of foraminifer-bearing greenish gray clay. As estimated from smear slides, the foraminifer concentrations are low (probably <5%), but they are visible to the naked eye on the split core. This unit is characterized by relatively high spectrophotometric lightness values (Fig. F10). Unit II
Lithostratigraphic Unit II is composed of clay with fine sand laminae and organic-rich black clay. We divided the unit into four subunits. Subunit IIA (3.33–6.2 mbsf)Lithostratigraphic Subunit IIA is characterized by clay with silt laminae <1 cm thick. In the interval of Subunit IIA, the sediment undergoes a color change from gray-green to black. Smear slides suggest that silt laminae are dominated by quartz with minor amounts of carbonate and organic matter. Some silt beds are black and may include significant amounts of organic matter and associated pyrite. Subunit IIB (6.2–12.0 mbsf)The top of lithostratigraphic Subunit IIB is defined just below a silt lamina at 6.2 mbsf. Subunit IIB is mostly composed of homogeneous black, organic-rich clay (Fig. F11) and records some of the lowest spectrophotometric lightness values (Fig. F10). The black clay rapidly oxidized after opening; it had turned green 24 h after splitting when the close-up core photographs were taken. Only the first millimeters at the surface were oxidized; the original color was preserved within the sediment. Subunit IIC (12.0–14.0 mbsf)Lithostratigraphic Subunit IIC is a thin, 2 m thick interval consisting of green clay with silt and fine sand laminae as thick as 1 cm (Fig. F12). Laminae are locally graded. Subunit IIC includes rare shell fragments and mud clasts. Subunit IID (14.0–17.25 mbsf)Lithostratigraphic Subunit IID is greenish gray clay, intensely bioturbated, and lacking sand and silt laminae. Foraminifers were not macroscopically visible in this subunit. Subunit IID is interpreted as a transitional unit between the turbidite unit above and the foraminifer-bearing Unit III below. Unit III
Lithostratigraphic Unit III consists of intensely bioturbated dark gray to greenish gray clay with foraminifers macroscopically visible on the split core face. The overall percentage of foraminifers is estimated at <5%. However, analysis of smear slides commonly indicates the presence of broken carbonate grains in the sediment. A distinct 2 cm thick ash layer occurs at 22.73 mbsf (Fig. F13). Analysis of smear slides indicated the ash is principally composed of undevitrified volcanic glass (Fig. F14). Overall, Unit III shows relatively high values of spectrophotometric lightness. A sharp drop in lightness values defines the base of the unit, even though this lightness extends deeper than the limit of obvious presence of foraminifers in the cores. Unit IV
Lithostratigraphic Unit IV is composed dominantly of green clay with a central section of very fine sand and silt laminae and beds with local grading. Unit IV shows a lower degree of lightness than the flanking foraminifer-bearing clays. Unit V
Lithostratigraphic Unit V consists of foraminifer-bearing clay and a thin (3 cm thick) foraminifer sand bed. Although thin, Unit V shows a distinctive narrow peak in spectrophotometric lightness (Fig. F10). Smear slides show a clear dispersed carbonate component in the clay. Unit VI
Lithostratigraphic Unit VI comprises greenish gray to reddish brown clay (Fig. F15). The major lithology of the unit is punctuated by dark gray to black, locally pyritic layers, ranging from millimeters to centimeters in thickness. The greenish gray and reddish brown clays are locally distinct and range from ~1 to several decimeters thick. More commonly these layers are disrupted by burrowing, imparting an overall mottled appearance to the unit. Smear slide analysisSmear slides are a valuable tool to identify mineralogical and biological constituents in core. Because the process of making the smear slides may not preserve relative percentages of components and any estimates of percentages are qualitative, the data have to be used with care. Nevertheless, the compiled results (Fig. F16) clearly show diagnostic features of particular units. For example, the ash layer of lithostratigraphic Unit III is dominantly volcanic glass, foraminifers are evident in smear slides from Units I and III, and a consistent organic component is present in the lower portion of Unit III. Interpretation of lithostratigraphyInterpretation by unitLithostratigraphic Unit I is enriched in planktonic foraminifers and lacks any turbidite beds or laminae; it is thus interpreted as Holocene drape. With a thickness of 3.33 m, Unit I is only slightly thinner than the thickness of a similar lithology observed in piston cores from this area by Mallarino et al. (in press). The subunits of lithostratigraphic Unit II provide a link to at least three major events in the history of this basin. The thin, locally graded sand and silt laminae of Subunit IIA indicate turbidite influx during deposition. As such, it correlates with the major phase of turbidite accumulation represented by the wedge-shaped fan structure that has prograded from the northeast (Beaubouef et al., 2003). The black organic-rich clays of Subunit IIB suggest a period of abundant organic matter input. The homogeneous nature of Subunit IIB, the complete absence of laminae and other sedimentary structures, and the presence of wood fragments suggests that it was emplaced rapidly, perhaps as a single event bed. We tentatively interpret this unit to be the result of a mass transport deposit (MTD), although additional shore-based analyses are needed to confirm this interpretation. Turbidite influx into the basin occurred during accumulation of lithostratigraphic Subunit IIC. Although only 2 m thick, Subunit IIC correlates with an acoustically reflective unit in the basin center, which is interpreted to be the result of turbidity current deposition (see “Lithostratigraphy” in the “Site U1320” chapter) (Mallarino et al., in press; Beaubouef et al., 2003). Subunit IID lacks evidence of turbidite deposition and is transitional to Unit III. Lithostratigraphic Unit III, characterized by microfossil-bearing clay and lacking turbidite features, suggests a period of hemipelagic sedimentation. Unit III correlates with a hemipelagic unit of Mallarino et al. (in press) that drapes over preexisting units in high-resolution seismic profiles. Unit III (6.25 m thick) is marginally thinner than the 8–10 m thickness observed by Mallarino et al. (in press) in piston cores. Unit III contains a distinctive 2.5 cm thick ash bed. This same ash bed has been observed in a nearby piston core and was interpreted as ash Layer Y8, representing the fallout from the Los Chocoyos (Guatemala) eruption at 84 ka (Mallarino et al., in press; Drexler et al., 1980). Lithostratigraphic Unit IV, characterized by clay with very fine sand laminae, records another pulse of turbidite input to the basin. Lithostratigraphic Unit V, a thin foraminifer- and nannofossil-bearing clay, records a period of minimum siliciclastic input to the basin and hemipelagic-dominated sedimentation. Given the occurrence of the ash Layer Y8 above, we tentatively assign this interval to the last interglacial (marine isotope Stage [MIS] 5e). Finally, lithostratigraphic Unit VI is a thick and monotonous interval dominated by bioturbated clay with rare foraminifers and lacking obvious evidence of sandy turbiditic input. The faint color banding (greenish gray and black) of the organic-rich clays is interpreted to represent fluctuations in organic matter and siliciclastic input. The general lack of coarse (silt and sand) grains suggests deposition from muddy plumes or from a nepheloid layer. Summary interpretationBecause Site U1319 is at the edge of Brazos-Trinity Basin IV, its sedimentary section is extremely condensed and minimally influenced by turbidite influx to the basin. Nevertheless, subtle but interpretable sedimentary signals are present. Overall, the sediments at Site U1319 show an alternation of turbiditic and hemipelagic deposition in a basin-margin setting. Accumulation of the first and fifth hemipelagic units (lithostratigraphic Units I and V) is related to the major sea level highstands at the present and at ~125 ka. These units bracket another interval of hemipelagic input (lithostratigraphic Unit III), which represents a significant pause in turbidity current influx to this basin margin site. |
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