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doi:10.2204/iodp.proc.308.104.2006

Lithostratigraphy

Hole U1320A penetrated a thick portion of the sedimentary infill of Brazos-Trinity Basin IV and into the underlying sediments. The 299.6 m thick sedimentary succession was divided into five lithostratigraphic units based on visual core description and analysis of smear slides (Table T3). Color spectrophotometry data showed only minor variations downhole and were not used to distinguish units. Because parts of the sedimentary succession were incompletely recovered, we used well logs, particularly gamma ray curves, to assist in interpreting lithologies and refining unit boundaries. A summary of the lithostratigraphic column is presented in Figure F2. Figures F3, F4, F5, F6, F7, and F8 illustrate the range of features and facies in each of the lithostratigraphic units, and Figure F9 summarizes the results of smear slide analyses.

Description of lithostratigraphic units

Unit I

  • Interval: Sections 308-U1320A-1H-1, 0 cm, through 1H-2, 115 cm
  • Depth: 0–2.65 mbsf
  • Age: Holocene/(?) late Pleistocene
  • Lithology: foraminifer-bearing clay

Lithostratigraphic Unit I consists of greenish gray clay with foraminifers and sponge spicules. As estimated from smear slides, the foraminifer concentration is low (probably <5%), but foraminifers are visible to the naked eye on the split core. The lower boundary is defined at the top of the first sand lamina observed downcore.

Unit II

  • Interval: Sections 308-U1320A-1H-2, 115 cm, through 17X-2, 91 cm
  • Depth: 2.65–137.5 mbsf
  • Age: late Pleistocene
  • Lithology: sand and mud

Lithostratigraphic Unit II is 134.85 m thick. The lower boundary is defined at the top of bioturbated foraminifer-bearing clay that corresponds closely with seismic Reflector R30. Unit II consists of ~51% mud and clay and 49% sand. The sandy units occur as packages as thick as 10 m, containing thick and medium beds of massive sand. The packages are separated by bioturbated muddy intervals ranging in thickness between 1 and 10 m. The thick sandy units were partly disturbed by the coring process and were very soupy after core splitting, but core recovery was good, particularly with the APC (Fig. F2). Despite the deformation, the thickness of the sands is well preserved, given the good correlation between core and logging data (Fig. F2). Based on arrangement of the sand and mud intervals and/or their sand content, we divided Unit II into five subunits (Fig. F2; Table T3).

Subunit IIA (2.65–27.6 mbsf)

Lithostratigraphic Subunit IIA consists primarily of fine-grained sand (up to 70%) occurring in massive beds with graded tops and containing shell fragments, plant debris, and mud clasts (Fig. F3A–F3D). The base of Subunit IIA contains numerous thin laminae and thin beds of very fine to fine sand. The sand beds are organized in three packages that increase in thickness upward within the unit, from ~4 m thick at the base to ~5 m thick in the middle and ~8 m thick at the top. Each sand package has a sharp base and is capped by thin beds and laminae of silt (upward thinning). Individual bed thickness ranges from decimeters to several meters, and the thickest beds were soupy in the cores.

Subunit IIB (27.6–37.8 mbsf)

Lithostratigraphic Subunit IIB consists of a 10.16 m thick clay interval that lacks any silt or sand laminae with sharp top and base. Freshly split cores showed ephemeral black mottling that disappeared a few hours after splitting, requiring several millimeters of scraping to reveal the mottling. Rare millimeter-scale silt-filled burrows and small wood fragments occur throughout the unit. A particularly intriguing feature is observed over parts of this interval (Fig. F3E), interpreted as the result of bioturbation or, alternatively, differential oxidation associated with core microcracking.

Subunit IIC (37.76–82.0 mbsf)

Lithostratigraphic Subunit IIC consists of a 44.24 m thick interval with high sand content. The top of the unit is defined by a sharp contact with the bioturbated clay above. The upper part of Subunit IIC shows an overall upward-fining and thinning succession from ~43 mbsf to the top of the Subunit at 37.8 mbsf. In this interval we observed thick (1–2 m) beds of sand, silt, and sandy silt that are normally graded and rich in plant debris. These beds are grouped into two packages several meters thick in the upper part of the unit (Fig. F2). Below these two packages is an interval of poor recovery (Cores 308-U1320A-10X and 11X). The gamma ray log indicates the presence of two sand intervals between 75 and 82 mbsf separated by a muddy interval. The base of Subunit IIC is defined at the base of the sand-rich interval interpreted from the gamma ray log at 82 mbsf. This contact corresponds closely with seismic Reflector R20. Subunit IIC consists of four packages of sand intercalated with three clay-rich intervals. With sand content as high as 77%, this interval is the sandiest of the entire succession penetrated in Hole U1320A and corresponds closely with an acoustically transparent interval between seismic Reflectors R10 and R20. The sand packages show overall upward increasing thickness, starting with the basal package at ~2 m thick, then ~5 m, ~6 m, and ~14 m for the uppermost package. The sand beds are characterized by upper fine to lower medium sand. Although very thick beds (>1–2 m) were soupy after splitting the core, they appear massive with no discernible grain size grading (Fig. F3F). Most sand beds are rich in shell fragments and plant debris. Each of the sand packages is capped by clays and thin-bedded silt and very fine sand laminae, grading upward to mottled and bioturbated clay. Contorted bedding occurs locally within the clay-rich package at 57–65 mbsf, possibly related to loading by the upper sand package or to a mass transport deposit (MTD).

Subunit IID (82.0–124.6 mbsf)

Lithostratigraphic Subunit IID is defined within an interval of relatively poor core recovery (average = ~20%), but the combined interpretation of cores and well logs indicates that this subunit is composed of an average of 68% clay and 32% sand, mostly within thick sand packages (Fig. F2; Table T3). The recovered sands within Subunit IID are mostly very fine to fine grained with normally graded tops. Abundant plant debris is observed in the thickest sand recovered at 93 mbsf (Fig. F3G), but most of the sand beds recovered are centimeters to decimeters thick. Clay intervals recovered in this subunit are characterized by bioturbation and black mottling, with rare thin silt laminae commonly disrupted by burrowing. Smear slides contain 5%–10% organic matter dispersed in the clays and a local presence of dispersed volcanic glass and CaCO3 fragments (Fig. F9).

Based on the gamma ray logs, we interpret two sand packages with sharp bases and gradual upward-thinning and -fining trends at the top (Fig. F2): the lower package is ~3.5 m thick and the upper package is ~7 m thick (see “Downhole measurements”). Similar to Subunit IIC, the sand beds within Subunit IID appear to be grouped into packages separated by bioturbated clays, with overall increasing in thickness uphole (Fig. F2).

Subunit IIE (124.6–137.5 mbsf)

Lithostratigraphic Subunit IIE consists of an interval dominated by clay (~90%) with only two sand beds, one at the base and one in the middle portion of the subunit (Figs. F2, F3). The key distinguishing characteristics of this subunit are the occurrence of beds containing mud clasts of various colors (Fig. F3H), folding, steep erosional contacts (Fig. F3I), and steep apparent bedding dips (Fig. F4). In some intervals (Sections 308-U1320A-16X-6 and 16X-7), we observed “chevron” or “wood-grain” patterns indicative of coring-induced biscuiting of folded or steeply dipping sediment similar to that observed on the Amazon Fan (Flood, Piper, Klaus, et al., 1995). Borehole images in this interval indicate the presence of steeply dipping beds, confirming our interpretation of coring-induced deformation for the “wood-grain” patterns (Fig. F4; “Downhole measurements”).

Unit III

  • Interval: Sections 308-U1320A-17X-2, 91 cm, through 17X-CC, 20 cm
  • Depth: 137.50–145.3 mbsf
  • Age: late Pleistocene
  • Lithology: foraminifer-bearing clay with ash layer

Lithostratigraphic Unit III is a 7.8 m thick interval consisting of bioturbated clay with variable coloration between greenish gray and brownish gray. The unit is distinct by the presence of foraminifers (<5%) over most of the unit. Locally within small burrows and thin discontinuous laminae we observe abundant foraminifer tests and test fragments. Several styles of bioturbation and bioturbation intensity can be discerned on the core face (Fig. F6).

Unit III in Hole U1320A is correlative with lithostratigraphic Unit III at Site U1319 based on the position of seismic Reflector R30, and therefore we expected to encounter the same ash layer encountered in Hole U1319A. After careful inspection and examination of several smear slides, we encountered a thin, discontinuous lens containing ~90% volcanic glass shards at 143.4 mbsf in Section 308-U1319A-17X-6 (Fig. F6). The grain size and appearance of the volcanic glass shards are very similar to those encountered in the ash layer in Hole U1319A (Fig. F6).

Unit IV

  • Interval: Sections 308-U1320A-17X-CC through 21X-1, 0 cm
  • Depth: 145.3–174.4 mbsf
  • Age: late Pleistocene
  • Lithology: Clay with thin sand/silt beds

Lithostratigraphic Unit IV is dominated by clay with thin beds of silt and sand. The clays in this interval have only minor amounts of silt (<20%) and show varying degrees of bioturbation and color changes (Fig. F7A–F7D). Color variations include both light and dark gray clays and black mottles. The color variations are ephemeral and disappeared within a few hours of exposure in the laboratory, particularly the black bands and mottles. Small silt blebs occur within burrow fills dispersed through the interval, which are often enriched in bioclastic components and probably represent bioturbated silt laminae. A few thin-graded beds and laminae of sand and silt occur within this interval (Fig. F7E). Primary sedimentary structures were very disrupted by biscuiting resulting from XCB coring.

The base of this unit is defined from the gamma ray log within an interval of poor core recovery and corresponds closely with seismic Reflector R40 (Fig. F2). Gamma ray logs indicate the presence of a ~3 m thick sand and silt interval in Hole U1320A (FMS-sonic logging run) that is as thick as 8 m in Hole U1320B (LWD) (Fig. F2; “Downhole measurements”). We defined the base of Unit IV at the base of the sandy interval in Hole U1320B. Because of the variability in gamma ray response between Holes U1320A and U1320B, which are 20 m apart, it appears that this boundary represents a dipping erosional surface. The sand content of Unit IV is ~23%, but if we exclude the anomalously thick sand interval interpreted from the gamma ray log at the base, the sand content of Unit IV is only 8%.

Unit V

  • Interval: Sections 308-U1320A-21X-0 cm, through 33X-CC, 39 cm
  • Depth: 174.4–299.6 mbsf
  • Age: late Pleistocene
  • Lithology: bioturbated clay

Lithostratigraphic Unit V, spanning to 299.6 mbsf at the base of Hole U1320A, is composed of greenish gray to reddish brown clay (Fig. F8). The clays are punctuated by dark gray to black organic-rich, locally pyritic layers ranging from millimeters to centimeters thick. The greenish gray and reddish brown clays are locally distinct and range from ~10 cm to several decimeters in thickness (Fig. F8B–F8D). More commonly, these layers are disrupted by burrowing, imparting an overall mottled appearance to the unit. Smear slides show that sediments throughout the unit have a minor component of bioclastic origin, including CaCO3 fragments and foraminifers. Bioclastic sediments are locally enriched within burrow fills and thin laminae (Fig. F9).

The top of Unit V occurs in an interval of poor core recovery across Cores 308-U1320A-20X and 21X. Based on seismic correlation with Site U1319, we expected to encounter a foraminifer-bearing hemipelagic clay near seismic Reflector R40. Based on the well logs, we tentatively interpret a ~2 m thick hemipelagic clay at the top of Unit V at a gamma ray peak observed in the wireline log in Hole U1320A (see “Downhole measurements”). It is possible that this hemipelagic clay interval is eroded in Hole U1320B (Fig. F2).

Interpretation of lithostratigraphy

Interpretation by unit

Lithostratigraphic Unit I is dominated by hemipelagic sedimentation. Lacking silt and sand beds, it represents a cessation of turbidity current influx into Brazos-Trinity Basin IV during the present interglacial period.

Lithostratigraphic Unit II, with an average of 49% sand beds, represents the main period of turbidity current influx into Brazos-Trinity Basin IV and corresponds to the major portion of the basin infill. It consists of a series of sand packages with sharp bases, with medium to thick fine-sand beds. Bioturbated clays at the top of each sand package may represent a pause in turbidite sedimentation within the basin or lower accumulation rates laterally distal from the main depocenters of sand in the basin.

Lithostratigraphic Unit III is dominated by hemipelagic sedimentation and represents a pause of turbidity current flux into the basin. The top of lithostratigraphic Unit III corresponds closely with seismic Reflector R30. The unit straddles an ash layer, tentatively correlated with ash Layer Y8 dated at 84 ka (Mallarino et al., in press; Drexler et al., 1980).

Lithostratigraphic Unit IV corresponds to the lowermost part of the onlapping fill of Brazos-Trinity Basin IV between seismic Reflectors R30 and R40. Immediately above the unconformity marking the base of the onlapping fill (seismic Reflector R40), there is an 8 m thick sand interval in Hole U1320B compared to a ~2–3 m thick sand in Hole U1320A, 20 m away. This difference is interpreted to result from a local erosional surface, perhaps associated with a channel with an apparent dip of ~14°. The remainder of lithostratigraphic Unit IV is interpreted to be the result of low-density turbidity currents forming thin beds and silt laminae and a few thin sandy turbidites. The basal part of lithostratigraphic Unit IV is silty, and the number of distinct turbidite beds and laminae increases upward. Bioturbated clays, representing a reduction in the influx of turbidity currents into the basin, cap the succession.

Lithostratigraphic Unit V is a thick and monotonous interval dominated by bioturbated clays with rare foraminifers and lacking obvious evidence of turbidity current input such as silt and sand laminae. 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 river plumes or from distal turbidites spilling from adjacent intraslope basins. Lithostratigraphic Unit V correlates with lithostratigraphic Unit VI described at Site U1319. The uppermost part of lithostratigraphic Unit V in Hole U1320A was not recovered and occurs immediately below a sandy interval identified in the well logs.

Summary interpretation

Hole U1320A penetrates a thick portion of Brazos-Trinity Basin IV and contains a nearly complete lithostratigraphic record of the onlapping fill of the basin above seismic Reflector R40 and of the conformable succession below. Ash Layer Y8, together with high-resolution seismic data, allows for excellent correlation of lithostratigraphic units between the basin margin (Site U1319) and the basin center (Site U1320). The underlying succession is represented by bioturbated clays, which are likely derived from river plumes and/or distal turbidity currents. The basin began to fill soon after the last interglacial, at ~125 ka. This lower portion of the basin fill is the result of dominantly muddy flows, probably spilling from the basins updip. The notable exception is that the very onset of this basin fill is represented by a thick sandy interval, which contrasts with the remainder of lithostratigraphic Unit IV, characterized mostly by muddy turbidites. The early onset of turbidte infill is then interrupted by hemipelagic deposition that draped the entire basin (lithostratigraphic Unit III) between ~90 and ~47 ka, according to the age model of Mallarino et al. (in press) and to the shipboard age determination (see “Biostratigraphy”). A clay-rich MTD occurs immediately above the hemipelagic interval containing clasts of foraminifer-bearing clay possibly derived from the underlying hemipelagic units. This MTD is 12.9 m thick in Hole U1320A but thickens considerably laterally away and is the result of mass wasting of the basin flanks (Winker, 1996).

The overlying 132.2 m represents the main portion of the basin infill, containing a series of sandy turbidite packages interbedded with bioturbated clays and muds and a few thin muddy MTD beds (Subunits IIA–IID) (Fig. F2). The sand packages consist of medium- to thick-bedded turbidites, mostly containing massive fine-sand beds with sharp bases and upward-thinning tops capped by 1–10 m of bioturbated clay. These packages represent lateral shifting of channels and lobes within the basin as indicated by seismic data (Winker, 1996; Beaubouef et al., 2003). The bioturbated muds between sand packages may represent distal deposition resulting from lobe shifts and/or reduced sedimentation rates throughout the basin. Some of these muddy intervals can be correlated laterally across the basin (e.g., Subunit IIB below seismic Reflector R10).

Within the onlapping fill above seismic Reflector R40, both sand content and thickness of turbidite packages increase upward. This is the result of increased sediment flux into Brazos-Trinity Basin IV, a consequence of lowering of sea level since the last interglacial and/or progressive increase in the spilling of turbidity currents from Brazos-Trinity Basin II updip.