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Stratigraphic correlation

Seismic sequence identifications

Three generations of seismic data guided the site selection of Hole M0028A. These include the Ewing 9009 (Ew9009) at ~15 m vertical resolution and the Oceanus 270 (Oc270) and Cape Hatteras 0698 (CH0698) at ~5 m vertical resolution. The higher resolution Oc270 and CH0698 imaged several seismic sequence boundaries not previously noted on Ew9009 and older, lower resolution data (see Table T10 in the "Methods" chapter).

Hole M0028A lies within a CH0698 grid of closely spaced lines at the crossing of dip profile 207 common depth point (CDP) 11390 and strike profile 218 CDP 4274 (Fig. F7 in the "Expedition 313 summary" chapter). CH0698 line 207 coincides with the regional Oc270 line 529. Hole M0028A is at CDP 5062 on Oc270 line 529 (Fig. F47). Time-depth correlations for this hole were based on semblance velocity data from Oc270 like they were for other Expedition 313 holes (see "Stratigraphic correlation" the "Methods" chapter).

Holes M0027A–M0029A were designed to sample different locations along the same transect of seismic sequences. Site M0028A samples features downdip from those sampled in Hole M0027A and updip from equivalents in Hole M0029A. Pleistocene through lower Miocene seismic surfaces were targeted in Hole M0028A. Pleistocene and middle Miocene surfaces comprised the first series of seismic boundaries intersected in Hole M0028A. Sheridan et al. (2000) used Geopulse data to identify Pleistocene surfaces that were named marine isotope chron (MIC) 3a, MIC3c, and MIC4. However, these reflectors were not cored, as recovery proved difficult in similar materials in Hole M0027A. Likewise, Miocene seismic sequence boundaries m1, m3, and m4 were rotary drilled without collecting samples. The seismic facies comprising m1, m3, and m4 contains low to moderate amplitudes and chaotic reflectors. This facies complicates tracking these surfaces through the long distances required to predict corresponding depths in Hole M0028A. Two new surfaces named m4.1 and m4.5 were traced into Hole M0028A from reflectors farther offshore. Surface m4.1 is a high-amplitude continuous reflector representing three merged surfaces as it tracks into this location. It directly overlies seismic surface m4.5, also a high-amplitude continuous reflector at the drill site. No previous drilling supplied chronological information regarding these two surfaces.

Strong seismic definition of lower and middle Miocene seismic surfaces continues in Hole M0028A (Figs. F54, F55). Surfaces m5, m5.2, and m5.3 are moderate- to high-amplitude continuous sigmoidal reflectors. Hole M0028A lies proximal to their clinoform rollover positions. Clinoform rollover positions are at CDPs 4848, 4860, and 4857 on Oc270 profile 529 for m5, m5.2, and m5.3, respectively. The seismic facies between surfaces m4.5 and m5 is uniformly transparent. Below surface m5, the seismic facies remains relatively transparent, with the addition of two to three high-amplitude reflectors displaying toplap against surface m5. Several moderate- to high-amplitude reflectors with both toplap and downlap terminations occur within a transparent seismic facies between surfaces m5.2 and m5.3 and a newly defined surface m5.32. A surface marking a series of well-defined downlapping reflectors between CDPs 5411 and 4934 on Oc270 profile 529 across Hole M0028A marks surface m5.32 before it begins to steepen and merges with overlying surface m5.3.

Below surface m5.32, the seismic facies becomes low to moderate amplitude, discontinuous, and subparallel to a new surface named m5.33 (Figs. F54, F55). This surface is moderate amplitude and discontinuous on Oc270 profile 529. It appears as a moderate- to high-amplitude continuous reflector on strike lines within the CH0698 200 hazard grid as well as strike line CH0698 profile 24. Surface m5.33 merges with surface m5.4 at its clinoform rollover position at CDP 5767 on Oc270 profile 529 and downlaps onto surface m5.4 at CDP 4389 on Oc270 profile 529. Below surface m5.33, the seismic facies is low to moderate, discontinuous, and hummocky to subparallel, with hints of onlap onto a moderate-amplitude continuous reflector at 533 ms in Hole M0028A. This surface marks a downhole change in seismic facies of moderate-amplitude subparallel inclined seaward reflectors. They display both onlap and downlap against the underlying m5.4 surface, here a moderate- to high-amplitude sigmoidal reflector. Hole M0028A intersects sigmoidal surface m5.4 seaward of its change back to a gentle seaward-dipping slope.

Three closely spaced seismic surfaces, m5.45, m5.47, and m5.6 in descending order, lie below surface m5.4 (Figs. F54, F55). Both m5.45 and m5.47 are moderate-amplitude continuous reflectors. Lower amplitude downlapping reflectors are imaged in the 15 ms space between these two surfaces. No sigmoidal clinoforms are observed on surfaces m5.45 and m5.47 on Oc270 profile 529. However, both of these surfaces track into the CH0698 data along strike, where they display seismic terminations characteristic of seismic sequence boundaries (Monteverde et al., 2008). Reflector m5.47 displays an incised character terminating at CDP 5398 on Oc270 profile 529, just west of Hole M0028A. It truncates the underlying m5.6 surface that emerges at CDP 5516 on Oc270 line 529. Data suggest that the original sigmoidal clinoform structure of reflector m5.6 was subsequently removed by the m5.47 surface where it displays a highly rugose pattern (CDPs 7033–5386 on Oc270 profile 529).

The next surface intersected is m5.7 (Figs. F54, F55). It is a moderate-amplitude continuous reflector east of Hole M0027A that decreases in amplitude, becoming discontinuous. Moderate-amplitude discontinuous reflectors that toplap against surface m5.47 and downlap against surface m5.7 define a seismic facies bounded by surfaces m5.47, m5.6, and m5.7. The final surface predicted to be sampled by Hole M0028A is m5.8, a moderate-amplitude continuous reflector that displays an amplitude increase at this site.

Core-seismic sequence boundary integration

The Pleistocene to upper middle Miocene section was not cored in Hole M0028A. The uppermost reflectors cored were two previously unnamed reflections, m4.1 and m4.5. The former was predicted at 231–234 mbsf. No prominent surfaces were observed in Cores 313-M0028A-2R through 7R, but a series of nodules in Sections 313-M0028A-6R-1, 61 cm, 6R-1, 110 cm, and 6R-2, 26 cm (236.1, 236.6, and 237.3 mbsf) cause density peaks and an impedance contrast associated with reflector m4.1 (Fig. F56). We interpret this zone as a possible maximum flooding surface (MFS), consistent with its interpretation based on core observations (see "Lithostratigraphy"). A surface in Section 313-M0028A-8R-2, 101 cm (244.23 mbsf), consists of homogeneous gray clay that abruptly changes downsection to medium quartz sand. No impedance contrast is associated with this surface, and it is interpreted as a flooding surface. A surface in Section 313-M0028A-9R-1, 135 cm (246.06 mbsf), includes bored nodules and a shell bed. The core is somewhat disturbed, but MSCL data show density peaks in this layer, which is interpreted as a transgressive surface (Fig. F57); a core magnetic susceptibility change at 236.8 mbsf may mark the top of the underlying unit. An erosional surface in Section 313-M0028A-10R-1, 5 cm (247.78 mbsf), is interpreted as a sequence boundary; it consists of granuliferous shoreface sand abruptly overlying offshore brown silty clay. The interpretation is consistent with its interpretation based on core observations (see "Lithostratigraphy"). Density decreases from clays to sands. The change in impedance between Cores 313-M0028A-9R and 10R is associated with reflector m4.5, which was predicted at 241–244 mbsf. A surface in Section 313-M0028A-12R-1, 19.5 cm (254.02 mbsf), consists of clayey silt over sands and is interpreted as a flooding surface.

Seismic sequence boundary m5 was predicted at 258–263 mbsf, placing it within a medium-coarse sand package (252–267 mbsf). Cores in this interval and immediately below (down to 275 mbsf) are disturbed and poorly recovered. We tentatively place reflector m5 in a coring gap from 266 to 269 mbsf near a major increase in downhole gamma ray values. This placement associates the m5 sequence with a seismically transparent facies interpreted as sands.

Two closely spaced surfaces associated with core density peaks occur near the level where seismic sequence m5.2 is predicted at 340–348 mbsf. A surface in Section 313-M0028A-34R-2, 95 cm (310.92 mbsf), is a sharp erosional contact between offshore above and shoreface–offshore transition below that is interpreted as a transgressive surface. A surface in Section 313-M0028A-35R-2, 48 cm (313.48 mbsf), is an erosional contact with shoreface–offshore transition fine–medium sand over offshore shelly clay that is interpreted as a sequence boundary (Fig. F58). Both surfaces yield density peaks, and the seismic reflector is a composite of these surfaces. This agrees with interpretations based on core observations (see "Lithostratigraphy").

A heavily burrowed surface in Section 313-M0028A-37R-2, 143 cm (320.61 mbsf), is a contact of clay over very fine sand interpreted as an MFS (Fig. F59).

We targeted seismic sequence m5.4 in Hole M0028A where it approaches its greatest thickness (predicted from ~347 to 497 mbsf). Two internal reflectors within this sequence were named m5.32 and m5.33, at 370–375 and 393 mbsf, respectively. Neither shows any prominent core expression, though recovery in this interval was not good. The lower reflector, m5.3, is predicted immediately below a downhole gamma log increase and may be a flooding (though not MFS) surface. Core observations suggest that this may be a candidate sequence boundary at ~390 mbsf (see "Lithostratigraphy"). A tentative MFS is identified at ~462 mbsf based on core observations. The lithologic expression of the basal unconformity predicted at 489–497 mbsf is likely a subtle surface in Section 313-M0028A-105R-1, 58 cm (495.19 mbsf) (Fig. F60). This is the base of a sand bed on top of sandy silt that causes a density peak. A downhole gamma log peak occurs just below this (498 mbsf), associated with the finer grained bed below.

Seismic sequence boundary m5.45 (predicted depth of 510–517 mbsf) is placed in Section 313-M0028A-110R-2, 110 cm (512.29 mbsf), at the base of toe-of-clinoform-slope apron fine poorly sorted silty sand over deep offshore shelly clayey silt (Fig. F61). A surface in Section 313-M0028A-113R-1, 68 cm (519.70 mbsf), consists of toe-of-clinoform-slope apron glauconitic sands over deep offshore claystones at a burrowed surface (Fig. F62). This is likely seismic reflector m5.47, which is predicted at 521–530 mbsf. An indurated zone at the top of Core 313-M0028A-116R (525.65 mbsf) likely contributes to the impedance contrast imaged as reflector m5.47. The surface at 519.70 mbsf is interpreted as a candidate sequence boundary based on core observations.

Seismic sequence boundary m5.6 (predicted depth of 540–550 mbsf) is in a coring gap between Cores 313-M0028A-126R and 127R (544.55–546.47 mbsf) at a prominent downhole gamma log peak. Core observations did not identify a candidate sequence boundary near 545 mbsf (see "Lithostratigraphy"), reflecting difficulty in identifying sequence boundaries in stacked amalgamated toe-of-slope apron deposits.

Core surfaces associated with seismic sequence m5.7 (predicted depth of 589–599 mbsf) are uncertain. There is a deeply burrowed surface in Section 313-M0028A-152R-1, 76 cm (611.6 mbsf), with toe-of-clinoform-slope apron glauconite sand burrowed down into clay over a contact zone in interval 313-M0028A-152R-1, 77–142 cm (Fig. F63). This surface is either a sequence boundary or a MFS; such heavily burrowed MFSs have not been observed onshore in New Jersey but have been interpreted in the Miocene at Bethany Beach, Delaware (USA) (Browning et al., 1996). Sediments above the glauconite sands from Section 313-M0028A-147R-1 to the top of Core 149R are shelly quartz sands rich in plant debris that could be interpreted as delta front deposits, which are in turn overlain by clinoform apron slope deposits in Core 147R and above; Core 148R is a coring gap. Thus, the unconformity may lie in the coring gap (600.3/604.42 mbsf), though the surface in Core 313-M0028A-151R is a major density contrast that contributes to if not causes reflector m5.7 in Hole M0028A.

Seismic sequence boundary m5.8 (predicted depth of 649–657 m) is placed in Section 313-M0028A-169R-1, 61 cm (662.98 mbsf), at a major contact of silty glauconite medium sand above and sandy siltstone below a burrowed contact that is interpreted as a core sequence boundary (see "Lithostratigraphy"). The density contrast clearly begins above the sequence in the glauconite sands at ~660 mbsf.

An acoustic impedance log was computed for Hole M0028A following the OSP to aid in core-seismic integration. The impedance log was constructed using MSCL bulk densities; velocities were taken from MSCL velocity logs only because downhole sonic logs were not available. The resultant acoustic impedance logs shown on the synthesis figures (Figs. F56, F57, F58, F59, F60, F61, F62, F63, F64, F65, F66, F67, F68) allow evaluation of seismic-core correlation made by other means: (1) we first predicted depth based on the velocity-depth function (Table T14); (2) we then slightly adjusted our predicted depths based on core surfaces, downhole log, and MSCL changes. In general, the impedance log at M0028 versus the other sites is hampered by recovery and reliance on MCSL data alone. Major downhole increases in impedance are associated with levels where we placed reflectors m4.5 (244.23 mbsf) and m5.45 (512.29 mbsf), whereas minor downhole impedance increases are associated with reflectors m5.2 (313.48 mbsf), m5.4 (495.19 mbsf), and m5.47 (519.70 mbsf). We cannot evaluate acoustic impedance contrasts associated with reflectors m4.1, m5, m5.3, m5.32, m5.33, m5.6, and m5.7 because of coring gaps.

Core-seismic-log synthesis

Figures F56, F65, F66, and F67 summarize the ties among sequence boundaries, sedimentary facies, and chronostratigraphy developed in Hole M0028A and are discussed below, emphasizing the relations between core sequences, logs, and seismic facies within a lithostratigraphic framework. (These figures are combined in Fig. F68.) These figures illustrate the same information as those for Hole M0027A (see "Stratigraphic correlation" in the "Site M0027" chapter), but bulk sediment density plots from MSCL measurements are included for this site. The MSCL plot appears discontinuous because of recovery gaps, but it shows significant changes that helps define seismic boundaries. In particular, several high-density (mineralized) intervals are found in this hole; the peaks of sediment density help correlate lithology to the impedance contrasts detected on seismic profiles.

Lithostratigraphic Unit II (223.33–335.37 mbsf)

Lithostratigraphic Subunit IIA is based by an unresolved seismic reflector correlated to a sediment surface picked in the core at 254.03 mbsf and corresponding to a flooding surface at the top of the shoreface sand of lithostratigraphic Subunit IIB, marking the transition to offshore silt and clay (Subunit IIA). This reflector is positioned at the base of a fine-grained clayey silt resting on poorly sorted coarse sand and granules (the Subunit IIA/IIB contact) and a corresponding abrupt downhole decrease in NGR and TGR values. A moderate impedance contrast is present at this depth as well. Subunit IIA is composed of two fining-upward intervals, corresponding basically to shoreface–offshore transitions in depositional environments. Several surfaces observed in the cores can be possibly correlated to the two seismic reflectors recognized within Subunit IIA: m4.1 and m4.5.

Seismic reflector m4.1 is predicted at 231–234 mbsf and matches the MFS surface placed between 236 and 237 mbsf. In fact, three distinct density peaks observed in Sections 313-M0028A-6R-1 (61 and 110 cm) and 6R-2 (26 cm) appear to generate the impedance contrast observed on seismic reflection (Fig. F56).

The depth range calculated on velocity for surface m4.5 corresponds to the surfaces detected in the sedimentary record. The surface m4.5 predicted interval (241–244 mbsf) covers the lithological horizon that includes the seismic boundary at 247.78 mbsf (Section 313-M0028A-10R-1, 5 cm), a transgressive surface at 246.06 mbsf (Section 9R-1, 135 cm), and a flooding surface at 244.23 (Section 8R-2, 103 cm). The maximum for sediment density correlates with the flooding surface (carbonate nodules occurring at the condensed interval; see "Core-seismic sequence boundary integration"). Fairly good chronological data suggest that the base of the sequence is ~14 Ma (Sr isotope age of 13.8 Ma ± 17 m.y.; see "Chronology").

Poor recovery does not allow precise placement of seismic sequence boundary m5, which is predicted between 258 and 263 mbsf. Nonetheless, it can be tentatively positioned at the base of Core 313-M0028A-18R in an interval spanning from 266 to 269 mbsf. Reflector m5 also marks a change in seismic facies that passes from transparent to subparallel reflectors and a lithological transition clearly seen in TGR/NGR values (Fig. F56).

Subunit IIC is based by seismic reflector m5.2. It is predicted at 310–316 mbsf and contains two surfaces described in the cores, corresponding to two distinctive peaks in density values. The first one is interpreted as a transgressive surface and has been described at 310.92 mbsf (Section 313-M0028A-34R-2, 95 cm). The second is placed at 313.48 mbsf (Section 313-M0028A-35R-2, 48 cm) and correlates to a sequence boundary described in the core as an erosional contact associated with a high impedance contrast. TGR values decrease between the two surfaces, related to the low Th concentration in fine-grained sand (Fig. F56). Changing lithology corresponds to fining-upward grain size in Subunit IIC, with shelly sands at the base passing to silt containing shell debris and wood fragments, indicating a passage from shoreface–offshore transition into an offshore environment. This part of the succession coarsens uphole into Subunit IIB (river-influenced shoreface). Variations in bulk density, TGR values, and impedance contrasts are visible on the logs in this interval (between ~270 and 310), but no seismic reflection can be identified that corresponds to these changes.

Lithostratigraphic Unit III (335.37–512.29 mbsf)

The depth range based on velocity for surface m5.3 is predicted between 340 and 348 mbsf, but an associated surface was not identified in cores from this interval. However, this predicted depth range correlates approximately with the transition between a ~7 m coring gap and the contact between lithostratigraphic Units II and III. We place the seismic boundary associated with reflector m5.3 in this coring gap between 335 and 342 mbsf (Fig. F56).

Reflectors m5.3 and m5.33 bracket lithostratigraphic Subunit IIIA, which is composed of massive coarse sand with sparse glauconite, interpreted as having been deposited in a shoreface environment. The predicted depth for reflector m5.33 is at 393 mbsf, located at the transition in seismic facies from subparallel to weak and discontinuous reflectors. Gamma ray values show a peak at ~391 mbsf related to enrichment in U content. A peak in density is also observed at ~390 mbsf (cemented sand), but sedimentary facies are poorly documented in this interval because of discontinuous recovery. Nonetheless, the density increase creates a sharp impedance contrast we conclude is the cause of reflector m5.33 (Fig. F65).

Sequence reflector m5.32 is predicted between 370 and 375 mbsf, but no sedimentary discontinuity can be detected in the cores (poorly recovered interval). Nonetheless, a clear change in seismic facies can be observed at this level, passing from subparallel-inclined reflectors to weak discontinuous reflectors downhole (Fig. F65).

Lithostratigraphic Subunit IIIB (river-influenced shoreface deposits) is a poorly recovered interval that correlates with very low gamma ray values (coarse-grained sand). Subunit IIIB represents the top of a shallowing-upward sequence that begins at seismic sequence boundary m5.4. The predicted depth for this latter reflector (489–498 mbsf) matches a surface in the core (Section 313-M0028A-105R-1, 58 cm; 495.19 mbsf). From the m5.4 sequence boundary uphole, the coarsening-upward sedimentary succession (most of Subunit IIID and all of Subunits IIIC and IIIB) is clearly marked by blocky stepwise decreasing gamma ray values. The seismic facies associated with this sequence begins at the base as strong, continuous reflectors onlapping the m5.4 clinoform slope and changes abruptly to subparallel and discontinuous reflectors close to an interval of poor core recovery at 450 mbsf. No change in log character is apparent at this depth (Fig. F65). The ~17 m interval between reflectors m5.4 and m5.45 (the base of lithostratigraphic Unit III) corresponds to coarse sand and gravel deposited in an apron at the toe of a clinoform and is overlain by a rapidly fining-upward package ending at surface m5.4 (Fig. F66).

Lithostratigraphic Unit IV (512.29–525.52 mbsf)

Lithostratigraphic Unit IV is separated from Unit III by seismic reflector m5.45. The predicted depth for sequence boundary m5.45 is between 510 and 517 mbsf and correlates well with the discontinuity in Section 313-M0028A-110R-2, 114 cm (512.33 mbsf). A decrease in gamma ray values coincides with this surface being marked by a shelly lag (Figure F66). The upper part of lithostratigraphic Unit IV lies between the m5.45 and m5.47 seismic sequence boundaries. This interval is characterized by a seismic facies showing clear parallel reflectors, possibly related to sediment lithology and consisting of alternating silt and thin sand beds deposited in a deep-offshore environment.

The predicted depth to seismic sequence boundary m5.47 (521–530 mbsf) is slightly deeper than the discontinuity noted in the core (519.70 mbsf). In addition, sequence m5.47 will need to be better constrained in age because the assignment to dinocyst Zone DN2 (early Miocene) appears to be too old compared to constraints on the overlying sequence (NN4; for sequence boundary m5.45, see "Chronology"). Nonetheless, it correlates well with a downhole interval to low TGR values (coarse sand bed overlying offshore clay) (Fig. F66).

Lithostratigraphic Unit V (525.52–611.19 mbsf)

Lithostratigraphic Unit V is mostly composed of coarse sand–bearing glauconite. An indurated zone between 530 and 545 mbsf corresponds to a K-enriched zone (glauconite). The predicted depth of reflector m5.6 closely matches the base of this interval of indurated beds. Glauconite and high-K gamma log values decrease steadily below this depth, and the remainder of lithostratigraphic Unit V is predominantly quartz rich, occasionally coarse-grained toe-of-slope debrites.

The calculated depth for sequence boundary m5.6 is between 540 and 550 mbsf (Fig. F66). The sedimentary discontinuity associated with this reflector is placed in a coring gap between Cores 313-M0028A-126R and 127R.

Seismic sequence boundary m5.7 bounds the base of lithostratigraphic Unit V (Subunit VC) and is predicted at 589–599 mbsf, but a corresponding surface in recovered cores is not clearly defined. Three surfaces are possible: 600.3 mbsf (base of Core 313-M0028A-147R), 604.42 mbsf (top of Core 149R), and 611.6 mbsf (Section 152R-1, 108 cm), as shown in Figure F66.

The possible sequences bordered by boundaries m5.47, m5.6, and m5.7 are poorly constrained in age. Between 545 and 610 mbsf, several peaks in TGR and MSCL bulk density are visible on the log (as well as significant impedance contrasts), but no additional seismic reflectors were identified. The interval is characterized by strong hummocky downlapping seismic facies (Fig. F66).

Lithostratigraphic Unit VI (611.19–662.98 mbsf)

Lithostratigraphic Unit VI is bordered by sequence boundaries m5.7 and m5.8. Both reflectors have been correlated to abrupt changes in impedance, although their predicted depths leave this association uncertain and in need of further analysis. The entire unit is assigned to mid-Zone NN2 and Zone DN2 (middle early Miocene).

Seismic surface m5.8 is predicted between 649 and 657 mbsf, positioned slightly shallower than the sedimentary discontinuity described in the core (Section 313-M0028A-169R-1, 61 cm) where a major burrowed contact correlates with a density contrast and the beginning of a peak on the gamma ray values (increasing K concentration in correspondence with glauconite sand; Fig. F67).

Lithostratigraphic Unit VI is a mostly fine-grained, well-laminated interval composed of clay and silt alternations deposited in a river-influenced offshore setting. The seismic facies characterizing this interval consists of transparent-parallel reflectors, generating a moderate impedance (Fig. F67).

In summary, log data are only available from 223 mbsf in Hole M0028A, covering lithostratigraphic Units II–VII (Fig. F64). Sequence boundaries m4.5, m5.2 (Unit II), m5.45 (Unit III), and m5.8 (Unit VII) picked in the cores correlate perfectly with downlogging data and seismics, whereas the position of seismic reflectors m5 (Unit II), m5.32, m5.33 (Unit III), and m5.6 (Unit V) remain problematic because of core gaps. Nonetheless, sequence boundary m5.6 corresponds to a prominent peak in TGR values at the base of indurate glauconite sandstone. Thus, reflector m5.6 can be confidently correlated to the TRG peak at the passage between Cores 313-M0028A-126R and 127R, despite the coring gap. Multiple seismic surfaces are possible for sequence boundary m5.7. The correlation with NGR and TGR values appears inconclusive; the unconformity may lie in a core gap. Finally, despite a prominent drop in impedance, a strong lithologic change, and a major gamma log change, the correlation of sequence boundary m5.8 to the core record remains unclear.