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doi:10.2204/iodp.proc.313.103.2010

Stratigraphic correlation

Seismic sequence identification

Site Survey Oceanus 270 (Oc270) and Cape Hatteras 0698 (CH0698) seismic data formed the framework for predicting seismic sequence boundaries in Hole M0027A (see Fig. F7 in the "Expedition 313 summary" chapter). Reflectors interpreted as lower and middle Miocene sequence boundaries (Miller and Mountain, 1994) were traced through the lower resolution Ewing 9009 (Ew9009) (~15 m vertical resolution) seismic grid from the outer shelf/slope region across the New Jersey shelf. These surfaces were subsequently correlated to the higher resolution Oc270 and CH0698 grids (Monteverde et al., 2008). Hole M0027A lies within a CH0698 hazard grid at the crossing of dip profile 107 common depth point (CDP) 11165 and strike profile 102 CDP 5704. CH0698 line 107 resampled part of Oc270 line 529. Time-depth correlations are based on semblance velocity data from Oc270 (see "Stratigraphic correlation" in the "Methods" chapter).

Targeted horizons in Hole M0027A range from the Pleistocene through the Eocene. The uppermost seismic reflectors intersected in Hole M0027A were Pleistocene surfaces identified on Geopulse data as MIC3a, MIC3c, and MIC4, with predicted depths on Oc270 line 529 at 6, 31, and 46 ms two-way traveltime (TWT) (Sheridan et al., 2000) corresponding to 12, 24, and 36 mbsf, respectively (see "Chronology" for age control on Pleistocene surfaces). Corresponding surfaces in the hole were identified as described below.

For Miocene and older sequences, seismic sequence boundaries were identified (see "Stratigraphic correlation" in the "Methods" chapter), and TWTs were picked by G.S. Mountain (unpubl. data) on OC270 line 529 and used during drilling operations. Monteverde et al. (2008) and this study (see "Stratigraphic correlation" in the "Methods" chapter) reevaluated loop correlations through the Oc270 seismic grid and provide slightly different TWT picks on some surfaces, as discussed below (Table T14). The velocity-depth function thus yielded two slightly different predictions of seismic sequence boundary correlations in Hole M0027A, and a third one was provided based on a post-OSP revised velocity analysis (see "Stratigraphic correlation" in the "Methods" chapter). The three estimates provided close agreement, except for differences noted below. Corresponding surfaces in the hole were identified as described below.

Tracking Miocene seismic sequence boundaries m1, m3, and m4 to the Expedition 313 sites is tenuous because of the distance to ODP Leg 150 drill sites and the nature of chaotic seismic facies in the vicinity of Hole M0027A. Depth predictions of these seismic surfaces are similar between seismic interpretations. Five new surfaces have been identified between m4 and m5 seaward of Hole M0027A, but only two, m4.1 and m4.5, track into the drill site. Here, surface m4.1 is a merged reflector of surfaces m4.1, m4.2, m4.3, and m4.4, from youngest to oldest. Surface m4.5 merges with middle Miocene seismic sequence boundary m5 <1 km seaward (east) of Hole M0027A at CDP 6995 (Fig. F63) on line 529. Surface m4.2 is defined as a seismic sequence boundary, whereas surfaces m4.3 and m4.4 are reflector terminations that are suggestive of MFSs. Surface m4.5 has some weak indications as a possible sequence boundary on the limited number of dip lines within the Oc270 and CH0698 seismic grids. Lower and middle Miocene seismic surfaces m5.4, m5.3, and m5.2 are more clearly defined as moderate- to high-amplitude continuous reflectors and are imaged clearly. Hole M0027A intersects these surfaces landward of their corresponding rollover (clinoform inflection) positions. A new surface was defined at the base of well-defined downlapping reflectors between CDPs 5411 and 4934 on line 529 across Hole M0028A (Figs. F63, F64). This surface, lying between m5.4 and m5.3 and named m5.32, was traced following a moderate- to high-amplitude, discontinuous, slightly undulating reflector into Hole M0027A. No rollover/inflection point is associated with seismic surface m5.45 along line 529. However, surface m5.45 correlates to a seismic sequence boundary within the CH0698 grid to the south (Monteverde et al., 2008; Monteverde, 2008).

The nature of the next deepest seismic sequence boundary, m5.47, is more complex. Boundary m5.47 correlates to a highly bisected surface between Holes M0027A and M0028A (CDPs 7033–5386 on Oc270 line 529). This surface truncates seismic surface m5.6 at CDP 7133 on line 529 immediately east of Hole M0027A. The eastern continuation of surface m5.6 emerges from the erosional truncation immediately west of Hole M0028A at CDP 5516 on line 529. Surface m5.7 was sampled in Hole M0027A ~100–200 m landward of its rollover/inflection point. This surface is a high-amplitude continuous reflector along the seaward-dipping clinoform front. Near its rollover/inflection point, however, it becomes a low- to moderate-amplitude discontinuous reflector. Immediately above and below the m5.7 rollover/inflection point are low- to moderate-amplitude hummocky reflectors. This seismic facies continues from 449 to 525 ms TWT below seafloor at the drill site, and below that, the facies changes to low- to moderate-amplitude, parallel to subparallel seismic facies to 566 ms TWT. A sharp high-amplitude continuous reflector separates the previous seismic facies above from a thin zone (20 ms) of well-imaged high-amplitude reflectors that show both downlap and onlap seismic terminations. Seismic surface m5.8 forms the base of this seismic unit. Hole M0027A is downdip (seaward) of the m5.8 rollover/inflection point. Reflector m5.8 is placed in the core at 489.4 mbsf, slightly below its predicted depth (470–480 mbsf). Post-OSP revision of the time-depth correlation has reduced this disagreement between depths calculated from TWT and presumed surface in the core (see "Stratigraphic correlation" in the "Methods" chapter).

Surface m6 underlies m5.8 as a moderate-amplitude, discontinuous, undulating reflector. It traces to a sigmoidal structure with a rollover at CDP 9395 on line 529. A new surface identified as o.5 (G.S. Mountain, unpubl. data) is a moderate- to high-amplitude discontinuous reflector that traces westward to a major downlap surface near CDP 7500 on line 529; this was originally identified as reflector m6 (see Table T10 in the "Methods" chapter), but loop correlations now indicate that it is older than reflector m6. Surface o.5 approximates the Miocene/Oligocene boundary (see "Chronology" for age control). The deepest predicted seismic surface encountered in Hole M0027A is surface o1, imaged as a moderate- to high-amplitude continuous reflector. Mountain et al. (1994) dated surface o1 as the Oligocene/Eocene boundary; however, in Hole M0027A it is clearly middle Oligocene (see "Chronology" for age control). The lack of Oligocene reflections between o.5 and o1 is due to the relative thinness. Numerous Oligocene sequence boundaries cored onshore (e.g., Pekar et al., 2000) are below seismic resolution.

Core-seismic sequence boundary integration

Core-seismic sequence boundary correlations are shown in Figures F65, F66, F67, F68, and F69. They are based on the integration of seismic and lithostratigraphy, log analysis, and age datings.

Pleistocene

A surface at Section 313-M0027A-5R-1, 11 cm (10.41 mbsf), marks the base of a coarsening-upward section, assigned an age of <90 ka based on nannofossils (Fig. F70). We interpret this surface as sequence boundary MIC3a (Sheridan et al., 2000). Gravel at the bottom of Core 313-M0027A-14R (26.38 mbsf) is dated at 125–250 ka based on nannofossils (Fig. F71). We interpret this as a sequence boundary predicted to be MIC3c (Sheridan et al., 2000). The section below this is older Pleistocene (NN19 with Sr isotope age estimates averaging 1.35 Ma in Section 313-M0027A-15R-CC). The base of this sequence is predicted at ~36 mbsf (Sheridan et al., 2000); it is poorly defined in cores because of poor recovery. It is tentatively placed at the gravel at Section 313-M0027A-19R-1, 39 cm (31.9 mbsf), though the position of this core is uncertain (Fig. F65).

Miocene

Reflectors m1–m4 are not well located relative to cores. Seismic reflector m1 is predicted (87–91 mbsf) in a poorly recovered zone, though we place it at 96 m log depth at a distinct decrease downsection in gamma ray values. Section 313-M0027A-50R-1 (97.12 mbsf) recovered a paleosol (Fig. F72) associated with moderate gamma values that decrease farther downsection and may mark the top of the underlying sequence m3. Reflector m3 was not cored but is placed near its predicted depth (105–109 mbsf) at the base of a downhole shift in gamma log values at 115 mbsf. Reflector m4 is predicted at 138–142 mbsf in an uncored interval, with little log expression (Fig. F73).

Reflector m4.1 marks a concatenated surface (at seismic resolution of ~5 m) of reflectors m4.1, m4.2, m4.3, and m4.4. Despite this probable condensed section or actual erosion, there is no apparent lithostratigraphic surface or change in log character at its predicted depth of 196 mbsf.

Reflector m5 is merged with reflector m4.5 at the site. This composite reflector m4.5/m5 (predicted depth of 210–215 mbsf) is tentatively placed at Section 313-M0027A-75R-2, 68 cm (218.39 mbsf), at the base of a granuliferous sandy silt over silty clay (Fig. F74). It is marked by a distinct impedance contrast caused by an increase in velocity and density measured by the MSCL (Fig. F66). It is interpreted as a candidate sequence boundary based on core observation (see "Lithostratigraphy"). The seismic placement is slightly above this depth, and this interval requires further study.

Reflector m5.2 (predicted depth of 225–230 m) was placed at Section 313-M0027A-80R-1, 10 cm, at the base of granuliferous very coarse sand over silty clay and corresponds to the base of a gamma log minimum (Fig. F66). There appear to be high MSCL density values at this level as well, although the recovery and data quality are too poor to be certain that this is an impedance contrast. It is interpreted as a candidate sequence boundary based on core observation (see "Lithostratigraphy").

A surface noted at the top of Section 313-M0027A-82R-1 (231.46 mbsf) has no obvious seismic expression. It consists of coarse sand in burrows at the top of the section and very fine sand below. The section above this (Section 313-M0027A-81R-2) changes downsection from clay to glauconite sand, consistent with its status as a candidate sequence boundary (see "Lithostratigraphy"). A downsection increase in MSCL density values is consistent with this section being a minor impedance contrast (Fig. F66).

Reflector m5.3 (235–240 mbsf predicted depth) is placed at Section 313-M0027A-83R-2, 127 cm (236.17 mbsf), at a contact between sand above and tight clay below (Fig. F75). There is a relatively large downhole decrease in MSCL density values across this surface that yields an impedance contrast (Fig. F66). It is interpreted as a candidate sequence boundary based on core observation (see "Lithostratigraphy").

A newly identified surface, m5.32 (predicted depth of 258–260 mbsf), appears to be a sequence boundary expressed as a surface at Section 313-M0027A-90R-1, 33 cm (256.19 mbsf). The surface juxtaposes granuliferous shelly silty sand over sandy silt (Fig. F76) and is interpreted as a candidate sequence boundary based on core observation (see "Lithostratigraphy").

Seismic sequence boundary m5.4 (predicted depth of 278–284 mbsf) appears to have an expression at Section 313-M0027A-95R-1, 10 cm (271.21 mbsf), at a sharp sedimentary facies change from laminated clayey silt above to sand below (Fig. F77). Core observation suggests this may be a transgressive surface, with the candidate sequence boundary immediately below (Fig. F66; see "Lithostratigraphy").

A surface noted at Section 313-M0027A-102R-2, 105 cm (295.01 mbsf), has no obvious seismic expression but is interpreted as a candidate sequence boundary based on core observations (see "Lithostratigraphy") (Fig. F78). It consists of very poorly sorted slightly glauconitic shelly sand deposited in shoreface–offshore transition environments overlying clayey silt deposited in offshore environments.

Seismic sequence boundary m5.45 (predicted depth of 325–330 mbsf) correlates with a surface at Section 313-M0027A-114R-2, 135.5 cm (331.9 mbsf), with a shelly, silty very fine sand overlying a sharp, irregular surface with heavy bioturbation that overlies sandy silt deposited in offshore environments (Fig. F79). There is a slight increase in MSCL density and velocity at this level. This level corresponds to a bioturbated surface and is interpreted as a candidate sequence boundary based on core observation (see "Lithostratigraphy").

Seismic sequence boundary m5.47 (predicted depth of 335–340 mbsf) correlates with a surface at Section 313-M0027A-116R-1, 90 cm (336.06 mbsf). This is a sharp, heavily bioturbated erosion surface separating clayey very fine sandy silt deposited in offshore environments from underlying silty, poorly sorted fine–very fine glauconitic sand with granules and pebbles deposited as a transgressive lag deposit (Fig. F63). The bioturbation extends from 72 to 101 cm. This surface is interpreted as a candidate sequence boundary based on core observation (see "Lithostratigraphy").

Seismic sequence m5.5 is truncated at this site, but seismic sequence m5.6 (predicted depth of 345–350 mbsf) appears to correlate with Section 313-M0027A-125R-1, 140 cm (355.53 mbsf). This zone is indurated, with shelly glauconite sand from a poorly recovered zone of indurated quartz sands below (Fig. F80). This zone shows a major downhole increase in core density and NGR (Fig. F67) and is interpreted as a candidate sequence boundary based on core observation (see "Lithostratigraphy"). Seismic sequence m5.7 is predicted close to this level (350–360 mbsf), and we tentatively place it in a coring gap below Section 313-M0027A-127R-2, 22 cm (361.28 mbsf), at a major downhole change in gamma log values.

Sequence m5.8 is very thick (~127 m) and contains a strong intrasequence reflection predicted at 450–460 mbsf. This intrasequence surface appears to be the MFS identified in Core 313-M0027A-162R (465 mbsf) based on benthic foraminifer paleobathymetry (see "Paleontology"), though the downhole gamma log may indicate its placement at a peak in Core 313-M0027A-163R (Fig. F68). Core observations place the MFS slightly higher (~458 mbsf; see "Lithostratigraphy"). A gradual increase in MSCL density between 465 and 475 mbsf appears to be the cause of this reflection. Seismic sequence boundary m5.8 is predicted at 470–480 mbsf. It is best placed deeper than predicted; we tentatively place it at the top of cemented glauconite sands in Section 313-M0027A-170R-CC (489.39 mbsf) or the top of Core 171R (489.39 cm), with cemented glauconite sands continuing below.

Seismic sequence boundary m6 was initially predicted at 540–545 mbsf, but this surface was retraced during the OSP as a sequence in the Oligocene (o.5). Based on this revision, the m6 reflection is thus predicted at 475–503 mbsf, which we correlate with Section 313-M0027A-174R-1, 111 cm (494.87), at the base of indurated glauconitic sandstones and the top of silty glauconite sands. This level is a candidate sequence boundary based on core observation (Fig. F69) (see "Lithostratigraphy"). However, at this stage of analysis, this reflector could be placed as low as a large downhole gamma log increase at ~499 mbsf.

Reflector o.5 was predicted at 540–545 mbsf and is tentatively correlated with the top of Core 313-M0027A-192R (539.51 mbsf). The o.5 sequence is uppermost Oligocene to ?lowermost Miocene (see "Chronology"). There is no obvious lithologic expression of this reflection, and it may be in a core gap between Cores 313-M0027A-193R and 195R (545.5–548.66 mbsf) or possibly at the base of a shelly zone in Section 313-M0027A-193R-1, 134 cm (543.90 mbsf) (Fig. F69). However, Section 313-M0027A-192R-CC is dated as older than 28 Ma and has Sr isotope and paleontological ages in congruence with the section below (see "Chronology"), though it is possible that the shells are reworked in the base of the sequence. Post-OSP studies are needed to resolve placement of the reflector and hiatus with respect to the cores.

Reflector o1 was predicted at 585–590 mbsf and is correlated with a surface at Section 313-M0027A-209R-1, 22 cm (585.48 mbsf). At this depth, a heavily bioturbated surface separates muddy glauconitic very fine sand from underlying clayey silts; it is associated with a significant downhole increase in MSCL density values (Fig. F81), suggesting that this is the source of an impedance contrast (Fig. F82). Reflector o1 was dated on the continental slope by Leg 150 and Alvin cores as the EOT (Miller et al., 1998); however, o1 here is clearly a younger sequence boundary because it is well dated as middle Oligocene (see "Chronology").

Two Oligocene surfaces identified in cores appeared to be possible sequence boundaries based on lithostratigraphic characteristics but were too subtle and closely spaced to be imaged well with available seismic data at this burial depth. A tentative surface at Section 313-M0027A-212R-2, 30 cm (596.30 mbsf), is a thin bed of shells and foraminifers sandwiched between glauconitic silty clay in an interval of heavy biscuiting, but it is associated with a major gamma kick in logs and cores (Fig. F69).

Calcareous nannofossils indicate a major hiatus between upper Oligocene Zone NP24 in Sample 313-M0027A-218R-2, 103–106 cm, and lower mid-Oligocene Zone NP23 in Sample 313-M0027A-220R-2, 49–52 cm. A faint surface at Section 313-M0027A-219R-1, 24 cm (617 mbsf), is a candidate for a sequence boundary associated with this hiatus, but we do not recognize a potentially correlative seismic surface.

The EOT is associated with a major erosional surface at Section 313-M0027A-223R-1, 90 cm (628.86 mbsf). Nannofossils in Sample 313-M0027A-223R-1, 26 cm, indicate lower Oligocene Zone NP22, whereas Sample 223R-1, 146 cm, indicates lower Zone NP21 and probable uppermost Eocene. Seismic and core expressions of Oligocene sequences are muted in Hole M0027A, and dating of hiatuses by biostratigraphy and Sr isotopes should provide a means of resolving sequence boundaries.

An acoustic impedance log was computed for Hole M0027A following the OSP to aid in core-seismic integration. The impedance log was constructed in two ways using MSCL bulk densities; velocities were taken from both MSCL velocity logs and downhole sonic logs. The resultant acoustic impedance logs shown on the synthesis figures (Figs. F65, F66, F67, F68, F69, F73, F82) allow evaluation of seismic-core correlation made by other means (e.g., the velocity-depth function, matching to core surfaces, downhole log, and MSCL changes). The impedance log in the top 200 m (Figs. F65, F73) uses only MSCL data, and core-log seismic integration is hampered by poor recovery. Comparison with the positions of reflections m4.5, m5.2, flooding surface m5.3, m5.32, and m5.4 with the impedance log is compelling: reflections are placed in the core by other criteria precisely at acoustic impedance contrasts except for m5.3, which is <2 m offset (Fig. F66). Acoustic log quality is not as high for sections encompassing seismic sequence boundaries m5.45, m5.47, m5.6, and m5.7 (Fig. F67) but are generally consistent with the core placement. Acoustic impedance data support placement of sequence boundaries m5.8 and m6 and are generally consistent with o1 but suggest that seismic sequence boundary o.5 may be too deep by ~5 m (Fig. F67).

Core-seismic-log synthesis

Figures F65, F66, F67, F68, F69, F73, and F82 summarize the ties between sequence boundaries, sedimentary facies, and chronostratigraphy developed in Hole M0027A and are discussed below, emphasizing the relationships between core sequences, logs, and seismic facies within a lithostratigraphic framework. (These figures are combined in Fig. F83.) Correlations include a synthesis of the lithology accompanied by an indication of depositional environments and paleodepth estimated by benthic foraminifer assemblages (see "Paleontology"). TGR (downhole logging data), NGR (MSCL measurements), and the relative concentration of isotope decay products (K, U, and Th) are also shown. Chronostratigraphy is established on the basis of biostratigraphic analyses (see "Paleontology"), whereas indication of age is given by the Sr dates (see "Chronology"). The data set is completed with a preliminary characterization of the seismic facies and with the correlation between the sequence boundaries detected in the cores and the predicted depth range of seismic reflections. Moreover, predicted depths from post-OSP revision to the time-depth function are shown in the figures, but they are not discussed because they were not available during the OSP (Figs. F65, F66, F67, F68, F69, F73, F82).

Lithostratigraphic Unit I (0–167.74 mbsf)

Seismic reflector MIC3c (Sheridan et al., 2000), with a predicted depth of 24 mbsf, is associated with a core sequence boundary at 26.29 mbsf (base of Core 313-M0027A-14R) and marks the boundary between lithostratigraphic Subunits IA and IB and the upper versus lower Pleistocene (Fig. F65). Seismic reflector MIC4, with a predicted depth of 36 mbsf, is associated with a major gap between the Pleistocene and the Miocene. The predicted depth from Geopulse seismic profiles (Sheridan et al., 2000) and the velocity function used during the OSP (see "Stratigraphic correlation" in the "Methods" chapter) is 36 mbsf, slightly deeper than the depth interpreted in the core lithology (Section 313-M0027A-19R-1, 56 cm; 31.9 mbsf). The succession passes downhole to a poorly recovered and barren interval (possibly fluvial deposit infilling a valley incision) of supposed Miocene age that includes lithostratigraphic Subunits IC and IB. The position of reflector m1 is based on well log data (decreasing gamma ray values at ~96 mbsf) and corresponds to the transition between lithostratigraphic Subunits IC and ID. The predicted depth range (89–94 mbsf) for reflector m1 is slightly shallower than the position inferred from gamma ray log values. The transition to possibly coarse-grained sediment is also marked on the seismic profile by the abrupt transition from parallel reflectors to channel-fill seismic facies.

Recovery is very poor down to Core 313-M0027A-58R (176 mbsf; lithostratigraphic Subunit ID). The section mainly corresponds to very low gamma ray values with granules and pebbles at the base, as evidenced in Cores 313-M0027A-55R and 56R. Reflector m3 has been positioned at 115 mbsf by relying on the well-log data, whereas the predicted depth range lies between 89 and 94 mbsf (Fig. F73). The log data signature was also used to locate reflector m4, where the predicted depth range based on velocity is 138–143 mbsf. The transition between lithostratigraphic Units I and II is not cored but is evidenced by downhole increasing gamma ray values and falls into an interval characterized by chaotic seismic facies.

Lithostratigraphic Unit II (167.74–236.16 mbsf)

The lack of a surface in the core or a change in either log or MSCL character at the predicted depth of reflector m4.1 (196 mbsf) requires further examination. Similarly, more work is needed to understand why the predicted depth for reflector m4.5 (208–211 mbsf) is shallower than the position interpreted in the core (Section 313-M0027A-75R-2, 68 cm; 218.39 mbsf). In contrast, reflectors m5.2 (Section 313-M0027A-80R-1, 10 cm; 225 mbsf) and m5.3 (Section 83R-2, 127 cm; 237.3 mbsf) match fairly well with the predicted depths (225–230 mbsf for m5.2 and 235–242 mbsf for m5.3) as shown in Figure F66. Reflector m5.3 coincides with the transition between lithostratigraphic Units II and III.

Several fining- and deepening-upward intervals (corresponding to transgressive shoreface evolving to shoreface–offshore transition; see "Lithostratigraphy") are identified between the predicted depths of reflectors m4.5, m5, and m5.3 (lithostratigraphic Unit II of middle Miocene age, as indicated by nannofossils). Each succession is truncated at the top, and the basal lag is marked by low values in gamma ray, K, U, and Th. The sequence overlying reflector m4.5 becomes homogeneous toward the top and bears a MFS (195 mbsf) that has no log expression. Nonetheless, it has been estimated on lithological facies (Section 313-M0027A-66R-2) in association with a downhole log gamma peak.

Lithostratigraphic Unit III (236.16–295.01 mbsf)

Lithostratigraphic Unit III (early middle Miocene) is composed of two distinct fining-upward sequences resting on reflectors m5.32 (256.2 mbsf) and m5.4 (271.21 mbsf) (Fig. F66). The predicted depths of the seismic reflectors lie slightly below the surfaces interpreted in the cores (258–260 mbsf for m5.32 and 278–284 mbsf for m5.4). Reflector m5.4 marks a significant change in seismic facies from parallel (above reflector m5.4) to subparallel reflectors.

The base of lithostratigraphic Unit III was positioned at 295 mbsf, corresponding to a major erosion surface (Section 313-M0027A-102R-2; see "Lithostratigraphy"). It corresponds to a peak in NGR-TGR values and a locally sharp increase in impedance derived from the sonic log (Fig. F67), but no seismic expression has yet been identified.

Lithostratigraphic Unit IV (295.01–335.93 mbsf)

Lithostratigraphic Unit IV contains reflectors m5.45 and m5.47 (Fig. F67). Reflector m5.45 is correlated at 331.9 mbsf at the base of the predicted depth interval (328–331 mbsf). It is marked by a peak in NGR values and a bioturbated surface. Reflector m5.47 is positioned at 336 mbsf (base of Unit IV), within the predicted depth range (335–343 mbsf), where a major lithological change is observed (coarse sand–silt transition) at the base of Unit IV (Fig. F84). In general, lithostratigraphic Unit IV is characterized by shallowing-upward successions from silt-prone offshore to shoreface–offshore transition with intercalated storm beds. Changes in the K-gamma log support these observations. Abrupt changes in impedance values (Fig. F67) suggest there ought to be strong seismic reflections in this unit as well. Indeed, there are corresponding reflections between reflectors m5.47 and m5.3, but they have very short lateral continuity (<1 km) on line 529, consistent with a series of retrograde parasequences.

Lithostratigraphic Unit V (335.93–355.72 mbsf)

The predicted depth of reflector m5.6 (346–350 mbsf) is 6–10 m above the preferred location at the base of lithostratigraphic Unit V (355.53 mbsf); hence, this unit is bracketed by reflectors (m5.47 at the top and m5.6 at the base) with no other seismic feature noted in between. Recovery in this unit was comparatively poor, and in general, the cores showed poorly sorted glauconite-rich sand with quartz and lithic granules interpreted as a transgressive shoreface environment. Perhaps this high-energy environment did not preserve the lateral continuity of stratal features that need to be resolved by seismic profiling. A Fresnel radius of ~100 m would be expected with the Oc270 air gun array at this depth, meaning that features on the order of a few tens of meters in lateral extent may not be detectable.

Lithostratigraphic Unit VI (355.72–488.75 mbsf)

Lithostratigraphic Unit VI is bracketed by reflectors m5.6 at its top and m5.8 at its base. It is a thick (~127 m) coarsening-upward succession of prodelta silt becoming clean shoreface quartz sand at top. The gamma log character shows this pattern of values smoothly decreasing uphole, with the exception of a local increase near 363 mbsf (Fig. F65) that corresponds to locally finer sediment in Core 313-M0027A-127R. This depth is close to the range of predicted depths for reflector m5.7 (350–360 mbsf) and is the position chosen for its most likely correlation. However, there is little other supporting evidence to justify this core-seismic correlation. In general, lithostratigraphic Unit VI corresponds to a large clinoform buildup comprising sequence m5.8 plus the poorly recovered ~8 m between reflector m5.7 and the top of Unit VI at reflector m5.6. The seismic facies show a subtle change from crudely parallel but discontinuous in the basal finer grained portion to faintly hummocky, irregular, and even more discontinuous in the coarser sands near the top of Unit VI.

Reflector m5.8 (Fig. F68) is placed in the core at 489.4 mbsf, slightly below its predicted depth (470–480 mbsf); as noted above, we attribute this to a possible overestimate of velocity. Within this sequence, a significant lithological change from medium-coarse grained sand (top) to silt (bottom) is associated with a gradual transition from very low to relatively high gamma ray values (transition between lower Miocene lithostratigraphic Subunits VIA and VIB and corresponding to river-influenced offshore to river-influenced shoreface–offshore transition). There is no seismic reflection associated with this coarsening upsection from silt to sand, but a change of seismic facies can be observed: slightly hummocky reflectors characterize Subunit VIA, and parallel reflectors match Subunit VIB (Fig. F68).

Lithostratigraphic Unit VII (488.75–625.60 mbsf)

A 5 m interval of coarse-grained, carbonate-cemented sand occurs at the top of lithostratigraphic Unit VII and creates large impedance contrasts (Fig. F69). The top of this interval is marked by a sharp downhole increase in impedance and provides a placement for reflector m5.8 within a few meters of its predicted depth. The bottom of this indurated interval creates a sharp downhole decrease in impedance and corresponds to reflector m6, also within a few meters of its predicted depth.

Downhole (between Cores 313-M0027A-176R and 180R), fluctuations of U concentration may indicate variable amounts of organic matter (and/or phosphorite?) in the sediments. These fluctuations are visible through the succession to Reflector o.5 (539.51 mbsf). This surface is predicted to lie between 540 and 545 mbsf, which matches reasonably well with the sedimentological interpretation (1–5 m deeper than the core pick). Surface o.5 spans the Miocene/Oligocene boundary, based mainly on dinoflagellate biostratigraphy at the top of Core 313-M0027A-192R. Below this surface, highly variable K concentrations could be related to glauconite content in the sand, but no evident surface has been identified in the sediment core.

At 585.48 mbsf, Reflector o1 was tied to Section 313-M0027A-209-1R, 22 cm, where it correlates perfectly with its predicted depth (585–590 mbsf).

Lithostratigraphic Unit VIII (625.60–631.15 mbsf)

The very bottom of Hole M0027A consists of deepwater (>100 m) sediments (see "Lithostratigraphy") moderately rich in glauconite. Downhole log data stop at ~600 mbsf, but the high amplitude of NGR measurements are possibly related to the presence of glauconite.

The Oligocene–Eocene transition is detected in Section 313-M0027A-223-1R, 90 cm (628.86 mbsf), with no associated seismic reflection.

The core-seismic-log synthesis (Fig. F70) in Hole M0027A shows fairly good correlation between the seismic surfaces and the log data in lithostratigraphic Unit I. However, the correlation with surfaces in the core is hardly possible in the Miocene succession because of poor recovery. Thus, the correlation is mainly based on downhole logging data. Lithostratigraphic Units II, III, IV, and V are almost totally recovered. An excellent correlation between the surface picked in the cores and reflectors is possible here. Reflectors m4.5, m5.2, m5.3, and m5.4 also match TGR-NGR values.

Lithostratigraphic Unit VI is poorly characterized in terms of sequence boundaries; the position of reflector m5.8 was problematic because of possible overestimation of velocity. The lowermost Miocene reflector (m6 in lithostratigraphic Unit VII) is well evidenced both in lithology and TGR values and correlates well with reflector m6. For positioning Oligocene reflectors o.5 and o1, further studies are necessary because no evident surfaces have been identified in the cores.

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