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Stratigraphic correlation

Stratigraphic correlation consisted of synthesizing the following: (1) seismic sequence and facies analysis, (2) seismic-core (sedimentary facies) correlation, and (3) incorporation of lithostratigraphy with other data sets (paleontology, Sr isotope). Correlation between seismic profiles and cores used a velocity-depth function tested by comparison with major core surfaces, downhole logs (particularly total gamma ray, spectral K as a proxy for glauconite, spectral U for organic-rich sediments, sparse velocity logs, and magnetic susceptibility), and MSCL logs (density and magnetic susceptibility). This procedure required the distillation of information based on lithology, interpreted sedimentary facies and paleoenvironments, paleodepths inferred by benthic foraminifer distribution, chronostratigraphic ages, seismic facies, and comparisons with physical property boundaries. Chronostratigraphic correlations were also an important part of the correlation loop; these are discussed in "Chronology."

Sequences in seismic profiles

Holes M0027A, M0028A, and M0029A are located on seismic profiles that embed several generations of seismic data (Fig. F1 in the "Expedition summary" chapter). Greenlee et al. (1988) first identified reflectors of mid-Cenozoic age in proprietary Exxon profiles that crossed the outer Baltimore Canyon Trough (middle to outer continental shelf off the mid-Atlantic United States). An informal workshop to correlate features on many of these proprietary data, attended by researchers from Exxon, Lamont-Doherty Earth Observatory, and Rutgers in 1990, resulted in the recognition of 17 surfaces as probable sequence boundaries based solely on their seismic character (Table T10). Eight of these were traced to a new grid of improved vertical resolution (~15 m) public data (Ew9009) and tied to drill sites with their ages estimated by Greenlee et al. (1992) (Table T10). From oldest to youngest these were green, pink-2, blue, yellow-2, tuscan, red-1, yellow-1, and pink-1. Drilling on the continental slope during ODP Leg 150 (Mountain, Miller, Blum, et al., 1994) sampled all but one of the original 17 candidate sequence boundaries previously identified, and each was renamed to an alphanumeric system with an age assigned (Table T10). Higher resolution (~5 m vertical) site survey Oc270 reflection data were collected in 1995 mainly on the outer shelf and slope but included line 529 (Fig. F5 in the "Expedition summary" chapter), which duplicated the lower resolution line 1003 of the earlier Ew9009 data set and extended to within 15 km of the shoreline. A reconnaissance grid of similar quality data (~5 m resolution) was collected during Cruise CH0698; this grid focused on the middle to inner shelf along New Jersey and included three grids of variably spaced lines (150–600 m; each roughly 6 km in length) that met safety assessment criteria across proposed drill sites MAT-1, MAT-2, and MAT-3 (Proposal 564), which were subsequently drilled as Expedition 313 Holes M0027A, M0028A, and M0029A, respectively (Fig. F7 in the "Expedition summary" chapter).

Seismic sequence boundaries were transferred from the older data sets to site survey CH0698 and loop correlated throughout this new seismic grid (Monteverde et al., 2008; Monteverde, 2008). Several additional lower Miocene sequence boundaries were identified in the process and, in keeping with the alphanumeric labeling scheme, were inserted into the hierarchy based on stratigraphic position. These new boundaries include, from oldest to youngest, m5.8, m5.7, m5.5, m5.47, m5.45, and m5.3 (Monteverde et al., 2008; Monteverde, 2008) (Table T10). Seismic sequences have since been named according to their basal reflector boundary, such that Sequence m5.5 lies on Reflector m5.5, for example.

Coreholes were drilled in the New Jersey coastal plain as part of ODP Legs 150X and 174AX (Miller et al., 1994; Miller, Sugarman, Browning, et al., 1998), and despite the obvious lack of seismic profiles tying these sites, correlations have been made (Table T10). Miller et al. (1997) bases ties on chronostratigraphy; Monteverde (2008) bases ties on synthetic seismograms of well data both offshore and onshore to profiles of the CH0698 grid. These latter onshore ties between onshore and offshore sequences do not agree in all cases (Table T10). One source of error is the uncertainty of ties from the outer shelf and slope (where Greenlee et al., 1992, and Miller et al., 1996, 1997, dated sequence boundaries) to the inner shelf, where sequence boundaries m5 to m6 are best expressed.

Prior to Expedition 313, seismic reflections were correlated for age control to industry wells on the outer continental shelf (Greenlee et al., 1992) and to coreholes onshore and on the continental slope (Browning et al., 2006; Monteverde et al., 2008; Monteverde, 2008) (Fig. F1 in the "Expedition summary" chapter). Ties for sequence boundaries m1–m4 between Holes M0027A, M0028A, and M0029A and drill sites on the slope provide relatively uncertain ages because of ambiguities resulting from long-distance seismic correlations (75 km and more). Correlations to onshore sequences are more reliable because seismic profiles were collected close to shore and pass within 1–3 km of onshore Leg 150X coreholes at Island Beach; Atlantic City, NJ (USA); and Cape May. In addition, synthetic seismograms provide a close match between those profiles and the coreholes (Monteverde, 2008).

Seismic profiles in two-way traveltime have been correlated to Expedition 313 cores using a velocity-depth function that provided approximate depths below seafloor to features of interest in the upper 1000 ms of reflection time (Fig. F18A, F18B, F18C). The velocity to depth conversion was developed as follows. Stacking velocities (Vrms) were examined at 22 common depth point (CDP) gathers along Oc270 MCS line 229. This profile is the seaward continuation of MCS line 529, which crosses the Expedition 313 drill sites (Fig. F1 in the "Expedition summary" chapter). The distance between adjacent CDP gathers was roughly 500 m, and all crossed a nearly level seabed (two-way traveltime = 77–80 ms) chosen because of the uniformity of the underlying acoustic structure down to 1000 ms; Vrms values at greater traveltimes showed increased scatter and were not included further. No meaningful along-profile variation in Vrms was observed, and each of the five to eight velocity-traveltime pairs at every gather was graphed and a third-order polynomial was fit to the entire set of data pairs above 1000 ms. This provided a single Vrms versus traveltime relationship to which the Dix (1955) equation was applied to derive interval velocities in 10 ms increments from 0 to 800 ms. The seafloor was within a few milliseconds of 80 ms in all of the Vrms-traveltime pairs used in this preparation. Accordingly, pairs above 80 ms were then discarded, leaving only those data corresponding to Vrms versus traveltime below the seafloor. This provided a means to calculate depths in mbsf versus traveltime in 10 ms increments, and to this was fit another third-order polynomial.

The polynomial used during the OSP was unconstrained; a later effort followed this identical procedure but forced this time-depth regression to pass through 0 ms and 0 mbsf. The resulting two polynomials differ by only a small amount (e.g., a traveltime of 500 ms predicts a depth of 433 mbsf for the former, 443 mbsf for the latter) (Fig. F18A, F18B, F18C). For simplicity we have used the former (unconstrained) function in the site chapter discussions, but in some figures in "Stratigraphic correlation" in each site chapter we show depth predictions using both. This technique provided a relationship of two-way traveltime below seafloor versus traveltime below seafloor that could be applied to a sediment column in any water depth, providing the pattern of velocity versus burial depth was sufficiently close to that along line 229. Initially, this was viewed as a means to simply estimate depths of reflectors at Expedition 313 sites. However, the match of these predicted depths to physical features in the cores and/or wireline logs thought to cause reflections proved to be remarkably accurate, with errors generally <3% of the predicted depth. Comparison to depths predicted by the independently measured VSP showed close agreement in most of the intervals with some exceptions that are discussed in "Stratigraphic correlation" in each site chapter. Because of various aspects of the VSP acquisition (see "Downhole measurements") there were gaps or modest/severe data degradation in some intervals (Fig. F18B). Despite the uncertainty of relying on a single time-depth relationship at three sites along line 529 that have such obvious subseafloor differences in stratigraphy, we chose to continue using the predictions derived from the stacking velocities. The correlations provide a model testable with data sets collected during Expedition 313.

Seismic facies interpretation through the CH0698 and Oc270 seismic grids was instrumental in predicting sedimentary facies for Expedition 313 (all data from CH698 and Oc270 were collected in opposite phase, i.e., a positive pulse in one is a negative pulse in the other, meaning that black in one data set is white in the other). High-amplitude, continuous horizontal reflectors that become sigmoidal at their corresponding clinoform rollovers and display downlapping reflector terminations along their relatively level topsets and onlapping terminations along their dipping foresets have been identified as sequence boundaries (Mitchum et al., 1977). Each was loop correlated through the CH0698 grid and to Oc270 line 529. Packages of discontinuous, low-amplitude reflectors that onlap the seaward front of clinoform rollovers have been interpreted as lowstand facies. Reflectors within the same packages terminate with downlap onto underlying sequence boundaries at their distal locations. Retrograde stacking of reflectors above and landward of the clinoform rollover comprises packages that are typically too thin to resolve with clarity with available seismic data. Where detected, these packages are marked by limited reflector onlap landward of the preceding rollover and by discontinuous, moderate amplitude, generally parallel reflectors. Transgressive surfaces within these packages are generally so close to the underlying sequence boundary as to be undetectable with existing seismic data. Downlap terminations are more clearly imaged seaward of the rollover where the flooding surfaces generally steepen and merge with the underlying sequence boundary. Reflectors within aggradational to progradational highstand facies are generally parallel to the underlying horizontal flooding surface with limited evidence of downlapping terminations.

Mapping surfaces throughout the CH0698 seismic grid showed shoreline-parallel changes in the thickness of the lower to middle Miocene sequences due largely to changing locations of sediment supply (Browning et al., 2006; Monteverde et al., 2008). In addition, postdepositional failure and erosion appear to have contributed to these along-strike thickness variations. Several seismic surfaces displaying all the necessary reflector terminations to be defined as sequence boundaries on several lines flatten out along strike, no longer display typical clinoform geometry, and merge with overlying surfaces. Such variations can be seen on Oc270 line 529 where sequence boundaries m5.5 and m5.47 merge to form a single, nearly planar surface. This merged surface locally truncates part of the m5.6 sequence boundary between Holes M0027A and M0028A (Fig. F15 in the "Expedition 313 summary" chapter). Sequence boundary m5.45, elsewhere associated with a clinoform buildup, correlates to a nearly planar surface across the Expedition 313 drill sites, apparently due to postdeposition erosion.

Late Miocene boundaries m1–m4, which track near Hole M0029A, are continuous, moderate-amplitude planar reflectors that become broadly discontinuous, lower amplitude, and subparallel in the landward direction across Holes M0027A and M0028A. Seismic facies shallower than 250 ms in the vicinity of Expedition 313 drill sites are less clear than those below. These shallower reflectors are discontinuous, low to moderate amplitude, dominantly subparallel to locally chaotic, and difficult to trace. Isolated incised channels are clearly imaged in the upper 100 ms.

Sequences in cores

The preliminary velocity function provided predictions for the depth of each seismic sequence boundary in the cores (see tables in "Stratigraphic correlation" in each site chapter), but the sequences in cores were defined on the basis of physical stratigraphy (see "Lithostratigraphy"), age breaks (see "Chronology"), and downhole and core log data (see "Downhole measurements" and "Physical properties").

In general, predicted depths of seismic sequence boundaries closely matched surfaces noted in cores (within ±5 m). In some cases, multiple lithostratigraphic surfaces were encountered within a 10 m interval, and the correlation to any of several nearby seismic sequence boundaries was uncertain. The possible placement of these and all seismic sequence boundaries was made on the basis of significant variations in sediment physical properties, lithologic changes, or the combination of the two parameters. The choice of placement of any single surface was preserved in tables and figures (e.g., Fig. F83 and Table T14 in the "Site M0027" chapter). Moreover, in some cases, a disagreement of as much as 10 m can be observed in the position of predicted seismic boundaries and corresponding surfaces picked in the core. No single explanation can be generally applied to decipher all mismatches; individual cases are discussed in each site chapter.

Sequence boundaries and MFSs are often associated with downhole and MSCL gamma peaks. In addition, density and velocity core logs are occasionally useful in picking out surfaces in the cores (see "Core-seismic sequence boundary integration" in each site chapter). All MSCL data provided useful indicators of lithologic variations. Physical properties of sediments depend to a large extent on lithology, grain size, and the proportion of the different components of the sediment (e.g., Austin et al., 1998). Bulk density in water-saturated sediments is related to porosity and partially controlled by grain size. Acoustic velocity is controlled by porosity, and magnetic susceptibility may reflect changes in lithology, particularly the proportion of biogenic components (carbonate and silica) to lithogenic components. Log data have been visually examined to detect trends and tentatively grouped according to their physical properties and corresponding stratigraphy.

In most cases, the changing patterns of physical properties are in good agreement with the boundaries and the nature of the major stratigraphic units. Sequence boundaries typically correspond to low total gamma ray values, but this is not always reliable because their definition is based on stratal geometry and not on lithic composition. In particular, downhole logging data proved useful in filling core gaps due to poor recovery in unconsolidated sandy intervals or where a spot coring strategy was adopted.

An acoustic impedance log was computed for Expedition 313 holes to aid in core-seismic integration. For Holes M0027A and M0029A, the impedance (velocity × density) log was constructed using MSCL bulk densities and velocities from both MSCL velocity logs and downhole sonic logs. For Hole M0028A, only MSCL data were available, as a sonic log was not obtained. MSCL density quality is generally good, though MSCL velocity suffers from artifacts (e.g., cores not filling liners and uneven distribution of drilling fluids in liner). The downhole sonic velocities are more reliable, though they also suffer in some intervals (e.g., washout zones); however, velocity logs were only obtained on about ⅓ of the Expedition 313 section.

Tables and stratigraphic synthesis figures in each site chapter provide a distillation of information used to identify fundamental stratigraphic surfaces, particularly sequence bounding unconformities. There is general agreement among methods used to identify surfaces, with predictions from seismic sequence-core correlations, lithostratigraphic discontinuities, changes in stacking patterns, and abrupt changes in physical and log properties converging on the identification and placement of major stratal surfaces (e.g., Fig. F66 in the "Site M0027" chapter and Figs. F55 and F57 in the "Site M0029" chapter). However, there are a few cases where differences in interpretation and placement of surfaces differ amongst techniques:

  • Some unconformities recognized on seismic profiles have subtle or little lithostratigraphic signature (e.g., seismic sequence m5.6 in Hole M0029A; Fig. F63 in the "Site M0029" chapter).

  • A few lithostratigraphic breaks that appear as candidate sequence boundaries occur within seismic sequences (Fig. F56 in the "Site M0028" chapter; ~290 mbsf).

  • There are cases where correlation of multiple stratal surfaces noted by their lithologic character cannot be unequivocally matched to seismic predictions (e.g., Fig. F66 in the "Site M0028" chapter; 500–530 mbsf).