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doi:10.2204/iodp.proc.313.101.2010

Principal results

Lithostratigraphy

The lithostratigraphic description of sediments cored in Holes M0027A–M0029A on the New Jersey shelf shows that sediments were deposited in two general contexts: (1) mixed storm- and river-dominated shelf with well-sorted silt and sand deposited in offshore to shoreface environments and (2) intrashelf clinoform rollover, clinoform slope, and toe-of-slope settings are dominated by poorly sorted coarse-grained debrites and turbidites with interbedded silt and silty clays. Fine-grained deposits are usually silt-rich and show a notable paucity of clays. The open shelf experienced frequent periods of dysoxia with cyclical repetitions. We found no evidence of exposure at the clinoform rollover (depositional shelf break). However, the periodic occurrences of shoreface facies along the slope of clinoforms and of offshore facies on the topset of the clinoforms in the same seismic sequence suggest large-amplitude changes in relative sea level. All statements below regarding depositional setting are interpretations based on Expedition 313 Science Party core observations. Refer to "Chronology" for refined ages, "Biostratigraphy" for paleodepth estimates, and "Stratigraphic correlation of seismic and sedimentary sequences" for comparison of key stratigraphic surfaces identified in core, well logs, and seismic data.

Hole M0027A

A summary of the lithologic units and major lithologies in Hole M0027A is provided in Figure F8. The lithology at the base of Hole M0027A (Unit VIII) is clay (Unit VIII; 631.15–625.60 meters below seafloor [mbsf]; late Eocene) deposited in a deep offshore environment. The overlying Unit VII (625.60–488.75 mbsf; Oligocene to lowermost Miocene [Aquitanian]) comprises a large-scale coarsening-upward sequence from silt to very fine sand to poorly sorted glauconite-rich coarse sand debrites and turbidites. Cyclic changes in average grain size and glauconite content occur upcore on the scale of 5–10 m; however, sedimentary structures are in places obscured by bioturbation. The succession records the progradation of clinoform slope apron systems over deep (>200 m?) distal clinoform toesets. Unit VI (488.75–355.72 mbsf; upper Aquitanian to lower Burdigalian) marks the outbuilding (progradation) of a thick storm-dominated river-influenced delta (offshore to shoreface) over a toe-of-slope apron. A thick shoreface succession (Subunit VIA; ~57 m thick) comprises clean quartz sand. The overlying Unit V (355.72–335.93 mbsf; lower mid-Burdigalian) marks an abrupt change in lithofacies and mineralogy to poorly sorted glauconite-rich sands with quartz and lithic granules whose environment of deposition in a clinoform rollover position is poorly constrained. Univ IV (335.93–295.01 mbsf; mid-Burdigalian) comprises a deepening-upward shoreface–offshore transition to offshore succession lacking a regressive facies. An erosional surface separates lithostratigraphic Units IV and III (295.01 mbsf), which begins with ~1.5 m of very coarse glauconitic sand representing a condensed transgressive lag deposit. Unit III (295.01–236.16 mbsf; upper Burdigalian to lower Langhian) is composed of deepening- and shallowing-upward packages of silty offshore and shoreface–offshore transition environments with a major storm influence. Unit II (236.16–167.74 mbsf; Langhian) consists of a series of fining- and deepening-upward sedimentary cycles that are interpreted as transgressive shoreface evolving to shoreface–offshore transition deposits. These cycles are thought to be incomplete depositional sequences from which regressive facies successions have been subsequently eroded. The upper part of Unit II consists of a clay-rich offshore succession. Unit I (167.74–0 mbsf; upper Miocene and upper Pleistocene, with no identified Pliocene) comprises sands and gravels deposited in a range of fluvial, coastal plain, estuarine, shoreface, and incised valley environments.

Hole M0028A

A summary of the lithologic units and major lithologies in Hole M0028A is provided in Figure F9. The oldest stratigraphic unit in Hole M0028A (Unit VII; 668.66–662.98 mbsf; mid-upper Aquitanian) comprises dark brown siltstone with thin-walled articulated shells deposited in a low-energy deep offshore environment. Unit VI (662.98–611.19 mbsf; Aquitanian to lowermost Burdigalian[?]) is a pale brown clayey silt with intercalated very fine and fine sand beds representing a river-dominated offshore (prodelta) environment. The contact with overlying Unit V (611.19–525.52 mbsf; lower Burdigalian) is abrupt and bioturbated. Unit V is divided into three subunits of poorly sorted coarse-grained gravity flow deposits. The poorly sorted coarse sediments are interpreted as deposits from high-concentration flows at the toe of slope of a degraded clinothem. Subunits VB and VC are dominated by debrites and Subunit VA by turbidites. Unit IV (525.52–512.29 mbsf; mid-Burdigalian) consists of sediment gravity flows, possibly from river flood events, deposited in an offshore environment. Unit III (512.29–335.37 mbsf; mid–upper Burdigalian) is divided into four subunits that mark the shallowing-upward and outbuilding of a storm-dominated river-influenced delta. Subunit IIID consists of toe-of-clinoform apron deposits of coarse sand with gravel abruptly overlying silts from Unit IV. Subunits IIIB and IIIC contain interbedded silts and two-part sands, interpreted as storm flows deposited in a shoreface–offshore transition setting. Poorly recovered Subunit IIIA comprises clean quartz sandstone deposited in a shoreface setting. Unit II (335.37–223.33 mbsf; upper Burdigalian to lower Serravalian[?]) is also divided into four subunits that mark a series of offshore to shoreface cycles. Subunit IID is poorly sorted and dominantly coarse grained, with a mix of mud, coarse sand, and gravel deposited at a clinoform rollover. The position may be gully fills and/or small deltas. These sediments fine uphole into offshore silts. A sharp contact separates Subunit IID from Subunit IIC, which consists of shoreface–offshore transition deposits. The succession initially fines and then gradually coarsens uphole from clay and silt to medium sand, indicating a transition from an offshore to a river-influenced shoreface–offshore transition setting. Contact with Subunit IIB is characterized by in situ glauconite in an offshore setting. The overlying intervals of poorly sorted quartzose gravelly sandstone are high-concentration sediment gravity flow deposits within a channelized environment. The fining then coarsening of the overlying succession reveals a deepening then shallowing from offshore to shoreface–offshore transition settings. The contact between Subunits IIB and IIA is marked by an abrupt grain-size break separating poorly sorted coarse sand with granules (below) from clayey silt (above). In Subunit IIA, an erosional surface that cuts into offshore silts with rare storm event beds is overlain by coarse to fine sand with bored nodules and glauconite grains, indicating condensed deposition (hiatus) followed by seafloor erosion and exhumation, which is overlain again by clay in an offshore environment. Unit I was not cored in Hole M0028A.

Hole M0029A

A summary of the lithologic units and major lithologies in Hole M0029A is provided in Figure F10. Unit VII (756.33–747.27 mbsf; lower Oligocene to lower Miocene) consists of siltstone with glauconite sand and thin-walled articulated shells deposited in a low-energy deep offshore environment. Unit VI (747.27–728.55 mbsf; late Aquitanian to early Burdigalian) is a pale brown clayey silt with intercalated very fine and fine sands deposited in a river-dominated offshore (prodelta) setting. Units V through III (728.55–640.51 mbsf; middle early Miocene to early middle Miocene) contain a series of granuliferous quartz and glauconite coarse sand packages separated by bioturbated silts. Sands are generally sharp based and often grade uphole into silts. Sands are interpreted to represent toe-of-clinoform-slope apron systems deposited during lowered sea level and silts to represent deep offshore sediments deposited during high sea level. Subunit IID (640.51–602.25 mbsf) contains two packages of poorly sorted glauconitic medium to coarse sand overlain by bioturbated silt. The depositional environment was toe-of-clinoform-slope apron at base and deep offshore on top.

Unit II (640.51–325.12 mbsf; middle Miocene [Langhian]) is divided into several subunits. Sediments in Subunit IIC (602.33–502.01 mbsf), comprising a monotonous succession of very fine sandy silt and silt, were deposited in a deep offshore environment below storm wave base. Subunit IIB (502.01–448.49 mbsf) contains three sediment packages. The two packages at the base coarsen uphole from medium to coarse sand grading to silt. The upper package contains poorly sorted slightly shelly silt with gravels. These sediments likely represent sediment gravity flow deposition on a clinoform slope, either within a submarine channel or intraslope apron environment. Subunit IIA (448.49–325.12 mbsf) is another monotonous very fine sandy silt and silt with rare sandier units. These represent offshore river-influenced offshore environments. Unit I (325.12–3.85 mbsf; upper Serravalian[?] and upper Pleistocene) was spot-cored to identify major reflectors. Sediments recovered were likely deposited in a range of shelf settings, from shallow marine shoreface to foreshore, coastal plain, and estuarine environments.

Biostratigraphy

Study of the fossils found in Holes M0027A, M0028A, and M0029A provides important constraints on the age, paleoenvironment, and paleowater depth of sediments deposited on the New Jersey shallow shelf. Age assignments based on calcareous nannofossils, planktonic foraminifers, and dinocysts are in generally good agreement (Figs. F11, F12, F13), although they do show some evidence of diachrony of biostratigraphic markers across unconformities between Holes M0027A and M0029A. Calcareous microfossils are more abundant and more consistently present downhole, allowing for better age control at the more distal Hole M0029A. Benthic foraminifer biofacies indicate paleobathymetric changes within a sequence stratigraphic and lithologic framework, including both shallowing-upward and deepening-upward successions. Pollen studies identified a hemlock horizon across all three holes, indicating the presence of temperate forests and humid conditions on the Atlantic coastal plain during the early Miocene (early Burdigalian). Middle Miocene pollen assemblages record the expansion of grasses and sedges, indicating increasing aridity at that time. The main results for the three drilled holes follow.

Hole M0027A

Pleistocene, Miocene, Oligocene, and possible uppermost Eocene sections were identified from calcareous nannofossils, planktonic foraminifers, and dinocysts (Fig. F11) and integrated with Sr isotope stratigraphy to establish a chronostratigraphic framework for Hole M0027A (Fig. F14). There is generally good agreement between the biohorizons of the different microfossil groups and the Sr isotope ages in this hole. The exception is within the Oligocene section; calcareous nannofossils indicate an expanded upper Oligocene section, whereas dinocysts do not.

The abundance and preservation of calcareous microfossils and dinocysts varies significantly throughout the hole, with barren intervals coinciding with coarse-grained sediments in the Miocene and younger sections. The prevalence of sands, particularly in the middle Miocene, may have resulted in depressed biostratigraphic last occurrences, particularly within the calcareous microfossils. Reworking of Paleogene material also made biostratigraphy challenging, predominantly within the lower Miocene sediments.

Paleobathymetry and paleoenvironments determined from benthic foraminifers, dinocysts, and terrigenous palynomorphs show that paleodepths varied throughout Hole M0027A, ranging from inner neritic (0–50 m) to outer neritic (100–200 m). Paleobathymetric fluctuations indicate shallowing- and deepening-upward trends that correlate with the evolution of depositional environments. In general, lithofacies and benthic foraminifer biofacies correlate well. Similarly, benthic foraminifer water depth estimates and palynological estimates of proximity to shoreline are consistent. Palynological data support previous reconstructions of a warm, humid early Neogene climate during the Eocene–Oligocene transition and early Neogene, whereas the Oligocene witnessed intervals of drier and cooler conditions, causing the spread of herbal taxa and conifer forests.

Hole M0028A

Middle and lower Miocene sections were identified from calcareous nannofossils, planktonic foraminifers, and dinocysts (Fig. F12) and integrated with Sr isotope stratigraphy to establish a chronostratigraphic framework for Hole M0028A (Fig. F14). Abundant reworking of Paleogene material made assigning ages to some intervals difficult, but there is generally good agreement between the planktonic microfossil groups and ages based on Sr isotopes. As in Hole M0027A, barren intervals coincide with coarse-grained sediments, diminishing biostratigraphic age control within intervals of the lower Miocene.

Paleodepths vary through the Miocene section of Hole M0028A, ranging from inner to middle neritic (0–100 m). In several sequences, paleobathymetric fluctuations indicate shallowing-upward successions. In general, lithostratigraphic units and benthic foraminifer biofacies correlate well. Similarly, benthic foraminifer water depth estimates and palynological estimates of proximity to shoreline are consistent. Palynological data support previous reconstructions of a warm, humid early Neogene climate.

Hole M0029A

Pleistocene and middle and lower Miocene sections were identified from calcareous nannofossils, planktonic foraminifers, and dinocysts (Fig. F13) and integrated with Sr isotope stratigraphy to establish a chronostratigraphic framework for Hole M0029A (Fig. F14). There is generally good agreement among the ages obtained from the different planktonic microfossil groups, which is particularly important in this hole because the Sr isotope ages within the middle Miocene strata have substantial scatter. Microfossils are also more abundant in this hole, allowing for age refinements within the lower Miocene sections that are barren of planktonic microfossils in the previous holes. As in the previous holes, reworked Paleogene material occurs throughout the Miocene sections, although it is more concentrated in the lower Miocene, making age assignments somewhat difficult in certain intervals.

Paleobathymetric estimates for Hole M0029A are based on benthic foraminifer occurrences, which indicate that paleodepths fluctuated from the outer neritic zone (100–200 m) to the inner neritic zone (0–50 m). Benthic foraminifer biofacies changes indicate that paleobathymetric fluctuations occur within a sequence stratigraphic framework, with several sequences showing a shallowing-upward succession and one showing a deepening-upward succession. Benthic foraminifer water depth estimates and palynological estimates of proximity to the shoreline show excellent agreement. As with the previous holes, palynological data support reconstructions of a warm, humid early Neogene climate.

Paleomagnetism

The main objective of the shore-based paleomagnetic studies was to produce a polarity magnetostratigraphy in as much detail as possible within the time constraints of the Onshore Science Party. At selected depth intervals for all three holes, attempts were also made to characterize the remanence carrier and to make preliminary estimates of relative paleointensity. These objectives were achieved through measurements of natural remanent magnetization (NRM) and alternating-field (AF) demagnetization of discrete samples taken from Expedition 313 cores.

Sediments from Holes M0027A–M0029A generally possess a weak or unstable NRM. However, clay-rich horizons in all holes exhibit much stronger magnetic moments, accompanied by peaks in magnetic susceptibility. AF demagnetization up to 15–30 mT, depending on lithology, typically removes a low-coercivity overprint with normal polarity, sometimes revealing a higher coercivity stable component. Inclination data show prevailing normal polarity, suggesting that the first component is a viscous overprint. In Hole M0028A, the inclination data also showed frequent reversals carried by stable components. When compared to the general age-depth constraints, it is suspected that either the normal or reversed parts of the interval in question (Cores 313-M0028A-7R and 8R) were caused by a chemical remanent magnetization (CRM). During AF demagnetization a few samples acquired what was identified as gyroremanent magnetization (GRM). The effect of GRM was removed by applying an anti-gyroremanent demagnetization procedure.

Two reversal boundaries were successfully identified in Hole M0027A within the clay sequence in upper Unit II and could be assigned to either C5ACn or C5ADn, based on age constraints from Sr and biostratigraphic analyses. In Hole M0028A, a polarity magnetization could only be established for two intervals from ~223 to 255 mbsf and ~610 to 669 mbsf. Within these intervals, six reversal boundaries were identified: C5ABr or C5ACr (between 242.30 and 242.59 mbsf), C5AAn or C5ABn (between 226.23 and 226.53 mbsf), C6Ar (between 665.91 and 666.41 mbsf), C6An.2n (between 655.92 and 656 34 mbsf), C6An.1r (between 622.65 and 622.90 mbsf), and C6An.1n (between 616.52 and 617.02 mbsf). In Hole M0029A, five boundaries were tentatively identified: C5Aar (between 331.11 and 330.91 mbsf), C5AAn (between 327.95 and 327.53 mbsf), C5Acn through C5ADN (thick zone of normal polarity between 370 and 470 mbsf), C6An.1r (between 733.29 and 733.49 mbsf), and part of all of C6An.2n (~747 mbsf).

Magnetic mineral dissolution was observed in thin section and may explain, in part, the weak NRM of these sediments. Additionally, authigenic magnetic minerals associated with glauconite pellets were identified in thin section, explaining the observed relationship between high magnetic susceptibility and glauconite-bearing sediments.

Petrophysics and logging

Petrophysical and downhole log data collected during Expedition 313 are essential for correlation between sedimentological observations and seismic interpretations, thus aiding our understanding of the sequence stratigraphy of the New Jersey margin.

Offshore, the petrophysics program included wireline logging and collection of high-resolution, nondestructive measurements on whole cores using the Geotek multisensor core logger (gamma density, transverse compressional wave velocity, electrical resistivity, and magnetic susceptibility). A graphical summary of the downhole measurements made during Expedition 313 is given in Figure F15.

Onshore, the petrophysics program involved measuring natural gamma radiation (NGR) and thermal conductivity (TC) on whole cores, split-core digital line scan imaging, and split-core color reflectance. Lower resolution measurements on discrete samples for P-wave velocities and moisture and density were also performed. Some petrophysical and downhole measurements display variation primarily with depositional sedimentary changes, whereas some are controlled by other factors such as the degree of postdepositional cementation or diagenesis and the type of interstitial water.

For all three holes, preliminary results from the petrophysics and logging program are of a methodological character. These data enable correlation of sedimentologic observations, downhole logs, and seismic reflections, thereby providing the backbone for sequence stratigraphic interpretations. The continuity and quality of the downhole through-pipe gamma log data in all three holes is especially valuable, particularly in intervals where no core was obtained. There is excellent correlation between all open-hole logs with the through-pipe gamma ray logs, both in depth and in correlation of distinctive features. Repeat sections of open-hole spectral gamma were collected in Hole M0027 to provide a comparison and calibration to the through-pipe spectral gamma ray data. This led to a very high level of confidence in using the continuous through-pipe gamma as a basis for stratigraphic correlations between holes. Furthermore, natural gamma ray (NGR) measurements on the unsplit cores will enable an even higher degree of core-log-seismic correlation by providing precise core depth positioning with respect to log depth.

Where NGR measurements were not distinctive, other logs, notably magnetic susceptibility, were found to be useful for core-log correlation. Magnetic susceptibility can be matched to line scan images and can indicate mineral variation at a high resolution (e.g., darker layers showing increased magnetic susceptibility related to minerals associated with pyritization). Also, the magnetic susceptibility in conjunction with the K/Th ratio is an excellent detector of glauconite, a mineral thought to indicate dysoxic, sediment-starved conditions.

Downhole acoustic images acquired during Expedition 313 are a useful tool for identifying important sequence stratigraphic surfaces in the intervals where they were collected (Fig. F15). The highest resolution images were obtained from a 30 m section from Hole M0028A. Images from digital line scanning proved very helpful in detecting sedimentological changes, enabling reliable comparison of features observed in the core with the in situ images of the borehole wall.

Information from the sonic velocity logs and gamma-derived density logs provide a basis for identifying impedance contrasts. These data sets give insight into both lithology and potential seismic reflectors and will aid interpretation of the sequences. Two high-quality sections of sonic velocity data were acquired in Hole M0027A (Fig. F15) and one section in Hole M0029A.

Changes in density and porosity very closely match lithology changes (grain size and mineral content). Overall, the porosity of specific grain size ranges (clay, silt, and sand) decrease exponentially with burial depth. Anomalously low porosity in the upper ~200 m of Holes M0027A and M0029A is tentatively related to the coring technique. In general, large-scale coarsening-upward sequences and abrupt decreases in grain size can be recognized by changes in density and porosity (as well as gamma ray).

Interesting links between the geochemical results and petrophysical data were identified, as highlighted by the conductivity logs and resistivity data on the core. Electrical conductivity trends are mainly controlled by the chlorinity of the pore fluids in Holes M0027A, M0028A, and the upper part of Hole M0029A (Figs. F8, F9, F10). In contrast, although both conductivity and chlorinity values are high in Hole M0029A, especially chlorinity below 320 mbsf, lithology controls the numerous electrical conductivity variations in Hole M0029A. In all holes, except in the lower part of Hole M0029A, the highest chlorinities are generally correlated with a low gamma ray signal indicative of coarse-grained formations.

In Hole M0029A, particularly distinct conductivity, sonic, and magnetic susceptibility values clearly identified boundaries between lithologic units. This was especially valuable in the several instances where boundaries occurred in a coring gap. This also occurred in some intervals in Holes M0027A and M0028A.

Interstitial water chemistry

A total of 222 interstitial water samples were taken from the cores using Rhizon samplers and by squeezing, and 179 sediment samples for analysis of headspace gas were collected offshore. We measured pH, alkalinity, salinity, and ammonium in the interstitial water onboard. Chlorinity in these waters was measured by electrochemical titration at the University of Hawaii in June and July 2009. With the assistance of the technical staff at the University of Bremen, we measured an additional 17 chemical species in the interstitial water, including chloride, bromide, and sulfate by ion chromatography and Li, Na, K, Mg, Ca, Sr, Ba, Mn, Fe, B, Al, Si, P, and S by inductively coupled plasma–atomic emission spectrometry (ICP-AES). We also analyzed the sediment for total C and S, total organic carbon, and mineralogy by X-ray diffraction.

The upper several hundred meters of sediment beneath the continental shelf off New Jersey is dominated by freshwater interlayered with saltwater of nearly seawater chlorinity in all three holes. These layers may be correlatable from hole to hole. At greater depths the chloride concentration increases to seawater value in Hole M0027A and to higher concentrations in the other two holes. In Hole M0029A, brine is encountered toward the bottom of the hole. Although fresh to saltwater chemistry dominates, the waters have reacted with sediment and these reactions are in part microbially mediated.

Hole M0027A

The salient characteristics of interstitial water from Hole M0027A are how fresh it is and how frequently and abruptly it alternates from fresh to salty downhole. The upper part of the hole (to 419 mbsf) contains five distinct layers of relatively fresh water, some thin and some thick, which alternate with salty layers and are separated from them by sharp gradients in chloride concentration (Fig. F8). The mean chloride concentration in the hole is 273 mM, indicating that overall nearly half of the pore water in Hole M0027A is fresh. A central task in interpreting the pore water chemistry is to explain how these sharp salinity gradients are maintained in the face of chemical diffusion, which tends to soften such gradients and then, over time, erase them altogether. Both the fresh and saltwater layers have distinct chemistries that may enable us to correlate them from hole to hole. Below the deepest fresh layer chloride increases linearly to its concentration in seawater while, relative to chloride, Na, K, and B decrease and Li, Mg, and Sr increase. Br and Ca show no large net change. Sulfate is reduced to low concentrations by microbial oxidation of organic matter, causing alkalinity, ammonium, and Ba to increase and holding Ca steady by precipitation of CaCO₃. The largest anomaly in pore water composition, of unknown origin, occurs at 394 mbsf as a large peak in Mn, Fe, Si, B, Li, K, Ca, and Sr, corresponding with the lowest Na/Cl ratio measured in the hole.

Hole M0028A

Like Hole M0027A, the pore water chemistry of Hole M0028A is dominated by the alternation of relatively fresh and salty layers (Fig. F9). Although we did not core the upper part of the hole, the shallowest sample recovered, from 225 mbsf, has only 14% of the chloride content of seawater. Below that depth there are at least two additional layers of fresher water alternating with saltier layers. Neglecting the brine near the bottom of the hole at 627–664 mbsf, the mean chloride concentration is 309 mM, indicating that overall ~40% of the pore water in the hole is fresh. Although the mean water is not as fresh as that in Hole M0027A, relatively fresh water extends to a greater depth, 536 mbsf. Below that depth, salinity increases until the water becomes a brine, with 608 mM chloride. Microbes appear to have been much more successful in reducing sulfate within the fresher water layers than within the saltier ones, in the process of oxidizing organic matter in the sediment, as the fresher layers have <1 mM sulfate and very low sulfate/chloride ratios, whereas the intervening saltier layers have values typical of seawater. As in Hole M0027A, the major fresh and saltwater layers appear to have distinct chemistries. These chemistries and the unique shapes of some of the depth profiles, particularly for chloride and sulfate, are proving useful for correlating the various fresher and saltier layers between Holes M0027A and M0028A.

Hole M0029A

Like Holes M0027A and M0028A, Hole M0029A displays alternating layers of fresher and saltier water within the upper 300 m of the hole, including the lowest chloride concentration we measured in any of the holes, 19 mM (Fig. F10). Unlike the earlier holes, Hole M0029A contains an increasingly saline brine from 345 mbsf to the deepest sample at 748 mbsf, which reaches 995 mM, nearly twice the concentration of seawater. This brine appears to be similar to that encountered at several holes drilled on the upper to mid-slope during Leg 150. For the depth interval above the brine, the mean chloride concentration is 314 mM, nearly identical to the 309 mM in Hole M0028A, indicating that overall ~40% of the pore water is fresh. The fresher layers again appear to have distinct, perhaps unique, compositions. Relative to the chloride concentration in seawater, the brine is depleted in Na, K, Mg, and sulfate and enriched in Br, Li, B, Ca, Sr, Fe, alkalinity, and ammonium.

Stratigraphic correlation of seismic and sedimentary sequences

A primary objective of Expedition 313 is to tie seismic sequences to cores and logs. Depths of seismic sequence boundaries (identified by onlap, downlap, and erosional truncation of reflectors on a regional seismic grid) were calculated at each site using a velocity-depth function derived from stacking velocities that had been used to process these same profiles. We examined cores and/or logs at these calculated depths (Fig. F16), and in the following section we discuss the extent of agreement between these "predictions" and the actual lithologic or log expression of demonstrably disconformable surfaces. Several additional unconformities not recognized on seismic data were found in the cores (three in Hole M0027A and three in Hole M0029A). Furthermore, three flooding surfaces were also identified in Hole M0029A that had no apparent seismic expression. By contrast, some seismic features could not be attributed to a single surface in the cores, but rather to a set of distinct yet closely spaced lithologic discontinuities that were either sequence boundaries, surfaces of transgression, or maximum flooding surfaces. Minor inconsistencies between seismic and sedimentologic features in cores and on MSCL and downhole logs are probably due to inaccuracies in the time-depth function that would be sampled at downdip locations in subsequent holes.

Hole M0027A

Hole M0027A provided the first test of tying seismic sequence boundaries to cores, logs, and age control. The site was chosen to enable us to sample seismic sequences updip of clinoform rollovers, while at the same location sample older sequences downdip of their clinoform rollovers landward of this hole.

There were insufficient data in nearby wells to guide our estimates of where the base of the Pleistocene section would be at any of the Expedition 313 sites. Slight angular discordance in shallow reflectors in MCS line 529 crossing Hole M0027A suggested the base of the Pleistocene would be at roughly 36 mbsf. The deepest occurrence of Pleistocene nannofossils and the top of a thick interval of fluvial/estuarine sediments barren of microfossils suggests this disconformable Pleistocene/Miocene contact is more likely at 32 mbsf. Studies using higher resolution Geopulse profiles (Sheridan et al., 2000) placed the base of the Pleistocene at roughly 36 mbsf if the time-depth conversion used in this present study is applied.

Discontinuous coring in the upper Miocene succession (32 to roughly 200 mbsf) made it unlikely that we would recover sediments exactly straddling sequence boundaries m1 and m3. Time-depth calculations predicted these surfaces would be at 91 and 108 mbsf, respectively. We detected a downhole decrease in gamma ray values at about 96 m and another at 115 mbsf, and we tied these to seismic events m1 and m3, respectively. The first of these, m1, matches the base of a paleosol, whereas m3 matches the top of a ~55 m interval of fluvial/estuarine sand.

Calculated depths of seismic sequence boundaries m4.5, m5.2, m5.3, m5.32, m5.45, m5.6, and m5.7 match remarkably well (within 1–7 m; i.e., better than ±3%) with lithologic features consistent with depositional sequence boundaries. The calculated depth of seismic sequence boundary m5.47 most closely matches a flooding surface but occurs within 4 m of a facies change interpreted to be a depositional sequence boundary. Seismic sequence boundary m5.7 coincides with a gamma log kick, whereas we link seismic sequence boundary m5.8 to a lithologic sequence boundary that is ~10 m deeper than the calculated depth. The calculated depth of seismic sequence boundary m6 has no definite equivalent in the core, and its precise placement is uncertain, though it is expected to mark a major (>1 m.y.) hiatus. The depth of reflector o.5 (not mapped in the existing seismic grid) was predicted at 540–545 m. There is no immediately recognizable log or lithofacies feature that is reasonably close to this depth. However, there is a 4 m.y. hiatus implied at 540 mbsf by Samples 313-M0027A-191R-CC (late Oligocene, Zone DN1) and 192R-CC (middle Oligocene >28 Ma). Reflector o1 is associated with an impedance contrast (see Fig. F69 in the "Site M0027" chapter) and a minor lithologic break (see Fig. F81 in the "Site M0027" chapter). Three other Oligocene surfaces noted in the cores appear to be sequence boundaries with significant hiatuses but are below seismic resolution.

Hole M0028A

Hole M0028A is located 12 km downdip from Hole M0027A. Seismic sequences that had been sampled along their topsets in Hole M0027A were targets for sampling in Hole M0028A near their clinoform rollovers. Older sequences in Hole M0027A that were sampled at clinoform slopes would be sampled at this site in a toeset setting. In this way, Holes M0027A and M0028A provide the opportunity to test models of depositional sequence development by sampling sequences at several key locations.

The upper 220 m in Hole M0028A was drilled without coring. The first seismic features sampled were two previously unnamed reflectors. The calculated depth of the first, m4.1, corresponds to a maximum flooding surface and several small but closely spaced high density values; the calculated depth of the second, m4.5, matches a significant downhole increase in impedance generated predominantly by a sharp increase in density measured by the MCSL.

Calculations predict seismic sequence boundary m5 is between 258 and 263 mbsf. No obvious feature was observed in log data at this depth or in the corresponding cores, but core recovery was unusually poor in this exact interval. The most likely feature we detected is the base of a quartz sand interval at 269 mbsf. Seismic sequence boundary m5 marks a change in seismic facies: the ~15 m interval above is acoustically transparent, and immediately below m5 there are several closely spaced, subparallel reflectors.

The calculated depth of seismic sequence boundary m5.2 is exactly the depth of the lower of two closely spaced impedance contrasts due to sharp density increases (see Fig. F56 in the "Site M0028" chapter). The lower surface has the lithofacies character of a flooding surface; 2 m above it, the other is interpreted as a depositional sequence boundary.

It is difficult to match surfaces in the core with sequence boundaries m5.3, m5.32, and m5.33 because of incomplete core recovery. The calculated depth of seismic boundary m5.4 is associated with a sharp impedance contrast at 495 mbsf (see Fig. F66 in the "Site M0028" chapter). It lies at the base of a very thick coarsening-upward succession and is consistent with the interpretation of a depositional sequence boundary. The calculated depth of seismic sequence boundary m5.45 likewise matches a lithologic expression of a sequence boundary and a sharp impedance contrast; the core expression of m5.47 appears to be ~5 m shallower than the calculated depth of the seismic event.

The calculated depth for seismic sequence boundary m5.6 closely matches the sedimentary discontinuity placed in a coring gap that corresponds to a significant downhole decrease in gamma ray values and coincident sharp decrease in impedance. The core surface associated with seismic sequence boundary m5.7 (predicted depth of 589–599 mbsf) is uncertain. However, three surfaces are noted in the core between 600 and 610 mbsf, and the reflection may be a response to one or more of these features.

The lithologic expression of seismic sequence boundary m5.8 is placed 9 m deeper than the calculated depth. We find the most reasonable match to be at a major burrowed contact separating silty glauconite sand above from sandy siltstone below that matches a large and sharp downhole decrease in density.

Hole M0029A

Drilling in Hole M0029A completed the program designed to sample individual clinoforms at three different positions: at the clinoform rollover, in topset beds landward of that position, and seaward of that position in toeset beds. Hole M0029A sampled the thickest section of middle Miocene found during Expedition 313, and seismic data showed that two previously unnamed seismic unconformities within the middle Miocene would be sampled.

Seismic correlations to locally high gamma ray log values in Holes M0027A and M0028A were traced to Hole M0029A, and these depths guided offshore attempts to spot core similar facies in the upper 256 m. Several surfaces in estuarine, fluvial, and possible paleosol sediments were recovered, and the calculated depths of seismic reflectors m1, m3, and m4 in Hole M0029A match sharp changes in the gamma ray logs. There was insufficient time during the Onshore Sampling Party (OSP) to narrow the range of any uncertainties in these links.

Continuous coring began at 256 mbsf, and, as expected, we found that the calculated depths of seismic reflectors matched flooding surfaces in cores more often in this hole than in the other two. This was true for reflectors m4.1, m4.3, and m4.4, whereas the calculated depth for m4.2 correlates to a sedimentary unconformity marked also by a major kick in bulk density and a minimum in the gamma ray values. Calculated depths of seismic reflectors m4.5 and m5 occur at very sharp, localized highs in MSCL density values and major impedance contrasts (see Fig. F53 in the "Site M0029" chapter). The predicted depth of seismic reflector m5.2 corresponds fairly well (there is a 6 m, or roughly 1%, discrepancy) to a sequence boundary described in the core and a major impedance contrast.

Because of the downdip location of Hole M0029A, the sequences below m5.2 are significantly thinner than they are in the other two holes. Combined with the subdued lithologic contrasts between sequences, this fact led to uncertain seismic-core correlations because of the overlapping error ranges of depth calculations (a few percent of total depth) and the subtle petrophysical features that generate the seismic reflections. Consequently, at this stage in our analysis there are several possible seismic-core correlations for reflectors m5.3, m5.4, m5.45, and m5.47. The predicted depth of reflector m5.6 occurs in a coring gap that correlates to a significant peak in gamma ray values due to high K concentrations; a major impedance contrast occurs across this gap (see Fig. F63 in the "Site M0029" chapter). We match a surface to reflector m5.7 that is ~10 m shallower than the calculated depth. However, the sedimentological expression of this discontinuity is an obvious and major lithologic break between siltstone with glauconite sand overlying pale brown clayey silt. It is unclear whether Hole M0029A penetrated seismic sequence boundary m5.8. There are several possible placements, including two major coring gaps. The upper one at the peak of gamma ray values, corresponding to glauconitic clays, is higher than the predicted depth. Two other possible horizons match well the predicted depths but lie in coring gaps. Only the refinement of biostratigraphy, core, and seismic correlations can clarify whether this seismic surface was penetrated and if so at what position in the core.

Chronology

Expedition 313 chronology for the uppermost Eocene? to Pleistocene sections is based on integrating biostratigraphy and Sr isotopic ages obtained in Holes M0027A, M0028A, and M0029A (Fig. F14). Below we describe preliminary age assignments of depositional sequences, each named by the seismic sequence boundary tentatively correlated to the base of each depositional unit as described in the previous section of this report.

Hole M0027A

The uppermost sequence above 10 mbsf is upper Pleistocene (<90 ka), one or more underlying Pleistocene sequences occur between 90 and 250 ka, and there is a thin lower Pleistocene sequence (~1 m.y.; 26–32 mbsf). No ages are available for sequences m1, m3, and m4 at this location. Reflector m4.1 is interpreted as a maximum flooding surface that cannot be resolved on profiles from three older surfaces (m4.2, m4.3, and m4.4, dated in Hole M0029A) and must represent a significant hiatus that is not discernible with the available age control. A series of relatively thin (<25 m) sequences (m5.2, an unnamed sequence, 5.3, 5.32, 5.4, and yet another unnamed sequence) span the lower/middle Miocene boundary (16.2 Ma). Preliminary ages for these sequences appear to be younger than in Holes M0028A and M0029A, which we attribute to depressed last occurrences of biostratigraphic markers in this updip sandy hole. Sequences m5.4, an unnamed sequence (295 mbsf), m5.45, and m5.47 have basal age estimates ranging from ~17.3 to 18.4 Ma but have fairly large errors because of limited data. Future work should improve the age estimates. A 37 m thick m5.45 sequence and a thin m5.47 sequence are estimated as ~18.0 and ~18.3 Ma, respectively, and are reasonably well constrained with a ±0.5 m.y. age resolution. Thin sequences m5.6 and m5.7 have no current calcareous nannofossil–derived age constraint (barren/non–age diagnostic nannofossils) except superposition, although they contain shells and further analyses are expected to produce ages based on dinocysts and Sr isotopes. A thick (128.1 m) m5.8 sequence has a lower boundary dated as 21.1 Ma. A thin lowermost Miocene sequence (m6) is dated only by assignment to mid–calcareous nannofossil Zone NN2 (~21–21.5 Ma).

The Oligocene succession is substantially thicker (>129 m) than has previously been found onshore, and there are five Oligocene sequences and part of an uppermost Eocene(?) to lowermost Oligocene sequence based on lithofacies. The uppermost Oligocene sequence is poorly dated at present. The o1 sequence is well dated by all fossil groups and Sr isotopes as mid-Oligocene (28.5–29.0 Ma) and may link well to astronomical predictions of insolation. Two underlying Oligocene sequences are poorly dated. The lowermost Oligocene sequence is assigned to lower Oligocene calcareous nannofossil Zone NP22 (23.46–32.8 Ma), whereas the base of the hole is in lower Zone NP21 (>32.8 Ma) based on the common occurrence of Ericsonia formosa, suggesting that Chron C13n and some of upper Chron C13r (the Eocene/Oligocene boundary) is represented by a hiatus across a sequence boundary.

Hole M0028A

Chronology for the middle to lower Miocene section in Hole M0028A is based on integrating biostratigraphy and Sr isotopic ages. The upper Miocene to Pleistocene was not cored. Sequence m4.5 is moderately well constrained to ~13–15 Ma by Sr isotopes (13.8 Ma ± 1.17 m.y.), calcareous nannofossil Zones ?NN6 and NN5 (12.6–15.6 Ma), planktonic foraminifer Zone N10/M7 (12.7–14.8 Ma), and foraminifer datum levels. Sequence m5 is constrained by Sr isotopes (14.0 Ma ± 1.17 m.y.) and dynocyst Zones DN5 (13.2–15.2 Ma) and NN5 (13.6–15.6 Ma) to yield a preliminary age of ~14–15.2 Ma. Sequence m5.2 is constrained by Sr isotopes (~16 Ma ± 0.8 m.y.) and Zones DN4 (15.2–16.8 Ma), NN5, and NN4 (15.2–16.8 Ma). The NN4/NN5 zonal boundary provides a firm age constraint (15.6 Ma) within the sequence. Sequence m5.3 is constrained in age by the combination of Sr isotopes (16.5 Ma ± 0.6 m.y.), Zone NN4 (15.6–18.2 Ma), and Zone DN2–DN3 (16.8–22.0 Ma) to yield an age estimate of 16–17 Ma and a possible basal age of ~16.6 Ma. Sequence m5.4 is assigned to Zones NN4, N6/M3 (17.3–18.8 Ma) or older, and DN3 (16.8–19.2 Ma) and Sr isotope ages of ~17 Ma ± 0.8 m.y., with a basal age of 17.3–18.3 Ma. Sequence m5.45 is assigned to Zones NN4 (<18.42 Ma) and DN2–DN4 and Sr isotope ages of 18.6 Ma ± 0.6 m.y. The base of the sequence is clearly younger than 18.4 Ma but could be as young as ~17 Ma with a best estimate of ~18.2 Ma. Sequence m5.47 has little age constraint other than superposition. Sequence m5.6 is only constrained by the presence of Globorotalia praescitula (<18.5 Ma) and the presence of Zone NN4 (<18.42 Ma) at the top. Sequence m5.7 has no constraints other than superposition (18.6 to ~20.5 Ma). Sequence m5.8 is assigned to mid-Zone NN2 (~20–21.5 Ma) and Zone DN2 (19.2–22.0 Ma). Provisional magnetostratigraphy identifies a thick (>18 m) normal magnetozone in the lower part of this sequence that may be Chron C6AN.2n, suggesting a basal age of ~21.5 Ma. Sequence m6 was just penetrated at the bottom of the hole, with an Sr isotope age of 20.7 Ma ± 0.6 m.y.

Sedimentation rates before decompaction are difficult to estimate with certainty in Hole M0028A based on these preliminary age constraints. Typical sedimentation rates are 40 m/m.y. Sedimentation rates during deposition of the targeted m5.4 sequence in a position near its greatest thickness were ~100–145 m/m.y.

Hole M0029A

The Pleistocene to upper middle Miocene section was spot-cored in Hole M0029A, and the hole bottomed in the lower Miocene. In general, age control at this site is better than at the updip locations. Calcareous nannofossils suggest that the uppermost sequence (above 14 mbsf) is upper Pleistocene (Zone NN21; <250 ka). There are no other age constraints on ?Pleistocene sequences and seismic sequence boundaries m1, m3, and m4 in Hole M0029A. Seismic sequence m4.2 is poorly dated. It is assigned to calcareous nannofossil Zones NN6 (12.6–13.2 Ma) and NN6–NN7, dinocyst Zones DN6–DN8 (younger than ~13.2 Ma), and planktonic foraminifer Zone N14 or older (>11.4 Ma), with scattered Sr isotope ages yielding an average of 12.8 Ma ± 0.8 m.y. The underlying tentative sequence termed m4.5 is assigned to dinocyst Zones DN6–DN8 (<13.2 Ma) and DN5 (13.2–15.1 Ma), nannofossil Zones NN6 (11.8–13.6 Ma) and NN5 (13.6–15.6 Ma), a foraminifer datum level of ~13.8 Ma, and Sr isotope ages of 13.8 Ma ± 0.7 m.y., yielding an age of ~13.5–14.6 Ma for the sequence. Sequence m5 is assigned to Zones NN5 (13.6–15.6 Ma) and DN5 (13.2–15.1 Ma), together with a foraminifer datum level of 14.8 Ma and Sr isotope ages of 14.2 Ma ± 0.8 m.y.; this yields an age assignment of 14.6–15.4 Ma, with a possible basal age of 15.0–15.4 Ma. The age of sequence m5.2 is well constrained as 15.6–16.1/16.2 Ma by biostratigraphy (Zones NN4 and NN5, DN4–DN5, and N8/M5). The finding of the Zone NN4/NN5 boundary (15.6 Ma) in the middle of this sequence both in this hole and in Hole M0028A contrasts with its placement within sequence m5.3 in Hole M0027A, suggesting a depressed last occurrence in the updip site. The age of seismic sequence m5.3 is constrained to Zones NN4 (15.6–18.2 Ma) and upper Zones DN3–DN4 and a Sr isotope age of 16.9 Ma ± 0.6 m.y.; its basal age is estimated as ~16.2–16.9 Ma. The ages of sequences m5.4, m5.45, and m5.47 cannot be precisely estimated because they are assigned to the long Zone NN4 (<18.2 Ma) and dinocyst Zone DN3 or older (>16.7 Ma). There are no current Sr isotope age estimates, but subsequent work should constrain the ages of these sequences. Sequence m5.6 has reasonable age constraints in Hole M0029A versus the updip sites. Here, for the first time, Zone NN3 (18.3–19.6 Ma) was identified, remarkably consistent with an Sr isotope age of 18.3 Ma ± 0.6 m.y. Sequence m5.7 is assigned to the middle part of calcareous nannofossil Zone NN2 (19.6–21.5 Ma and Zone DN2 (19 to ~20.2 Ma), with a best estimate of 19.6–20.2 Ma. Sequence m5.8 is assigned to the mid-upper part of Zone NN2 (older than 19.6 and younger than 21.5 Ma) and Zone DN2 (19– 22.2 Ma). It has a Sr isotope age estimate of 21.0 Ma ± 0.6 m.y., suggesting that the sequence is 20.4–21.6 Ma. We may have drilled through sequence boundary m5.8 and into the underlying sequence m6. Dinocysts suggest that the base of Hole M0029A was in Zone DN1 (>22.2 Ma). However, nannofossils suggest that the base of Hole M0029A was younger than 21.5 Ma, remarkably consistent with an Sr isotope age of 21.3 Ma ± 0.6 m.y. These ages are more consistent with assignment to sequence m5.8 based on regional correlations.

Sedimentation rates before decompaction are difficult to estimate in Hole M0029A from the preliminary age constraints. Typical sedimentation rates are ~80 m/m.y. Sedimentation rates during deposition of the targeted m5.2 sequence in a position near its greatest thickness were ~176 m/m.y.

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