Craig S. Fulthorpe

Tracking the last sea-level cycle: seafloor morphology and shallow stratigraphy of the latest Quaternary New Jersey middle continental shelf

Seafloor geomorphology and surficial stratigraphy of the New Jersey middle continental shelf provide a detailed record of sea-level change during the last advance and retreat of the Laurentide ice sheet (∽120 kyr B.P. to Present). A NW–SE-oriented corridor on the middle shelf between water depths of ∼40 m (the mid-shelf “paleo-shore”) and ∼100 m (the Franklin “paleo-shore”) encompasses ∼500 line-km of 2D Huntec boomer profiles (500–3500 Hz), an embedded 4.6 km2 3D volume, and a 490 km2 swath bathymetry map. We use these data to develop a relative stratigraphy. Core samples from published studies also provide some chronological and sedimentological constraints on the upper <5 m of the stratigraphic succession. The following stratigraphic units and surfaces occur (from bottom to top): (1) “R”, a high-amplitude reflection that separates sediment >∼46.5 kyr old (by AMS 14C dating) from overlying sediment wedges; (2) the outer shelf wedge, a marine unit up to ∼50 m thick that onlaps “R”; (3) “Channels”, a reflection sub-parallel to the seafloor that incises “R”, and appears as a dendritic system of channels in map view; (4) “Channels” fill, the upper portion of which is sampled and known to represent deepening-upward marine sediments ∼12.3 kyr in age; (5) the “T” horizon, a seismically discontinuous surface that caps “Channels” fill; (6) oblique ridge deposits, coarse-grained shelly units comprised of km-scale, shallow shelf bedforms; and (7) ribbon-floored swales, bathymetric depressions parallel to modern shelf currents that truncate the oblique ridges and cut into surficial deposits. We interpret this succession of features in light of a global eustatic sea-level curve and the consequent migration of the coastline across the middle shelf during the last ∼120 kyr. The morphology of the New Jersey middle shelf shows a discrete sequence of stratigraphic elements, and reflects the pulsed episodicity of the last sea-level cycle. “R” is a complicated marine/non-marine erosional surface formed during the last regression, while the outer shelf wedge represents a shelf wedge emplaced during a minor glacial retreat before maximum Wisconsin lowstand (i.e., marine oxygen isotope stage 3.1). “Channels” is a widespread fluvial subarial erosion surface formed at the late Wisconsin glacial maximum ∼22 kyr B.P. The shoreline migrated back across the mid-shelf corridor non-uniformly during the period represented by “Channels” fill. Oblique ridges are relict features on the New Jersey middle shelf, while the ribbon-floored swales represent modern shelf erosion. There is no systematic relationship between modern seafloor morphology and the very shallowly buried stratigraphic succession.

Marshall Paraconformity: a mid-Oligocene record of inception of the Antarctic circumpolar current and coeval glacio-eustatic lowstand?

Abstract The sedimentary fill of the Canterbury Basin, New Zealand, is the product of a long-term (80 Ma), tectonically controlled relative sea-level cycle with a megasequence geometry analogous to the sequence stratigraphic model of Vail (Am. Assoc. Petrol. Geol. Stud. Geol No. 27, 1, 1–10, 1987). The condensed section of the megasequence, resolvable in detail in outcrop and on seismic profiles, comprises a basin-wide pelagic to hemipelagic limestone interval. A regional mid-Oligocene unconformity, the Marshall Paraconformity, lies within the limestone interval onshore and correlates with hiatuses in at least two, and possibly three, offshore exploration wells and with a temporary lithological change from limestone to quartz sand at a fourth. Strontium isotopic age estimates confirm that a 2–4 Ma hiatus is associated with onshore outcrops of the Marshall Paraconformity (between ∼32 and 29 Ma), which correlates with the opening of the Pacific sector of the Southern Ocean and the postulated mid-Oligocene sea-level fall of Haq et al. (Science235, 1156–1167, 1987; Spec. Publ. Soc. Econ. Paleonotol. Mineral. No. 42, 71–108, 1988). Lowering of base level, coupled with cooling and enhancement of current activity, may have caused the temporary cessation of limestone deposition and a regional hiatus. This hypothesis reconciles the apparently contradictory palaeogeographical evidence for a regional highstand. The Marshall Paraconformity may exemplify the signature by which similar glacio-eustatic events can be recognized in offshore platform facies.

Evolution of Pliocene to Recent abyssal sediment waves on Bounty Channel levees, New Zealand

Abstract Levees bordering Bounty Channel 900 km east of New Zealand accommodate a 400 m-thick sequence (maximum) of sediment waves that have formed since Pliocene times. These bedforms, with amplitudes of 2–17 m and wavelengths of 0.6–6 km occur in 4100–4900 m of water and were formed by turbidity currents, as indicated by their restriction to levee backslopes, the frequent occurrence of turbidites in cores and the preferential but not exclusive development of waves on the left-bank levee in accord with the Southern Hemisphere coriolis deflection. The wave field was instigated in the Late Pliocene when glacially lowered sea level allowed rivers draining the Southern Alps of South Island to discharge directly into Bounty Channel and its attendant canyons. The field grew vertically through the coalescence of small waves into larger bedforms that continually migrated across and up levee backslopes at an average rate of 5.6m/100 yrs. Wave growth decreased into the Late Pleistocene probably in response to progressive containment of turbidity currents as the relief of Bounty Channel increased to 200 m or more. The glacial periods of wave growth were interrupted by interglacial interludes of quiescence when the field was draped mainly by pelagic calcareous ooze.

Sequence concepts at seismic and outcrop scale: the distinction between physical and conceptual stratigraphic surfaces

Abstract The sequence stratigraphic terms maximum flooding surface and downlap surface, as currently applied, are ambiguous. Examples of these intra-sequence surfaces are summarised from high frequency mid-Pleistocene sequences and from a Cretaceous–Recent seismic megasequence, both from New Zealand. At any one locality, a mid-Pleistocene sequence contains up to four stratal discontinuities, in ascending order: the sequence boundary, ravinement surface, local flooding surface and downlap surface. These physical surfaces, which occur in outcrop, are regionally diachronous and should be differentiated from theoretical isochronous horizons such as the time of maximum flooding (horizon corresponding to maximum shoreline transgression) and the time of peak eustatic or local relative sea-level (horizons corresponding to the highpoint of the eustatic and relative sea-level cycles, respectively). In seismic studies, the boundary between the transgressive and highstand systems tracts is usually located at the downlap surface. On the basis of a major thermo-tectonic sea-level cycle in the Canterbury Basin, it is shown that the downlap surface is not a single regional surface, and that the change in slope associated with toes of successive prograding clinoforms rises in stratigraphic height basinwards. The downlap surface therefore does not usually coincide with the maximum flooding horizon. In Plio–Pleistocene cyclothems, a discrete unit — the mid-cycle shellbed — straddles the contact between the transgressive and highstand systems tracts. This unit might be classified within its own systems tract (the condensed section systems tract; CSST). Alternatively, the position of the boundary between the transgressive and highstand systems tracts can remain unspecified or unknowable.

Three-dimensional architecture of shelf-building sediment drifts in the offshore Canterbury Basin, New Zealand

Abstract A grid of high-resolution, multichannel seismic profiles from the offshore Canterbury Basin, New Zealand, reveals that at least 11 large (up to 1000 m thick, >50 km long, along-strike, and 20 km wide, down-dip) elongate sediment drifts developed within the lower Miocene to Recent shelf-slope sediment prism. The drifts overlie a condensed section of late Eocene to late Oligocene limestone and cover an area of ∼5000 km2. The drifts were deposited in water depths of 300–750 m, probably by a northward-flowing contour current, and aggraded to shelf depths. The drifts exhibit mounded morphologies with channel-like moats along their landward flanks. Erosion of the landward flanks creates prominent unconformities; these unconformities are diachronous and, therefore, not sequence boundaries. The internal architecture of the drifts defines two end members of elongate drift, which we describe as simple and complex. Early (early to middle Miocene) simple drifts are small ( 50 km long and up to 20 km wide) and occur in the northeastern part of the survey area. These late, simple elongate drifts are subdivided into three parts (base, core, and crest) based on seismic facies. These facies form in response to progressive confinement of current flow within the moat. Complex drifts may be multi-crested or multistage. Multi-crested drifts form in response to rapid lateral shifts in position of the moat, perhaps associated with relative sea-level change, modulated by paleoslope inclination and orientation. Such drifts, together with the observation that several drifts were often active simultaneously, indicate that flow patterns were complex and involved multiple pathways. Multistage drifts comprise superimposed subdrifts whose retrogradational and progradational stacking patterns indicate fluctuations in the rate of sediment supply. Complex drift formation may require intermediate shelf relief: high enough to sustain drift development during changes in sea level and rate of sediment supply, but low enough so that such changes are still able to influence moat position. In addition, coeval climatic cooling may amplify changes in sea level, current intensity and sediment supply, thereby contributing to complex drift formation.

Continental-shelf progradation by sediment-drift accretion

Multi-channel seismic profiles from the Canterbury Basin on the eastern margin of the South Island of New Zealand reveal the importance of current activity in shaping a Neogene shelf sediment prism. The shelf prism prograded across a broad, near-horizontal platform in water depths of 1,000 to 1,750 m. The platform was formed above a condensed section of late Eocene to late Oligocene limestones which overlie Cretaceous to Paleogene rift-fill and transgressive sediments. The Neogene sediment prism contains sediment drifts which are as much as 25 km long and 15 km wide and extend up to 1,600 m (uncompacted) vertically. Individual drifts migrated westward and can be traced between dip profiles, revealing that the long axes of most are subparallel to the present coastline and shelf-edge. Channel-like features at the landward edges of the drifts correspond to residual space left between the landward-prograding off-shelf sediment drift and the adjacent shelf foreslope. Erosion or slow deposition characterized the foreslope. Progradation of the shelf was by the accretion of successive sediment drifts. Before ca. 11.5 Ma (= Pink Horizon), the shelf-edge-parallel drifts were distributed across the central part of the basin, whereas subsequently they were concentrated to the northeast. The seismic architecture of the Neogene sediment prism results from the interplay of an abundant western sediment source and an offshore boundary current system. Present-day ocean circulation involves northward flow along the east coast of the South Island. The basin may have been subjected to a middle Miocene to late Pliocene phase of intensified flow, caused by local topographic enhancement and/or global paleoceanographic events. Current activity has played a crucial role in the sedimentary evolution of the Canterbury Basin Neogene shelf prism.

Geological controls on seismic sequence resolution

A seismic sequence analysis of the Canterbury basin, eastern South Island of New Zealand, has illustrated the roles of subsidence, sediment supply, and current activity as controls on sequence resolution and architecture during basin evolution. The rates of sediment supply and subsidence determine the background depositional regime (transgressive or regressive),and effectively determine the frequency response of the continental margin sedimentary section to input signals with a broad range of frequencies, including eustasy. A regressive (progradational) depositional regime and minimal current erosion favor the preservation of high-frequency sequences, particularly at fourth-order level. Under less favorable conditions, the record of sequences is incomplete or ambiguous. Such frequency response characteristics must be considered when inverting sequence records to derive the frequency of the input cyclicity, and when making global comparisons of regional sequence stratigraphic studies. Clastic basins with simple subsidence histories and uniform or increasing rates of sediment supply develop from a transgressive phase, characterized by ramplike major sequence boundaries, to a mature, progradational shelf phase with clinoform sequences and optimum sequence resolution. The mature phase constitutes the preferred setting for sequence stratigraphic analyses.

Controls on sequence stratigraphy of a middle Miocene–Holocene, current-swept, passive margin: Offshore Canterbury Basin, New Zealand

The offshore Canterbury Basin exemplifies sequence development on a prograding passive margin influenced strongly by submarine currents. Nineteen middle Miocene–Holocene, regional, sequence-bounding unconformities are interpreted by using high-resolution multichannel seismic data. The sequences can be grouped into larger units based on seismic geometry and facies that reflect different combinations of controls on sequence architecture. Correlation with oxygen isotope records suggests that eustasy controls the timing of sequence boundaries. The number of sequences is similar to that of coeval cycles on a temperature-adjusted, Miocene and early Pliocene δ 18 O record. The late Pliocene–Pleistocene sequence record is of lower frequency than the isotopic record of this period, either because of the limitations of seismic resolution or because of removal of sequence boundaries by erosion associated with high-amplitude eustasy. However, the last two sequence boundaries correlate well with the last two 100 k.y. isotopic cycles. In contrast, sequence architecture is influenced strongly by local processes. Along-strike currents create large, elongate sediment drifts that control sequence thickness; current erosion in drift moats forms diachronous unconformities. Drifts focus deposition on the slope, reducing the rate of basinward advance of the shelf edge, but increasing that of the slope toe, thereby reducing slope inclination. Replacement of along-strike processes by downslope processes increases rates of shelf-edge progradation, and the slope steepens as the reduced accommodation space over the expanded slope is filled. Clinoform geometries along strike from active drifts suggest that currents might influence clinoform formation even in locations lacking seismic evidence of current reworking.

Buried fluvial channels off New Jersey: Did sea-level lowstands expose the entire shelf during the Miocene?

High-resolution multichannel seismic profiles from the New Jersey continental margin reveal that some middle to late Miocene sea-level falls exposed the entire continental shelf. At several sequence boundaries, fluvial channels occur landward of the clinoform breakpoints that mark paleo-shelf edges. The seismically observed progradation therefore resulted from sediment delivery to the shelf edge by rivers during lowstands. Suspended sediment crossing the relatively shallow water (30–40 m) shelf also fostered progradation during highstands. River systems reaching the outermost shelf were small and closely spaced; they approximated a line source of sediment. This finding helps to explain the observed linearity of Miocene shelf edges. Although these systems discharged near clinoform tops, they did not form canyons incising the clinoforms; such canyons were rare in this active depositional setting, in contrast to their prevalence on the modern continental slope.

Shallowly buried, enigmatic seismic stratigraphy on the New Jersey outer shelf: Evidence for latest Pleistocene catastrophic erosion?

Chirp seismic profiles reveal a prominent seismic facies boundary 0–20 m below the seafloor within an 8.6 × 10.2 km area of the New Jersey outer shelf. This irregular boundary separates seismically transparent facies from underlying stratified facies. The irregularities form two populations of incisions trending northeast and east-northeast. Stratified blocks within the transparent facies, occasionally within presumed incisions, together with lithologic evidence, indicate that some of the transparent facies was formed by disruption of the stratified facies. Preservation of steep (50°–90°) incision flanks implies that deposition of the transparent facies closely followed disruption. Catastrophic erosion and redeposition following the multiple breaching of glacial lake dams to the north ca. 19–12 ka constitute the likeliest mechanisms both for producing facies-boundary incisions and emplacing the transparent facies, implying that local forcing can generate erosional unconformities on periglacial shelves in the absence of base-level change. This facies boundary adds complexity to the already complex, shallowly buried stratigraphic record of the last glacio-eustatic cycle on the New Jersey continental shelf, long considered typical of periglacial shelves worldwide.