James A. Austin

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.

Deep penetrating MCS imaging of the rift-to-drift transition, offshore Douala and North Gabon basins, West Africa

We describe a regional grid of West African PROBE Study (WAPS) deep penetrating multi-channel seismic reflection (MCS) and potential field data crossing the transition from rifted continental crust (RCC) to normal oceanic crust (OC) in the offshore Douala and North Gabon Basins. In profiles oriented sub-parallel to oceanic fracture zones RCC is shown to terminate seaward into a very different form of crust interpreted to be composed of highly attenuated blocks of RCC, wedges of seaward-dipping reflectors representing intermixed mafic volcanics and sediments, mafic lower crust, and possibly exhumed mantle rocks, all of which form a complex faulted terrain. This heterogenous type of crust is referred to here as ‘proto-oceanic crust’ (POC). POC grades seaward into normal to thick OC. In the Douala Basin, sedimentary units deposited in an Early Cretaceous continental rift now form marginal basin highs overlain by < 1 km younger of sediment beneath the shelf. RCC here has undergone little attenuation, as more highly attenuated RCC is believed to occur below the conjugate Sergipe-Alagoas Basin in eastern Brazil. In the North Gabon Basin, RCC has been markedly thinned and resulting rift structures are deeply buried by post-rift deposits. Reflective lower crust within RCC forms discontinuous bands, where normal dip-slip faults sole into it and transform related faults crossthrough it. Interpreted Moho reflections atthe base of RCC form a discontinuous horizon that is sometimes offset at crustal deformation zones; elsewhere it transects the reflective grain of crustal deformation zones. Interpreted WAPS MCS data show that the margin is composed of rift-units separated at ca 40 km intervals by NE trending fracture zones and transfer fault zones of variable length, causing the margin to step progressively southwestward by alternating between normal and transformfaulted segments. Correlation of rift structures between these West African basins and the conjugate Sergipe-Alagoas Basin show that the two margins formed a connected rift-branch that underwent NE-SW oriented extension, ca 35° oblique to the rift axis. This orientation favoured occupation of NE-SW trending preexisting shear fabrics situated in the Late Proterozoic Pan-African/Braziliano mobile belt. At the RCC-POC boundary, wedges of seaward-dipping reflectors abut against RCC and overlie non-reflective lower crust and reflection Moho. They correlate with pronounced negative magnetic anomalies (−200 to −600 nT), probably caused by the juxtaposition of weakly magnetised RCC and more magnetic, mafic volcanic rocks of the POC. Steeply-dipping faults are interpreted to occur at transform-faulted segments, where they separate blocks of RCC from POC, or in some places OC. These NE trending transfer fault zones propagate into oceanic fracture zones (FZs) that compartmentalise and isolate both continental and oceanic rift-units. For example, the Ascension FZ is shown to intersect the African continent south of Bata, Equatorial Guinea, where it forms a 200 km long transform-faulted boundary between RCC and POC. The thickness of POC from the top of the seaward-dipping reflectors to reflection Moho ranges between 2–5 s two-way travel time (twtt). This POC probably formed by volcanic outpouring of partial melt at the time of continental rupture, and then during initial seafloor spreading. Phases of magmatic and amagmatic extension formed within rift-unit spreading cells, owing to variable widths of POC (20–200 km wide) trending parallel to flow lines before evolving seaward into normal OC. Aptial salt extends over RCC and POC, and in some places, salt has been mobilised farther seaward (<10 km) as horizontal sills emanating from salt diapirs. The northernmost known occurrence of this Aptial salt lies 100 km south of Douala, Cameroon. There is no evidence in the MCS data that salt was deposited behind structural highs rimming the margin of the salt basin. Instead, salt overlies sub-horizontal reflectors (0.2–1 s twtt thick) believed to represent sedimentary rocks that drape POC and OC of presumably Aptial age. This interpretation implies that salt in the part of West Africa was deposited in nearshore to deeper basin margin environments without restriction from central basin waters.

Progradation along a deeply submerged Oligocene–Miocene heterozoan carbonate shelf: How sensitive are clinoforms to sea level variations?

We combine two- and three-dimensional seismic stratigraphic interpretation with paleobathymetric analysis from benthic foraminifera to understand the genetic significance of prominent seismic discontinuity surfaces typically mapped as sequence boundaries and flooding surfaces in the late Paleogene–early Neogene northern Carnarvon Basin.The progradational succession, dominated by heterozoan carbonate sediments, is divided into 5 northwest-prograding clinoformal sequences and 19 subsequences. Clinoform fronts progress from smooth to highly dissected, with intense gullying apparent only after the middle Miocene optimum. Once initiated, gullies become the focus for sediment distribution across the front. Bottomsets remain relatively sediment starved without the development of aprons on the lower slope and basin. Small-scale variability suggests heterogeneous sediment dispersal through the slope conduits. Along-strike sediment transport superimposed on progradation changes from southwest directed in the late Oligocene to northeast directed in the late middle Miocene, suggesting a major reorganization of circulation in the southeastern Indian Ocean.Prominent seismic discontinuity surfaces represent both intervals of shallow paleowater depth and flooding of the shelf. Partial exposure of the shelf indicated by karst morphology is coeval, with middle to outer neritic paleowater depths on the outer shelf. Instead of building to sea level, progradation occurs with shelf paleowater depths at the clinoform rollover greater than 100 m. Therefore, in the northern Carnarvon Basin, onlap onto the clinoform front is not coastal, and the sensitivity of the clinoforms to sea level changes is muted.

High-resolution 3D seismic reflection and coring techniques applied to late Quaternary deposits on the New Jersey shelf

Studies of variable, small-scale features typical of Quaternary glacimarine environments require an interdisciplinary approach: precisely navigated, high-resolution 3D seismic surveys nested within a regional 2D seismic framework and integrated with a precisely navigated coring program designed to sample specific shallow acoustic facies. We illustrate this approach with examples from continuing studies of sediments on the New Jersey continental shelf. Surficial stratigraphy associated with the latest Pleistocene-Holocene deglaciation has been imaged twice in true 3D using the Huntec® DTS system, a deep-towed, high-frequency (500–3500 Hz) boomer source. In both cases, nominal profile spacing was 10 m (2.5 m shot points). Penetration of ~40 ms (~30 m) was achieved. Seismic facies in the upper 5 m have been interpreted using differential GPS-navigated vibra-cores. Accuracy of position of both seismic profiles and core locations approaches 5 m, much smaller than the wavelengths of the geologic features being imaged/sampled. Survey results show multiple, sediment-filled channels buried beneath the modern outer shelf meandering south and southeast, in the general direction of the shelf edge. Channels are typically a few meters deep and 100–200 m across. Lateral continuity between the channeled surfaces within the outer-shelf wedge and the mid-shelf cannot be demonstrated because of later erosion, but both sets of channels incise reflector ‘R’, believed to be a pre-Late Wisconsinan exposure surface. AMS14C dates on benthic foraminifers from cores show that channels on the midshelf are cut into sediment >45,000 yrs old, and filled with muddy sediment as young as ~12,500 yrs, whereas channels on the outer shelf are filled with coarser, and perhaps older, sediment. Both sets of filled channels are buried beneath a surficial sand layer. The integrated approach described here can remove some uncertainties and considerably ‘sharpen’ the Quaternary stratigraphic picture. Without the perspective obtained from 3D seismic surveys and equally precise sampling, interpretations based on broader regional seismic surveys and spot coring are inevitably constrained by the potential for mismatching shallow acoustic and sedimentary facies.

Sequence Biostratigraphy of Prograding Clinoforms, Northern Carnarvon Basin, Western Australia: A Proxy for Variations in Oligocene to Pliocene Global Sea Level?

Abstract Sequence biostratigraphic analyses from five industry wells in the Northern Carnarvon Basin (NCB), Western Australia, are tied to seismic stratigraphic interpretations from a set of 3D and 2D seismic data. Distribution patterns of ∼286 benthic and 73 planktonic foraminiferal taxa in sidewall cores and ditch cuttings from Eocene to Pliocene intervals are documented and supplemented with observations of other fossil groups (e.g., fragments of ostracodes, bryozoans, corals, and mollusks) and lithological components such as calcite cement and quartz sand. Preservation of foraminiferal assemblages is extremely variable in latest Eocene to Pliocene stratigraphy, depending upon the location of wells and the interval investigated. Nonetheless, consistent, detectable faunal signals correlate between wells and with prominent seismic horizons and sequences. The late Oligocene to middle Miocene is characterized by deeper-water benthic assemblages dominated by infaunal taxa and a high planktonic abundance. Stratigraphic events in the middle Miocene, including turnover in benthic foraminifera, are interpreted to record a regional flooding event (equivalent to cycle Tejas B (TB) 2.3) at the beginning of the mid-Miocene climatic optimum (∼16–14.5 Ma). Following this event, seismically defined geomorphic features include karstification on the shelf and incision on the clinoform front. All wells show a major transition to shallow-water, warm conditions on the shelf in the middle and late Miocene, with benthic assemblages dominated by larger foraminifera. This transition appears higher in more-basinward wells and appears to be a result of progradation. Geomorphic features in the late middle Miocene (∼12 Ma) identified from 3D seismic analyses show an intensification of earlier gully formation, resulting in the development of submarine canyons. Detailed analyses of faunal patterns also provide evidence of higher-frequency sea-level fluctuations (0.5–3 Ma), not detected in the seismic stratigraphic patterns.

Upper Triassic—Lower Jurassic salt basin southeast of the Grand Banks

Abstract A grid of multichannel seismic reflection (MCS) profiles has been used to delineate and map a large, apparently tectonically controlled basin and associated diapiric province beneath the continental slope and rise southeast of the Grand Banks of Newfoundland. The depocenter, for which we propose the name Salar basin, is defined both landward and seaward by pronounced hinges in acoustic basement. It is approximately 400 km long, 35–70 km wide, and covers an area of approximately 20,000 km 2 . The southern end of the basin is marked by the intersection of the Southeast Newfoundland Ridge with the South Bank High on the Grand Banks, and the northern end is bounded by the southwestern margin of Flemish Cap. The Salar basin generally parallels the much smaller Carson basin, which lies beneath the Grand Banks immediately to the west, and it merges with this basin north of 45°22′N. Based upon correlation with contiguous deposits sampled by drilling in the Carson basin, we infer piercements filling the Salar basin to be mobilized Late Triassic-Early Jurassic evaporites, probably predominantly halite. A consequent early Mesozoic age for the Salar basin implies a multi-stage rifting history for the Newfoundland Basin, as has been documented for the adjacent Grand Banks. Initial extension, which probably formed the Salar basins bounding basement hinge zones, occurred in Late Triassic—Early Jurassic time, and was associated with rifting of North America from Africa and extension in western Europe. The presence of early Mesozoic evaporites in a basin southeast of the Grand Banks suggests development of a significant, but shallow and restricted, seaway between the Grand Banks and the Iberian peninsula, probably connected northward with coeval salt provinces of northwestern Europe. A second rift phase related to separation of Iberia and the Grand Banks affected the Grand Banks and Salar basin in Late Jurassic to Early Cretaceous time, after a period of epeirogenic subsidence during the Early and Middle Jurassic. Uplift associated with this extension led to the development of the widespread “U” (Avalon) unconformity across the Grand Banks, the Salar basin and the Newfoundland Basin out to the J-Anomaly and may have induced the first halokinesis in the Salar basin. The second rift phase culminated in the initiation of seafloor spreading in mid-Early Cretaceous (Aptian) time at a location now marked by the J-Anomaly.

Buried Periglacial Drainage Channels on the New Jersey Outer Continental Shelf

At the time of maximum Cenozoic glaciation, the New Jersey outer continental shelf, lying south of the ice margin (thought to have been along the southern shore of Long Island), occupied a periglacial, shallow water environment. Thick wedges of sediment on the mid and outer shelf are thought to be composed of material delivered via the Hudson River drainage system during melting and glacial retreat.