Glacial Unconformities Research Articles

Structural and fluid-migration control on hill-hole pair formation: Evidence from high-resolution 3D seismic data from the SW Barents Sea

Hill-hole pairs are subglacial landforms consisting of thrust-block hills and associated source depressions. Formed by evacuation of material where ice sheets have been locally frozen to the substrate, they give insights into paleo-ice-sheet dynamics. The aim of this study was to document the relationships between ancient hill-hole pairs identified on a buried glacial unconformity with the structure of the underlying sedimentary deposits, and then to determine if the basin geology and glacial fluid migration pathways promoted local subglacial freeze-on during the hill-hole pair formation. The study is based on seismic geomorphological interpretation of four high-resolution 3D seismic cubes covering an area of 800 km2 in the SW Barents Sea, and fluid seepage data from 37 gravity cores.The seismic datasets allowed the identification of 55 hill-hole pairs along the buried unconformity. The hills are characterized by chaotic to homogenous seismic facies forming up to 19 m high mounds, each covering areas of 2000–644,000 m2. The holes form depressions between 1 and 44 m deep and 2000–704,000 m2 in areal extent, which cut into preglacial Mesozoic bedrock and later infilled by glacial till. The holes are often found above fault terminations. High-amplitude reflections identified along the faults and in the strata below the holes are interpreted as shallow gas migrating upward towards the glacial unconformity. Geochemical data of the seabed sediment cores further indicates an association between hill-hole pair occurrence and present-day thermogenic hydrocarbon seepage.The hill-hole pairs geometries were also used to identify five paleo-ice-flow directions along the glacial unconformity. These ice flows exhibit polythermal regimes, and four of them are parallel to ice-stream flow sets interpreted from glacial lineations. The integrated interpretation supports localized fault-related basal freezing of the Barents Sea Ice Sheet which resulted in the formation of hill-hole pairs when the ice sheet moved. In this context, the faults functioned as migration pathways for deep thermogenic fluids, possibly sourced from leaking Jurassic reservoirs.This study highlights the importance of the underlying geology for ice-sheet dynamics: While hill-hole pairs above glacial till appear to be commonly associated with dispersed gas hydrates, hill-hole pairs above bedrock additionally indicate a link to underlying fault systems and hydrocarbon reservoirs. Freeze-on of underlying bedrock to the basal ice along the strike of faults in sedimentary bedrock explains deeper hill-hole pairs with smaller extents along the glacial unconformity compared to areally larger but shallow hill-hole pairs detected above glacial till on modern seabeds. Such close association between paleo-thermogenic gas seepage and the location of hill-hole pairs strongly support that hill-hole pairs are excellent markers revealing exit points of fluid migration pathways in petroleum system models.

Porosity and free gas estimates from controlled source electromagnetic data at the Scanner Pockmark in the North Sea

We present porosity and free gas estimates and their uncertainties at an active methane venting site in the UK sector of the North Sea. We performed a multi-disciplinary experiment at the Scanner Pockmark area in about 150 m water depth to investigate the physical properties of fluid flow structures within unconsolidated glaciomarine sediments. Here, we focus on the towed controlled source electromagnetic (CSEM) data analysis with constraints from seismic reflection and core logging data. Inferred background resistivity values vary between 0.6–1 Ωm at the surface and 1.9–2.4 Ωm at 150 mbsf. We calibrate Archie’s parameters with measurements on cores, and estimate porosities of about 50±10% at the seafloor decreasing to 25±3% at 150 mbsf which matches variations expected for mechanical compaction of clay rich sediments. High reflectivity in seismic reflection data is consistent with the existence of a gas pocket. A synthetic study of varying gas content in this gas pocket shows that at least 33±8% of free gas is required to cause a distinct CSEM data anomaly. Real data inversions with seismic constraints support the presence of up to 34±14% free gas in a 30–40 m thick gas pocket underneath the pockmark within the stratigraphic highs of a till layer above the glacial unconformity in the Aberdeen Ground Formation.

Distribution of pore pressure and fracture pressure gradients in the paleozoic sediments of Takouazet field, Illizi basin, Algeria

A comprehensive pore pressure and fracture gradient (PPFG) characterization is a basic requirement for subsurface geomechanical modeling. This study deploys indirect approaches to estimate subsurface pressure profiles in the Mesozoic and Paleozoic successions of eastern Illizi basin, Algeria. Amoco exponent has been modeled for pseudo density generation and overburden gradient has been deciphered from a composite density profile. PP calculated from sonic and resistivity logs have been calibrated and validated with direct downhole measurements. Results indicate normal pore pressure regime in the Mesozoic and Devonian sediments. Disequilibrium compaction induced abnormal pore pressure in Silurian marine shales with a maximum gradient of 12.7 PPG. Top of geopressure has been marked at Silurian top (2050m). Pore pressure drops sharply from Silurian to Ordovician sediments, across the glacial unconformity (2332 m). Fracture gradients have been interpreted by Eaton's Poisson's ratio based model and Mathews & Kelly's effective stress ratio based approach. Based on the vertical distribution of subsurface pressure, effective vertical stress, a safe mud window has been recommended for optimum drilling fluid design.

Scratching the surface: Footprint of a late Carboniferous ice sheet

AbstractField observations in conjunction with aerial images from an unmanned aerial vehicle were used to create the first map of a glacial unconformity underlying the late Carboniferous Dwyka Group of South Africa. Crosscutting relationships reveal that the glacial unconformity at Oorlogskloof, in which flutes, grooves, and striae were ploughed into unconsolidated sand, formed in a three-phased process charting a periodic shift in the locus of subglacial erosion. The unconformity formed by a periodically decoupled ice sheet in a probable tidewater setting. This model contrasts with earlier views that the structures simply record progressive ice-margin liftoff during transgression, and they provide unique insight into the complex temporal development of a 300 Ma subglacial environment.

Sedimentary processes at ice sheet grounding‐zone wedges revealed by outcrops, Washington State (USA)

AbstractGrounding‐zone wedges (GZWs) mark the grounding terminus of flowing marine‐based ice streams and, in the presence of an ice shelf, the transition from grounded ice to floating ice. The morphology and stratigraphy of GZWs is predominantly constrained by seafloor bathymetry, seismic data, and sediment cores from deglaciated continental shelves; however, due to minimal constraints on GZW sedimentation processes, there remains a general lack of knowledge concerning the production of these landforms. Herein, outcrop observations are provided of GZWs from Whidbey Island in the Puget Lowlands (Washington State, USA). These features are characterized by prograded diamictons bounded by glacial unconformities, whereby the lower unconformity indicates glacial advance of the southern Cordilleran Ice Sheet and the upper unconformity indicates locally restricted ice advance during GZW growth; the consistent presence of an upper unconformity supports the hypothesis that GZWs facilitate ice advance during landform construction. Based on outcrop stratigraphy, GZW construction is dominated by sediment transport of deformation till and melt‐out of entrained basal debris at the grounding line. This material may be subsequently remobilized by debris flows. Additionally, there is evidence for subglacial meltwater discharge at the grounding line, as well as rhythmically bedded silt and sand, indicating possible tidal pumping at the grounding line. A series of GZWs on Whidbey Island provides evidence of punctuated ice sheet movement during retreat, rather than a rapid ice sheet lift‐off. The distance between adjacent GZWs of 102–103 m and the consistency in their size relative to modern ice stream grounding lines suggests that individual wedges formed over decades to centuries. © 2018 John Wiley & Sons, Ltd.

Chalk-glacitectonite, an important lithology in former glaciated terrains covering chalk and limestone bedrock

A glacitectonite is defined as a brecciated sediment or a cataclastic sedimentary rock formed by glaciotectonic deformation (Pedersen 1988). The term tectonite was initially introduced by Sander (1912), mainly for tectonically brecciated metamorphic rocks in the Alps. In the classic work on cataclastic rocks, Higgins (1971) stated that the term covered all rocks with fabric displaying coordinated geometric features related to continuous flow during deformation.Therefore brecciated lithologies formed by glaciotectonic deformations can be termed tectonites. Banham (1977) suggested the prefix glaci- to clarify the relation to glacial dynamics. Furthermore, Pedersen (1988) suggested the application of the bedrock prefix. Thus, a chalk-glacitectonite is a brecciated chalk formed by shear deformation during a glacial advance over an exposed bedrock surface of chalk (Fig. 1). Hence the term describes a sedimentary rock in which the primary structures are so disturbed that they cannot be continuously traced, and a glacitectonic fabric developed as joint fractures or shear surfaces superimposed on the lithology. The significance of recognising chalk-glacitectonite from chalk and limestone bedrock is the difference in textural properties, which is fundamental in geological modelling. In areas dominated by glaciotectonic complexes, which include thrust sheets of pre-glacial sedimentary rocks, the sheets are subject to shearing and dragged along the sole of the ice during its movement over the glaciotectonic complex. Due to truncation and shear-drag, the glacitectonite forms at the base of the deformational layer in a lodgement till. From the source area, which typically is a detachment anticline, the glacitectonite thins out in the direction of transport from 1–2 m (Fig. 2) to a thin shear zone only a few centimetres thick over a distance of one to a few kilometres (Pedersen 1996). Moreover, brecciation of thrust sheets displaced by glacial thrusting occurs within glaciotectonic complexes. The deformation ranges from initially anastomosing jointing (Figs 1, 3) to brecciation with bedrock clasts in crushed bedrock matrix (Fig. 4). The tectonic breccia distributed from the décollement zone at the base to the truncating glacial unconformity at the top may additionally be termed glacitectonites. Here we describe the occurrence and identification of chalk-tectonites.

In Situ Temperature Measurements at the Svalbard Continental Margin: Implications for Gas Hydrate Dynamics

AbstractDuring expedition MARIA S. MERIAN MSM57/2 to the Svalbard margin offshore Prins Karls Forland, the seafloor drill rig MARUM‐MeBo70 was used to assess the landward termination of the gas hydrate system in water depths between 340 and 446 m. The study region shows abundant seafloor gas vents, clustered at a water depth of ∼400 m. The sedimentary environment within the upper 100 m below seafloor (mbsf) is dominated by ice‐berg scours and glacial unconformities. Sediments cored included glacial diamictons and sheet‐sands interbedded with mud. Seismic data show a bottom simulating reflector terminating ∼30 km seaward in ∼760 m water depth before it reaches the theoretical limit of the gas hydrate stability zone (GHSZ) at the drilling transect. We present results of the first in situ temperature measurements conducted with MeBo70 down to 28 mbsf. The data yield temperature gradients between ∼38°C km−1 at the deepest site (446 m) and ∼41°C km−1 at a shallower drill site (390 m). These data constrain combined with in situ pore‐fluid data, sediment porosities, and thermal conductivities the dynamic evolution of the GHSZ during the past 70 years for which bottom water temperature records exist. Gas hydrate is not stable in the sediments at sites shallower than 390 m water depth at the time of acquisition (August 2016). Only at the drill site in 446 m water depth, favorable gas hydrate stability conditions are met (maximum vertical extent of ∼60 mbsf); however, coring did not encounter any gas hydrates.

Volcano–ice–sea interaction in the Cerro Santa Marta area, northwest James Ross Island, Antarctic Peninsula

We present here the results of detailed mapping, lithofacies analysis and stratigraphy of the Neogene James Ross Island Volcanic Group (Antarctic Peninsula) in the Cerro Santa Marta area (northwest of James Ross Island), in order to give constraints on the evolution of a glaciated volcanic island. Our field results included recognition and interpretation of seventeen volcanic and glacial lithofacies, together with their vertical and lateral arrangements, supported by four new unspiked K–Ar ages. This allowed us to conclude that the construction of the volcanic pile in this area took place during two main eruptive stages (Eruptive Stages 1 and 2), separated from the Cretaceous bedrock and from each other by two major glacial unconformities (U1 and U2). The U1 unconformity is related to Antarctic Peninsula Ice sheet expansion during the late Miocene (before 6.2Ma) and deposition of glacial lithofacies in a glaciomarine setting. Following this glacial advance, Eruptive Stage 1 (6.2–4.6Ma) volcanism started with subaerial extrusion of lava flows from an unrecognized vent north of the study area, with eruptions later fed from vent/s centered at Cerro Santa Marta volcano, where cinder cone deposits and a volcanic conduit/lava lake are preserved. These lava flows fed an extensive (>7km long) hyaloclastite delta system that was probably emplaced in a shallow marine environment. A second unconformity (U2) was related to expansion of a local ice cap, centered on James Ross Island, which truncated all the eruptive units of Eruptive Stage 1. Concomitant with glacier advance, renewed volcanic activity (Eruptive Stage 2) started after 4.6Ma and volcanic products were fed again by Cerro Santa Marta vents. We infer that glaciovolcanic eruptions occurred under a moderately thin (~300m) glacier, in good agreement with previous estimates of paleo-ice thickness for the James Ross Island area during the Pliocene.

Seismic stratigraphic evidence for glacial expanse during glacial maxima in the Yakutat Bay Region, Gulf of Alaska

[1] Modern shelf bathymetry bordering the Gulf of Alaska exhibits shelf-crossing sea valleys that suggest focused pathways for ice flow during glacial conditions. Using an integrated seismic data set between the present Yakutat and Alsek Sea Valleys, we investigate the glacial stratigraphic record in order to improve our understanding of the regional glacial system during maximum glacial conditions. Our investigations reveal four glacial unconformities, of which, the latter two are overlain by sediment packages hundreds of meters thick. We suggest that these unconformities are indicative of ice advance phases during the Little Ice Age (LIA), the Last Glacial Maxima (LGM), and two pre-LGM advances with glacial retreat sequences preserved from the youngest two. The advances were dominated by ice expanse from the Malaspina Glacial system and Alsek River districts rather than the Hubbard Glacial system and only show distinctive morainal bank development near the shelf edge and near the mouth of, or within, modern bays, fjords, or river valleys. This observation strongly supports rapid and continuous retreat from glacial maxima conditions during climatic warming. All, except the inferred LIA sequence, transgress the shelf and exhibit concentrated erosion in overdeepened troughs, analogous to cross-shelf troughs similar to those observed on other high-latitude glaciated shelves. The two Alaskan troughs discussed here may be end-member examples of high-sediment flux systems due to the temperate glacial setting combined with an actively exhuming orogen.

Fourier Grain-Shape Analysis of Antarctic Marine Core: The Relative Influence of Provenance and Glacial Activity On Grain Shape

Abstract Grain shape is useful as a depositional, environmental, diagenetic, and provenance proxy. Here we conduct the first study of down-core grain-shape variation in Antarctica. This study further examines the utility of grain shape as a depositional proxy, namely of glacial influence, in the James Ross Basin and Joinville Plateau of the Antarctic Peninsula from the Eocene to the present. Fourier grain-shape analysis was used to quantify the grain shape of 31 samples spanning the glacial history of the Antarctic Peninsula Ice Sheet. A total of 6,442 quartz grain peripheries were digitized and described through the twenty-first Fourier harmonic. Sediment provenance has a greater influence on grain roughness than glacial activity in the study area. However, provided a similar sedimentary source, secondary changes in roughness correlate with changes in the extent of the Antarctic Peninsula Ice Sheet since the late Oligocene. Grain roughness increased with increased glacial influence during the middle Miocene, after an early Pliocene warm period, and just above a Pliocene regional glacial unconformity. Conversely, grain roughness decreased with less glacial influence during the late Oligocene and during an early Pliocene warm period. This study further illustrates the usefulness of grain shape in distinguishing sediment provenance and environmental conditions in Antarctic settings.

Ice-sheet evolution in James Ross Basin, Weddell Sea margin of the Antarctic Peninsula: The seismic stratigraphic record

The stratigraphic architecture of the James Ross Basin, NW Weddell Sea continental shelf, shows three major phases of deposition: preglacial, ice-sheet growth, and ice-sheet dominated. The preglacial phase shows stratigraphic sequences that are similar to low-latitude margins. During the ice-sheet growth phase, there is evidence for initial ice-sheet grounding in the southern part of the basin, which migrated in steps northward. Sediment supply was also high during the ice-sheet growth phase. During the ice sheet–dominated phase, both north and south parts of the basin inherited all the classic features indicative of glacial strata: large glacial troughs hundreds of meters deep and tens of kilometers wide, truncation of sequences overlain by chaotic seismic facies and topped with coherent laminated reflections, and an oversteepened continental slope. The James Ross Basin strata are divided into five units (S5–S1). Each unit is characterized based upon seismic facies and stratigraphic architecture, and the age of each unit is inferred from a seismic stratigraphic age model. End-member strata (S5 and S1) were drilled and dated during SHALDRIL and help to constrain this age model. In total, 34 grounding events of the Antarctic Peninsula Ice Sheet (APIS) are recorded on the continental shelf. This is similar to the 31 grounding events imaged on the Pacific side of the peninsula. Ice grounding on the continental shelf is shown to have occurred as early as late middle Miocene to early late Miocene in the southern part of the basin. The ice sheet advanced northward to encompass the northern peninsula by late Miocene time. The seven oldest glacial unconformities are believed to predate all previously identified unconformities on the peninsula continental shelf. An expanded section of late Pliocene–Pleistocene deposits shows a minimum of 10 grounding events; this record more closely matches the deep-sea oxygen isotope record than previous seismic studies of Antarctic margins have shown.

New insights on the post-rift seismic stratigraphic architecture and sedimentary evolution of the Antarctic Peninsula margin (Central Bransfield Basin)

A seismic stratigraphic study of the Antarctic Peninsula margin of the Central Bransfield Basin was conducted using seismic and bathymetric data. The study focused on seismic sequences and units deposited after the basin opened, and yield a record of the evolution of the northern-most Antarctic Peninsula Ice Sheet. Two seismic sequences compose the post-rift stratigraphic succession on the Central Bransfield Basin slope platforms. Both sequences are composed of seismic units that record glacial/interglacial cycles and show evidence of the Antarctic Peninsula Ice Sheet having grounded on the continental shelf and slope platform on several occasions in the past. This evidence includes glacial and glaciomarine related sedimentary and erosional features within the seismic units (glacial unconformities, glacial troughs, till wedges, prograded wedges, trough mouth fans, glaciomarine sheet deposits and moraines), which are bounded by glacial unconformities. The Antarctic Peninsula lower slope also registered in its eastern sector the deposition of gravity flow deposits during ice sheet retreat and interglacial stages. The deposition of seismic units on the middle slope produced the present-day step-like profile of the margin composed of wide flat slope platforms and narrow and steep upper and lower slopes. Seismic units show changes in thickness and distribution that record the effect of the physiography and climatic changes on margin sedimentation. These variations produced an overall change from more line-sourced deposition to more point sourced deposition as ice streams evolved. The stacking of units shows a change in the stratigraphic architecture through time, from dominantly progradational (S2) to progradational/aggradational (S1). This change is correlated with a change in the frequency of ice sheet grounding events which was probably driven by a higher rate of eustatic rise and fall in relation with the Mid Pleistocene global climatic change.

The origin and glaciodynamic significance of sandstone ridge networks from the Hirnantian glaciation of the Djado Basin (Niger)

AbstractThe Djado Basin (Niger) was located beneath the inner part of the Late Ordovician ice sheet. The Felar‐Felar Formation consists mainly of glaciomarine deposits, associated with the major ice sheet recession within the glaciation, and is bounded by two glacial unconformities. Structures corresponding to sandstone ridges are found within the Felar‐Felar Formation. Sandstone ridges are several metres high, about 10 m wide and hundreds of metres long. These structures are organized in extensive anastomosed to sub‐polygonal networks. The association of sandstone ridge networks with the later glacial unconformity and with other glacial evidence suggests sub‐glacial conditions for their origin. Sandstone ridge sedimentological characteristics indicate that sandstone ridges result from the scouring of the Felar‐Felar Formation by sub‐glacial, turbulent and pressurized meltwater; then sub‐glacial cavities were infilled with sand derived from glacial abrasion. Sandstone ridge networks are comparable with tunnel channels and document unusual drainage structures of the inner part of the palaeo‐ice sheet.

Isotope stratigraphy of the Lapa Formation, São Francisco Basin, Brazil: Implications for Late Neoproterozoic glacial events in South America

The Lapa Formation is a thick carbonate sequence (∼900m) that constitutes the upper part of the Vazante Group on the São Francisco craton, Brazil. It conformably overlies a previously unrecognized glacial diamictite unit of poorly constrained age. The sequence, above the glacial unconformity, consists predominantly of organic-rich shale, subtidal rhythmic dolomicrites and microbialaminites, and intertidal stromatolites. Four boreholes, spanning different depositional settings, were sampled at high-resolution and investigated for their petrographic and chemical criteria to evaluate their degree of preservation.The δ13C and δ18O values of well preserved Lapa carbonate microsamples range from −8.2‰ to 3.3‰ (VPDB) and from −13.6‰ to −0.9‰ (VPDB), respectively. Each of the δ13C profiles of the investigated cores reveals two strong negative excursions of up to 8‰, an event in post-glacial dolomicrites immediately above the glaciogenic unit and a 10m interval of organic-rich shale, and a second near the top of the sequence associated with a shale interval. Based on the observation of dropstones and sedimentary iron formation in the underlying diamictite, as well as the distinguishable carbon isotope trends, the Lapa Formation is considered as a cap carbonate lithofacies. The age of the Lapa Formation is presently unknown but the least radiogenic 87Sr/86Sr value (∼0.7068), associated with a negative carbon isotope excursion, matches that from the Rasthof Formation in Namiba on the Congo craton, which is radiometrically constrained to be younger than ca. 750Ma.

Were West Antarctic Ice Sheet grounding events in the Ross Sea a consequence of East Antarctic Ice Sheet expansion during the middle Miocene?

Seismic correlation of glacial unconformities from the Ross Sea outer continental shelf to chronostratigraphic control at DSDP sites 272 and 273 indicates that at least two West Antarctic Ice Sheet (WAIS) expansions occurred during the early part of the middle Miocene (i.e. well before completion of continental-scale expansion of the East Antarctic Ice Sheet (EAIS) inferred from δ 18O and eustatic shifts). Therefore, if the volume of the EAIS was indeed relatively low, and if the Ross Sea age model is valid, then these WAIS expansions/contractions were not a direct consequence of EAIS expansion over the Transantarctic Mountains onto West Antarctica. An in-situ development of the WAIS during the middle Miocene suggests that either West Antarctic land elevations were above sea level and/or that air and water temperatures were sufficiently cold to support a marine-based ice sheet. Additional chronostratigraphic and lithologic data are needed from Antarctic margins to test these speculations.

Qasim Formation: Ordovician Storm- and Tide-Dominated Shallow-Marine Siliciclastic Sequences, Central Saudi Arabia

ABSTRACT The Middle to Upper Ordovician Qasim Formation is well exposed in the Qasim region of central Saudi Arabia and is recognized from many wells to the north and east. It consists of the Hanadir, Kahfah, Ra’an, and Quwarah members that are arranged in two coarsening-upward progradational sequences that have a total thickness of about 200 m in outcrops. Two sections were measured in each sequence. The lower sequence is a storm-dominated, shallow-marine depositional system composed of the late Llanvirn to middle Caradoc Hanadir and Kahfah. At least five progradational beach parasequences were identified. The Hanadir overlies deltaic deposits of the upper Saq Formation and marks a major marine transgression onto Gondwana. It is composed of laminated fissile shale formed in an offshore marine environment together with siltstone laminae that indicate periodic influxes of low-density turbidity currents. The overlying Kahfah is composed of shale and sandstone. The upper progradational sequence consists of the middle Caradoc to middle Ashgill Ra’an and Quwarah members. The Ra’an is a succession of homogeneous fissile shale and minor siltstone laminae formed in an offshore marine environment following a major transgression onto the Kahfah. The overlying Quwarah consists of sandstone and minor amounts of siltstone and shale in its lower parts, and a thickly bedded sandstone facies above. The diagnostic sedimentary structures in the Quwarah are large-scale lens-shaped tidal sand waves composed of sigmoidal bundles deposited in a mesotidal setting of barrier bars, tidal channels, ebb-dominated deltas and lagoons. In the subsurface, the Formation thickens northeastward (basinward) to over 4,000 ft (1,220 m). In the same direction, the sand-dominated Kahfah and Quwarah gradually become shale-dominated, small-scale bed forms are more abundant, and identification of members based on lithofacies is more difficult. After deposition of the Quwarah, present-day Saudi Arabia was affected by a Late Ordovician (late Ashgill) glaciation. The Qasim and older formations are deeply incised by glacial channels at the base of the Zarqa and Sarah formations. Basinward, the glacial unconformities (or their submarine erosion surface equivalents) become less significant and a thick succession of the Formation is preserved.

Did the Antarctic ice sheets expand during the early Pliocene?

Seismic data show that glacial unconformities are located within lower Pliocene strata on the Antarctic continental shelves. The glacial unconformities are significant because they provide direct evidence that the Antarctic ice sheets advanced despite the generally warmer climates and elevated sea levels that characterized most of the early Pliocene. The magnitudes of peak eustatic lowstands and 18 O enrichments indicate that the ice volume on Antarctica may have exceeded today9s ice volume by approximately 18%, which suggests that the ice-sheet grounding events on the shelves probably were associated with larger than present ice volumes on two to three occasions during the early Pliocene.

Relative temporal stability of the Antarctic ice sheets during the late Neogene based on the minimum frequency of outer shelf grounding events

The stability of the Antarctic cryosphere involves two fundamental aspects: the magnitude of ice volume changes, and the frequency of ice volume changes. In this article, we synthesize results from three regional seismic stratigraphic studies of the continental shelf to evaluate the relative frequency of extreme advances of the ice sheets during the late Neogene for different sectors of the Antarctic ice sheet. Detailed analyses of glacial unconformities on the eastern and western Ross Sea outer continental shelf indicate that there were at least eight episodes during the late Neogene when the East Antarctic ice sheet (EAIS) and the West Antarctic ice sheet (WAIS) were significantly larger. The seismic results from the shelf do not support the conventional view that the land-based EAIS was relatively stable and that the marine-based WAIS was relatively dynamic. Glacial unconformities on the Pacific continental shelf of the Antarctic Peninsula indicate that there were at least 30 outer shelf ice sheet expansions during the late Neogene. This suggests that the small land-based Antarctic Peninsula Ice Cap (APIC) may have been the most dynamic component of the Antarctic cryosphere. Conversely, the greater number of grounding events on the Antarctic Peninsula shelf may be a result of the margin’s younger thermal age and hence greater potential to preserve units and unconformities during the late Neogene. We acknowledge that the available chronostratigraphic control from the Antarctic margin is poor, and that more chronological data are needed to confirm our late Neogene age assignments.

Seismic data from the Northern basin, Ross Sea, record extreme expansions of the East Antarctic Ice Sheet during the late Neogene

Approximately 3700 km of intermediate-resolution single-channel seismic data from the Northern basin were analyzed to investigate the late Neogene history of East Antarctic Ice Sheet (EAIS) grounding events on the shelf, and to evaluate how glacial unconformities on the shelf are manifested on the upper slope. The Northern basin was chosen as the site of this study because ice-sheet reconstructions show that the basin received sediment by ice emanating from East Antarctica . In addition, seismic correlations to DSDP Site 273 suggest that a relatively thick late Neogene section exists on the basin's outer shelf and upper slope. On the Northern basin shelf, glacial unconformities exhibiting broad, low-angle relief, topset truncation, and cross-cutting relationships reveal a dynamic history of expansions and contractions during which the EAIS was larger than present on at least eight occasions during the late Neogene. On the upper slope, the correlative conformities of the glacial unconformities are indistinct reflections within thick trough-mouth fan (TMF) depocenters at the mouths of Drygalsky and Joides basins. The glacial unconformities and correlative conformities define TMF sequences, and each TMF sequence contains several topset-truncated prograding-slope reflectors. We infer that the correlative conformities on the continental slope correspond to the interface between prograding glaciogenic deposits (glacial maximum) and diatomaceous glacial-marine sediments (glacial minimum). The seismic–stratigraphic analysis and regional mapping indicate that the upper slope does not contain a more complete late Neogene section than that which exists on the shelf. We infer that diatomaceous glacial-marine sediments on the slope may be relatively undisturbed, and hence may provide a means of dating the TMF sequences. It is hoped that these results will stimulate efforts to core the late Neogene section in the Northern basin TMFs to investigate how these EAIS expansions and contractions relate to other records of late Neogene climate and eustasy .