Paul Gammon

Glacial-interglacial (MIS 1-10) migrations of the Subtropical Front across ODP Site 1119, Canterbury Bight, Southwest Pacific Ocean

Abstract Ocean Drilling Program (ODP) Site 1119 is located at water depth 395 m near the subtropical front (STF; here represented by the Southland Front), just downslope from the shelf edge of eastern South Island, New Zealand. The upper 86.19 metres composite depth (mcd) of Site 1119 sediment was deposited at an average sedimentation rate of 34 cm/kyr during Marine Isotope Stages (MIS) 1–8 (0–252 ka), and is underlain across a ∼25 kyr intra-MIS 8 unconformity by MIS 8.5–11 (277–367 ka) and older sediment deposited at ∼14 cm/kyr. A time scale is assigned to Site 1119 using radiocarbon dates for the period back to ∼39 ka, and, prior to then, by matching its climatic record with that of the Vostok ice core, which it closely resembles. Four palaeoceanographic proxy measures for surface water masses vary together with the sandy-muddy, glacial–interglacial (G/I) cyclicity at the site. Interglacial intervals are characterised by heavy δ13C, high colour reflectance (a proxy for carbonate content), low γ-ray (a proxy for clay content) and light δ18O; conversely, glacial intervals exhibit light δ13C, low reflectance, high γ-ray and heavy δ18O signatures. Early interglacial intervals are represented by silty clays with 10–105-cm-thick beds of sharp-based (Chondrites-burrowed), shelly, graded, fine sand. The sands are rich in foraminifera, and were deposited distant from the shoreline under the influence of longitudinal flow in relatively deep water. Glacial intervals comprise mostly micaceous silty clay, though with some thin (2–10 cm thick) sands present also at peak cold periods, and contain the cold-water scallop Zygochlamys delicatula. Interglacial sandy intervals are characterised by relatively low sedimentation rates of 5–32 cm/kyr; cold climate intervals MIS 10, 6 and 2 have successively higher sedimentation rates of 45, 69 and 140 cm/kyr. Counter-intuitively, and forced by the bathymetric control of a laterally-moving shoreline during G/I and I/G transitions, the 1119 core records a southeasterly (seaward) movement of the STF during early glacial periods, accompanied by the incursion of subtropical water (STW) above the site, and northwesterly (landward) movement during late glacial and interglacial times, resulting in a dominant influence then of subantarctic surface water (SAW). The history of passage of these different water masses at the site is clearly delineated by their characteristic δ13C values. The intervals of thin, graded sands–muds which occur within MIS 2–3, 6, 7.4 and 10 indicate the onset at times of peak cold of intermittent bottom currents caused by strengthened and expanded frontal flows along the STF, which at such times lay near Site 1119 in close proximity to seaward-encroaching subantarctic waters within the Bounty gyre. In common with other nearby Southern Hemisphere records, the cold period which represents the last glacial maximum lasted between ∼23–18 ka at Site 1119, during which time the STF and Subantarctic Front (SAF) probably merged into a single intense frontal zone around the head of the adjacent Bounty Trough.

Middle to Upper Eocene stratigraphic nomenclature and deposition in the Eucla Basin

The Eucla Basin has the largest onshore extent of Cenozoic marine sediments anywhere in the world. The sediments provide a record of the evolving marine environments of the Southern Ocean and the terrestrial hinterland of the Australian continent. However, owing to its size and remoteness, the Eucla Basin is comparatively understudied. This is exacerbated by the scattered and often deeply weathered nature of the outcrops along the margins of the basin, and the inaccessibility of exposures in the basin centre, except in cliffs and caves. The extent and isolation of the Eucla Basin over two states has resulted in conflicting and overlapping stratigraphic nomenclature, especially of the marginal sediments. Therefore, we propose rationalising the nomenclature of the Eocene rocks in the region based on three guiding principles: the use of consistent terminology across the region; the recognition of the importance of allostratigraphy in defining stratigraphic architecture, in particular two 3rd‐order cycles correlated with the Tortachilla and Tuketja transgressions; and continuity with past usage wherever possible, with a minimum of new terminology. We propose eight major changes to the existing nomenclature: (i) abandoning the term Bremer Basin for the marine and marginal marine to non‐marine Eocene sediments that infill palaeovalleys and form a veneer across crystalline basement in southwest Western Australia and including these sediments in the margin of the Eucla Basin; a similar situation exists in the east, where the Eocene sediments that have been included in the Polda Basin are likewise a marginal extension of the Eucla Basin; (ii) introducing the term Maralinga Formation for all Middle Eocene non‐marine to marginal marine sediments, including those previously included in the lower part of the Pidinga Formation in South Australia, and North Royal Formation for similar sediments in Western Australia; these replace the previous informal usage of lower Pidinga and lower Werillup Formation, respectively; (iii) restricting Hampton Sandstone to its original usage for a calcareous marine sand underlying the Wilson Bluff Limestone; (iv) raising the Paling Member of the Wilson Bluff Limestone to formation status; (v) using Pidinga Formation for all Upper Eocene carbonaceous sediments on the margins of the Eucla Basin in South Australia, and Werillup Formation for all such sediments in Western Australia, including the marginal palaeovalleys; terms such as Wollubar Sandstone in the palaeovalleys of the Yilgarn Craton, and Poelpena and Wanilla Formations in the Eocene part of the former Polda Basin should be abandoned; (vi) using the term Pallinup Formation for all Upper Eocene spicule‐rich sediments along the western margin of the Eucla Basin; (vii) recognising the formation status of the Upper Eocene spicular marine sediments in the eastern Eucla Basin that were formerly termed the Khasta Member of the Hampton Sandstone and the Bring Member of the Pidinga Formation, abandoning the term Bring Member, and including those rocks, and similar sediments of the Poelpena Formation in the Polda Basin, in the new Khasta Formation; and (viii) abandoning the term Toolinna Limestone previously applied to Upper Eocene grainstone along the western margin of the Eucla Basin as it is a facies of the Wilson Bluff Limestone, whereas the grainstone at the type locality at Toolinna Cove is in fact Abrakurrie Limestone and is indistinguishable from the rest of that formation. We believe this rationalisation emphasises the unity of stratigraphy across much of southern Australia and, thus, will facilitate research on the Eucla Basin as a whole.

Facies and sequence stratigraphy of Eocene palaeovalley fills in the eastern Eucla Basin, South Australia

Abstract The Eocene succession filling palaeovalleys in the northeastern Eucla Basin, South Australia, is interpreted using facies and sequence-stratigraphic models based on relative sea-level changes. The dominantly fluvial sediments were deposited in incised valleys which graded basinwards to an estuarine coastal plain under warm and humid palaeoclimatic conditions. Sedimentological examination suggests a tidal influence in this fluvial succession. Fluvial–estuarine-shoreline facies associations can be recognised in these (Eocene) sequences, each of which comprises a diverse assemblage of lithofacies that can be grouped into lowstand and/or transgressive and highstand system tracts. Since the palaeorivers had hydrological connection with the sea, deposition was dominantly controlled by sea-level changes. Results of the study indicate that two third-order Eocene eustatic cycles have largely controlled sedimentation. The resulting key surfaces (unconformity, and transgressive, tidal/wave ravinement, and maximum flooding surfaces) bound depositional sequences which extend over significant areas and may be used in basin-wide correlations of stratal packages.

The middle Pleistocene Merced-2 and -3 sequences from Ocean Beach, San Francisco

The Merced Formation comprises a 2-km-thick shallow marine and non-marine succession that was deposited in a small transtensional basin along the San Andreas Fault during the late Pliocene to middle Pleistocene. The sediments dip between 10° and 80° to the northeast, and are locally disrupted by small faults. During tilting, the beds have been rotated into subparallelism with the San Andreas Fault zone, splays of which bound the outcrop belt of Merced sediments to both the southwest and northeast. The Merced Formation contains more than 20 transgressive–regressive sedimentary rhythms (cyclothems, or sequences) that are generally between 40 and 120 m thick, and which were deposited mostly during interglacial time, under the influence of rising, highstand and early falling sea levels. Sequences Merced-2 [units M1–N of Soc. Econ. Paleontol. Mineral., Field Guidebook 3 (1984) 1] and Merced-3 (units O–P), though in fault contact, comprise typical Plio-Pleistocene shallow water cyclothems. The Merced-2 Sequence is 22+ m thick, and comprises a sandy and shelly transgressive systems tract, including a basal Type A shellbed, an in situ Type B mid-cycle shellbed, and a highstand systems tract of massive siltstone. The Merced-3 Sequence is 47 m thick, and comprises a basal compound shellbed, a thin highstand systems tract siltstone, and a sand-rich regressive systems tract. The RST comprises distal shoreface sands with an abundant in situ molluscan fauna, and upper shoreface and back-beach trough cross-bedded sands and pebbly sands. The top of the Merced-3 cycle comprises a beach sand capped by a palaeosol and lignite (the “Beetle Bed”), which together mark the subaerial exposure of the site during the ensuing glacial sea-level lowstand. Analysis of 10Be across the Merced-3 Sequence shows major peaks, indicative of sedimentation starvation, in the basal transgressive systems tract shellbed and in the capping lignite of the Beetle Bed. Smaller 10Be peaks are associated with a shellbed that is inferred to represent winnowing at the foot of the shoreface, and with a minor exposure surface that delimits a small paracycle in the top of the sequence. Otherwise, 10Be abundances decline regularly across the Merced-3 Sequence, consistent with an increasing sedimentation rate as shoreface progradation, and regression, progressed. The character of cycles Merced-2 and Merced-3 respectively resembles the Seafield and Rangitikei sequence motifs described from similar Plio-Pleistocene sediments in New Zealand. The cycles are of mid-Pleistocene age, and were probably deposited during interglacial oxygen isotope stages 21 and 19, respectively.