LATE QUATERNARY PALEOSEALEVELS AND PALEOENVIRONMENTS INFERRED FROM FORAMINIFERA, NORTHERN SPENCER GULF, SOUTH AUSTRALIA

Abstract

Spencer Gulf is an elongate marine embayment extending northwards ca. 300 km inland into southern continental Australia to the apex at Port Augusta, with a narrow estuarine extension northwards. Since the post-glacial sealevel maximum, prograding coastal sedimentation has been effective through the trapping and binding actions of seagrasses, mangroves and cyanobacterial mats, and a well-defined zonation of subtidal, intertidal and supratidal sedimentary facies is characteristic of the northern gulf. Mollusks and foraminifera are prolific, especially within seagrass meadows, and their abundant remains, entire and comminuted, form bioclastic carbonate sediments. Distinctive assemblages of foraminifera are associated with the various estuarine and marine environments. The hypersaline estuary north of Port Augusta is characterized by the textulariids Ammobaculites barwonensis and Trochammina inflata, the latter most common where mangroves are present. Two species of Triloculina, T. inflata and T. oblonga are the only common miliolids, while the rotaliids are represented by Ammonia beccarii, Elphidium articulatum and Nonion depressulus. South of Port Augusta, in euryhaline intertidal waters, Trochammina inflata, A. beccarii and E. articulatum are commonly associated. The shallow subtidal Posidonia seagrass meadows support an abundant fauna which is dominated by three species, Nubecularia lucifuga, Peneroplis planatus and Discorbis dimidiatus. These species continue into the deeper waters, ca. 20 m, where they are subordinate to Quinqueloculina lamarckiana, Massilina milletti, Elphidium crispum and E. macelliforme. A vibrocore from 20 m water depth in Northern Spencer Gulf recovered 4 m of late Quaternary fossiliferous sediment. Amino acid racemization (AAR) and radio-carbon ages derived from fossil mollusks revealed four chronostratigraphic packages of sediment: 400–360 cm, penultimate interglacial, oxygen isotope stage 7; 360–240 cm, last interglacial, oxygen isotope substage 5e; 240–180 cm, interstadial, oxygen isotope stage 3; and 180 cm to the top of the core, postglacial, oxygen isotope stage 1. Species of foraminifera within the core are mostly also living in the modern gulf, thus the preserved assemblages permit plausible interpretations of paleoenvironments and paleosealevels. Large numbers of Q. lamarckiana and M. milletti in the substage 5e interval indicate that the last interglacial sealevel in southern Australia was only slightly higher than that of today. N. lucifuga is abundant in the stage 3 sediment, thus paleowaterdepth at the core site was ca. ≤2 m. This evidence further supports interstadial paleosealevels above −30 m, as previously determined for Gulf St Vincent. Basal sediments of the postglacial interval preserve a monospecific assemblage of Miliolinella labiosa which signifies an estuarine setting having many of the characteristics of a saline lake. Maximum Holocene sealevel is signified by the acme of M. milletti at 90 cm. Subsequent local hydroisostatic relative fall of sealevel is shown by changes up-core from 90 cm in both the general foraminiferal record, for example by declining numbers of M. milletti, and also by reduced numbers of E. macelliforme compared with those of E. crispum.