Brian McGowran

Biogeographic impact of the Leeuwin Current in southern Australia since the late middle Eocene

Abstract The Leeuwin Current can be tracked from the western margin of the Australian continent to the southern margin by the record of fossilized organisms typical of warm-water marine environments. This transport smudges the normal latitudinal asymmetry in the biotas of opposing oceanic and continental margins, in which the eastern margins of oceans are cooler than the western margins and warmer biotas are restricted to lower latitudes in the east. The most comprehensive record is in the large benthic foraminifera, although fossils of benthic invertebrates, nektonic nautiloids and planktonic protists are also informative. In addition, organic biomarker hydrocarbons in stranded bitumens and resins demonstrate that they have travelled the same route from their ultimate sources in Cenozoic sedimentary basins and modern tropical rainforests of the Indonesian Archipelago. The earliest spoor of the current is in the later middle Eocene, at which time the part-deflection of counter-gyral circulation in the Indian Ocean to the southeast was stimulated by the accelerated opening of the oceanic gap between Australia and Antarctica. Thus the origin of the Leeuwin Current is twice the age of the previously suggested origin in the Miocene. The current turns on and turns off in the Great Australian Bight during the late Quaternary in concert with the interglacials and the glacials at scales of 10 5 yr. The switch can be seen in the faunal succession of planktonic foraminifera which are consistent with the neritic benthic faunas of the central gulfs: both communities show that, at the warm peak of the last interglacial, the current transported biota across the Bight more strongly than it has during the Holocene. The Cenozoic record of the past 40 Ma is in the same mode at 10 6 yr scales: the relevant fossils are found concurrently at major marine transgressions and warming reversals of the overall fall in global temperature. However, the fossil pattern is due to transport on the activated Leeuwin Current, not merely to general warming and the spread of friendly environments to higher latitudes.

Cenozoic stratigraphic succession in southeastern Australia

Strata of Cenozoic age occur around the southern margin of Australia as thin and discontinuous outcrops, interpolated and fleshed out by economic exploration onshore and offshore. The neritic strata fall into four sequences or allostratigraphic packages of (I) Paleocene — Early Eocene, (II) Middle Eocene — Early Oligocene, (III) Late Oligocene — Middle Miocene, and (IV) Late Miocene — Holocene age: a four‐part pattern that can be seen also in the flanking pelagic and terrestrial realms including regolith deep weathering. Problems of correlation and age determination (predominantly biostratigraphic) have included biogeographical constraints (endemism in neritic molluscs and terrestrial palynomorphs, mid‐latitude assemblages in calcareous plankton), and slow progress in magnetostratigraphy and chemostratigraphy. Sequence I largely repeats the Cretaceous siliciclastic‐coal, marginal‐marine facies (carbonate‐poor, with marine and non‐marine palynomorphs and agglutinated foraminifers) punctuated by marine ingressions with microfaunas and sparse macrofaunas. Sequence II contains the first carbonates in the region since the Palaeozoic and the most extensive coals of the Cenozoic anywhere. Sequence III contains the most extensive neritic carbonates and the last major coals. Sequence IV is more strongly siliciclastic than the two preceding. Each of these four second‐order entities (107 years duration) comprises third‐order packages each with an unconformity and marine transgression. These packages hold true right along the southern Australian margin in the sense that the hiatuses and transgression do not display significant diachroneity at the relevant time‐scales (105–106 years). Recognised, delimited and correlated independently of the putatively global Exxon sequences, they are remarkably consistent with the latter, thereby providing a significant regional test. There are two widespread emphases on southern Australian geohistory and biohistory: (i) to regard the regional story as part of the global story of accreting continents, an expiring Tethys, and an episodically cooling planet; and (ii) a somewhat contrary emphasis, with the region being a special case of rapid longitudinal motion towards the equator. Both emphases are plausible with the former being the more heuristic. The stratigraphic record is strongly punctuated, the four sequences being separated by both tectonic and climatic events. Thus: the sequence I/II gap involved extensive plate‐tectonic reorganisation and a new spreading regime from ca 43 Ma, coevally with early growth of Antarctic ice; in the II/III gap, deformation in marginal basins is coeval with a global low in cooling, large ice sheet and falling sea‐level to ca 30 Ma; and the III/IV gap is marked by widespread cessation or contraction of stratal accumulation and withdrawal of thermophilic taxa coevally with the major expansion at ca 14 Ma of the Antarctic ice sheet, onset of intense canyon cutting, and plate‐wide basin inversion.

Australian Cretaceous shorelines, stage by stage

Abstract Twelve maps of shoreline configuration are presented, ten of which are continent-wide; the maps represent critical times in the sea-level history of Australia. Before the Aptian, most of the continent was emergent. The most extensive coverage by the sea, which occurred in the Aptian-Albian, was followed by Late Cretaceous fluctuations and general withdrawal. The development of interior seaways and basins marginal to the continent relates to eustasy as well as to local balance between sedimentation and subsidence. For basins at the continental margin, vertical tectonics relating to continental separation are significant.

Silica burp in the Eocene ocean

The Eocene was a time of greatly increased silica accumulation in the ocean, and the peak was in the early middle Eocene at about 50 Ma. The responsible geohistorical configuration included the following elements: extensive volcanism about 4 m.y. earlier, as part of the Chron 24 plate reorganization; early Eocene warming, with deep weathering to high latitudes and accumulation of the released silica in a sluggish ocean; and sharp cooling in the earliest middle Eocene, stimulating oceanic upwelling and biosilicification. It is possible, on the evidence of carbon and oxygen isotopic patterns, that the trigger for the exhalation of silica was a reverse greenhouse effect.

The Tertiary of Australia: Foraminiferal overview

The important problems in Tertiary geohistory are circumglobal and concern events of the order of 106 years duration. Those problems are essentially tectonic and climatic and their formulation arises out of stratigraphic configurations, continental and oceanic. Their solutions depend on international and interdisciplinary correlation and calibration. This review of the Australian Tertiary record is based on planktonic foraminiferal biostratigraphy and its evidence for recognizing four sequences with intervening hiatuses, regression or non-identification — a broadly isochronous pattern which is transcontinental and which transcends local tectonic and sedimentary environments (facies): (1) Paleocene to Early Eocene: bracketed by regression and hiatus at the Cretaceous/Tertiary and Early/Middle Eocene boundaries; maximum extent during Late Paleocene time (Zone P4). (2) Middle to Late Eocene: transgression in Middle Eocene (Zone P12) and most extensive in Late Eocene; Early to Middle Oligocene relatively very restricted in distribution. (3) Late Oligocene to Middle (Late) Miocene: the most prominent episode of transgression and basin formation, beginning in Zones N3–N4, reaching maximum extent in the vicinity of the Early/Middle Miocene boundary but punctuated by a widespread hiatus in Zones N8–N9; concluding with restricted record of the later Middle to Late Miocene. (4) Latest Miocene to Quaternary: more varied, facies-wise, being the youngest and also the time of pronounced vertical tectonism; the record begins high in Zone N17. The distribution of “larger” benthonic foraminiferal assemblages in terms of the P- and N-zones reveals a series of short-lived, extratropical excursions. Five Eocene and four Oligocene—Miocene excursions show a good fit, on the whole, with oxygen isotope curves. This mutual correlation supports (a) the reality of rapid, far-reaching and bipolar climatic fluctuations, and (b) the resolving power of isotopic and foraminiferal—biogeographic data. Most of the interfaces of the Indo-Pacific, larger foraminiferal “Letter Stages” can be related to eustatic and climatic changes. From the reasonably coherent links between rapid climatic fluctuation and planktonic foraminiferal biostratigraphy, I conjecture that “laterite” and “silcrete” have alternated several times as the dominant duricrust of the Australian Tertiary landscape. All of the foregoing relies more on correlation against the P- and N-zones than on their calibration. A survey of circum-Australian tectonism in a frame-work of biostratigraphic, radiometric and geomagnetic chronology suggests that it is broadly isochronous and episodic (outstanding problems — time of “obduction” and clarification of mid-Tertiary metamorphism — notwithstanding). Some form of tectonic climax close to the Paleocene/Eocene and Eocene/Oligocene boundaries and the Middle Miocene is succeeded by plate tectonic readjustment and the conclusion of the respective sequence. This is the primary control on climatic change and, via shelf/basin carbonate fractionation, on change in oceanic CCD.

Bryozoan growth habits; classification and analysis

Bryozoans are an important part of the benthic marine fauna in a wide variety of modern environments and are found in rock forming abundance in a number of settings throughout much of the Phanerozoic. Bryozoologists and nonspecialists have grouped taxa into colonial growth forms (e.g., erect fenestrates or encrusting sheets), both to simplify analyses and because correlations exist between some colony growth forms and the environmental conditions in which the organism lived. These correlations allow for the possibility of paleoenvironmental analyses based on skeletons alone. Existing bryozoan colonial growth from classifications do not, however, fully exploit the ecological information present in colony form. A new scheme is proposed here (Analytical Bryozoan Growth Habit Classification), which provides a list of colony-level morphological characteristics for bryozoan growth habits. This differs from previous approaches to bryozoan growth form analysis in that it is a classification of growth habit characteristics rather than a classification of morphological groups as such. The classification is based on eleven character classes, which describe the orientation of the colony and its occupation of, and placement in space. The overall colony shape is described based on the arrangement of modules in colonial growth. This classification provides a common ground for systematic comparison of character states among varied bryozoan growth habits. This approach allows for the evaluation of correlations among observed morphological character states and specific environmental conditions in which they develop. In addition, these growth habit characters can be used to recognize, characterize, evaluate, and apply more traditional growth form groups in broader studies.

Bryozoan colonial growth-forms as paleoenvironmental indicators; evaluation of methodology

Bryozoans have played a significant ecological role in many shallow marine benthic communities since the Ordovician and are important contributors to carbonate sediment production in many modern cool-water marine environments. Correlation between bryozoan colonial growth forms and environments in which the organisms lived allows for the application of growth forms as paleoenvironmental indicators. This can be done as either (1) a characterization of regional environmental or distributional data within a comprehensive study; or (2) as a predictive tool applied in an unknown setting using limited data. A number of workers have demonstrated this potential in biological, paleontological, and sedimentological studies. Growth-form distributions established independently from, and later compared to, environmental factors provide for the greatest predictive utility. Problems encountered in methodology need to be addressed before bryozoan colonial growth forms can achieve their full potential as paleoenvironmental indicators. Methodological problems include those associated with specimen abundance versus species richness, numeric versus volumetric frequency, relative versus absolute abundance, and changes within growth forms among localities versus changes among growth forms within localities (facies). A procedure is proposed that combines species richness and specimen abundance, as well as information about distributions within growth forms and within localities, into a single, comparable data set. An example is provided using bryozoans from the cool-water Lacepede Shelf of southern Australia.

Palaeoceanographic significance of recent foraminiferal biofacies on the southern shelf of Western Australia: a preliminary study

The southern shelf of Western Australia lies close to the Subtropical Convergence, in a region strongly influenced by the warm Leeuwin Current from the north and the cold, more massive Western Australian Current from the south. Fresh Holocene and relict Pleistocene foraminiferal specimens are mixed in dredged sediment samples, similar in composition to those from other parts of the southern Australian margin. The Holocene planktonic assemblages are dominated in the west by subtropical forms (Globigerinoides trilobus s.l., Globorotalia menardii and Neogloboquadrina dutertrei ) and in the east by the temperate species Globorotalia inflata. Three Holocene benthic assemblages are distributed from the inner shelf to the upper slope, also showing a strong longitudinal gradation from west to the east. The change from warm-water assemblages to temperate assemblages is progressive and continuous, but the southwest corner is marked by the southerly limit of some larger benthic taxa including Heterostegina, and the area off Esperance is the furthermost extent of an abundant, though patchy, living Amphisorus‐Marginopora association. This W! E gradation indicates that the Leeuwin Current has played a key role in influencing the distribution of foraminifera as from at least the last interglacial, because this pattern exists not only in the Holocene but also in the relict foraminiferal biofacies.

The later Eocene transgressions in southern Australia

Correlations based mostly on planktonic foraminiferal assemblages indicate four continent-wide transgressions: two in the late Middle Eocene (high Zone PI2; near Zone P14/P15 boundary) and two in the Late Eocene (near top Zone P15; low Zone P17). The first (Wilson Bluff) is part of the Indo-Pacific Khirthar Transgression at the boundary between the Laramide and Himalayan global tectonic regimes; the Khirthar was an ‘immediate“ response to renewed seafloor spreading (geomagnetic Anomaly 19) after India/Asia collision. However, the later transgressions were more widespread in this region. Although data are patchy and correlations are somewhat strained, there is some evidence that transgressions fall between lowstands in the incomplete global curve and that they are coeval with isotopic and biological (oceanic, neritic, continental) evidence for warming. The later Eocene as a whole marks a significant intervening reversal of the global climatic deterioration that occurred across the Early/Middle Eocene and E...

Stratigraphic record of Early Tertiary oceanic and continental events in the Indian Ocean region

Abstract Primarily on the evidence of planktonic foraminifera, correlations are made among oceanic sections (northern and eastern Indian Ocean and southwest Pacific Ocean) and composite continental successions (India—Pakistan, Australian region). The paper discusses the stratigraphic patterns thus perceived and their possible meaning in terms of chronological relationship to tectonic, oceanic and climatic/biogeographic events. In the oceans, the stratigraphy includes: hiatus across the Paleocene/Eocene boundary; a sharply defined but allochronous top to the Eocene “cherts” (early Middle Eocene, Indian; Middle/Late Eocene boundary, southwest Pacific); hiatus and sporadic accumulation of pelagic carbonates in the Middle and Late Eocene. Events in the continental record are similarly widespread: regardless of local environment one can recognize two sequences, of Paleocene—Early Eocene and Middle—Late Eocene age respectively and separated by regression, hiatus or non-identification across the Early/Middle Eocene boundary. The stratigraphy reflects isochronous, “sudden”, platewide geohistory. The available evidence indicates plate-tectonic rearrangement at about the time of the Paleocene/Eocene boundary with responses then and subsequently in the stratigraphic, climatic and foram-iniferal-biogeographic record.