David M. McKirdy

A chemostratigraphic overview of the late Cryogenian interglacial sequence in the Adelaide Fold-Thrust Belt, South Australia

Within the Neoproterozoic sequence of the Adelaide Fold-Thrust Belt, South Australia, two of the most severe ice ages in Earth history (Sturtian and Marinoan) are recorded in the glacigenic rocks which mark the base and top of the 4.5 km-thick Umberatana Group, the focus of the present chemostratigraphic study. Rock units from three drillholes and ten measured outcrop sections, located in the southern and central Flinders Ranges and on the Stuart Shelf, were sampled for isotopic analysis of their co-existing calcite (δ13Ccal), dolomite (δ13Cdol) and kerogen (δ13Corg). Strontium and sulphur isotopic analyses were also run on selected samples. The sample sites are in tectonic domains where regional metamorphism has been insufficient to significantly alter the isotopic composition of the kerogen. The resulting data, when integrated with the lithostratigraphy of the Umberatana Group, yield one of the most complete records of secular variation in the C-isotope signature of the late Cryogenian ocean between ∼750 and 680 Ma. Its composite δ13Ccarb curve begins with a sharp positive excursion (−3 to +1.5‰ in the dolomitic Tindelpina Shale which caps Sturtian glacial diamictites (Wilyerpa Formation). Then follows a steady climb through the shoaling upward Tapley Hill Formation reaching +4‰ in the Brighton Limestone and culminating in a plateau of 13C enrichment within the platformal carbonates of the Etina Formation (+8 to +10‰). The upper part of the temporal trend shows a decline to +6 to +7‰ in the stromatolite reef facies of the Enorama Shale; and a dramatic drop to −8 to −9.5‰, followed by a recovery to –3‰, in the youngest carbonate unit (Trezona Formation) beneath the Marinoan glacigenic Elatina Formation. The latter is capped by a thin dolostone, the Nuccaleena Formation, which records a consistent up-section decrease in δ13C (−1 to −3‰). Superimposed on this overall pattern of C-isotopic variation are second and third-order fluctuations attributable to eustatic sea-level change. Pyritic sulphur in the lower Tapley Hill Formation (δ34S=+9 to +40‰) is distinctly heavier than that in the underlying Wilyerpa Formation (+4 to +20‰). Minimum 87Sr/86Sr values obtained from the Brighton Limestone (0.7071) and Etina Formation (0.7076) are consistent with their late Cryogenian age. This Sturtian to Marinoan interglacial succession is more complete than equivalent sequences elsewhere, and thus provides a useful reference for global chemostratigraphic correlation. Notable features of its C-isotope curve are the strong post-Wilyerpa positive excursion, also seen in key sections from Svalbard and western Mongolia; the ‘Etina plateau’ of 13C enrichment which is remarkably similar to maxima in the terminal Proterozoic C-isotope records of western Mongolia, north-western Canada and Brazil; and the spectacular depletion of 13C in the lower Trezona relative to similar carbonate facies in the underlying Etina Formation and Enorama Shale. A preglacial offset in δ13Ccarb values of such proportions (∼14‰) is almost unprecedented, except perhaps for that recently documented in the upper Ombaatjie Formation beneath tillites of the Ghaub Formation, Otavi Group, Namibia. It represents a key piece of evidence in the case presented by the proponents of a Neoproterozoic ‘snowball’ Earth.

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.

Early Organic Evolution

This volume presents both an overview and final synopsis of the work performed between 1978-1989 by an international task force of geologists, geochemists, and paleontologists rallied around the charter of IGCP-Project 157 (Early Organic Evolution and Mineral and Energy Resources). Apart from several review papers summarizing the state of the art in selected facets of the subject, the bulk of the contributions reflects current research activities in the field (including modern analogs of ancient processes) ranging from early evolution of life on this planet to questions of Precambrian weathering, metallogeny and petroleum formation. Dealing with processes at the interface between evolutionary biology, organic geochemistry and economic geology, the book gives an updated summary of the current state of one of the most challenging frontiers in Earth Sciences.

Oil generation by igneous intrusions in the northern Gunnedah Basin, Australia

Abstract The origin of extensive oil staining, recorded over a 120 m interval of the Jurassic Pilliga Sandstone in the fully-cored Bellata-1 well, northern Gunnedah Basin, has been investigated using conventional organic geochemical techniques. The borehole intersects 451 m of Permian and Triassic strata, overlain by 640 m of Jurassic and Cretaceous sediments. Igneous intrusions occur mainly within the lower part of the Triassic Napperby Formation where the vitrinite reflectance profile shows the effect of significant local heating ( R v up to 2.43% within the intruded interval). Lower reflectance values ( R v =0.57–0.74%) occur in the unaffected parts of the Permian sequence, with even lower values in places due to either marine influence or liptinite-rich lacustrine organic facies. Hydrocarbon biomarkers in the oil stain in the Pilliga Sandstone indicate its derivation from a higher plant-rich, shaley source. A methylphenanthrene index of 1.48 (calculated vitrinite reflectance R c =1.29%) for the oil suggests expulsion from a late-mature source rock. None of the potential source rock samples analysed appear to have attained such a maturation level, except those from the lower part of the Napperby Formation affected by intrusions. Source-specific biomarkers from this formation show several parallels with those of the oil stain, but differ from those in the Permian Goonbri and Maules Creek Formations and the upper parts of the Napperby Formation. A Jurassic source can also be excluded based on the immaturity of that part of the section and the absence of Jurassic-specific biomarkers in the oil stain.

The record and stratigraphic implications of organic-walled microfossils from the Ediacaran (terminal Proterozoic) of South Australia

Two assemblages of organic-walled microfossils have been recognized in drillcore samples from the late Proterozoic Rodda Beds in the eastern Officer Basin, South Australia. The fossils include tube-like remains and large, simple and sculptured acritarchs. Lithostratigraphic studies and seismic information, in conjunction with previous (albeit limited) acritarch finds, allow local correlation of the Rodda Beds with Ediacaran or terminal Proterozoic sequences in the northern Adelaide Fold Belt (site of the nominated Ediacaran stratotype). The new palynofloras are comparable with acritarch assemblages in the Amadeus Basin of central Australia, and suggest tentative correlations with sequences in China and the U.S.S.R. The presence of isotopically heavy marine carbonate in the lower fossiliferous horizons of the Rodda Beds ((PCPDB = +3 to +6%0) is consistent with isotopic data from the equivalent interval in China. In contrast, the upper fossiliferous strata occur higher in the Rodda Beds where carbonate is significantly lighter (# 13 CPDB = — 1 to + 3%o).

Ediacaran ice-rafting and coeval asteroid impact, South Australia: insights into the terminal Proterozoic environment

Isolated quartzose pebbles, clusters of quartz granules, orthogonal aggregates of poorly sorted quartzose coarse sand, and ovoid pellets (≤2 mm long) of quartz silt occur in hemipelagic marine mudstone of the mid-Ediacaran Bunyeroo Formation exposed in the Adelaide Geosyncline (Adelaide Rift Complex), and ovoid pellets of quartz silt in cores of the correlative marine Dey Dey Mudstone from deep drillholes in the Officer Basin, South Australia. This detritus is interpreted respectively as dropstones, dumps, and frozen aggregates dispersed by sea ice possibly of seasonal origin, and till pellets transported by glacial ice. The ice-rafted material in the Bunyeroo Formation only has been found <10 m stratigraphically below and above a horizon of dacitic ejecta related to the 90 km diameter Acraman impact structure in the Mesoproterozoic Gawler Range Volcanics 300 km to the west. Furthermore, till pellets have been identified 4.4 to 68 m below distal Acraman ejecta in the Dey Dey Mudstone >500 km northwest of the impact site. The Acraman impact took place at a low paleolatitude (∼12.5°) and would have adversely affected the global environment. The stratigraphic observations imply, however, that the impact occurred during, but did not trigger, a cold interval marked by sea ice and glacial ice, although the temporal relationship with Ediacaran glaciations elsewhere in Australia and on other continents is unclear. Release from the combined environmental stresses of a frigid, glacial climate near sea-level and a major impact in low latitudes may have been a factor influencing subsequent Ediacaran biotic evolution.

Oceanic transport of fossil dammar resin: The chemistry of coastal resinites from South Australia

Abstract Fragments of resin and fossilised resin (resinites) occasionally wash ashore along the southern Australian coast. Chemical and isotopic analyses were carried out on a suite of coastal and reference resinites to determine the likelihood of a local, as opposed to a distant, origin. All of the coastal resinites were found to contain a polymer based on the sesquiterpenoid cadinene skeleton and were markedly different to the diterpenoid resinites found in local Victorian coals. The coastal resinites closely resemble both fossil and recent dammar resin—a material associated primarily with the tropical angiosperm hardwoods of Southeast Asia, and one which has no known Australian source. Minor variations in the composition of our resinite samples are attributed to differences in their thermal history. These findings confirm the viability of long-distance oceanic transport, not only for the resinites but also for the waxy bitumens that strand along the same coastline. Analytical data on the coastal resinites also help to clarify the role of dammar resin in generating bicadinanes—a class of source and age-specific triterpanes found commonly in the Cainozoic oils and sediments of Southeast Asia.

Precambrian Petroleum: A Decade of Changing Perceptions

Multidisciplinary studies during the last decade have identified hitherto unrecognized controls on the distribution and composition of organic matter preserved in the Earth’s Precambrian sedimentary rock record (> 0.54 Ga). Carbonaceous shales and cherts containing isotopically inhomogeneous kerogen of prokaryotic origin occur sporadically in Archean metasedimentary sequences. The oldest recognizable petroleum source rocks were deposited in anoxic microbasins where methylotrophic bacteria played a key role in the production of oil-prone organic matter highly enriched in 12C. These late Archean sulfide-rich, black shales (TOC up to 15%) are now thermally overmature. During Early Proterozoic time (2.5–1.6 Ga) epeiric seas developed, allowing widespread epicratonic sedimentation and burial of organic matter from an expanding planktonic biomass. The molecular fossil (or biomarker) record indicates that eukaryotic algae became significant kerogen precursors at least 1.7 Ga ago. Along the southeastern margin of the Canadian Shield Aphebian black argillites (or, at Elliot Lake and Blind River, stratiform uraniferous kerogen layers in the Matinenda Formation) were probable sources for widespread vein pyrobitumens found in ~ 2 Ga-old rocks of the Great Lakes region, USA and Canada. Shungite, another epigenetic pyrobitumen in the Jatulian Series of Karelia, USSR, originated from interbedded carbonaceous, pyritic shales. Indigenous Early Proterozoic dry gas was encountered by mineral drillholes in the Pechenga Series, Kola Peninsula, USSR, and McArthur Group, Northern Territory, Australia. Optimum conditions for the formation of thick, extensive petroleum source beds were first attained in Early Proterozoic intracratonic or rift-related tectonic settings, as exemplified by the organic-rich mudstones of the Franceville Basin, Gabon (2 Ga), and the Pine Creek Geosyncline (1.8–2.2 Ga) and McArthur Basin (1.6–1.8 Ga), northern Australia. These ancient source rocks (or carbonized residues of their expelled crude oil) are variously associated with U, Au, Pb, Zn, Ag and Cu mineralization. Among the oldest sediments currently being explored for hydrocarbon resources are those of the 1.4-Ga-old Roper Group, McArthur Basin, and the 1.1-Ga-old Oronto Group, Mid-Continent Rift, USA. Here, extensive organicrich shales and siltstones were deposited in marine and lacustrine environments, respectively; aliphatic Type I–II kerogen is of appropriate thermal maturity for oil generation; and oil shows are common. The Late Proterozoic (1.0–0.54Ga) was a time of prolonged oceanic anoxia and high rates of accumulation of organic matter in marine sediments (TOC contents locally as high as 20–30 %). Rich source rocks and the world’s oldest commercial oil and gas reserves, much of them in giant fields, occur on the Siberian Platform (Lena-Tunguska region) and Arabian Shield (Oman) where late Riphean and Vendian siliciclastics and carbonates pass upwards into thick evaporite deposits which acted as regional seals. Peritidal dolomitic carbonates of Sinian age in the Sichuan Basin, southeastern China, include basal argillaceous source beds and the main reservoir of the giant Weiyuan gas field. Proterozoic oils and pyrobitumens have light carbon isotopic signatures (δ13C = — 30 to — 45%o). The unusual biomarker distributions of the oils and associated organic-rich sediments (where they contain well-preserved kerogen) cast new light on the membrane and cellular lipids of Proterozoic microbial ecosystems.

Fernenes and other triterpenoid hydrocarbons in Dicroidium-bearing Triassic mudstones and coals from South Australia

Tandem sector mass spectrometry of the aliphatic hydrocarbon fraction of selected Late Triassic sub-bituminous coals, mudstones and plant fossils from the Leigh Creek coalfield, Telford Basin, South Australia, has revealed three pseudohomologous series of triterpenoid hydrocarbons. Two of these series have been identified as fern-8-enes (C29-C31) and fern-9(11)-enes (C29-C30). The third series (C29-C31) comprises saturated hydrocarbons tentatively identified as fernanes. These hydrocar- bons appear to be related to pteridosperms (seed ferns) of the genus Dicroidium, fronds of which are preserved throughout the sedimentary sequence. # 1998 Elsevier Science Ltd. All rights reserved

Molecular fossils in coastal bitumens from southern Australia: signatures of precursor biota and source rock environments

Abstract Weathered waxy crude oil and black asphaltite (tar) are regularly washed ashore along the coastlines of South Australia and western Victoria. These bitumens generally strand on medium to high-energy, sandy beaches in west or southwest facing bays. Some very weathered bitumens have been discovered in Holocene dune deposits up to 200 m from the present-day coastline. Differences in the sulphur content and carbon, hydrogen and sulphur isotopic compositions of the coastal bitumens suggest that they belong to at least four oil families, one of which may be emanating from submarine seeps along Australias highly faulted southern continental margin. The remainder appear to be flotsam carried south from S.E. Asia, possibly Sumatra. The waxy bitumens (Families 1–3) range from paraffinic to aromatic-intermediate in bulk composition and contain up to 2.6% S. Their density (13–38° API) dictates that they are near-surface drifters. They have low pristane/phytane ratios (pr/ph ⩽ 2), abundant 4α-methyl steranes (C 30 with 24-ethyl substitution) and, in most cases, high concentrations of botryococcane. This biomarker assemblage indicates accumulation of freshwater algal source material (including the remains of Botryococcus sp. and dinoflagellates) under anoxic to suboxic conditions. The additional presence of 28,30-bisnorhopane, dinosterane and 24- n -propylcholestane among the molecular fossils in Family 3 bitumens suggests derivation from a lacustrine organic facies subject to marine incursions; or, alternatively, mixing in the reservoir of two end-member (lacustrine and marine) oil types. Trace amounts of oleanane, and isomeric bicadinanes in the waxy bitumens constrain the age of their source rocks (Late Cretaceous or younger) and also preclude their local derivation; an Indonesian origin is considered likely, implying long-distance transport by surface ocean currents. The Family 4 bitumens (∼ 5° API) are bottom drifters. These sulphur-rich (3–6% S), aromatic-asphaltic crudes have molecular signatures that include: pr/ph ∼ 1; C 27 > C 29 > C 28 > C 30 desmethyl steranes; dinosterane dominant over 4α-methyl-24-ethylcholestane; an enhanced concentration of 28,30-bisnorhopane relative to hopane; and the absence of 2- or 3-methylhopanes. A distal, anoxic marine facies of the Late Cretaceous Belfast Mudstone in the eastern Otway Basin, or its lithostratigraphic equivalent in the Duntroon Basin, are possible local sources of the asphaltic bitumens.