Department of Earth Sciences

Vertical burrowing is a biological activity deemed to have evolved during the earliest Cambrian, the older soft-bodied fossils of the Ediacaran Period, that thrived between 575 and 541 Ma, seemingly being restricted to occupying the seafloor/ocean interface. Yet at two levels within the pre-Ediacaran sequence of the Amadeus Basin, central Australia, are vertical tubular features that have pre- viously been reported as possibly of biogenic origin: a suite of such tubes occurs within the Areyonga Formation, dated at about 700 Ma, while two other tubular structures occur within the older Heavitree Formation that is perhaps 970–950 million years old. Should these structures prove, on re-examination, to indeed have been biologically formed, then vertical burrowing initially evolved possibly as much as 435 million years earlier than previously believed and these features would make the Amadeus Basin an isolated crucible of, seemingly, two evolutionary trials of vertical burrowing, albeit trials that ultimately failed for reasons that are discussed. Additionally, the same two stratigraphic levels each contain another structure that was also initially described as of biogenic origin, one being a possible body-fossil.

Review and analysis of 1332 gas chromatography (GC) n-alkane traces of oils from the Cooper and Eromanga basins indicate the shape of any GC trace profile is primarily controlled by the degree of organic maturity (early, peak or late) at which the oils were expelled from the parent source rock, rather than indicating the depositional environment, and hence organic composition, of that source rock. The depositional environment of a source rock may still be inferred, however, from the position of the n-alkane maximum on the GC traces of early expulsion oils in association with the pour point of the oil. Departures of GC trace profiles from the standard early, peak or late expulsion profiles can indicate mixing of oils of different maturities, while variations in the GC trace profiles of oils within adjacent reservoir units may indicate phase separation of the parent liquid, or possible seal breach by an accumulation that exceeds the capacity of its overlying seal. KEY POINTS 1. GC trace profiles of 1332 oils from across the Cooper and Eromanga basins of central Australia have been reviewed. 2. Organic maturity, rather than organic composition, of the parent source rock controls the shape of any GC trace profile. 3. All early maturity oils display a consistent GC trace profile shape that is different from all peak maturity oils and different again from all late maturity oils. 4. Depositional environment of the source rocks within a basin can be inferred from the relative pour points of the resultant oils.

Recently acquired seismic data in the eastern portion of the Amadeus Basin of central Australia have enabled disconformities at the base of both the Sturt and Elatina glaciogenic sequences within the Neoproterozoic succession to be mapped on a semi-regional scale. On a more detailed scale, along the shallowly shelving eastern flank of the basin, seismic data have revealed features that demarcate the transition from a grounded to floating, or possible absent, Sturt ice-sheet. Coincident with this transition is an extensive mound-like accumulation of glacial debris, some 10 to 15 km wide, 300 m high and extending at least 150 km subparallel to the northeast-southwest-oriented basin margin. This glacial debris entered the basin subglacially through meandering tunnel valleys, one of which can be mapped seismically.

The Seychelles microcontinent became isolated between the Somali\ Mascarene and Arabian basins of the Indian Ocean as a result of the Mesozoic fragmentation of Gondwana[ Major rifting events occurred during the Triassic!Middle Jurassic and Late Cretaceous "Cenomanian!Santonian and Maastrichtian# during which shaly source rock facies accumulated in principally marginal marine:deltaic environments[ Between these times\ post!rift passive margin deposition within restricted to open marine environments produced shaly source rocks during late Middle Jurassic!Early Cretaceous\ Campanian!Maastrichtian and Paleocene times[ Recent geochemical analysis of cuttings from the Seagull Shoals!l well has identi_ed an oil!prone liptinitic "Type II# coaly shale within early Middle Jurassic abandoned deltaic deposits[ This coaly source rock is regionally developed\ having also been identi_ed in the Majunga and Morondava basins of Madagascar[ Oil!prone Type II organic matter has also been identi_ed in the Owen Bank A!0 well within restricted marine shales of late Middle Jurassic age[ These shales are part of a thick post!rift source rock sequence that extends into the Early Cretaeous and is in part correlative with the proven Late Jurassic Uarandab Shale of Somalia[ Analysis of Campanian marine shales from Reith Bank!0 well identi_ed signi_cant dilution of total organic carbon content in composite\ compared to picked\ well cuttings samples[ This _nding supports a published inference that these post!rift shales have source rock potential[ The very limited drilling of the various Jurassic:Cretaceous source rock sequences in Seychelles "only three penetrations\ all within a 02 km radius# indicates that they span the entire maturity window and that some have generated hydrocarbons[ However\ the petroleum system associated with the Triassic!Early Cretaceous rift:drift sequence has been tested only by three invalid wells at the western extremity of the Seychelles Plateau\ whilst that associated with the Late Cretaceous rift:drift sequence remains untested[ Þ 0887 Elsevier Science Ltd[ All rights reserved[

Tarballs have been found stranded as traction deposits on a beach on northwest Mahe Island, Seychelles, with every ebbing tide. Weekly collections from mid-1993 to mid-1994 gathered nearly 4750 samples of two types of tar: a dominant black malleable tar and a less common brownish-black hard tar. Both tars represent moderately degraded extracts of crude oils seemingly derived from mature source rocks dominated by a combination of oil-prone phytoplanktonic and algal organic matter over gas-prone land-plant organic matter. Source rock deposition occurred principally in paralic to open-marine environments. The presence of minor oleanane and bicadinane, and a general preponderance of diasteranes and tricyclic terpanes in the hard tar suggest derivation from a Late Cretaceous clastic source rock with possible carbonate inter- beds. By contrast, no samples of the malleable tar contain oleanane or bicadinane, but all contain significant norhopane and tetracyclic terpanes, which, along with significant sulfur and low diasterane contents, indicate derivation from a carbonate source rock of perhaps pre-Late Cretaceous age. Both these tars differ markedly from a tar that is stranded only on Coetivy Island that contains minor amounts of steranes, oleanane, bicadinane, and botryococcane, but has high pristane and tricyclic terpane contents, suggesting derivation from a late Cretaceous clastic deltaic source rock. All three tar types can be related to depositional environments that developed during the multiphase rift-drift origin of the Seychelles microcontinent from east Africa, Madagascar, and India. Additionally, maturity and limited geochemical data from proven and potential source rocks present in the offshore wells enable the malleable and hard tars to be equated with specific units in the Seychelles stratigraphy; the malleable tar is seemingly derived from a Middle Jurassic carbonate of Tethvan affinity and the hard tar possibly originated from either Campanian or Maastrichtian-Paleocene shales. These tars are thus concluded to
be natural seepage products derived from source rocks indigenous to Seychelles, and their persistent beach stranding significantly reduces the risk of hydrocarbon exploration in this frontier province.

The Amirante ridge/ trough complex developed along the Late.
Cretaceous transform boundary that separated the Sevchelles/India
and Madagascar/ Somali Basin plates. Motion between these plates was
complex, comprising sinistral N-S strike-slip movement coupled and coeval with counter-clockwise rotation induced when seafloor
spreading developed in only the southern portion of the transform. The
overall morphology of the complex comprises a series of arcuate ridge
and trough segments. These segments were successively produced by
tectonic and volcanic activity within the zone of migrating plate contact adiacent to the rotational pivot where compression was replaced by
extension along the transform boundary. In the extensional regime to the south of this contact zone the Mascarene oceanic basin developed,
whilst in the compressional zone further north island arcs developed and the ophiolites of Baluchistan were obducted from the Somali Basin

The Seychelles region developed as a unique continental sliver through the three phases of rift tectonism that, since the Late Permian, created the western Indian Ocean. The history of rifting in Seychelles can perhaps be traced back to ~ 700 Ma when the granites that form the central islands were emplaced. The low §180 of the high-temperature minerals within these slightly alkaline granites on Mahe indicate an intraplate origin, and the fracturing of the lithosphere consequent to their emplacement created a locus ofweakness for future rift tectonism to exploit. Thermally induced extensional rifting, initiated during the Carboniferous and rekindled in the Late Permian (~ 225 Ma), eventually severed Super-Gondwana in the Middle Jurassic (~ 160 Ma) into East and West Gondwana. A proto-Indian Ocean, the Somali Basin which separated East and West Gondwana, ceased to expand in the Early Cretaceous (~ 115 Ma). This rift / drift event is recorded in Seychelles by block-faulted sequences of distal fluviatile, through near-shore to open marine deposits along its western margin and possibly at Farquhar. Within East Gondwana, transform rifting then separated Madagascar and Seychelles/India in the Late Creta- ceous. At 84 Ma a spreading ridge developed between these blocks forming the Mascarene Basin as a new arm of the evolving Indian Ocean. A thick progradational wedge of sediment, initially alluvial followed by lacustro-marine and eventually open marine, resulted from this event along the southwest Seychelles margin. Finally, hotspot-related thermally induced rifting, causing block-faulting and fluviatile to marine deposition, eventually separated Seychelles from India at the Cretaceous/ Tertiary boundary. This separation coincided with the Deccan volcanic event and the development of the Carlsberg Spreading Ridge, which replaced the Mascarene Spreading Ridge and initiated the opening of the northwestern Indian Ocean Basin. Sedimentation on the Seychelles microcontinent throughout the Tertiary produced predominantly shelf carbonates.

A revised nomenclature is proposed for the dominantly clastic sequence of sediments comprising the lower Wilpena Group (late Adelaidean) with in the Adelaide fold belt. This sequence. herein termed the Brachina Subgroup, has sharp, locally disconformable lower and upper boundaries. between which formations are defined on the basis of mappable lithozones.
T'he Brachina Subgroup displays an upward coarsening sequence which represents a complete phase fo regressive sedimentation, during which energy conditions became increasingly vigorous. As the regression progressed, depositional regimes changed from low energy settling (from suspension) in a subtidal environment to high energy traction deposition
in deltaic plain, barrier-bar and intertidal mud-, and sand-flat environments. Toward the peak of regression at least two islands emerged in the central portion of the basin producing local high energy fluvial to marginal marine conditions.

The Late Precambrian Brachina Subgroup was deposited in the Adelaide Geosyncline in response to a massive influx of coarse detritus generated on the Gawler Craton to the west by a phase of tectonism and volcanic activity. The palaeogeographic configuration of the geosyncline at the time of initial detrital influx was that of a vast aerobic tidal shelf in the western and central regions skirting a shallow anaerobic basin in the eastern region.
The sediment debouchment accumulated as an intertidal deltaic pile which prograded across the western region, halting at the gentle palaeoslope leading into the deeper eastern region. Wave activity acted upon the leading edge of the delta creating a fringing barrier-bar system. Delta characteristics indicate this delta to have been a fluvial and tide modified, wave-dominated system fed by stable distributary channels.
Basin shallowing accompanied the continued sediment influx and the eastern and central regions shallowed to form an extensive intertidal shelf. Tidal activity thus replaced wave action as the dominant influence on delta morphology, and the delta evolved to an unbarred fluvial modified, tide-dominated system fed by braided distributary channels.
Delta development ceased when tectonic instability directly affected the western margin of the basin, causing partial erosion of the delta with reworking and dispersal into a vast, thin intertidal sandflat deposit throughout the central region of the basin.

Enigmatic, subcylindrical to conical, sediment-infilled structures occurring in the Trezona Formation and the lower and upper parts of the Moorillah Formation of the late Precambrian of the central Flinders Ranges, South Australia, resemble certain fossil burrows such as Berganeria (Prantl), and also late Precambrian body fossil remains including the sack-shaped, soft-bodied form Ernietta (Pflug) from South West Africa (Namibia). The structures occur up lo 3,300 m stratigraphically below the well known Ediacara assemblage. The lack of disruption or vents in their matrix seemingly negate the possibility that they are water escape structures, but studies of their infilling and literature research indicate that their true origin is almost certainly inorganic, their resemblance of various fossil remains being fortuitous. 'Their recognition emphasizes the apparent non-occurrence of metazoan remains below the Ediacara assemblage in the local rock record, lending negative support to recent theories postulating either late evolution of the Metazoa or rapid diversification of animal life in the latest Precambrian.

ASS'tRACT: Casts of shrinkage cracks found within sedimentary sequence are frequently cited as being diagnostic of depositional environments periodically subjected to subaerial exposure. The term shrinkage cracks, however, encompasses a broad suite of sedimentary structures having various origins. Shrinkage cracks can form not only at the sediment-air interface by desiccation processes but also at the sediment-water interface or substratally by synaeresis processes. Shrinkage cracks are induced in muddy sediments by variations in the salinity of the depositing medium, sediment compaction, and temperature in the case of some desiccation mudcracks. Crack morphology is influenced by the interplay of numerous factors, such as sedimem composition, bed thickness, and bed surface configuration. Factors controlling desiccation mudcracks also include the rate of initial drying, total exposure time, depth of the groundwater table, and direction of surface drainage. Unfortunately, because...