School of Earth Sciences - Theses
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ItemThe platinum-group element geochemistry and petrogenesis of the Heazlewood River mafic-ultramafic complex, Tasmania( 1990)The Heazlewood River mafic-ultramafic complex (HRC) comprises well-layered olivine- and orthopyroxene-rich cumulates, gabbronorite dykes, tonalites and low-Ti tholeiitic basalt and boninite lavas. The complex was emplaced as part of a large, low-angle thrust sheet during the middle Cambrian and subsequently deformed during the Devonian, so that the original stratigraphical relationships are obscured. The cumulate succession incorporates two distinct blocks, viz. the western HRC, comprising primitive adcumulates, and the eastern HRC, consisting of more evolved orthocumulates and mesocumulates. These two cumulate blocks are interpreted to represent stratigraphically equivalent parts of a single magma chamber. In this scenario, the western HRC represents an axial part of the intrusion where high heat flows, due to repeated injections of primitive magma, promoted the development of a compositionally zoned magma chamber. In contrast, the eastern HRC is believed to constitute a marginal facies of the intrusion, where sidewall cooling caused rapid crystallisation of successive magma additions and inhibited adcumulate growth and the formation of a compositionally stratified liquid column. Results from a detailed study of the mineral compositions and whole-rock geochemistry of the HRC suggest that all of the cumulates and most of the dykes and tonalites were derived from boninitic parental magmas. This hypothesis is substantiated by empirical models which were calculated using both major and trace element approaches. The models also show that the low-Ti basalts (second-stage melts) and boninites (third-stage melts) were probably derived from component-induced progressive partial melting of a MORB-depleted spinel lherzolite source. Partial melting of the refractory mantle source was initiated and sustained by the continued influx of slab-derived Si02-, LREE-, Zr-enriched hydrous fluids. The proposed petrogenetic model for the HRC is most consistent with an island arc setting for the complex, with melting occurring in MORB-depleted forearc lithosphere overlying a subduction zone. The HRC is not an ophiolite sensu stricto, despite the fact that it is more similar to the upper portions of the so-called 'island-arc ophiolites' (eg. Troodos) than to any other type of ultramafic intrusion. It is best perceived as a high-level boninitic magma chamber which developed immediately beneath a platform of genetically-related submarine lavas. The composition of the boninitic parental magmas was the principal control on the PGE geochemistry of the cumulate sequences. Despite representing PGE-enriched, S-undersaturated second-stage melts similar to the parental (U-type) magmas for the ultramafic portions of the Bushveld complex, the boninites were unable to form a Merensky-reef type PGE deposit because they did not come into contact with S-saturated (A-type) magmas. In the absence of cumulus sulphides, the PPGE (Pt, Pd, Rh) were partitioned into the residual liquids, whereas the IPGE (Os, Ir, Ru) were strongly fractionated into early-formed olivine-chromite cumulates. These features are highlighted by the extremely low IPGE tenor of the boninites, and the relatively high IPGE tenor of the dunites in comparison to the more evolved cumulates. Three types of chromitites are recognised in the HRC. Type I and type II chromitites occur as magmatic schlieren which probably formed during replenishment events. Type III chromitites occur as layers, pods and irregular patches developed in an unusual xenolith-bearing plagioclase peridotite. It is interpreted to have formed due to mixing between ascending xenolith-bearing, hydrous intercumulus liquids and resident ultramafic magma along the floor of the magma chamber. Chromitite occurrences in the HRC are enriched in PGE by up to two orders of magnitude relative to their ultramafic host rocks, and most strongly-enriched in Ru and/or Pt and Rh. Their PGE tenor reflects the early crystallisation of laurite, followed by Pt and Rh sulpharsenides, in response to increasing S and As activities which developed primarily due to magma mixing. The low Os and Ir abundances in the chromitites is believed to reflect their formation from Os- and Ir-depleted boninitic magmas. The HRC and the Adamsfield complex were the world's major suppliers of Os-Ir-Ru alloys during the early part of this century. The alloys occur in alluvial deposits that are spatially associated with primitive olivine-rich cumulate sequences. The latter are commonly suspected to represent the source for the alloys, but recent exploration programs have yet to define a bedrock occurrence of Os-Ir-Ru alloys in Tasmania. The results from the present study provide important constraints on the genesis of these alloys. Silicate inclusions found in the alloys suggest that they formed at mantle temperatures and pressures and were transported to crustal magma chambers by boninitic magmas. The alloys may have crystallised during ascent, or alternatively, represent residual mantle phases which became incorporated into the boninites during partial melting. Most of the observations pertaining to the Os and Ir geochemistry of the HRC suggest that the alloys probably occur in thin magmatic concentrations that were deposited along the base of the intrusion from the most primitive of the boninitic magmas involved in the generation of the cumulate sequences. Future exploration should focus on delineating the cumulate products of these primitive magmas and specifically, in defining the horizons which demarcate fresh influxes of these liquids.
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ItemExploring the palaeoclimate potential of South East Australian speleothems( 2013)The paucity of palaeoclimatic data existing for the Southern Hemisphere and the regional bias of new data from the Northern Hemisphere has meant conclusions regarding the global response to the numerous climatic events of the last 20 kyr is both widely disputed and poorly understood. Despite being one of the Southern Hemisphere’s largest landmasses, Australia in particular displays a limited pool of palaeoclimatic information and the production of a new, robust record providing an insight into the response and timing of key climatic events is paramount to generating a more comprehensive characterisation and improved understanding of palaeoclimate in this region. Speleothems (cave deposits) are valuable archives of palaeoclimate variation, characterised by their extensive growth intervals and large geographic extent. They contain a multitude of ‘proxy’ records both directly and indirectly linked to climatic fluctuations and are typically robust, displaying high preservation potential with no post-depositional alteration. Key to their success is their amenability to radiometric dating, allowing the establishment of robust and reliable chronologies to which their multi-proxy records can be anchored. Consequently speleothems provide a clear opportunity to explore and expand palaeoclimatic knowledge at sites across the globe. This thesis describes and explains the use of state of the art technology to exploit relatively recent advances in U-series dating to construct reliable and detailed records of south east Australia’s response to palaeoclimatic fluctuations over the last 50 kyr using samples collected from cave sites from across the states of Victoria and New South Wales. 28 speleothem samples have been analysed in terms of both their coincident growth intervals and stable isotope variation to provide records with palaeoclimatic implications at a range of time scales. The production of a chronological template of speleothem growth intervals has enabled the assessment of south east Australia’s response to some of the key local and global millennial scale climatic events of the last glacial to interglacial transition and detailed stable isotope analysis of selected samples have been interpreted with the aid of a thorough cave monitoring programme, identifying increased variability in the region’s climate during the late Holocene. The palaeoclimatic records developed in this thesis represent a significant step forward in Southern Hemispheric palaeoclimatology. These records offer valuable new data for both palaeoclimatologists exploring south east Australia’s past climate and those investigating climatic fluctuations at a hemispheric to global scale. The high resolution and robust chronology of the records produced means that they provide a benchmark to which future records might be anchored.
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ItemThe geology and geochemistry of the Agnew Intrusion: implications for the petrogenesis of early Huronian mafic igneous rocks in Central Ontario, Canada( 1996-07)The Early Proterozoic Agnew Intrusion is a well-preserved leucogabbronoritic to gabbronoritic layered intrusion that is a member of the East Bull Lake suite of layered intrusions (ca. 2490-2470 Ma) occurring in central Ontario. These intrusions are related to the development of the Huronian Rift Zone, which may be part of a much more widespread rifting event that involved the Fennoscandian Shield. Structural data suggest that these intrusions have been subjected to ductile deformation and are erosional remnants of one or more sill-like bodies originally emplaced along the contact between Archaean granitic rocks of the Superior Province and an Early Proterozoic Huronian continental flood basalt sequence in the Southern Province.
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ItemMineralogy, geochemistry and origin of the Kalgoorlie gold deposits, Western Australia( 1978)Rich gold-telluride lodes (steeply dipping and flatly dipping) and minor gold-quartz stockwork mineralization characterize the Kalgoorlie gold-field. The origin of these gold deposits, the relationship between deposits and then nature of the host rocks are the major problems considered in this thesis. Extensive diamond drilling at the essentially unmineralized southern end of the field provided excellent material for stratigraphic studies and for country rock analysis whilst ore samples were obtained from both mines and drill core.
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ItemVolatile and precious metal geochemistry of the Mount Isa ores and their host rocks( 1986)Geochemical and petrographic investigations of Pb-Zn-Ag mineralization (12 orebody) and Cu-Co mineralization (1100 orebody) from Mount Isa were undertaken. Over one hundred and twenty carefully selected samples were analyzed for major and minor elements and for some or all of the following volatile metals: Au, Ag, Cd, As, Sb, Se, Bi, Co and Tl. A strong Tl enrichment is observed in (pyritic) unmineralized lateral equivalents of 12 orebody for several kilometers to the north of the mine sequence. The Se and As contents, S/Se ratios and S isotope relationships in the Pb-Zn ores and their host pyritic shales preclude a magmatic or deep-seated hydrothermal S Source. The data suggest that sulfide S in the Urquhart Shales was derived from reduction of a “seawater”/evaporitic/pore water sulfate source. Lateral variations in the thickness of mineralized intervals, the nature of the sulfide-gangue textures in the ores, the pervasive K and Tl enrichment in the host rocks and other chemical features of the Pb-Zn ores indicate that much of the Mount Isa mineralization formed epigenetically within the unconsolidated Urquhart Shales. The Pb-Zn-Ag ores contain very little Au and it is argued that this feature is best explained by the hydrothermal solutions that formed the Pb-Zn ores being cool (<<200°C) and moderately oxidized. The “silica dolomite” (the host to all the Mount Isa Cu mineralization) formed from “normal” Urquhart Shale as a result of intense fault-related hydrothermal activity (Perkins, 1984). The alteration has silicified the shales adjacent to the fault, and dolomite, phyllosilicates and “immobile” elements liberated during the silicification have been re-deposited at higher levels up-dip in the silica dolomite bodies. For the most part primary sulfide textures have not been preserved. It is argued that the distribution of several elements (notably Co, Bi, As, Fe and S) in 1100 orebody and its location down-dip from a strongly pyritic section of Urquhart shale are good evidence that stratiform Co (and Cu) mineralization was present in pyritic Urquhart Shales prior to formation of the silica dolomite. Chemical and isotopic evidence suggest that the Cu mineralization had a similar S-source and formed from similar solutions to the Pb-Zn-Ag ores. A new co-genetic model for the Mount Isa Cu and Pb-Zn-Ag deposits in which the mineralization formed from cool oxidized solutions in the upper few meters of the unconsolidated Urquhart Shales is presented. The metal-bearing solutions were expelled from their source rocks (oxidized clastic sediments lower in the Moust Isa Group) during the course of normal basin compaction and dewatering. Base metal sulfides were fixed by sulfate reduction processes occurring in the diagenetic environment of the Urquhart Shales. Weathered mafic volcanic detritus may have been and important component of the source.
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ItemThe geochemistry and geochronology of Tanzanian kimberlites( 2012)The Tanzanian Craton is host to numerous kimberlite intrusions, although few have been studied in any detail. Due to its proximity to the East African rift zone, this region also provides a unique opportunity to investigate the effect of rifting on kimberlite volcanism. To date, however, no geochemical studies have been conducted on Tanzanian kimberlites, largely due to their altered and weathered nature, and only four intrusions have published age constraints. This thesis seeks to redress these deficiencies by the application of modern high precision analytical techniques to the study of kimberlites from this previously neglected region. The study focused on 30 separate kimberlite intrusions located on the Tanzanian Craton. Due to their highly altered and weathered character, perovskite was used as a proxy to obtain geochemical and geochronological information on the host kimberlites. Perovskite is a common accessory mineral in kimberlites and contains substantial U, Sr and Nd and lesser amounts of Pb and Hf. Consequently, perovskite provides the means to determine both age constraints and initial isotopic signatures for the host kimberlites. Perovskite extracted from the Tanzanian kimberlites was utilised for U-Pb geochronology and analysed for trace element and Sr-Nd-Hf isotopic compositions. Similar to kimberlites worldwide, these samples show extreme enrichment in large ion lithophiles and LREE, whereas they are heavy REE depleted. They are characterised by moderately radiogenic 87Sr/86Sri compositions (0.70268 – 0.70476) and ɛSr values (-14.56 to 4.46), restricted 143Nd/144Ndi compositions (0.51138 to 0.51277) and ɛNd values (1.8 to 3.8), and very restricted 176Hf/177Hfi compositions (0.282058 to 0.282966) and ɛHf values (0.8 to 8.2). All samples plot within the range previously defined for Group I kimberlites. This study provides compelling evidence for the utility of perovskite analysis in highly altered kimberlitic samples that would otherwise be intractable to study. The ages obtained for these samples define two periods of kimberlite volcanism for the Tanzanian Craton. Early Mesoproterozoic kimberlites include the Itanana kimberlite (1083 Ma) and the previously dated Bubiki kimberlite (1097Ma). All other kimberlites for which ages were obtained are significantly younger, with ages ranging from 44-80 Ma. The latter group includes some of the youngest reliably dated kimberlites known; the Kikhomango (44 Ma) and X073 (47 Ma) intrusions. Ages from both the older and younger Tanzanian kimberlites coincide with periods of known kimberlite activity worldwide. Previous studies of the Tanzanian kimberlites have focused on the thermobarometry of garnet macrocrysts derived from disaggregated mantle xenoliths. Garnet compositions vary with proximity to the East African Rift, suggesting that the SCLM in this region was affected by rift related heating prior to kimberlite emplacement. If the source of these kimberlites was within the SCLM, systematic geographic variation in the isotopic and trace element compositions in relation to their proximity to the rift would be expected. This has not been observed in this study, with the entire region displaying no significant geographic variation in geochemical composition. Therefore the current results suggest a deeper source for the Tanzanian kimberlite magmas, below the influence of rifting in the region.
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ItemThe timing and origin of orogenic gold mineralisation in the western Lachlan Orogen, southeast Australia: constraints from 40Ar/39Ar dating and halogen and noble gas geochemistry( 2012)The Ballarat East gold deposit (408t) is the second largest orogenic gold deposit in the Western Lachlan Orogen, southeast Australia. The western Lachlan Orogen is characterised by a thick package of Ordovician turbiditic sedimentary rocks overlying Cambrian oceanic volcanic sequences. The region was variably affected by multiple major deformation/metamorphism and magmatism events during the Cambrian to Devonian. The Ballarat East gold deposit is located in the Bendigo structural zone of the Western Lachlan Orogen and is hosted in Ordovician sediments of the Castlemaine Supergroup. Gold mineralisation in the Ballarat East deposit is sited in quartz and quartz-carbonate veins within goldfield-scale, west-dipping reverse faults. Two major lode types are present: 1) lode type ‘1’ is characterised by arsenopyrite-dominated quartz veins associated with early movement on reverse faults, whereas 2) lode type ‘2’ is related to structurally later, shallow east-dipping, pyrite-sphalerite-galena-white-mica dominated veins, emanating from reverse faults. Previous studies have suggested that gold mineralisation in the Western Lachlan Orogen occurred at ~440Ma, as a result of metamorphic devolatilisation reactions in the lower crust. However the age of mineralisation at the Ballarat East deposit is only broadly constrained to a period between 460 and 370 Ma, and the source of the gold-bearing fluids could include metamorphosed volcanic rocks, sedimentary rocks and/or granites. In order to provide a more robust chronological framework for gold mineralisation at the Ballarat East deposit, several samples of detrital and hydrothermal potassium-rich minerals were collected and analysed by 40Ar/39Ar dating. In addition, fluid inclusions in portions of quartz and quartz-carbonate veins were characterised by micro-thermometry and halogen/noble gas isotopic tracer methods to further constrain the source(s) of the gold mineralising fluids. The 40Ar/39Ar data obtained from detrital muscovite grains yield ages between 530 – 460 Ma and are concordant with previously published detrital ages. The vein muscovite/sericite ages fall into three age groupings as follows: 445 – 435 Ma (lode type ‘1’), 420 – 415 Ma (lode type ‘2a’) and 380 – 370 Ma (lode type ‘2b’). The gold-bearing quartz veins (from both lode types) contain low salinity (average 4 wt.% NaCl eq.) aqueous H2O inclusions and mixed H2O-CO2 fluid inclusions. Fluid inclusion 40Ar/36Ar values range from 322 (close to Air Saturated Water; ~296) up to a maximum of 4503, and 40Ar/36Ar is strongly correlated with Cl/36Ar. Fluid inclusions have variable Br/Cl values between 1.66 10-3 and 2.91 × 10-3 and I/Cl values between 153 × 10-6 and 501 × 10-6, with a strong correlation between Br/Cl and I/Cl. The fluid inclusion 84Kr/36Ar and 129Xe/36Ar values are variable but show a systematic enrichment in the heavier noble gases. The 40Ar/39Ar ages suggest gold mineralisation at the Ballarat East deposit occurred in three main episodes at ca. 445 Ma, ca. 420 Ma and ca. 380 – 370 Ma. All episodes of mineralisation are associated with fluid inclusions of similar composition. This fluid is suggested to reflect a deeply sourced fluid, possibly originating by devolatilisation of altered volcanic rocks (e.g. basalts). In this scenario, the fluid would have acquired additional noble gases and organic Br plus I by interaction with sedimentary rocks, including organic-rich shales that are found beneath and surrounding the deposit. The data are compatible with genetic models for orogenic Au in which gold mineralisation was initiated by metamorphic devolatilisation in the lower crust, linked to Lachlan Orogenesis at ca. 440 Ma.
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ItemGeochemistry and mineralisation of primary and secondary platinum-group elements in the ultramafic "Alaskan-type" Owendale complex and laterites in the Fifield Region, New South Wales, Australia( 1995)The Owendale Complex belongs to a family of ultramafic-mafic intrusions that is characterised by a zonal, nonstratiform arrangement of the principal ultramafic units. The ultramafic rocks of the Owendale Complex are virtually identical to many of the Alaskan-type intrusions, however the associated gabbroic rocks (wehrlites) are K-rich and Si-undersaturated, in contrast to the tholeiitic gabbroic rocks of the Alaskan examples. The intrusion history of the Owendale Complex is thought to have involved emplacement of a gabbroic intrusion that was invaded by an ultrabasic magma, possibly while the former was still only partly solidified. Emplacement of both magmas probably occurred during Late Devonian tectonism and deformation synchronous with emplacement and crystallisation is necessary to explain the present non-stratiform arrangement of the rock units. The most obvious linkage factor between the two proposed parent magmas (gabbroic and ultrabasic) of the Owendale suites is their mutual affinity with tholeiitic basalt magmas and the similarities of their products with intrusions of alkalic basalt derivation. This suggests the possibility that the Owendale Complex rocks and those of other tholeiitic intrusions of the regions are comagmatic products of an ancestral magma that may have also produced the widespread assemblage of complexes. Viewed from this perspective, the ultramafic rocks of Owendale Complex would thus represent a very minor product of a period of regional magmatic activity. Most alloys, erlichmanite, cooperite and some grains with exclusion texture of Pt-Os-Ir-Pd-Rh are considered to represent a primary high-temperature paragenesis. Concentration of PGE in pegmatoidal units of dunite-wehrlite is explained by the accumulation of platinum-rich alloys that segregated directly from the melt at an early stage in the evolution of the complex. The high-temperature PGM segregate directly from a silicate melt and were not generated by exsolution from spinels or magmatic sulphides. These suggest that fS2 was generally low (subordinate sulphide formation) and, after some influence at the beginning, has given way to rising fO2 (chromite, olivine and Pt-Fe-Cu-Ni alloys formation). After lithification, the ultramafic rocks become subject to "reducing" conditions, i.e., conditions of lower O2 and S2 activities. Ni-Fe alloys, native Fe and Bi formed in cracks which filled the serpentine matrixes. The former PGM (erlichmanite, cooperite and Pt-Fe alloys) were exposed to the reducing conditions via cracks were desulphurated to form porous cooperite with Pt-Fe alloys and multiphase textural Os-Ir-Ni, Pt-Ir aggregates. It is plausible that the veinlets and aggregates of unnamed Rh-Sb-S, (Pt, Ir)2(Fc, Cu)3(S, Sb, AS)3 in the dunites may also have been formed by reduction of Ni-rich sulphides and erlichmanite, Pt-Fe alloys or cooperite. Late PGM are dominated by sperrylite-geversite solid solution resulting from the reaction of early PGM with a fluid phase. A hydrothermal origin is also indicated for native Fe, native Bi and awaruite (NiFe) and the base-metal sulphides (pentlandite, chalcopyrite, sphalerite, arsenopyrite, pyrite, pyrrhotite, and some Ni-Co-Fe sulfide). The cause of the reducing conditions may have been related to H2 production accompanying hydrous alteration of the dunites and clinopyroxenites. The laterites overlying the ultramafic complexes in the Fifield region are exceptionally well-developed and well-preserved weathering profiles. Field, textural and geochemical data all support a chemical weathering origin for the profiles and compatible with meteoric and ground water origins. Meteoric water with intermediate Eh and pH and negligible dissolved species sinks into the laterite where these parameters are modified. The Eh rises and pH decreases to the conditions typical of lateritic soils and the concentration of dissolved species increases. In this state the water is able to take PGE and Au into solution from a finely disseminated form in the bedrock as a part of the process of lateritisation. When the soil solution transports the PGE and Au towards a transitional interface must exist between the ferruginous and saprolite zones with lower Eh, neutral pH and lower concentration of dissolved salts. At this transitional region, deposition of the PGE and Au occurred. The presence of magnetic Pt-Fe-Cu-Ni alloys suggests that hydrothermal solutions play a later role in the Fifield region, and the alloys have grown in situ in a lateritic soil by a process involving laterite water solution in the high Eh, low pH conditions prevalent in such soil, followed by deposition when the conditions become less extreme. Some examples of the Pt-Fe alloys from such an environment become frequently strongly magnetic with larger size. It is assumed that the temperature of the hydrothermal solution is in the range of 300° - 500° C (Bowles, 1990). PGE mineralisation in the primary rocks and laterite in this region has demonstrated a good example of multi-stage process mineralisation including primary high temperature magmatic formation; low temperature postmagmatic hydrothermal alteration and residual lateritic enrichment.
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ItemThe geochemistry and petrology of the Enterprise dolerite, Ora Banda, Western Australia( 1998)The Enterprise Dolerite was emplaced as an intrusive tholeiitic sill within the Ora Banda Sequence at Ora Banda in the Eastern Goldfields Province of the Yilgarn Craton. The Enterprise Dolerite is now a metamorphic body with modifications in both the mineralogy and geochemistry of the rocks. Careful analysis of petrographic features integrated with geochemical trends have made it possible to interpret the original igneous characteristics of the sill. It is proposed here that the order of crystallisation in the Enterprise Dolerite is plagioclaseolivine- clinopyroxene-quartz. Furthermore, plagioclase and olivine accumulated through crystal settling before a switch to in-situ crystallisation in the remainder of the sill. The bulk chemistry of the Enterprise Dolerite is equivalent to that of the Mt Ellis Sill which occurs at the same stratigraphic position, and it is proposed here that they are continuations of the same intrusive body. This intrusive body is related to the other mafic members of the Ora Banda Sequence, with all members forming a differentiation trend and in which the Big Dick Basalt represents a primary mantle magma. The Enterprise Dolerite/Mt Ellis Sill has evolved in composition along the trend from this primary magma. Finally, the addition and removal of phases has produced a chemically evolving system with differentiation progressing to maxima in silica and iron concentrations which provide very good conditions tor gold deposition. This study proposes that both the Enterprise Dolerite and the Mt Ellis Sill be examined for future potential gold mineralisation.
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ItemThe geology, petrology and geochemistry of the Otway formation volcanogenic sediments( 1983)The geology, petrology and geochemistry of the Early Cretaceous Otway Formation have been investigated in detail and used to determ ine the nature of the source rocks and to develop a model for the diagenetic and low-grade metamorphic readjustments. The fluviatile Otway Formation was deposited in continental rift grabens that stretched some 1000 km along the southern coast of Australia during the Early Cretaceous. The main areas of deposition in the Otway, Gippsland and Bass Basins contain an estimated 100,000 cubic kilometres of detritus. The major part of this detritus was derived from pyroclastic material which has been shown by the fission track dating studies to have been derived from contemporaneous volcanism. The pile of volcanogenic material comprising the Otway Formation is at least 3 to 4 km thick in the main basins. The sediments are entirely non-marine and were deposited by large scale multichannel streams cut in extensive floodplains. The streams deposited thick multistorey channel sandstones in sheet-like bodies and a diverse spectrum of overbank mudstones and fine-grained sandstones. The complex channel sandstones fine upwards but have numerous erosional breaks indicating repeated flood cycles. Whereas the channel deposits have internal features consistent with braided stream channels the overall system has a large proportion of floodplain which was been considered in the past to have been a feature of meandering channels. The oversupply of volcanogenic detritus is considered to have been responsible for the development of the multiple channel depositional system in a climate of high seasonal rainfall. Whole rock chemical analyses of all lithologies in the sedimentary suite, recalculated i.nto a set of normative minerals, have proved useful in the distinction and description of sedimentary rocks in general. P20S was found to be useful for the identification of Early Cretaceous soil forming processes. The study of the chemical composition of detrital minerals has demonstrated the usefulness of this approach in the identification of the nature of the source magmas of volcanogenic sediments. For the Otway Formation, analyses of clinopyroxenes, amphiboles, feldspars and sphene in particular, have shown that high potassium dacitic to shoshonitic volcanism dominated during Early Cretaceous rifting. The new data on the geology and mineralogical and chemical features of the Otway Formation have application to the study of diagenesis and low-grade metamorphism in volcanogenic sediments in general. (From Abstract)