published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

The Proterozoic period saw some of the most fundamental revolutions in the biological and geological world. During this period, life diversified and set the stage for the radiation of multicellular life, altering the face of the planet in the process. The fossil record of this time is not yet fully understood, and a revisitation of a historically reported fossil deposit in the 1 Ga Torridon rocks of northwest Scotland shows that they host the fossils of the earliest non-marine eukaryotes, as well as a full and diverse fossil assemblage preserved in sedimentary phosphates and shales. Fine scale sedimentology of the fluvio-lacustrine rocks of the Torridon Group reveals them to be laid down in a laterally variable basin with distinctly different palaeoenvironments. The resident biota is seen to be similarly variable between lithofacies. New criteria for classifying taphonomic effects are presented, and used to characterise assemblages from different palaeoenvironments, with broader applications beyond this study. The Torridon rocks are also host to macrostructures on the surfaces and soles of beds, and these are interpreted as of likely biological origin, with their variability mapped between different lithofacies. High-resolution studies of both the preserved biota and the mineralogy of the preserving medium reveal in detail not only the fine scale structure of the fossil organisms, but also the reasons for their exceptional preservation. Phosphate is analysed in detail to explain its enigmatic occurrence in Proterozoic lakes. To place the Torridon deposits in context, both older and younger rocks were examined in comparison, from the 2 Ga Gunflint Formation of Ontario, Canada, and the Precambrian-Cambrian successions of eastern Newfoundland respectively. New finds of phosphate in these rocks help to reveal biochemical interactions and evolution on the early Earth, with implications for further understanding life on our own planet and elsewhere.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2017. / Massif-type anorthosites formed in the Proterozoic Eon are the most voluminous anorthosite occurrences on Earth, reaching tens of thousands of square kilometers in aerial extent. While they formed throughout the Proterozoic, most formed during a 700 Ma period between 1800 and 1100 Ma. The rocks are dominated by plagioclase (typically 70 – 95 volume %) of intermediate composition (An40-65). Olivine, orthopyroxene, clinopyroxene and Fe-Ti oxides make up the minor mafic proportion. While most researchers agree that the anorthosites formed from a high-alumina basaltic parental magma, there are disparate views on how that parental magma was generated. Whether the parental magma formed by partial melting of the lower crust, or by mantle melting, is a topic of much debate. The anorthosites commonly have crust-like isotopic signatures, but this could be produced by melting of the lower crust, or by crustal contamination of mantle-derived magmas. Many Proterozoic anorthosite complexes consist of both olivine-bearing and orthopyroxene-bearing anorthosites. This has been attributed to variable amounts of crustal contamination of mantle-derived magmas, based on evidence from isotopes and field relations. While geochemical and petrologic evidence for crustal contamination is plentiful, existing experimental work shows that a thermal divide exists for high-alumina basalts fractionating at lower crustal depths, casting doubts on whether fractionation of a mantle melt could produce anorthosite. Here I use high-pressure experiments to test whether the fractionation of high-alumina basalt can form anorthosites, and to what extent crustal contamination affects the fractionation sequence. The results are compared to new geochemical and petrologic data from the Kunene Anorthosite Complex (KAC), in Angola and Namibia. The KAC is one of the largest anorthosite complexes in the world, with an area of ~18 000 km2. The KAC (1438 – 1319 Ma) has an elongate shape and intruded into Palaeoproterozoic to Mesoproterozoic country rocks (~2200 to 1635 Ma) at the southern margin of the Congo craton. It is associated with a suite of granitoid rocks of variable composition, which are akin to the granitoids associated with nearly all Proterozoic anorthosites. The granitoids have been shown to be coeval with the anorthosites, but were from a chemically independent magma series. The most distinctive granitoids in the KAC are the Red Granites, which outcrop around the southern margins of the complex, and also cross-cut the complex in a NE-SW linear belt, dividing the complex roughly into northern and southern domains. The rocks of the KAC are highly variable in terms of mode, mineral chemistry, and texture, but there is a general trend of more olivine-bearing anorthosites north of the granite belt, and orthopyroxene-bearing anorthosites to the south. The olivine-bearing rocks (or leucotroctolites) typically contain plagioclase and cumulus and/or intercumulus olivine, with lesser interstitial orthopyroxene and/or clinopyroxene, Fe-Ti oxides, and biotite. The orthopyroxene-bearing anorthosites (or leuconorites) contain cumulus plagioclase ± cumulus orthopyroxene, and interstitial orthopyroxene, clinopyroxene, oxides and biotite. The leucotroctolites are characterized by more calcic plagioclase (An56-75), while the leuconorites contain more intermediate plagioclase (An48-56). The variability of the rocks across the complex suggests that the KAC consists of several coalesced plutons with different histories. The petrologic data and field observations in this study are consistent with the leuconorites of the complex being derived from a mantle-derived magma that experienced contamination by silica-rich rocks, crystallizing orthopyroxene rather than olivine, and less calcic plagioclase. The leucotroctolites experienced less or no contamination. To test whether the mineral dichotomy and the variations in plagioclase chemistry observed in Proterozoic anorthosites are due to variably contaminated mantle-derived magma, piston cylinder experiments were conducted on a synthetic high-alumina basalt (HAB) composition, as well as a mixture of this HAB with 30% of a Red Granite composition. Experiments were conducted at 10 kbar, to simulate the depth at which anorthosite differentiation most likely begins (based on Al-in-orthopyroxene geobarometry of highly aluminous orthopyroxene megacrysts that occur in many massifs). The uncontaminated experiments produced olivine as the first liquidus phase, followed by plagioclase (An65-68), and then by clinopyroxene, pigeonite and ilmenite at progressively lower temperatures. Residual liquids evolve towards more silica-rich compositions with decreasing temperature. The contamination experiments produced liquidus orthopyroxene, followed by plagioclase (An51-56), and then by pigeonite at lower temperatures. The experiments show that contamination of a primitive HAB magma by granitic material, most likely produced by partial melting of the lower crust during anorthosite formation, can shift the mineral assemblages of the crystallizing anorthosite from olivinebearing to orthopyroxene-bearing, and produce less calcic plagioclase than the uncontaminated HAB magma. This could explain the observation of olivine-bearing and orthopyroxene-bearing anorthosites in the KAC and many other Proterozoic anorthosites. Previous high-pressure experimental studies, using a slightly more evolved HAB composition, indicated the presence of a thermal divide, which causes liquids to evolve to more Si-poor compositions. The experimental results presented in this study however, do not show a thermal divide, indicating that small variations in experimental starting composition can cause large differences in the liquid line of descent. The results of this study indicate that partial melting of the mantle can produce anorthosite parental magmas, and that the range in mineral assemblages of the anorthosites can be accounted for by crustal contamination of a mantle-derived magma. Fractionation of the experimental starting compositions was also modeled using the MELTS algorithm. These calculations produce a close match to the experimental liquid trends. This allows for modeling of a variety of compositional and environmental variables. The MELTS modeling shows that as little as 10% contamination of HAB magma with a granitic composition may position the magma in the orthopyroxene stability field, forming orthopyroxene-bearing anorthosites. The modeling also shows that a variety of silica-rich contaminants, including granites, granodiorites and tonalities, produce similar results and liquid evolution trends, so a range of granitoid compositions may successfully produce the shift in mineral assemblages of the anorthosites. This suggests that crustal contamination of mantle-derived HAB could be a widespread process and the primary mechanism that produces the distinctive crust-like signatures in Proterozoic anorthosites. In summary, the mineralogical and chemical diversity observed in Proterozoic anorthosites can be produced by variable amounts of crustal contamination of mantle-derived, highalumina basaltic magma. The experimental results in this study combined with field observations, and geochemical and isotopic data, provide evidence for a model of massif-type anorthosite petrogenesis. Orthopyroxene-bearing rocks formed from an originally highalumina basaltic magma that experienced contamination by granitic partial melts of the lower crust, during ponding of the magma at the Moho. This process preconditioned the surrounding crust and possibly prevented further anatexis. Following emplacement of orthopyroxene-bearing anorthosites, subsequent magma pulses ponded at the Moho did not assimilate any/as much granitic material, as they were interacting with preconditioned crust, and formed olivine-bearing anorthosites. With better constraints on the parental magma composition, magma source, and crustal contamination processes, addressing aspects such as the tectonic setting and emplacement mechanisms of these massive intrusions should be prioritized. Understanding these enigmatic aspects of anorthosite petrogenesis is leading the anorthosite community towards answering the ultimate questions of why massif-type anorthosites are restricted to the Proterozoic. / XL2018

Many aspects of Proterozoic massif-type anorthosite petrogenesis have been, and remain, controversial. Mafic lower crust and depleted mantle have both been proposed as mutually exclusive sources of these near-monomineralic, temporally restricted batholiths. The debate surrounding the magma source has also led to uncertainty regarding the tectonic setting of these massifs, with a range of possibilities including convergent, divergent and anorogenic settings. The dramatic geochemical effects of crustal contamination in these massifs are well known and strong crustal signatures are evident in most, if not all, Proterozoic anorthosite massifs. The source debate, in the simplest sense, reduces to whether the ubiquitous crustal signature is derived principally from melting of a lower crust or is an effect of crustal assimilation. The origin of this crustal signature, and whether it obscures the original isotopic composition of the magmas or not, has fuelled the debate surrounding the source of the anorthosites. Using major element, trace element and isotopic compositions, as well as energyconstrained assimilation-fractional-crystallisation (EC-AFC) modelling from samples representing various stages of the polybaric crystallisation history of the magmas, including high-pressure megacrysts, anorthosites and their internal mineral phases, I remove the obfuscating effects of possible crustal contamination and probe the source of the magmas. In order to assess the effects of crustal contamination, if any, anorthosites from three massifs – the Mealy Mountains Intrusive Suite, Nain Plutonic Suite (both in eastern Canada) and Rogaland Anorthosite Province (Norway), have been analysed – all of which intrude into crust of significantly different age and chemical character. Sm-Nd geochronology of high-Al, high-pressure orthopyroxene megacrysts, as well as the comagmatic, host anorthosites, indicate that the magmatic system is long-lived, with an age difference between the megacrysts and hosts of ~110-130 million years. Isotopic compositions of primitive megacrysts qualitatively show that the magmas were derived from melting of the depleted mantle. Strong links between the isotopic offset from depleted mantle evolution and the age and composition of the surrounding crust confirm that the geochemical nature of the crustal contaminant plays a significant role in the petrogenesis of the anorthositic rocks. The geochronological indications of a long-lived magmatic system point to Proterozoic anorthosite formation in a continental magmatic arc – one of the only environments capable of supplying geographically-localised magma and heat to the base of the crust for over 100 million years. Proposed divergent or ‘anorogenic’ settings could not plausibly supply magma to the base of the crust for over 100 m.y. without initiating ocean formation or continental break-up. Anorthosite emplacement at mid-crustal levels may coincide with late- to post-orogenic events in several terranes, but evidence presented for a long-lived magmatic system is incongruent with this proposed setting. In this thesis, I propose that the petrogenesis of these intrusives must span both orogenic and post-orogenic periods. An overlap in megacryst crystallisation age with the onset of calc-alkaline orogenic magmatism in the Sveconorwegian Orogen, both occuring ~100 m.y. before anorthosite emplacement, confirms that initial magma and megacryst formation coincides with the main phase of magmatic and orogenic activity in a convergent magmatic arc. These geochronological constraints have implications for regional geodynamics in the Sveconorwegian Orogen (and the Labrador region) with the evidence providing corroboratory support for a long-lived accretionary orogen, as opposed to the widely-held view that the Sveconorwegian orogeny was predominantly collisional. Compositions of high-pressure megacrysts, anorthosites and analysis of internal isotopic disequilibrium indicates that lower crustal contamination has a significant influence on the isotopic composition of the rocks, with relatively minor contributions from the mid- to upper crust. Energy-constrained AFC modelling confirms that significant lower crustal contamination occurs during ponding of magmas at the Moho and is able to reproduce the observed isochronous isotopic compositions of the megacrysts as well as the compositions of the host anorthosites. Evidence of varying degrees of internal isotopic disequilibrium reinforces the significant role that assimilation of crust of different age and chemical nature have on the compositions of Proterozoic anorthosites. Unexpected patterns of isotopic disequilibrium show that anorthosite petrogenesis is not a “simple” case of progressive crustal contamination during polybaric ascent of viscous, partially-molten 4 magma mushes, but is more likely to involve significant differentiation and solidification at lower crust depths, followed by ascent of high-crystallinity bodies (> 50 % crystallinity) to upper crustal levels. Although the composition of the bulk continental crust is different to plagioclase-rich Proterozoic anorthosites, both are missing a mafic component. It is unclear how this missing mafic component was generated in the continental crust, because most of the evidence for these crustal differentiation processes is sequestered below or near the Moho. However, Proterozoic anorthosites, formed by viscous, plagioclase-rich mushes, entrain rare cumulate megacrysts from these depths and consequently preserve evidence of magmatic differentiation processes at the Moho. The evidence for the formation and sequestration of dense ultramafic cumulates in ponding magmas at the Moho can not only explain the missing mafic component in Proterozoic anorthosites, but also suggests that cumulate formation in crust-forming, arc environments is a significant process and should be taken into account in models dealing with evolution and differentiation of the continental crust. Sampling and petrographic and geochemical analysis of five pegmatitic segregations, or “pods”, from anorthosites of the Mealy Mountains Intrusive Suite reveal a diverse range of compositions from mafic, Fe-rich and Si-poor, to Fe-poor and Sirich felsic compositions and from monzogranite through quartz-monzodiorite and monzodiorite to Fe-P-rich gabbronorite. Each pod shows a range of noteworthy graphic, myrmekitic and symplectic textures on a variety of scales, along with distinctive mineralogical assemblages and highly-enriched trace element compositions. Derivitive minerals (e.g. apatite and zircon), high concentrations of Fe, Ti, P (and in some cases SiO2) and 10-1000 times chondrite enrichment suggest that many of the pods are highly fractionated. U-Pb zircon geochronology reveals that all the pods are the same age as the anorthositic hosts and confirms that the Mealy Mountains Intrusive Suite was emplaced between 1654 and 1628 Ma. Using the aforementioned evidence, I show that the pods represent the fluid-bearing, late-stage crystallisation products of a residual liquid in the massif anorthosite system and provide a window into the final stages of crystallisation in the anorthosite system. A range of rock types (monzonites, monzonorites, ferrodiorites and jotunites) observed in similar pod-like structures, as well as dykes and plutons, have also been documented in other Proterozoic anorthosite massifs. These have, at one time or another, controversially been interpreted as the residual liquids of anorthosite crystallisation. The observation of in-situ pods with similar compositions to all of the aforementioned rock types and displaying textures indicative of late-stage crystallisation support the notion that these associated lithologic units are comagmatic with, but residual to, the anorthosites and are not residual liquids of other crustally-derived rocks, immiscible liquids, parental magmas or cumulates. Isotopic compositions of these highly-fractionated, late-stage pods also overlap with those of anorthosites, lending further evidence to the case that upper crustal contamination plays only a minor role in developing the chemical signature of the anorthosites. With these results I propose that the nature/composition of the residual liquids of Proterozoic anorthosite magmas can vary dramatically, depending on geochemical differences in the original magma pulses and by mixing of mobilised, independently-evolved segregations of residual liquids. This process could explain why so many varied rock types associated with Proterozoic anorthosites have been suggested as residual liquids: these rocks all represent residual liquids resulting from varying degrees of differentiation, subsequent mobilisation, mixing and final solidification as plutons or dykes. Proterozoic anorthosite petrogenesis is an inherently polybaric process and so by its very nature produces a range of complicated and contradictory features which have clouded interpretation of numerous aspects of the rocks formation. In analysing crystallisation products from numerous stages of the anorthosites polybaric history, I have been able to probe the magmatic processes operating at different stages of Proterozoic anorthosite petrogenesis. In doing so I show that the magmas are derived from melting of the depleted mantle in continental-arc-like settings – two controversial aspects of Proterozoic anorthosite petrogenesis. These constraints on the source and tectonic setting will allow renewed investigation into the ultimate question surrounding Proterozoic anorthosites: why are these rock types restricted to the Proterozoic and what clues does this temporal restriction offer about Earth’s geodynamic evolution during this period? The assertion in this thesis that 5 Proterozoic anorthosites formed in arc environments dictates that subduction processes or geodynamic conditions during the Proterozoic favoured the production of voluminous masses of plagioclase, because modern-day magmatic arc terranes show no evidence of anorthosites with similar compositions. However, calcic anorthositic inclusions and xenoliths are observed in modern-day volcanic and continental arcs suggesting that anorthosites may be forming in these environments, but that conditions such as water content or style of subduction are different to the Proterozoic, producing less and compositionally different plagioclase and anorthosite. The results of this thesis shed new light on and refine the petrogenesis of Proterozoic anorthosites, but the focus of research must now shift to explaining the temporal restriction of these intrusions and the implications of this restriction for the geodynamic evolution on Earth during the Proterozoic.

Includes one folded map in pocket in back cover. / Includes bibliographical references. / iv, 214, [46] leaves, [10] leaves of plates : ill. (some col.), col. maps ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Geology and Geophysics, 1994

published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

Microbes, including Bacteria, are globally important mediators of elemental transformations in the marine water column, but not until recently has their biomass been suggested to contribute significantly to carbon export flux. Here I characterize lipid and carbon isotopic signatures in marine particulate organic matter (POM) explicitly at microbial size scales, and I quantitatively explore how these signatures are transferred down the water column. In the North Pacific Subtropical Gyre (NPSG) an isotopically-enriched pool of submicron POM appears to dominate export to mesopelagic depths, supporting recent observations that bacterioplankton communities contribute to export flux in proportion to their biological abundance. In the Eastern Tropical North Pacific (ETNP) complex pathways emerge for the flux of POM to the deep ocean. I use the largest data set to date for natural \(^{13}C\) signatures of individual water column lipids to reveal that submicron and larger-size suspended POM size classes are isotopically distinct. Results point to de novo production of lipids above and within the oxygen minimum zone. I develop quantitative models to deconvolve the signatures of sinking and in situ sources of these lipids. Results converge on a best-fit model for downward flux in the ETNP that includes both surface-derived and sub-photic zone lipids. Overall results from the modern ocean suggest that approximately half of total suspended POM is submicron in size, much of it is bacterial in origin, and despite the small size of this material, it participates dynamically in water column export flux. These results also suggest some revised interpretations of organic matter signatures in the geologic record. I formulate a quantitative model of marine microbial production and degradation, and reproduce "inverse" isotopic signatures found in lipids and organic matter preserved in Proterozoic sedimentary rocks. Results suggest that the disappearance of this inverse \(^{13}C\) pattern was a consequence of the shift from Bacteria to Eukarya as dominant producers of marine autotrophic biomass. Together, results of this thesis reveal that heterogeneity in the isotopic signatures of marine suspended POM is associated with particle size, and by extension, must be a function of the composition of the total planktonic community. / Earth and Planetary Sciences

The snowball Earth theory. that low-latitude continents became glaciated after sea ice extended from the poles to the equator during the Neoproterozoic (1000 - 545 Ma), has sparked a flood of numerical modelling studies investigating all phases of the proposed extreme climatic cycle. Modelling studies have both supported and refuted the original ‘hard’ snowball theory, as well as offered alternative theories such as the ‘soft’ snowball theory, where glaciers covered all land masses but sea ice did not extend equatorward of -25° latitude. Presented here are a number of sensitivity studies investigating the effects of different sea ice models on snowball inception and how large ranges in accepted albedo values affect the amount of radiative forcing required for deglaciation. These experiments shed light on disparities between previous modelling results. This study demonstrates the importance of a sea ice component's thermodynamic formulation by comparing the physically complete thermodynamic sea ice model in the UVic Earth System Climate Model to an incomplete sea ice model used in an early version of the Fast Ocean Atmosphere Model. As well as the importance of a sound thermodynamic sea ice model is the inclusion of sea ice dynamics. Sea ice dynamics have not previously been included in snowball simulations and their inclusion suggests that the alternative `soft' snowball theory may not be plausible. With a purely thermodynamic sea ice component. the 'soft' snowball state is stable whereas with the inclusion of sea ice dynamics it is not. Also gained from this study was the influence of wind speed on the level of CO2 required to produce a hard snowball solution: greater wind speed results in a cooler ocean and easier snowball inception. Finally, although albedo values are critical for the albedo feedback that initiates snowball inception, they are even more important for determining the amount of radiative forcing required to deglaciate the snowball Earth, as the entire planet is covered by snow and ice. Through a suit of sensitivity studies, the amount of forcing required to deglaciate a hard snowball Earth is found to be extremely sensitive to the snow albedo, sea ice albedo and snow masking depth.

abstract: ABSTRACT The accretion of juvenile island-arc lithosphere by convergent tectonism during the Paleoproterozoic, in conjunction with felsic volcanism, resulted in the assembly, ductile to partial brittle deformation, uplift, and northwest-directed thrusting of rocks in the McDowell Mountains region and adjacent areas in the Mazatzal Orogenic belt. Utilizing lithologic characteristics and petrographic analysis of the Proterozoic bedrock, a correlation to the Alder series was established, revising the stratigraphic sequences described by earlier works. The central fold belt, composed of an open, asymmetric syncline and an overturned, isoclinal anticline, is cut by an axial-plane parallel reactivated thrust zone that is intruded by a deformed Paleoproterozoic mafic dike. Finite strain analyses of fold geometries, shear fabrics, foliations, fold vergence, and strained clasts point to Paleoproterozoic northwest-directed thrusting associated with the Mazatzal orogen at approximately 1650 million years ago. Previous studies constrained the regional P-T conditions to at least the upper andalusite-kyanite boundary at peak metamorphic conditions, which ranged from 4-6 kilobars and 350-450⁰ Celsius, although the plasticity of deformation in a large anticlinal core suggests that this represents the low end of the P-T conditions. Subsequent to deformation, the rocks were intruded by several granitoid plutons, likely of Mesoproterozoic age (1300-1400 Ma). A detailed analysis of Proterozoic strain solidly places the structure of the McDowell Mountains within the confines of the Mazatzal Orogeny, pending any contradictory geochronological data. / Dissertation/Thesis / M.S. Geological Sciences 2012

Ogilvie Mountains breccia (OMB) is in Early (?) to Late Proterozoic rocks of the Coal Creek Inlier, southern Ogilvie Mountains, Yukon Territory. Host rocks are the Wernecke Supergroup (Fairchild Lake, Quartet and Gillespie Lake groups) and lower Fifteenmile group. Distribution and cross-cutting relationships of the breccia were delineated by regional mapping. OMB was classified by clast type and matrix composition. Ogilvie Mountains breccia crops out discontinuously along two east-trending belts called the Northern Breccia Belt (NBB) and the Southern Breccia Belt (SBB). The NBB extends across approximately 40 km of the map area, and the SBB is about 15 km long. Individual bodies of OMB vary from dyke- and sill-like to pod-like. The breccia belts each coincide with a regional structure. The NBB coincides with a north side down reverse fault—an inferred ruptured anticline—called the Monster fault. The SBB coincides with a north side down fault called the Fifteenmile fault. These faults, at least in part, guided ascending breccia. The age of OMB is constrained by field relationships and galena lead isotope data. It is younger than the Gillespie Lake Group, and is at least as old as the lower Fifteenmile group because it intrudes both of these units. A galena lead isotope model age for the Hart River stratiform massive sulphide deposit that is in Gillespie Lake Group rocks is 1.45 Ga. Galena from veinlets cutting a dyke that cuts OMB in lower Fifteenmile group rocks is 0.90 Ga in age. Therefore the age of OMB formation is between 1.45 and 0.90 Ga. Ogilvie Mountains breccia (OMB) has been classified into monolithic (oligomictic) and heterolithic (polymictic) lithologies. These have been further divided by major matrix components—end members are carbonate-rich, hematite-rich and chlorite-rich. Monolithic breccias with carbonate matrices dominate the NBB. Heterolithic breccias are abundant locally in the NBB, but are prevalent in the SBB. Fragments were derived mainly from the Wernecke Supergroup. In the SBB fragments from the lower Fifteenmile group are present. Uncommon mafic igneous fragments were from local dykes. OMB are generally fragment dominated. Recognized fragments are up to several 10s of metres across and grade into matrix sized grains. Hydrothermal alteration has locally overprinted OMB and introduced silica, hematite and sulphide minerals. This mineralization has received limited attention from the mineral exploration industry. Rare earth element chemistry reflects a lack of mantle or deep-seated igneous process in the formation of OMB. However, this may be only an apparent lack because flooding by a large volume of sedimentary material could obscure a REE pattern indicative of another source. The genesis of OMB is significantly similar to modern mud diapirs. It is proposed that OMB originated from pressurized, underconsolidated fine grained limey sediments (Fairchild Lake Group). These were trapped below and loaded by turbidites (Quartet Group) and younger units. Tectonics and the initiation of major faults apparently triggered movement of the pressurized fluid-rich medium. The resulting bodies of breccia are sill-like and diapir-like sedimentary intrusions. Fluid-rich phases may have caused hydrofracturing (brittle failure) of the surrounding rocks (especially in the hanging wall). Breccia intrusion would have increased the width of the passage way while encorporating more fragments. Iron- and oxygen-rich hydrothermal fluids apparently were associated with the diapirism. Presumably these fluids are responsible for the high contents of hematite and iron carbonate in fragments, and especially, in the matrix of the breccias. Exhalation of these fluids may have formed the sedimentary iron formations that are spatially associated with the breccias. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate