Yvonne Bone

Bryozoans as carbonate sediment producers on the cool-water Lacepede Shelf, southern Australia

Abstract Modern sediments on the Lacepede Shelf and adjacent slope are typical mid-latitude, cool-water, palimpsest deposits, dominated by bryozoan, mollusc and quartz particles. Bryozoan bioclasts form autochthonous and, to a lesser extent, allochthonous accumulations across the grain-size spectrum, from mud to boulder size. The bryozoan skeletons are grouped, on the basis of architecture and style of disarticulation/fragmentation, into nine morphotypes. Erect rigid forms comprise (1) fenestrate, (2) foliose, (3) flat, robust branching and (4) delicate, branching morphotypes. Large Adeona sp. skeletons are recognised as a separate type of fenestrate form because of their distinctive shape, mineralogy and restricted environment. Erect flexible forms are either (5) articulated branching or (6) articulated zooidal. Other important morphotypes are (7) encrusting, (8) nodular/ arborescent and (9) vagrant. Differences in skeletal architecture result in distinct particle types. Encrusting morphotypes on soft organic substrates and the wide variety of erect rigid forms are broken into fragments. Erect flexible forms disintegrate upon death into definitive sand and silt size grains. Encrusting morphotypes on hard substrates, nodular/arborescent and vagrant forms remain whole, and when large, accumulate in place. Morphotypes are mineralogically distinct and so bryozoan sediment composition is environmentally controlled. Shallow water (

Holocene carbonate sedimentation on the west Eucla Shelf, Great Australian Bight: a shaved shelf

The southern continental margin of Australia is a cool-water carbonate sedimentary province located in a high-energy, swell-dominated oceanographic setting. A vibrocore transect of 14C-dated sediments across the centre of the Eucla Shelf is the first record of Holocene shelf deposition in the Great Australian Bight. Much of the seafloor shallower than 70 m water depth, the base of wave abrasion, is bare Cenozoic limestone, in some places encrusted by (?) Late Pleistocene, coral-rich, limestone that is cemented by high-magnesium calcite (12 mole% MgCO3). The areally extensive, 100 km-wide, hard, bored substrate supports an epibiota of coralline algae, minor bryozoans and soft algae or is covered by patches of Holocene sediment up to 1.5 m thick; generally a basal bivalve lag (< 3 ka) overlain by quartzose-bioclastic palimpsest sand. This pattern of active carbonate production but little accretion on the wave-swept mid- to inner-shelf is similar to that on other parts of the southern Australian continental margin. The term shaved shelf is proposed for this style of carbonate platform, formed by alternating periods of sediment accretion, cementation and erosion. The palimpsest sand is typically rich in bivalves, coralline algae and locally, detrital dolomite. Outer shelf Holocene sediment, below the base of wave abrasion but inboard of the shelf edge, is a metre-thick unit of fine, microbioclastic muddy sand with minor delicate bryozoans overlying a 9–13 ka rhodolith gravel. Some of this outer shelf sediment appears to have been resedimented. The shelf edge is a sandy and rocky seafloor with active bryozoan growth and sediment production. The Holocene sediments are enriched in coralline algal particles and conspicuous large foraminifers (cf. Marginopora) and depleted in bryozoans, as compared to coeval deposits on the Lacepede and Otway shelves off southeastern Australia. These differences are interpreted to reflect warmer waters of the Leeuwin Current and prevalent downwelling in this area as opposed to the general upwelling and colder waters in the east.

Petrogenesis of Cenozoic, Temperate Water Calcarenites, South Australia: A Model for Meteoric/Shallow Burial Diagenesis of Shallow Water Calcite Sediments

ABSTRACT The Gambler Limestone is one of several extensive, shallow-water shelf carbonates of Eocene to Miocene age exposed along the southern margin of Australia. It is a muddy to grainy bryozoan calcarenite, with accessory benthonic foraminifers and echinoids. The sediments, originally composed almost entirely of calcite or Mg-calcite, have been in vadose and phreatic environments for over 10 Ma, yet are virtually unlithified. The only cements of any consequence are epitaxial on echinoids. Numerous karst features, dolines, caves, speleothems and surface karren attest to prolonged residence in the meteoric zone. The Gambler Limestone is presently one of the best fresh water aquuifers in Australia. Most flow occurs through intergranular pores in sediments with over 30% porosity. Cementation is by minor intergranular pressure solution which has developed under overburden of less than 100 m. The overlying Naracoorte Limestone (Miocene), a calcarenite of warmer water aspect, contains numerous aragonite molds and is well-cemented. We propose that such cool water limestones are a better model for the meteoric diagenesis of calcite sediments of all ages than are aragonite-rich tropical sediments. It is probable that many similar early and middle Paleozoic calcite limestones may have been in the meteoric zone for prolonged periods yet contain little or no petrographic or geochemical record of such exposure.

Fluid inclusion and vitrinite-reflectance geothermometry compared to heat-flow models of maximum paleotemperature next to dikes, western onshore Gippsland Basin, Australia

Abstract Nine basalt dikes, ranging from 6 cm to 40 m thick, intruding the Upper Jurassic–Lower Cretaceous Strzelecki Group, western onshore Gippsland Basin, were used to study maximum temperatures ( T max ) reached next to dikes. T max was estimated from fluid inclusion and vitrinite-reflectance geothermometry and compared to temperatures calculated using heat-flow models of contact metamorphism. Thermal history reconstruction suggests that at the time of dike intrusion the host rock was at a temperature of 100–135°C. Fracture-bound fluid inclusions in the host rocks next to thin dikes ( T max systematically increases towards the dike margin to at least 500°C. The estimated T max next to the thickest dike (thickness ( D )=40 m) suggests an extended zone of elevated R v-r to at least a distance from the dike contact ( X ) of 60 m or at X / D >1.5, using a normalized distance ratio used for comparing measurements between dikes regardless of their thickness. In contrast, the pattern seen next to the thin dikes is a relatively narrow zone of elevated R v-r . Heat-flow modeling, along with whole rock elemental and isotopic data, suggests that the extended zone of elevated R v-r is caused by a convection cell with local recharge of the hydrothermal fluids. The narrow zone of elevated R v-r found next to thin dikes is attributed to the rise of the less dense, heated fluids at the dike contact causing a flow of cooler groundwater towards the dike and thereby limiting its heating effects. The lack of extended heating effects suggests that next to thin dikes an incipient convection system may form in which the heated fluid starts to travel upward along the dike but cooling occurs before a complete convection cell can form. Close to the dike contact at X / D R v-r often decreases even though fluid inclusion evidence indicates that T max is still increasing. Further, fluid inclusion evidence indicates that the evolution of water vapor or supercritical fluids in the rock pores corresponds to the zone where R v-r begins to decrease. The generation of the water vapor or supercritical fluids near the dike contact seems to change vitrinite evolution reactions. These metamorphic conditions, closer to the dike than X / D =0.3 make vitrinite-reflectance unreliable as a geothermometer. The form of the R v-r profile, as it indicates T max , can be interpreted using temperature profiles estimated from various heat-flow models to infer whether the dike cooled by conduction, incipient convection, or a convection cell. A contact aureole that consists of decreasing R v-r or T max extending out to X / D ≥2 and that has a T contact ≫( T magma + T host )/2 appears to be a signature of simple conductive cooling. Incipient convection is indicated by a R v-r profile that decreases to background levels at X / D R v-r profile and consistently high R v-r that may not decrease to background levels until beyond distances of X / D >1.5.

Bryozoan growth habits; classification and analysis

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

Cool-water Carbonate Production from Epizoic Bryozoans on Ephemeral Substrates

Abstract Bryozoan skeletons are a dominant constituent of cool-water carbonate sediments in the Cenozoic of southern Australia. The primary substrate on much of the modern continental shelf is loose sediment that is reworked intermittently to 200+ m water depth by storm waves. Availability of stable substrate is a limiting factor in the modern distribution of bryozoans in this setting. As a result, a significant proportion of the sedimentologically important modern bryozoans (30–250 m water depth) live attached to sessile, benthic invertebrate hosts that possess organic or spicular skeletons. Hosts such as hydroids, ascidian tunicates, sponges, soft worm tubes, octocorals, and other lightly-calcified and articulated bryozoans provide ephemeral substrates; after death, host skeletons disarticulate and decay, leaving little or no body fossil record. The calcareous sediments produced by these epizoic bryozoans from ephemeral substrates result in loose particles that rarely preserve substratal relationships, but potentially retain diagnostic basal attachment morphologies. Although the best known examples of epizoic carbonate production on ephemeral substrates are from the southern Australian margin, this may be an important phenomenon both globally and in the fossil record. Bryozoan sediment production from epizoans on ephemeral substrates would seem, however, to have a scant record prior to the Cretaceous.

Shallow burial dolomitization and dedolomitization of Mid-Cenozoic, cool-water, calcitic, deep-shelf limestones, southern Australia

ABSTRACT Oligocene to mid-Miocene, deep-shelf, bryozoan-rich limestones across southern Australia are variably altered to gray to orange, Ca-rich, medium-crystalline, sucrosic dolomite. Degree of replacement ranges from scattered rhombs to complete dolostone units several tens of meters thick and many kilometers in areal extent. The locale and timing of dolomitization are tightly constrained to shallow burial and mid- to late Miocene. Dolostone varies from friable to dense and well lithified. Dolomitization is fabric specific; muddy sediments are preferentially replaced; calcite bryozoans and brachiopods form biomolds. Geochemistry suggests that dolomitization was predominantly by seawater-limestone interaction, but admixing of continental possibly meteoric groundwater is required by data on stable and radiogenic isotopes and trace elements. Sr isotopes confirm a mid-Miocene age for the dolomite, if precipitated from seawater with no inherited limestone values. Dolomite crystals have undergone variable degrees of dissolution, and meteoric calcite cement has locally filled the resultant rhombic voids. Dissolution began in the crystal cores, implying some sort of metastability, and expanded outward until, in some ins ances, the whole dolomite crystal was dissolved, leaving dolomolds in otherwise unaffected limestone. This dolomite occurrence, well constrained by regional geology, shows that widespread, porous and permeable, fabric-destructive, medium-crystalline, sucrosic dolostone can form in the shallow subsurface soon after sediment deposition. Such dolomite, however, may be metastable in the presence of variably mixed continental-marine waters, and newly formed crystals can undergo dedolomitization soon after precipitation.

Brachiopod δ18O values do reflect ambient oceanography: Lacepede Shelf, southern Australia

Although commonly used as proxies for attributes of ancient ocean waters, the δ 18 O values of brachiopods from modern seas are little studied. To evaluate the utility of brachiopods as recorders of regional oceanography, modern shells from the Lacepede Shelf (25 000 km 2 ) of southern Australia were analyzed for δ 18 O, and the results were compared to the values of ambient seawater. Southern Ocean waters cover this area of extensive cool-water carbonate deposition, but there are distinct sectors of seasonal upwelling and lesser fluvial outflow. δ 18 O values of brachiopods across the environmental spectrum from 40 to 300 m water depth are in general isotopic equilibrium with surrounding seawater. Nevertheless, δ 18 O values from individual sample sites vary as much as 0.60‰. The area of cold-water upwelling in particular is clearly delimited by a group of high δ 18 O values. The range of values across this one shelf, on the order of 2.5‰, is similar to the range of values postulated on the basis of similar results for secular changes in many ancient oceans.

Stable isotope aspects of modern molluscs, brachiopods, and marine cements from cool-water carbonates, Lacepede Shelf, South Australia

The isotopic composition of some modern gastropods, bivalves, and brachiopods from the Lacepede Shelf of South Australia were investigated to determine the isotopic equilibrium and dis-equilibrium states of biogenic and non-biogenic (inorganic) carbonates and to understand the kinetic and metabolic isotopic effects on precipitating carbonates and the isotopic fractionation between these different carbonates and dissolved bicarbonate. Seawater and high Mg-calcite (HMC) marine cements from different depths were also analyzed to determine equilibrium isotopic composition. This study shows that the shells of some cool-water gastropods and bivalves precipitate in isotopic equilibrium with their ambient waters while brachiopod shells show 18O values heavier than equilibrium values and coralline algae, and ahermatypic corals exhibit significant metabolic and kinetic effects. The isotopic equilibrium precipitation seen in these biogenic carbonates results from lower precipitation rates that are, in turn, caused by the lower bottom temperatures in this area.

Evolving mineralogy of cheilostome bryozoans

Abstract Cheilostomes dominate bryozoan faunas today and are the only order of bryozoans to have evolved aragonitic, calcitic, and bimineralic skeletons. New XRD analyses of 177 recent species and 34 Cretaceous–Eocene species are combined with published data to probe the mineralogical evolution of cheilostomes. This is undertaken with particular reference to the effects of the late Paleogene transition from calcite to aragonite seas believed to have been driven by the increasing Mg/Ca ratio in seawater. Aragonite was absent from all of the Cretaceous and Paleocene cheilostomes analyzed, even though most came from deposits preserving aragonitic mollusk shells, but was detected in four distantly related cheilostomes from the middle Eocene (Lutetian). Examples of cheilostomes preserved as partial molds, however, suggests that bimineralic species with aragonitic outer skeletal layers may have originated as early as the Maastrichtian. A strong latitudinal gradient was evident in cheilostome mineralogy, with the proportion of aragonitic and bimineralic recent species increasing towards the tropics. Unfortunately, relatively few low-latitude bryozoans have been described from the Cretaceous and Cenozoic fossil record, where aragonitic species are likely to be most numerous and may have their oldest occurrences. A combined database of cheilostome mineralogy shows aragonite to be widely distributed across Cheilostomata, occurring in numerous genera and families belonging to the three most diverse subgroups (Flustrina, Umbonulomorpha, and Lepraliomorpha) as well as one genus of Malacostegina. In spite of the lack of a robust phylogeny for cheilostomes, it is clear that aragonite has evolved independently on multiple occasions, the earliest acquisitions antedating the onset of aragonite seas, although apparently accelerating after this transition.