Nancy Chow

Magnetic hysteresis of limestones: facies control?

Abstract The hysteresis properties of 116 non-red, marine limestones from 92 localities indicate that remanence is carried by magnetite of pseudo-single-domain (PSD) and small multidomain (MD) size. Pelagic limestones have paramagnetic matrices and hysteresis properties compatible with larger PSD or MD grain sizes of magnetite, probably associated with detrital clay minerals introduced by pelagic rain-out. Thus they may be less suitable recorders of stable remanence. Other limestone facies (excepting dolomitized examples) have diamagnetic matrices. They include shallow—subtidal limestones which tend to have smaller PSD sizes of magnetite, as do backreef—lagoonal, undifferentiated-shelf and reef facies. It is believed that the wide geographical and temporal range of samples minimizes effects related to post-compaction groundwater flow (late diagenesis) and that the associations recognized should be tested in future studies.

The depositional record of the Frasnian/Famennian boundary interval in a fore-reef succession, Canning Basin, Western Australia

Abstract The effects of the global Frasnian/Famennian (F/F) biotic crisis on the shallow-marine reef-builders and associated biota in a laterally extensive late Frasnian reef complex on the Lennard Shelf, northern Canning Basin, are recorded in the well-exposed fore-reef succession (Napier Formation) of the Napier Range. The conodont-defined F/F boundary lies within a distinctive and mappable stratigraphic interval up to 15 m thick which separates highly fossiliferous Frasnian from markedly less fossiliferous Famennian carbonate and siliciclastic lithofacies. Marked lateral facies variation within the interval means that the actual boundary is not represented by a discrete bed or surface. The interval is characterised by a range of lithofacies that record very low sedimentation rates on the slope and reworking and bioturbation of slope strata indicating a condensed interval with negligible sediment input from contemporaneous carbonate platform production. Siliciclastic lithofacies and abundant sponge debris are locally developed. Macrofossil content is generally very low but there is no evidence for anoxic or hypoxic conditions during deposition at a range of palaeowater depths. Allochthonous blocks of stromatoporoid-bearing limestone are absent from the uppermost Frasnian fore-reef strata and stromatoporoid debris is significantly reduced prior to the end of the Frasnian. This suggests that extinction of the stromatoporoid reef margins occurred before the F/F boundary with microbial communities dominating the platform margins prior to the Famennian. Sedimentological and biostratigraphic evidence suggests that a series of sea-level fluctuations, in response to basin tectonism, generated recurring or ongoing environmental stress in the stromatoporoid-dominated reefal communities because of recurring reductions in living space, influxes of siliciclastic sediment supply from the proximal highlands and quite possibly generation of toxic (too saline or too nutrient-rich) lagoonal water. These conditions were potentially superimposed on a global regime of incipient cooling climate leading to widespread glaciation in the Late Carboniferous.

Origin of Authigenic Mn-Fe Carbonates and Pore-Water Evolution in Marine Sediments: Evidence from Cenozoic Strata of the Arctic Ocean and Norwegian-Greenland Sea (Odp Leg 151)

Early diagenetic Mn-Fe carbonates, occurring in thin layers, nodules, and burrows, were recovered from Cenozoic silts and clays from the Arctic Ocean and the Norwegian-Greenland Sea during Ocean Drilling Program Leg 151. At Sites 909 (Fram Strait; Miocene-Quaternary) and 911 (Yermak Plateau; Pliocene-Quaternary), fine-crystalline siderite, enriched in Ca and/or Mg, is the predominant carbonate. Variable δ13C values (-21.8 to +7.9‰ PDB) suggest that dissolved carbon was derived from the Fe-reduction suboxic zone, the oxidation of marine organic matter in the bacterial sulfate-reduction zone, and the early stages of methanogenesis. The range of δ18O values for siderite (-6.3 to +5.1‰ PDB) indicates precipitation over a temperature range of 4-56°C during successive burial. At Site 913 (East Greenland Margin; Eocene-Quaternary), concentrically zoned microspherules and rhombohedra of ferroan rhodochrosite and manganoan siderite, 20-600 μm in diameter, are the main authigenic carbonates. Although they have low δ13C values (-22.3 to -13.8‰ PDB), elevated pore-water sulfate concentrations indicate low sulfate-reducing activity and suggest that dissolved carbon may have been partially derived from thermogenic methane. The variable δ18O values for rhodochrosite (-11.9 to +1.5‰ PDB) and siderite (-11.4 to -10.3‰ PDB) suggest precipitation at elevated temperatures (60-100°C) during progressive burial. The close association with barite cement suggests that carbonate precipitation at Site 913 may have been influenced by hydrothermal fluids. However, active circulation of hydrothermal fluids is precluded by the occurrence of geochemical gradients in Site 913 pore waters. Pore waters at the three sites reflect various evolutionary pathways that are partly due to variations in the extent of interaction with volcanic ash layers and the underlying basaltic crust, which led to an overall decrease in Mg2+, K+, and δ18O and an increase in Ca2+ with depth. However, pore waters at Site 913 have evolved more extensively to Na-Ca-Cl brines. The sources of freshwater that cause the downhole decrease in Cl- and Na+ concentrations at Sites 909 and 913 are poorly constrained. The possible influence of meteoric-water incursion is suggested by the δD-δ18O relationship of pore waters and the low δ18O values of Site 913 authigenic carbonates.

Oxic facies and the Late Devonian mass extinction, Canning Basin, Australia

The close association of anoxic or dysoxic sedimentary rocks and the major Late Devonian (Frasnian–Famennian) mass extinction has focused considerable attention on anoxia as the major cause or as a major factor in a multicausal scenario. The record of the Late Devonian biotic crisis in the well-known reef complexes of northwestern Australia (Canning Basin), in contrast to many localities elsewhere, does not display sedimentological evidence of anoxia through the Frasnian–Famennian boundary interval. Analysis of continuous drill core through this interval has yielded three positive δ 13 C isotopic excursions, only one of which coincides with total organic carbon (TOC) maxima in our data. Multi-element geochemical proxies suggest that TOC maxima preceding positive shifts in δ 13 C most likely resulted from higher productivity caused by nutrient influx from continental weathering, given the close association between TOC maxima and regional relative sea-level falls. Our interpretation supports the view that anoxia was not a fundamental driver of mass extinction and stresses the importance of integrated data sets and understanding regional controls on environmental changes and/or stresses.

Syndepositional fault control on lower Frasnian platform evolution, Lennard Shelf, Canning Basin, Australia

Syndepositional faulting was a major control on internal platform stratigraphy in a Frasnian reef complex on the southeastern Lennard Shelf, northern Canning Basin, Australia. By combining platform (mostly backreef) facies distributions with key stratal surfaces and biostratigraphic data, we have developed a temporal framework for the Hull Range area. Platform evolution was controlled by normal faults via an initial tilt block geometry and subsequent differential subsidence and accommodation across the platform. Three third-order, flooding surface–bounded platform phases are recorded. Basal shallow marine siliciclastic facies were deposited in topographic lows adjacent to the hanging wall and close to internal faults. Early carbonate deposition in dominantly deep subtidal environments suggests an overall ramp-style setting that deepened toward the southeast. This phase is capped by a sequence boundary represented by multiple paleokarst surfaces in the northwest that pass laterally into a major flooding surface to the southeast, above which the platform expanded toward the Mount Elma–Painted Rocks fault system. The overall stacking pattern above the sequence boundary–flooding surface is aggradational to progradational with higher-frequency shoaling trends, and locally developed exposure surfaces, related to higher-order relative sea-level changes. This second phase of platform growth ended with major flooding and a pronounced backstep of the leeward margin.

Frasnian reef evolution and palaeogeography, SE Lennard Shelf, Canning Basin, Australia

Abstract Frasnian reef complexes of the southeastern Lennard Shelf (northern Canning Basin) developed on tilt-block highs and their evolution was controlled by fault-related subsidence. Tectonic control on relative sea-level changes was, therefore, a major factor influencing Early–Middle Frasnian palaeogeography of the Lennard Shelf. However, palaeogeographic reconstruction is not consistent with simple landward (northward) backstepping and younging of reef complexes in response to basin extension and subsidence of fault blocks. Using a sequence-stratigraphic approach, in conjunction with sedimentological and biostratigraphic data, we propose that two neighbouring fault blocks (Lawford area on the eastern side of Bugle Gap and the Hull platform to the north) record a similar history and that the reef complexes on those blocks were initiated at similar times. Seven phases of Early–Middle Frasnian platform growth (Fr2–8) are identified. All are bounded by third-order flooding surfaces associated with backstepping of platform margins and three surfaces (defined by conodont Zone 4, Zone 6 and late Zone 6) correlate across the two fault blocks. Only one sequence boundary has been clearly identified and a second relative sea-level fall is proposed based on a major collapse event following progradation and associated coarse siliciclastic facies. We propose that the correlation of flooding events across the SE Lennard Shelf is related to episodes of basin-margin faulting centred on the large, long (c. 25 km) faults which border these blocks (shelf-parallel faults for the Lawford block and an oblique north-trending transfer zone for the Hull block). There is limited evidence for relative sea-level falls and those recognized most likely resulted from eustatic events. The correlation between the Lawford block and Hull platform suggests linkage between major NW-trending shelf-parallel and oblique transfer faults and an evolved rift system by the Early Frasnian.