Sequence Stratigraphy in Proterozoic Successions

Sedimentological logging and facies mapping have been used to identify depositional sequences bounded by subtle but regionally persistent unconformities in rocks of Proterozoic age in the western United States, South Australia, and northwestern Canada. We conclude from these studies that the sequence stratigraphic approach is of considerable importance for intrabasinal time correlation in the Proterozoic and for facies interpretation and basin analysis in Proterozoic rocks

Sequence Stratigraphy and Evolution of a Basin-Slope Succession: The Late Proterozoic Wonoka Formation, Flinders Ranges, South Australia

A shelf to basin‐slope transition is vertically and laterally exposed within the Late Proterozoic Wonoka Formation in the northern Flinders Ranges of South Australia. The shelf to basin‐slope transition can be divided into four units (C to F) which are defined on the basis of facies, sedimentary structures, contacts, stratal geometry, and the type and abundance of down‐slope mass movement. The lowest unit (C) is mudstone dominated and parallel laminated with rare synsedimentary slides. Unit D, a thin, resedimented siliciclastic‐carbonate unit deposited on a sequence boundary at the end of unit C progradation, displays a lateral facies change from well bedded ‘outer shelf deposits in the east to basin‐slope debris flows in the west. Unit E forms a shallowing and coarsening upward succession from ‘outer shelf siltstone to ‘inner shelf storm wave influenced sandstone deposits. The unit thickens westwards, in the interpreted down‐slope direction, where it becomes finer grained and thinner bedded and displays an increasing abundance of synsedimentary slides. Unit F, deposited on an inferred shelf to basin‐slope transition, coarsens and shallows upward, thickens to the west and contains the highest percentage of sandstone and synsedimentary slides. Unit G, deposited at shelf depths, also shallows and coarsens upward from a thin, basal carbonate‐siliciclastic member, with sandstone increasing upsection to a gradational contact with the Pound Subgroup. Three sequences can be defined within this transition on the basis of facies, stratal terminations, and facies discontinuities at inferred sequence boundaries. Each sequence is marked by a transgressive base, overlain by a shallowing‐upward succession. On the inferred shelf and near the shelfbreak, toward the top of the succession, facies discontinuities at sequence boundaries are more obvious, with distinct contrasts in lithology and inferred palaeoenvironments; farther down‐slope and stratigraphically lower in the succession, the boundaries are cryptic, and only lateral tracing of the contacts from the shelf to the slope or the observation of stratal terminations permits them to be recognized

Depositional Sequence Analysis Applied to Late Proterozoic Wilpena Group, Adelaide Geosyncline, South Australia

The initial application of depositional sequence analysis to selected stratigraphic sections through outcropping Late Proterozoic strata of the Adelaide Geosyncline in South Australia has identified major depositional sequences within the several‐kilometre‐thick Wilpena Group. Sharp facies shifts in vertical stratigraphic sections are proposed as actual sequence boundaries which, provided they are the result of eustatic sea level variations, may be key elements for future attempts at inter‐regional chronostratigraphic correlation. Two major sequence boundaries are identified, one at the base of the Nuccaleena Formation (boundary A) and a second at the top of the Brachina Subgroup (boundary B). These are attributed to significant basinward shifts in coastal onlap resulting in subaerial exposure and at least localized erosion, followed in each case by establishment of relatively deepwater environments. A somewhat different boundary (boundary C) is associated with an interval of diagenetic dolostone interbeds and is interpreted either as a downlap surface within a sequence, or as a combined deepwater sequence boundary and downlap surface. It may have developed during an episode of reduced sediment input in response to a period of maximum transgression. Alternatively it may represent a hiatus at the termination of a depositional sequence, prior to subsequent downlap or onlap of the succeeding sequence. Boundary C lies a few metres below the stratigraphic level from which kilometre‐deep canyons have incised underlying sequences. These canyons, which are infilled by a complex succession of carbonate breccias, conglomerates, sandstone and mudstone, may have been eroded in a submarine setting by turbidity currents. Such a model requires a significant increase in rate of eustatic sea level fall or a decrease in the rate of tectonic subsidence, in order to move the locus of coastal onlap to the vicinity of the shelf edge. If the cause was eustatic, evidence for it should be found at an equivalent sequence boundary in Late Proterozoic basins remote from the Adelaide Geosyncline. Alternatively, the canyons may have been eroded in a subaerial setting and infilled by coastal sediments during an ensuing period of relative sea level rise. In this model a considerably greater drop in relative sea level is required, most likely related to localized tectonic uplift

Working Hypotheses for the Origin of the Wonoka Canyons (Neoproterozoic), South Australia

Recent attempts to apply concepts of sequence stratigraphy to the Neoproterozoic Wilpena Group of the Adelaide "geosyncline" in South Australia have provided an important new method for improving the resolution of intrabasinal correlation in sparsely fossiliferous and unfossiliferous strata. Eight regional unconformities are now recognized within or bounding the Wilpena Group. The most prominent of these, at or near the base of the Wonoka Formation, is expressed by a series of spectacular incised valleys or canyons, some more than 1 km deep and dated as approx 630 to 580 Ma. The canyons developed following an interval of continental rifting that took place between about 800 and 700 Ma and prior to a second phase of accelerated subsidence of uncertain origin in Early Cambrian time (after about 560 Ma). Subsidence during the intervening span of more than 140 my was in part of thermal origin and in part due to the withdrawal of buried salt at depth, but it may also have involved additional extension for which little direct structural evidence is preserved. The canyons are incised into a succession of shallow marine mainly terrigenous strata that accumulated in a broad north- and east-facing ramp. They are exposed in two distinct belts within and east of the Flinders Ranges, in an area that is about 275 km in a north-south direction and about 175 km east-west. The canyons are inferred to have been filled by shallow marine sediments primarily on the basis of sedimentary structures interpreted as combined flow and oscillation ripples and hummocky cross-stratification. If this is correct, development of the canyons was related to regional lowering of depositional base level by more than 1 km. Recent work also indicates a second phase of valley incision at an unconformity immediately above the main canyons and involving a relative sealevel fall of at least 200 m. Two working hypotheses are advanced to account for the origin of the Wonoka canyons: regional uplift and an evaporitic lowering of sealevel in an isolated basin, analogous to the Messinian event in the Mediterranean. Any regional uplift would likely have been of tectonic origin. Diapirism associated with buried salt cannot account for the wide distribution of erosion or for pronounced uplift in an extensional setting lacking evidence for basin inversion or compressional deformation coeval with sedimentation. One possible mechanism for tectonic uplift involves inhomogeneous extension of the lithosphere, with the amount of extension balanced at all levels on a regional scale possibly by means of detachment faults. Possible difficulties with this hypothesis are the requirement of relatively uniform uplift over distances of hundreds of kilometers and the fact that repeated large-scale lowering of base level implies oscillatory vertical motions that are not readily explained. An evaporitic drawdown accounts for the wide distribution and scale of the canyons and for repeated lowering of base level. Possible difficulties in this case are the presence within the canyon fill of facies that have been interpreted to be of tidal origin; the fact that unlike the Messinian crisis in the Mediterranean, the Wonoka canyons do not appear to have been drowned rapidly; and the lack of direct evidence for evaporities of appropriate age. Neither hypothesis accounts for the apparent absence of appreciable meteoric diagenesis in areas far removea from sites of canyon incision. Two additional conclusions are as follows. First, neither of the hypotheses precludes eustasy as an important control on sedimentation. Sequence stratigraphic comparisons with other basins of the same general age should focus primarily on the time of formation of sequence boundaries not on the geometry of the boundaries or the facies involved. Second, a drawdown in excess of 1 km implies that the adjacent basin was originally at least this deep and hence likely underlain at least locally by highly attenuated continental crust or oceanic crust. Either hypothesis therefore has important implications for the tectonic development of the Adelaide geosyncline

Sequence Stratigraphy and the Interpretation of Neoproterozoic Earth History

The application of sequence stratigraphy to Neoproterozoic successions is important for improving the resolution of time-correlation within individual sedimentary basins and potentially at a global scale. The methodology is illustrated in this paper by reference to two contrasting examples from the Flinders Ranges (Adelaide geosyncline) of South Australia, where the younger part of the Neoproterozoic to earliest Cambrian succession (∼ 770 Ma to ∼ 540 Ma) has been divided into thirteen unconformity-bounded depositional sequences. One of the most prominent sequence boundaries, at or near the base of the Wonoka Formation, is characterized by a series of buried canyons as much as 1 km deep. High-resolution sequence-stratigraphic studies at Umberatana syncline continue to support the view that the canyons were cut subaerially and filled by fluvial and shallow-water sediments. In contrast to the Wonoka canyons, sequence boundaries interpreted at the base of the Nuccaleena Formation/Seacliff Sandstone and near the top of the ABC Range Quartzite are relatively subtle, with only limited evidence for erosion and valley incision. Four sequence boundaries, at the level of the Sturtian and Marinoan (Varanger?) glacial deposits and in the vicinity of the Precambrian-Cambrian boundary, are thought to correlate with surfaces in the Amadeus basin of central Australia. Other prominent sequence boundaries, including the Wonoka canyons and surfaces within the upper part of the Wonoka Formation and at the base of the Ediacara Member of the Rawnsley Quartzite, correspond with a relatively condensed section in the Amadeus basin, and their lateral persistence beyond the Adelaide geosyncline is therefore difficult to evaluate. Given the lack of precision in biostratigraphy and isotope geochemistry in Neoproterozoic rocks, and in a marked departure from Phanerozoic practice, we recommend placement of a terminal Proterozoic GSSP at a sequence boundary. A prime candidate in Australia is the sequence boundary at the base of the Nuccaleena Formation/Seacliff Sandstone, immediately above the Marinoan glacial rocks in the Adelaide geosyncline, and its likely correlative at or near the base of the Gaylad Sandstone in the Amadeus basin

Late Proterozoic Patsy Springs Canyon, Adelaide Geosyncline: Submarine or Subaerial Origin?

A significant aspect of Late Proterozoic sedimentation in the Adelaide Geosyncline, South Australia, is the presence of kilometre-deep erosional incisions which have been termed canyons. These structures were formerly described to be of submarine origin, cut and filled in an inferred basin-slope setting by subaqueous processes. Subsequent detailed research, particularly on a specific incision known as Patsy Springs Canyon, indicates that sedimentary structures within some of the canyon-filling sediments are indicative of deposition above fair weather wave base. In addition, an unusual carbonate unit, which is observed to veneer upper portions of canyon shoulders and to contribute to carbonate breccias interbedded with canyon-fill, has a stable isotope signature which may imply a non-marine origin. The presence of the carbonate veneer, where it is in situ, suggests that at least upper portions of the canyons could have been emergent during the canyon-filling phase. Considering these observations, and combining them with regional stratigraphical relationships, an alternative model for canyon genesis is proposed involving subaerial erosion and subsequent filling by coastal onlap. Such a model requires base-level changes of the order of 1 km, in order to account for observed canyon cutting and filling. Vertical movements associated with halokinesis, or thermally-induced uplift of the order of 1 km, could have resulted in the observed erosional events. Alternatively, a Messinian-style evaporitic lowering of base-level is currently receiving serious attention. With present knowledge this mechanism most satisfactorily explains all observations

Structural Properties of Central Galaxies in Groups and Clusters

Using a representative sample of 911 central galaxies (CENs) from the SDSS DR4 group catalogue, we study how the structure of the most massive members in groups and clusters depend on (1) galaxy stellar mass (Mstar), (2) dark matter halo mass of the host group (Mhalo), and (3) their halo-centric position. We establish and thoroughly test a GALFIT-based pipeline to fit 2D Sersic models to SDSS data. We find that the fitting results are most sensitive to the background sky level determination and strongly recommend using the SDSS global value. We find that uncertainties in the background translate into a strong covariance between the total magnitude, half-light size (r50), and Sersic index (n), especially for bright/massive galaxies. We find that n depends strongly on Mstar for CENs, but only weakly or not at all on Mhalo. Less (more) massive CENs tend to be disk (spheroid)-like over the full Mhalo range. Likewise, there is a clear r50-Mstar relation for CENs, with separate slopes for disks and spheroids. When comparing CENs with satellite galaxies (SATs), we find that low mass (<10e10.75 Msun/h^2) SATs have larger median n than CENs of similar Mstar. Low mass, late-type SATs have moderately smaller r50 than late-type CENs of the same Mstar. However, we find no size differences between spheroid-like CENs and SATs, and no structural differences between CENs and SATs matched in both mass and colour. The similarity of massive SATs and CENs shows that this distinction has no significant impact on the structure of spheroids. We conclude that Mstar is the most fundamental property determining the basic structure of a galaxy. The lack of a clear n-Mhalo relation rules out a distinct group mass for producing spheroids, and the responsible morphological transformation processes must occur at the centres of groups spanning a wide range of masses. (abridged)Comment: 22 pages, 14 figures, submitted to MNRA

Ubiquitous Presence of Fe(II) in Aquatic Colloids and Its Association with Organic Carbon

Despite being thermodynamically less stable, small ferrous colloids (60 nm to 3 μm in diameter) remain an important component of the biogeochemical cycle at the Earth’s surface, yet their composition and structure and the reasons for their persistence are still poorly understood. Here we use X-ray-based Fe L-edge and carbon K-edge spectromicroscopy to address the speciation and organic–mineral associations of ferrous, ferric, and Fe-poor particles collected from sampling sites in both marine and freshwater environments. We show that Fe(II)-rich phases are prevalent throughout different aquatic regimes yet exhibit a high degree of chemical heterogeneity. Furthermore, we show that Fe-rich particles show strong associations with organic carbon. The observed association of Fe(II) particles with carboxamide functional groups suggests a possible microbial role in the preservation of Fe(II). These finding have significant implications for the behavior of Fe(II) colloids in oxygenated waters, and their role in different aquatic biogeochemical processes

Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system

Author Posting. © Nature Publishing Group, 2007. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature 450 (2007): 407-410, doi:10.1038/nature06273.Continental erosion controls atmospheric carbon dioxide levels on geological timescales through silicate weathering, riverine transport and subsequent burial of organic carbon in oceanic sediments. The efficiency of organic carbon deposition in sedimentary basins is however limited by the organic carbon load capacity of the sediments and organic carbon oxidation in continental margins. At the global scale, previous studies have suggested that about 70 per cent of riverine organic carbon is returned to the atmosphere, such as in the Amazon basin. Here we present a comprehensive organic carbon budget for the Himalayan erosional system, including source rocks, river sediments and marine sediments buried in the Bengal fan. We show that organic carbon export is controlled by sediment properties, and that oxidative loss is negligible during transport and deposition to the ocean. Our results indicate that 70 to 85 per cent of the organic carbon is recent organic matter captured during transport, which serves as a net sink for atmospheric carbon dioxide. The amount of organic carbon deposited in the Bengal basin represents about 10 to 20 per cent of the total terrestrial organic carbon buried in oceanic sediments. High erosion rates in the Himalayas generate high sedimentation rates and low oxygen availability in the Bay of Bengal that sustain the observed extreme organic carbon burial efficiency. Active orogenic systems generate enhanced physical erosion and the resulting organic carbon burial buffers atmospheric carbon dioxide levels, thereby exerting a negative feedback on climate over geological timescales

New genetic loci implicated in fasting glucose homeostasis and their impact on type 2 diabetes risk

Levels of circulating glucose are tightly regulated. To identify new loci influencing glycemic traits, we performed meta-analyses of 21 genome-wide association studies informative for fasting glucose, fasting insulin and indices of beta-cell function (HOMA-B) and insulin resistance (HOMA-IR) in up to 46,186 nondiabetic participants. Follow-up of 25 loci in up to 76,558 additional subjects identified 16 loci associated with fasting glucose and HOMA-B and two loci associated with fasting insulin and HOMA-IR. These include nine loci newly associated with fasting glucose (in or near ADCY5, MADD, ADRA2A, CRY2, FADS1, GLIS3, SLC2A2, PROX1 and C2CD4B) and one influencing fasting insulin and HOMA-IR (near IGF1). We also demonstrated association of ADCY5, PROX1, GCK, GCKR and DGKB-TMEM195 with type 2 diabetes. Within these loci, likely biological candidate genes influence signal transduction, cell proliferation, development, glucose-sensing and circadian regulation. Our results demonstrate that genetic studies of glycemic traits can identify type 2 diabetes risk loci, as well as loci containing gene variants that are associated with a modest elevation in glucose levels but are not associated with overt diabetes