Frequency Of Depositional Events Research Articles

Trace-fossil suites and composite ichnofabrics from meandering fluvial systems: The Oligocene Lower Freshwater Molasse of Switzerland

The Oligocene Lower Freshwater Molasse of Switzerland is host to spectacular outcrops of fluvial deposits. In distal areas of the Swiss Molasse Basin, deposits of a north-eastward draining meandering system are exposed. Combined sedimentologic and ichnologic observations in the Chli Sunnhalde and Roggwil quarries allow an improved understanding of animal-substrate interactions in deposits of meandering fluvial systems. Strata include meander-belt sandstone, levee sandstone and mudstone, crevasse-channel and splay sandstone, floodplain mudstone and sandstone, and paleosols. While the meander belt and the more proximal crevasse-channel deposits are typically unbioturbated, the more distal deposits show a clear biogenic overprint, with distal crevasse-channel, crevasse-splay, and floodplain deposits being intensely bioturbated.Simple grazing and locomotion trails (included in the Cochlichnus anguineus-Helminthoidichnites tenuis suite) were formed subaqueously in ponded areas of the floodplain. Meniscate burrows (Taenidium isp. suite) and root trace fossils occur along the margin of these floodplain water-bodies and at the top of crevasse-splay deposits. Some of the floodplain, crevasse-splay, and crevasse-channel deposits are pervasively bioturbated by crayfish burrows (Camborygma litonomos suite). Crevasse-splay deposits locally host escape trace fossils.Where cross-cutting relationships are apparent, a colonization sequence can be established as follows: the Cochlichnus anguineus-Helminthoidichnites tenuis suite is cross-cut by the Taenidium isp. suite, which in turn is cross-cut by the Camborygma litonomos suite and root trace fossils. Composite ichnofabrics are common in these meandering river deposits, recording progressive lowering of the water table in overbank subenvironments. Frequency of depositional events was a first-order control on degree of bioturbation. Position of the water table and degree of substrate consolidation exerted considerable influence over the types of trace fossils produced. Distinct bioturbation phases may result in the formation of different trace-fossil suites - representative of disparate ichnofacies - preserved on a single bedding plane. This study further supports the notion that meandering fluvial systems are characterized by trace-fossil suites belonging to the Scoyenia, Camborygma and/or Mermia ichnofacies.

Event Sedimentation, Deposition Rate, and Paleoenvironment Using CrowdedRosseliaAssemblages of the Bluesky Formation, Alberta, Canada

Abstract Spectacular examples of well-preserved, stacked Rosselia occur in cores of the Lower Cretaceous Bluesky Formation from Alberta's Peace River oil-sands deposit. Stacked Rosselia segments reflect burrow readjustments of single tracemakers following erosion and sedimentation events. Where present, these traces help to refine paleoenvironmental interpretations and can be used as proxies for assessing the magnitude and frequency of depositional events. In this study, six cores were logged in detail. Two representative cores containing densely packed (crowded), stacked Rosselia were described sedimentologically and ichnologically and burrow lengths and numbers were measured. The Bluesky sections are interpreted to represent storm-influenced, wave-dominated delta-front deposits on the basis of modal sedimentation measurements and the presence of storm- and wave-generated sedimentary structures. The stacked Rosselia burrows record up to four post-depositional re-establishments per tracemaker, with each readjustment representing a response to abrupt sediment accumulation. Decimeter-length burrows occurring in the core datasets suggest multiple decimeter-scale depositional events in a relatively short time frame, i.e., months to perhaps a few years, depending on the lifespan and growth rate of the organism. The use of Rosselia in this study illustrates a method of evaluating sedimentation events within a brief temporal window, one lying beyond the resolution of more traditional dating methods. Furthermore, the assemblages of crowded, stacked Rosselia are presented as the first documented occurrence from the Mesozoic and the first recognized in a cored succession.

Ichnological and sedimentological features of the Hongguleleng Formation (Devonian–Carboniferous transition) from the western Junggar, NW China

Two sections of the Late Devonian to Early Carboniferous Hongguleleng Formation in the Sha'erbuerti and Xiemisitai mountains of western Junggar, NW China were logged in detail and studied with particular reference to their trace fossil content and sedimentary features. The Hongguleleng Formation is composed essentially of shallow marine deposits formed in a volcanic arc-related setting. It contains trace fossils including indeterminate ellipsoid structures, indeterminate shallow-pit structures, Chondrites intricatus, Chondrites targionii, Chondrites isp., Dolopichnus gulosus,?Multina isp., Nereites missouriensis, Nereites isp., Palaeophycus tubularis, ?Phycosiphon isp., ?Phymatoderma isp., Rhizocorallium commune, Rosselia cf. socialis, Teichichnus rectus, Teichichnus ispp., Thalassinoides isp., and Zoophycos ispp. These ichnofossils and related bioturbation structures constitute five distinct groups of ichnofabrics: Chondrites–?Phymatoderma, Rhizocorallium commune, anemone dwelling/burrowing, Teichichnus, and Zoophycos ichnofabrics, conforming to an upward trend from the Cruziana to Zoophycos ichnofacies. The volcanic arc setting provides a natural laboratory to investigate the colonisation styles of benthic faunas during the Devonian, especially for environments with widespread and relatively frequent depositional events disturbing background deposition. The distal event beds are especially suitable for the distinction of the relationship between background and post-event ichnofaunas. Simple and composite ichnofabrics have been distinguished. The simple ichnofabrics are typified by the indeterminate ellipsoid “ichnofabric” made by sea anemones, representing short colonisation event and in situ death and burial resulted from relatively high sedimentation rates even during the periods between volcanic explosions. The composite ichnofabrics are characterised by the Chondrites–?Phymatoderma, ?Multina–Zoophycos and multi-tier Zoophycos ichnofabrics, representing vertical migration of benthic faunas in pace with either distal event deposition or steadily accreted sea floor. The colonisation window in the arc-related setting is closely related to sedimentation rates and frequency of depositional events, determining the completeness of burrow system structure and maturity of benthic communities.

Sequence stratigraphy and the resolution of the fossil record

Sequence stratigraphy, a major theoretical achievement of earth sciences, integrates facies associations and stratal architecture within a chronological framework of the geological record. The sequence stratigraphic model implies that sediment supply to the basin, preservation potential of individual horizons, and the resolution of paleontological data co-vary with base-level fluctuations, especially for siliciclastic depositional systems. Using Holocene transgressive-regressive successions of the Po Plain (Italy), we assessed the model’s hypotheses by analyzing 249 marine mollusk shells dated individually using 14 C-calibrated amino acid racemization methods. As postulated by the model, the temporal resolution of the fossil record, frequency of depositional events, and net accumulation rates decreased upward concomitantly through the transgressive systems tract reaching minima in the condensed section (maximum flooding zone). The reverse trend, with increasingly frequent, thicker, and less time-averaged beds, was observed throughout the overlying highstand systems tract. The results quantify the postulated sequence stratigraphic asymmetry in temporal resolution of the fossil and sedimentary records.

The role of event beds in the preservation of organic carbon in fine-grained sediments: Analyses of the sedimentological processes operating during deposition of the Whitby Mudstone Formation (Toarcian, Lower Jurassic) preserved in northeast England

Geologists have recently observed that source rocks commonly contain a wide variety of microfabrics, including diminutive ripples and burrows. The existence of these fabrics challenges the assumption that persistent bottom-water anoxia and low-energy conditions are necessary prerequisites for enhanced organic carbon preservation in these units. In order to enhance our understanding of the depositional and diagenetic processes responsible for organic carbon preservation in these strata, the lower Jurassic Whitby Mudstone Formation, which contains mudstones with up to 14.2% total organic carbon, has been investigated using optical, electron optical and geochemical methods.Four thinly-bedded mudstone lithofacies were identified: (a) silt-bearing, clay-rich mudstones; (b) clay-rich mudstones; (c) clay-, calcareous nannoplankton-, and organic carbon-bearing mudstones; and (d) cement-rich mudstones. Internally individual beds are typically sharp-based and, normally-graded. They are either laminated, partially bioturbated or homogenized. Where depositional fabrics are preserved, pellets, silt lags, ripple lamination, shell pavements, wave enhanced sediment gravity flows of fluid mud, and organo-minerallic aggregates have been observed.The presence of up to 14.2% TOC in units that are sharp based and normally-graded suggests that mass flows, possibly generated by storms likely influenced sedimentation during deposition of these mudstones. The abundant organo-minerallic fabrics in these units indicates that at least some of the organic carbon was delivered to the seafloor as marine snow aggregates, that likely formed in the water column following phytoplankton blooms. Homogenization of bed tops indicates that between sediment delivery episodes, an infauna was able to colonize the seafloor, implying that the bottom waters were at least partly oxygenated during these intervals.These fabrics suggest that during deposition of the Whitby Mudstone Formation, organic carbon was being preferentially preserved below regions of high primary organic production. The close association between sharp-based, graded beds and organo-minerallic aggregates indicates that large volumes of organic carbon were delivered episodically to the seafloor, and that conditions at the sediment-water interface were episodically dynamic. Here organic carbon is likely to have been preferentially preserved because the frequency of depositional events was sufficiently high to minimize both oxidant diffusion into the sediment and organic carbon mineralization. While bottom water anoxia may have developed during the phytoplankton blooms, it was unlikely to have been persistent, because the microfabrics present indicate that there was sufficient time and oxygen available for an infauna to disrupt the sediment between deposition events.

Possible evidence for underlying non-linear dynamics in steep-faced glaciodeltaic progradational �successions

Steep-faced glaciodeltaic progradational successions are often studied in order to reconstruct the behaviour of the glacial feeder system, or changes in the sediment sink. This paper analyses the magnitude and frequency of depositional events associated with steep-faced glacier-fed progradational successions recorded in Scandinavia and Ireland. The successions exhibit depositional patterns that may be interpreted as a function of underlying non-linear dynamics. A number of the sequences display fractal scaling in the frequency and thickness of foreset units. Other successions demonstrate chaotic patterns and strong relationships between delta-front angle and bed thicknesses, suggesting that the progradation of such sequences is self-organized, and to an extent occurs independently of forcing by the feeder system that provides sediment to the delta front. These patterns of sedimentation appear to be a function of the steepness of the delta front and/or the textural characteristics of the sediment. This paper provides further evidence for the simultaneous presence of order and chaos in Earth surface processes and calls into question the extent to which palaeoenvironmental reconstructions may be made from steep-faced progradational successions. Copyright © 2000 John Wiley & Sons, Ltd.