Distal Storm Deposits Research Articles

Storm deposits and trace fossils of Xinji Formation (Cambrian Series 2) in the southern margin of North China Plate

The Xinji Formation (Cambrian Series 2) in the southern margin of the North China Plate developed typical siliciclastic storm deposits and abundant trace fossils. These deposits are characterized by essential indicators, such as erosional surfaces, lag deposits, blocky beddings, hummocky cross stratifications, horizontal or wavy beddings. Storm deposits in the study area are dominated by the development of hummocky cross stratifications, which is categorized into five storm deposit types based on the arrangement of different sedimentary units in the profile. These five types of storm deposits from I to V show the transition from proximal storm deposits to distal storm deposits, the gradual weakening of storm intensity, and the associated growth of quiescent intervals. The trace fossils are mainly developed in the sandstone-mudstone interbeds between storm events and include 14 ichnospecies in 11 ichnogenera: Didymaulichnus isp., Gordia marina, Helminthopsis tenuis, Oldhamia radiata, Palaeophycus striatus, Palaeophycus tubularis, Phycodes isp., Planolites beverleyensis, Planolites montanus, Planolites punctatus, Skolithos isp., Taenidium isp., Torrowangea rosei, Treptichnus pedum. By analyzing the ichnological index (burrow size, BPBI, ichnodiversity and ichnodisparity) from different storm deposits, it can be seen that from storm deposit type I to storm deposit type V, the ichnological index shows an increasing trend. Combined with the relationship between storm deposits and trace fossils, it is inferred that differences in storm intensity result in different colonization windows, and thus variations in ichnological index of preserved trace fossils. Based on the characteristics of storm deposits and distribution of trace fossils, the preservation forms of trace fossils under storm influence are explored.

Ichthyoliths of the Corumbataí Formation: new occurrences and interpretations for São Paulo state, Brazil (northeastern Paraná basin)

The Permian ichthyofauna of the Passa Dois Group is well known for the ubiquitous presence of disarticulated elements of a variety of fish taxa that remains largely understudied. Lower actinopterygians (Palaeonisciformes) are the most abundant, together with isolated teeth of Xenacanthiformes chondrichthians. This study brings new information on the external micro-ornamentations of actinopterygian teeth and rhomboid scales of the Corumbataí Formation collected for the first time in Santa Rosa de Viterbo region in the State of São Paulo, as well as a new proposition to approach this remains by using cluster analysis. These fossils were found in distal storm deposits (tempestite) and were separated from the rock matrix for analysis. Six distinct actinopterygian tooth morphotypes were identified. Curved forms are dominant in the assemblage, with ornamentations varying from smooth to ribbed shafts and others with grooves and tubercles. The scales are very fragmented showing surface micro-punctuations and longitudinal ridges. The Xenacanthiformes tricuspid teeth were very scarce in the assemblage and no complete specimen was recovered. Nonetheless, it was possible to identify them by their subtriangular shape and drop-shaped basal tubercle. Both ichthyodonts and scales show characteristics similar to at least other 26 occurrences, spread over more than 1000 km, and already described in the literature stratigraphically positioned above and below the Corumbataí Formation, attesting the wide temporal and spatial distribution of these fossils.

Taphonomic pathway of exceptionally preserved fossils in the Lower Ordovician of Morocco

The Fezouata Shale in Morocco is the only Lower Ordovician Lagerstätte to yield a diverse exceptionally preserved marine fauna. Sediments of this formation have yielded soft to lightly sclerotized taxa that were previously unknown from the Ordovician. Yet the taphonomic pathway of fossils from this formation remains poorly understood. Here, based on drill core material, a close association between exceptional preservation and a specific sedimentary facies is evidenced in the Fezouata Shale. This facies corresponds to calm sea-bottoms, sporadically smothered by distal storm deposits. The patterns of exceptional preservation in this facies indicate that most animals were dead and decayed on the seafloor prior to their burial by distal storm deposits. Furthermore, contrasted elemental and molecular compositions between fresh-cored and altered materials show that surface deposits of the Fezouata Shale were substantially affected by recent weathering. This weathering resulted in the leaching of organic materials from fossils originally preserved as carbonaceous compressions and the transformation of pyrite into iron oxides. Understanding the processes behind the current patterns of soft tissue preservation in the Fezouata Shale is essential prior to any palaeontological description, especially of taxa with no current representatives.

STORM-INDUCED COMMUNITY DYNAMICS IN THE FEZOUATA BIOTA (LOWER ORDOVICIAN, MOROCCO)

In the Central Anti-Atlas (Morocco), the lower part of the Fezouata Shale has yielded locally abundant remains of soft-bodied to lightly sclerotized taxa, occurring in low diversity assemblages characterized by strong spatial and taxonomic heterogeneities, and frequently, by the occurrence of small-sized individuals. Size frequency analyses of Celdobolus sp., Wosekella sp. (both linguliformean brachiopods) and Babinka prima (babinkid bivalve) collected in deposits of the Fezouata Shale and associated with distinct paleoenvironmental conditions show that short-lived communities of epifaunal and shallow infaunal taxa were regularly smothered and killed by distal storm deposits. Small-sized individuals more likely represent juveniles, rather than ‘dwarfed' adults (Lilliput Effect). Consequently, unstable environmental conditions (regular storms, and possibly low oxygenation of the water column) probably explain the unusual community dynamics of late Tremadocian assemblages of the Fezouata Biota (high density of individuals, low α-diversity, and high γ-diversity), interpreted as short-lived, opportunistic populations. This process has wider implications for the understanding of occurrences of small individuals elsewhere in the fossil record.

Oligocene storm-induced lacustrine deposits in the Yaxi Area of the Jiuxi Basin, northeastern margin of the Tibetan Plateau

Storm-induced deposits in the sedimentary record can now be easily identified on account of a better understanding of processes of unidirectional and oscillatory flow. Thin-bedded sandstone, siltstone, and interlayered mudstone deposits of the Oligocene Jianquanzi Member in the Yaxi Area of the Jiuxi Basin along the northeastern margin of the Tibetan Plateau were formed by shallow lacustrine storm-generated flows. Petrologic analysis reveals that the sandstones exhibit moderate to high compositional maturity but are texturally sub-mature to immature. Based on investigation of drill cores, six storm-deposit lithofacies are identified, including massive or graded sandstone, parallel and trough cross-bedded sandstones, hummocky cross-stratified sandstone, wave-rippled sandstone and siltstone, massive or horizontal bedded mudstone, and structurally deformed sandstone and mudstone. In a vertical succession, these lithofacies occur as three fining-upward lithofacies associations, each of which represents different architectural elements (and depositional processes) of proximal to distal storm deposits. The relatively coarse proximal architectural element is formed by strong density flows and unidirectional-dominated combined flows; the transitional architectural element is dominated by unidirectional- and oscillatory-dominated combined flows to pure oscillatory flows, resembling Tb-e divisions of the classic Bouma sequence; the relatively fine distal architectural element is dominated by unidirectional-dominated combined flows with a very weak oscillatory flow component. The origin of storms responsible for these deposits is attributed to Asian winter monsoons associated with the paleoclimatic and paleogeographic setting.

A new Silurian ophiuroid from the west of Ireland

Silurian echinoderms from Ireland are poorly known; hitherto, only three nominal species have been described, all crinoids and all from the Telychian (Upper Llandovery, Lower Silurian) Kilbride Formation in County Galway. A new species from this formation, Crepidosoma doylei, is the first recorded Irish Silurian ophiuroid (brittle star). The new species is described from five articulated specimens; all are mouldic and none retains a counterpart. The disc and immediately adjacent arm lateral ossicles of the new species differ in shape from those of the type species in that these ossicles are comparatively flat rather than angular or ridged, and the series is marked by a well-defined channelway that appears to have extended from within the disc for the full length of the free arm. Ambital framework ossicles are smooth, and not S- or Z-shaped and ridged as in the type species. The new specimens of C. doylei were recovered from distal storm deposits that are thought to be deep-water equivalents of Benthic Assemblage Zone 5 (Clorinda Community).

A new Silurian ophiuroid from the west of Ireland

Silurian echinoderms from Ireland are poorly known; hitherto, only three nominal species have been described, all crinoids and all from the Telychian (Upper Llandovery, Lower Silurian) Kilbride Formation in County Galway. A new species from this formation, Crepidosoma doylei, is the first recorded Irish Silurian ophiuroid (brittle star). The new species is described from five articulated specimens; all are mouldic and none retains a counterpart. The disc and immediately adjacent arm lateral ossicles of the new species differ in shape from those of the type species in that these ossicles are comparatively flat rather than angular or ridged, and the series is marked by a well-defined channelway that appears to have extended from within the disc for the full length of the free arm. Ambital framework ossicles are smooth, and not S- or Z-shaped and ridged as in the type species. The new specimens of C. doylei were recovered from distal storm deposits that are thought to be deep-water equivalents of Benthic Assemblage Zone 5 (Clorinda Community).

Life mode of in situ Conularia in a Middle Devonian epibole

Abstract: Exceptionally abundant specimens of Conularia aff. desiderata Hall occur in multiple marine obrution deposits, in a single sixth‐order parasequence composed of argillaceous and silty very fine sandstone, in the Otsego Member of the Mount Marion Formation (Middle Devonian, Givetian) in eastern New York State, USA. Associated fossils consist mostly of rhynchonelliform brachiopods but also include bivalve molluscs, orthoconic nautiloids, linguliform brachiopods and gastropods. Many of the brachiopods, bivalve molluscs and conulariids have been buried in situ. Conulariids buried in situ are oriented with their aperture facing obliquely upward and with their long axis inclined at up to 87 degree to bedding. Most specimens are solitary, but some occur in V‐like pairs or in radial clusters consisting of three specimens, with the component specimens being about equally long or (less frequently) substantially different in length. The compacted apical end of Conularia buried in situ generally rests upon argillaceous sandstone. With one possible exception, none of the examined specimens terminates in a schott (apical wall), and internal schotts appear to be absent. The apical ends of specimens in V‐like pairs and radial clusters show no direct evidence of interconnection of their periderms. The apical, middle or apertural region of some inclined specimens abuts or is in close lateral proximity to a recumbent conulariid or to one or more spiriferid brachiopods, some of which have been buried in their original life orientation. The azimuthal bearings of Conularia and nautiloid long axes and the directions in which conulariids open are nonrandom, with conulariids being preferentially aligned between 350 and 50 degree and with their apertural end facing north‐east, and nautiloids being preferentially aligned between 30 and 70 degree. Otsego Member Conularia were erect or semi‐erect, epifaunal or partially infaunal animals, the apical end of which rested upon very fine bottom sediment. The origin of V‐like pairs and radial clusters remains enigmatic, but it is probable that production of schotts was not a regular feature of this animal’s life history. Finally, conulariids and associated fauna were occasionally smothered by distal storm deposits, under the influence of relatively weak bottom currents.

Biostratigraphy and palaeoenvironment of the Agrio Formation (Lower Cretaceous), Mendoza Province, Neuquén Basin, Argentina.

A bioestratigraphical and palaeoenvironmental study of the Agrio Formation, at the Cuesta del Chihuido section, southern Mendoza Province (35º45'S/69º34'W), Argentina, is presented. Thirty seven species of nannofossils, thirty four of foraminifers and six of ostracods have been recognized. Nannofossils association corresponding to Zones CC4A y CC4B indicates an Upper Valanginian-Upper Hauterivian age. At the base of the section, the presence of the benthonic foraminifer Epistomina loncochensis Ballent and radiolaria suggests eutrophic conditions of surface waters and oxygen depressed sea-bottom conditions. In the middle part of the section, diversity of lagenid foraminifers and nannofossils indicates a shelf and nearshore environment with clear and well-oxygenated waters. Abundant polymorphinids, spirillinids and involitinids towards the top of the section corroborate a general regressive tendency. Mudstones and silty-sandstone intercalations containing allochthonous adherent foraminifers and adult carapaces of cytheroid ostracods represent distal storm deposits.

Storms versus tsunamis: Dynamic interplay of sedimentary, diagenetic, and tectonic processes in the Cambrian of Montana

The Upper Cambrian Deadwood Formation of north-central Montana is composed mainly of fine-grained calcareous sandstone and intercalated silty shale, and contains scattered intraclastic flat- pebble conglomerate. These rocks, typical of many mixed carbonate- siliciclastic, subtidal shelf successions of Proterozoic through Early Ordovician age, are distal storm deposits in which the conglomerates are conventionally thought to record occasional, very high energy events that scoured incipiently cemented layers. In the Deadwood Formation, most sandstone beds have a linear to crudely reticulate pattern of vertical cracks filled with injected silt and clay. The cracks exhibit a consistent northwest-southeast trend, and are very shallow burial deformation structures caused by the passage of seismic waves possibly originating from strong earthquakes in Idaho. Most conglomerates are composed of angular to subangular sandstone intraclasts, the polygonal plan view of which is similar in outline to the areas between cracks in the sandstones. The deformation implied by the cracks, sporadic distribution of conglomerates, high degree of scouring indicated, and angularity of the intraclasts argue that, rather than storms, the conglomerates were generated by occasional tsunamis. The sweep of these tsunamis across the shallow intracratonic sea created extraordinarily strong oscillating bottom currents at the deeper reaches of storm wave base. “Tsunamites” are therefore identified with confidence virtually for the first time in shallow subtidal shelf deposits.

Depositional Gradient Analysis in Transgressive Systems Tracts and Highstand Systems Tracts, Eocene of the Brazos River Valley Section, Texas

Middle-late Eocene third-order sequences in the Brazos River section are composed of transgressive and highstand systems tracts of shelfal marine and marginal marine sediments. Diagenesis has destroyed most fossils (except palynomorphs, arenaceous forams, and traces), but sedimentary structures and fabric provide evidence for determination of depositional gradient. Tidalite deposits dominate shallower parts of the gradient and proximal and distal storm deposits dominate deeper parts of the gradient. As seen in the Weches, Crockett, Yegua, Caddell, and Manning formations, a characteristic shoaling gradient consists of a coarsening-upward section of deeper water mudstones with thin distal storm deposits overlain by sediment with proximal storm deposits (with upward-increasing storm bed content), by planar cross-bedded sands and by intensely bioturbated shore-zone sands. A characteristic deepening gradient is a fining-upward section of cross-bedded sands overlain by flaser bedded (and plant-fragment rich) tidalites, by proximal storm deposits (usually fossil and glauconite rich), and by shales with thin distal storm deposits. Maximum flooding surfaces (associated with sediment starvation) are indicated by submarine exposure surfaces and zones of amalgamated storm beds. The lithologic character of storm deposits varies with availability of grains (sand, shell, glauconite pellets, silt) and distance of transport. Gradient analysis was used to locate themore » maximum flood of the Jackson Stage in the Brazos River section. It lies in a parasequence within the lower lignite-bearing portion of the Manning Formation, as verified by the last appearance datum of Textularia hockleyensis.« less

Short-term paleoclimatic fluctuations expressed in lower Mississippian ramp-slope deposits, southwestern Montana

Lower Mississippian ramp-slope deposits (Paine Member) of southwestern Montana are composed of thin, rhythmically interbedded limestone and argillaceous limestone (argillite). Millimeter-thick graded layers typical of limestone beds represent distal storm deposits, whereas argillite layers containing abundant whole, delicate fossils represent quiet-water deposition during times of little or no storm activity. Spectral analyses of the fluctuating insoluble-residue content (quartz, muscovite-illite, organic matter) indicate a dominant periodicity of 0.6-2.85 ka in the ramp-slope deposits; no spectral peaks corresponding to typical Milankovitch-type periods ({approximately}20-100 ka) were observed. Similar {approximately}2.5 ka paleoclimatic periodicities are recorded in Quaternary continental and alpine glaciers, Quaternary deep-sea sediments, C variations in Holocene tree rings, and Permian deep-water evaporite varves. These short-term paleoclimatic fluctuations may represent one of several harmonics of the precessional (19-23 ka) or obliquity (41 ka) orbital cycles or may be related to variations in solar activity.

An Appalachian-sourced deltaic sequence, northeastern Alabama, USA: biofacies-lithofacies relationships and interpreted community patterns

The thin sequence of Lower Pennsylvania rocks along Sand Mountain, Plateau coal field, northeastern Alabama, U.S.A., records the deposition in a deltaic coastal-plain paleoenvironment along the ancient Appalachian seaway. The section is laterally continuous, well exposed, and preserves a rich macrobiota. Identified coexisting paleodepositional environments contain distinctive biofacies. Specific paleoenvironments of deposition contain unique biofacies in this Late Carboniferous (Westphalian A) sequence. The criteria established for the recognition of these biofacies can be utilized to assist in refined interpretations of deltaic sites in Carboniferous coastal paleoenvironments. Identifiable biofacies include those preserved under a variety of forested wetland (swamp) conditions, distributary and crevasse-splay channels, coastal bays (“interdistributary” and lagoonal), barrier sands, and distal storm deposits (Fig. 1; Gastaldo et al., 1989). Vegetation in forested wetlands grew either in clastic substrates or peat substrates. The principal biofacies preserved in clastic substrate swamps were lycophyte-dominated, and can be recognized by either a predominance of canopy litter or subterranean stigmarian appendages (Gastaldo, 1986). The canopy litter that has accumulated on the forest floor was preserved under unique sedimentological conditions, and reflects the ecological gradient associated with the distribution of lycophyte genera in the swamp (Gastaldo, 1987). A monotypic assemblage of lycophytes characterized edaphically stressed sites. In sites proximal to the levee, a mixed assemblage of lycophytes, calamiteans, pteridophytes and pteridosperms is common. In the absence of compressed canopy macrodetritus, subterranean axes with helically arranged appendages (“rootlets”) crosscutting the bedding may be preserved. Macro-invertebrates are restricted to traces and trails, reflecting behavioral traits when conditions were conducive for their movement into these sites. Peat-colonizing vegetation parallels that of the clastic swamp. Deep distributary channels contain sandstone-cast and compressed aerial trunks of lycophytes and spenophytes. These occur in bedload deposits along with quartz and quartzose pebbles and cobble-size phyllite clasts. Degradation of external morphology usually precludes assignment of logs to a systematic position lower than order. In shallower, en-echelon stacked crevasse sands occur a mixture of lycophyte, calamitean and pteridosperm “woody” parts. Additionally, scoriaceous fern-like foliage may be found. Little evidence exists for macro-invertebrate communities in these unstable settings. Coastal bays preserve in situ macro-invertebrate communities, as well as allochthonous macrodetritus that was derived principally from levee vegetation. Four phases of biofacies development can be delineated (Gibson and Gastaldo, 1987). Stress-tolerant inarticulate brachiopods dominate the initial transgressive phase. Individuals are found isolated in the siltstone, commonly preserved by authigenic cementation in siderite concretions. Rarely are patches or clusters of individual encountered. Where clustering does occur it is associated with the colonization of “woody” plant parts. Transition to the molluscan-dominated phase is accompanied by the establishment of a rich ichnofauna. Continued transgression and the development of more normal salinities under lagoonal conditions are paralleled by an increase in species richness and abundance. The third biofacies phase remains molluscan dominated, but the assemblages at any particular point in time are represented by monospecific genera. Plant macrodetritus was utilized by the macro-invertebrate communities, and that which is preserved is restricted to highly fragmentary, unindentifiable remains. The fourth biofacies phase reflects the development of lagoonal conditions. This change can be recognized by macro-invertebrate body and ichnofossil fauna (Seilacher's Cruziana ichnofacies) diversification. An increase in the abundance of brachiopods and other invertebrates is considered to represent more open-marine conditions. Barrier sandstone and distal storm deposits contain disarticulated epifaunal brachiopods, bivalves, gastropods, and trilobites. These stenohaline forms are concentrated in a shell lag at the base of migrating barrier-sand bodies. The overlying sandstone is devoid of macro-invertebrate body fossils and plant macrodetritus, but is characterized by the colonization of Zoophycos traces.

The Bailey Hill Formation: Ludlow Series turbidites in the Welsh Borderland reinterpreted as distal storm deposits

The mainly Gorstian (Lower Ludlow, Silurian) Bailey Hill Formation, defined here, has previously been interpreted as a ‘bathyal’ turbidite sequence deposited in the north–south elongated Montgomery Trough. It has been seen as the Ludlow equivalent of the Wenlock and Llandovery turbidite systems preserved further west. Slowly deposited background sediments are punctuated by event deposits interpreted here not as turbidites but as distal storm deposits accumulated in an outer shelf setting. The model envisages entrainment of sediment above storm wave base on the inner shelf, lateral transport in the storm forced flow, and redeposition below storm wave base under the residual influence of this same flow. The distribution of the formation reflects the interplay between the palaeoslope of a broad shelf and palaeooceanographic factors such as storm wavebase, during a period of high sea level. Contemporaneous true turbidites to the northwest suggest that a depositional low existed outboard of the area of Bailey Hill Formation deposition at this time, in continuity with earlier Silurian turbidite systems. The revised palaeogeography is consistent with other aspects of Ludlow regional geology. By mid-Ludlow time a tectonic modification of palaeogeography was underway, marked by extensive submarine slides.

The envjronment of deposition of the Caradoc Alternata Limestone and contiguous deposits of Salop

AbstractThe middle Caradoc rocks of Salop are a mosaic of sedimentary facies deposited in nearshore environments. The uppermost Horderley Sandstone in the Onny Valley is interpreted as formed by migrating marine bars and proximal and distal stonn deposits. The time equivalent Chatwall Sandstone of the Chatwall district is interpreted as a beach sequence. The overlying Alternata Limestone is a product of both proximal and distal storm deposits, associated with marine bars, in which different faunal assemblages are mized. Correlation of local sections is difficult due to facies and faunal changes and non‐deposition events. Based on a sedimentary model and palaeoecological information it is suggested that the Alternata Limestone in the Chatwall area is older than elsewhere, and that the base of the Cheney Longville Flags youngs southwards. Four faunal events are defined based on transect collecting, discriminant function and cluster analysis. These are the Howellites antiquior, Dalmanella lepta and Bancroftina robuste Associations and the Heterorthis alternata interregnum.

The envjronment of deposition of the caradoc alternata limestone and contiguous deposits of salop

AbstractThe middle Caradoc rocks of Salop are a mosaic of sedimentary facies deposited in nearshore environments. The uppermost Horderley Sandstone in the Onny Valley is interpreted as formed by migrating marine bars and proximal and distal stonn deposits. The time equivalent Chatwall Sandstone of the Chatwall district is interpreted as a beach sequence. The overlying Alternata Limestone is a product of both proximal and distal storm deposits, associated with marine bars, in which different faunal assemblages are mized. Correlation of local sections is difficult due to facies and faunal changes and non‐deposition events. Based on a sedimentary model and palaeoecological information it is suggested that the Alternata Limestone in the Chatwall area is older than elsewhere, and that the base of the Cheney Longville Flags youngs southwards. Four faunal events are defined based on transect collecting, discriminant function and cluster analysis. These are the Howellites antiquior, Dalmanella lepta and Bancroftina robuste Associations and the Heterorthis alternata interregnum.