Grain-size distribution and mineralogical composition are independent variables in most detrital sediments. Separate classifications are necessary to describe these sediment attributes. Essential characteristics are given when the following format is used: (sorting term) (size term): (cement) (prominent nondetrital) (detrital composition). This polynomial evokes a clear mental image of the sediment. Descriptive nomenclature and procedure are modified from earlier Folk publications. The resultant classification system is objective and practical. Gravel, sand, silt, and clay are end members of triangular diagrams that are divided into named textural categories in accord with apparent natural groupings. Modal size and sorting terms supplement the names of some categories. Rudite, arenite, and lutite are used in a compositional sense for detrital sediments whose texture is respectively indicated by the terms gravel (conglomerate or breccia), sand (sandstone), and mud (mudstone, siltstone, or clayston...

https://www.tandfonline.com/doi/pdf/10.1080/00288306.1970.10418211

Detrital sedimentary rock classification and nomenclature for use in New Zealand

Synopsis The River Endrick carries gravel and finer sediment in a pool and riffle channel. Point bars and riffles, which alternate along most of the channel, are foci of active particle-size segregation within the general downstream size decline: they are therefore centres of facies change. This differentiation results in sediment of a particular size being systematically and repeatedly placed in similar parts of the channel: on point bars, for example, proportionately more gravel is found at the apex and more sand in the cross-over reaches of the meander. Cross-stratal dip direction in these sediments is roughly parallel to the local flow and is therefore different in sand and gravels. It also follows that sand has a higher standard deviation of cross-bed dip orientation than gravel. The bed forms of the river often have only a superficial thickness of related sediment: they were built by a lower stage of flow than most of the sediment enclosed by the form, and in this sense forms give an erroneous measure of structure. The meanders move phase downstream so that at point bars crescents of differentiated sediment accrete on the inside, as older crescents are eroded from the outside or cut bank of the channel. In this way the river builds up a facies mosaic whose interface zig-zags downstream, and the cut bank exposes a section with a lateral size decline opposite to that of the facing point bar. Sedimentary successions show more variability within than between bars and the use of this fact, together with the variation in dip of cross-strata and other criteria, facilitates the independent reconstruction of the scale and sinuosity of the river.

Sedimentation in the meandering River Endrick

Aspidella terranovica Billings, 1872 was first described from the late Neoproterozoic Fermeuse Formation (St. John's Group) on the Avalon Peninsula of eastern Newfoundland, approximately 1km stratigraphically above the famous Ediacaran biota at Mistaken Point, and several kilometres below the base of the Cambrian. Aspidella has been reinterpreted perhaps more than any other Precambrian taxon, and has variously been regarded as a fossil mollusc or ‘medusoid’, a gas escape structure, a concretion, or a mechanical suction mark. Our studies indicate that Aspidella includes a wide variety of preservational morphs varying from negative hyporeliefs with a raised rim and ridges radiating from a slit (Aspidella-type preservation), to flat discs with a central boss and sharp outer ring (Spriggia preservation), to positive hyporeliefs with concentric ornamentation (Ediacaria preservation). Specimens occur in a continuum of sizes, with preservational styles dependent on the size of the specimen and the grain size of the host lithology; the elongation of specimens is tectonic. Aspidella is confirmed as a body fossil from observations of complex radial and concentric ornamentation, mutually deformed borders in clusters of specimens, and occurrence on the same bedding planes as certain distinctive Ediacaran taxa. Aspidella is indistinguishable from, and has priority over, several of the most common genera of late Neoproterozoic discoidal body fossils worldwide. Similar fossils from Australia are interpreted as holdfasts of frond-like organisms. The density of specimens in the Aspidella beds suggests levels of benthic biomass in the Neoproterozoic that could rival those of modern marine communities. The serial growth forms, PalaeopascichnusIntrites, Neonereites renariusYelovichnus, associated with Aspidella, are interpreted as body fossils of unknown affinities rather than trace fossils. A new, trilobed, Ediacaran body fossil, Triforillonia costellae gen. et sp. nov., is described from the Aspidella beds of the Fermeuse Formation.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.0031-0239.2000.00134.x

The first named Ediacaran body fossil, Aspidella Terranovica

The trace fossil genus Skolithos Haldeman, 1840, included species, and related genera placed in synonymy, are reviewed. From among the 35 nominal species in this group, five distinctive species of Skolithos are recognized: Skolithos linearis Haldeman, S. verticalis (Hall), S. magnus Howell, S. ingens Howell, and S. annulatus (Howell). Specimens of Skolithos, vertical burrows made by such animals as annelids and phoronids, commonly in sandy sediments, are widely distributed in Lower Paleozoic and younger rocks, and are indicative of shallow water.

Systematic review of the genus Skolithos

Scottish streams of low sinuosity have four kinds of bars, three are bank-attached (lateral), and one is medial. Bar type is related to sinuosity, and as streams often increase in sinuosity away from source, there tends to be a variation in bar type successively downstream. The morphology and structure of the bars are related to grain size—gravel bars do not have the same bed forms and structures as those made of sand—and variations in flow stage. The bar head, composed mostly of gravel, forms during the high-flow stage, and as the flow falls so sediments of the lee face record the changing flow pattern. With further drop in water level the locus of sedimentation shifts to the bar tail where the dwindling flow may build diversely oriented bed forms and structures. Changes in the rate of fall, or differences in the maximum rise of water level, result in different proportions of sediment type being deposited.The mean orientation of the directional structures is not always alined with the mean stream direction, and the diversity of orientation is comparable with the diversity in orientation of directional structures in meandering streams. The variability in orientation is due to the effects of flood-produced bed forms in diverting the low-stage flow, stream sinuosity, and the changing orientation of the channel as it sweeps across the floodplain.

C. E. B. Conybeare

Papers

Manual of sedimentary structures