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Stratigraphy
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Some vertical variations in stratigraphy are cyclic. River deposits, for example, may vary with season in response to changing water flow—greatest in spring and least in winter—depositing coarse material during times of high flow and finer material at other times. The sediments thus may contain sedimentary packages that repeat every year. In ocean settings, many marine deposits form from turbidites, sedimentary gravity flows that slide down continental slopes to flat ocean plains during times of high sedimentation. Multiple turbidites often occur in the same place and so produce sedimentary rocks containing individual strata reflecting many different submarine flows. And, many glacial sediments are seasonal, such as the varves shown in Figure 8.38 of Chapter 8.
Minerals and rocks
Published in A.C. McLean, C. D. Gribble, Geology for Civil Engineers, 2017
In graded bedding, a sediment containing a wide range of grain sizes is sorted vertically such that there is a continuous gradation from coarse particles at the bottom of the sedimentary layer to fine grains at the top (Fig. 2.31). Certain thick sedimentary sequences are characterised by a rhythmic alternation of thin sandstones and shales. The sandstones (or greywackes) show graded bedding. These are believed to have been deposited by turbidity currents, probably flowing off ocean shelf areas into deep water carrying a slurry of sand-laden muddy water, which forms turbidites.
Sedimentary Rocks
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
Greywacke (or German, ‘grauwacke’) is dark, often grey-coloured, and containing many coarse angular grains of quartz and feldspar which have been sedimented with little sorting, together with mica and small rock fragments (e.g. of slate) and fine matrix material. Grey-wackes are formed in the seas adjacent to rapidly uplifted land masses. Such an association commonly accompanies the development and filling of orogenic troughs or geosynclines (Fig. 2.3) when much coarse detritus is washed into the area of deposition. As a result of the eventual compression of the contents of the trough, typical grey-wackes are now found in areas of sharply folded strata, for example among the Lower Palaeozoic sediments of central Wales and the Southern Uplands of Scotland. Many greywackes show graded-bedding (Fig. 2.1), in which the sediment passes from coarser to finer particles from the bottom of a bed upwards; this structure is produced by the settling of a mixture of sand and mud in water, after movement over the sea floor as a turbidity current; a form of slump (Fig. 6.6). (Turbidity currents give rise to a mass of disturbed sediment which on later consolidation forms the rocks known as turbidites.) Elongated projections called flow-casts or sole-markings are frequently found on the undersides of greywacke beds; they show the direction of currents which operated at the time of their formation.
Sedimentology in metamorphic rocks, the Willyama Supergroup, Broken Hill, Australia
Published in Australian Journal of Earth Sciences, 2018
B. P. J. Stevens, G. M. Bradley
The simple and generally quite thin graded beds in the Broken Hill and Sundown groups do not conform to the ‘classic’ deep-water turbidite model of Bouma (1962). The Bouma (1962) model was widely accepted and applied globally but the classic turbidite model is undergoing serious revision. Individual turbidite units consist of five divisions: Te: Hemi-pelagic to pelagic mud at topTd: Planar laminated mud from suspension settlingTc: Ripple or climbing ripple laminated sandTb: High velocity planar laminated sandTa: Massive or upward fining sandDivisions Ta to Td were considered to be deposited from a single turbidity current, while division Te was interpreted as hemi-pelagic/pelagic mud settling between turbidity flows. In genuine Phanerozoic deep-water turbidites, shallow-water fossils can be found in divisions Ta–Td, while only deep-water fossils are found in division Te. The turbidite concept has been applied to many sequences of rocks of different ages around the world. These ‘classical turbidites’ are tens to many hundreds of metres thick, and are characterised by interbedded sandstones and mudstones with very uniform bed thickness and great continuity (e.g. Figure 3a). The sandstones have sharp, flat bed tops and bases, with scouring rarely greater than a few centimetres. There is an overall progression of grainsize from relatively coarse sand in the basal Ta division to mud in the Td and Te divisions, but gradational upward fining of grainsize tends to be confined to the basal Ta division.