The Stretton Group is a group of rocks associated with the Longmyndian Supergroup of Ediacaran age, in Shropshire , England . The rocks are located within the tract between two elements of the Welsh Borderland Fault System , the Church Stretton Fault and the Pontesford-Linley Lineament .
15-471: The Stretton Group is a predominantly sedimentary group with a range of facies attributable to that of a closing ocean. The Wentnor Group overlies the Stretton Group of rocks and although the units are separate, together they show a good geological progression. At the base of the Stretton Group the rocks are of basinal oceanic facies and as time goes a coarsening occurs with increased terrigenous input from
30-596: A facies series . Johannes Walther Johannes Walther (July 20, 1860 – May 4, 1937) was a German geologist who discovered important principles of stratigraphy , including Walther's Law . Walther came from a religious home and studied botany , zoology , and philosophy at the University of Jena . In 1882 he successfully completed this course with a doctorate. Then he studied geology and palaeontology in Leipzig and later Munich . The following year he worked at
45-478: A depositional environment "migrates" laterally, sediments of one depositional environment come to lie on top of another. In Russia the law is known as Golovkinsky-Walther's law, honoring also Nikolai A. Golovkinsky . A classic example of this law is the vertical stratigraphic succession that typifies marine transgressions and regressions . Ideally, a sedimentary facies is a distinctive rock unit that forms under certain conditions of sedimentation , reflecting
60-577: A geochronological time of 566Ma+/-2.9. The base of this formation is defined as the base of the rhyolitic tuffs of the Buxton Rock Member. It coarsens upwards from thin to thick bedded turbidite lobe facies. Above the turbidite facies are shallow marine mudstones and then deltaic sandstones. The uppermost unit, the Cardingmill Grit, at the top is interpreted as a fluvial deposit. Beltanelliformis minutae and Beltanelliformis brunsae occur in
75-435: A group of "facies descriptors" which must be distinct, reproducible and exhaustive. A reliable facies description of an outcrop in the field would include: composition, texture, sedimentary structure(s), bedding geometry, nature of bedding contact, fossil content and colour. The sequence of minerals that develop during progressive metamorphism (that is, metamorphism at progressively higher temperatures and/or pressures) define
90-529: A particular process or environment. Sedimentary facies are either descriptive or interpretative. Sedimentary facies are bodies of sediment that are recognizably distinct from adjacent sediments that resulted from different depositional environments. Generally, geologists distinguish facies by the aspect of the rock or sediment being studied. Facies based on petrological characters (such as grain size and mineralogy ) are called lithofacies , whereas facies based on fossil content are called biofacies . A facies
105-558: Is sandier and coarser than the Synalds and Lightspout formations but also interpreted as alluvial plain and fluvial deposits. The base is marked by the Huckster Conglomerate which is thought to be deposited in a braided fluvial (palaeo)environment. Facies In geology , a facies ( / ˈ f eɪ ʃ ɪ . iː z / FAY -shih-eez , US also / ˈ f eɪ ʃ iː z / FAY -sheez ; same pronunciation and spelling in
120-638: Is usually further subdivided. The characteristics of the rock unit come from the depositional environment and from the original composition. Sedimentary facies reflect their depositional environment, each facies being a distinct kind of sediment for that area or environment. Since its inception in 1838, the facies concept has been extended to related geological concepts. For example, characteristic associations of organic microfossils, and particulate organic material, in rocks or sediments, are called palynofacies . Discrete seismic units are similarly referred to as seismic facies. Sedimentary facies are described in
135-509: The Stazione Zoologica in Naples as a lecturer, staying for two years. Among other things, he ran extensive sedimentological and biological studies. In 1885 he returned to Jena and habilitated there in 1886 with a thesis on crinoids . After travelling, he was appointed as a professor at Jena in 1890. Walther moved to the University of Halle in 1906, staying until 1929. Whilst there, he
150-578: The Wentnor Group . Below we carry on from the underlying Portway Formation (Stretton Group). This is a unit of coarse epiclastic sediments that may pass upward into the Helmeth Grit. Its position is stratigraphically tentative. A grey/green shale with normally graded siltstone which is interpreted as a distal turbidite with basin floor facies. The Helmeth Grit is located at the base of the unit (medium-grained sandstone). The Stretton Shale has been assigned
165-670: The continent. Turbidites are observed and deltas form latterly with alluvial plains with occasional marine washovers. This creeps up into the Wentnor Group where alluvial plains latterly with fluvial and alluvial deposits noted in the uppermost (youngest) Bridges Formation. The information below is present oldest to youngest. The progradational Longmyndian Sequence from oldest to youngest is: Ragleth Tuff Formation; Stretton Shale Formation; Burway Formation; Synalds Formation; Lightspout Formation; Portway Formation; Bayston-Oakswood Formation; Bridges Formation. The latter two units belong to
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#1732852353861180-450: The plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formation) and the changes that may occur in those attributes over a geographic area. A facies encompasses all the characteristics of a rock including its chemical, physical, and biological features that distinguish it from adjacent rock. The term "facies"
195-497: The succession. A mudstone lithology with inter-bedded, laminated and cross stratified sandstones is interpreted as alluvial plain, fluvial and possibly deltaic deposits. Mudstones are green at the base of the unit and red at the top. Beltanelliformis miuntae and Beltanelliformis brunsae occur in the succession. Laminated mudstones that coarsen upwards with upward fining sandstones interpreted as alluvial plain, fluvial and possibly deltaic facies. Beltanelliformis miuntae occurs in
210-413: The succession. The succession is geochronologically dated at 555.9Ma+/-3.5. The formation also contains matrix and clast supported conglomerate members with sub-rounded lithic clasts and sub-angular sedimentary clasts. These are interpreted as braided fluvial deposits. The unit is barren of fossils. This succession comprises red mudstones and siltstones interbedded with fine grained sandstones although it
225-470: Was introduced by the Swiss geologist Amanz Gressly in 1838 and was part of his significant contribution to the foundations of modern stratigraphy , which replaced the earlier notions of Neptunism . Walther's law of facies, or simply Walther's law, named after the geologist Johannes Walther , states that the vertical succession of facies reflects lateral changes in environment. Conversely, it states that when
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