The Charmouth Mudstone Formation is a geological formation in England , dating to the Early Jurassic ( Sinemurian – Pliensbachian ). It forms part of the lower Lias Group . It is most prominently exposed at its type locality in cliff section between Lyme Regis and Charmouth (alongside the underlying Blue Lias ) but onshore it extends northwards to Market Weighton , Yorkshire , and in the subsurface of the East Midlands Shelf and Wessex Basin . The formation is notable for its fossils, including those of ammonites and marine reptiles and rare dinosaur remains. The formation played a prominent role in the history of early paleontology, with its Lyme Regis-Charmouth exposure being frequented by fossil collectors including Mary Anning .
19-508: The Shales With Beef Member is around 28–30 metres thick in the Lyme Regis-Charmouth region and predominantly consists of thinly bedded medium to dark grey mudstone, blocky calcareous pale-weathering mudstone and brown-grey organic-rich mudstones with frequent bedding parallel veins of fibrous calcite ("beef"), that are usually less than 10 centimetres thick. Several beds of nodular and tabular limestone are also present. It
38-438: A stratum , is the smallest formal unit in the hierarchy of sedimentary lithostratigraphic units and is lithologically distinguishable from other layers above and below. Customarily, only distinctive beds, i.e. key beds , marker beds , that are particularly useful for stratigraphic purposes are given proper names and considered formal lithostratigraphic units. In case of volcanic rocks, the lithostratigraphic unit equivalent to
57-526: A bed is a flow . A flow is “...a discrete, extrusive, volcanic rock body distinguishable by texture, composition, order of superposition, paleomagnetism, or other objective criteria.” A flow is a part of a member as a bed of sedimentary rock is a part of a member. In geotechnical engineering a bedding surface often forms a discontinuity that may have a large influence on the mechanical behaviour (strength, deformation, etc.) of soil and rock masses in tunnel , foundation , or slope construction. These are
76-509: A combination of local deposition on the inclined surfaces of ripples or dunes , and local erosion . Graded beds show a gradual change in grain or clast sizes from one side of the bed to the other. A normal grading occurs where there are larger grain sizes on the older side, while an inverse grading occurs where there are smaller grain sizes on the older side. Bed thickness is a basic and important characteristic of beds. Besides mapping stratigraphic units and interpreting sedimentary facies,
95-418: A partial skeleton including cranial material, cervical, dorsal, and caudal vertebrae, ribs, right pectoral girdle, partial limb bones, and an osteoderm An early diverging thalattosuchian . Dimorphodon Bed (geology) In geology , a bed is a layer of sediment , sedimentary rock , or volcanic rock "bounded above and below by more or less well-defined bedding surfaces". A bedding surface
114-442: A result of changes in environmental conditions. As a result, a bed is typically, but not always, interpreted to represent a single period of time when sediments or pyroclastic material accumulated during uniform and steady paleoenvironmental conditions. However, some bedding surfaces may be postdepositional features either formed or enhanced by diagenetic processes or weathering . The relationship between bedding surfaces controls
133-422: Is three-dimensional surface , planar or curved, that visibly separates each successive bed (of the same or different lithology ) from the preceding or following bed. Where bedding surfaces occur as cross-sections, e.g., in a 2-dimensional vertical cliff face of horizontal strata, are often referred to as bedding contacts . Within conformable successions, each bedding surface acted as the depositional surface for
152-726: Is conformably overlain by the Dyrham Formation in some areas, but in the Charmouth area there is an erosive unconformable boundary with the much younger Early Cretaceous ( Albian ) aged Gault clays. Archaeonectrus A. rostratus Charmouth Arnioceras semicostatum - Echioceras raricostatum ammonoid zone BMNH 38525 (holotype skeleton) A marine sauropterygian, member of the family Rhomaleosauridae Attenborosaurus A. conybeari Asteroceras obtusum , Arnioceras semicostatum - Echioceras raricostatum ammonoid zones A marine sauropterygian, early member of
171-530: Is frequently used in textbooks, e.g., Collinson & Mountney or Miall. Both definitions have merit and the choice of which one to use will depend on the focus of the specific study on a case by case basis. Typically, but not always, bedding surfaces record changes in either the rate or type of accumulating sediment that created the underlying bed. Typically, they represent either a period of nondeposition, erosional truncation, shift in flow or sediment regime, abrupt change in composition, or combination of these as
190-538: Is the lowest unit of the formation and directly overlies the Blue Lias Formation , with the boundary being marked by a prominent bioturbated horizon. Notable persistent marker beds within the member include the laminated calcareous siltstone "Fish Bed", "Table Ledge", which consists of lens beds of limestones with mud content with nests of rhynchonellid brachiopods , the Devonshire Head and
209-672: The Spittles limestones and the Birchi Nodules (which are septarian concretions ) The upper boundary with the Black Ven Marl Member is marked by the prominent laterally persistent limestone Birchi Tabular Bed. The Black Ven Marl Member is around 43 metres thick consists of thinly bedded dark mudstones, with several laterally persistent cementstone horizons, notable horizons include the Lower and Upper Cement beds and
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#1732772153074228-594: The Stellare nodules. The Belemnite Marl Member is around 20 to 27 metres thick, and consists of interbedded pale and dark grey calcareous mudstone, with numerous belemnites , hence the name. The top of the member is marked by the Belemnite Stone Bed The Green Ammonite Member is up to 31 metres thick predominantly consists of medium grey mudstones, with 3 limestone horizons, Lower Limestone; Red Band, and Upper Limestone, it
247-439: The accumulation of younger sediment. Specifically in sedimentology , a bed can be defined in one of two major ways. First, Campbell and Reineck and Singh use the term bed to refer to a thickness-independent layer comprising a coherent layer of sedimentary rock, sediment, or pyroclastic material bounded above and below by surfaces known as bedding planes. By this definition of bed, laminae are small beds that constitute
266-533: The analysis of bed thickness can be used to recognize breaks in sedimentation, cyclic sedimentation patterns, and gradual environmental changes. Such sedimentological studies are typically based on the hypothesis that the thicknesses of stratigraphic units follows a log-normal distribution . Differing nomenclatures for the bed and laminae thickness have been proposed by various authors, including McKee and Weir, Ingram, and Reineck and Singh. However, none of them have been universally accepted by Earth scientists. In
285-1004: The family Plesiosauroidea inside Plesiosauria . The typical plesiosaur, and one of the most common found on the formation. Ichthyosaurus I. anningae Charmouth Stonebarrow Marls Member DONMG:1983.98 Subadult specimen An ichthyosaurid ichthyosaur Ichthyosaurus sp. West of Westhay Water, Stonebarrow Stonebarrow Marls Member NHMUK R15907, partial skeleton Considered to belong to I. communis by Bennett et al. 2012, considered indeterminate within Ichthyosaurus by Lomax and Massare 2015 Leptonectes L. moorei Seatown Belemnite Marls Member BMNH R14370 partial anterior skeleton with most complete skull A leptonectid ichthyosaur L. solei Seatown Acanthopleuroceras valdani Subzone, Tragophylloceras ibex Zone, lower Pliensbachian NMW 91.296.2.2 "a large, isolated and incomplete forefin" Turnersuchus T. hingleyae Belemnite Marl Member LRM 2021/45,
304-483: The family Pliosauridae . It was first identified as Plesiosaurus conybeari . ? Plesiosaurus ? P. sp. Prodactylioceras davoei ammonoid zone A marine sauropterygian, type member of the family Plesiosauroidea inside Plesiosauria . A juvenile specimen that resembles those assigned to the genus Plesiosaurus . Its assignation is dubious. Plesiosaurus P. dolichodeirus Echioceras raricostatum ammonoid zone A marine sauropterygian, type member of
323-530: The gross geometry of a bed. Most commonly, the bottom and top surfaces of beds are subparallel to parallel to each other. However, some bedding surfaces of a bed are nonparallel, e.g., wavy, or curved. Differing combinations of nonparallel bedding surfaces results in beds of widely varying geometric shapes such as uniform-tabular, tabular-lenticular, curved-tabular, wedge-shaped, and irregular beds. Types of beds include cross-beds and graded beds . Cross-beds, or "sets," are not layered horizontally and are formed by
342-742: The practice of engineering geology, a standardized nomenclature is used for describing bed thickness in Australia, the European Union, and the United Kingdom. Examples of widely used bed thickness classifications include Tucker (1982) and McKee and Weir (1953). According to both the North American Stratigraphic Code and International Stratigraphic Guide, a bed is the smallest formal lithostratigraphic unit that can be used for sedimentary rocks. A bed,
361-438: The smallest (visible) layers of a hierarchical succession and often, but not always, internally comprise a bed. Alternatively, a bed can be defined by thickness where a bed is a coherent layer of sedimentary rock, sediment, or pyroclastic material greater than 1 cm thick and a lamina is a coherent layer of sedimentary rock, sediment, or pyroclastic material less than 1 cm thick. This method of defining bed versus lamina
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