The lithostratigraphy of the Shales-with-Beef Member of the Charmouth Mudstone Formation, Lower Jurassic
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Abstract:
The Shales-with-Beef Member of the Charmouth Mudstone Formation (Lower Jurassic) crops out in almost continuous cliff and
foreshore sections over a distance of c. 5 km between Pinhay Bay, east Devon and Charmouth, west Dorset. A fault-bounded
outlier, 3.5 km west of Pinhay Bay, exposes the lower part of the member. At its type section on the foreshore and in cliffs below
Black Ven, Charmouth, the member consists of c. 30 m of thinly interbedded organic-rich mudstones and calcareous mudstones
with numerous thin beds of fibrous calcite (‘beef’) and several beds of tabular and nodular limestone. Many of the individual beds
of mudstone are richly fossiliferous and this has previously been used, in combination with the lithological variations, to divide
the succession into over 100 numbered and lettered beds. However, as noted in the original study, many of the thinner beds are
laterally impersistent and few can be recognised with confidence away from the type section. In addition, the type section is
separated from the main Shales-with-Beef Member outcrop in the Lyme Regis area by a penecontemporaneously active fault belt
that had an effect on sedimentation. Other fault belts at Lyme Regis and westwards from there divide the outcrop into areas with
successions that differ in detail from the type section and from one another. As a result, few of the numbered beds can be traced
from one of these areas into the adjacent area. A simplified system of bed numbering is described here based on sections that crop
out on the west and east sides of Lyme Regis, supplemented by the successions proved in cored site-investigation boreholes drilled
at Lyme Regis. The proposed lithostratigraphy provides a framework that takes account of the lateral variations in the member over
its full outcrop distance, and enables material collected from any part of the exposure to be placed in its correct stratigraphical
context.Keywords:
Outcrop
Syncline
Bedding
Lithology
Lithostratigraphy
Subaerial
Breccia
Fault block
Section (typography)
Meander (mathematics)
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The outcrop of the Pecan Gap Chalk in northeastern Texas can be divided into two parts at the change in strike from westbound to southbound. The eastern part, about 60 km in length, is composed of chalk with partings and thin beds of marl, which overlie the Wolfe City Sandstone unconformably. The upper contact with the Marlbrook marl varies from disconformable to questionably conformable. The southern part, about 30 km in length, is composed of a silty marl unit, hitherto unnamed, and an overlying chalk and calcarenite with silty quartz. These two units are conformable to slightly unconformable with one another and apparently conformable with the Wolfe City and Marlbrook. At the junction of the two parts of outcrop, there is an area of about 2 sq km with exposures of a qu rtzose arenite and calcarenite that is distinguished by abundant glauconite-phosphorite, abundant foraminifers, coccoliths, pelecypod prisms, thin, even bedding, and prominent mottles of burrows. These three units are described as new members of the Pecan Gap, and the formation is emended accordingly. The relations of the three units to one another and to the chalk of the eastern belt are unclear, particularly in the environs of Farmersville, Texas, where the sequence seems discordant with that elsewhere. The appearances of the planktonic foraminifers, Globotruncana ventricosa (White) and G. calcarata Cushman, seem to bear out the stratigraphic complexity of the Farmersville area; elsewhere they suggest absence of the silty marl along the eastern outcrop and approximate equivalence of the chalk and calcarenite of the southern outcrop and the Pecan Gap of the northern part. They indicate that much of the section of the southern part of the outcrop is missing farther south and along the eastern outcrop. End_of_Article - Last_Page 1687------------
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An almost complete section through the Mercia Mudstone Group is exposed in the cliffs
between Sidmouth and Axmouth on the south Devon coast. This is the only section in
North West Europe that exposes such a complete sequence of the mid to late Triassic
'Keuper Marl' facies. The c. 450.m-thick succession dips gently eastwards in a series of
long, continuous sections that are separated from one another by minor faults which, with
one exception, do not cut out any of the succession. At Seaton about 50 m of strata occur
within a complexly faulted zone in which fault-bounded fragments of the succession
cannot be readily correlated with the main outcrop. The availability of a continuous core
from a nearby borehole has enabled the sequence in the faulted section to be pieced
together for the first time, and to provide a complete stratigraphy for the group. The
previously un-named lower part of the Mercia Mudstone Group (the beds below the Blue
Anchor Formation) exposed in the Devon coastal sections can be divided into three
formations and nine members based on gross lithology. The lowest and highest of these
formations consists of relatively monotonous red mudstones for which the names
Sidmouth Mudstone and Branscombe Mudstone are proposed. The middle formation, for
which the name Dunscombe Mudstone is proposed, consists of a 35 m-thick sequence of
laminated green, purple and grey mudstones which expands in some inland successions
proved in boreholes to over 100 m by the addition of thick beds of salt. The lithologies
exposed in the coastal sections can be correlated with the geophysical-log signatures of the
Mercia Mudstone Group successions proved in inland boreholes throughout the Wessex
Basin.
Lithostratigraphy
Marl
Outcrop
Lithology
Sequence (biology)
Red beds
Sequence Stratigraphy
Stratigraphic unit
Section (typography)
Siltstone
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The cliff and foreshore exposures in the Devon part of the Dorset and East Devon Coast World Heritage Site expose an unbroken
late Triassic to early Jurassic succession. The change from the terrestrial, red-bed facies of the Triassic Mercia Mudstone Group to
the fully marine conditions of the Jurassic Lias Group takes place via the Penarth Group, a succession of mudstones, siltstones and
limestones deposited in lagoonal and sheltered shallow-marine environments of varying salinities. The Penarth Group as
currently defined is divided into the Westbury Formation overlain by the Lilstock Formation, based on type sections in the Severn
Estuary area. The lithology and sedimentology of the Westbury Formation strata exposed on the east Devon coast are closely
comparable with those of the type area, but those of the Lilstock Formation are not. It is therefore proposed on lithological and
historical grounds that this formation should be replaced by a Cotham Formation overlain by a White Lias Formation. This would
reinstate, without any change in their original definitions, two of the oldest formally defined stratigraphical names in the British
Phanerozoic. All three formations are lithologically distinctive, and are separated from their neighbours by erosion surfaces that
represent non-sequences. Those at the bases of the Westbury and Cotham formations are overlain by pebble beds rich in
vertebrate remains (‘bone beds’). The Cotham Formation is a highly condensed succession comprised of thinly interbedded
mudstones and limestones with ripple trains, stromatolites, desiccated surfaces and slumped beds, the last of which have been
attributed to earthquakes or a bolide impact. The limestones of the White Lias Formation exposed on the east Devon coast are
sedimentologically complex with channels, slumps and desiccated surfaces. The position of the Triassic-Jurassic boundary is
currently under review. Possible positions include the base of the Cotham Formation, a horizon within the formation, the base of
the White Lias, the base of the overlying Blue Lias Formation or a horizon within the Blue Lias Formation.
Lithology
Conglomerate
Early Triassic
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Lithology
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ABSTRACT The outcrop of the Pecan Gap Chalk in northeastern Texas can be divided into two parts at the change in strike from westbound to southbound. The eastern part, about 60 kilometers in length, is composed of chalk with partings and thin beds of marl, which overlie the Wolfe City Sandstone unconformably. The upper contact with the Marlbrook marl varies from disconformable to questionably conformable. The southern part, about 30 kilometers in length, is composed of a silty marl unit, hitherto unnamed, and an overlying chalk and calcarenite with silty quartz. These two units are conformable to slightly unconformable with one another and apparently conformable with the Wolfe City and Marlbrook. At the junction of the two parts of outcrop, there is an area of about 2 square kilometers with exposures of a quartzose arenite and calcarenite that is distinguished by abundant glauconite-phosphorite, abundant foraminifers, coccoliths, and pelecypod prisms, thin, even bedding, and prominent mottles of burrows. These three units are described as new members of the Pecan Gap, and the formation is emended accordingly. The relations of the three units to one another and to the chalk of the eastern belt is unclear, particularly in the environs of Farmersville, Texas, where the sequence seems discordant with that elsewhere. The appearances of the planktonic foraminifers, Globotruncana ventricosa (White) and G. calcarata Cushman, seem to bear out the stratigraphic complexity of the Farmersville area; elsewhere they suggest absence of the silty marl along the eastern outcrop and approximate equivalence of the chalk and calcarenite of the southern outcrop and the Pecan Gap of the northern part. They indicate that much of the section of the southern part of the outcrop is missing farther south and along the eastern outcrop.
Calcarenite
Marl
Outcrop
Conformable matrix
Arenite
Glauconite
Graded bedding
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In a former communication to the Geological Society I presented an outline of the features of the coast in the vicinity of Lyme Regis; but having there described the lias only in a general manner, it is my present purpose to offer a more detailed account of that formation, as I am not acquainted with any situation where its geological structure and composition, and the organic remains which it contains, can be better studied. The relative position of the cliffs referred to in the following description is represented in the section connected with my former paper upon this subject*:—And the annexed sketch (Pl. III. fig. 1.) represents the order of superposition of the inferior oolite sands, the lias formation, and the upper beds of the red marl, as they appear in the section between Down Cliff and Culverhole Point; the beds being supposed to be placed horizontally above each other, and the superincumbent chalk and green-sand removed. The uppermost beds of the lias rise from beneath the sands of the inferior oolite at Down Cliff, on the west of Bridport Harbour; where a section is afforded of about one hundred and eighty feet of sand, in which large flattened masses of limestone occur, containing Pectens, Belemnites, Terebratulæ, &c. These sands are calcareo-siliceous, their colour is a ferruginous yellow, and they contain an abundance of mica in their lower parts. A bed of grey marl occurs in them about one hundred feet above the commencement of the lias. The calcareous
Marl
Cliff
Section (typography)
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Summary The lithological and faunal succession in the beds above the Carboniferous Limestone over an area of about 50 square miles in North-West County Clare is described. This area has the town of Lisdoonvarna at about the central point and includes the Cliffs of Moher on the Atlantic coastline. Some 1300 feet of beds are preserved above the limestone and are to be referred to the upper part of the Homoceras and Lower Reticuloceras stages of the Namurian. Seventy-three fossiliferous bands, belonging to 10 faunal horizons, have been found and vertical sections measured to show their relationships. The succession, in this part of County Clare, is shown to be very condensed in its lower portion. There exists a considerable non-sequence above the Carboniferous Limestone. The Geological Survey map is shown to embody certain errors and a geological sketch-map is given which attempts to rectify these.
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Summary The region comprises the extreme south-western portion of the Dinantian outcrop in the southern Pennines. A survey on the 1/10,560 scale has revealed a succession ranging from Tournaisian to Namurian. The beds are of marginal facies, lying between the Derbyshire Rigid Block and the Cheshire basin; they contain both reef and bedded limestones. A lower series consists of an upward sequence of cement-stones, massive limestones, and black cherty limestones. The major reef limestones and the cementstones contain a pericyclid fauna equal to a C 1 age, and the higher beds are referred, with a little evidence, to C 2 -S 1 horizons. The upper series overlies these with unconformity, and represents the D 1 P 2 zones. Facies changes from north to south, related to a central reef belt, occur in both D 2 and P horizons, in the following fashion :— Minor unconformities occur at the base of the “ crassiventer ” beds and the base of the P zone. In the west, early Namurian beds rest with slight transgression upon P 2 ; this discordance increases eastwards, and it is clear that the main folding movements were pre-Namurian. Two periods of folding have been recognized, one of pre-D 1 age along north-west and south-east lines; the main late-Viséan folding has a north-and-south trend. The reef limestones have been little affected by the folding, the less competent bedded limestones being folded and shattered against them.
Manifold (fluid mechanics)
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FLAGS register
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The Westphalian A Colindale Member comprises the upper portion of the Port Hood Formation in western Cape Breton Island. The member is exposed at Colindale (type section) in the Port Hood area, and at the Evan's Coal Mine, MacRae's Beach, and Chimney Comer in the St. Rose area. A· ~umber of diamond drill cores, drilled as part of a coal resource evaluation, intersected portions of the Colindale Member, providing additional documentation of this stratigraphiC unit in both areas. The Colindale Member.is both conformable and transitional with bounding strata. The contact with the underlying Margaree Member of the Port Hood Formation is marked by a transition from a sandstone-dominated succession with red overbank deposits, to a succession dominated by grey, fine grained overbank deposits with fewer abundant channel sandstone bodies. This contact can be seen at Chimney' Corner and MacRae's Beach, and occurs within a thin concealed interval in the Colindale type section. The upper contact of the member with the overlying Henry Island Formation is known in cores from two holes drilled on Port Hood Islap.d. There, the member is transitional through intercalation with overlying fine grained, red overbank materials of the Henry Island Formation. The Colindale Member reaches more than 830 m thick in the St. Rose Chimney Corner area, and is approximately 620 m thick in the Port Hood region. Thick intervals of dark grey to black shales, mudstones, and siltstones typify the Colindale Member, with intercalated sandstones as thick channel sandstone bodies, and as thinner beds within the overbank succession. The fine grained grey rocks of the member consist of shales containing abundant bivalves and ostracodes, mudstones characterized by blocky weathering with slickensides and root traces (paleosols), laminated siltstones, and coal seams with associated carbonaceous shales. Coal seams of present and former economic significance occur in the upper part of the Colindale Member. The No. 5 seam, up to 2.6 m thick, is a useful local datum for correlating strata of the Colindale Member in the St. Rose Chimney Corner area. The so-called 6-Foot seam in the Port Hood area provides a comparable local datum. The overall similarity of the member in each of these respective outcrop areas suggests that these two relatively thick coal seams may be correlative.
Overbank
Westphalian sovereignty
Coal measures
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