The stratigraphy of the Kimmeridge Clay Formation (Jurassic) of the Vale of Pickering, Yorkshire, UK
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The Upper Jurassic Kimmeridge Clay Formation (KCF) underlies much of the Vale of Pickering where it is almost wholly concealed by the Cretaceous Speeton Clay Formation and Quaternary deposits. There are few KCF inland or coastal exposures in Yorkshire with the result that the succession was stratigraphically poorly known until the 1970s oil crisis when the British Geological Survey drilled continuously cored boreholes at Marton and Reighton to examine the formation as a possible source of hydrocarbons. These were supplemented in 1987 by continuously cored boreholes drilled at Marton, Reighton, Ebberston and Flixborough by the Institut Français du Pétrole for hydrocarbons research. Taken together, the boreholes have enabled the lithological, palaeontological, geochemical and geophysical characters of the full thickness of the formation to be examined. Comparison of the KCF successions proved in Yorkshire with those in the adjacent North Sea, the East Midlands and the Dorset coast type area, shows marked variations in thickness related to penecontemporaneous faulting. However, there are only minor variations in the lithologies and faunas at any particular stratigraphical level. This appears to be due to a combination of Milankovitch-driven climatic fluctuations and pulsed variations in sea level which combined to produce similar depositional conditions throughout the English KCF at any one time. The chronostratigraphical classification of the KCF developed in southern England has therefore been shown to be applicable to the Yorkshire outcrop and the southern North Sea. The changes in sea level may be eustatic rather than regional events, but there is insufficient palaeontological evidence to enable them to be correlated with confidence with those of the standard Jurassic sea-level curve.The Jefferson and Three Forks Formations, mostly of Late Devonian age, are recognized in western Wyoming and adjacent areas far south of the classic localities of southwestern Montana. The Jefferson is divided into an unnamed lower member and a prominent ledge-forming dolomite called the Birdbear Member. Intraformational correlation of laterally persistent carbonate ledges and detrital interbeds in closely spaced measured sections demonstrates that the Darby Formation of west-central Wyoming is equivalent to the lower member of the Jefferson Formation. The Logan Gulch and Trident Members of the Three Forks are equivalent to the sandy Beirdneau Formation in the miogeosyncline of southeastern Idaho and northeastern Utah. Members of the Jefferson and Three Forks are truncate eastward in Wyoming and are overlain by a dark shale unit of Devonian and Mississippian age.
Devonian
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Continental Margin
Red beds
Diachronous
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The major stratigraphic aspects of lacustrine rock units are geometry (thickness and lateral extent), facies patterns, and vertical sequence. Sizes and shapes of modern lakes show wide ranges, but many large ones are subcircular to elongate. In cross section most thick lacustrine units are broadly lenticular with maximum thickness near the center of the basin where subsidence is greatest. Bottom sediments of modern lakes encompass a wide variety of lithofacies. If clastic sediments dominate, there may be concentric belts of gravel, sand, sandy marly mud, and mud, which are controlled by wave base and overall energy gradients. Facies patterns in chemical and organic sediments are not so easily predicted. However, two carbonate models are recognized, one with increasing carbonate content toward the center of the lake and the other with higher carbonate concentrations near the margins. The former results from nearshore dilution by terrigenous sediment and the latter from greater carbonate productivity in shallower water. Similarly, two organic facies patterns predominate. Offshore increase in organic matter results from deposition and preferred preservation below wave bas . In contrast, nearshore concentrations of organic matter are mostly caused by in-place accumulations of organic remains. Few ancient lacustrine sequences are either sufficiently End_Page 837------------------------------ well preserved or studied in sufficient detail for construction of even general facies maps. One obvious exception is the Green River Formation of Peleocene(?) to Eocene age, the most extensively studied lacustrine rock unit in the world. In the Green River Formation, the general facies pattern in northeast Utah and northwest Colorado is one of marginal coarse clastics and centralized organic-rich mudstone; a general basinward increase in carbonate rock is also notable. Most lakes pass through more than one cycle of expansion and retreat. The resulting vertical sequence is a composite of many complete and incomplete cycles. Lacustrine rocks display a variety of allocyclic sequences: glacial and nonglacial varves, transgressive-regressive cycles, and various composite groupings represented by bundles of varves or other cyclic deposits. End_of_Article - Last_Page 838------------
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Sedimentation
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The Triassic system is represented by marine sandstones, siltstones, limestones, and shales and by non-marine redbeds, all of Early Triassic age. These beds are correlated with the Dinwoody, Woodside, and Thaynes formations of southeastern Idaho, western Wyoming, and northeastern Utah. The system is widely distributed and ranges in thickness from zero in Silver Bow County to more than 1,500 feet in southern Beaverhead County. These sediments were deposited under relatively stable to mildly unstable conditions along the margins of the geosyncline, the axis of which lay west of this region during the Triassic, as well as during the Paleozoic. The Jurassic system is represented by marine sediments that are correlated with the Sawtooth, Rierdon, and Swift formations of the Middle and Upper Jurassic Ellis group and by non-marine deposits that represent the Upper Jurassic Morrison formation. Jurassic strata are thin or missing in the northern and central parts of the region, as the result of the development of a small but prominent positive area, and they thicken eastward into Gallatin County and southward and westward in Beaverhead County. The marine Jurassic rocks were deposited under shelf conditions, and the axis of the geosyncline still lay west of this region during the Jurassic. The presence of subgraywackes in the Morrison formation presaged the eastward shift of the geosynclinal axis into this region, which occurred i the Cretaceous.
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Cretaceous facies in Peru commonly fall into a lower, mainly clastic, group of formations and an upper group consisting of limestones, dolomites and shales. The clastic formations are largely non-marine. Over much of the country they range in age from Valanginian to Late Aptian or Albian, though in eastern Peru they extend into the Upper Cretaceous. The basal part of the limestone and shale group of formations is commonly Albian in age, though it is younger in the east. Tuffs and flows form an important part of the Cretaceous sequence of coastal Peru, but are not common elsewhere in the country. These Cretaceous units are overlain by redbed formations, some of which may be as old as the Campanian. The relationships between the redbeds and the underlying units range from c nformity to slight angular unconformity. The Cretaceous formations range in thickness from about 3,000 meters on the western flank of the Andes and 2,000 m. in eastern Peru to 1,000 m. or less in the intervening region. On this basis the Andean belt is believed to have consisted of two troughs (the West Peruvian trough and East Peruvian trough of this report) separated by a relatively positive area called the Maranon geanticline. The clastic sequence in the West Peruvian trough was probably derived from the geanticline and from tectonic lands on the southwest. The clastics in the East Peruvian trough were contributed by the Brazilian shield, and probably by the geanticline. Although there were temporary marine advances into the troughs during the Neocomian and Aptian the main transgression did not begin until the Albian. The West Peruvian trough and the Maranon geanticline were submerged by the Medial Albian, and marine conditions began to spread into the northern part of the East Peruvian trough. The latter was not completely submerged, however, until the sea reached its greatest extent during the Coniacian. Although there was Late Albian tectonism in parts of the Andean belt, widespread emergence did not begin until the Santonian or Campanian, when the West Peruvian trough was uplifted. Subsequently the whole of the belt was folded and uplifted by orogenic phases which took place possibly in the Miocene and Pliocene. The Andean belt in central Peru may be divided into structural provinces, which are from southwest to northeast: Paleozoic massifs; gently folded and bIock-faulted Mesozoic sediments and volcanics; batholith; strongly folded Cretaceous formations; folded and metamorphosed Paleozoic formations overlain by gently folded Mesozoic and Cenozoic formations; and moderately folded Mesozoic and Cenozoic formations.
Trough (economics)
Aptian
Red beds
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The name Cincinnatian Series, a time-stratigraphic term, is restricted to rocks of Late Ordovician age, wherever they occur. Most of the formations assigned to the Cincinnatian Series are actually biostratigraphic zones. The name Cincinnati group is revived for the body of rocks continuous with the type Cincinnatian Series in the Cincinnati arch region, without time connotation. The Cincinnati group in subsurface is composed, in descending order, of the Maysville-Richmond formation and the Eden shale. Upper Ordovician rocks in western Indiana and eastern and southern Illinois are assigned to the Maquoketa group, composed, in descending order, of the Orchard Creek shale, Cape limestone (Templeton and Willman, in press), and Eden shale. Upper Ordovician rocks in northwestern Illinois and eastern Iowa are assigned to the Maquoketa shale. Contrary to existing correlations, rocks representing Edenian and Maysvillian, as well as Richmondian deposition, are present in Illinois and Iowa.
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A southeastward-plunging synclinal mountain ridge, capped by Mississippian rocks, rises from a base of Cambrian rocks in the Sunwapta Pass area. Detailed sections involving Silurian and Devonian rocks were studied from the Nigel Peak area on the north southward to the Sunset Pass area. The Mt. Wilson quartzite, heretofore referred to the Devonian, is shown to be Ordovician or Silurian in age and a probable correlative of the Wonah formation of British Columbia. Halysites-bearing beds of probable early Silurian age rest unconformably on the Mt. Wilson quartzite and may correlate with the Brisco formation of British Columbia. The Devonian consists of three formations, the coral-bearing Fairholme at the base, followed by the scarp-forming Palliser limestone, and capped by the Exshaw shale and limestone. Tentative correlations are made between the Leduc section and the Sunwapta Pass area for Devonian rocks. Mississippian Banff shale rests unconformably on the Exshaw formation.
Devonian
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