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34-749: The Kapthurin Formation is a series of Middle Pleistocene sediments associated with the East African Rift Valley . Part of the East African Rift System , it is also an important archaeological site in the study of early humans who occupied the area and left stone tools and animal bones behind. It outcrops in Kenya west of Lake Bogoria and northwest of Lake Baringo in the Kenya Rift Valley , exposed on

68-448: A solid solution series with intermediate compositions termed anorthoclase . Exsolution of an albite phase does occur; resulting cryptoperthite can best be observed in electron microprobe images. In addition to its presence in the groundmass of felsic rocks, sanidine is a common phenocryst in rhyolites and, to a lesser extent, rhyodacites . Trachyte consists largely of fine-grained sanidine. Fallout ash beds in sedimentary rock of

102-461: A complete solid solution exists between sanidine and albite. Rapid cooling of the sanidine freezes the composition, though most sanidine is cryptoperthitic, showing separate layers of low-sodium sanidine and albite at a sub-micron scale that can be detected only by X-ray crystallography or electron microscope methods. The crystal structure of ideal potassium feldspar has four sets of tetrahedral sites, each capable of accepting either an aluminum or

136-609: A lake environment, creating thin, well-defined layers. Detailed evidence of environmental change is seen in a series of three tufa carbonate beds spanning 1–2 metres (3 ft 3 in – 6 ft 7 in) in the Middle Salts and Gravels beneath the Upper Kasurein Basalt in the Lake Baringo Kapthurin outcrops. Together, they represent freshwater environmental changes that occurred in

170-582: A silicon ion. These are labeled the T 1 o, T 1 m, T 2 o, and T 2 m sites. In sanidine, the aluminum and silicon are distributed randomly among all four sites, and the T 1 o and T 1 m are mirror images of each other, as are the T 2 o and T 2 m sites. This produces a crystal with monoclinic symmetry. With slow cooling, the aluminum becomes concentrated in the T 1 sites but remains randomly distributed between T 1 o and T 1 m sites. The resulting orthoclase crystal retains monoclinic symmetry but with different crystal axis lengths. Further cooling causes

204-421: Is also overlain by either a thin clay layer or paleosol . Alongside a heavy oxygen isotope signature in the paleosols that suggests high evaporation, this change in texture indicates cyclical changes in water level. At its maximum, the lake would have covered about 1 km (0.39 sq mi). While the changing environment would have impacted early hominin movements and resource exploitation, more research

238-483: Is associated with anatomically modern Homo Sapiens. The gradual transition and regionally differentiated tools suggest a "long term evolutionary process". The departure from the long-standing Acheulean industry may also link to environmental changes in the freshwater spring environment that existed more than 500,000 years ago near present day Lake Baringo as well. Argon–argon dating of volcanic ash overlying ochre fragments found there has dated what may represent some of

272-676: Is required to understand the relationship between water level, resource availability, and hominin activity. The Kapthurin Formation contains the oldest evidence of blade production , or the repeated manufacture of blades from a single larger stone core , dating to ~500,000 years ago. This is 150,000 years older than the earliest evidence for blade production in Europe. Specifically, within the Kapthurin Formation outcrop west of Lake Baringo, Kenya, archaeologists have found tools from

306-895: The Acheulian - Middle Stone Age transition. The Kapthurin is on the floor of the basin of a half-graben that forms the Kenya Rift. This is one of two half-grabens in the Eastern portion of the East Africa Rift Valley. Because of nearby North-South striking normal faults that form this half-graben, the Kapthurin and other sedimentary formations are on a fault block tiled to the West. The formation contains lacustrine , fluvial , and volcanic rocks (specifically basalts and trachytes ). Generally, clastic sediments dominate

340-529: The K–Ar dating method. Although the ideal composition of sanidine is 64.76 wt% SiO 2 , 18.32 wt% Al 2 O 3 , and 16.72 wt% K 2 O , natural sanidine incorporates significant sodium , calcium , and iron(III) . Calcium and sodium substitute for potassium (with concurrent substitution of additional aluminum for silicon, in the case of calcium) while ferric iron substitutes for aluminum. A typical natural composition is: At elevated temperature,

374-408: The monoclinic crystal system. Orthoclase is a monoclinic polymorph stable at lower temperatures. At yet lower temperatures, microcline , a triclinic polymorph of potassium feldspar, is stable. Due to the high temperature and rapid quenching, sanidine can contain more sodium in its structure than the two polymorphs that equilibrated at lower temperatures. Sanidine and high albite constitute

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408-521: The 2nd and 3rd carbonate beds shows how the later lakes sustained more life and had a consistent freshwater source. This could have been a spring source in either the Tugen Hills to the East, or a paleoclimatic change leading to increasing rainfall, per current hypotheses. Lastly, each of the tufa three beds, from bottom to top, progresses from a spongy texture to a dense crystalline cap. Each tufa bed

442-472: The Acheulean industry, characterized by large cutting tools such as hand axes in addition to flaking. In sediment over 285,000 years old, Acheulean tools are interstratified with what is considered Middle Stone Age (300-250,000 years old) technology. Levallois point technology, which is typically characterized as a Middle Stone Age phenomenon, appears among Acheulean tools beneath Upper Basaltic Tuff in

476-705: The Bedded Tuff Member of the Kapthurin. Levallois flakes also appear below this tuff at the Acheulian-dominated "Leakey Handaxe Area." From this interstratification, archaeologists conclude that there was a gradual transition from Acheulean to Middle Stone Age technology than began roughly 280,000 years ago. During and after the transition, regional differences between tools present an even more complex picture. In particular, at sites where they are interstratified with Acheulean tools, Levallois points and flakes do coexist. Middle Stone Age technology

510-595: The Grey Tuff, Argon-Argon dating brackets several sites associated with hominin activity around the time Homo Sapiens evolved. It consists of three geochemically distinct mafic tuffs, defined as Lower, Upper, and Evolved tuffs based on their magnesium oxide content and listed here from oldest to youngest, stratigraphically. In addition, the Bedded Tuff has two more felsic tuffs, the Koimolot Tuff (between

544-820: The Kapthurin is less than 700,000 years old. The unconformity between the Chemeron and Kapthurin likely represents a significant gap in the geologic record, then. The majority of radiometric dates in the Kapthurin are Argon-Argon dates from Tuffs (described below), The Upper (deposited 537,000-567,000 years ago) and Lower Kasurein Basalts (deposited 650,000-570,00 years ago), and a Trachyte (deposited 542,000-548,000 years ago). The Kapthurin preserves information from volcanic eruptions in consolidated ash, of tuff. While tuff deposits vary with outcrop location and there are smaller tuff beds in members primarily categorized as silt or gravel, there are three well-researched tuff deposits within

578-481: The Kapthurin. The oldest is the Pumice Tuff deposit. It was initially dated to 600,000-800,000 years ago by radiometric dating of sanidine in the bed, however, paleomagnetic data indicates that the bed deposited prior to 700,00 years ago. More recent Argon-Argon dating techniques suggest that this tuff is instead 542,000–548,000 years old. The Grey Tuff, which is 500,00-518,000 years old, deposited well after

612-475: The Loboi Formation) overlies the Kapthurin unconformably, with sediments being of Holocene or Late Pleistocene Age. These sediments represent the present day erosional activity and deposition. Stratigraphically, a major faulting episode separates the Kapthurin and Kokwob. Dating of the Kapthurin formation's members is described in the following section. Localized faulting is common in this outcrop, and

646-518: The North American Stratigraphic Code, and are permitted under International Commission on Stratigraphy guidelines only in exceptional circumstances. A supergroup is a set of two or more associated groups and/or formations that share certain lithological characteristics. A supergroup may be made up of different groups in different geographical areas. A sequence of fossil -bearing sedimentary rocks can be subdivided on

680-693: The Upper and Evolved basaltic tuffs) and the Pumiceous Trachytic Tuff (overlying the Evolved Basaltic tuff). Only the latter has been radiometrically dated due to high sanidine content for argon-argon dating, with a wide range of deposition dates from ~233,000-296,000 years old. Still, the fact that Middle Stone Age tools have been found beneath the Pumiceous Trachytic Tuff member confirms this industry's appearance in this region of Africa prior to any other location. Detailed chronology of

714-414: The aluminum to concentrate in the T 1 o sites, breaking the monoclinic symmetry and producing triclinic microcline. Each transition requires exchange of ions between tetrahedral sites, which takes place at measurable rates only at high temperature. Pure sanidine melts incongruently at 1150 °C, yielding solid leucite and liquid. A mixture of sanidine with silica in the form of tridymite melts at

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748-531: The basin ~500,000 years ago. Sediment below the carbonates indicates a fluvial environment, whereas sediment above the series represent a lacustrine environment. The first of the three tufa carbonate layers represents a shallow lake environment fed by groundwater through cracked rocks, as evidenced by their high Magnesium content and interpretations of high water temperature via oxygen isotopes . Subsequent carbonate beds represent deeper, open lake environments. Fossil evidence and associated elevated strontium levels in

782-522: The basis of the occurrence of particular fossil taxa . A unit defined in this way is known as a biostratigraphic unit, generally shortened to biozone . The five commonly used types of biozone are assemblage, range, abundance, interval and lineage zones. Sanidine Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi 3 O 8 ). Sanidine is found most typically in felsic volcanic rocks such as obsidian , rhyolite and trachyte . Sanidine crystallizes in

816-491: The basis of their shared or associated lithology . Formally identified lithostratigraphic units are structured in a hierarchy of lithostratigraphic rank , higher rank units generally comprising two or more units of lower rank. Going from smaller to larger in rank, the main lithostratigraphic ranks are bed, member, formation, group and supergroup. Formal names of lithostratigraphic units are assigned by geological surveys . Units of formation or higher rank are usually named for

850-546: The bedded tuff member also enables an in depth look at the gradual transition from the Acheulean to the Middle Stone Age tool industries (see Archaeology: Tool Production). Geologic evidence in the Kapthurin reveals a dynamic freshwater environment in this area during the Middle Pleistocene. Coarsely-bedded conglomerates record evidence of flash floods, and the Bedded Tuff might represent ash fall into

884-458: The boundaries do not need to be sharp. To be formally recognised, a formation must have sufficient extent to be useful in mapping an area. A group is a set of two or more formations that share certain lithological characteristics. A group may be made up of different formations in different geographical areas and individual formations may appear in more than one group. Groups are occasionally divided into subgroups, but subgroups are not mentioned in

918-495: The case of a marker horizon . A member is a named lithologically distinct part of a formation. Not all formations are subdivided in this way and even where they are recognized, they may only form part of the formation. A member need not be mappable at the same scale as a formation. Formations are the primary units used in the subdivision of a sequence and may vary in scale from tens of centimetres to kilometres. They should be distinct lithologically from other formations, although

952-480: The earliest human aesthetic sensibility to 285,000 years ago. The ochre fragments must have been brought to the site by human agency and may have been used as body adornment. Stratigraphic unit Units must be mappable and distinct from one another, but the contact need not be particularly distinct. For instance, a unit may be defined by terms such as "when the sandstone component exceeds 75%". Sequences of sedimentary and volcanic rocks are subdivided on

986-1007: The formation, but evidence of volcanic activity from tuffs and rocks representing lava flows are found throughout. The Kapthurin overlies the Chemeron Formation, dated to roughly 1.57 million years ago, unconformably . The bulk Kapthurin formation has been dated to the Middle Pleistocene based on fossil evidence. An idealized stratigraphic characterization divides the formation into five members , listed here in descending order with their geologic abbreviations and intercalated tuffs and lava flows (also in descending order). K5) Upper Silts and Gravels K4) The Bedded Tuff K3) Middle Silts and Gravels * Gray Tuff * Upper Kasurein Basalt K2) Pumice Tuff K1) Lower Silts and Gravels * Lower Kasurein Basalt The Kokwob formation (also called

1020-409: The pumice tuff. It follows the deposition of lake sediments and basalts. Notably, in certain Kapthurin formation outcrops, the Grey Tuff overlies hominin remains, making it an important bed for relative dating. Additionally, the Grey Tuff underlies archaeological sites with evidence of tool industry transitions. The Bedded Tuff contains tools from Middle Stone Age and Acheulean tool industries. With

1054-429: The stratigraphy described here is not representative of every Kapthurin outcrop. As a lower bound on the age of the Kapthurin, Chemeron Formation has been dated to 1.57 million years ago from Potassium-Argon dating of basaltic rocks. However, the most recent radiometric date in the oldest members of the Kapthurin is ~610,000 years old. Other radiometric dates and paleomagnetic geochronology of tuffs also suggest that

Kapthurin Formation - Misplaced Pages Continue

1088-755: The surface in a 150 km (58 sq mi) area. It also outcrops in portions of the Tugen Hills farther east. The ~125 metres (410 ft) of sediment that comprises the Kapthurin formation represents more than 600,000 years of depositional history. Clastic sediments , tuffs , and carbonate beds, in the Kapthurin give information on past river and lake environments. Additionally, intercalated tuffs and extrusive igneous rocks associated with Rift Valley volcanic activity have allowed for multiple argon–argon dating studies. The high resolution dating enables archaeological studies regarding changing hominin behavior. The Kapthurin Formation has been used to study

1122-467: The unit's type location , and the formal name usually also states the unit's rank or lithology. A lithostratigraphic unit may have a change in rank over a some distance; a group may thin to a formation in another region and a formation may reduce in rank for member or bed as it "pinches out". A bed is a lithologically distinct layer within a member or formation and is the smallest recognisable stratigraphic unit. These are not normally named, but may be in

1156-445: The western United States have been classified in part by whether sanidine phenocrysts are present and, if present, whether they are sodium-enriched. W-type rhyolite ash beds contain sodium-poor sanidine; G-type rhyolite ash beds contain sodium-rich sanidine; and dacite fallout ash beds frequently lack sanidine. Because of their high potassium content, sanidine phenocrysts are also very useful for radiometric dating of rhyolite ash beds by

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