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The contrast in lithology between formations required to justify their establishment varies with the complexity of the geology of a region. Formations must be able to be delineated at the scale of geologic mapping normally practiced in the region; the thickness of formations may range from less than
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of their rock strata, which can vary widely. They are usually, but not universally, tabular in form. They may consist of a single lithology (rock type), or of alternating beds of two or more lithologies, or even a heterogeneous mixture of lithologies, so long as this distinguishes them from adjacent
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of
Arizona. The names must not duplicate previous formation names, so, for example, a newly designated formation could not be named the Kaibab Formation, since the Kaibab Limestone is already established as a formation name. The first use of a name has precedence over all others, as does the first
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The boundaries of a formation are chosen to give it the greatest practical lithological consistency. Formations should not be defined by any criteria other than lithology. The lithology of a formation includes characteristics such as chemical and mineralogical composition, texture, color, primary
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may be defined. Long-established formations dating to before the modern codification of stratigraphy, or which lack tabular form (such as volcanic formations), may substitute a type locality for a type section as their stratotype. The geologist defining the formation is expected to describe the
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in his theory of the origin of the Earth, which was developed over the period from 1774 to his death in 1817. The concept became increasingly formalized over time and is now codified in such works as the North
American Stratigraphic Code and its counterparts in other regions.
231:
Geologic formations are typically named after a permanent natural or artificial feature of the geographic area in which they were first described. The name consists of the geographic name plus either "Formation" or a descriptive name. Examples include the
363:"Formation" is also used informally to describe the odd shapes (forms) that rocks acquire through erosional or depositional processes. Such a formation is abandoned when it is no longer affected by the geologic agent that produced it. Some well-known
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A type section is ideally a good exposure of the formation that shows its entire thickness. If the formation is nowhere entirely exposed, or if it shows considerably lateral variation, additional
309:. Members differ from formations in that they need not be mappable at the same scale as formations, though they must be lithologically distinctive where present.
62:) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exposed in a geographical region (the
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A formation must be large enough that it can be mapped at the surface or traced in the subsurface. Formations are otherwise not defined by the
351:. Intrusive igneous rocks and highly metamorphosed rocks are generally not considered to be formations, but are described instead as
195:
is composed of alternating thin beds of two lithologies, mudstone and sandstone, penetrated by veins of a third lithology, gypsum.
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100:, allowing geologists to infer the tectonic history of a region or predict likely locations for buried mineral resources.
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Although formations should not be defined by any criteria other than primary lithology, it is often useful to define
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is underlain by the
Mutitjulu Arkose, a formation composed almost entirely of a single lithology (arkosic sandstone).
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were described and put in chronological order by the geologists and stratigraphers of the 18th and 19th centuries.
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The concept of a geologic formation goes back to the beginnings of modern scientific geology. The term was used by
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132:. These strata make up much of the famous prominent rock formations in widely spaced protected areas such as
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stratotype in sufficient detail that other geologists can unequivocally recognize the formation.
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of formations and white numbers correspond to formations (click on picture for more information)
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showing where various formations are exposed at the surface are fundamental to such fields as
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301:, and the formation is the fundamental unit of stratigraphy. Formations may be combined into
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The concept of formally defined layers or strata is central to the geologic discipline of
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As with other stratigraphic units, the formal designation of a formation includes a
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North
American Commission on Stratigraphic Nomenclature (November 2005).
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on geochemical criteria, and these are included in stratigraphic codes.
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regarded as rock-forming particles, or other organic materials such as
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Structural geology : fundamentals and modern developments
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American Commission on Stratigraphic Nomenclature 2005
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North
American Commission on Stratigraphic Nomenclature 2005
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North
American Commission on Stratigraphic Nomenclature 2005
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North
American Commission on Stratigraphic Nomenclature 2005
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North
American Commission on Stratigraphic Nomenclature 2005
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North
American Commission on Stratigraphic Nomenclature 2005
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North American Commission on Stratigraphic Nomenclature 2005
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to correlate geologic strata across wide distances between
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324:. Formations were at first described as the essential
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a valid lithological basis for defining a formation.
583:
The Birth and Development of the Geological Sciences
312:The definition and recognition of formations allow
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665:. John Wiley & Sons. pp. 111–114, 200.
586:. Baltimore: The Williams & Wilkins Company.
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339:Geologic formations can be usefully defined for
140:. From top to bottom: Rounded tan domes of the
328:markers, based on their relative ages and the
682:Stratigraphy : terminology and practice
680:Rey, Jacques; Simone Galeotti, eds. (2008).
563:Principles of sedimentology and stratigraphy
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660:
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148:, cliff-forming, vertically jointed, red
249:name applied to a particular formation.
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27:Fundamental unit of lithostratigraphy
228:a meter to several thousand meters.
166:Glen Canyon National Recreation Area
593:"North American Stratigraphic Code"
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684:. Paris, France: Editions OPHRYS.
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25:
740:
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565:(1st ed.). Merrill Pub. Co.
66:). It is the fundamental unit of
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638:(1st ed.). Pergamon Press.
661:Brookfield, Michael E. (2008).
128:demonstrate the principles of
42:. Black numbers correspond to
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1:
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285:on the age of the rocks, and
580:Frank, Adams Dawson (1938).
401:
7:
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663:Principles of Stratigraphy
305:of strata or divided into
152:, slope-forming, purplish
134:Capitol Reef National Park
283:chronostratigraphic units
164:sandstone. Picture from
138:Canyonlands National Park
293:Usefulness of formations
287:chemostratigraphic units
236:, named for the town of
561:Boggs, Sam Jr. (1987).
391:List of rock formations
332:. The divisions of the
202:depositional structures
156:, layered, lighter-red
359:Other uses of the term
320:and exposures of rock
275:biostratigraphic units
196:
184:
169:
87:Abraham Gottlob Werner
47:
547:, pp. 1570–1572.
511:, pp. 1553–1554.
487:, pp. 1561–1562.
439:, pp. 1567–1569.
334:geological time scale
193:Summerville Formation
190:
175:
160:, and white, layered
124:area of southeastern
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33:
715:at Wikimedia Commons
634:Ghosh, S.K. (1993).
396:List of fossil sites
330:law of superposition
64:stratigraphic column
52:geological formation
729:Geologic formations
713:Geologic formations
620:10.1306/07050504129
612:2005BAAPG..89.1547.
451:, pp. 221–222.
420:, pp. 545–547.
367:formations include
258:which is usually a
104:Defining formations
343:layers, low-grade
267:reference sections
244:, named after the
238:Morrison, Colorado
234:Morrison Formation
197:
185:
170:
158:Moenkopi Formation
98:structural geology
48:
18:Geologic formation
711:Media related to
606:(11): 1547–1591.
345:metamorphic rocks
150:Wingate Sandstone
146:Kayenta Formation
68:lithostratigraphy
16:(Redirected from
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341:sedimentary rock
242:Kaibab Limestone
179:(Ayers Rock) in
162:Cutler Formation
154:Chinle Formation
142:Navajo Sandstone
122:Colorado Plateau
82:bodies of rock.
74:or rock layers.
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654:Further reading
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70:, the study of
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701:External links
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349:volcanic rocks
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246:Kaibab Plateau
220:of fossils is
144:, layered red
120:strata of the
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130:stratigraphy
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54:, or simply
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40:Grand Canyon
373:stalagmites
369:stalactites
34:A geologic
572:0675204879
555:References
533:Frank 1938
473:Ghosh 1993
449:Frank 1938
418:Boggs 1987
353:lithodemes
314:geologists
281:criteria,
255:stratotype
240:, and the
402:Footnotes
181:Australia
79:thickness
60:lithology
56:formation
723:Category
625:8 August
379:See also
318:outcrops
218:taxonomy
118:Jurassic
116:through
608:Bibcode
307:members
214:kerogen
206:fossils
168:, Utah.
114:Permian
38:of the
688:
669:
642:
569:
347:, and
322:strata
303:groups
216:. The
72:strata
44:groups
596:(PDF)
177:Uluru
686:ISBN
667:ISBN
640:ISBN
627:2020
567:ISBN
371:and
365:cave
210:coal
191:The
136:and
126:Utah
112:The
616:doi
277:on
222:not
212:or
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