A geological formation , or simply formation , is a body of rock having a consistent set of physical characteristics ( lithology ) 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 stratigraphic column ). It is the fundamental unit of lithostratigraphy , the study of strata or rock layers.
26-510: The Emu Bay Shale is a geological formation in Emu Bay, South Australia , containing a major Konservat- Lagerstätte (fossil beds with soft tissue preservation). It is one of two in the world containing Redlichiidan trilobites . The Emu Bay Shale is dated as Cambrian Series 2, Stage 4, correlated with the upper Botomian Stage of the Lower Cambrian . Its mode of preservation is the same as
52-594: A geologic formation goes back to the beginnings of modern scientific geology. The term was used by Abraham Gottlob Werner 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. Geologic maps showing where various formations are exposed at
78-526: 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 name applied to a particular formation. As with other stratigraphic units, the formal designation of a formation includes a stratotype which is usually a type section . A type section
104-420: A non-calcified exoskeleton that consists of an articulating head shield (or cephalon ), thorax and tail shield (or pygidium ), and there are no constrictions where these parts meet. The cephalon is semi-circular and has a straight back margin. The thorax consists of 3 or 4 narrow segments. The pygidium is 1-2× as long as the cephalon and has a distinct border furrow. The mouth plate (or hypostome ) that sits on
130-412: A number of rarer elements. The Big Gully trilobites rarely preserve any trace of non-biomineralized tissue; a small number of specimens of Redlichia have been reported with antennae. Taxa documented from a quarry located inland of the shoreline exposure at Big Gully include Oestokerkus , a genus of leanchoiliid closely related to the well-known Leanchoilia , the early chelicerate Wisangocaris and
156-594: 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 Morrison Formation , named for the town of Morrison, Colorado , and the Kaibab Limestone , named after the Kaibab Plateau of Arizona. The names must not duplicate previous formation names, so, for example,
182-567: A range of environmental settings during the Cambrian. Some Emu Bay fossils display extensive mineralization of soft tissues, most often of blocky apatite or fibrous calcium carbonate , including the oldest phosphatized muscle tissue – along with records from Sirius Passet in Greenland, the first thus far reported from the Cambrian . Mid-gut glands are preserved three-dimensionally in calcium phosphate in
208-449: Is not a valid lithological basis for defining a formation. 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 a meter to several thousand meters. Geologic formations are typically named after
234-520: Is Australia's only known Burgess-Shale-type Konservat-Lagerstätte, and includes faunal elements such as Anomalocaris , Tuzoia , Isoxys , and Wronascolex , in common with other Burgess-Shale-type assemblages, notably the Chengjiang Biota in China, the closest palaeogeographically, although somewhat older. A few genera of non-biomineralized arthropods, among them Squamacula , Kangacaris , and
260-430: 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 cave formations include stalactites and stalagmites . Emucarididae Emucarididae is an extinct family of soft-shelled trilobite -like arthropods ( nektaspids ) from
286-560: Is central to the geologic discipline of stratigraphy , and the formation is the fundamental unit of stratigraphy. Formations may be combined into groups of strata or divided into members . 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. The definition and recognition of formations allow geologists to correlate geologic strata across wide distances between outcrops and exposures of rock strata . Formations were at first described as
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#1732876866497312-455: 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 reference sections 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
338-512: Is unconformable on the White Point Conglomerate and yields soft-bodied fossils in addition to the trilobites, including two species of the giant predator Anomalocaris ( A. briggsi and Anomalocaris cf. canadensis ), Isoxys , Tuzoia , two species of the nektaspid arthropod Family Emucarididae ( Emucaris fava and Kangacaris zhangi ), the palaeoscolecid worm Wronascolex , the problematic Myoscolex and Vetustovermis , and
364-597: The Burgess shale , but the larger grain size of the Emu Bay rock means that the quality of preservation is lower. More than 50 species of trilobites , non-biomineralized arthropods , palaeoscolecids , a lobopodian , a polychaete , vetulicolians , nectocaridids , hyoliths , brachiopods , sponges , chancelloriids , and a chelicerate are known from the Emu Bay Shale. The Emu Bay Shale of Kangaroo Island , South Australia,
390-470: The Lower Cambrian of South Australia and South China. It contains only two genera – Emucaris and Kangacaris . Two species were described in 2010 from specimens recovered from Emu Bay Shale Lagerstätte , one species in 2012 from the Maotianshan Shales . It is classified under the order Nektaspida , and is a sister-group to the families Liwiidae and Naraoiidae . The Emucarididae have
416-556: The arthropods Isoxys and Oestokerkus , as in related species from the Burgess Shale. The type section of the Emu Bay Shale crops out on the east side of Emu Bay where it conformably overlies the White Point Conglomerate. Here it yields a rich assemblage of Estaingia , Redlichia , hyolithids , brachiopods , and the scleritome -bearing Chancelloria . At the Big Gully locality (8 km east of White Point), its presumed correlative
442-399: The belly-side of the cephalon is elongated and divided by a straight left-to-right suture. The frontal portion is approximately square with two pointed horns that extend from the frontal margin to the sides (like in a capital T), the back portion is longer than wide with a round back margin. The pair of relatively short antennas are implanted behind the horns of the hypostome and stick out to
468-550: The essential geologic time markers, based on their relative ages and the law of superposition . The divisions of the geological time scale were described and put in chronological order by the geologists and stratigraphers of the 18th and 19th centuries. Geologic formations can be usefully defined for sedimentary rock layers, low-grade metamorphic rocks , and volcanic rocks . Intrusive igneous rocks and highly metamorphosed rocks are generally not considered to be formations, but are described instead as lithodemes . "Formation"
494-522: The eyes of Anomalocaris . The latter specimens are consistent with anomalocaridids being closely related to arthropods as had been suspected. The find also indicated that advanced arthropod eyes had evolved very early, before the evolution of jointed legs or hardened exoskeletons. The eyes were 30 times more powerful than those of trilobites, long thought to have had the most advanced eyes of any species contemporary with Anomalocaris and which were only able to sense night or day. With more than 16,000 lenses,
520-522: The formation is expected to describe the stratotype in sufficient detail that other geologists can unequivocally recognize the formation. Although formations should not be defined by any criteria other than primary lithology, it is often useful to define biostratigraphic units on paleontological criteria, chronostratigraphic units on the age of the rocks, and chemostratigraphic units on geochemical criteria, and these are included in stratigraphic codes. The concept of formally defined layers or strata
546-464: The greatest number of thoracic segments known for Trilobita as a whole (a record of 103 in one Balcoracania specimen), and so far entirely restricted to Australia and Antarctica. The sedimentary depositional environment of the majority of Burgess-Shale-type assemblages is outer shelf, deeper water. The Emu Bay Shale in contrast, appears to represent deposition in restricted basins on the inner shelf, indicating that soft tissue preservation occurred in
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#1732876866497572-461: The megacheiran Tanglangia , are known only from the Emu Bay Shale and Chengjiang. The site is also the source of high-quality specimens of trilobites such as Redlichia takooensis , Emuella polymera , Balcoracania dailyi , Megapharanaspis nedini , Holyoakia simpsoni , and Estaingia (=Hsuaspis) bilobata . Balcoracania and Emuella are the only known genera of the distinctive Redlichiina family Emuellidae , known for possessing
598-491: The resolution of the 3 centimetres (1.2 in) wide eyes would have been rivaled only by that of the modern dragonfly , which has 28,000 lenses in each eye. After Paterson et. al.(2015). NOTE: Much of the text of this article was used with permission of Sam Gon III from his below referenced web site, in particular from the Emu Bay page References about Australian trilobites: Formation (stratigraphy) A formation must be large enough that it can be mapped at
624-643: The surface are fundamental to such fields as structural geology , allowing geologists to infer the tectonic history of a region or predict likely locations for buried mineral resources. 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 depositional structures , fossils regarded as rock-forming particles, or other organic materials such as coal or kerogen . The taxonomy of fossils
650-432: The surface or traced in the subsurface. Formations are otherwise not defined by the thickness 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 bodies of rock. The concept of
676-506: The type species of a monotypic genus of artiopodan arthropod, Australimicola . An armoured lobopodian of the Family Luolishaniidae is known from a single specimen that closely resembles an unnamed species from the Burgess Shale popularly known as Collins' Monster. In 2011, seven fossils of large, isolated compound eyes were described from the inland quarry site at Emu Bay , as well as the first well-preserved visual surfaces of
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