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25-578: The Pilar Formation is a geologic formation that crops out in the Picuris Mountains of northern New Mexico . It has a radiometric age of 1488 ± 6 million years, corresponding to the Calymmian period . The Pilar Formation consists of a dense gray-black to black hard carbonaceous phyllite or schist with thin white schistose layers that are interpreted as a metatuff. The rock has very irregular slaty cleavage, with tiny muscovite flakes visible in

50-589: A chart such as below can be used for interpreting depositional environments , with increasing water velocity going down the chart. Ripple marks usually form in conditions with flowing water, in the lower part of the Lower Flow Regime. There are two types of ripple marks : Antidunes are the sediment bedforms created by fast, shallow flows of water with a Froude number greater than 1. Antidunes form beneath standing waves of water that periodically steepen, migrate, and then break upstream. The antidune bedform

75-594: 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

100-526: 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

125-594: 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,

150-403: A vortex in the lee side of the dune. As the upper flow regime forms, the dunes become flattened out, and then produce antidunes . At higher still velocity, the antidunes are flattened and most sedimentation stops, as erosion takes over as the dominant process. Typical unidirectional bedforms represent a specific flow velocity, assuming typical sediments (sands and silts) and water depths, and

175-449: 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

200-476: Is 700 m (2,300 ft). The contact with the underlying Rinconada Formation is an unconformity representing a 200 million year gap, while the contact with the overlying Piedra Lumbre Formation is gradational. Because of its distinctive appearance, the formation is an important structural marker, which provided evidence that the Hondo Group of which it is a part fills an inverted syncline . Its carbon content

225-436: 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 . Sedimentary structures Sedimentary structures include all kinds of features in sediments and sedimentary rocks , formed at

250-560: 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

275-578: Is characterized by shallow foresets , which dip upstream at an angle of about ten degrees that can be up to five meters in length. They can be identified by their low angle foresets. For the most part, antidunes bedforms are destroyed during decreased flow, and therefore cross bedding formed by antidunes will not be preserved. A number of biologically-created sedimentary structures exist, called trace fossils . Examples include burrows and various expressions of bioturbation . Ichnofacies are groups of trace fossils that together help give information on

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300-455: 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

325-579: Is one of the earliest traces of life in northern New Mexico. The formation was originally designated as the Hondo Slate by Evan Just in his 1937 survey of pegmatites in northern New Mexico. However, this name conflicted with other unit names, and it was renamed the Pilar Phyllite Member of the Ortega Formation by Arthur Montgomery in 1953 Bauer and Williams promoted it to formation rank within

350-512: The Vadito Group in their sweeping revision of the stratigraphy of northern New Mexico in 1989. With the realization that the formation was much younger than the lower Vadito Group, the formation was reassigned to the Trampas Group by Christopher G. Daniel and coinvestigators in 2013. Geologic formation A formation must be large enough that it can be mapped at the surface or traced in

375-484: The depositional environment of the sediment. There are two kinds of flow structures: bidirectional (multiple directions, back-and-forth) and unidirectional. Flow regimes in single-direction (typically fluvial ) flow, which at varying speeds and velocities produce different structures, are called bedforms . In the lower flow regime , the natural progression is from a flat bed, to some sediment movement ( saltation etc.), to ripples, to slightly larger dunes. Dunes have

400-598: The Pilar Formation yields an age of 1488 ± 6 Mya , considerably younger than the Rinconada Formation. This suggests that the Pilar Formation and Piedra Lumbre Formation should be removed from the Hondo Group (and placed in the informal Trampas Group) and provides evidence supporting the Picuris orogeny . The base of the formation is 2-3 meters of garnet-bearing quartzite that is blue-black in color. The total thickness

425-467: The depositional environment. In general, as deeper (into the sediment) burrows become more common, the shallower the water. As (intricate) surface traces become more common, the water becomes deeper. Microbes may also interact with sediment to form microbially induced sedimentary structures . Soft-sediment deformation structures or SSD, is a consequence of the loading of wet sediment as burial continues after deposition. The heavier sediment "squeezes"

450-550: 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"

475-522: 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

500-425: The hand lens. It contains many quartz veins and some limonite augens . Its composition is 50% to 75% quartz and 15% to 30% muscovite with carbonaceous material finely disseminated throughout the quartz and muscovite grains. Montgomery rejected its designation as a slate , as it has undergone middle-grade metarmophism along with the underlying staurolite -bearing Rinconada Formation . A metamorphosed tuff bed in

525-564: The sediment is deposited. Secondary sedimentary structures form after primary deposition occurs or, in some cases, during the diagenesis of a sedimentary rock . Common secondary structures include any form of bioturbation , soft-sediment deformation, teepee structures , root-traces, and soil mottling. Liesegang rings , cone-in-cone structures , raindrop impressions , and vegetation-induced sedimentary structures would also be considered secondary structures. Secondary structures include fluid escape structures , formed when fluids escape from

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550-407: 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

575-643: 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

600-522: The time of deposition . Sediments and sedimentary rocks are characterized by bedding , which occurs when layers of sediment, with different particle sizes are deposited on top of each other. These beds range from millimeters to centimeters thick and can even go to meters or multiple meters thick. Sedimentary structures such as cross-bedding , graded bedding , and ripple marks are utilized in stratigraphic studies to indicate original position of strata in geologically complex terrains and understand

625-449: The water out of the underlying sediment due to its own weight. There are three common variants of SSD: Bedding Plane Structures are commonly used as paleocurrent indicators. They are formed when sediment has been deposited and then reworked and reshaped. They include: These structures are within sedimentary bedding and can help with the interpretation of depositional environment and paleocurrent directions. They are formed when

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