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24-550: The Baca Formation is a geologic formation in southern New Mexico and Arizona . It preserves fossils dating back to the Eocene period . The formation consists of coarse conglomerate , red and white sandstone , and red clay . Its total thickness is 694 feet (212 m), of which 80–140 feet (24–43 m) is conglomerate. The formation rests unconformably on the Cretaceous Mesaverde Group and

48-597: 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

72-469: A heterogeneous mixture of lithologies, so long as this distinguishes them from adjacent bodies of rock. The concept of 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

96-542: 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

120-617: 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,

144-403: 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

168-449: 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

192-436: 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

216-560: 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

240-578: 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

264-455: 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

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288-508: 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 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

312-532: Is overlain by the Oligocene Spears Formation . The formation was deposited in a basin that developed in Eocene time in eastern Arizona and west-central New Mexico. The basin was mostly a system of braided streams subject to frequent flash floods , with meandering streams restricted to the easternmost part of the basin. Paleocurrent directions showed that stream flow was generally to

336-483: The Bear Mountains . Formation (geology) 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 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

360-484: 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

384-531: The titanotheres Paleosyops and Manteoceras , an artiodactyl fossil trackway and Protoreodon fossil, and a small mammalian fauna at Mariano Mesa. The titanotheres are characteristic of the Bridgerian age while the trackways indicate a late Eocene age or younger. The formation was first described by R.H. Wilpolt and his coinvestigators in 1946 and named for exposures in Baca Canyon in

408-467: 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"

432-625: The east-northeast. Shallow floodplain lakes and a large, shallow closed lake in the eastern part of the basin were also present, though lacustrine beds are nearly absent due to extensive river delta formation. Deposition ended with the eruption of the Mogollon-Datil volcanic field , which disrupted the Baca drainage and buried the basin in volcaniclastic rock. The formation is relatively poor in vertebrate fossils, and only six of biostratigraphic significance have been found. These are

456-550: 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"

480-522: 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

504-400: 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

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528-564: 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

552-522: 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

576-449: 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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