Misplaced Pages

#387612

25-584: The Buda Limestone is a geological formation in the High Plains and Trans-Pecos regions of West Texas and in southern New Mexico , whose strata date back to the Late Cretaceous . Pterosaur remains are among the fossils that have been recovered from the formation. The Buda Limestone is a dense white to yellow sparry limestone with numerous calcite veins that weathers to yellowish or pinkish due to its iron content. In far western Texas,

50-597: 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

75-402: A conchoidal fracture. The middle member is about 30 feet (9.1 m) of an argillaceous or marly nodular grayish-white limestone. The upper member is up to 60 feet (18 m) of limestone similar to the lower member. Foraminifera are poorly preserved, but may include Globigerina . Gastropods, a few Gryphaea oysters, and the bivalve Alectryonia carinata has also been found in

100-469: 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

125-542: 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

150-617: 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,

175-403: 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

200-449: 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

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

SECTION 10

#1732786716388

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

325-566: Is now being exploited using techniques such as horizontal and underbalanced drilling . While the vertical wells relied on the porosity of the formation (up to 6%), the horizontal wells exploit a system of natural vertical fractures in the formation. The formation was originally named the Shoal Creek Limestone, but this name was preempted, and the formation was renamed the Buda Limestone by T.W. Vaughan in 1900. By 1935,

350-508: 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

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

400-467: 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"

425-550: 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"

450-516: The formation had been included in the Washita Group . 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

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

500-801: The formation overlies the Del Rio Formation and is in turn overlain by the Eagle Ford Group . The total thickness is about 100 feet (30 m). In trans-Pecos Texas, the formation rests on the Grayson Formation and is overlain by the Boquillas Formation . In the Big Bend National Park area, the formation is divided into three informal members. The lowest is 25 feet (7.6 m) of hard microgranular grayish-white limestone that breaks with

525-509: The formation. Abundant bivalve fossils are found in the formation east of Big Bend National Park. Exposures in Hays County, Texas have yielded fossil remains of Pterodactyloidea , Ornithocheiroidea and Ornithocheiridae . Type locality for Graptocarcinus texanus Roemer, 1887(Decapoda: Brachyura). The Buda Limestone is an important petroleum reservoir that has been exploited for decades via conventional vertical drilling. It

SECTION 20

#1732786716388

550-400: 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

575-564: 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

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

#387612