Stratigraphy is concerned with the order and relative position of strata and their relationship to the geological time scale.
The image at the right shows rock strata in Cafayate, Argentina, the subject of stratigraphy.
Def. the "study of rock layers and the layering process" is called stratigraphy.
Smith's first law, the law of superposition states: in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence, an object cannot be older than the materials of which it is composed.
Newer rock beds lie on top of older rock beds unless the succession has been overturned.
Since there is no overturning, the rock at the bottom in the image on the right is older than the rock on the top by the Law of Superposition.
Smith's second law, the Law of Strata identified by fossils states that each stratum in the succession contains a distinctive set of fossils, which allows beds to be identified as belonging to the same stratum even when the horizon between them is not continuous.
The Principle of Original Horizontality states that layers of sediment are originally deposited horizontally under the action of gravity.
In the second image down on the right, the sediments composing these rocks were formed in an ocean and deposited in horizontal layers.
In the image on the left, these strata make up much of the famous prominent rock formations in widely spaced protected areas such as Capitol Reef National Park and Canyonlands National Park. From top to bottom: Rounded tan domes of the Navajo Sandstone, layered red Kayenta Formation, cliff-forming, vertically jointed, red Wingate Sandstone, slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layered Cutler Formation sandstone. Picture from Glen Canyon National Recreation Area, Utah.
The principle of lateral continuity states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous. As a result, rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous.
Layers of sediment do not extend indefinitely; rather, the limits can be recognized and are controlled by the amount and type of sediment available and the size and shape of the sedimentary basin. As long as sediment is transported to an area, it will eventually be deposited. However, as the amount of material lessens away from the source, the layer of that material will become thinner.
Often, coarser-grained material can no longer be transported to an area because the transporting medium has insufficient energy to carry it to that location. In its place, the particles that settle from the transporting medium will be finer-grained, and there will be a lateral transition from coarser- to finer-grained material. The lateral variation in sediment within a stratum is known as sedimentary facies.
Cross-cutting relationships is a principle of geology that states that the geologic feature which cuts another is the younger of the two features:
- Structural relationships may be faults or fractures cutting through an older rock.
- Intrusional relationships occur when an igneous pluton or dike is intruded into pre-existing rocks.
- Stratigraphic relationships may be an erosional surface (or unconformity) cuts across older rock layers, geological structures, or other geological features.
- Sedimentological relationships occur where currents have eroded or scoured older sediment in a local area to produce, for example, a channel filled with sand.
- Paleontological relationships occur where animal activity or plant growth produces truncation. This happens, for example, where animal burrows penetrate into pre-existing sedimentary deposits.
- Geomorphological relationships may occur where a surficial feature, such as a river, flows through a gap in a ridge of rock. In a similar example, an impact crater excavates into a subsurface layer of rock.
Second down on the left, an igneous intrusion is cut by a pegmatite dyke, which in turn is cut by a dolerite dyke. These rocks are of Precambrian (Proterozoic) age and they are located in Kosterhavet National Park on Yttre Ursholmen island in the Koster Islands in Sweden. The oldest igneous rocks in this photo show features caused by magma mingling or magma mixing.
The law of included fragments states that clasts in a rock are older than the rock itself. A xenolith, a fragment of country rock that fell into passing magma as a result of stoping, or a derived fossil, a fossil eroded from an older bed and redeposited into a younger one, are included fragments.
Def. a "body of rock with specified characteristics reflecting the way it was formed" is called a facies.
A facies is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formation), and the changes that may occur in those attributes over a geographic area and is the sum total characteristics of a rock including its chemical, physical, and biological features that distinguishes it from adjacent rock.
Law of Facies, or simply Walther's Law, states that the vertical succession of facies reflects lateral changes in environment; conversely, it states that when a depositional environment "migrates" laterally, sediments of one depositional environment come to lie on top of another. In Russia the law is known as Golovkinsky-Walther's Law, "The fundamentals of the facies law, known in the West as Walther's Law and in Russia as Golovkinsky-Walther's Law, were also described in Golovkinsky's work long before Walther drew his conclusions on this subject. The present paper shows that the fundamentals of sequence stratigraphy were first set forth in the work of N. A. Golovkinsky."
The changes in the relative proportions of trace elements and isotopes within and between lithologic units vary with time and can used to map subtle changes that occurred in the paleoenvironment.
Dates have been assigned to specific geologic stratigraphy frames, columns, or columnar units.
As an example of a stratigraphic column, the image at the right shows one for the Dinosaur National Monument, Utah and Colorado, USA.
Each geographic location on the rocky surface of the Earth has a stratigraphic column. Correlating each stratum that has been shown to be in a geologic time period with others around the world is part of the fun of stratigraphy.
At the right is a small portion of the stratigraphic column between the Hatherton and MacKay glaciers in Antarctica. The top rock layer is a greyish red siltstone. The next downward is a greenish grey siltstone penetrated by sinuous tubes that may be roots or root-like structures. Underlaying this is "a zone of calcareous nodules."
"The Beacon Supergroup (Barrett, 1970) in the Transantarctic Mountains is largely a flat-lying, nonmarine sequence from Devonian or older to Jurassic in age. It consists of the Taylor Group (Devonian or older), a quartzose sandstone sequence, and the Victoria Group (Permian and Triassic), dominantly a coal-bearing sandstone-siltstone sequence (Harrington, 1965)."
"The Taylor Group comprises up to 1,450 m of quartzose sandstone, with smaller conglomerate, arkosic and shaly units [...]. [The] youngest Taylor Group unit [is] the Aztec Siltstone [of which the image at the right exhibits]."
Glacial sediment layers
The cross section of a sandur deposit on the right from Iceland shows strata of various sand-like material atop dead ice from a former surge of the glacier Brúarjökull.
In the second image down on the right: "The pebble beach at Weybourne marks the start of the cliff section of the Norfolk coast that extends in easterly direction. This change in the character of the coastline is due to the properties of the chalk, which is harder to the east. The cliffs resting on this chalk base are composed of layers of glacial sediments of flints and fossils."
Def. "a pair of sedimentary layers, a couplet, that form in an annual cycle as the result of seasonal weather changes" is called a varve.
"Typically formed in glacial lakes a varve couplet consists of a coarser grained summer layer formed during open-water conditions, and a finer grained winter layer formed from deposition from suspension during a period of winter ice cover. Many varve deposits contain hundreds of couplets."
Def. a "stratigraphic group from the lower Jurassic period, consisting of thin layers of blue limestone [present in parts of southern England]" is called a lias.
"The [Calabrian] GSSP occurs at the base of the marine claystone conformably overlying sapropelic bed ‘e’ within Segment B in the Vrica section. This lithological level represents the primary marker for the recognition of the boundary, and is assigned an astronomical age of 1.80 Ma on the basis of sapropel calibration."
"The boundary falls between the highest occurrence of Discoaster brouweri (below) and the lowest common occurrence of left-coiling Neogloboquadrina pachyderma (above), and below the lowest occurrences of medium-sized Gephyrocapsa (including G. oceanica) and Globigerinoides tenellus."
In the image on the right, the Vrica section includes specifically the GSSP of the Calabrian Stage fixed at the top of layer ‘e’.
"The base of the Quaternary System [shown in the image on the right] is defined by the Global Stratotype Section and Point (GSSP) of the Gelasian Stage at Monte San Nicola in Sicily, Italy, currently dated at 2.58 Ma."
"The astrochronological age of sapropel MPRS 250 (mid-point), corresponding to precessional cycle 250 from the present, is 2.588 Ma (Lourens et al., 1996), which can be assumed as the age of the boundary."
"The boundary-stratotype of the stage is located in the Eraclea Minoa section on the southern coast of Sicily (Italy), at the base of the Trubi Formation. The age of the Zanclean and Pliocene GSSP at the base of the stage is 5.33 Ma in the orbitally calibrated time scale, and lies within the lowermost reversed episode of the Gilbert Chron (C3n.4r), below the Thvera normal subchron."
In the chronostratigraphic correlation in the Piacenzian section, the base of the Zanclean is marked as the '0' point.
"The GSSP section near El Kef contains one main feature that allows for a direct correlation of this marine section with continental sections: the Ir anomaly at the base of the Boundary Clay."
The Global Boundary Stratotype Section and Point for the base of the Danian Stage is also the base GSSP for the Paleocene, Paleogene, "Tertiary", and Cenozoic at El Kef, Tunisia.
In the diagram on the right, the Permian-Triassic boundary is at the base of the Induan limestone that occurs within the Yinkeng Formation.
"The Global Stratotype Section and Point (GSSP) of the Permian-Triassic boundary [...] is defined at the base of Hindeodus parvus horizon, i.e. the base of Bed 27c of Meishan section D, Changxing County, Zhejiang Province, South China."
"Hindeodus parvus is now recognized as the index fossil" occurring in the Zone above the P-T boundary.
"The boundary for the Frasnian/Famennian Stage Global Stratotype Section and Point (GSSP) [...] is drawn [above] in a section exposed [in the second image above] near the Upper Coumiac Quarry in the southeastern Montagne Noire, France."
A specimen of Clymenia laevigata from the Upper Devonian Famennian of Poland is on the right.
On the left is a fossil of Platyclymenia intracrostata also from the Famennian of Poland.
On the right is an image of the type locality for the Telychian base GSSP indicated by an arrow which points parallel to the bedding. Older bedding of the Aeronian is to the right. The Telychian GSSP is in the Wormwood Formation, Cefn Cerig quarry.
In the section below for the Aeronian, the lower Telychian is marked with a Ⓣ.
The diagram above has the GSSP for the base of the Aeronian symbolized by a Ⓐ. The upper GSSP for the end of the Aeronian is symbolized by a Ⓣ.
On the right is the type locality for the base of the Aeronian indicated by the arrow. Actual beds are perpendicular to the arrow. The base of the Aeronian is in the Cefngarreg Sandstone Formation (formerly Trefawr Formation), Trefawr track section, Crychan Forest, Central Wales.
"The Lower Sandbian Nemagraptus gracilis Zone comprises one of the most widespread, and easily recognizable graptolite faunas in the Ordovician System. The base of the N. gracilis Zone also marks the base of the Upper Ordovician Series, and is internationally defined by the FAD of the eponymous species, with the Global Stratotype Section and Point (GSSP) located at Fågelsång in Scania, southern Sweden (Bergström et al., 2000, 2009)."
"The Global boundary Stratotype Section and Point (GSSP) for the base of the Guzhangian Stage (Cambrian Series 3) is defined at the base of a limestone (calcisiltite) layer 121.3 m above the base of the Huaqiao Formation in the Louyixi section along the Youshui River (Fengtan Reservoir), about 4 km northwest of Luoyixi (4 km southeast of Wangcun), in northwestern Hunan, China."
"The GSSP level contains the lowest occurrence of the cosmopolitan agnostoid trilobite Lejopyge laevigata [in the image on the left] (base of the L. laevigata Zone)."
"In the central Flinders Ranges the 4.5 km thick Umberatana Group encompasses the two main phases of glacial deposition (see Thomas et al., 2012). The carbonaceous, calcareous and pyritic Tindelpina Shale Member, of the interglacial Tapley Hill Formation, caps the Fe-rich diamictite and tillite formations of the Sturt glaciation. The upper Cryogenian glacials of the Elatina Formation are truncated by the Nuccaleena Formation at the base of the Wilpena Group and the Ediacaran System."
"In 2004, the Global Stratotype Section and Point (GSSP) for the terminal Proterozoic was placed near the base of the Nuccaleena Formation in Enorama Creek in the central Flinders Ranges [in the image on the right], thus establishing the Ediacaran System and Period (Knoll et al., 2006). As the Nuccaleena Formation has not been accurately dated, a date of c. 635 Ma from near-correlative levels in Namibia and China is presumed for the base of the Ediacaran (Hoffmann et al., 2004; Condon et al., 2005; Zhang et al., 2005)."
Sediment cores may be obtained "by drilling or jack-hammering a steel rod or shoving a hand auger or hollow "push core" into a beach or marsh or water bottom, and pulling up sediment samples for analysis."
"You can think of a sediment core as being more or less a tape recorder of time. Within that sediment core, we work with proxies, or environmental proxies, and these can be very simple measures of grain size or composition or some organic geochemical property or maybe pollen."
"There are environments that preserve storm records that are buried in the sea bed, so that you can go down through time and actually develop a record of the intensity and frequency of cyclonic storms. That's something that's pretty high up on the radar for coastal inhabitants. Kind of understanding the pattern of these storms through time helps us to understand what might be coming down the pike."
"You take a sediment core through a barrier island and under that is marsh, bay, marsh, mainland. You have maybe an old forest, roots. Looking back in time at that location, hundreds of thousands of years ago, you get this vertical succession of these different layers."
"A VIMS field crew [assembled in the image on the right] collects sediment cores on the landward side of the Plum Island barrier island in May 2014. These cores were collected with a Geoprobe drill rig [shown] and went as much as 60 feet below the surface of the island. Cores were collected in 4 feet sections and brought back to VIMS for processing."
- To obtain stratigraphic columns in locations where exposures do not occur, corings may provide alternatives.
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