Триас кезеңінің соңғы жасы: Ратев дәуірі

Latest age of the Triassic Period

The Rhaetian is the latest age of the Triassic Period (in geochronology) or the uppermost stage of the Triassic System (in chronostratigraphy). It was preceded by the Norian and succeeded by the Hettangian (the lowermost stage or earliest age of the Jurassic). The base of the Rhaetian lacks a formal GSSP, though candidate sections include Steinbergkogel in Austria (since 2007) and Pignola Abriola in Italy (since 2016). The end of the Rhaetian (and the base of the overlying Hettangian Stage) is more well defined. According to the current ICS (International Commission on Stratigraphy) system, the Rhaetian ended ± 0.2 Ma (million years ago). In 2010, the base of the Rhaetian (i. e. the Norian Rhaetian boundary) was voted to be defined based on the first appearance of Misikella posthernsteini, a marine conodont. Most recently, aspects of the "short Rhaetian" hypothesis have been revived by radiometric dating of Peruvian bivalve extinctions and magnetostratigraphy at the Pignola Abriola GSSP candidate. These studies suggest that the base of the Rhaetian was close to 205.5 Ma. Rigo et al. (2020) found this same pattern in the nearby Mt Volturino and Madonna del Sirino sections, as well as the Kastelli section of Greece. They also found it in East Panthalassan sediments (Kennecott Point of British Columbia and New York Canyon of Nevada) and West Panthalassan sediments (Wombat and northern Carnarvon Basins of Australia and the Kiritehere section of New Zealand). It was construed to be related to the same event responsible for the Norian Rhaetian extinction, which heavily impacted ammonoids, bivalves, conodonts, and radiolarians. The Norian Rhaetian extinction may have been caused by the eruption of the Angayucham large igneous province in Alaska, or the asteroid responsible for the Rochechouart impact structure in France. However, the dating of these geological events and their effects on life are uncertain at best. A second formal GSSP candidate was not provided until Rigo et al. (2015) proposed the Pignola Abriola section of southern Italy. This is a sequence of the Norian Rhaetian Calcari con Selce ("Cherty limestone") Formation named after two nearby towns. It preserves a diverse array of conodonts (including the Misikella hernsteini posthernsteini morphocline) as well as pronounced radiolarian zones.

Gallet et al. (2007) argued in support of a "short Rhaetian" (where the Rhaetian lasts under 5 million years) based on the Oyuklu section, a sequence from Turkey. This sequence was largely normal polarity dominated, and presented two potential Norian Rhaetian boundaries (since the defining biostratigraphy of the Rhaetian was not resolved at the time). Defining the boundary based on the appearance of Misikella posthernsteini placed it in a reverse polarity section (B−) near the base of Oyuklu. Defining the boundary based on the extinction of Epigondolella bidentata placed it at magnetozone G+, the first of several major normal polarity sections. The early reverse polarity zones (B− to D−) were correlated with PM11r, a reverse polarity section at the top of Pizzo Mondello, a similar Carnian Norian sequence in Sicily. The inferred overlap between these reverse polarity sections was located above a normal polarity section (A+ in Oyuklu and PM11n in Pizzo Mondello). This underlying normal polarity section was correlated with either magnetozone E21n or E23n of the Newark sequence. Although the upper portion of Oyuklu was mostly normal, it did have a few reverse sections (H− and J−) which were at odds with the almost entirely normal last few Triassic magnetozones of Newark. Palynomorph turnovers and changes in tetrapod faunas similar to Norian events in Europe have also been used to support this hypothesis. Many of the biostratigraphers who argue in favor of a Newark hiatus use similar techniques to support a "long Tuvalian" hypothesis, in which the Tuvalian (late Carnian) extends into a period of time commonly believed to be early Norian. When the International Commission on Stratigraphy updated their Geologic Time Scale in 2012, the "short Rhaetian" and "long Tuvalian" hypotheses were equated with each other. The combined "short Rhaetian/long Tuvalian" hypothesis as described by Ogg (2012) was ultimately not chosen by the ICS when compared to its competition, which was supported by a more diverse array of methods.

The "short Rhaetian" hypothesis has been criticized for its reliance on the assumption that a hiatus existed at Newark. This hiatus was presumed to lie within the normal polarity dominated end of the Rhaetian, after a very short reverse polarity section (E23r) and just before the first CAMP eruptions. However, the lithology and astrochronology of Newark seem to be continuous and this precludes any assumed unconformity. In addition, the magnetic signature of at the end of Newark basin has been found worldwide, with sequences in Morocco, Nova Scotia, Italy, the U. K., and possibly Turkey all preserving E23r equivalent magnetozones underlying the Rhaetian Hettangian boundary. It would be very improbable for all of these sites of varying geology and deposition rates to experience an unconformity erasing an equivalent amount of time. Kent, Olsen, & Muttoni (2017) additionally found convincing correlations between the magnetozones of the upper Passaic Formation and Rhaetian strata in England. They suggest that the apparent delay between Newark and Europe fauna and flora may instead be biogeographic differences due to climatic variation over time and latitude, a factor which has manifested at other points in the Triassic. Various studies have supported a "long Rhaetian" hypothesis (where the Rhaetian lasts 5–10 million years) based on magnetostratigraphy. Muttoni et al. (2010) studied a pair of Triassic sequences in northern Italy: the Norian Rhaetian Brumano section and the Rhaetian Hettangian Italcementi section. In Brumano, M. posthernsteini first appeared quite a distance below the oldest reported magnetozone, BIT1n, which was correlated with E20n at Newark. The opposite is true in Pizzo Mondello, where M. posthersteini appears above the youngest complete magnetozone, PM12n (equivalent to E17n at Newark). This suggests that the Norian Rhaetian boundary lies in the range of Newark magnetozones E17r to E19r, or 207 210 Ma. The authors expressed skepticism towards the substantial overlap between Oyuklu and Pizzo Mondello proposed by Gallet et al. (2007). Ikeda & Tada (2014) provided an astrochronologically constrained chert sequence in Japan which suggested that the Norian Rhaetian boundary occurred 208.5 ± 0.3 Ma, based on the extinction of the Norian radiolarian Betraccium deweveri. Hüsing et al. (2011) was a prominent study arguing in favor of a long Rhaetian. This was based on biostratigraphy and magnetostratigraphy of the Steinbergkogel section in Austria, which is a candidate GSSP for the base of the Rhaetian. They proposed two options for defining the base of the Rhaetian, either at the first occurrence of Misikella hernsteini or the first appearance datum of Misikella posthernsteini (sensu lato). At Steinbergkogel, M. hernsteini first occurred at the top of a normal polarity section while M. posthernsteini (sensu lato) first appeared at the base of a shorter overlying reverse polarity section. These sections were correlated with magnetozones E16n and E16r of the Newark Basin. Hüsing et al. (2011) preferred to define the Rhaetian based on M. hernsteini, and estimated a date for the Norian Rhaetian boundary of 209.8 Ma based on that of Newark's magnetozone E16n. Partially inspired by the work of Hüsing et al. (2011), the ICS's 2012 Geologic Time Scale utilized a tentative 208.5 Ma date for the Norian Rhaetian boundary.

The Norian Rhaetian boundary was finally provided with radiometric dating in a study by Wotzlaw et al. (2014). They studied a sequence of the Aramachay Formation in Peru which records the extinction of large Monotis bivalves. This prominent biotic event is closely associated with the Norian Rhaetian boundary. The last Monotis specimens lie between ash beds which are Uranium Lead dated to 205.70 ± 0.15 Ma and 205.30 ± 0.14 Ma. This allowed them to conclude that the Norian Rhaetian boundary occurred somewhere between these ash beds, 205.50 ± 0.35 Ma. This date corresponds to "short Rhaetian" predictions, but Wotzlaw et al. (2014) also agreed with "long Rhaetian" proponents who argued that there was no good evidence for a hiatus in the Newark Basin sequence. Wotzlaw et al. (2014) estimated that the Norian Rhaetian boundary was concurrent with a lengthy reverse polarity section (E20r.2r) of Newark magnetozone E20. Golding et al. (2016) utilized U Pb dating at a part of the Black Bear Ridge section of British Columbia which is considered early Rhaetian based on its conodont fauna. Their estimated 205.2 ± 0.9 Ma date for this early Rhaetian section agrees with the results of Wotzlaw et al. (2014). This compromise between "short Rhaetian" and "long Rhaetian" hypotheses has been supported by other studies. Maron et al. (2015) elaborated on the dating of an upcoming GSSP candidate for the Rhaetian in the form of the Pignola Abriola section in Southern Italy. This section recorded the Norian Rhaetian boundary as tracked by the first occurrence of Misikella posthernsteini, the base of the Proparvicingula moniliformis radiolarian zone, and a prominent negative δ13C anomaly. Magnetostratigraphy correlated MPA5r (the Pignola Abriola magnetozone surrounding the Norian Rhaetian boundary) with the early part of Newark's E20. This provided an estimated date of 205.7 Ma for the Norian Rhaetian boundary, very similar to Wotzlaw et al. (2014)'s estimate. This debate has led some biostratigraphers to suggest avoiding the use of conodonts in Triassic chronostratigraphy altogether, a proposal which itself has been criticized by Triassic conodont specialists. Writing on behalf of the ICS, Ogg (2016) stated that there were two possible dates for the Norian Rhaetian boundary: 209.5 Ma (using M. posthernsteini sensu lato and Steinbergkogel as a GSSP) or 205.8 Ma (using M. posthernsteini sensu stricto and Pignola Abriola as a GSSP). A recent update of Newark stratigraphy by Kent, Olsen, & Muttoni (2017) combined magnetostratigraphy with astrochronology to form the longest astrochronostratigraphic polarity time scale (APTS) known in the fossil record. The Newark sequence was affected by astrochronological (Milankovitch) cycles as recorded by climate induced changes in lake depth and geology, although depositional rate is remarkably consistent within the Newark basin. The most consistent and regular of these cycles are 405,000 year cycles known as McLaughlin cycles. By tracing McLaughlin cycles backwards from the radiometrically dated CAMP basalts, the boundaries between each formation and magnetozone in the Newark sequence could be assigned a precise age. Magnetozone E20r.2r lasted from 206.03 to 204.65 Ma according to this method, suggesting that the Rhaetian began ~205.5 Ma. This agrees with the dates for the Norian Rhaetian boundary obtained by Wotzlaw et al. (2014) and Maron et al. (2015).

The end date of the Rhaetian currently in use by the ICS (201.3 ±0.2 Ma) is based on a study by Schoene et al. (2010) involving ammonite bearing strata in Peru. They used CA ID TIMS Uranium Lead dating to date ash beds slightly below and slightly above the first appearance of Psiloceras in the Pucará Basin. The overlying ash bed was dated to 201.29 ±0.16 Ma while the underlying was 201.36 ±0.13 Ma. This allowed the first appearance of Psiloceras to be given a date of 201.31 ±0.18/0.43 Ma (assuming minimum/maximum uncertainty).