Norian-Rhaetian boundary: "short Rhaetian" hypotheses 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.
Norian-Rhaetian boundary: "long Rhaetian" hypotheses magnetostratigraphy at
Steinbergkogel, a candidate
GSSP near
Hallstatt, Austria 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.
Norian-Rhaetian boundary: Recent compromises '' in Peru 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).
Rhaetian-Hettangian boundary The end date of the Rhaetian currently in use by the
ICS (201.4 ±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). == Notable formations ==