The base of the Anisian. A Candidate Stratotype Section And Point from Chios Island (Greece)

In recent years there have been reappraisals of a variety of significant stratigraphic sections across the Spathian/Anisian (Early/Middle Triassic) boundary (Wang, 1985; Bucher, 1989; Tozer and Talon, 1990; Gradinaru, 1991; Gaetani et al., 1992). At present, sections in Oman, Dobrudgea (Romania) and Chios (Greece) appear to be the most relevant sections described in pelagic Tethyan environments. All of them are comprised of pinkish to reddish nodular limestones with low sedimentation rates. The Himalayan sections do not seem to be suitable, essentially because the boundary occurs within the so-called Niti Limestone (Nicora et al., 1984), where ammonoids are badly preserved and almost impossible to chisel out. As regards magnetostratigraphy it is worth noting that, although a paleomagnetic survey has not been attempted, the Color Alteration Index of conodonts (4 to 5) suggests that the region underwent high regional heating (more than 300 °C?) that may have reset the magnetization. In SE China (Yangtze Platform) the critical interval is condensed and is characterized by mixed ammonoid faunas (Wang, 1985), whereas in Nevada the critical interval around the boundary falls within the poorly fossiliferous "Brown Sandstones" (Silberling and Wallace, 1969). As Bucher (1989) reports, the first ammonoid-bearing level above this terrigenous unit is the J. welteri horizon. Thus, the most suitable sections for the Global Stratotype Section and Point (GSSP) of the base of the Anisian seem to be, at present, the western Tethyan Oman, Dobrudgea and Chios sections.