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The Earliest Marine Triassic Rocks: Their Definition, Ammonoid Fauna, Distribution and Relationship to Underlying Formations
The base of the Otoceras woodwardi Zone (stratotype in Himalayas) is accepted to define the base of the Triassic. Adoption of this convention is necessary because the Buntsandstein of Germany, the base of which provides the definition of the Permian-Triassic boundary, is not characterized for recognition throughout the world. All rocks older than the Woodwardi Zone are regarded as pre-Triassic. In terms of the four Lower Triassic stages (Griesbachian, Dienerian, Smithian, Spathian) the Woodwardi Zone is Griesbachian.
Griesbachian ammonoids are assigned to 9 (possibly 11) genera. Of the 9, one is a survivor of the dominantly Paleozoic Prolecanitida. Two, assigned to Xenodiscidae, are Ceratitida with obvious Permian relatives. One, Otoceras, is the survivor of another Permian ceratitid stock. Five, paced in Ohpicer-atidae and Meekoceratidae, may be described as novel Ceratitida. The Griesbachian ammonoids thus include Paleozoic holdovers and novel Ceratitida.
In Canada Lower and Upper substages are discriminated within the Griesbachian. Otoceras and Xenodiscus characterize the Lower Griesbachian. Ophiceratidae and Proptychites (Meekoceratidae) dominate the fauna of the Upper Griesbachian, in which Otoceras is unknown. These two substages are clearly recognizable in east Greenland, probably also in Siberia, Kashmir and at Spiti (Himalayas) and are thus of more than local significance. In Canada, Siberia, Greenland, and Spitzbergen the earliest Triassic beds have Otoceras but no Ophiceratidae or Meekoceratidae. Only in the Himalayas, where the sections are extraordinarily thin, and where individual beds may contain fossils of more than one age, is there any evidence that Ophiceratidae and Meekoceratidae occur in the earliest Triassic beds. The Lower Griesbachian is characterized by a meagre ammonoid fauna of which only Otoceras is diagnostic. It is thus impossible to identify earliest Triassic beds unless Otoceras is present. The Ophiceras-bearing beds of the Salt Range are certainly not necessarily earliest Triassic, but are more probably Upper Griesbachian.
Lower Griesbachian has been definitely identified only in the Himalayas, Arctic Canada, Alaska, east Greenland, Spitzbergen and northeast Siberia. These Lower Griesbachian beds commonly, if not invariably, rest concordantly upon Permian rocks, but nowhere are the underlying Permian rocks demonstrably the youngest known beds of that System. As yet there is no known place where the youngest known Permian (e.g. the Paratirolites beds of Armenia) is followed by the earliest Triassic. The boundary everywhere seems to mark a hiatus with one or more units of the chronostratigraphic scale missing. Where the record is preserved, a world-wide event, probably an eustatic change in sea level, evidently interrupted marine sedimentation immediately before the Lower Griesbachian. A record may have been made in the ocean basins, but unless preserved in the Wharton Basin, west of Australia, it was presumably long ago swept into a subduction zone where its identity has been lost forever.
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