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Reference
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| Title: |
The Late Triassic Bivalve Monotis in Accreted Terranes of Alaska
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1997
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ABSTRACT--Late Triassic bivalves of the genus Monotis occur in at least 16 of the lithotectonic terranes and subterranes that together comprise narly all of Alaska, and they also occur in the Upper Yukon region of Alaska where Triassic strata are regarded as representing non-accretionary North America. On the basis of collections made thus far, 14 kinds of Monotis that differ at the species or subspecies level can be recognized from Alaska. These are grouped into the subgenera Monotis (Monotis), M. (Pacimonotis), M. (Entomonotis), and M. Eomonotis. In places, Monotis shells of one kind or another occur in rock-forming abundance. On the basis of superpositional data from Alaska, as well as from elsewhere in North America and Far Eastern Russia, at least four distinct biostratigraphical levels can be discriminated utilizing Monotis species. Different species of M. (Eomonotis) characterizes two middle Norian leves, both probably within the upper middle Norian Columbianus Ammonite Zone. |
| Report by: |
Norman J. Silberling
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J. A. Grant-Mackie
,
K. M. Nichols
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| | Age: | Norian (late middle Norian) |
| Formation: | Glenn Shale |
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| Comment: | Plate 6 explanation: Monotis (Eomonotis) ?pinensis and the associated ammonite Himavatates. 6-11. USGS Mesozoic loc. M1715 (several meters stratigraphically above loc. M1714). 6. Right and left valves. 7-8. Left valves. 9-10. Right valves. 11. ?Himavitites cf. H. multiauritis. ---- "In east-central Alaskan strata, Monotis shells are so abundantly stacked together that the compression has impressed one shell into another forming a confused patter of interfering ribbing and incomplete shell outlines (pl. 6, fig. 13; pl 7, figs. 1-2). The abundance of shells themselves makes it difficult to obtain positively identifiable specimens. Species of Eomonotis, such M. (Eo.) daonellaeformis (listed by Silberling as M. "scutiformis typica" in Brabb, 1968, p. 112) and probably M. (Eo.) pinenensis, are particularly well represented. In younger beds, specimens that belong to M. (P.) subcircularis are well represented, although invariably poorly preserved. Other late Norian species are not known, most lackly because of lack of exposure." (Silberling et al., 1997, p. 9). ----- "In the Upper Yukon, M. (Eo. daonellaeformis (loc. M1714; pl. 6, figs. 1-5) occurs 3-10 m below M. (Eo.) ?pinensis (loc. M1715; pl. 6, figs. 6-16). This is the only unquestioned occurrence of M. (Eo.) daonelliformis in Alaska, but in northeastern Asia, Dagys and others (1979) define a M. daonellaeformis subzone of the "M. scutiformis zone,", overlain by a M. pinensis subzone." (Silberling et al., 1997, p. 12-13) |
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