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Abstract

Eight short stratigraphic sections within the Moenkopi Formation of northeastern Arizona verify a network of magnetic polarity stratigraphy previously observed in that area. The magnetostratigraphic signature was used to test the time relationships of numerous vertebrate faunal occurrences and to test the synchroneity of a major change in facies over this part of the depositional basin. The magnetic polarity data show that the vertebrate occurrences are not all of the same age; two or three different ages of fauna are indicated. Parallelism of the changes of magnetic polarity and lithology indicate a relatively rapid spread of sand-laden streams across the depositional basin. The near time-synchroneity of this lithologic change over much of the area indicates that the revised definition of the boundary between the Moqui and Holbrook members of the Moenkopi Formation as the first occurrence of a persistent, ledge-forming sandstone (Purucker and others, 1980) is a well-founded definition. A paleopole position was calculated from those samples that exhibited demagnetization behavior that is univectorial to the origin of orthogonal axes plots; this early Middle Triassic (early Anisian) paleopole is located at 94.8°E, 58.5°N (alpha-95 = 3.4°). Global correlation of the magnetostratigraphic pattern of the Moenkopi Formation to the patterns of two marine sequences indicates that Moenkopi deposition began in the Early Triassic mid-Griesbachian Stage and continued until it probably was interrupted by a hiatus that represents much of the Smithian. Deposition resumed in late Smithian and continued until late Spathian. The latest Spathian is represented by a hiatus. Deposition resumed again in the early Middle Triassic (early Anisian), thus the Moenkopi Formation provides a heretofore unknown record of geomagnetic field polarity during this time interval. A widespread Smithian hiatus, i.e., a lowstand, is suggested by comparisons of observations in south China, the Moenkopi Formation, and the Chugwater Group to those of the Arctic stratotypes.

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