Revised Lithostratigraphy of the Sonsela Member (Chinle Formation, Upper Triassic) in the Southern Part of Petrified Forest National Park, Arizona

PLOS ONE, Feb 2010

Background Recent revisions to the Sonsela Member of the Chinle Formation in Petrified Forest National Park have presented a three-part lithostratigraphic model based on unconventional correlations of sandstone beds. As a vertebrate faunal transition is recorded within this stratigraphic interval, these correlations, and the purported existence of a depositional hiatus (the Tr-4 unconformity) at about the same level, must be carefully re-examined. Methodology/Principal Findings Our investigations demonstrate the neglected necessity of walking out contacts and mapping when constructing lithostratigraphic models, and providing UTM coordinates and labeled photographs for all measured sections. We correct correlation errors within the Sonsela Member, demonstrate that there are multiple Flattops One sandstones, all of which are higher than the traditional Sonsela sandstone bed, that the Sonsela sandstone bed and Rainbow Forest Bed are equivalent, that the Rainbow Forest Bed is higher than the sandstones at the base of Blue Mesa and Agate Mesa, that strata formerly assigned to the Jim Camp Wash beds occur at two stratigraphic levels, and that there are multiple persistent silcrete horizons within the Sonsela Member. Conclusions/Significance We present a revised five-part model for the Sonsela Member. The units from lowest to highest are: the Camp Butte beds, Lot's Wife beds, Jasper Forest bed (the Sonsela sandstone)/Rainbow Forest Bed, Jim Camp Wash beds, and Martha's Butte beds (including the Flattops One sandstones). Although there are numerous degradational/aggradational cycles within the Chinle Formation, a single unconformable horizon within or at the base of the Sonsela Member that can be traced across the entire western United States (the “Tr-4 unconformity”) probably does not exist. The shift from relatively humid and poorly-drained to arid and well-drained climatic conditions began during deposition of the Sonsela Member (low in the Jim Camp Wash beds), well after the Carnian-Norian transition.

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Revised Lithostratigraphy of the Sonsela Member (Chinle Formation, Upper Triassic) in the Southern Part of Petrified Forest National Park, Arizona

Arizona. PLoS ONE 5(2): e9329. doi:10.1371/journal.pone.0009329 Revised Lithostratigraphy of the Sonsela Member (Chinle Formation, Upper Triassic) in the Southern Part of Petrified Forest National Park, Arizona Jeffrey W. Martz 0 William G. Parker 0 Andrew Allen Farke, Raymond M. Alf Museum of Paleontology, United States of America 0 Division of Resource Management, Petrified Forest National Park, Petrified Forest , Arizona , United States of America Background: Recent revisions to the Sonsela Member of the Chinle Formation in Petrified Forest National Park have presented a three-part lithostratigraphic model based on unconventional correlations of sandstone beds. As a vertebrate faunal transition is recorded within this stratigraphic interval, these correlations, and the purported existence of a depositional hiatus (the Tr-4 unconformity) at about the same level, must be carefully re-examined. Methodology/Principal Findings: Our investigations demonstrate the neglected necessity of walking out contacts and mapping when constructing lithostratigraphic models, and providing UTM coordinates and labeled photographs for all measured sections. We correct correlation errors within the Sonsela Member, demonstrate that there are multiple Flattops One sandstones, all of which are higher than the traditional Sonsela sandstone bed, that the Sonsela sandstone bed and Rainbow Forest Bed are equivalent, that the Rainbow Forest Bed is higher than the sandstones at the base of Blue Mesa and Agate Mesa, that strata formerly assigned to the Jim Camp Wash beds occur at two stratigraphic levels, and that there are multiple persistent silcrete horizons within the Sonsela Member. Conclusions/Significance: We present a revised five-part model for the Sonsela Member. The units from lowest to highest are: the Camp Butte beds, Lot's Wife beds, Jasper Forest bed (the Sonsela sandstone)/Rainbow Forest Bed, Jim Camp Wash beds, and Martha's Butte beds (including the Flattops One sandstones). Although there are numerous degradational/ aggradational cycles within the Chinle Formation, a single unconformable horizon within or at the base of the Sonsela Member that can be traced across the entire western United States (the ''Tr-4 unconformity'') probably does not exist. The shift from relatively humid and poorly-drained to arid and well-drained climatic conditions began during deposition of the Sonsela Member (low in the Jim Camp Wash beds), well after the Carnian-Norian transition. - Geologists and paleontologists are ultimately historians whose objective is to construct an accurate narrative of the history of the Earth and its living organisms, and to understand why these events occurred. Biostratigraphy, magnetostratigraphy, radioisotopic dating, the interpretation of depositional systems and paleoclimatology, are all tools for deriving a historical narrative from the rock record. However, if the basic superpositional relationships of the fossils, mag-strat samples, volcanic minerals, and lithologic units used to acquire this information are misunderstood, the interpretation derived from them will be inaccurate. The order and timing of events will be wrong, and any attempt to understand cause and effect will be in vain. Lithostratigraphy is therefore the foundation of paleontology as a historical science. Developing an accurate and detailed lithostratigraphic framework is the first and most essential step before anything collected from these strata can be used to construct a narrative. The Chinle Formation of the Colorado Plateau, and related strata throughout the western United States, preserve some of the most extensively exposed and well-studied Late Triassic continental deposits in the world [13]. These strata also preserve one of the best-studied terrestrial vertebrate faunas from this critical period in the Earths history (e.g., [4]). The Upper Triassic strata and vertebrate fossils in Petrified Forest National Park (hereafter PEFO) in northeastern Arizona (Figures 12) are arguably the most intensively studied in the Western Interior for several reasons: 1. PEFO and the surrounding area has had a long history of research, with significant investigations into the sedimentary geology and paleontology of the Chinle Formation dating back to the first half of the 20th century (e.g., [56]). The Chinle Formation remains a rich source of plant and animal fossils, the collection and description of which is ongoing by researchers from various institutions, including the park staff (e.g., [79]). 2. Almost the full section of the Chinle Formation is exposed within PEFO. Most of the park has excellent exposures of the middle part of the Chinle Formation, which has traditionally been referred to as the Petrified Forest Member, and has more recently been formally divided into the Blue Mesa, Sonsela, and Petrified Forest (or Painted Desert) Members [3,1012] (Figure 3). Lowermost Chinle Formation strata (variously referred to as the Monito (...truncated)


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Jeffrey W. Martz, William G. Parker. Revised Lithostratigraphy of the Sonsela Member (Chinle Formation, Upper Triassic) in the Southern Part of Petrified Forest National Park, Arizona, PLOS ONE, 2010, Volume 5, Issue 2, DOI: 10.1371/journal.pone.0009329