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.
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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)