New Mexico Geological Society Annual Spring Meeting — Abstracts

In 1978, Pipringos and O'Sullivan named the erosional surface underlying the Chinle Group on the Colorado Plateau the Tr-3 unconformity. This surface is expressed either as scours beneath coarse-grained channel-fill siliciclastics of the Shinarump Formation or as a complex, pedogenically-modified zone of siliCiclastics, either of which is typically developed on the Lower-Middle Triassic Moenkopi Group or, rarely, on older strata. The pedogenically modified zone has traditionally been assigned to an informal lithostratigraphic unit termed the "mottled strata" throughout much of the southern Colorado Plateau. In southeastern Utah, this pedogenically modified zone was named the Temple Mountain Formation of the Chinle Group, and correlative strata in SE Nevada were named the Spring Mountains Formation of the Chinle Group.

Pedogenically modified sediments associated with the base of the Chinle Group exhibit a wide array of lithologies and apparently originated in several ways. Some of these strata represent pedogenesis of underlying strata that were originally deposited during either the Permian or Early/Middle Triassic. Others are remnants of deposits that were generated by fluvial systems that existed after the end of Moenkopi Group deposition but that predate Chinle Group depOSition. Other deposits may represent pedogenically modified fluvial and floodplain facies associated with deposition of the Shinarump Formation. Regardless of their apparent origin, we consider all of these facies to be units of the Chinle Group, in part because of their continuity with Chinle deposits and because this facilitates mapping of the base of the Chinle Group, and therefore the erosional surface associated with the Tr-3 unconformity.

Strata we assign to the Moenkopi Group or to older units predate pedogenesis and typically consist of flaggy sandstones similar in lithology to Moenkopi Group litharenites, except that they have been bleached and bear numerous reduction spots or else are extensively color mottled. Although most original sedimentary structures have been obliterated by pedogenesis, these strata retain a flaggy weathering profile that may correspond to original bedding. These strata have traditionally been assigned to the "mottled strata."

The other primary lithologies assigned to the mottled strata are more extensively modified horizons of "porcellanites." Almost all original sedimentary structures have been disrupted by pedo-and bioturbation. These strata, too, are typically assigned to the "mottled strata."

The Temple Mountain Formation is used to describe only slightly pedogenically modified deposits, some of which are likely contemporaneous with at least part of the of the "mottled strata" as currently defined. Temple Mountain strata still retain some original bedding features and probably correspond to the youngest deposits associated with the Tr-3 erosional surface They may reflect the earliest part of the base level rise recorded by the Shinarump Formation.