Field of Science

Lithostratigraphy and biostratigraphy of the Chinle Formation in Lisbon Valley, Utah

The stratigraphy of the Chinle Formation in Lisbon Valley, Utah has been somewhat controversial. This paper is the result of several seasons of fieldwork and provides an intense revision of the local stratigraphy as well as discussion of the fossils found in these rocks. Martz et al. provide a large amount of information and are careful to make sure that their work is repeatable as possible. To this end they provide coordinates and labelled photos of their measured sections as well as a list of voucher specimens for all of the fossil taxa. Martz's work continues to raise the bar not only for studies in the Upper Triassic of Utah, but also for stratigraphic and biostratigraphic work in general.

Martz, J. W., Irmis, R. B., and A.R.C. Milner. 2014. Lithostratigraphy and biostratigraphy of the Chinle Formation (Upper Triassic) in southern Lisbon Valley, southeastern Utah; pp. 397-448 in MacLean, J.S., Biek, R.F., and J.E. Huntoon (eds.), Geology of Utah's Far South. Utah Geological Association Publication 43.

Abstract - We present here a detailed study of the lithostratigraphy and preliminary vertebrate biostratigraphy of the Upper Triassic Chinle Formation in Lisbon Valley, southeastern Utah. Triassic salt tectonism resulted in a period of erosion and possibly non-deposition that removed the top of the Lower Permian Cutler Group, the Early-Middle Triassic Moenkopi Formation, and the Late Triassic (earliest Norian) Shinarump Member of the Chinle Formation. Chinle Formation deposition in Lisbon Valley began or resumed sometime during the middle-late Norian or Rhaetian and terminated before the end of the Rhaetian. Chinle Formation sediments are mostly siltstone to fine-grained sandstone dominated by planar cross-bedding and climbing ripple cross-lamination; these sediments generally exhibit poor paleosol development, with interbedded conglomerates dominated by clasts composed of reworked intrabasinal sediments and containing only minor extrabasinal silica. The regional climate during Chinle deposition was becoming increasingly arid, with fluctuating seasonal rainfall. Deposition by the braided and meandering rivers of the lower Kane Springs beds filled paleovalleys incised into the Cutler Group, and was followed by a poorly drained interval of poorly oxygenated swamps and lakes crisscrossed by small streams that produced the middle Kane Springs beds. These conditions transitioned back to the slowly aggrading braided and meandering rivers of the upper Kane Springs beds, probably by Rhaetian time. The Kane Springs paleoenvironment, probably at least partially syndepositional with that of the Owl Rock Member, was inhabited by conifers, freshwater ostracods, bivalves and gastropods, indeterminate phytosaurs, and the aetosaur Typothorax. The shift to the overlying Church Rock Member was gradational and probably involved only subtle shifts in the depositional system, largely related to better-drained sediments. Braided channels with seasonally variable discharge crossed well-drained, rapidly aggrading and well-oxygenated floodplains. Rare paleosols (including entisols, vertisols, and aridisols) indicate seasonal wetting and drying in a generally arid climate. Rivers and lakes were inhabited by the phytosaur taxon Machaeroprosopus (including the derived form “Redondasaurus”), rare metoposaurids, coelacanths, a diverse actinopterygian fish fauna, conchostracans, ostracods, bivalves, and gastropods. The flora included conifer trees, giant horsetails, Cynepteris (a fern), Zamites (a bennettitalean), the small shrub-like conifer Pelourdea, and the enigmatic Sanmiguelia. Terrestrial tetrapods included the aetosaur Typothorax (suggesting that the genus endured into the Rhaetian), paracrocodylomorphs, and small theropods. Eventually, wind-blown eolian deposits entered the region, and late in the Rhaetian, prior to 201.3 Ma, the eolian Wingate erg swamped small braided channels still inhabited by the phytosaur “Redondasaurus,” actinopterygian fishes, and small theropods.

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