Stratigraphy and correlation of the Morrison Formation (Late Jurassic-?Early Cretaceous) of the western interior, United States, with emphasis on southeastern Wyoming

The Morrison Formation of southeastern Wyoming is one of the most prolific fossil-bearing units in the world. This study was initiated to understand the local stratigraphy of the formation and to look at long-distance correlations using radiometric dating, lithostratigraphy, and biostratigraphy.; A radiometric age range was obtained for the upper part of the Morrison Formation in southeastern Wyoming which constrains this part of the formation to the Late Jurassic. This does not, however preclude an Early Cretaceous age for the uppermost strata of the formation in this region. These new radiometric data also demonstrate that primary volcanic ash was indeed a part of the input to the formation.; Lithostratigraphic correlations within southeastern Wyoming were demonstrated to be valid. Previously suggested long-distance temporal correlations, however, including those based on a regional change in clay mineralogy, were shown to be unreliable.; Fossil fern spores were identified from the uppermost Morrison Formation in southeastern Wyoming that may be indicative of an Early Cretaceous age. Complications exist, however, with biostratigraphic zonation using palynomorphs that may render them ineffective for age determinations in the Late Jurassic and Early Cretaceous. Also, several studies that use biostratigraphic data to constrain the age of the Morrison Formation were shown to have major flaws.; This work also determined that the faunas of Quarry Nine at Como Bluff and the Purbeck Group of southern England most probably were diachronous. It is possible, therefore, that either the faunas are misdiagnosed or the rate of mammalian evolution at that time was so slow as to make fossil mammals ineffective for biostratigraphy in the middle Mesozoic.; This study concludes that although local correlations within the Morrison Formation probably are reliable, until precise radiometric dates can be obtained across the depositional area stratigraphers and paleontologists need to be extremely cautious in attempting long-distance correlations. Once a radiometrically based stratigraphic framework for the entire formation is established, workers can then place the biostratigraphic data into context and make useful comparisons both within the formation and worldwide.