A REVIEW OF MORRISON FORMATION PALEOGEOGRAPHY AND TESTS FOR TIME-TRANSGRESSION AND FAUNAL PROVINCIALISM

The flagellicaudatan Suuwassea emilieae, from the Morrison Formation of south-central Montana, exemplifies a trend in sauropod dinosaurs that has only recently become manifest: taxa from its northern reaches (northern Wyoming and Montana) differ in size, and possibly taxonomy, from their better-known counterparts from the southern portion of the depositional basin. Factors that may contribute to this disparity are age and paleoenvironmental/paleoecological differences, or both. Whether or not there is an age difference between the northern and southern portions of the Morrison Formation has not been previously examined in detail. Except in its southwest corner, the Morrison Formation overlies marine sediments of the northward regressing Sundance Sea. Given that the Morrison is almost exclusively terrestrial, two hypotheses are possible: (1) the formation is time-transgressive northward (low lithostratigraphic units in the north correlate with higher units in the south), or (2) Sundance Sea level fall caused its southern margin to regress into northernmost Montana/southern Canada before any terrestrial sedimentation occurred, so Morrison deposition was synchronous throughout the basin (lithostratigraphic = chronostratigraphic units). Dating of glaucony from the Sundance/Swift Formation immediately below the Morrison Formation and zircons recovered from a smectitic (altered volcanic ash) horizon high in the Morrison section at the Suuwassea type locality can be compared to those previously recovered from the formation on the Colorado Plateau. If the ages from units of stratigraphically roughly equivalent levels are younger in the north, it would support the time-transgressive hypothesis, and both paleoecological and age differences may explain why the sauropod fauna appears different in the north. If the ages are equivalent, then age cannot be a factor contributing to the faunal distinction.