Jennings, Debra S.¹ 
(1) University of Kansas, Lawrence, KS

ABSTRACT: Identification of Dinosaur Tracks in Palustrine-lacustrine Deposits of the Morrison Formation: Their Significance to Lacustrine Sequence Stratigraphy

A new dinosaur quarry in the Morrison Formation near Thermopolis, Wyoming has yielded dozens of shed theropod teeth closely associated with juvenile Camarasaurus remains that have been trampled in a smectite-rich carbonate mudstone. Stratigraphic and mineralogic data from the site indicates that the interlayered mudstone - carbonate mudstone making up the sequence containing this quarry were deposited along the shore of a lake. Dinosaur tracks extending through carbonate mudstone to the mudstone below define subtle disconformity surfaces throughout the outcrop. Features including localized loading, micro-faults, and claystone injected into the carbonate mudstone reveal the location of these obscure tracks. The base of the tracks coincides with a sharp, but subtle, boundary between mudstones containing pedogenic carbonates, clastic dikes, and mudcracks and mudstones containing ostracodes, conchostracans, and dinosaur remains. Pedogenic mudstones indicate regressive events while units containing tracks and invertebrate fossils suggest a transgressive event. 
Although lacustrine transgressive regressive events provide valuable paleoenvironmental and paleoclimatic information, they may be difficult to distinguish in interlayered fine-grained sediments of palustrine-lacustrine sequences. The hypothesis of this study is that dinosaur tracks can be identified in palustrine-lacustrine sequences by definitive features and used to identify subtle bounding surfaces delineating transgressive-regressive cycles in the history of the lake. Results indicate that previously overlooked dinosaur tracks can now be located in fine-grained lacustrine sequences and used as valuable tools in sequence stratigraphic analysis of lacustrine systems.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.