A REGIONALLY EXTENSIVE LANCIAN SEISMITE SERVES AS A TIME SYNCHRONOUS STRATIGRAPHIC MARKER FOR MAPPING DINOSAUR BONEBEDS IN NORTHEASTERN WYOMING

WEEKS, Summer, Southwestern Adventist University, Keene, TX, United States;
CHADWICK, Arthur, Southwestern Adventist University, Keene, TX, United States

The Lance Formation in northeastern Wyoming and other correlative Upper Cretaceous deposits in the region are notoriously resistant to stratigraphic studies. The beds can change character completely over a few meters, and it is not unusual to find an entire change in sediments between adjacent outcrops. An informal stratigraphic look at the Lance Formation leaves the impression that prominent sandstone ledges could serve as stable marker beds, but these are secondarily cemented with carbonate, and they can disappear and reappear at a higher or lower stratigraphic position without consistency. Without a dependable stratigraphic framework, it is difficult to correlate remote bone outcrops with the main quarry area. In the past, the most useful stratigraphic marker had been the major bonebed which is exposed for several hundred meters. The persistence of the bonebed suggested to us that a stratigraphic framework was possible. During the 2011 season, we identified a prominent seismite in the region of our quarries that is traceable across the extent of the quarries. The seismite is a tan, fine-grained, immature sandstone which exhibits distorted, undulating to crumpled bedding features and overlies an undisturbed sandstone. A flat, undisturbed sandstone lies above the seismite. We have thus far been able to map the seismite over an area of 50 square kilometers and we are presently working to ensure the seismite is everywhere a single event and to extend the seismite as far as possible. Using this time synchronous bed as a mappable horizon, we have been able to tentatively establish the relationship between the thirteen quarry sites we have worked. The majority of the quarries (eleven) occur in the main bonebed at a horizon 27 meters below the seismite. A microsite quarry is 15 meters below the seismite and a unique bonebed is 37 meters below the seismite. While the eleven quarries are paucispecific Edmontosaurus sites, the unique bonebed is remarkable in that excavation thus far (500 bones, teeth and mappable fragments) has revealed no Edmontosaurus remains, but bones and teeth from most other taxa of dinosaurs reported from the Lance (Pachycephalosaurus, Triceratops, Nanotyrannus, Tyrannosaurus, Dromaeosaurus, Nodosaurus, Struthiomimus, and several not yet identified) as well as remains of turtles, lizards, frogs, crocodiles and mollusks. We now know the stratigraphic relationship of this bed to the main bonebed, and we are now working to piece together a more comprehensive taphonomic model for the entire site.

poster presented at SVP 2012 meeting, Raleigh, NC, Oct 2012

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