Constraints on the Formation of Marine Bone Concentrations

Laura A. Vietti¹ and Ryan Littlewood^2
1University of Minnesota, Minneapolis, MN
²Coastal Environments, Inc., Baton Rouge, LA

Marine bone concentrations can occur as distinct widespread stratigraphic intervals approximately .2m -1m thick; for example the Shark Tooth Hill bonebed, CA (Miocene), Zone 12 in the Calvert Fm (Miocene), and the PaleoAngola bonebed in Africa (Cretaceous). To date, two hypotheses are generally proposed for non-catastrophic bonebed origin: 1) an offshore concentration where bone input > sediment input, or 2) a winnowed and/or transported lag caused by eustatic sea-level change or storm events. Here, we scrutinize conditions necessary to generate an offshore bone concentration caused by sediment starvation through the use of a quantitative model to evaluate the feasibility of this hypothesis. In the absence of bone decay, we found that sedimentation rates must be lower than 1cm/1000years and the equivalent of 1 large whale or more must die every 1000 km² per year. Under these conditions, a bone bed .5m thick will take a maximum of 110,000 years to a minimum of 30,000 years to form. However, when bone decay rates observed on recent whale fall skeletons and shipwrecks are introduced to the model, bones will degrade faster than the maximum possible sedimentation rates can bury them, making it impossible to generate a bonebed under the given assumptions. Our results indicate that marine bone concentrations are unlikely to develop in sediment-starved environments without the aid of other unidentified processes. Thus, we tentatively propose that sediment winnowing and/or bone transport processes are necessary in order to generate an attritional marine bone concentration.

AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013