Howell, Charles D.¹, Janok P. Bhattacharya¹, James A. MacEachern² 
(1) University of Texas at Dallas, Richardson, TX 
(2) Simon Fraser University, Burnaby, BC

ABSTRACT: Estimates of Sedimentation Rates from Sediment and Faunal Interactions within an Ancient Delta Lobe, Wall Creek Member, Frontier Formation, Powder River Basin, Wyoming, U.S.A

Combined sedimentology and ichnology of an ancient river-influenced delta lobe yields seasonal and annual time estimates for sediment accumulation and delta front progradation. Episodic sediment accumulation, most likely seasonal in origin, is recorded by changes in sedimentology, stratification type and trace maker behavior. The dominant lithofacies that mark rapid sedimentation and progradation are stacked simple and compound bed-sets of discrete delta front turbidites consisting of very fine- to fine-grained flat-stratified to structureless sandstones and mudstones. During these times of rapid sedimentation, escape structures (fugichnia) prevail. This contrasts markedly with finer-grained, poorly stratified, bioturbated sandstones and mudstones deposited during quiescent periods, within which the archetypal Cruziana ichnofacies is preserved. Homogenization of these quiescent lithofacies results from the activity of infaunal, epifaunal and nektonic organisms near the sediment-water interface. 
Estimated life spans of 1/2-2 years for individual trace-makers constrain sedimentation rates within the rapidly deposited proximal facies. Fossilized bivalves record life spans of 5-10 years, and constrain sedimentation rates within quiescent inter-distributary facies. We estimate basinward delta front progradation in a low accommodation setting to lie between 10-24 meters per year and vertical accretion rates of up to 1 m per year. Lateral variation of vertical sediment accretion within the delta lobe results from the changing ratios of inter-distributary to proximal facies contained within any vertical measured section. Time constraints are conservative, given that most observable traces appear to have been produced during the adult stages of the trace maker’s existence. 

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