Extreme Ichnology: Burrowing in Gravel

Shahin E. Dashtgard¹, Murray K. Gingras², and George S. Pemberton^2
1Earth Sciences, Simon Fraser University, Burnaby, BC, Canada
²Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada

Ichnological variations in modern marginal-marine depositional environments result from differences in the physico-chemical stresses of the environment, which can be grouped into three categories: 1) basin-derived stresses; 2) locally variable stresses; and 3) grain-size induced stresses. Basin-derived stresses include oxygen content of the water, temperature, turbidity, and predation. Locally variable stresses include: wave energy, subaerial exposure (related to tidal regime), salinity, and sedimentation rate. Wave energy and subaerial exposure-generated stresses are strongly influenced by basin-derived processes, and sedimentation rate by grain size. Grain size directly or indirectly controls food availability, sediment pore-water content, oxygen content of the sediment, and substrate consistency.

In the megatidal (>8 m tidal range) Bay of Fundy, Canada, the neoichnological character of three coarse-grained systems can be predicted as a function of sediment caliber and subaerial exposure. In the backshore, burrowing is developed only within sandy sediments - preferentially medium-grained sand. The upper intertidal is devoid of burrowing, whereas burrows in the middle intertidal zone are mainly developed in sands and firm mud, and in sands and fine gravel with an overlying mud veneer. In the lower intertidal and shallow subtidal zone, burrows are predominantly developed in sand and firm mud, although there is also a significant increase in burrowing within gravels. Burrowing in gravels is characterized by a low density and diversity of lined- and/ or robust traces, where the degree of bioturbation decreases rapidly onshore (across the intertidal zone).

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas