Grain-size controls on the occurrence of bioturbation

Grain size and grain-size related stresses impart a significant influence on the ichnological character of marginal-marine deposits. This is evident on the New Brunswick coastline of the Bay of Fundy, Canada, where three coarse-grained marginal-marine deposits are studied to assess grain-size controls on the occurrence and type of bioturbation. Firm mud and sand substrates exhibit the greatest diversity and density of bioturbation (i.e., bioturbation intensity). The types of organisms colonizing sands and firm-mud substrates are variable; however, the resultant trace assemblages are similar. Thixotropic muds exhibit significantly reduced trace diversity and density relative to firm mud, reflecting the additional stress placed on the organisms by the relatively soupy consistency of the sediment. A significant change in the trace assemblage occurs when sediment caliber passes the gradational sand—fine gravel boundary. Four main conclusions can be drawn from this study. First, for mixed sand and gravel, fine gravel, and coarse-gravel deposits, the degree of bioturbation (diversity ⁎ density) decreases more rapidly onshore (across the intertidal zone) than is noted in sand or mud deposits. Second, there is a decrease in the degree of bioturbation with increasing grain size for substrates composed of sand-sized and larger clasts. Third, burrows in gravels tend to be lined and/or robust, likely to maintain a stable environment within the burrow. Fourth, in coarse-gravel substrates or substrates with a significant component of coarse gravel, burrows are developed between the clasts and tend to be more permanent structures (than those developed in sand or mud), which are generally continuously occupied. The degree of burrowing noted in these modern gravel deposits contrasts with the relative paucity of biogenic structures reported in conglomerates preserved in the rock record. Based on the intensity of burrowing observed in the gravels, we hypothesize that ancient marginal-marine conglomerates are likely bioturbated, but that these burrows are likely distorted during burial and compaction.