MP9: Chapter 16: Burrows Storing an Otherwise Lost Sedimentary Record
by A. Wetzel.
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This chapter is from GAC's Miscellaneous Publication 9: "Ichnology: Papers from Ichnia III" editted by Dr. Duncan McIlroy. To purchase the entire book on disc, please see the Miscellaneous Publications section of our bookstore at http://www.gac.ca/publications/view_pub.php?id=262.
Trace fossils may store a sedimentary record that is not otherwise preserved due to living organisms actively filling their burrows with material from the sediment surface, or the passive filling of open burrows after their abandonment. The potential of burrow-fill to unravel the incomplete stratigraphic record, in shallow water as well as deeper settings, appears to have been underestimated and hence, under-investigated.
In the case of actively filled traces, burrow producers transfer sedimentary particles from the sediment surface downwards into their burrows. Normally these actively in-filled particles initially formed the top of the seafloor, resulting from either event deposition, or seasonal sediment input, such as blooms or river plumes. The top layer is easily mixed with other sediment by bioturbation, especially if the layer is thin and the sediment is soft. Surface-feeding endobenthic animals, however, often transfer benthic food deposited on the layer, together with layer material, into their burrows and hence, store the information about the sediment present on the previous seafloor in an otherwise by bioturbation homogenized deposit. Volcanic ash is among the best known material preserved in this way, but also organic detritus, foraminifera and other components have been found stored in burrows.
Open, abandoned tubes connected to the seafloor can become passively filled by particles settling from the water column, transported on the sediment surface, or temporarily deposited material. Ideally, different burrows form at different stages during the formation and induration of a discontinuity surface. The fill of these different burrows can therefore provide information on different phases of deposition and sediment (net-) bypass. Sometimes exotic material is found, for instance, iron-ooids occurring in micritic limestone, mud containing planktic foraminifera in borings in oncoids, or sediments preserving a tidal signature in tubes (tubular tidalites), in otherwise totally bioturbated sediments.
Geological Association of Canada
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