UNDERSTANDING BURGESS SHALE-TYPE PRESERVATION: NEW INSIGHTS FROM THE WHEELER SHALE, UTAH

Cambrian konservat-lagerstätten are the most significant fossil deposits for our understanding of the initiation of Phanerozoic life. Although many modes of preservation may occur, most frequently these deposits contain "soft-bodied" fossils preserved in the form of kerogenized carbon films, a rare yet important taphonomic pathway that has not previously been explained for any unit by a comprehensive model. The middle Cambrian Wheeler Formation of Utah, one of these lagerstätten, contains abundant kerogenized preservation of nonmineralized tissues. Within the Wheeler Formation, this style of preservation occurs within a distinctive facies that accumulated under the following conditions: 1) inboard sequestration of silt and coarser clastics, resulting in domination of the siliciclastic fraction by clay-sized particles, 2) close proximity to a carbonate platform, which resulted in exclusively fine-grained, mixed carbonate–clay sediments, 3) a well-developed oxygen minimum, precluding benthic colonization and burrowing, and 4) relative proximity to oxic bottom-waters, facilitating transport of organisms from a habitable environment to one that favored their preservation. We propose that preservation of nonmineralized tissues in the Wheeler Formation may have resulted from a combination of influences that reduced permeability and, thus, lowered oxidant flux, which in turn may have restricted microbial decomposition of some nonmineralized tissues. Those influences include near bottom anoxia, preventing sediment irrigation by restriction of bioturbation; reducing conditions near the sediment-water interface that may have acted to deflocculate aggregations of clay-minerals, resulting in low permeability face-to-face contacts; early diagenetic pore occluding carbonate cements; and an absence of coarse grains such as silt, skeletonized microfossils, fecal pellets, or bioclasts. This model may be applicable to kerogenized preservation of macrofossils in other fossil lagerstätten. If minimal porosity is indeed important to the organic preservation of nonmineralized tissues, secular trends in the type and quantity of biogenic contributions to sediments may provide an interesting comparison with the secular decline in Burgess Shale-type preservation.