CAMBRIAN BURGESS SHALE-TYPE DEPOSITS WORLDWIDE SHARE A COMMON TAPHONOMIC MODE

It has long been recognized that deposits yielding exceptionally preserved biotas are unusually abundant in Cambrian strata. Together these “Burgess Shale-type” (BST) deposits capture an extraordinarily complete record of marine life immediately following the initial Phanerozoic radiation. Although the phenomenon of exceptional preservation in the Cambrian has received considerable attention, the nature of BST preservation remains controversial. In the last decade, research has highlighted taphonomic differences in BST fossils found at different BST localities. This has led to a view that local conditions led to BST preservation in each case and that no single mechanism can explain the global occurrence of BST fossils. Much of the discussion has focused on fossils from the two most important BST deposits: the Burgess Shale and the Chengjiang biota. Both deposits are problematic, however; the former has been transformed by greenschist-facies metamorphism and the latter by extensive weathering in a humid environment. Here we present new data from SEM-EDX study of the preservation of BST fossils from five of the nine principal Cambrian BST deposits, the Kinzers, Kaili, Spence, Wheeler, and Marjum deposits, as well as from the BST facies of the late Neoproterozoic Doushantuo Formation. The six deposits examined have not undergone the degree of metamorphism experienced by the Burgess Shale, nor the extensive weathering that characterizes the Chengjiang deposit. The new data indicate that the major morphological aspects of the fossils in each of these six formations are preserved as carbonaceous compressions. Secondary aspects of some fossils are preserved by mineralization in calcium phosphate or pyrite in some of the deposits. Together with the findings of Gabbott et al., 2004 (Chengjiang) and Butterfield et al., 2007 (Burgess), these data suggest that BST preservation worldwide involves a single process that presumably shares a common cause. The taphonomy of BST deposits requires a mechanism for the suppression of organic decay; early authigenic mineralization may sometimes occur in association.