Description and Stable Isotope Record of Newly Discovered Neoproterozoic (CRYOGENIAN?) Tube Structures, Northern Utah

Tube structures in Neoproterozoic post-glacial cap carbonates are extremely rare, documented in only five localities worldwide: Death Valley, Brazil, N. and S. Namibia, and SW Mongolia. A new locality for these structures has recently been discovered in the cap-carbonate sequence that sharply overlies the Mineral Fork Formation in northern Utah. If current correlations between Cryogenian diamictites in northern Utah and southeastern Idaho are valid, these Utah tube structures may be the first reported occurrence in a non-Marinoan post-glacial cap carbonate. To compare the Utah cap carbonate and tube structures with those reported from other locations -and to address the question of how they may have formed- we report primarily on detailed macroscopic and microscopic analyses similar to those used by previous workers. This includes 1) field measurements of size/type/spacing of tube structures in host rock, 2) hand-sample analysis of tube and host rock composition, texture, and structure 3) petrographic thin section and scanning-electron microscope (SEM) analysis of tube and host rock composition and texture. In addition, we present a carbon-isotope profile for a measured section within the cap carbonate and a carbon-isotope data set comparing the d-13C values of tube structure fill vs. host rock laminations. Initial analyses indicate the Utah tube structures have an average width of 0.60 cm, are typically spaced 1.90 cm apart, and are mainly infilled with sparry cement, although micrite and "ghost" fill also occurs. Most tube structures are oriented perpendicular to bedding and some tubes change along their vertical length from one fill type to another, suggesting that the fill type may be the result of diagenesis rather than an original feature. The tube fill appears laminated in some tubes but not in others; when laminae are present, they do not always display obvious concave-up geometry. Within the tube structure interval (~ 1.5 m thick), undulose lamination is common, some of which is locally disrupted. Tubes are clustered in m-scale, laterally discontinuous lengths and clusters are separated by areas of massive to laminated strata. The carbon-isotope profile of the Utah cap carbonate shows relatively little variability, and values are consistent with those reported from other Neoproterozoic cap carbontes. Tube structure fill and host rock C-isotope values are not significantly different, suggesting syngenetic formation of tubes and host rock rather than generation of tube structures via gas-escape. Continuing analyses will provide more information about tube structure and host rock composition and interactions, ultimately yielding clues about the origin and occurrences of these enigmatic structures.