Parasequence development in the Ediacaran Shuram Formation (Nafun Group, Oman): high-resolution stratigraphic test for primary origin of negative carbon isotopic ratios

摘要

Neoproterozoic carbonates are known to show exceptional variations in their carbon isotopic ratios, and in the absence of biostratigraphy and a firm geochronological framework, these variations are used as a correlation tool. However, it is controversial whether the carbon isotope record reveals a primary oceanographic signal or secondary effects such as diagenesis. The Shuram Formation of the Nafun Group of Oman allows a stratigraphic test of this problem. The Nafun Group (Huqf Supergroup, Oman) in the Huqf area of east-central Oman consists of inner carbonate ramp facies of the Khufai Formation overlain by marine, storm-generated, red and brown siltstones of the Shuram Formation. Towards its top, the Shuram Formation is composed of distinctive shallowing-upward, 4-17-m-thick parasequences cropping out continuously over 35 km, which show recessive swaley cross-stratified siltstones capped by ledges comprising wave-rippled, intraclast-rich ooidal carbonate. These storm-dominated facies show a regional deepening in palaeobathymetry towards the south. The carbonates of the Shuram Formation are marked by an extreme depletion in C-13 in bulk rock. delta C-13 values quickly reach a nadir of -12 parts per thousand just above the Khufai-Shuram boundary and steadily return to positive values in the overlying mainly dolomitic Buah Formation. The Shuram excursion is thought to be ca. 50 Myr in duration and extends over 600 m of stratigraphy. Carbon isotopic values show a systematic variation in the parasequence stack, with values varying both vertically through the stratigraphy (similar to 2 parts per thousand per 45 m) and laterally in the progradation distance (similar to 1 parts per thousand over 35 km). This supports a primary, oceanographic origin for these extremely negative carbon isotopic values and independently argues strongly against diagenetic resetting.