CHARACTERIZING THE EDIACARAN-CAMBRIAN TRANSITION IN MONGOLIA AND BEYOND

Ediacaran to Cambrian (E-C) strata record one of the most fundamental biological transitions in the geological record, and yet, despite much interest and research, characterizing and identifying the boundary has proven to be difficult, particularly in siliciclastic-poor successions. The GSSP PC-C boundary in Newfoundland is defined by the first appearance datum (FAD) of Treptichnus pedum, which is considered to be roughly coincident with the FAD of small shelly fossils (SSFs) and a large negative carbon isotope excursion. However, assessing the coincidence of these markers has been difficult due to taphonomy and lithological limitations.

Here we present integrated sedimentology, carbon isotope chemostratigraphy, and paleontology from multiple measured sections in the Zavkhan Basin of southwestern Mongolia. The Zavkhan Basin hosts a thick, mixed carbonate-siliciclastic succession of E-C strata and offers the opportunity to better characterize the E-C boundary. In Mongolia, the FAD of T. pedum occurs in the upper part of the Bayangol Formation, hundreds of meters above multiple horizons of abundant and diverse small shelly fossils and multiple large, negative carbon isotope excursions. In successions in Death Valley, Northwestern Canada, and Oman, it has been suggested that a negative carbon isotope excursion occurs at the boundary. Importantly, the geochronological constraints on the boundary come from Oman, where ashes dated at 542.3±0.2 Ma and 541.0±0.2 Ma with U-Pb CA-ID-TIMS (Bowring et al., 2007) occur below and during a negative carbon isotope anomaly, respectively, and are consistent with the U-Pb age of 540.61±0.88 Ma from an ash that is unconformably below the FAD of T. pedum in Namibia (Grotzinger et al., 1995). In Mongolia, if we choose to define the E-C boundary directly below the largest carbon isotope excursion, it is broadly consistent with the FAD of SSFs but well below the FAD of T. pedum. These data highlight some of the problems in identifying the boundary in the Zavkhan Basin and, more broadly, the way in which the boundary is globally defined.