Two out of three ain’t bad: Dealing with proxy paucity when correlating the base of the Cambrian

Abstract

Global Boundary Stratotype Section and Points (GSSP’s) are often regarded as immutable points, that once ratified should “remain fixed in spite of discoveries stratigraphically above and/or below” (Cowie, 1986, p. 79). The Precambrian–Cambrian boundary, defined at Fortune Head by the FAD of the trace fossil Treptichnus pedum, has been a source of controversy and debate since its ratification. Treptichnus pedum has proven to be a difficult marker to apply to global correlation, and lack of other markers at Fortune Head (skeletal fossils, chemostratigraphic, magnetostratigraphic or radiometric data) has prompted the use of other “unofficial” indicators of the Precambrian–Cambrian boundary in many sections around the world. δ13C chemostratigraphy has become a standard global correlation tool, and the integration of δ13C isotopes with other “multi-proxy” data is an approach that is becoming increasingly adopted. Discoveries and advances in such methods and techniques demonstrate that new data can (and should) enable the fine-tuning of stratigraphic boundaries. Recent work in southwestern Mongolia through the Precambrian–Cambrian boundary section at Bayan Gol demonstrates the utility of multi-proxy stratigraphic data to defining and correlating the base of the Cambrian. Here, two proxies are used in concert to identify the Precambrian–Cambrian boundary; the first occurrence of the small shelly fossil Protohertzina anabarica and the nadir of the BACE (δ13C excursion). This section demonstrates 1) the value of carbonates in their capacity to preserve a wider variety of stratigraphic proxies than siliciclastics, 2) the importance of systematically measured and sampled stratigraphic sections (rather than composite sections) for regional and global correlation and 3) the need for a redefined Cambrian GSSP, with a supporting ASSP as a pathway toward global correlation of the base of the Cambrian.