Magnetostratigraphy susceptibility correlations for the Guadalupian-Lopingian boundary and the placement of the North American Ochoan Series: Texas (USA) and South China

Abstract

Stratigraphic studies have emphasized the use of multiple, geochemical and geochemical proxies to investigate critical geologic boundaries, events and their correlation throughout the entire Phanerozoic. In this study, an emerging geophysical proxy, mass-specific, low-field magnetic susceptibility (χ) is used to investigate the global correlation of the Guadalupian-Lopingian (G-L) boundary, to better resolve and supplement biostratigraphic correlations, including inferred G-L boundary sections defining the base of the North American Ochoan Series. The North American Ochoan Series of West Texas, defined lithostratigraphically by the contact between the Bell Canyon and Castile Formations, represents a unique and well-studied sequence in the Delaware Basin, West Texas, but it has not yet been possible to tie the Ochoan to international type sections, including the Guadalupian-Lopingian (G-L) Global Boundary Stratotype Section and Point (GSSP), due to biostratigraphic ambiguities. χ results, and their correlations may be used for global, isochronous correlation tied to both eustasy and global climate throughout distributed marine basins. This study presents regional and global χ results for sections in Texas and China to investigate the correlation of the G-L boundary, the correlation of both global conodont biozones and lithofacies, and the placement of the North American Ochoan Series. These χ correlations reinforce the biostratigraphically defined position of key conodont biozones in both Texas and China and indicate that the G-L boundary very nearly corresponds to the start of the Ochoan Series in Texas, and to the base of unit E in the West Texas EF section, despite the absence of the defining conodont C. postbitteri postbitteri. Lithostratigraphic correlations, depositional facies and χ trends were interpreted to represent distinct systems tracts, which were tied to changes in eustatic sea-level. The correlation of high-frequency lithostratigraphic cycles may represent a globally correlated fifth-order climate cyclicity. Coincident changes in global facies, in China and Texas, suggest that the G-L boundary represents a period of significant environmental change and warming, producing global evaporite sequences in the US and Europe, coinciding with a eustatic highstand.