High-resolution stratigraphy of the deep-water lower Cloridorme Formation (Ordovician), Gaspé Peninsula, based on K-bentonite and megaturbidite correlations

Abstract

The Ordovician Cloridorme Formation is a thick foreland-basin turbidite succession that for several decades has served as a natural laboratory for the long-distance physical tracing and study of single turbidites. A robust temporal correlation of these deposits is a prerequisite for the evaluation of sedimentary processes. Published stratigraphic subdivisions of the lower part of the Cloridorme Formation have impeded rather than facilitated such studies because of errors in the physical correlation of contemporaneous deposits. Tracing of the deposits of exceptionally large gravity flows, known as "megaturbidites," was previously recognized as a useful stratigraphic tool. Here, we strengthen and revise the stratigraphic framework based on megaturbidites by considering nine widely traceable and geochemically fingerprinted tuffs (K-bentonites). Two discriminant functions incorporating the abundances of six trace elements (V, Sm, Nd, Th, Er, Zr) permit the unambiguous distinction of the lower four tuffs. The other tuffs can be successfully distinguished using bivariate plots of TiO2, Th, Sc, and Eu. The tracing and correlation of 71 megaturbidites and the nine K-bentonites permits a high-resolution subdivision of the lower Cloridorme Formation. Particularly thick megaturbidites mark the boundaries of three newly defined allostratigraphic members (St-Hélier Allomember, St-Yvon Allomember, and Petite-Vallée Allomember). This precise temporal framework shows that the stratigraphic heights of the contacts of lithostratigraphic members defined by other workers in 1969 and 1991 were miscorrelated by more than 400 m over short lateral distances!