Isochronline in chemostratigraphy for global correlation and an exception of the Walther's law of facies

Abstract

Chemostratigraphy based on various proxies has been well developed in recent decades for stratigraphic correlation by higher resolution, gap-less sampling, and environmental records. At present, various types of samples are measured quantitatively in higher accuracy and wider application than traditional biostratigraphy. Even the benthonic index fossils are widely replaced by pelagic data for biostratigraphic correlation, fossil gaps also could cause data break in a section. However, chemostratigraphy remains challenge as its data from sediments is to resolve mixtureof local depositional environment and global correlatable signal. How to separate these two parts and extract the local imprints free from the mixture is the subject of this study. An isochronlines is a newly introduced auxiliary line for the globally correlatable signal extraction from the mixture signals. An isochronline is a virtual synchronous line on plane of an equal depositional environment, and can evaluate the global signal unlike the chemostratigraphic data that provides environmental records. Based on the data obtained from lateral stratigraphic investigation along a bed strike, or from grid mapping on a wide-open outcrop of a bedding plane, or on a surface of a lithological unit, the facies-independent chemostratigraphic proxies could be extracted. A set of methods was designed including lateral profiling and grid mapping on the outcrop to extract the facies-independent geochemical signal for global correlation based on isochronlines identification, and an exception of the Walther's law of facies was revealed. Meanwhile, the author presents examples to avoid misleading of the global chemostratigraphic correlation based on isochronline analysis. This method included measurement of two-wing lateral-vertical strata, determination of non-lithological lens from geochemical abnormities of diagenesis, and microhabitat. An example from Laibin Auxiliary boundary Stratotype Section and Point of Lopingian-Guadalupian of Permian was measured by grid mapping, that revealed the isochronlines, extracted facies-independent geochemical proxies for global correlation, and hidden geochemical lenses.