Chemical criteria to identify the depositional environment of chert: general principles and applications

Abstract

Many chert sequences are located at key stratigraphic intervals or in structurally allochthonous packages, and because such sequences provide critical information regarding the formation of sedimentary basins and tectonic orogens it is important to determine precisely their original depositional position. Chemical methods of delineating environments are becoming increasingly sophisticated and useful, in part because even slight post-depositional recrystallization may obscure primary rock textures. Most chemical criteria have developed only on a case-by-case and local basis, however, with limited broad applicability. Here I synthesize major, trace, and rare earth element (REE) data in bulk chert from 49 sequences from continental margin, pelagic, and ridge-proximal environments, ranging in age from Early Paleozoic to Neogene, and develop a set of depositional chemical criteria applicable to cherts regardless of age, diagenetic history, or tectonic region. The criteria allow for analytical limitations and diagenetic modifications (precluding use of Si, Ca, Mn, Mg, P, Sr, and Ba, and other elements) intrinsic to chert chemistry. Major element ratios, capitalizing on the affiliations of Al2O3 and TiO2 to terrigenous matter and of Fe2O3 to metalliferous input, can distinguish continental margin from ridge-proximal sedimentation, but cannot uniquely identify pelagic deposition. Trace element data are too few to interpret rigorously, with substantial overlap occurring between depositional regimes. REE ratios (i.e., Lan/ Cen) allow the best resolution of all three depositional regimes and are independent of diagenetic modification. I present new diagrams illustrating the chemical depositional criteria, and apply them to several sequences described in the literature that have historically yielded problematic genetic interpretations.