Some fundamental considerations in the dating of glauconies: a comment on “A test of the reliability of Rb [sbnd]Sr dates for selected glauconite morphologies of the upper cretaceous (Navesink formation) of New Jersey”, by R.L. Montag and D.E. Seidemann

Abstract

In an attempt to understand the geochemical behaviour of Rb and Sr in glauconies (see below) used for dating purpose, Montag and Seidemann [1] stated several hypotheses in contradiction with some facts generally known to the specialists in these difficult problems. In their introduction these authors define the word glauconite as a mineral but unfortunately also use it to describe the green sedimentary pellets in which glauconitic minerals occur. The term has unfortunately come to be used loosely to cover two concepts: on the one hand a concept of facies: something green, usually in pelletal form, lying in a marine sediment; on the other hand a mineralogic concept which designates a mica [2]. The two concepts cover diverse possibilities. This duality of significance leads us, amongst others [3], to propose to solve this confusion by using: glaucony or pelletal glauconite.., for the facies and glauconitic minerals for the authigenic components of the green pellets [4]. These glauconies may be more or less evolved and fundamentally contain two kinds of components: ( 1 ) t h e inherited ones, which originate from the initial substrate of glauconitization (quartz, mica, clay, calcareous debris) [5]; there is no glauconitization without this preliminary substrate; (2) the authigenic ones, which are the glauconitic minerals, ranging from a smectitic end-member component to a micaceous endmember. Only the latter should actually be named glauconite, (the suffix -ite characterizes a mineral). This is a convention, other conventions are possible and we do not intend to impose a solution; but it is certain that the confusion of the two concepts and, withirl each concept, the non-recognition of their diversity, can only lead to complete confusion in the application to geochronology. Ages of glauconies are not generally 10-20% younger than the age of sedimentation; this old assumption was based on the study of deeply buried samples from North America (which had been rejuvenated) or on a biased time scale [6]. Numerous recent studies have shown that the main problems with the dating of giauconies are, in chronological order: (1) the zero-time problem (see Tisserant and Odin [7] and Fig. 1); (2) possible influences from tectonic and deep burial [8], and (3) possible changes due to weathering [9]. Whatever the colour or the form, the green pellets may lack reliability, for geochronology, due to the three main causes listed above. If the light-green pellets give apparent ages which are often different from the age of sedimentation, it is well recognized today that this is not related to their colour but to the fact that either they are little-evolved or that