Discussion on the calpionellid biozones and proposal of a homogeneous calpionellid zonation for the Tethyan Realm

Abstract

Calpionellids are a group of planktonic microfossils usually occuring in large numbers in pelagic deposits from the upper lower Tithonian to upper Valanginian, and showing a fairly uniform stratigraphic distribution all over the Tethyan Realm and in the southeastern Pacific margin. A preliminary calpionellid scheme was first proposed by Jürgen Remane in 1963 and 1971 consisting of five zones and six subzones informally named: A (A1, A2 and A3), B, C, D (D1, D2 and D3), and E. This biozonal scheme based on specific assemblages, acmes, and partial or total ranges of some calpionellid genera, has constituted, so far, a valid zonal framework towards a homogeneous and universal calpionellid scheme. It only required improvements at the levels of subzones and lower subdivisions. Subsequently, several authors have established numerous new biozonations often inconsistent and varying from one region to another. Such discrepancies noticeably hamper the purpose of calpionellids for biostratigraphic and long-distance correlations. These discrepancies between local schemes often originated from misguided choice of zonal index taxa. Quite a number of subzones are grounded on either rare species with short-range and sporadic occurrences, rare and atypical species, or long-ranging taxa in some stratigraphic levels that yield species with more reliable stratigraphic value. Consequently, further consistent stratigraphical use of this fossil group requires a homogeneous framework with unambiguous definition of zones and subzones, preferably based on specific assemblages. We propose herein a coherent framework of 7 biozones, 16 subzones, and 6 intervals or horizons based upon the analysis of several outcrop sections and the critical insights of formerly published works.