Strontium‐isotope stratigraphy as a constraint on the age of condensed levels: examples from the Jurassic of the Subbetic Zone (southern Spain)

Abstract

Condensed levels are often characterized by reworked fossils that may lead to incorrect age assessments. Strontium‐isotope stratigraphy is an important chronostratigraphic tool that can be used to verify the biostratigraphic information from condensed beds. This paper describes a study of the 87Sr/86Sr isotope ratios of 56 belemnite samples collected from 28 stratigraphic sections of the boundary between the Upper Member of the Gavilán Formation and the Zegrí Formation (Pliensbachian, Subbetic Zone). The petrographic and geochemical data (δ18O, δ13C, concentrations of Fe, Mn and Mg, and the Sr/Mn ratio) suggest that the belemnites have preserved their original marine geochemical composition. After plotting the samples in diagrams of 87Sr/86Sr values against time according to their biostratigraphic age, four different groups (A, B, C and D) were obtained with respect to the reference curve. In groups A and B, the age deduced from the Sr‐isotope ratio is in total or partial agreement, respectively, with the biostratigraphic age; therefore the 87Sr/86Sr ratio is a good method for the dating, correlation and assessment of biostratigraphic results. In groups C and D, the SIS age and the biostratigraphic age do not coincide. A graphic procedure is presented as a suitable methodology to constrain the age of the samples showing an SIS age that differs from the relative age deduced (by biostratigraphy or stratigraphic correlation) for the bed they were collected in. These situations are interpreted as being the result of reworking of the belemnites (group C) or ammonites (group D) that are included in condensed levels. These condensed levels formed during the maximum flooding event that led to the drowning of the Gavilán carbonate platform. The methodology supplied in this paper represents a valuable tool in identifying reworking processes, improving correlation and constraining biochronostratigraphic results. The values of 87Sr/86Sr represent a new contribution to the data set of 87Sr/86Sr ratios for the Pliensbachian.

Strontium isotopic ages of the Torinosu-type limestones (latest Jurassic to earliest Cretaceous, Japan): Implication for biocalcification event in northwestern Palaeo-Pacific

“…The global trend from the Ordovician to the Present has already been established (McArthur et al, 2001) and applied for age-determination of the Phanerozoic carbonates (e.g. Steuber et al, 2005;Kano et al, 2007a,b;Takayanagi et al, 2007;Nieto et al, 2008).…”

Análisis de elementos traza en braquiópodos del Jurásico Inferior del Paleomargen Sud-Ibérico (SE de España). Correlación con las señales bióticas e implicaciones ambientales en torno al Evento de Extinción Masiva del Toarciense inferior

“…In the Eastern Subbetic, the Lower Jurassic succession is typified by the Gavilán and Zegrí formations (e.g. García-Hernández et al, 1979;Ruiz-Ortiz et al, 2004;Nieto et al, 2008) (Fig. 2B), which have been presented in the composite section herein arranged (Fig.…”

Stratigraphy of the upper Vijlen Member (Gulpen Formation; Maastrichtian) in northeast Belgium, the southeast Netherlands and the Aachen area (Germany), with special reference to belemnitellid cephalopods

“…Due to its sedimentation history, it may be speculated that reworked belemnite rostra can be expected to occur within the entire Vijlen Member. With reworking being a common feature of Vijlen Member belemnites, and in consideration of the fact that without additional geochemical analyses remanié material can barely be recognised (compare Nieto et al, 2008), we refrain from subdividing the Vijlen Member by belemnitellid cephalopods.…”

“…sedimentological evidence for downslope transport or fossil preservational state) was beyond the scope of the investigation. In future studies, it would be worthwhile to document whether or not isotopic and elemental composition (Nieto et al, 2008) might be the key to distinguish between remanié and indigenous belemnite rostra in the study area. Seen in this light, the lack of isotopic continuity between belemnite rostra on the one hand and inoceramid shells and chalk matrix on the other (as documented for the Altembroeck section; see Elorza et al, 1997), might be re-interpreted in terms of belemnite reworking.…”