The significance of iron ooids from the middle Eocene of the Transylvanian Basin, Romania
Graphical abstract
Introduction
Young (1989, p. ix) proposed the term ironstone “as a petrological term and as an informal lithostratigraphic term” following original definition by Kimberley (1978) that ironstone is a rock of any age having an iron content > 15 wt%. Ironstones and iron-rich limestones regularly occur as components of recurrent time-specific facies in the terminal Paleoproterozoic (Lin et al., 2019) and throughout the Phanerozoic (Brett et al., 2012) and mostly concentrate during the Ordovician to Early Devonian and the Jurassic to Paleogene (Matheson and Pufahl, 2021 and references therein). Cenozoic occurrences of ironstone are, however, less numerous compared to previous times (McGregor et al., 2010) and, in general, iron ooidal episodes appear to get rarer and of limited chronological extension moving towards recent times, as also observed for other oxygen-related facies (e.g., black shales; Negri et al., 2009a, Negri et al., 2009b). In addition, Phanerozoic ironstones mostly occur within condensed sequences (Oggiano and Mameli, 2006).
The genesis of iron ooids and ironstones is still far from understood and both abiotic and biologically-triggered models have been proposed (Ferretti, 2005, Ferretti et al., 2012, Matheson and Pufahl, 2021 and references therein). Chamosite, berthierine, hematite, goethite, siderite and rarer magnetite are the main mineralogical phases detected throughout the Phanerozoic. The source and release mechanism of Fe remain poorly constrained (see, among others, Young, 1989, Kimberley, 1994, Matheson and Pufahl, 2021). Iron could have been transported to the oceans through increased fluvial input during greenhouse climates (“proximal source”; e.g., Hunter, 1970, Young, 1989, Van Houten, 2000, Taylor et al., 2002, Yilmaz et al., 2015). An alternative view of ironstone deposition due to a distal ironstone factory (“distal source”) with coastal upwelling delivery of ferruginous seawater to a storm dominated continental shelf was recently suggested for the Ordovician of the Iberian Margin (Pufahl et al., 2020) and for the Middle Ordovician ironstone of the Welsh Basin by Dunn et al. (2021). Modern occurrences of iron ooids were reported in areas of intense hydrothermal activity which include Cape Mala Pascua in Venezuela (Kimberley, 1994), Mahengetangin Indonesia (Heikoop et al., 1996), and Panarea in Italy (Di Bella et al., 2019, Di Bella et al., 2021, Ferretti et al., 2019), whereas a similar genesis for the fossil counterpart was suggested (e.g., Di Bella et al., 2019).
Luan et al. (2018) recently grouped iron-bearing structures in three main categories according to their bathymetric position: i) a shallow-water type, mainly represented by ooidal ironstones or ferruginous laminated structures, deposited close to land; ii) a relatively deep-water type, associated to condensation or sediment-starved conditions; and iii) a deep-ocean type, associated to low sedimentation rates, sea floor erosion, and vent systems (hydrothermal vents and cold seeps). Furthermore, a microbial contribution for all proposed iron-bearing types has been outlined (Luan et al., 2018).
In spite of such an active and enduring discussion, additional information is still required to get to a global interpretation of ironstones. Size and mineralogical composition of iron ooids have proved to reflect chemical properties and processes acting in the water column (Dunn et al., 2021); however, “ironstone is an underutilized proxy of oceanic and biotic instability” (Matheson and Pufahl, 2021, p. 1).
Focusing on Eocene times, only 15 occurrences of ironstones are known (Table 1), all from the northern hemisphere and specifically from both the northern and southern side of the Neotethys (James, 1966, Van Houten, 1992, Petránek and van Houten, 1997, Salama et al., 2012, Garnit and Bouhlel, 2017), however they remain unrelated to any specific climatic, geographical, or environmental setting. Here we present a study on iron ooids from the middle Eocene of Romania. These ooids occur just below a spectacular nummulite bank in Capuş, west of Cluj-Napoca, and are strictly associated with the distinctive species Nummulites perforatus, the main component of the bank itself.
Nummulitic accumulations known as “nummulite banks” (Arni, 1965) represent a fundamental component of the Eocene sedimentary record in the Neotethyan Realm (Jorry et al., 2006, Papazzoni and Seddighi, 2018, Bindiu-Haitonic et al., 2021, Kövecsi et al., 2022). They are particularly common and well documented in the Bartonian sedimentary sequence in the north-western part of the Transylvanian Basin (Fig. 1). Detailed microfacies and paleoenvironmental studies were recently run by Pleş et al., 2020, Bindiu-Haitonic et al., 2021, Kövecsi et al., 2022 who confirmed that these bioaccumulations originated in a shallow-water inner-shelf paleoenvironment with hydrodynamic oscillations, but no emphasis was given to the “bio-ooidal carbonates” there recorded.
The purpose of this paper is two-fold: characterize the chemistry and mineralogy of the middle Eocene iron ooid horizon from Capuş and clarify the connection of this occurrence with penecontemporaneous ironstone beds along the Neotethyan Realm.
Section snippets
Geological setting
The stratigraphical record of the Transylvanian Basin documents the existence of successive sedimentary basins with distinctive sequences (Krézsek and Bally, 2006, Filipescu, 2011). The Paleogene sequence was deposited in an extensional setting between the Eocene and the early Oligocene (Krézsek and Bally, 2006). Resting on the Upper Cretaceous, the Paleogene consists of continental and marine formations representing the post-tectogenetic cover of the deformed Carpathian structures (Fig. 2).
Material and methods
Bed-by-bed description and sampling were carried out in this study throughout the Căpuş Formation in the Căpuş section (N46°48′39″, E23°14′30″). Ironstone occurs there as a discrete 5 m-thick horizon (samples ROM1106, ROM1107/ROM1906, ROM1907 and ROM1108/ROM1908), which represents the source of the material investigated in this study. Sample preparation was undertaken in the laboratories of the Department of Chemical and Geological Sciences of the University of Modena and Reggio Emilia, Italy.
Microfacies
The investigated horizon documents an unusual association of iron ooids and nummulite shells (Fig. 3a). Nummulites of different sizes (Nummulites discorbinus and N. perforatus, A forms with average diameter 2.5 and 6 mm respectively) and apparently lacking any preferred orientation are embedded in a matrix of unsorted and variable ovoidal to sub-elliptical ooids sparse throughout the section. Ooids are dark red to rusty brown in colour. Large foraminifera keep their whitish walls and iron ooids
Discussion
The genesis of the iron ooids from the Transylvanian Paleogene has been debated since their discovery. Stoicovici and Mureşan (1964) proposed a connection with the alteration of volcanic and/or metamorphic rocks. Mureşan and Stoicovici (1987) provided a detailed mineralogical characterization of the ironstone (regarded as “limonitic oolitic layer”), recording a gradual passage from a goethite to a glauconite facies moving from a littoral setting to the open sea. However, their study was run at
Conclusions
This study focuses on the characterization of a Bartonian (middle Eocene) ironstone from north-western Romania. Based on mineralogical and geochemical analyses on the samples from the Căpuş section, several conclusions are made as follows.
- 1.
Iron in the Transylvanian Basin ooids occurs in the form of cryptocrystalline goethite.
- 2.
The nucleation of goethite on ferruginous material indicates cortex precipitation followed a phase of iron enrichment of the cores on the sea bottom intermixed or alternated
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank Mona Seddighi (formerly PhD student at the University of Modena and Reggio Emilia), who contributed to the fieldwork and collected part of the samples for this study. We also express sincere thanks to Massimo Tonelli and Mauro Zapparoli (Scientific Instruments Facility, University of Modena and Reggio Emilia) for their technical support.
Author Contributions
C.A. Papazzoni and A. Ferretti conceived the project. S. Filipescu and C.A. Papazzoni collected rocks with fossils and ooids. C.A. Papazzoni, A. Ferretti, M.F. Brigatti and L. Medici acquired SEM and ESEM measurements. L. Medici collected XRD data. A. Ferretti and B. Cavalazzi collected and discussed the optical data. B. Cavalazzi, F. Foucher and F. Westall collected RAMAN information. All authors provided critical input to data analysis and contributed to data interpretation. A. Ferretti and
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