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Aalenian-Bajocian iron-coated grains: evolutionary stages, sequence distribution and genetic significance (Iberian Basin, Spain)

Garcia-Frank A., Ureta S., Mas R.

Volumina Jurassica

2006

Vol. 4, no. 1

88-89

The study area is situated in the northwestern extreme of the Iberian Range (northern Spain). Some sections ranging from uppermost Toarcian to Bajocian have been studied for the analysis of the iron rich particles. Petranek & Van Houten (1997) documented about 550 well-known Phanerozoic ooidal ironstone outcrops, showing tat in the Jurassic of Europe, ironstones occur mainly in the Lower-Middle Jurassic and in the Middle-Upper Jurassic transitional beds. The main minerals forming these particles are berthierine, chamosite and goethite. The particles recorded from the Iberian Range, according to Flugel’s (2004) classification, consist on iron-skeletal grains, iron-coated grains (cortoids, ooids and oncoids) and iron-grain aggregates, mostly composed of the ferrous Fe-phyllosilicate berthierine, and in a less amount goethite. Five stages of increasing complexity in the habit of these particles are documented in the Aalenian/Bajocian boundary, which are correlated with a more condensed sedimentary record in the stratigraphical succession. A trend ranging from iron cortoids (stage 1) in the lower part of the series, passing to small-sized iron ooids and oncoids (stage 2), and finally, more complex iron-grain aggregates and larger iron ooid-oncoid mixed particles (stage 3). In condensed sediments, a rim of berthierine can be developed around any of the previously documented particles (stage 4), or even, a total dissolution of the particle can occur (stage 5). A relationship between these stages and the main stratigraphical discontinuities affecting the sequences is acknowledged. Stages 1 and 2 appear in the lower sequences (Lower Aalenian), in which, each individual discontinuity, represent a sedimentary gap equivalent at least to one ammonoid subzone. Stages 3 to 5, occur in the upper part of the succession, and as previously mentioned, related with more condensed sediments, and where, discontinuities represent more extent temporal gaps. The active tectonic context of the basin, might be associated with the Fe-mineralogy of the particles by exhalative sources. The rare earth elements (REE) distribution of the particles and samples of the coeval volcanism documented in the Iberian Range, shows similar patterns. It is therefore probable to situate the studied area in an active tectonic framework within the Aalenian-Bajocian, where the source of iron could be associated with exhalative events.