Sedimentology and biostratigraphy of a peritidal succession across the Triassic/Jurassic boundary: Monte Sparagio, north western Sicily

Abstract

A 490 m thick section of shallowing upward peritidal cycles spanning the Triassic-Jurassic boundary is exposed along the northern slope of Mount Sparagio in the San Vito Lo Capo Peninsula (Western Sicily). A detailed microfacies analysis allowed defining the aspects of cyclic sedimentation of this section and especially the stratigraphic occurrence patterns of benthic foraminifera and algae across the T/J boundary. The lower part of cycles commonly shows medium to low energy subtidal facies, typical of the inner lagoon of the carbonate platform. Subtidal facies such as coral bafflestone in some cycles suggest more open lagoonal conditions (patch reefs), while some higher energy layers such as ooidal grainstones or megalodont rudstone/floatstones are interpreted as storm layers. The intertidal facies consist of planar stromatolites and loferites, while the upper part of cycles is commonly composed by flat and black pebbles conglomerates capped by Terra Rossa paleosols. The thickness of cycles is quite irregular and reflects irregular variations of accommodation space on the flat topped platform. It ranges from a few tens of cm to 3-4 m (in case of the presence of thick paleosols), with an average thickness of about 1-1.5 m. Often, the upper part of the cycles and, in particular, the paleosols, is truncated by a ravinement surface generated by the periodic flooding of the platform. The distribution pattern of megalodontids allows to differentiate three different informal units: a lower unit containing abundant and large megalodontids (A unit); an intermediate unit characterized by small and rare megalodontids (B unit) and an upper unit lacking pelecypods (C unit). The foraminiferal assemblages in the units A and B (Triasina hantkeni, Aulotortus permodiscoides, Glomospirella friedli, Aulotortus sp.) match the Triasina hantkeni range zone. Upsection, the abrupt disappearance of all the benthic foraminifera in the subtidal units and the onset of an oligothipic association dominated by Thaumatoporella parvovesiculifera, is assumed as the Rhaetian/ Hettangian boundary. A gradual recovery of benthic associations is documented upsection by the appeareance of siphovalvulinids such as Siphovalvulina gilbraltarensis.