El desarrollo de la Capa de Arroyofrío (límite Calloviense-Oxfordiense) al Sur de Zaragoza, entre Ricla y Aguilón: bioestratigrafía y facies

Abstract

New biostratigraphic data on the Middle-Upper Jurassic (i.e. Callovian-Oxfordian) boundary bed (the socalled: Arroyofrío Bed), at the South of Zaragoza (NE Iberian Range) show that the Callovian-Oxfordian boundary is located at the base of this irregular stratigraphic interval. Both the facies and fossil content of the bed (ammonites, belemnites, brachiopods, bivalves, crinoids, stromatolite structures and planktic foraminifera) are indicative of marine sedimentary conditions. However, the revision of biostratigraphic data, as well as taphonomic analysis, confirm the presence of a basin-extent stratigraphic and registratic gap involving at least the upper Lamberti and Mariae zones. Taphonomic features shown by ammonite internal moulds, such as disarticulation surfaces, truncation surfaces, phosphate infilling, and in some cases ellipsoidal abrasion facets and annular furrows, are evidence of taphonomic reelaboration. The latter abrasion features indicate that ammonite reelaborated moulds stood for long time on the sea bottom after being exhumed and were abraded by the action of shallow directional currents under extremely shallow intertidal to even supratidal conditions, resulting in a probable widespread sub aerial exposure of the platform. All this supports the hypothesis of the boundary bed being formed in punctuated sedimentary events during lower and, most probably, early middle Oxfordian. These data are of primary importance to understand the palaeogeographic setting of the northeastern Iberian Platform at the turn of Middle-Upper Jurassic