El Lias Superior y el Dogger inferior en Gorgo a Cerbara (Apenino Central): Aspectos bioestratigráficos y sedimentológicos

Abstract

The present note reports on a detailed survey of a part of the Jurassic - Lower Cretaceous deepening sequence exposed along the Candigliano River west of the hamlet Gorgo a Cerbara in the Umbrian-Marchean Apennines (Italy). The section examined spans the late Pliensbachian to? Bajocian interval, with a major documentary gap in the Toarcian (accounted for by a Neogene fault), and comprises the top of the Corniola Fm. and parts of overlying Bosso Fm. (inclusive of the Mbs. Rosso Ammonitico and Calcari e Marne a Posidonia). Facies of "autochthonous" rock types -mostly lime mudstones and nodular wackestones- invariably imply a deeper-water origin; moreover, inasmuch as gravity-displaced sediment makes up about half the thickness of the section, a depositional setting can be inferred in thc lower reaches or at the base of an unstable slope flanking a structurally- controlled intrabasinal high. The in situ hemipelagic limestones in the upper part of the Corniola Fm. (our lithologic units 1 and 2) have yielded ammonites at 6 separated levels. The ammonite assemblages recovered indicate a late Domerian to earliest Toarcian age. The ammonites collected systematically across the lower nodular limestone unit of the Bosso Fm. (our lithologic unit 3) have enabled the upper Toarcian and part of Aalenian to be recognized and resolved to the zonal and, in certain cases, subzonal level. From the middle Aalenian (Murchisonae Zone p.p.) on, a higher dissolution intensity in the depositional and very shallow burial environment, most likely related to increasing bottom depth, causes the remains of originally aragonitic skeletal components to disappear from the sedimentary record, thus precluding an age control by means of ammonite biostratigraphy.