Cyclostratigraphic investigations in the Calcare Massiccio (Early Jurassic, Umbria-Marche Basin) through photogrammetry

In this paper, we present a case study to demonstrate the potential of photogrammetry in cyclostratigraphic applications. To this end, we considered an ~300-m-thick section exposing the Lower Jurassic Calcare Massiccio Formation in the Marche Apennines of central Italy. The Calcare Massiccio comprises a thick succession of peritidal shallow-water carbonates displaying a prominent sedimentary cyclicity, where supratidal and subtidal facies alternate. The section investigated in this study is exposed on the wall of an active quarry and is almost completely inaccessible because it is vertical and because of safety and liability regulations. This setting prevents the application of standard sampling and facies analysis techniques on the whole series. An accurate three-dimensional model of the quarry wall was therefore produced by processing ~360 digital images through photogrammetry and generating a high-resolution (centimeter-scale) point cloud of the outcrop with red-green-blue (RGB) values associated with each point. An ~150-m-long log representing color variations on a continuous portion of the exposed succession was then extracted from the point cloud by converting the original RGB values to grayscale values. The main facies were directly investigated in an ~10-m-long accessible section that was logged and sampled, and it was established that supratidal facies with planar stromatolites and teepee structures are darker in color, while subtidal facies, made of bioturbated mudstones to floatstones with gastropods and oncoids, display lighter color. This provided ground-truth data with which to interpret the grayscale variations in terms of facies alternations. Time-series analysis was then carried out on the grayscale series, and this revealed prominent cyclicities. Because the biochronostratigraphic framework of the Calcare Massiccio is poor, the potential orbital origin of these frequencies was tested with the average spectral misfit technique. Preliminary results suggest that the observed spectral features are compatible with Milankovitch periods and that astronomical forcing might have been a major driver in the deposition of the Calcare Massiccio Formation. Furthermore, they testify to the great potential of photogrammetry in cyclostratigraphic applications, especially when large-scale, inaccessible outcrops have to be investigated.