Stratigraphic Interpretation through UAV-Based Digital Outcrop Modelling in a Desert Environment: the Case Study of the Canyon Gramonal (Ica Province, Peru)

In the last decade, the use of Digital Outcrop Models (DOM) in geosciences has drastically increased, given the greater affordability and capability of user-friendly photogrammetric-related utilities for solving geological problems. The use of Unmanned Aerial Vehicles (UAV) provided the significant advantage of investigating key outcrops otherwise physically inaccessible. The latter is the case in the present study, where several DOMs of the Canyon Gramonal (Ica province) were produced for stratigraphic analysis. The stratigraphic details along the canyon walls were captured over a total length of 1.5 km.

The study area, located in a remote desert environment, presented multiple challenges affecting the UAV survey (e.g., high wind velocity, poor visibility, etc.). Therefore, the photogrammetric survey consisted of flights under different conditions which required an extra effort in the attempt to optimize a non-conventional outcrop scan.

Besides the commonly adopted workflow for Structure from Motion (SfM) photogrammetry, this study involved an extensive post-production phase. The pictures, in RAW format, were edited in graphic software to prevent issues related to improper alignment, possibly linked to the color flatness, typical in desert areas. Secondly, the produced DOM dense point cloud was imported in the CloudCompare software for a cleaning phase before creating the final 3D digital surface model. Finally, the DOM was scaled and georeferenced by natural ground targets identified in aerial imagery.

The stratigraphic interpretation was performed in both the 3D model and high-resolution orthomosaics of the canyon walls. The sedimentary succession exposed at Canyon Gramonal is part of the lower Miocene Chilcatay Formation and comprises a composite sediment wedge interpreted as a coarse-grained, mixed carbonate-siliciclastic subaqueous marine delta. The mapping of stratigraphic surfaces and the outcrop study of bed surfaces and facies distribution show that the sediment wedge is composed of three successive sets of delta-scale sigmoidal clinothems that are separated by seaward-dipping erosion surfaces and are preserved in an overall southeastward prograding stacking pattern.