Photogrammetric and spatial analysis of a bioeroded Early Miocene rocky shore, western Portugal

A method for systematically acquiring and analyzing images of Miocene bivalve borings (Gastrochaenolites lapidicus and Gastrochaenolites torpedo) is applied for analysis of spatial patterns in Foz da Fonte (Sesimbra, Portugal). Space point pattern analysis is used to describe patterns of locations of fossils and to test whether there is a significant occurrence of clustering in the study area. Emphasis is placed in close-range photogrammetry methods for data acquisition and spatial statistics for data analysis. Thresholds for the occurrence of clustering at various scales of observation were computed based on a series of Monte Carlo simulations of random spatial point patterns. Goals of the analysis are focused on the statistical description of the borers population, the investigation of some spatial relationship among G. lapidicus of different dimensions and, finally, the investigation of a preferential orientation of the G. torpedo in order to draw paleogeographic and paleoenvironmental conclusions. Both G. lapidicus and G. torpedo were significantly clustered during life of the borers. The preferential cluster location of different diameter classes of G. lapidicus indicates the relative location and orientation of the paleoshore. G. torpedo populations show preferential orientations interpreted as being perpendicular to the predominant swell moving direction during a Middle Burdigalian transgression, around 18 My ago.