Bedding attitudes as a sequence stratigraphy proxy: A case study from borehole images, Integrated Ocean Drilling Program Expedition 313, Site M28

Abstract

In this study we investigate the relationship between the dips of seismic reflectors, which are used to define sequence boundaries, and the orientation (dip and dip direction) of bedding surfaces at core scales. Sequence boundaries from seismic data and lithostratigraphic boundaries from cores and logs are compared with the bedding orientations measured on borehole images of Miocene siliciclastic sediments at Integrated Ocean Drilling Program Expedition 313 Site M28. It is not surprising that bedding orientations show huge variations at scales that are too small to be detected on seismic profiles. However, changes of orientation defined as rotation between two successive intervals match the depths of approximately half of the seismic sequence boundaries. While they do not match boundaries between lithostratigraphic units, changes of orientation frequently correlate with maxima and minima in the gamma ray signal, suggesting that they are related to changes in depositional processes rather than to changes in lithology. This study suggests for the first time that bedding attitudes can be used as a stratigraphic tool at various scales from bed to bed across depth intervals of tens of meters.