Structural vs Stratigraphic Dip
Learning objectives
- Split an observed dip into a structural tilt plus a stratigraphic (depositional) dip
- Remove the structural dip so bedding planes go flat
- Read the residual foreset dips as a paleocurrent, the sediment transport direction
- See that structural dip is a vector, a magnitude and an azimuth
Two Dips in One
The dip a tool measures is the sum of two things. One is the structural dip, the regional tilt the whole interval shares because the basin was later deformed. The other is the stratigraphic dip, the depositional inclination of the bedding itself, cross-bed foresets, channel margins, the lee face of a dune. To read the depositional story you must first take the structural tilt back out.
Removing the Structural Tilt
The operation is a rotation. Find the structural dip, rotate it back to horizontal, and carry every measured plane with it. On the polar plot the bedding planes, which carried only the structural tilt, collapse to the center; they are flat once the tilt is gone. The cross-bed foresets, which carried an extra depositional dip, do not vanish. They gather into a single tight cluster.
The Paleocurrent
That cluster is the paleocurrent, the direction the sediment was carried when the bed was laid down, and it is one of the most valuable things an image log gives a geologist. Notice that the structural dip is a vector: the bedding goes truly flat only when both the magnitude and the azimuth of the removal are right. Estimate it wrong in either and a residual tilt is left behind, smearing the paleocurrent.
References
- Bengtson, C. A. (1981). Statistical curvature analysis techniques for structural interpretation of dipmeter data. AAPG Bulletin, 65(2), 312-332.
- Rider, M. and Kennedy, M. (2011). The Geological Interpretation of Well Logs, 3rd ed. Rider-French.