# Fault and stratigraphic interpretation

Series Geophysical References Series Problems in Exploration Seismology and their Solutions Lloyd P. Geldart and Robert E. Sheriff 10 367 - 414 http://dx.doi.org/10.1190/1.9781560801733 ISBN 9781560801153 SEG Online Store

## Problem 10.6a

In Figure 10.6a, the reflection at about 0.6 s appears to be faulted at SP 5; draw in the fault and describe its probable type and characteristics.

### Background

See problem 10.5 for a discussion of types and evidences of faulting. For definitions of geological terms, see Sheriff (2002) or Jackson (1997).

Aggradation (up-building) is associated with rising relative sea level (or subsiding land level) and progradation with a sea-level stillstand. A sequence begins with a fall of sea level and ends with the next sea-level fall.

Figure 10.6b shows terms used to describe the angularities where reflections terminate. Onlap of reflections during rising relative sea level following a fall may indicate a sequence boundary. Toplap often marks no significant change (a stillstand) of relative sea level, and erosional truncation a fall. Downlap and apparent truncation generally result from starvation, that is, not enough sediment being available to permit resolution.

### Solution

The fault surfaces in Figure 10.6c are curved and concave upward (listric) and down-thrown to the right and then soling out into bedding planes. Faulting was probably occurring at the time the sediments were being deposited, often a characteristic of growth faults. The dashed fault is not as reliable because the overlying fault may cause fault-shadow effects which cause distortions of deeper data.

## Problem 10.6b

How can changes in the intervals between different reflections in Figure 10.6c be explained?

### Solution

The most prominent reflections have been lettered in Figure 10.6c Most of these reflections are at unconformities and may be minor sequence boundaries. In seismic stratigraphy, an unconformity is a break in the time sequence of sediments (a hiatus), and may represent erosion or simply nondeposition. While many unconformities are sequence boundaries, not all are. We have somewhat overinterpreted this section; we believe that, early in an interpretation, one should consider all possibilities, later discarding ideas that appear to be unlikely based on other evidences.

The interval between the seafloor and reflection ${\displaystyle A}$ thins to the right, probably because the source of sediments is far away, that is, relative sea level is high during this time so that the coastline is a long way landward. Downlap onto ${\displaystyle A}$ and toplap below it support this concept.

The interval from ${\displaystyle A}$ to ${\displaystyle B}$ thickens as it approaches the fault, probably because the fault was active during deposition. There is also downlap and perhaps onlap onto ${\displaystyle B}$. The sediments above ${\displaystyle A}$ and ${\displaystyle B}$ are probably transgressive associated with rises in relative sea level.

Figure 10.6a.  Marine seismic section (from Hatton et al., 1986).
Figure 10.6b.  Terminology for reflection terminations.
Figure 10.6c.  Interpretation of Figure 10.6a.

There is onlap onto ${\displaystyle C}$. The intervals from ${\displaystyle D}$ to ${\displaystyle K}$ all show thickening to the right and there is onlap onto all of these reflections, Events ${\displaystyle H}$.${\displaystyle J}$, ${\displaystyle K}$, and ${\displaystyle M}$ suggest shelf edges or offlap breaks.

There is a prominent bulge above reflection ${\displaystyle K}$; it appears that there are two shelf edges with some associated slumping. Following the deposition of ${\displaystyle K}$, sealevel must have fallen below the shelf edge on ${\displaystyle K}$, as is evident from the onlap onto ${\displaystyle K}$ below the ${\displaystyle K}$ shelf edge. Following this, sealevel must have risen and onlapped onto the shelf of ${\displaystyle K}$, then formed the shelf edge ${\displaystyle J}$ and fallen again. Note onlap onto the lower part of the bulge and downlap onto thc top of thc bulge.

Note also the probable fault-plane reflection that lies just below the fault that forms the lower boundary of ${\displaystyle L}$, onto which several reflections onlap. There may be another bulge on the fault between 1.15 and 1.20 s that may be a slope fan.

Data quality is very poor from ${\displaystyle M}$ to ${\displaystyle N}$. The poor reflection quality may indicate little acoustic impedance contrast. The conflict of dips here may indicate both primary reflections that are probably dipping appreciably to the right and also multiples that are nearly flat. There are probably other faults soling out in this region also. Although ${\displaystyle N}$ is of poor quality, it is probably a major sequence boumdary onto which reflections downlap.