Fault and stratigraphic interpretation

ADVERTISEMENT
From SEG Wiki
Jump to: navigation, search

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 Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\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 Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle A} and toplap below it support this concept.

The interval from Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle A} to Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle B} thickens as it approaches the fault, probably because the fault was active during deposition. There is also downlap and perhaps onlap onto Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle B} . The sediments above Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle A} and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\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 Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle C} . The intervals from Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle D} to Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle K} all show thickening to the right and there is onlap onto all of these reflections, Events Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle H} .Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle J} , Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle K} , and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle M} suggest shelf edges or offlap breaks.

There is a prominent bulge above reflection Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle K} ; it appears that there are two shelf edges with some associated slumping. Following the deposition of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle K} , sealevel must have fallen below the shelf edge on Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle K} , as is evident from the onlap onto Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle K} below the Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle K} shelf edge. Following this, sealevel must have risen and onlapped onto the shelf of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle K} , then formed the shelf edge Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\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 Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\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 Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle M} to Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\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 Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle N} is of poor quality, it is probably a major sequence boumdary onto which reflections downlap.

Continue reading

Previous section Next section
Mapping faults using a grid of lines Interpretation of salt uplift
Previous chapter Next chapter
Data processing Refraction methods

Table of Contents (book)

Also in this chapter

External links

find literature about
Fault and stratigraphic interpretation
SEG button search.png Datapages button.png GeoScienceWorld button.png OnePetro button.png Schlumberger button.png Google button.png AGI button.png