Unconformities

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Problem

An obvious unconformity () is evident at approximately 1.5 s in Figure 10.13a; precisely where would you position it? Is it associated with the same event at opposite sides of the section?

Figure 10.13a.  Section with angular unconformity (from Yilmaz, 1987).

Background

An unconformity is a surface which at one time was subjected to erosion, either subaerial or submarine, that removed some of the section. An unconformity is characterized by a hiatus, a period of time for which no sediments are present. In stratigraphic interpretation, periods during which sediments were not being deposited are often lumped with erosonal unconformities, because the reason for the missing section may not be evident.

There is a reasonable probability that depositional conditions changed during a hiatus and hence the sections above and below an unconformity are apt to be different. Hence, an unconformity is often marked by a fairly strong reflection, and unconformities often provide the dominant reflections. The reflection patterns above and below an unconformity are often different, and, thus, the dip is apt to be different, especially if the area was tilted during the hiatus. An unconformity is thus often marked by angularities with the unconformity reflection, both below and above the unconformity over at least some portions. Differences between the rocks below and above an unconformity at different locations cause the unconformity reflection to change character and sometimes even polarity. Unconformities may be difficult to recognize if they do not involve a change in dip.

Solution

The left-hand 40% of the section in Figure 10.13a shows a different dip than the right-hand 60%. A hinge-line occurs near that is especially evident at the unconformity but is also evident in the shallower reflections, for example at 0.4 s. Above 1.4 s the reflections on the right side of the section are nearly horizontal and parallel, whereas, on the left side, a number of intervals show thickening to the left.

The interval is somewhat an exception to this as it thins to the left. The character near the top of interval seems to be slightly different from to , possibly suggesting a reef at the shelf edge. Unit also downlaps at its base.

The hinge also appears below the unconformity . Note that the interval on the left portion of this section maintains roughly uniform thickness, although the top of the unit changes its character, whereas interval just below it thickens appreciably to the right. Velocity data might add to the understanding of this hinge and help determine the role of the section between and in what we observe. For example, if this section is low-velocity compactible rock, compaction may be partly the cause of the hinge, and the hinge in the interval may be at least partly a velocity anomaly.

Where the section from to the top of unit subcrops at the unconformity, the unconformity reflection seems to sag and change character somewhat.

Note the changes in below , especially the curved event at the base of . The data at just below the interface between and may indicate another shelf edge or a fault. Changes in the character of the pre-unconformity section also call for explanation. The reason for the rapid thickening to the right of the section below is not clear, nor is the significance of the sharp upward curvature of event at its left end.

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