Page 99 - Carbonate Sedimentology and Sequence Stratigraphy

90 WOLFGANG SCHLAGER

Vail et al. (1977, p. 51) commence their classic expose always match. Ground truthing by bore holes and seismic
with the following statements: “Primary seismic reflections modelling of outcrops provide evidence of this mismatch.
are generated by physical surfaces in the rocks ... There- Three different situations may develop.
fore, primary seismic reflections parallel stratal surfaces and 1. Boundaries where both outcrop bedding and seismic
unconformitie”, and “... the resulting seismic section is a reflections abut against a surface. This situation is un-
record of the chronostratigraphic ... depositional and struc- problematic because the field geologist and the seismic
tural patterns and not a record of the time-transgressive interpreter will classify this feature as an unconformity.
lithostratigraphy ...”. 2. There are outcrop unconformities that cannot be seen in
I believe that these elegant, clear and simple statements seismics because their geometric expression consists of
are somewhat misleading because of their categoric style. In a microrelief on the scale of centimeters or decimeters
a questionaire that leaves only the options, right and wrong, while the large-scale bedding of the two units remains
the statement - seismic reflections are time lines - would parallel. Common in deep-sea sediments.
have to be classified as right. However, the refined state- 3. Seismics shows unconformities that correspond to tran-
ment - seismic reflections always represent time lines - is sitional boundaries in outcrop. Two situations must be
wrong and a number of authors have indicated this (e.g. distinguished:
Christie-Blick et al., 1990; Tipper, 1993; Emery et al. 1996, p. (a) Seismic unconformities where time lines converge
45). Sheriff and Geldart (1995, p. 150) succinctly summarize into an interval of continuous but very slow sedi-
the situation “ ... overwhelming evidence indicates that re- mentation and where the seismic tool portrays this
flections coincide with time boundaries except in occasional condensed interval as a lapout surface. In most
unusual circumstances”. In fact, Vail et al. (1977) them- instances this condensed interval meets the defi-
selves provide an example of such “unusual circumstances”, nition of unconformity by Vail et al. (1977) in so
shown in Fig. 6.8. The task before us as sedimentologists far as all deposits below the condensed unit are
and stratigraphers is to acknowledge the overwhelming ev- older than the oldest deposits above it. The differ-
idence for time-parallel reflections without losing sight of ence to the classical situation is merely that “non-
the, often important, exceptions such as the ones presented deposition” needs to be replaced by “slow deposi-
in the next section. tion”. For the seismic interpreter this is a minor
difference, but one that should be kept in mind
Unconformities in outcrop and seismic data when seismics is tied to boreholes or outcrops.
(Van Hinte, 1982)
The above discussion on sequences, time lines and seis- (b) Most disturbing are pseudo-unconformities where
mic reflections automatically leads to the stratigraphic un- a rapid facies change occurs in each bed at a sim-
conformity as a particularly important element in sequence- ilar position and the seismic tool merges these
stratigraphic theory. As sequence stratigraphy is applied to points of change into one reflection. Time lines
both outcrops and seismic data, it is important to realize that cross this reflection; it is therefore not an uncon-
outcrop unconformities and seismic unconformities do not

Fig. 6.8.— Seismic model of sand-
stone interfingering with shale. At 20-
Hz frequency, the lower reflection runs
oblique to bedding, approximately con-
necting the terminations of the sand-
stone tongues. At 50 Hz, the individual
sandstone tongues are revealed by re-
flections parallel to bedding. After Vail et
al. (1977). (Reprinted by permission of
20 Hertz pulse the AAPG whose permission is required
for further use).

50 Hertz pulse