Page 137 - Carbonate Sedimentology and Sequence Stratigraphy

128 WOLFGANG SCHLAGER

A well-constrained submarine example of pseudo- Pseudo-unconformities occur in at least 50 % of the dozen
downlap was discovered in the Neogene off eastern Aus- outcrop models constructed by our group or compiled
tralia (Fig. 7.29; Isern, Anselmetti, Blum et al., 2002). A very from the literature between 1989 and 1995 (Schlager, 1996).
similar configuration off western Australia, not tested by the The principal cause of the common occurrence of pseudo-
drill, was interpreted as a genuine unconformity by Cathro unconformities in carbonates may be drastic facies changes
et al. (2003). and rapid thickness variations at the periphery of carbonate
factories. Fig. 7.25 summarizes the most common situations
At present, we do not know what percentage of seismic
unconformities are pseudo-unconformities that correspond where these false lapouts have been observed.
to facies changes or drastic thickness variations in the rock
column. The percentage may be significant, though, and the
problem should not be brushed aside.

A)

B)
E D C B A
.650

time(s) .700

.750

100 m

Fig. 7.27.— Details of interfingering at the toe of a retreating platform slope at Picco di Vallandro (Southern Alps, Italy). Compiled after
Rudolph et al. (1989) and Bracco-Gartner and Schlager, (1999). A) Interfingering of slope carbonates and argillaceous basin sediments.
Slumps and debris flows deposited tongues of carbonate material that were emplaced at a time when accumulation in the basin was
faster than slope sedimentation such that basin sediments encroached on the lower slope. B) Vertical-incidence seismic model at 25 Hz
shows bedding-parallel reflections in the slope and basin domains, respectively. In addition, a nearly continuous reflection runs oblique
to bedding and creates the impression of basin sediments onlapping the slope.