ABSTRACT: Reefs and Unconformities in Outcrops and Seismic Records

SCHLAGER, WOLFGANG, and JAN STAFLEU, Free University of Amsterdam, Netherlands

Definitions of reefs and criteria for their recognition vary, depending on the techniques applied. Examples include ecologic reef, defined by biologic criteria, and stratigraphic reef, defined in geologic terms. We suggest adding the concept of "seismic reef" for reefs defined and delineated by criteria of seismic interpretation. The terms ecologic reef, stratigraphic reef, and seismic reef overlap in scope but they are not congruent. There are ecologic reefs that do not qualify as seismic or stratigraphic reefs, and both seismic reefs and stratigraphic reefs include more than the fossil remains of ecologic reefs.

A similar, potentially more disturbing situation exists with respect to unconformities. Comparison of outcrops and seismic records as well as seismic modeling of outcrops indicate that outcrop unconformity and seismic unconformity are not congruent terms. What appears as unconformity on a seismic line is not necessarily an unconformity in outcrop and vice versa. The most significant mismatches are pseudo-unconformities in seismic records. Seismic models of outcrops show that the seismic tool has a strong tendency to portray rapid changes in facies and dip as lap out patterns. We observed pseudo-downlap and pseudo-onlap at the base of prograding or retreating carbonate platform slopes respectively; we noticed pseudo-toplap at the margin of a rapidly prograding platform where the flat-l ing topsets of the platform roll over into slope clinoforms. None of these outcrops were correctly portrayed seismically at 25 Hz frequency. However, in some instances the true relationship (of rapid lateral facies change) would gradually emerge at frequencies of 50-100 Hz.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)