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Seismic Modeling of an Exposed, Progradational Carbonate-Platform Margin

Kurt W. Rudolph, Kevin T. Biddle, Wolfgang Schlager

It commonly is difficult to acquire high-quality reflection data in areas where the distribution of stratal surfaces and lithology are well known. However, the geologic significance of impedance boundaries in the subsurface is generally an interpretation. Seismic modeling provides a technique that can tie the understanding of outcrop geology to seismic reflections.

We have modeled a well-exposed progradational carbonate-platform margin from the Pico di Vallandro area of Dolomite Alps, northern Italy. These outcrops are the remnants of a carbonate platform with well-developed clinoforms that represent paleoslope deposits, and thinly bedded marls and limestones that we interpret as basin-floor units. The clinoforms are composed of megabreccias with a matrix of lime sand and mud, and show several scales of progradation and backstepping. The paleoslope deposits are completely dolomitized in their upper parts and partly dolomitized in their lower reaches. Individual beds within the prograding carbonate units interfinger with, and are onlapped by, the basinal marls and limestones.

We have reconstructed the geometry of the platform margin by measuring stratigraphic sections and by tracing and projecting major stratal surfaces. Velocities and densities of samples from the stratigraphic sections were then measured. P-wave velocities range from over 22,000 ft/sec for nonporous dolomite to about 14,000 ft/sec for basinal marls. These data were used to create impedance logs from the measured sections. We modeled the complicated toe-of-slope areas in detail and used this information to produce a general model of the platform margin. The resulting synthetic seismic lines indicate how the complicated geometry of this platform margin might be portrayed by conventional reflection seismic data.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.