The Seismic Modeling of a Jurassic Drowning Unconformity in Outcrop: Djebel Bou Dahar, Central and Eastern High Atlas, Morocco

CAMPBELL, A., Ewan, Instituut voor Aardwetenschappen, Vrije Universiteit, Amsterdam, The Netherlands

Along the southern margin of the High Atlas of Morocco, carbonate platforms of Early Jurassic age developed in a transtensional, rifted basin. The Djebel Bou Dahar platform was initiated during the Early Jurassic, grew to a vertical relief of 500 m, drowned, and was onlapped by deep marine sediments.

Exceptional outcrops allowed a detailed lithologic model of the platform to be made with a resolution of a few meters. Lithostratigraphic boundaries were modeled as being concordant with chronostratigraphic boundaries, each bounding a particular lithology that was assumed to be homogenous and invariate throughout the model. Petrophysical properties of the lithologies were determined and used to construct reflection coefficient time sections. These were convolved with 25 Hz to 100 Hz Ricker wavelets to produce synthetic seismic sections. Profiles were constructed using 1-ms and 4-ms sampling rates.

For 1-ms sampling rates, facies patterns were poorly resolved with a 25-Hz source and were represented by spurious amplitude events. With a 100-Hz source, features such as reef knolls and grainstone pockets of the platform margin, began to be resolved as a series lenses. A 1-ms sample rate and a 100-Hz source gave a possible vertical resolution of approximately 15 m. At sample rates of 4 ms, almost all platform facies relationships were unresolved. Only very large features, such as the drowning unconformity and the onlap of the basin-fill reflectors against the platform slope, were resolved at all frequencies and sampling rates. Although a common phenomenon in carbonate depositional systems, this is the first seismic modeling study of a drowning unconformity in outcrop.

AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)