Seismic Model of a Progradational Carbonate Platform, Picco di Vallandro, the Dolomites, Northern Italy (1)

Abstract

We present a seismic model of a well-exposed carbonate platform-to-basin transition from the Triassic succession at Picco di Vallandro, the Dolomites, northern Italy. The core of this model is a detailed lithologic cross section that represents 1500 m of stratigraphic section over a distance of 3.5 km. The cross section is based on detailed measured sections, photo interpretation, and careful projection of surface mapping into the plane of section. Samples from the measured sections provide control on velocity and density distribution, and these data were used to generate an impedance model from the cross section. The impedance model was convolved with a zero-phase, 25-Hz peak-frequency wavelet to produce the seismic model. The results illustrate how the platform-to-basin transition might be imaged by conventional seismic reflection data, how the reflections relate to the distribution of lithology and through-going geological surfaces, and how constructive and destructive interference and seismic resolution combine to produce individual reflections. The Picco di Vallandro section displays several scales of progradation and retrogradation, ranging from stacked buildups that are several hundreds of meters thick to the interfingering of individual beds. The seismic model displays this complexity with varying degrees of success. The individual buildups are well imaged but, as expected, smaller features are not as clearly reproduced by the model. Intermediate-scale sections that represent platform retrogradation are shown as surfaces of onlap in the seismic model, but field work shows that the platform was still shedding sediment while retreating and that no simple onlap surface exists in the outcropping section. A similar lack of stratigraphic resolution is shown by the model at the downlapping clinoforms, which do not correspond to iscrete stratal surfaces, but to the toe-of-slope position where major bedding units thin below seismic resolution. Smaller features, such as the position of downslope terminations of carbonate breccia wedges, are not resolved by the model. Toe-of-slope packages of limestone turbidites are represented only by reflection segments of higher amplitude. The decreasing seismic resolution is caused by limited seismic bandwidths and by the effects of constructive and destructive interference. Interference has the largest effects at the complexly interfingering buildup-to-basin transition, or at the most detailed part of our model. An understanding of these types of seismic limitations and their controls is important as we try to extract more and more stratigraphic information from seismic data. /P>