Abstract: Pulsed Tectonic Activity and its Control on the Sequence Stratigraphy of Zulia Oriental, Western Venezuela

Sarzalejo, S. - PDVSA; Hague, P. - Robertsons Research Int.; Truskowski, I. - PDVSA

Recent interest in the Cretaceous - Eocene section of Zulia Oriental, to the east of Lake Maracaibo has prompted the construction of a structural and sequence stratigraphic framework for the area. Previous studies in the Maracaibo Basin have attempted to correlate stratigraphic horizons with 3^rd order eustatic sequence boundaries. However, biostratigraphic control in Zulia Oriental is scarce such that absolute ages for individual horizons are somewhat subjective. Furthermore, Zulia Oriental was subjected to considerable tectonic activity during the Eocene, the effect of which cannot be ignored. In order to truly understand the stratigraphic and structural evolution of Zulia Oriental, it is important to understand the processes which were responsible for the stratigraphic horizons.

Figure 1 shows a N-S chronostratigraphic section through Zulia Oriental, whilst Figure 2 shows subsidence curves for two wells from Zulia Oriental superimposed on the eustatic sea level curve. The horizons used in previous studies clearly correspond to eustatic falls in sea level. In Zulia Oriental, however, the rate of eustatic sea level fall during the Paleocene - Eocene rarely exceeded the rate of subsidence. Exceptions at 58Ma, 54Ma and 49.5Ma may have resulted in erosion but all other eustatic sea level falls would have merely resulted in regressional ? transgressional boundaries. These may be correlated with unconformities in Lake Maracaibo but across-fault correlation is often difficult.

Sb44 shows angular truncation and is well constrained by biostratigraphic data throughout Lake Maracaibo but is difficult to identify in Zulia Oriental where key biostratigraphic assemblages are absent and the seismic data is dominated by parallel topsets with no obvious reflector terminations. In Zulia Oriental, this boundary is therefore picked at the top of a basin-wide regression which can be observed on the well logs. The eustatic sea level curve shows only a minor fall in sea level at 44Ma which is unlikely to have produced a major sequence boundary unless it was enhanced by a period of tectonic uplift.

The effects of tectonic uplift depend on the point at which the uplift takes place in the eustatic sea level cycle. If uplift occurs when eustatic sea level is falling then it only serves to enhance a eustatic sequence boundary. A purely tectonic sequence boundary can be produced during a period of eustatic sea level rise if the rate of uplift exceeds the rate of sea level rise. This is generally considered unlikely since rates of eustatic sea level change tend to exceed rates of regional uplift resulting in a composite sequence boundary such as that which marks the top of the Eocene. However, local unconformities exist in Zulia Oriental which cannot be attributed to eustacy. One such unconformity in southern Zulia Oriental has been assigned an age of 41.5Ma on the basis of biostratigraphic data. This coincides with a period of eustatic sea level rise such that the unconformity can only be attributed to a sharp pulse of local uplift. These concepts are illustrated with a series of graphs and examples from seismic and well data.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil