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 3rd 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