Bidimensional Reconstitution of the Geological History of Sedimentary Basins - Theory and its Application to the Campos Basin

Andre A. Bender, Ulisses T. Mello and Chang H. Kiang
Petrobrás Research Center, Cidade Universitária, Qd 7, Ilha do Fundão, Rio de Janeiro, RJ, CEP 21910, Brazil.

Expanded Abstract

Understanding both the geohistory and the tectonic subsidence is important for the quantitative reconstruction of sedimentary sections; this in turn is essential for determining the origin and evolution of a sedimentary basin. Calculating a section's geohistory entails determination of the paleobathymetries and paleodepths of its chronostratigraphic units. Backstripping provides information on the basement's tectonic subsidence. For both purposes. it is fundamental that sediment load is corrected since it accounts for around 65% of a basin 's total subsidence (SAWYER, 1985). This correction varies greatly depending on whether a local or regional isostatic mechanism is adopted. Adopting the former means assuming that the lithosphere presents no resistance to vertical shearing, i.e., it behaves much as a fluid. The regional, or flexural, compensation mechanism presupposes lithospheric flexural rigidity. A flexural compensation mechanism can thus be considered more realistic and for this reason was utilized in this study.

The technique of flexural backstripping is explained in detail, as are the procedures and formulas needed to obtain a basin's paleobathymetric estimates (and complete geohistory). As an exercise, Campos Basin geologic section 203-RL-76 has been reconstituted (geohistory, paleobathymetry, and backstripping).

Once an isostatic mechanism has been adopted (flexural in this case), the basement response can be obtained using a fourth-order differential equation (see text: formula 20). In this case it is possible to vary lithospheric flexural rigidity spatially.

A section's paleobathymetry is mathematically calculated by backstripping each layer of the sedimentary column, using formula 11 (see text). This set of operations entails removing one layer of the basin at a time, decompressing the remaining layers, and uplifting the basement (necessary to maintain isostatic equilibrium). The section 's paleobathymetry is the space between sea-level and the top of the sedimentary column during that time. Resultant paleobathymetries can be compared with those obtained through paleoecology.

Once paleobathymetry has been calculated, the basin's geohistory can be obtained by decompressing layers, relaying on porosity curves to do so. These curves are assumed to be invariable over time end volume of the lithological unit's mass is held constant.

Backstripping entails the sequential removal of a basin 's sedimentary loads, in accordance with pre-selected chronostratigraphic interval. Remaining sediments are then replaced with water and the basement isostatically readjusted. Basement depth is thus determined without the effect of sedimentary load, which can be obtained by applying formula 5 (see text).

Campos Basin geologic section 203-RL-76 was submitted to bidimensional flexural backstripping. This exercise relied on: paleobathymetric data obtained via paleoecology, an average porosity curve, physical properties of lithologies (such as grain density), a curve showing relative variation in sea-level, lithospheric parameters, and some physical constants (table 1). The effective elastic thickness values used for the lithosphere were considered equivalent to the depth of isotherm 150 *C.

Figures obtained through the application of this technique indicate that lithospheric extension only occurred seaward of the basin 's hinge zone (345 km; fig. 7). It is possible that the presence of sediments in the region located between the hinge zone and the continent (coastal plain) are the result of the flexural effect.

Flexural backstripping showed that offshore the lithosphere displays an extension factor (beta) which increases until reaching 3.0 at the Corvina-Parati Low. From that low until some kilometers beyond the shelf break, extension factors range around 1.65. Beta values obtained through local backstripping differ an average of 15-20% from those obtained under the regional technique. In the basement's deepest regions especially, the difference is sharper, reaching as much as 65%. On the other hand, practically no difference is observed between lithospheric extension values calculated under one or the other technique for wells located in basement highs, due to the flexural influence of sedimentary loads located in neighboring depocenters.

This study shows that a local isostatic approach overestimates rebound in sedimentary load correction, thus leading to overestimations in tectonic subsidence calculations. In other words, application of a local compensation model yields smaller lithospheric extension factors than does a flexural compensation model.

The technique used here for calculating paleobathymetry yields values which are coherent with estimates obtained through paleoecology. In the region of the present Campos Basin continental shelf, paleobathymetry remained shallow during the Tertiary, growing deeper during the Middle Eocene. However, this deeper Eocene environment is a distortion caused by gravitational faults, which was not fully corrected during mathematical processing. These faults also partially disguise the effects of sedimentary progradation, mainly verified during the Upper Oligocene through the Middle Miocene, but still perceivable in the geohistory of the section analyzed.

During the Upper Cretaceous, bathymetric levels increased (bathyal environment), while in some areas they remained slightly shallower, e. g., in the region of RJS-l 17's high. Estimated bathymetry for the Lower Cretaceous is neritic.