Ulisses T. Mello
Petrobrás Research Center, Cidade Universitária, Qd 7, Ilha do Fundão, Rio de Janeiro, RJ, CEP 21910,
Current address: IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598.
Determination of basement subsidence was based on the one-layer (McKenzie, 1978) and two-layer (Royden & Keen, 1980) stretching models of basin formation. These theoretical models describe initial, fault-controlled basement subsidence and ensuing thermal subsidence based on the degree of (lithospheric stretching. All driving subsidences that may be partially or totally infilled by sediments were assumed to be water-loaded.
Gravimetric modeling and analysis of basement subsidence (Mello. 1987) were used to estimate (lithospheric stretching factors.
Vertical heat flow is calculated by obtaining the Fourier series for the rift temperature structure and by solving the diffusive heat-flow equation (McKenzie, 1978). Lateral effects of heat flow were obtained by convolving the impulse or Green 's function for lateral heat conduction with the solution of the vertical heat flow equation (Karner, 1986).
The response of the lithosphere to applied loads is assumed to be analogous to the flexure of a thin elastic plate overlying a viscous fluid. The effective elastic thickness of the lithosphere is related both to the rheological properties of the lithosphere and to its temperature structure. Here. the effective elastic thickness is determined using the depth of the 300 *C isotherm. Deformation of the basement due to sedimentary loading is obtained by numerically solving a fourth-order elastic differential equation (Hetenyi, 1946), using either the Fast Fourier Transform (spatially constant elastic thickness, Karner, 1982) or finite difference (space-dependent elastic thickness, Bodine. 1981) techniques.
In terms of shape and depth. the basin 's paleobathymetry was similar to present-day bathymetry. Therefore, sediment supply should be adequate for filling in the basin as it subsides, maintaining a constant water-depth and preserving its "equilibrium profile."
Changing sedimentary loads produced the compaction of underlying sediments over time. Compaction was ascertained based on the mass rather than the thickness of deposited sediment; the mass of sedimentary grains contained in a layer was assumed to be constant during burial, and reduction of water volume was controlled using a porosity-depth curve for the basin.
Three eustatic sea-level variation curves presenting different magnitudes and wavelengths patterns were tested in the integrated model (Vail et al.. 1977, Watts & Thorne, 1984, and Hallam, 1984).
Erosion was assumed to be proportional to topographic elevation, the erosion rate decreasing exponentially with topographic denudation (Watts & Thorne, 1984).
Modeling of stratigraphic sections of the Potiguar basin in northeastern Brazil indicates that primary controls on sedimentation include:
(1) thermal subsidence, responsible for tectonic subsidence of the basin following rifting;
(2) long-term eustatic sea-level fluctuation, which has been of key importance in determining sediment geometry in the shallow water regions of the basin (e.g., coastal plain area);
(3) the increasing of flexural rigidity over time, producing coastal flexural onlap;
(4) erosion of exposed and uplifted areas;
(5) lateral heat flow following rifting, which led to thermal uplifting of rift shoulders. These shoulders may have become the source of sediments and limited the depositional area of the sedimentary sequences (e.g., Alagamar Formation, the early post-rift unit).
The interaction of sea-level fluctuations with subsidence had a definitive effect on the development of stratigraphic features near the coast (shallow water environments). Onshore in the basin, where the rate of subsidence after the Santonian was lower than the rate of fall of the sea-level, deposition ceased upon the sedimentary units last deposited. The resulting exposure may have occurred in the absence of tectonic uplift During the Late Cretaceous, flexural onlap (Açu and Jandaíra Formations was amplified offshore by the rising sea-level. During the Tertiary, the rate of fall of sea-level was greater than the rate of subsidence, exposing Upper Cretaceous units (Açu and Jandaíra Formations) and hence producing coastal erosional truncation over those sediments.
In conclusion, the forward stratigraphic modeling technique based on the integration of the aforementioned models can explain many stratigraphic features observed in he Potiguar Basin. A comparison between synthetic and observed basin stratigraphy allowed for recognition and ranking of factors influencing the stratigraphic evolution of the basin. Primary controls on sedimentation are: (1) availability of sediment supply, (2) thermal subsidence, (3) long-term eustatic fluctuations of sea-level, (4) flexural compensation, (5) erosion, and (6) lateral heat flow.