Coarse-Grained Facies in Intracontinental Rift Basins: Relations to Base Level, Depositional Systems and System Tracts

Hercules T. F. Da Silva, L. P. Magnavita

In intracontinental rift basins, two principal mechanisms control basin-fill: tectonics and climatic fluctuations. In these basins, base level is highly variable, depending on (1) low frequency fault-controlled subsidence, (2) high frequency climatic fluctuations and (3) a combination of both. Siliciclastic sediments reach the depositional area through three main pathways: (1) platform margin (low subsidence region), (2) axial depocenter (high subsidence area) and (3) escarpment margin (high relief related to a major fault). Rivers entering both platform margin and axial depocenter areas possess medium to large drainage areas, escarpment margin-related sediments, however, have small drainage areas.

The analysis of some Brazilian Eocretaceous basins reveals that three depositional systems account for most of the coarse-grained facies in syn-rift fills: fluvial, deltaic and basinal. Facies related to these systems can be subdivided into distinct sub-types, depending on how they reach or align within the depositional area. Facies f1, f2, f3 and f4 occur in fluvial systems; d1 through d4 are present in deltaic systems; and basinal facies include b1 through b4 sub-types. These coarse-grained facies can be reunited into systems tracts. Highstand systems tract comprises facies f2, f3, f4, d1, d2, b3 and b4. Transgressive systems tract includes mainly f3, estuarine facies and b4. Finally, lowstand systems tract encompasses f1, f2, f4, d3, b1, b2 and b4. Most of the facies discussed here are potential petroleum reservoirs. Variable conditions within each of the systems tracts control not only the development of reservoir, but also of source rock and seal, so determining the occurrence of petroleum accumulations.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995