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