Abstract: Tectonic Control on the Development of Evaporite Sequences across Passive Continental Margins

John P. McGinnis, Neal W. Driscoll, Garry D. Karner

Thick evaporite sequences are often preserved in the stratigraphic record along passive continental margins. Previous studies have suggested that margin evaporites mark the initial encroachment and desiccation of marine waters into the basin, thus possibly recording the rift-drift transition. We suggest, however, that continental margins associated with multiple hinge zone development during the rifting process are prime candidates for the generation of several evaporite sequences. Using a finite-rifting, basin modeling routine, in conjunction with seismic and exploratory well log data, we have determined the distribution, magnitude, and timing of lithospheric extension across the Congo continental margin, west Africa. The spatial and temporal distribution of extension across this mar in resulted in the formation of two major hinge zones, the Eastern and Atlantic. Consequently, these hinge zones played an integral role in controlling the paleowater depths and development of the margin stratigraphy. Along the west African margin, previous interpretations identified an evaporite layer developed across the entire margin which roughly coincides with the rift-drift transition (i.e., the late Aptian Loeme Formation). Our modeling indicates, however, that two distinct evaporite sequences were deposited, an earlier evaporite seaward (west) of the Atlantic hinge and the Loeme Formation between the hinge zones. An evaporite sequence seaward of the Atlantic hinge is inferred on the basis of extensive diapirs and salt tectonic structures observed in seismic data. In order to matc the observed post-rift stratigraphy across the basin, however, our model requires large paleowater depths west of the Atlantic hinge during the late Aptian. These large paleowater depths preclude the formation of a late Aptian evaporite sequence across this region. Consequently, we propose that the evaporites seaward of the Atlantic hinge were formed earlier during the syn-rift history of the margin and are not contemporaneous with the Loeme salts deposited between the hinge zones. Seismic reflection data also support this interpretation. This "double-salt" scenario is consistent with observations from the Brazil margin in which two distinct evaporite sequences occur within the Sergipe-Alagoas basin as a result of multiple hinge zones. Therefore, an important implication of our study is that continental margins with distributed extension across multiple hinge zones favor the formation of a number of distinct evaporite sequences.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994