Evaluation of the structural control over quaternary sedimentation on the continental shelf of Salvador, Bahia, using high-resolution seismic surveys

摘要

The continental shelf adjacent to the municipality of Salvador (CSS), state of Bahia, is the narrowest sector of the entire Brazilian continental shelf. During most of the Quaternary, this sector of the shelf was exposed to subaereous conditions, since sea level was approximately 45-60 meters below what is currently observed. This long subaereous exposure and resulting erosion were certainly controlled by the heterogeneity of the structural and stratigraphic framework of the Camamu and Jacuípe basins, highlighting relic physiographic features from both basins. The complex seascape that resulted from this prolonged erosion played an important role in controlling the characteristics of shelf sedimentation in episodes when the CSS was flooded during interglacial periods. The objective of the present study was to understand the geological evolution and the structural control of the Camamu /Jacuípe basins over sedimentation patterns of the Continental Shelf of Salvador, during the Quaternary, using seismic-stratigraphic interpretation of high-resolution shallow seismic lines obtained from a boomer plate sound source. Two major stratigraphic units were identified in the study area : the Cretaceous Sedimentary Basin and the overlapping Unconsolidated Quaternary Sediments. Isopach maps of Quaternary deposits and depth maps of the top of the Cretaceous sedimentary rocks showed that the structural framework of the Sedimentary Basin structurally controls shelf sedimentation. Compressional structures that affect the Cretaceous sedimentary rocks in areas near previously mapped transference faults seem to have originated topographic highs on the shelf where coarse bioclastic sedimentation predominates. These highs, in turn, delimitate low areas where fine-grained sediments accumulate. Shelf topographic highs are associated with antiforms in the Camamu/ Jacuípe basins, with their most deformed strata following a WNW-SSE trend. This information is important to aid marine environmental management strategies. Moreover, the present study will allow the evaluation of the controlling factors that determinate narrow shelf sedimentation and stratigraphy, with small accommodation area, as in the case of the CSS.