Unincised fluvial and tide-dominated estuarine systems from the Mesoproterozoic Lower Tombador Formation,Chapada Diamantina basin,Brazil |
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| Institution: | 1. Petróleo Brasileiro SA, PETROBRAS, Av. República do Chile 330, 9° andar, Torre Leste, 20031-170 Rio de Janeiro, RJ, Brazil;2. Universidade Federal do Rio Grande do Sul, UFRGS, Av. Bento Gonçalves 9500, Prédio 43137, Agronomia, 91501-970 Porto Alegre, RS, Brazil;3. University of Alberta, 8029, 112 Street, Edmonton, Alberta T6G 1K2, Canada;1. Department of Earth Sciences, Indian Institute of Engineering Science & Technology, Shibpur, Howrah, West Bengal 711103, India;2. Geocom Consulting, Valle del Portet 17, 03726 Benitachell, Spain;1. Korea National Oil Corporation, 305 Jongga-ro, Jung-gu, Ulsan, 44538, South Korea;2. Department of Geological Sciences and Geological Engineering, Queen''s University, Kingston, Ontario, K7L 3N6, Canada |
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| Abstract: | The Mesoproterozoic Lower Tombador Formation is formed of shallow braided fluvial, unconfined to poorly-channelized ephemeral sheetfloods, sand-rich floodplain, tide-dominated estuarine, and shallow marine sediments. Lowstand braided fluvial deposits are characterized by a high degree of channel amalgamation interbedded with ephemeral, intermediate sheetflood sandstones. Sand-rich floodplain sediments consist of intervals formed by distal sheetflood deposits interbedded with thin layers of eolian sandstones. Tide-dominated estuarine successions are formed of tide-influenced sand-bed braided fluvial, tidal channel, tidal sand flat and tidal bars. Shallow marine intervals are composed of heterolithic strata and tidal sand bars. Seismic scale cliffs photomosaics calibrated with vertical sections indicate high lateral continuity of sheet-like depositional geometry for fluvial–estuarine successions. These geometric characteristics associated with no evidence of incised-valley features nor significant fluvial scouring suggest that the Lower Tombador Formation registers deposition of unincised fluvial and tide-dominated systems. Such a scenario is a natural response of the interplay between sedimentation and fluctuations of relative sea level on the gentle margins of a sag basin. This case study indicates that fluvial–estuarine successions exhibit the same facies distributions, irrespective of being related to unincised or incised-valley systems. Moreover, this case study can serve as a starting point to better understand the patterns of sedimentation for Precambrian basins formed in similar tectonic settings. |
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| Keywords: | Unincised fluvial Unincised tide-dominated estuary Precambrian Tombador Formation Chapada Diamantina |
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