High-resolution sequence stratigraphy in a Mesoproterozoic intracratonic sag basin, the Tombador Formation, Chapada Diamantina Basin, Brazil

  • Author / Creator
    Magalhaes, Antonio J. C.
  • Studies applying sequence stratigraphy to the Precambrian sedimentary record have been limited and have focused on low-resolution applications due to problems in preservation, deformation/metamorphism, and diagenesis. However, higher-resolution studies are possible where geometry, facies definition, and facies relationships are well-definable, such as that presented by the Mesoproterozoic Tombador Formation in the Chapada Diamantina (diamond-bearing plateau) Basin. Despite its very old age, this unit underwent localized very low-grade metamorphism, and the exposure and preservation are exceptional, affording a unique opportunity to observe changes at the sub-seismic scale (high-resolution sequence stratigraphy). Furthermore, this unit enables researchers to unravel the processes that dominated the deposition and understand their stratigraphic evolution, as few such pristine data sets are available anywhere in the world for that time interval. The data for this study were collected from several sources, including detailed and regional seismic-scale photo-mosaics; 1869.8 m of vertical section with a detailed facies description at a 1:40 scale; 697 paleocurrent readings; 1870 m of gamma ray (GR) logs with a 20-cm space sample; a 6.2-km ground penetrating radar (GPR) survey; and the description of 106 thin sections. The vertical description of the facies and stratigraphic sections, calibrated with the GR logs, the photo-mosaics, the GPR survey, and the petrography of sandstones allowed for the interpretation of the depositional systems, the identification of key stratigraphic surfaces, and the external geometry of the sequences. Higher-rank depositional sequence boundaries are placed at the base of the extensive amalgamated fluvial sand sheets or at the base of the alluvial fan conglomeratic successions that indicate basinward shifts of facies. The hierarchy system that applies to the Tombador Formation includes sequences of different orders, which are defined as follows: sequences associated with a particular tectonic setting are designated as 'first order' and are separated by first-order sequence boundaries where changes in the tectonic setting are recorded; second-order sequences represent the major subdivisions of a first-order sequence and reflect change cycles in the stratal stacking pattern observed at 1E2 m scale (i.e., 200-300 m); changes in the stratal stacking pattern at 1E1 m scale indicate third-order sequences (i.e., 40-70 m); and changes in the stratal stacking pattern at 1E0 m scale are assigned to the fourth order (i.e., 8-12 m). Changes in paleogeography due to relative sea level changes are recorded at all hierarchical levels with a magnitude that increases with the hierarchical rank. Thus, the Tombador Formation corresponds to one first-order sequence, representing a distinct sag basin fill in the polycyclic history of the Espinhaço Supergroup in Chapada Diamantina. An angular unconformity separates the fluvial-estuarine to the alluvial fan deposits and marks the second-order boundary. Below the angular unconformity, third-order sequences register deposition of unincised fluvial and tide-dominated estuarine systems and exhibit a lowstand fluvial sand sheet, an undifferentiated transgressive-highstand sand-rich floodplain, a transgressive estuarine, and highstand shoreface strata. In contrast, the third-order sequences above the angular unconformity are characterized by fining-upward continental alluvial successions composed of conglomerates overlain by fluvial and eolian strata; however, the data are inconclusive regarding whether these successions were downstream or upstream controlled. Hence, no systems tract nomenclature was used. Fourth-order sequences are clearly recognized at the third-order transgressive systems tract, and they exhibit distinct facies associations depending on their occurrence at the estuarine or fluvial domains. At the estuarine domain, they are composed of tidal channel, tidal bar and overlying shoreface heterolithic strata. At the fluvial domain, there are two types of fourth-order sequences: a) those preserved at the sand-rich floodplain, which are composed of cyclic successions of sand-bed braided or distal ephemeral sheetflood deposits capped by eolian sand sheet or mudstone layers, and b) those preserved at the tide-influenced braided fluvial sand sheet, which consist of cyclic successions of fluvial strata bounded by tide-influenced intervals. Fine grained intervals from shoreface heterolithic and sand-rich floodplain deposits constitute fourth-order sequence boundaries that at the reservoir approach, constitute the most important horizontal heterogeneity and hence the preferable boundaries of production zones. The criteria applied to assign sequence hierarchies in the Tombador Formation are based on rock attributes, are easy to apply, and can be used as a baseline for the study of sequence stratigraphy in Precambrian and Phanerozoic basins placed in similar tectonic settings.

  • Subjects / Keywords
  • Graduation date
    Fall 2015
  • Type of Item
  • Degree
    Doctor of Philosophy
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    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.