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The enigmatic flat-pebble breccia of the Sete Lagoas Formation (Bambuí Group, Brazil): Evidences of seismic-induced deformation in an Ediacaran carbonate platform
Journal of South American Earth Sciences ( IF 1.704 ) Pub Date : 2019-12-11 , DOI: 10.1016/j.jsames.2019.102461
J. Okubo; L.V. Warren; G.L. Luvizotto; F.G. Varejão; F. Quaglio; G.J. Uhlein; M.L. Assine

The Sete Lagoas Formation (Ediacaran), located in the central part of the São Francisco Craton (Brazil), consists of limestones and dolostones deposited in very shallow waters in the inner part of a rimmed carbonate platform. Four breccia types occur throughout the stratigraphic succession: evaporitic breccia with tepees, flat-pebble breccia, hydrothermal breccia and brecciated stromatolites. Here we combined a detailed sedimentological and stratigraphic analyses of the flat-pebble breccia in order to determine its origin and the processes and environmental conditions that originated these brecciated facies. The studied interval consists of a 20 m-thick succession of tabular beds composed of flat-pebble breccia interbedded with laminated microbialites. In these breccia beds, the clasts are usually platy or oblate with angular edges and are mainly disposed horizontally within the sedimentary bed, suggesting that they were little transported or reworked. The presence of clasts with sharp edges and vertices in the Sete Lagoas flat-pebble breccia suggests that the lithification process started very early in diagenesis; even the sediments exposed at the bottom were, at least, partially lithified. Some breccia levels show bidirectional imbrication. Clast size analyses reveal that the higher aspect ratio clasts show a NE-SW orientation whereas square clasts tend to fill the space among oriented clasts. Breccia clasts are vertically oriented and show deformation features increasing upwards typically of deforming beds caused by ascendant expulsion of liquefied sediment. These features are found both in modern and ancient deposits of seismic influence, which suggests a similar origin for the Sete Lagoas flat-pebble breccia. Thus, the processes that led to the formation of the studied flat-pebble breccia are interpreted as seismically triggered, since: a) the breccia beds are laterally continuous and extend for several kilometers; b) the breccia beds are restricted to a 20 m-thick stratigraphic interval; c) the interbedding of breccia beds and laminated microbialite beds is recurrent; d) the rock layers are subhorizontal and present irregular upper and lower contacts; e) the presence of liquefaction structures and dyke injection. Thus, this seismic-triggered breccia deposits represent the product of the synsedimentary tectonism occurred within the São Francisco Craton during the terminal Ediacaran and correspond to a very well-defined local stratigraphic marker in the Bambuí basin. The exact location of the epicenter is uncertain, but it could be related to the NW regional faults in the regional Paleoproterozoic basement of the study area, page17which were reactivated during the deposition of the Sete Lagoas Formation.

更新日期:2020-01-04

 

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