| Popis: |
The stratigraphic relationships between the Chapada Diamantina and Bambuí (or Una) groups were practically establishedwith the papers of J. C. Branner, a century before. Sometimes that proposed lithostratigraphy was questioned, but we do knownow that these arguments were based up incomplete or inadvertent field observations. At the sheet of Mirangaba, centralnorthernpart of Bahia, both the angular and erosional unconformity between the above-mentioned groups are exposed of aconspicuous way, according to their three dimensions, what deserves the special mention here consigned. The quartzites andconglomerates of the Chapada Diamantina group are cropping out as folded and re-fold rock units (general strike ENE-WSW)and they present features of intense superposed erosional processes. Two paleosurfaces were formed and are being exhibited,with complementary features of erosional scarpments, as well as amphitheatres, suppression of some lithostratigraphic units,testimonies-hills (quartzitic islands), etc. The lower pediplan then formed (today with summits ≥ 500 m) was completelycovered by the limestones (Salitre Fm.), where calcilutites are predominating, presenting many algalic edifications. We are not able to affirm that the proterozoic limestone progradation have covered the higher pediplan (today with summits ca. 1,000 m), but there aresome testimonies-hills of limestone presenting tops up to 600 m, clearly above the present and predominating surface (Neogene, VelhasSurface), so reiterating that an intense erosional dissecation has occurred (Cenozoic in age) of the limestones. These limestones presentmonotonous subhorizontal structural attitudes, with the presence of some open folding (ondulations) when they are near of the contactswith the subjacent quartzites. Quite often these limestones were submitted to local ruptural displacements. The polyphasic deformationof the subjacent Chapada Diamantina group is being hypothetically attributed to the Brazilian cycle (as foreland domain of the Riacho doPontal fold system), but this proposed condition needs to be better constrained, as well as the northern limit of the São Francisco Craton(to the north) demands to be reviewed. The basal diamictites of the Bambuí Group (Bebedouro Fm.) is only locally cropping out, at thesouthern and at the northwestern part of the area. The time interval between the deformation of the quartzites and the beginning of thelimestones deposition was necessarily very long, some millions of years, and this is an obligatory target for future researches. |
