Open carbonate ramp facies, microfacies and paleoenvironments of the Gramame Formation (Maastrichtian), Pernambuco-Paraiba Basin, Northeastern Brazil

The monotonous carbonates of the Maastrichtian Gramame Formation can be div ided into 11 microfacies, but these do not correspond with the five major f acies distinguished in the field. The microfacies were grouped into six com posite microfacies which were used to construct a depositional model. In th is model, tectonic disturbances of a relatively steeply sloping carbonate r amp caused apparently random occurrences of coarser and sandier bioclastics and influx of elastic material into generally mid-outer-ramp environments. Only the central section of the carbonate ramp is exposed in a strike sect ion. Shallower and deeper facies must be inferred from the transgressive na ture of the sequence. Dolomitization is ubiquitous but apparently random. T he closest modern analogy of the Gramame Formation is the relatively steep West Florida ramp. However this does not have fault-bounded shallows, for w hich analogies can be found on the shelf of the Arabian Gulf. Combining fea tures from both of these allows a plausible reconstruction to be made of Gr amame Formation environments. We envisage a steeply sloping ramp cut by hor sts and grabens related to the opening of the south Atlantic ocean. (C) 199 9 Elsevier Science Ltd. All rights reserved.