The textural evolution and ghost matrices of the Cretaceous Barra Velha Formation carbonates from the Santos Basin, offshore Brazil

Abstract

The Aptian lacustrine carbonates of the Barra Velha Formation, offshore Brazil, have undergone a highly unusual paragenetic and diagenetic history and present significant terminological problems if current textural classifications are applied. Existing limestone classifications invoke the concept of textural maturity, whereby the ratio of matrix to grain is a critical property and can be used to infer energy levels and the degree of transport prior to deposition. In the Barra Velha Formation, some critical carbonate grains grew in a matrix which later dissolved, congruently, producing what appears to be primary intergranular porosity. In addition, apparent boundstone textures occur, not due to organic trapping or binding, but ones which can be shown to be diagenetic, and were also related in part to the former presence of labile matrices. Thus, applying some terms in existing classifications can lead to false interpretations, and to avoid misinterpretations criteria for identifying ghost (former) matrices are provided.