The Pucará Group platform carbonates (Upper Triassic - Lower Jurassic) were laid down in northern and central Peru in a NNW-SSE elongated basin (Fig. 1). They represent the first sediments of the Andean cycle, the beginning of which is marked by a Norian transgression (Mégard, 1978). The sedimentary evolution of the Pucará Group can be explained in terms of a large transgressive/regressive second order sequence which consists of predominantly shallow water carbonates including a maximum flooding period represented by ammonite-bearing bituminous calcareous shales. Detailed investigations in the southern part of the basin show that the Pucará Group thickens progressively from west to east in the form of a half-graben (Fig. 2). This can be explained by asymmetrical subsidence during sedimentation such being assisted by contemporaneous faulting along the eastern margin of the basin (perm...