Drowning unconformities on presalt carbonate platforms – Examples from the Itapema Formation (Lower Cretaceous), Santos Basin, offshore Brazil.

Drowning unconformities are surfaces characterized on seismic data by high amplitude contrast between deepwater and shallow water facies. Such surfaces are created by the covering of the platform by onlapping siliciclastic sediments or other non-platform material. The key aspect of this paper is the application of the drowning unconformity concept, commonly used in marine settings, to a more restrictive one: the coquinas of the Itapema Formation, in the pre-salt succession of the Santos Basin (Brazil). In the studied area the sequence boundaries of the deposits related to the Itapema Formation are often associated with high reflectivity seismic horizons, which are tied to radioactive peaks of low-energy sedimentary facies in the gamma ray logs and also to the siliciclastic supply rate increase. However, the stratigraphic significance of these reflectors still lacks further investigation and an adequate description. In this way, it is proposed that the space-time filling of the Itapema Formation occurred by the successive migration of proximal facies towards the structural highs, under continuous accommodation space generation conditions and high carbonate production rate, with the depocenters limited by rapid expansions of the basin. The results presented here, therefore, suggest that the sedimentary record of this section was punctuated by instantaneous transgressive surfaces that altered the ecological accommodation, forcing the backstepping of the coquina deposits and the development of a drowning unconformity in the final stage of the Itapema Formation deposition. This configuration produced at the top of this unit a specific facies association characterized by packstones and mudstones composed of micritized grains, which increase in volume and thickness towards the structural lows and tend to be thinner on the way up to the structural highs, where they may be eroded by the Pre-Alagoas Unconformity. The correct identification of such surfaces is essential for understanding the sedimentary filling of the studied area. Depending on which surface is mapped, they may be related to different processes and occur in opposite positions in the base-level cycles. Thus, we propose the existence of two surfaces at the end of the deposition of the Itapema Formation. The first limits the top of the coquinas and is represented by the contrast of low-energy sediments (packstones and micritized mudstones) and the coquinas deposits, configuring the platform drowning itself. The second one, in turn, is understood here as the Pre-Alagoas Unconformity, a subaerial unconformity associated with the regional platform exposure, developed after the drowning events recognized here. • Seismic and well characterization of drowning unconformities in the pre-salt succession. • Drowning unconformities in a non-marine environment setting. • Shut-down of the coquina factory associated with a rapid drowning in early Cretaceous. [ABSTRACT FROM AUTHOR]