Geomorphological analysis of different growth stages of a deeply buried submarine fan, Santos Basin, Brazil.

Seismic geomorphology has shown to be a powerful tool to assess deep-water systems, allowing to characterize the geometry and composition of depositional elements and to reconstruct erosion, transport and deposition. However, this approach has been applied mainly to describe a relatively short period of the depositional time, preventing the interpretation of long-term changes in geomorphology and the resulting depositional architecture of individual systems. In this research, we analyze the geomorphological evolution of a deeply buried submarine fan in the Upper Cretaceous of northern Santos Basin, SE Brazil. The submarine fan is 3100 m below seafloor, it has an area of ~700 km², 200 m of maximum thickness, and the external geometry is influenced by the topography of an underlying mass transport deposit (MTD) and salt domes. By using 3D seismic data, we mapped 5 horizons related to the submarine fan (Hz1 – fan base; Hz2 – lower fan; Hz3 – middle fan; Hz4 – upper fan; Hz5 – fan top). We generated isochron maps to define the overall geometry and to examine spatial changes in deposition during different stages of growth. We produced a coherence map of Hz1 to highlight slope and substrate irregularities. Spectral decomposition attribute was extracted from internal fan horizons (Hz2 to 4), which clearly revealed channel networks radiating from a feeder canyon. A total of 109 channel segments were measured to calculate sinuosity indexes (SI) considering three classes (SI<1.1 = straight; 1.1<SI<1.5 sinuous; SI>1.5 = meandering. The results show important aspects of fan development and changes in channel style with time. The isochron maps reveal lobe avulsion caused by compensation cycles (Hz2 to Hz3) and fan progradation towards the basin in an elongated shape. The submarine fan was first build up with straight channels and we observe an upward increase in channel sinuosity. Hz2 has 100% of straight channels, Hz3 shows 65% of straight, 30% of sinuous and 5% of meandering channels, and the Hz4 presents 51% of sinuous and 49% of straight channels. We interpret this overall increase in sinuosity as a result of a decrease in fan surface gradient caused by a progressive aggradation. The lack of MTDs within the fan and the few terminal depositional lobes observed in seismic attribute maps imply that superimposed overbank and channel-fill elements dominate the submarine fan architecture. Furthermore, the application of seismic attributes based on amplitude of acoustic impedance contrasts shows that channel-fill elements concentrate most of the sandy deposits.