Abstract
The existence of gas hydrate systems along Brazil’s vast continental margin has been known since the 1980s, based on observations of bottom simulating reflectors (BSRs) in two large shelf-slope depocenters: (1) the Amazon deep-sea fan in the Foz do Amazonas Basin and (2) Rio Grande Cone in Pelotas Basin. These depocenters are both undergoing gravitational collapse above deep detachment surfaces, resulting in upslope extensional and downslope compressional domains. The BSR is discontinuous across the Amazon deep-sea fan, mainly observed at water depths of 600–2800 m and at anticlines within an upper slope thrust-fold belt related to the compressional domain of the fan. Conversely, a fairly continuous BSR extends across Rio Grande Cone at water depths of 520–3500 m, within both extensional and compressional domains. Interestingly, the well-defined BSR that spans Rio Grande Cone rises to meet the seafloor at water depths of 515–520 m, forming an unusual ‘BSR outcrop’. This phenomenon has been observed previously in only a few locations worldwide. Gas hydrates have been recovered within piston cores taken from seafloor seeps in both depocenters, and analyses reveal that gas is dominated by methane of microbial origin.
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