Late quaternary evolution and shallow gas formation in a tropical estuarine environment: the case of the Bertioga channel, Brazil

Abstract

Shallow gas occurrences and the effects of these components in seismic waves are studied by several researchers in different parts of the world; these features can also provide important information regarding the origin and evolution of coastal and inner environments, once the gas signatures on seismic waves are mainly a function of density of gas and sediment properties. The Bertioga channel is located on the southern coast of Sao Paulo - Brazil, with estuarine characteristics that connects the city of Santos to the ocean by a 25 km channel. This is a low hydrodynamic system with maximum depths of 18 m, and is influenced by its tributaries from the Serra do Mar mountains. The present study was conducted with the seismic sources Pinger (24 kHz), Chirp (2 - 8 kHz) and Boomer (0.5 - 2 kHz), operating simultaneously. Surficial sediment sampling was carried with a Van Veen grab. Five cores were also acquired, of which the sedimentation rate was obtained. The seismic survey allowed the identification of several retaining gas facies and seepages. Between the identified gas facies, the most important, regarding the evolutionary context were: Acoustic Turbidity, acoustic blankets and black shadows as gas retaining facies; the highlighted gas seeps were acoustic plumes, intra-sedimentary Plumes and turbidity pinnacles. Once the patterns were seen, the Bertioga Channel could be split into three sectors: The Strait Sector, Candinho and Eastern Sector. In the Strait, the acoustic blankets were predominant with less variation in elevation (8.6 - 2.6), seepages occurred less compared to other areas and were mostly consisting of turbidity pinnacles, this was also the portion where the highest sedimentation rates were observed. In the Candinho sector, the shallowest facies occurred, black shadows and acoustic blankets about 1 to 2m of the bottom prevailed, with a high occurrence of seepages in the form of acoustic plumes. Finally, the Eastern Sector had the highest changes in the levels of acoustic blankets, with the deepest occurrences (14.0 - 4.3) and seepages in the form of intra-sedimentary plumes. Stratigraphic units were defined, which were different between each sector and contributed to the previous division based on gas occurrences. Four stratigraphic units were identified in the Eastern sector, two in the Candinho sector and only one stratigraphic unit in the Strait sector, once the seismic signal was totally limited by gas blankets in this area. Given these evidences, it was concluded that the Bertioga channel evolved as two distinct systems, which were separated during the marine regressions and reconnected during marine transgressions. The sudden change in the depths of occurrence in acoustic blankets between the Eastern sector and Candinho, and evidence of neotectonic faulting in the Late Pleistocene suggest that the connection of these environments (Strait sector + Candinho and Eastern sector) may have been abrupt.