Origin of buried, bottom current-related comet marks and associated submarine bedforms from a Paleogene continental margin, southeastern Brazil

Bottom current-related bedforms, such as depositional and erosive features, are useful indicators of oceanographic circulation and sedimentary balance conditions in both modern and ancient marine environments. The Eocene-Oligocene transition in northern Santos Basin is seismically expressed as a prominent, high-amplitude reflector (horizon H8.1) that truncates Eocene prograding clinoforms and records a major transgression in the basin associated to global and local environmental changes. This surface is punctuated by circular depressions, elongated depressions, mounds and conic features, which, based on seismic interpretation, are associated with the escape of gas on the paleoseafloor. Elongated negative features with a head and an erosive tail (comet marks) are associated with the erosive action of bottom currents that, when encounters an obstacle or bottom irregularity such as pockmarks, create a downflow, linear furrow (tail). The occurrence of carbonate mounds and mud volcanoes may affect the erosive power of the flow by deflecting and/or focusing undercurrents. In the study area, parallel to subparallel comet marks occur abundantly on the former outer shelf and upper slope, being up to 1.2 km long, 200 m wide, and 30 m deep. These dimensions allow the classification of such bedforms as giant comet marks, which form as a result of erosion of the irregular sea floor by currents as fast as 0.6 m s− 1. The strong alignment of the comet tails indicates paleocurrents to the NE on the Oligocene outer shelf and upper slope. Conditions for comet mark formation include the development of pockmarks due to gas seeps, reworking of pockmarks by vigorous and long-lived bottom currents and low sediment supply during the Eocene-Oligocene transition, a period marked by the opening of the Drake Pass and the establishment of undercurrents through the connection of the Atlantic and the Pacific oceans. Bottom-current related bedforms on H8.1 in Santos Basin may thus be related to such paleoenvironmental changes. Although no pre-Quaternary comet marks have been reported before, their identification and mapping through 3D seismic data is a unique opportunity to assess paleoenvironmental conditions and paleo-circulation on continental margins.