Echo-character identification and grain size analysis of the upwelling system of Cabo Frio, RJ

Abstract

The acoustic patterns variation can be used to understand the sediment distribution extrapolating the information to places where is not possible to take any sample. Many authors already use this integrated method to investigate interest areas in, both, commercial and scientific aspect. In this direction the current study initiated in 2009 in the frame of the Geochemistry Network PETROBRAS/CENPES, aims to track the quaternary evolution of a muddy bank situated at the coastal western boundary upwelling system (WBUS) off Cabo Frio (23°S) in the Atlantic Ocean. The WBUS is located at the north coast of the Rio de Janeiro State, Brazil. Here the coastline orientation differs from the default East-West direction of the Brazilian coast, concomitant with the boundary of the Campos and Santos Basins, the most important oil and gas basins of the country. The upwelling phenomenon occurs due to constant northeast winds. The same winds combined with the Northeast-Southwest coastline orientation also promote a curl confluence making the environment even more complex. Seismic data was acquired in cruises on board the RV Ocean Survey. Altogether 680 km was covered by a 3.5 KHz sub-bottom using the Geopulse Geoacustic System. Further, four piston cores were collected to be scan and described in physic parameters and sedimentological content, respectively. The scan was performed using a Geotek Multi Sensor Core Logger (MSCL), which measure and register the Vp, gamma density and porosity of the core. After the scan, the cores were sample and analyzed on sedimentological grain size distribution determined by a Cilas 1064 laser analyzer. The results were integrated in order to identify common trends. Three major echo-characters were described in seismic records. Grain size analysis presented a good relationship with these echos. Echo A) Transparent, represented by a sharp bottom reflector with no sub-bottom reflectors scatter of a transparent layer mapped in seismic records. The grain size distribution is more balanced with a slight higher content of sands (55 %) in the correspondent core. Echo B) Stratified described as a series of parallel sub-bottom horizons, its percentage of clays is the highest of the three cores (77 %). The last echo, C) Reflexive, is a prolonged echo with one or none sub-bottom reflector. This echo is related to the highest percentage of sands of the three cores (61%). The sediment analysis was extrapolated all over the echo-characters extension to understand the transport regime over the area. This criteria, allied to isopach map, made possible to recognize three distinct zones inside the muddy accumulation.