Northern Espírito Santo basin canyon models ancient sand transport

The channel system of the ancient Rio Doce (early Eocene) is dominated by salt, which controls the direction of turbidite flow. Sand rich canyons are higher amplitude: black (+) and red (-).

A second modern canyon system trends north-south and does not have a river feeding sediments into the system. The canyon is growing toward the shelf by headward erosion. Retrogressive slumping of the canyon walls and occasional debris flows from the shelf margin are the primary sources of sediment for this system. An assessment of seismic amplitudes suggests that this system is mostly sand-poor.

Models

Developing models for the transport and deposition of sand into the deepwater environment of various basins around the world is challenging. A model that fits data in one basin may not work in adjacent basins, and large basins with multiple points of sediment influx may require more than one depositional model. The authors propose a north-south strike-fed system for sand deposition in the ancient Rio Doce. This idea contrasts with the more conventional dip-fed systems proposed in the Campos basin.

Using 3D seismic data to interpret the top of salt, erosional unconformities, and important stratal relationships, Cenozoic canyon sequences containing sand-rich channel-levee systems can be mapped throughout the Espírito Santo basin. Older analogs of this modern system can be tracked back into the Cretaceous section of the basin and potentially linked to the ancient Regencia Canyon. The pathways of both modern and ancient canyon systems have been influenced by north-south trending salt structures. Many salt structures were emplaced during the Albian, produced by the onset of autochthonous salt deformation.

Mapping

Unconformity bound seismic mega-sequences from the basement rift to the seafloor in the Espírito Santo basin were mapped using over 10,600 sq km of 3D seismic data. Sequences were then divided into groups that related to the Cenozoic canyon systems (Rio Doce and an east-west canyon system) or the Cretaceous canyon system (Regencia Canyon). Within the shallow Cenozoic section, both seismic resolution and frequency content are much higher than in the Cretaceous. Beds/seismic events down to 5 m in thickness can be resolved. In the deeper Cretaceous section, no beds/seismic events smaller than approximately 18 m thick can be resolved. All the seismic is zero-phase SEG reverse polarity.

Sediments transported down any canyon system can be involved in erosional downcutting, non-erosional bypass, deposition, or some combination of these. The process that occurs depends on eustacy, sediment supply, basin subsidence, and tectonic controls.

For deposition within the Rio Doce Canyon we used a depositional model for infilling a sand-prone canyon system. Turbidity flows transport sand-prone sediments down the canyon and infill it. Lateral migration of fill deposits is caused by increasing deflection of turbidity flows away from the topographic highs of previous turbidite deposits.

In the Espírito Santo basin, the influence of intervening salt structures can be a dominant factor. These structures can confine sediment transport alongside or between the salt ridges. Where this occurs, canyon fill deposits may be stacked. Such intervening salt structures are the major reason this system drains north to south. In several locations sediments do breach the confining salt mastiff, but are subsequently retrapped due to recurrent movement of the salt.

A second modern canyon system (oriented north-south) has no associated river to feed in sediment. Sediment supply must come from a different, mostly non-shelf derived source. The model invokes a series of faults parallel to the canyon walls that allow slumping onto the canyon center. This produces an uplift of sediment in the canyon center where slump blocks collide.

The small uplift produced is above the baseline of equilibrium in the axis of the canyon. It will be eroded by any turbidite flows or the release of any buildup of water on the shelf caused by winds or storms. A model of this type allows the canyon system to grow larger (by headward erosion), but for the most part, it will be a mud-dominated system. Sands that do exist within this system are still exploration targets, but the reserves are usually too small for deepwater economics.

This generalized chronostratigraphy of the Espírito Santo basin shows sand deposition and the growth of channels through time.

Patterns of sediment transport into the Espírito Santo basin have constantly evolved since evaporite deposition in the Aptian. Initially, low to moderate amounts of sediment were fed into the basin from the west. Later in the Cretaceous, there was a reorganization of the coastal drainage patterns to the south in and around the Serra do Mar and the Serra da Mantiqueira coastal ranges (Campos basin).

This reorganization continued through the Eocene, directing sediment northward, and eventually increasing the sediment supply into the Espírito Santo basin. The uplift of these coastal ranges may also have trapped the ancient Rio Doce River into a specific canyon system. With the north-south trending salt ridges already in place offshore, the direction of sediment transport would also be north-south, parallel to the salt ridges.

Dominant controls on the stratigraphic evolution of the Espírito Santo basin include:

* Onset in Albian of autochthonous salt deformation

* Uplift of the Serra do Mar and Serra da Mantiqueira and a reorganization of the coastal drainage patterns starting in the Late Cretaceous

* Late Cretaceous sediment bypass in the area north of the transfer zone

* Late Cretaceous reorientation of canyons close to the shelf slope brake from east-west, following a transfer zone, to north-south when caught between the salt ridges

* Entrapment of the Rio Doce canyon system in the Eocene, delivering coarse grain sediments into deepwater.

In the Eocene and younger section of the Espírito Santo basin, high amplitudes within the canyons probably indicate coarse-grained sediment. Mapping the abrupt lateral changes in amplitude intensity and continuity is the easiest way to track the canyons through the basin. A timeslice one second below the water-bottom shows the sand rich canyon systems as well as some sandy canyon systems.

To identify deeper buried Cretaceous canyons, an interpretation of condensed sequences that bracket the canyons should be completed first. After flattening the base of the condensed section, cutting time slices upward seems to give the best results for tracking these older systems. In the deeper section, bi-directional downlap of reflections on top of the condensed sections is a positive indication of a canyon section being sand prone. Amplitudes in the Cretaceous section may not be a good indicator of coarse-grained sediment. To better identify Cretaceous canyons, the interpreter needs a methodology combining sequence stratigraphy, seismic attribute analysis, and depth imaging.

Acknowledgement

The authors thank CGG Americas for permission to use the seismic data in this study.