Giant Cretaceous Mixed Contouritic-Turbiditic Systems, Offshore Uruguay: The Interaction Between Rift-Related Basin Morphology, Contour Currents and Downslope Sedimentation

Abstract

The Cretaceous succession offshore Uruguay records the interaction between along- and down-slope processes with the underlying rift-controlled basin topography. A recently acquired 3D seismic survey, covering 13500km², images a spectacular array of sedimentary features including, in stratigraphic order, large sedimentary waves, giant sediment drifts and mixed turbiditic/contouritic systems. The sediment wave packages are related to two discrete sediment entry points. In planform the packages are approximately 30km wide and comprise a series of curvilinear ridges elongated parallel to the palaeo-slope; in dip section the packages are relatively tabular (c. 500m thick) but are internally composed of a series of low-angle clinoforms with down-stepping geometry, originating an apparent progradation. They are linked to an updip, thick prograding unit that delivered sediments into the basin until the Late Cretaceous. Above the sediment wave package a series of downdip converging, giant slope-attached sediment drifts can be traced along the continental slope for over 125km. The package exhibits a ‘pinch and swell’ architecture along strike with thicknesses varying between 100m in the moat regions to over a kilometer at the drift crest. Individually the drifts have wavelengths up to 45km and can be traced for over 100km perpendicular, and oblique, to the palaeomargin. Internally, the drifts comprise a series of packages that migrate from north to south, creating a series of laterally shifting depocentres. The location of these drifts is directly related to the interaction of a southwards flowing current with underlying basement topography. Enhanced drift development and clustering occurs up-current from basement highs (a topographic backstop). Horizon terminations on the stoss side of the drifts suggest that sediments were eroded and reworked within the moat region. Whilst the drifts are dominantly acoustically transparent the moat regions tend to have the highest amplitudes which are inferred to represent coarser grained material, transported downslope and reworked by currents. Downslope flows exploited the moat regions and subtle topographic lows within the drift, forming linear channel systems - some with local overspill – and lobes. Many of these features, which are only observed due to the size and quality of the dataset, pose questions about our fundamental understanding of margin morphologies and bedform development in the deep marine environment.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016