Paper Number: 3720
A Late Cretaceous mixed-drift system on the Uruguayan Margin: sedimentary and paleoceanographic significance
Creaser, A.1, Hernández-Molina, F. J.1, Badalini, G.2, Thompson, P. 2, Soto, M. 3, Walker, R.2 Tomasini, J.4
1Dept. Earth Science, Royal Holloway U. London, Egham, Surrey, UK (Adam.Creaser.2013@live.rhul.ac.uk)
2BG Group Exploration, 100 Thames Valley Park, Reading, UK
3Instituto de Ciencias Geológicas, Facultad de Ciencias, Iguá 4225, 11400 Montevideo, Uruguay
4ANCAP, Exploración y Producción, Paysandú s/n esq. Av. Del Libertador, Montevideo, Uruguay
Down-slope (gravity-driven) and along-slope (bottom-currents) sedimentary processes have been proven to be fundamental forces in controlling the morphology of continental margins [1]. While both processes may be isolated, spatially and temporally, they often coexist resulting in mixed (turbidite-contourite) systems. The consequences of these interactions, and resulting deposit geometries and internal stacking characteristics are poorly documented [2]. Using 2D and 3D seismic data from the Uruguayan margin, this presentation aims to offer insights into the evolution of a Late Cretaceous mixed-drift system across the Pelotas Basin of the Uruguayan Margin.
Four large (>25 km wide and up
to 100 km long) asymmetrical drifts have been identified propagating
obliquely from the middle-slope, before terminating on the lower slope.
Drifts have over 1 km of local relief over adjacent depressions, which
amalgamate towards the SE following underlying basement topography.
Internally, drifts are characterised by a series of convex-up, low to
medium amplitude reflections which prograde away from the slope, and
prograde/aggrade laterally towards the southwest. Basal reflections can
be traced across multiple drifts, though later, these terminate into
adjacent depressions or pinch-out against internal discontinuities.
Along lee slopes (SW), slumps displace sediment down into depressions,
prior to subsequent reworking by gravity flows.
Three stages can be recognised: an early Pre-drift stage is dominated by down-slope systems fed from the NW and SW, and ponding within inter-basin depocentres. Throughout the growth stage, down-slope systems remain active, though prolific growth (< 1 km) of drifts occur following a progradation of drifts towards the SE, prior to a lateral aggradation of drifts during later stages. The burial-stage is characterised by draping of reflections over underlying drift-topography, following the termination of turbidite activity and the dominance of the along-slope system.
Sediment availability appears fundamental in controlling the growth of channel-drifts. Prominent growth coincides with highly active down-slope systems and deltaic progradation along the shelf, correlating with the initiation and intensification of ocean circulation following the opening of tectonic gateways (e.g. Falkland-Agulhas Seaway, Walvis Ridge/Rio-Grande Rise). Bottom current activity during the Cretaceous Thermal Maximum controlled the morphology of subsequent deposits, forming mixed-drift system rather than a traditional channel-levee system. Similar characteristics have also being documented across the present-day the Antarctic Peninsula Pacific margin [3].
References:
[1] Rebesco, M. et al. (2014), Marine Geology, v. 352, p. 111-154.
[2] Mulder et al. 2008. In: Contourites, Developments in Sedimentology Vol 60, Elsevier, 435 – 456
[3] Rebesco et al. 1996. Geo-Marine Letters (1996) 16: 65-75