Origin of an enigmatic regional Mio-Pliocene unconformity on the Demerara plateau

Abstract

International audienceThe Demerara plateau, located offshore French Guiana and Suriname, is part of a passive transform continental margin particularly prone to develop slope instabilities, probably in relation to the presence of a free distal border along its steep continental slope. Slope failure occurred at different periods (Cretaceous to Neogene) and shows an overall retrogressive evolution through time. Upslope these failure headscarp, an enigmatic regional Mio-Pliocene unconformity has been discovered through the interpretation of new academic and industrial datasets. The aim of this work is to describe and understand the origin of this surface. Our analysis shows that this unconformity is made of a series of valleys that cross-cut sedimentary strata. Each one of these valleys has a short lateral extent and is closed along two perpendicular directions, which suggests that it could correspond to a highly meandering system, or to some sub-circular depressions. The infill of these features is equivalent to the regional stratigraphic strata found outside the structures, but in a subdued position. This seems to imply that the structures have originated by a local loss of sediments at their base or by sliding processes. Furthermore, these depressions intersect each other through time, while migrating progressively downslope.We discuss a series of hypotheses that try to explain the onset and evolution of these depressions forming the Mio-Pliocene unconformity (Canyons? Slope failures? Contourite moats? Hydrate pockmarks?). Having established that these structures are depressions formed by collapse, and have many similarities with structures recently described in the literature as pockmarks associated with gas hydrate dissolution, we favor this hypothesis.We propose that these hydrate pockmarks form with a mass failure that was triggered by fluid-overpressure development at the base of the hydrate stability zone

A synthesis of the sedimentary evolution of the Demerara Plateau (Central Atlantic Ocean) from the late Albian to the Holocene

“…4) is known from well investigations derived from hydrocarbon exploration (Gouyet, 1988;Mercier de Lépinay et al, 2016) and from ODP Expedition 207 drillings (Erbacher et al, 2004;Mosher et al, 2007). Sedimentary cover along the Demerara Plateau was deposited above an upper Albian regional unconformity (called reflector R4; Pattier et al, 2013;Pattier et al, 2015) that marks the end of the transform activity and corresponds to a sub-aerial erosive surface covered by shell debris that caps deformed upper Cretaceous and older rocks (Gouyet, 1988;Pattier et al, 2013). Post-Albian sedimentation consists of a sedimentary basin made of thick prograding wedges (up to 4 km in thickness on the continental shelf) that progressively thin seawards (Gouyet, 1988).…”

“…This succession records the lithological consequences of the Cretaceous/Tertiary boundary (called reflector R3; Pattier et al, 2013;Pattier et al, 2015) (Ingram et al, 2011;Mosher et al, 2007). Finally, as a probable result of the onset of the Amazon River (and probably the Maroni River), the Pliocene sediments are clay-rich.…”

Meticulous Depiction and Genetic Mechanism of Unconformity Belt Structure

“…Traditional understanding is that due to the uplift of underlying strata, the ground is exposed to water and subject to weathering and denudation (Marsset et al, 2015;Xu, 2015). Through data and discussion of outcrops, numerous structures of unconformity belt, in particular, the two-dimensional structure, does not conform to this deposition process, such as unconformity belts that formed under water or inside the stratum.…”

Description of a contourite depositional system on the Demerara Plateau: Results from geophysical data and sediment cores

“…Since the Oligocene, the external DP has been particularly affected by slope instabilities with a major sediment slope failure headscarp still visible in the bathymetry ( Fig. 3) and probably regularly reactivated (Loncke et al, 2009;Pattier, 2013;Pattier et al, 2013;Pattier et al, 2015). MTDs (Mass Transport Deposit) pile up for a thickness of nearly 600 m in the distal DP (Pattier et al, 2013).…”

“…obtained from boreholes and published in Pattier et al (2013) and Pattier et al (2015). On top of the MTDs, the unit thickness of the bedded sediment is variable.…”