Eastern Guyana Shelf Margin Stratigraphic Architecture
Abstract
Abstract
The Guyana margin is part of the Equatorial Atlantic continental passive margin. This was affected by two rifting episodes; the first one in Late Triassic-Early Jurassic time associated with the southward propagation of the central Atlantic rifting into the area, and a second rifting episode in the Early Albian inherited from the opening of the South Atlantic, occurring along a pre-existent fracture zone. A subsequent plate rearrangement in the Middle Albian resulted in a counterclockwise rotation of Africa with respect to South America. This rotation induced contraction, uplifting and structural inversion along the entire margin followed up by a period of tectonic quiescence driven by thermal subsidence.
In this tectonic scenario, the shelf, from the Albian to Maastrichtian, was dominated by proximal fluvial-deltaic-to-offshore depositional environments. In this study the Cretaceous-to-Tertiary sedimentary succession of Guyana's shelf is dissected into 9 low-order depositional sequences: sequence 1 (Aptian-Albian) rests unconformably on Jurassic clastic and volcanic rocks. It consists of shales and carbonates (foraminiferal limestone), which show an onlapping-aggrading stacking pattern, and undifferentiated volcanic rocks.
Sequences 2, 3 and 4 are encased in the Berbice paleo-incised valley. This is the largest depositional feature on the shelf, which acted as a conduit allowing sediments to reach deep-water areas. All sequences are bounded by sequences boundaries and a maximum flooding surface with progradational, retrogradational and progradational stacking patterns from bottom up. These are interpreted as lowstand, transgressive and late highstand system tracts respectively. Sequences 2 and 3 consist of fluvial-deltaic-to-shallow marine sandstones and marine mudstones respectively. The latter is equivalent to the Canje Formation, which is a proven source rock in the area. Sequence 4 consists of interbedded sandstones and shales in a prograding pattern.
Sequences 5 and 6 exhibit an agrading-prograding staking pattern. These appear detached from the Berbice; however their depositional environment is similar, a fluvial-deltaic system. Sequences 7 and 8 are characterized by carbonates and mudstones, which are part of a widespread carbonate platform in the area. The top sequence (Pliocene) consists of mudstones and siltstones with a prograding geometry.
AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016