Shallow Marine and Shelf Systems
Shelf deltas are by and large poorly documented in literature. The exception to this is in the Gulf of Mexico where Sutter and Berryhill (1985) developed the original models for shelf versus shelf edge deltaic deposits. Many shelf setting are characterized by more bypass than accommodation, leaving thin to absent regressive shelf deposits. Along many margins of the world, it is simply a matter of focusing accommodation space near the shelf edge that drives the preferential build up of shelf edge deltaic deposits during regression. The North Coast Marine Area is an exception to this trend. Its location along the tectonically active, transtensional/transpressional boundary of the Caribbean and South American plates creates a unique opportunity to build and thus examine these shelf deltaic deposits.
The North Coast Marine Area extends across ~7000 km2 of the northern Trinidad and Tobago shelf in water depths between 50 to 200 meters. In 2009, the North Coast Marine Area had two exploration blocks under active oil and gas exploration with gas production from the North Coast Marine Area totaling ~1.1 TCF since 2002. All natural gas discovered to date in the North Coast Marine Area has been interpreted as biogenic although one previous worker has speculated that a minor component of thermogenic gas is also present. The North Coast Marine Area is located within a complex tectonic environment characterized by oblique strike-slip displacements between the Caribbean and South American plates at a rate of about 20 mm/yr. The main faults of the 200-km-wide plate boundary zone include: (1) the El Pilar right-lateral strike-slip fault zone to the south on the island of Trinidad and the Gulf of Paria which GPS results indicate to be largely inactive; (2) the North Coast fault zone which coincides with the southern boundary of the Tobago basement terrain and appears to be slightly active, showing down-to-the-north, Miocene to recent oblique-slip movements on the North Coast fault zone producing accommodation space for deposition of sediments along the northern shelf of Trinidad and Tobago; and (3) the Hinge Line fault zone crossing through the North Coast Marine Area. The fault systems set up localized zones of transpression and transtension that influence that accommodation and bypass of shelf sediments bound for the distal basin.
In addition, these localized areas of complex faulting and folding provide important structural traps for Pliocene and Miocene gas reservoirs in the North Coast Marine Area north of the Hinge Line fault zone. Growth sequences along the Hinge Line fault zone indicate that the fault activated during the Miocene and continues to up to the late Pleistocene (~500 k.y.) and in some areas forms active scarps on the seafloor.
A seismic stratigraphic study of the area analyzed two Pleistocene fourth-order shelf and shelf-edge stratigraphic sequences deposited over the past ~500 key in the western part of the North Coast Marine Area. New micropaleontologic data tied to a well through the two sequences B and C constrain the initial deposition of each sequence ~450 ky (Sequence B) and ~260 ky (Sequence C). The lithologic well log shows that the sequences are sand, shale, and thin limestone. Seismic interpretation allows division of sequences B and C into eight system tracts which include: (1) lowstand system tracts, (2) transgressive system tracts, (3) highstand system tracts, and (4) falling stage system tracts.
Two lowstand systems tracts in sequences B and C are characterized by delta plain deposition of the Orinoco Delta with a north-eastward terrigenous source direction coming from the western side of Trinidad, through the Gulf of Paria. The falling stage systems tract of sequence C consists of a suite of ~20 to 45-m-high, 0.1°- to 0.25°-inclined, and north-eastward-prograding muddy, shelf deltaic clinoforms marking the paleo-shelf edge. Fault activity on the shelf influences accommodation in Sequence B forming accommodation “sinks” on the shelf that “thieve” sediments making their way to shelf edge locations. The results are shelf deltaic deposits that thicken landward. In contrast, faults appear to have become dormant, showing little subsidence during deposition of the overlying sequence C. These sediments appear to have aggraded in response to rising sea level.
Structural and isochron maps were made for four horizons underlying the northern shelf of Trinidad including top Mesozoic basement, top Miocene, top Pliocene, and seafloor. These maps indicated a change in terrigenous source area for the northern shelf of Trinidad: during the Miocene and early Pliocene, terrigenous sources were coming from the southeast through the Atlantic Ocean; during the mid-Pliocene to present the source area changed to the southeast through the Gulf of Paria. It is possible at the mid-Pliocene sediments became dominated by muds during highstand and falling stage forming unusually muddy clinoforms in the Pliocene to recent shelf packages.