Generalized
Stratigraphy
Guyana lies in the northern
part of the Guiana Shield which includes Guyana, Suriname and French
Guiana, and parts of Brazil and Venezuela. The rocks and sediments
of Guyana can be subdivided into four groups on the basis of their
ages.
The Precambrian rocks
(GSC) north of the Takutu Basin (TB)
include folded metasedimentary rocks and metavolcanic rocks as well
as coarse- and fine-grained sedimentary rocks with intercalations
of volcanic rocks. Intrusive bodies occur within the folded strata.
Intrusive rocks, volcanic
rocks and folded metasedimentary rocks comprise the Guiana Shield
Complex south of the Takutu Basin. Mylonitized zones within high
grade metamorphic rocks in the region have been related to an Upper
Proterozoic tectono-thermal event.
The intrusive (granitoid)
Precambrian rocks north and south of the Takutu Basin can be differentiated
on the basis of their mineral assemblages. The granitoid rocks south
of the basin are petrologically similar whereas those north of the
basin are rather variable in composition.
The Roraima Supergroup
(RSG), consisting of mainly continental
sedimentary rocks with interbedded volcanic rocks, unconformably
overlies Precambrian rocks and is intruded by sills and dykes. The
rocks of the Roraima Supergroup were previously assigned to the
Paleozoic Era. However, recent studies have shown that these deposits
are Precambrian in age.
Basic dykes of Mesozoic
age intrude the Precambrian bedrock north and south of the Takutu
Basin. These rocks are not shown on the accompanying map because
of their limited size and areal extent.
The Takutu Basin (TB)
is part of a graben that has been infilled mainly with sedimentary
rocks and minor volcanic rocks of Jurassic to Cretaceous age in
the lower part of the sequence. The thickness of the infill sequence
has been estimated at about 7,000 m. Evaporite deposits
and hydrocarbons have been reported from the basin.
The Cretaceous to Recent
Coastal Deposits (CD) consist of alternating
sands and clays of freshwater and marine origns. In some areas,
these sediments are underlain by alluvial and residual bauxite and
kaolin that, in turn, are underlain by Precambrian bedrock. Thin
peat layers are present in the upper part of the succession.
Selected
Mineral Deposits
Recent geological investigations
on the Guiana Shield have identified significant deposits of gold
in Guyana (Omai deposit) and in Venezuela (Las Cristinas deposit).
The auriferous deposits occur in Precambrian greenstone belts similar
to the Archean greenstone belts of the Canadian Shield.
In Guyana, primary gold deposits are commonly found near the margins
of granitic to dioritic stocks that intruded the greenstone belts.
Fault zones, shear zones and tectonic lineaments within the greenstone
belts are also potential areas of gold deposits.
The mining of gold and
diamond associated with placer deposits is common in the Essequibo
region. The source of the alluvial diamonds has not been established.
Bauxite,
associated with the Coastal Deposits, is mined at Linden, Aroaima
and Kwakwani. The bauxite was formed from the underlying Precambrian
bedrock during a mid-Tertiary weathering episode.
Petroleum
Exploration
Seismic surveys have
been conducted on the continental shelf and in the Takutu basin.
A small number of wells were drilled in both the offshore and onshore
areas on the basis of the seismic surveys. There has been renewed
interest particularly in the offshore area since 1997 and drilling
of a turbidite-fan target which was set to commence on June 3, 2000
was halted as a result of the maritime
boundary dispute with Suriname. |
