Lusitanian Basin
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The Lusitanian Basin is a
rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...
basin located on both the mainland and continental shelf off the west-central coast of
Portugal Portugal, officially the Portuguese Republic, In recognized minority languages of Portugal: :* mwl, República Pertuesa is a country located on the Iberian Peninsula, in Southwestern Europe, and whose territory also includes the Macaronesian ...
. It covers an area measuring and extends north-south from
Porto Porto or Oporto () is the second-largest city in Portugal, the capital of the Porto District, and one of the Iberian Peninsula's major urban areas. Porto city proper, which is the entire municipality of Porto, is small compared to its metropo ...
to Lisbon. The basin varies between approximately and in width and belongs to a family of periatlantic basins such as the Jeanne d'Arc Basin. To the east of the Lusitanian Basin lies the Central Plateau of the
Iberian Peninsula The Iberian Peninsula (), ** * Aragonese and Occitan: ''Peninsula Iberica'' ** ** * french: Péninsule Ibérique * mwl, Península Eibérica * eu, Iberiar penintsula also known as Iberia, is a peninsula in southwestern Europe, defi ...
. A marginal horst system lies to the west. The
Alentejo Alentejo ( , ) is a geographical, historical, and cultural region of south–central and southern Portugal. In Portuguese, its name means "beyond () the Tagus river" (''Tejo''). Alentejo includes the regions of Alto Alentejo and Baixo Alen ...
and Algarve Basins connect to the southern end of the Lusitanian Basin. In the north, it connects to the Porto and Galicia Basins via an undersea
ridge A ridge or a mountain ridge is a geographical feature consisting of a chain of mountains or hills that form a continuous elevated crest for an extended distance. The sides of the ridge slope away from the narrow top on either side. The line ...
.


Tectonic history

The Lusitanian Basin results from the opening of the North Atlantic Ocean due to
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Creta ...
extension. Rifting was initiated between
Newfoundland Newfoundland and Labrador (; french: Terre-Neuve-et-Labrador; frequently abbreviated as NL) is the easternmost province of Canada, in the country's Atlantic region. The province comprises the island of Newfoundland and the continental region ...
and Iberia and occurred primarily in two phases. The initial phase occurred in the
Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch ...
–earliest Jurassic followed by an episode of faulting and basin subsidence during the early to
middle Jurassic The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations ...
south of the Nazare Fault. The second phase of rifting occurred in the
late Jurassic The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987. In European lithostratigraphy, the ...
—extension shifted from a wide-rift mode that encompassed future proximal margins, and it focused at distal margins where continental crust eventually. Throughout the complete formation of the basin, from the Late Triassic to the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
, four phases of rifting can be defined. This first phase occurred during the Late Triassic to the Early Jurassic and is characterized by symmetrical
graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...
s and
half-graben A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults. Rift and fault structure A rift is a region where the lithosphe ...
s and is concentrated in the central areas of the basin. During the
Early Jurassic The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma  ...
, the second phase is characterized by minor salt movements along most substantial faults and occurs south of the substantial, centrally-located Nazare fault. The third phase is initiated during the beginning of the Late Jurassic. This phase is defined by the
onlap Onlap or ''overlap'' is the geological phenomenon of successively wedge-shaped younger rock strata extending progressively further across an erosion surface cut in older rocks. It is generally associated with a marine transgression. It is a more ge ...
ping of the Oxfordian succession on salt pillows that formed in association with fault activity. In between the third and fourth phases, there is a major period of tectonic dormancy. This is most likely due to the opening of the Central Atlantic. During this time, in the Late Jurassic, there is widespread fault-controlled subsidence. The final phase, initiated between the latest Late Jurassic and the earliest Early Cretaceous, triggered salt movement and the development of salt structures. Extensional tectonics ended during the Early Cretaceous.


Structural history

The most significant faults and salt structures throughout the Lusitanian Basin trend north-northeast, parallel to the elongation of the shoreline. The faults that trend northeast to east-northeast are fewer in number. However, they are fairly substantial—for example, the Nazare Fault. The north-trending faults are concentrated mainly in the central part of the Lusitanian Basin. These faults are a part of in important structural trend within the Estremadura Trough which highlights the Oxfordian extension within the basin. West-northwest trending faults are scattered throughout the basin. Both
thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...
s and normal faults are observed. The thrusting observed is resultant of the basement-attached inversion movements of the pre-existing normal faults during the
Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recent" ...
. Most of the faults within the basin are fairly high-angle, with some shallowing with depth observed. There is always faulting beneath salt structures and it is speculated that the movement of the salt structures is caused by basement-influenced faulting.


Divisions of Basin

Resultant of diapirism leading to the formation of salt pillows, the Lusitanian Basin can be divided into seven different sub-basins: * Southern Lusitanian Basin (SLB) * Central Lusitanian Basin (CLB) * Northern Lusitanian Basin (NLB) * Arruda Subbasin (ASB) * Turcifal Subbasin (TSB) * Bombarral Subbasin (BSB) * Monte Real Subbasin (MRSB)


Paleontology

The Lusitanian basin Triassic to Cretaceous rocks provided thousands of fossils, from plants, microfossils, invertebrates and vertebrates. The most productive formations are the Late Jurassic Lourinhã Formation, Alcobaça Formation, and Montejunto Formation and the Cretaceous Papo Seco Formation. This includes an outstanding abundance of Jurassic mammals and bones and tracks of dinosaurs.


Regional stratigraphy

The Lusitanian Basin is a late Triassic rift basin is filled with synrift siliciclastics and capped by post-rift
evaporite An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as oce ...
s. In the late Triassic, there was deposition of
fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluv ...
sand and clay that eventually evolved into deposition of shallow marine dolomites during the lower and middle Jurassic. These sediments are known as the Silves, Dagorda, and Coimbra Formations. Deposition of Carbonates of the Brenha and Candieros Formations are in shelf, ramp, and sub-marine fan environments and filled the basin during the early and middle Jurassic. The carbonate formations are commonly interbedded with shale and there are local
turbidite A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing Turbidites w ...
beds also present. Below the halite-bearing evaporites that compose the Dagorda formation are the synrift continental siliciclastics of the Silves formation. Above the Dagorda formation is the post-rift carbonate shelf environment—the Coimbra dolomite, Brenha
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms wh ...
, and Cardieros carbonate
grainstone Under the Dunham classification (Dunham, 1962) system of limestones, a grainstone is defined as a grain-supported carbonate rock that contains less than 1% mud-grade material. This definition has recently been clarified as ''a carbonate-dominat ...
. A substantial
unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...
, associated with significant tectonic activity, characterizes the top of the sequence. The carbonate shelf environment is still present in the Upper Jurassic—characterized by the Montejunto grainstone and reef facies, Cabacos organic-rich limestone (capped by
anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with t ...
). Above this sequence, deposition is dominated by the siliciclastics from the Meseta highlands. The Abadia formation is composed of shale, marl, siltstone, and minimal sandstone. The rest of the Upper Jurassic and Cretaceous is dominated by westward-prograding continental clastics.


Hydrocarbon exploration

In the basin, all Jurassic source-rocks could possibly be within the
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
generation window. However, this is not the case throughout the entire basin. This is due to the highly heterogeneous nature of basin subsidence, especially in the Late Jurassic. Around 100 exploratory wells have been drilled in the basin. 80% of these wells, from Triassic, Jurassic, and Cretaceous strata, had oil and gas shows while drilling and 27% recovered live oil or gas at surface. While there is substantial hydrocarbon potential in this basin alone, Portugal imports 100% of its fossil fuel. There are two major petroleum systems at work within the basin—Subsalt and Suprasalt. In the subsalt petroleum system, there are Paleozoic source rocks and synrift Triassic
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...
reservoirs that are sealed by Dagorda evaporites. In the suprasalt system, the oldest source rock is the Lower Brenha formation. The best reservoirs to mention include the Coimbra dolomite and Upper Dagorda carbonates, Candieiros oolitic and bioclastic grainstones, and the fractured carbonates of the Brenha formation. Seals are tight or marly carbonates of the Brenha and overlying basal Upper Jurassic. N. Pimentel1 and R. Pena dos Reis (2016) - PETROLEUM SYSTEMS OF THE WEST IBERIAN MARGIN: A REVIEW OF THE LUSITANIAN BASIN AND THE DEEP OFFSHORE PENICHE BASIN. Journal of Petroleum Geology, Vol. 39(3), July 2016, pp 305-326. {{Reflist Sedimentary basins of Europe Geology of Portugal Mesozoic rifts and grabens