The Casma Group ( es, Grupo Casma) is a

stratigraphic group In geology, a group is a lithostratigraphic unit consisting of a series of related formations that have been classified together to form a group. Formations are the fundamental unit of stratigraphy. Groups may sometimes be combined into superg ...

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 ...

(

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/ Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± ...

to Cenomanian) sedimentary formations exposed along the coast and within the Cordillera Occidental near

Casma Casma is a city in the coastal desert of Peru, located northwest of Lima. It is the capital of Casma Province and the third most populous city in the Ancash Region Ancash ( qu, Anqash; es, Áncash ) is a department and region in northern P ...

Peru , image_flag = Flag of Peru.svg , image_coat = Escudo nacional del Perú.svg , other_symbol = Great Seal of the State , other_symbol_type = Seal (emblem), National seal , national_motto = "Fi ...

Description

The sediments of the Casma Group reflect deposition in two distinct environments of the West Peruvian Basin. Some sediments were deposited near the

Casma Volcanic Arc Casma is a city in the coastal desert of Peru, located northwest of Lima. It is the capital of Casma Province and the third most populous city in the Ancash Region with an estimated population of 29,343 (2015). It is located in the lower Casma ...

in an intra-cratonic

sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence ...

that was occasionally connected to the ocean. Other sediments were deposited in what was once a

continental platform In geology, a platform is a continental area covered by relatively flat or gently tilted, mainly sedimentary strata, which overlie a basement of consolidated igneous or metamorphic rocks of an earlier deformation. Platforms, shields and the baseme ...

bounded by

reef A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic component, abiotic processes—deposition (geology), deposition of ...

s and with anoxic pockets.Compare et al, 1983 Together with the

Morro Solar Morro Solar (''morro'' in Spanish language, Spanish means: a rounded promontory) is a headland, situated in the district of Chorrillos District, Chorrillos, to the south of Lima, Peru. Morro Solar is notable for being a site of the Battle of San J ...

and Imperial Groups, the Casma Group contains

clastic Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rock ...

volcanosedimentary material derivative of the Mesozoic

.Pfiffner & González, 2013 The folding of the Casma Group sediments is the result of the Cretaceous Mochica Phase of the

Andean orogeny The Andean orogeny ( es, Orogenia andina) is an ongoing process of orogeny that began in the Early Jurassic and is responsible for the rise of the Andes mountains. The orogeny is driven by a reactivation of a long-lived subduction system along ...

. The group is

intruded Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: March ...

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

s of the

Coastal Batholith of Peru The Coastal Batholith of Peru ( es, Batolito costero peruano) is a group of hundreds, if not thousands, of individual plutons that crop out near or at the coast of Peru. The batholith runs a length of ca. 1600 km. Most of the plutons of the ba ...

. The Casma Group include the following lithologies:

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and dr ...

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 ...

, marl,

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

lava flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

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 ...

and

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** C ...

References

Bibliography

* * {{Major South American geological formations Geologic groups of South America Geologic formations of Peru Lower Cretaceous Series of South America Upper Cretaceous Series of South America Cretaceous Peru Cretaceous volcanism Albian Stage Cenomanian Stage Limestone formations Marl formations Sandstone formations Conglomerate formations Tuff formations Geography of Ancash Region Geography of Lima Region