Lower Cretaceous

Lower Cretaceous
Name	Lower Cretaceous
Color	#ffcc66
Time start mya	145.0
Time end mya	100.5
Major rock types	Sandstone; Shale; Limestone; Chalk; Conglomerate
Major fossils	Archaeopteryx; Iguanodon; Spinosaurus; Belemnites; Ammonites
Preceded by	Jurassic
Followed by	Upper Cretaceous

Lower Cretaceous The Lower Cretaceous marks the early part of the Cretaceous Period, spanning roughly from 145.0 to 100.5 million years ago and bridging major transitions after the Jurassic and before the Upper Cretaceous. This interval records key faunal turnovers, extensive carbonate and siliciclastic deposition, and tectonic reorganizations tied to the fragmentation of Pangea, with well-studied successions exposed in regions such as Western Europe, North America, Africa, Asia, and Australia.

Definition and Chronostratigraphy

The formal chronostratigraphic subdivision corresponds to the Early Cretaceous epoch recognized by the International Commission on Stratigraphy and subdivided into stages including the Berriasian, Valanginian, Hauterivian, Barremian, Aptian, and Albian. Global correlation relies on biostratigraphic markers such as ammonite zonations anchored by sections in England, France, Spain, Germany, Portugal, and Italy alongside magnetostratigraphy defined in reference sites like the Gubbio interval used since correlations with the Cretaceous Normal Superchron were established. Radiometric ages from igneous tuffs in the Sierra Madre Oriental, Canadian Rockies, Iberian Range, Pyrenees, Andes, and Scotland provide numerical calibration used by the International Chronostratigraphic Chart.

Geology and Paleoenvironments

Sedimentation during the Lower Cretaceous produced diverse lithologies, from chalk deposits of the White Chalk Formation equivalents across England to fluvial sandstones in the Wealden Group of Southern England and lacustrine sequences in the Isle of Wight, Neuquén Basin of Argentina, and Jehol Biota basins of China. Shallow epicontinental seas inundated parts of Eurasia and North America, forming carbonate ramps and platform carbonates analogous to the Badenian patterns, while deeper basins like the Exmouth Plateau and Porcupine Basin hosted organic-rich black shales. Climatic indicators from oxygen isotope records in Foraminifera and distribution of palaeofloras tied to the Greenland–Iberia reconstructions signal greenhouse trends punctuated by events such as the Aptian Oceanic Anoxic Event recorded extensively in the Tethys realm and sections in Italy, Spain, Egypt, Tunisia, and Morocco.

Paleontology (Fauna and Flora)

Fossil assemblages include iconic vertebrates and invertebrates: theropods like Spinosaurus and Velociraptor relatives, ornithopods such as Iguanodon and Muttaburrasaurus, sauropods documented from the Hypsilophodon-bearing units, and early birds related to Archaeopteryx-grade taxa recovered in Germany and China. Marine life yields diverse Ammonite faunas used for biostratigraphy, belemnites in basins from Japan to Argentina, and teleost fishes similar to those from the Santana Formation and Liaoning Province. Plant fossils include bennettitaleans, cycads, ginkgophytes such as specimens comparable to Ginkgo biloba relatives, and the diversification of angiosperms documented in associations like the Yixian Formation and floras of Portugal, Brazil, Australia, and North America. Trace fossils and ichnofacies in the Burgess Shale-age strata are absent here, but dinosaur tracksites in Spain, Portugal, Colorado, and Australia record diverse behaviors.

Major Formations and Regional Sequences

Key stratigraphic units include the Wealden Group and Chalk Group of England, the Cedar Mountain Formation and Benton Group equivalents in Utah and the Western Interior Seaway margins of North America, the Jehol Group and Yixian Formation of China, the Neuquén Group of Argentina, the Aptian-Albian successions in the Aptian Basin of Morocco, the La Amarga Formation in the Neuquén Basin, the Santana Formation and Crato Formation of Brazil, and the Eromanga Basin and Murraba units of Australia. African records include the Kem Kem Beds of Morocco and the Elrhaz Formation of Niger, while Eurasian sequences are well exposed in the Iberian Peninsula, Pyrenees, Carpathians, Transylvania, and the Volga-Ural region.

Tectonics and Paleogeography

Plate reorganization following the breakup of Pangea advanced with the opening of the South Atlantic and expansion of the North Atlantic gateways between Greenland and Iberia, influenced by spreading centers linked to the Mid-Atlantic Ridge. Continental drift produced isolated basins such as the Neuquén Basin, Gondwana fragments like Madagascar and India beginning northward drift, and microplates including the Apulian Plate interacting with the Tethys Ocean margins. Orogenic activity in the Alps, Pyrenees, and early stages of the Andean deformation reworked Lower Cretaceous strata; synrift sequences are recorded in the Saharan basins and the Gabon and Angola margins, while passive margin deposits formed along the Brazilian and West African continental shelves.

Economic Resources and Stratigraphic Uses

Lower Cretaceous rocks host petroleum systems in the North Sea, Gulf of Mexico, Gabon, Angola, Brazil, and Australia where source rocks, reservoirs, and seals occur within formations such as the Kimmeridge Clay-equivalent organic-rich shales and sandstone reservoirs of the Bakken Formation analogs. Coal and lignite seams in the Wealden and Cretaceous basins of China, India, Poland, and Germany supported regional energy resources, while phosphate deposits associated with marine transgressions are economically important in Morocco and Florida. Engineering stratigraphy and sequence stratigraphic frameworks developed from Lower Cretaceous cyclothems provide chronostratigraphic tie points used by the American Association of Petroleum Geologists and national surveys in United Kingdom, United States Geological Survey, Geological Survey of Canada, and Bureau of Mineral Resources-style agencies for exploration and geological mapping.

Category:Cretaceous