Lower Cretaceous

Chronology and scope

Measured on a modern geological timescale, the Lower Cretaceous begins after the end of the Jurassic and ends at the base of the Upper (or Late) Cretaceous. In absolute terms it extends from about 145 to 100.5 million years ago, a duration that exceeds several entire geological periods. Because the Cretaceous is subdivided differently in various parts of the world, the term Lower Cretaceous is often used in a regional sense where standard global stages are not emphasized. For background on time divisions see geological timescales and the broader Phanerozoic framework; the epoch sits within the eon that hosts abundant fossil life.

Characteristic rock groups and depositional environments

The most classic and well-studied exposures of the Lower Cretaceous occur in southern England. There the sequence is commonly divided into a lower set of freshwater to marginal-marine deposits and an upper set of marine sediments. The lower unit, historically called the Wealden, consists mainly of sandstones and mudstones laid down in fluvial plains, deltas and coastal lagoons; these reflect river-dominated, often brackish or freshwater conditions. The overlying Greensand formations are generally marine sandstones with an olive-green tint from glauconite, and these pass upward into the Gault Clay — a stiff, blue-gray clay deposited in quieter, relatively deeper shelf waters.

• Wealden (Lower): fluvial and lagoonal sands and clays, rich in plant and dinosaur remains.
• Greensands (Lower to middle): glauconitic sandstones formed in shallow marine settings.
• Gault Clay (Upper of the Lower Cretaceous): quieter, low-energy marine clay indicating deeper water.

Sea-level change and paleogeography

A defining feature of the Lower Cretaceous is the major stepwise rise in global sea level recorded at the boundary between the lower (Wealden-dominated) and upper (Greensand/Gault) parts of the series. Areas that had been large river deltas and coastal plains were progressively transgressed by shallow seas as shorelines moved inland. By the succeeding Upper Cretaceous, sea level reached some of its highest points in the Mesozoic, producing widespread continental shelf deposition and organic-rich marine sediments.

Life, significance and examples

Fossils from the Lower Cretaceous document a mix of terrestrial and marine ecosystems. Dinosaur remains, plant fossils and freshwater invertebrates are abundant in fluvial sequences like the Wealden, while marine invertebrates (ammonites, bivalves), early teleost fishes, and microfossils appear in the Greensand and Gault. The interval is important for understanding the early evolution of several modern groups and the responses of ecosystems to sea-level and climatic shifts. Classic localities that illustrate these changes provide teaching and research exposures widely cited in geological literature.

Notable distinctions and study history

The term Lower Cretaceous has a long usage in regional geology and helped define sea-level interpretations in the 19th and 20th centuries. It contrasts with the Upper Cretaceous in depositional style, fossil content and water depth. When consulting modern stratigraphic schemes, the Lower Cretaceous may be correlated with numbered global stages; specialists combine lithostratigraphic, biostratigraphic and chronometric evidence to refine correlations across continents.

Further reading and resources

• Cretaceous period overview
• Cretaceous stratigraphy
• Absolute time and radiometric dating
• Southern England stratigraphic examples
• Global sea-level change studies
• Sediment sources and transport
• Fluvial system reconstructions
• Clay minerals and depositional settings
• Coastal depositional environments
• Lagoonal facies
• Greensand composition and properties
• Marine depositional processes
• Calm-water sedimentation
• Upper Cretaceous contrasts
• Wealden paleoenvironments
• Nearshore facies mapping
• Continental shelf deposits
• Mesozoic sea-level maxima