2 - 1 Cretaceous Survey Flashcards
Global sea level in Mesozoic (1.2.2)
Eustatic sea level rose from E Jurassic (50m below present) to a maximum in L Cretaceous (250m above present).
Cause: rifting of Pangaea led to new oceans and greater length of mid ocean ridge.
Mesozoic climate (1.2.3)
Warming from Permian Ice Age through Mesozoic, with greenhouse state continuing into the Oligocene (c. 33Ma).
Evidence from: tillites, coals, evaporates, fossils of climate-sensitive organisms, oxygen isotopes, clay mineral assemblages.
Mesozoic biodiversity (1.2.4)
End of Permian: mass extinction (96% of species).
Despite mass extinction at end of Triassic, biodiversity increased through Mesozoic to unprecedented levels. Mass extinction at end of Cretaceous (c. 65Ma). Biodiversity today is higher.
Dinosaurs through Mesozoic, birds from Jurassic, angiosperm from E Cretaceous. Calcareous plankton from L Cretaceous.
Palaeogeography of Mesozoic (1.2.1)
Beginning (250Ma) Pangaea surrounded by Panthalassa Ocean, with eastern wedge Palaeotethys.
E Jurassic (200Ma) Pangaea begins to rift.
L Jurassic (150Ma) Tethys splits Pangaea into Laurasia and Gondwanaland.
Cretaceous: Atlantic Ocean spreads and Indian Ocean grows as India moves north (collides with Asia in Tertiary). Arctic Ocean small and enclosed with no deep water contact.
How was the Cretaceous period different to the present? (1.1)
Higher sea level (chalk, oxygen isotope, 250m).
Warmer climate (greenhouse, Arctic slope Alaska).
Carbonate production (Istria rudists).
Oceanic Anoxic Events (dark platy limestone e.g. Kasserine Tunisia, restricted deep flow).
Volcanism (Allison guyot, superplume).
How does the Cretaceous period fit into the context of the Mesozoic era? (1.2)
Rifting continues: Laurasia and Gondwanaland split with growth of Atlantic and Indian Oceans, while Tethys closes.
Eustatic sea level rises as rifting continues, to maximum in Late Cretaceous (250m).
Climate continues to warm, reaching maximum in Late Cretaceous.
Greenhouse Permian to Oligocene (33Ma).
Biodiversity increases from Permian extinction (96%), mass extinction 65Ma.
How do we date the Cretaceous period? (1.3)
Relative and absolute dating distinct.
Relative: 12 stages defined by basal boundary stratotype and correlated out through biostratigraphic zones (8.8Ma Aptian: 8 ammonite zones), chemostratigraphy (87Sr/86Sr, 13C/12C), magnetostratigraphic chrons, cyclostratigraphy (Milankovitch).
Absolute: radiometric dating of K40-Ar40, Ar40-Ar39, Rb87-Sr87, U238-Pb206. Range to 95% confidence.
What is the evidence of palaeogeographic changes? (2)
Geophysical: palaeomagnetism and magnetic chrons.
Fossil distribution: Milner et al 2000 (abelisaur and titanosaur). Marine fauna either side of Caribbean.
Lithostratigraphy: ODP N. W. Iberia, Scandinavia. Cross section Brazil - W Africa. Rudist limestones central Tethys e.g. Istria.
Complexity: Metcalfe 1988 (S. E. Asia, terranes). Hay et al 1999 (S Atlantic).
Rock texture: syn-rifting sediment HS9 derived from sediment, poor drawdown.
What were the palaeogeographic changes? (2.2)
N Atlantic unzip north (ODP).
C Atlantic shallow connection to Pacific through Caribbean (shared marine fauna).
S Atlantic parallel (Hay et al 1999) though isolated from C Atlantic by Brazil - W Africa.
Indian Ocean grows though promontories based around Kergeulen Ridge.
Pacific seismic active: chrons, superplume (mid Pacific Ridge, Allison Guyot etc).
Central Tethys shallow (Istria rudists): constriction.
North Tethys subduction and terranes.- Tethys closes as Africa rotates around Iberia and moves northwards.
What are the implications of these palaeogeographic changes upon the Cretaceous Earth system? (2.3)
CO2: weathering (drawdown): few mountains, syn-rifting sediments but mostly mineralogically mature, carbonates widespread, volcanism.
Distribution of heat: incursion of oceans into continents - maritime climate. But deep circulation limited (Caribbean, Brazil, central Tethys, S America tip, N Atlantic not reach Arctic), leading to OAEs. Possible source hypersalinity in Atlantic (Hay et al 1999).
Chemistry: rapid extensive spreading much hydrothermal circulation so enrichment of calcium and depletion in magnesium.
Albedo: increased ocean cover with high eustatic sea level, distribution of oceans shifts to higher latitudes. Little impact of mountains, though cloud-forming aerosols from volcanoes.
How do we measure eustatic sea-level change?
- Hypsometric curves. Continental relief has changed in time. Erosion of marine deposits. But good indication of relative.
- Correlation of tectonically stable areas, difference in present and depositional height. But subsidence e.g. Western Interior due to Farallon Plate subsidence: 1000m a.s.l. Similar uplift due to hot buoyant mantle e.g. S Africa.
- Guyots give depth, plus floor sinking at known rate. But mantle plumes common in Cretaceous cause renewed uplift.
- Hays and Pitman (1977) calculated displacement from spreading measured with chrons. Consistent with hypsometric. But Tethys ridge destroyed so cancel out with Indian.
- Vail et al (1977) argue that RSL cycles which correlate across areas can be assumed eustatic. Produced eustatic chart since updated (Haq et al 1988). But bias in oil data towards passive margins so would experience similar processes. Also difficult to correlate due to resolution. Recent charts Europe only. Nevertheless good for relative.
What do we know of eustatic sea-level in the Cretaceous?
- First order: the Great Cretaceous Transgression, peak Turonian. Height initially thought 500m (1973) but revised down to about 250m (Haq et al 1988). Still above 60m so not just melting of cryosphere.
- Shorter term oscillations proposed by Vail et al (1977) more problematic. Smaller amplitude could be local. Resolution not good enough, e.g. 8 ammonite zones in Aptian (8.8Ma) = 1.1Ma per cycle, far above most recent Pleistocene cycle 120ka. Study of 46m cycles in S Appenines found regular periodicities consistent with Milankovitch.
Relative ages of the Cretaceous (1.3.1)
Berriasian Valanginian Hauterivian Berremian Aptian Alban Cenomanian Turonian Coniacian Santonian Campanian Maastrichtian
