145.5 – 65.5 MILLION: CRETACEOUS

100 million years ago, Pangaea broke apart. The Atlantic Ocean poured in between Africa and the Americas. India broke away from the African continent, and Antarctica and Australia, still connected above sea level, were stranded near the South Pole.

Laurasia splits into North America and Eurasia;
Gondwana fractures into Africa, South America, Antarctica/Australia, and India (an island continent);
Shrinking Panthalassa becomes Pacific Ocean;
Atlantic Ocean forms as Tethys continues to shrink;
Climate warm and humid world-wide with vast forests;
Broad shallow continental seas stabilize temperatures;
Dinosaurs – in wide variety – dominate the land;
Birds proliferate as pterosaurs decline;
Flowering plants (angiosperms) appear & flourish with help from new insects;
Calcium-shelled marine life leave vast chalk beds;
Era ends when asteroid impact brings mass extinction;
Chicxulub impact might have triggered the enormous Poladpur, Ambenali, and Mahabaleshwar (Wai Subgroup) lava flows in the Deccan traps.

The **Cretaceous**, derived from the Latin “creta” (chalk), usually abbreviated K for its German translation Kreide (chalk), is a geologic period and system from circa 145 ± 4 to 66 Ma (million years ago).

Name	Diameter (km)	Age (megayears)	Dating method	Morphological type	Notes
Miami Florida Crater/Structure	~1	~140	After Florida Plateau deposition	IMPROBABLE Complex?	Underwater off Miami
Hotchkiss Structure, Alberta	3.9	120-330	Dated unconformity	PROPOSED Complex	Seismic research
Carswell, Alberta	39	115 ± 10	⁴⁰Ar/³⁹Ar	CONFIRMED Peak ring	Multi-ring impact?
West Hawk, Manitoba	2.44	100	Geological dating	CONFIRMED Simple	No reliable age
Deep Bay, Saskatchewan	9.5	99 ± 4	Geological dating	CONFIRMED Simple Flat-floored	Cretaceous sediments
Kentland, Indiana	~13	>97	Geological dating	CONFIRMED Complex	Quarry
Steen River Albera	25	91 ± 7	K-Ar pyroclastic vesicular rock	CONFIRMED Complex	Magnesioferrite MgFe₂O₄
Howell Creek, BC	~10	90-97	Between syenites and Cardium sands	IMPROBABLE Simple	Circular geologic formation
Lac du Bonnet, Manitoba	~3.8	75	Sedimentary cover depth	PROPOSED Simple	Circular magnetic anomaly
Bow City, Alberta	6	<75	Geological dating	PROPOSED Complex	No surface expression
Maple Creek, Saskatchewan	6	<75	Geological & radioactive decay dating	CONFIRMED Complex	Disrupts Late Cretaceous rocks
Purple Springs, Alberta	6	<75	Geological dating	PROBABLE Complex	No surface expression
Manson, Iowa	~35	73.8	Ar⁴⁰-Ar³⁹ melt rock	CONFIRMED Central peak	No surface evidence
Dumas, Saskatchewan	4	<70	Geological dating	PROBABLE Simple	Seismic and borehole data
Chicxulub, Yucatan, Mexico	150	66.043 ±0.011	⁴⁰Ar/³⁹Ar	CONFIRMED Peak ring basin	Cretaceous-Paleogene

66.043 ±0.011 mA – CRETACEOUS-PALEOGENE EXTINCTION

76% of all species lost — Ammonite 15 cm lengthThe delicate leafy sutures decorating this shell represent some advanced engineering, providing the fortification the squid-like ammonite required to withstand the pressure of deep dives in pursuit of its prey. Dinosaurs may have ruled the land during the Cretaceous period but the oceans belonged to the ammonites. But volcanic activity and climate change already placed the ammonites under stress. The asteroid impact that ended the dinosaurs’ reign provided the final blow. Only a few dwindling species of ammonites survived. Today, the ammonites’ oldest surviving relative is the nautilus. Will it survive the sixth great extinction?

Cretaceous–Paleogene extinction event

CRETACEOUS-TERTIARY/CRETACEOUS-PALEOGENE (K–Pg) BOUNDARY

A major stratigraphic boudary on Earth marking the end of the Mesozoic Era, best known as the age of the dinosaurs. The boundary is defined by a global extinction event that caused the abrupt demise of the majority of all life on Earth. It has been dated to 65 million years ago, coeval with the age of the 200-kilometer-diameter Chicxulub impact structure in Mexico.

Cretaceous–Paleogene (K–Pg) boundary, formerly known as the Cretaceous–Tertiary (K–T) boundary – at the Royal Tyrrell Museum Drumheller Alberta.

K/T Boundary: concentrations of the rare platinum group elements (PGEs; Ru, Rh, Pb, Os, Ir, and Pt) and other siderophile elements (e.g., Co, Ni) are enriched by up to 4 orders of magnitude in the thin clay layer marking the K-T boundary compared to those of normal terrestrial crustal rocks. Cretacious/Tertiary boundary (the C abbreviation is already assigned to the Cambrian system), at present practically synonymous with marking the giant mass extinction 65 Ma ago. The extinction of the dinosaurs at that time is only a subordinate part of this remarkable event. See Chicxulub impact structure.

Luis (left) and Walter Alvarez at a limestone outcrop near Gubbio, Italy. Walter’s right hand is touching the top of the Cretaceous limestone, at the K-T boundary. **Extraterrestrial Cause of the Cretaceous-Tertiary Extinction** a preChicxulub discovery paper-1980 (Image courtesy of Lawrence Berkeley National Laboratory).

The following impacts occurred around the time of the Cretaceous-Paleogene Extinction:The Cretaceous–Paleogene (K–Pg) extinction event, also known as the Cretaceous–Tertiary (K–T) extinction, was a mass extinction of some three-quarters of the plant and animal species on Earth—including all non-avian dinosaurs—that occurred over a geologically short period of time approximately 66 million years ago. With the exception of some ectothermic species in aquatic ecosystems like the leatherback sea turtle and crocodiles, no tetrapods weighing more than 55 pounds (25 kilos) survived. It marked the end of the Cretaceous period and with it, the entire Mesozoic Era, opening the Cenozoic Era that continues today.In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows high levels of the metal iridium, which is rare in the Earth’s crust but abundant in asteroids.
Evidence for impact Sedimentary layers found all over the world at the Cretaceous–Paleogene boundary contain a concentration of iridium many times greater than normal. Iridium is extremely rare in Earth’s crust because it is a siderophile element, and therefore most of it traveled with the iron as it sank into Earth’s core during planetary differentiation. Iridium is abundant in most asteroids and comets suggesting that an asteroid struck the Earth at the time of the K–Pg boundary. There were earlier speculations on the possibility of an impact event, but this was the first hard evidence of an impact.In a 2013 paper, Paul Renne of the Berkeley Geochronology Center reported that the date of the asteroid event is 66.043±0.011 million years ago, based on argon–argon dating. He further posits that the mass extinction occurred within 32 thousand years of this date.Renne, Paul R.; (et al) Jan (7 February 2013).

“Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary”. Science 339 (6120)

Extraterrestrial cause for the cretaceous–tertiary extinction. (1980 – preChicxulub discovery)

Name	Diameter (km)	Age (megayears)	Dating method	Morphological type	Notes
Chicxulub, Yucatan, Mexico	150	66.043 ±0.011	⁴⁰Ar/³⁹Ar	CONFIRMED Peak ring basin	Cretaceous-Paleogene