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Saturday, March 03, 2007

Cretaceous Stratigraphy

Period
Epoch (ICS, with added subdivision)
Harland Epoch
Age
ICS Base
ICS Duration
Paleogene
Paleocene: 9.7 My
Paleocene
Danian
65.5
3.8
Cretaceous80.0 My
Late Cretaceous II(End-Cretaceous)18.0 My
Senonian
Maastrichtian
70.6
5.1
Campanian
83.5
12.9
Late Cretaceous I(The "High Cretaceous")16.1 My
Santonian
85.8
2.3
Coniacian
89.3
3.5
Gallic
Turonian
93.5
4.2
Cenomanian
99.6
6.1
Early Cretaceous II(Aptian-Albian)25.4 My
Albian
112.0
12.4
Aptian
125.0
13.0
Early Cretaceous I("Neocomian")20.5 My
Barremian
130.0
5.0
Neocomian
Hauterivian
136.4
6.4
Valanginian
140.2
3.8
Berriasian
145.5
5.3
Jurassic
Late Jurassic: 15.7 My
Malm
Tithonian
150.8
5.3

i have used the Harland three-fold division of the Cretaceous into Neocomian, Gallic, and Senonian Epochs. As of this writing (040911) we are in the process of gradually converting to the ICS system, which recognizes only Early and Late Cretaceous epochs. Unfortunately, the ICS Cretaceous epochs are unreasonably long for our purposes, so we have taken the further step of dividing both of them into two.
Our "Neocomian" division is based largely on climatic considerations. This "Early Early Cretaceous" demi-epoch was a time of steadily rising seas and temperatures. According to one source, the ocean temperature increased an almost unbelievable 17 C° over the Early Cretaceous, and the bulk of this must increase must have occurred in the Neocomian division. The Aptian-Albian division continued the Neocomian trend, but at a slower rate. The Aptian-Albian is also the interval that produced the definitive Cretaceous dinosaur clades. These dinosaurs dominated the large herbivore guild in the Late Cretaceous: the ornithischian iguanodonts (including hadrosaurs), Ceratopsia (e.g. Triceratops), and various saurischian titanosaurs. In the oceans, a final radiation of pliosaurs also occurred at about this time.
The Early Late Cretaceous ("High Cretaceous") was marked by several critical events. The first was the widening Atlantic rift. The Atlantic Ocean: (a) had become wide enough to become a complete barrier to east-west dispersal over its entire length, except in the far north, and (b) was circulating meaningful amounts of ocean water north and south. The initial results seem somewhat paradoxical. On the one hand, the High Cretaceous experienced unprecedented uniformity of ocean temperatures from pole to pole, suggesting very good horizontal mixing of ocean waters. On the other hand, it is well known for sporadic deep ocean anoxia, which would indicate poor vertical mixing. It is tempting to speculate on the causes of this peculiar set of events. One strong line of evidence implicates methane and/or carbon dioxide outgassing. But most of this data comes from the Atlantic basin. In due course, we will have enough information from the Pacific to give us a better global perspective.
The second major event of the High Cretaceous was angiosperm dominance. Angiosperm plants had begun to spread at least as early as the middle Neocomian. However, during the High Cretaceous, angiosperms reached some critical mass or critical stage of development and became the dominant type of plant in most parts of the world. Finally, the long, gradual increase in sea levels which began in the Triassic reached its peak in the High Cretaceous. During the End-Cretaceous, sea levels began to retreat after 165 million years of advances. Miller et al. (2003) have recently reported that the peak and decline of sea levels in the Late Cretaceous is punctuated by a number of sudden, drastic, marine regressions. Apparently, the oceans retreated quite quickly, and, almost as quickly, returned to more or less their former depths. The pace of these changes appears to have been below the limit of geological resolution for their core samples, i.e. about <500Ky. Miller et al. state that their data are consistent only with the formation of short-lived, but rather extensive ice sheets in Antarctica. This conclusion is almost -- but not quite -- irreconcilable with what we know about Late Cretaceous climate. Miller's group coordinates data from a number of different regions in arriving at this result. While these sequences are diverse, they still cover only the Atlantic and Tethyan regions. Once again, we are badly in need of data from the Pacific Basin.

more to follow on cretaceous in join me in irvincalicut.blogspot.com

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