Page 49 - Introduction to Paleobiology and The Fossil Record

36 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Western Eastern Sea level
Periods North American Mega sequences on craton North American
orogeny orogeny High Low
Cretaceous Zuni

Jurassic
Nevadan
Triassic Appalachian
Permian Absaroka
U Sonoma
Carboniferous
L Acadian
Kaskaskia
Devonian Antler
Silurian Taconic
Tippecanoe
Ordovician
Cambrian Sauk
Figure 2.9 North American Phanerozoic sequences: the recognition of these large packages of rock or
what are termed “megasequences” formed the basis for the modern discipline of sequence stratigraphy,
established by the Exxon Corporation. (Based on various sources.)

ically or tectonically driven, or perhaps a ated with marine transgressions as more
mixture of both, and the room available for shallow-water habitats are created when con-
sediment, termed accommodation space. tinents are flooded. On the other hand, marked

Normal regressions, driven by increased sedi- regressive events have been associated with
ment supply, and forced regressions, driven by major extinctions through habitat loss. Nev-
base level fall, will both generate falls in sea ertheless it has been suggested by some authors
level, where base level is the level above which that such diversity changes are artifi cial.
deposition is temporary and prone to erosion. Transgressive units are generally more wide-
Transgressions are prompted by base level spread across continental areas, so increasing
rise, when this of course exceeds sedimenta- the chance to collect fossils; the converse may
tion rates. There are also six main types of be true for regressive events. But sampling
surface: subaerial unconformity, basal surface biases alone cannot account for apparent
of forced regression, regressive surface of changes in biodiversity through time; pro-
marine erosion, maximum regressive surface, cesses related to sea-level change and the for-
maximum flooding surface and ravinement mation and destruction of marine habitats

surface; the first three are associated with base have also provided controls on the origination

level fall and the last three with base level rise. and extinction of marine taxa (Peters
Finally there is a variety of systems tracts (Fig. 2005).
2.10): lowstand, transgressive, highstand,
falling stage and regressive systems tracts.
Changes in sea level seem to have had major Cyclostratigraphy: fi nding the rhythm
effects on the planet’s marine biotas through Quaternary geologists have accepted for some
time and sequence stratigraphy provides a time that recent climate change follows
framework to describe these effects (Box 2.4). repeated cycles of astronomical change. These
For example, shell concentrations may be short-term patterns are called Milankovitch
associated with stratigraphic condensation at cycles, named after the Serbian mathemati-
maximum flooding surfaces, i.e. the deepest- cian Milutin Milankovitch (1879–1958). Such

water facies where deposition is very slow or cycles are controlled by the additive effects of
they may lie near the top of highstand system the Earth’s movements through space (Fig.
tracts. Firmgrounds (see p. 522) and their 2.12a) and can directly affect global sedimen-
biotas, that include usually burrowers and tation patterns. Three main types of move-
encrusters, favor major fl ooding surfaces. ment occur: eccentricity (variation in the
Moreover, diversity increases are often associ- shape of the Earth’s orbit from nearly circular