Page 48 - Introduction to Paleobiology and The Fossil Record

FOSSILS IN TIME AND SPACE 35

Global United Kingdom North America
System Series Stage Series Stage Series Stage

Hirnantian Ashgill Hirnantian Gamachian

Upper Ordovician Katian Streffordian Cincinnatian Richmondian
Rawtheyan
Cautleyan
Maysvillian
Pusgillian
Edenian
Chatfieldian
Cheneyan
Sandbian Caradoc Aurelucian Mohaw- kian GSSP Turinian
Burrellian
Chazyan
Llandeilian
Ordovician Darriwilian Llanvirn Abereiddian Not

Middle Ordovician Fennian Whiterockian distinguished

Dapingian Arenig Whitlandian ? Rangerian

Lower Ordovician Floian Moridunian Ibexian Black Hillsian
Tulean
Tremado- cian Tremadoc Skullrockian
Migneintian
Stairsian
Cressagian
GSSP
Figure 2.8 Current status of the development of a new, internationally accepted
chronostratigraphy for the Ordovician System. New global series and stages are correlated with
the comparable chronostratigraphic divisions used in North American and the United Kingdom
and Ireland. GSSP, global standard section and point.

The dividing lines between transgressive sions defi ned within unconformity-bounded
and regressive system tracts are marked by sequences has proved valuable in hydrocar-
various types and degrees of unconformities bon exploration, where sequence boundaries
that may be recognized on seismic profi les. can be recognized at depth using seismic
Whereas most major sequence boundaries are geophysics.
probably due to global eustatic changes in sea Sequence stratigraphers have developed
level associated with climatic change or fl uc- their own specialist terminology (Fig. 2.10).
tuations in seafl oor spreading processes, A sequence is a unit of similar strata bounded
sequences can also be generated by more local by unconformities. Sequences are laid down
tectonic controls. Research teams in the Exxon in three-dimensional assemblages of lithofa-
Corporation expanded the concept of sequence cies linked by common depositional processes
stratigraphy to build global sea-level curves that can be divided into individual systems
for the entire Phanerozoic during the 1980s tracts. The architecture of sequences is con-
and 1990s. The description of succes- trolled by changes in sea level, whether eustat-