Page 47 - Introduction to Paleobiology and The Fossil Record

34 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

raphy was first established in the historic type stratigraphy, an approach that emphasizes the
area by defi nition of formations and members. importance of unconformities. In the early
On the basis of detailed collecting through the 1960s Larry Sloss recognized that the Pha-
stratigraphy a succession of biozones was then nerozoic rocks of the old North American

defined, based on the ranges of characteristic continent could be split into six main cycles
graptolite faunas. Finally, a succession of stages separated by unconformities (Fig. 2.9). These
was established together with two chronozones. were large-scale cycles describing the major
This remains the international yardstick for changes in sea level across an entire continent
Wenlock time. When discussing geological and through over 500 myr of Earth history.
time, we generally use the adjectives early, mid More minor sequences could be recognized
and late, but when dealing with rock the use of within these major cycles. The fact that sedi-
lower, middle and upper is more appropriate. mentary rocks can be described as packets of
strata, presumably deposited during trans-
gressive events (when the sea fl oods the land),
Sequence stratigraphy: using divided by periods of non-deposition during
transgressions and regressions
regressions (when the sea withdraws from
North American oil geologists developed a the land), forms the basis for sequence
whole new system in the 1960s called sequence stratigraphy.

Box 2.3 The Ordovician: a system on the move

The Ordovician System was born out of controversy, with Charles Lapworth taking the disputed
overlapping strata between Sedgwick’s Cambrian System and Murchison’s Silurian System (see
above). Despite the best efforts of British specialists (e.g. Fortey et al. 1995), they and many other
international experts have pointed out that – although the classic British series and stages have wide
global usage – they were based largely on endemic shelly faunas with only local and regional distri-
butions, some units are bounded by disconformities (minor gaps in deposition, where the rocks
below and above are oriented similarly, in contrast to the larger chunks of time represented by

unconformities), and some have significant overlaps with adjacent series. Moreover many of the key
sections are located in poorly exposed sections. In order to assemble a consolidated chronostratig-
raphy that would work internationally, definitions in new sections were necessary.

First, it was decided in the 1980s by the International Subcommission on Ordovician Stratigraphy,
a group of highly-qualified experts drawn from all over the world, that basal stratotypes for chro-

nostratigraphic units should be correlated by means of conodonts and graptolites, the most effective

of all Paleozoic zone fossils. Second, there should only be three series, defined as lower, middle and

upper; and third, new sections must be sought to define a new set of global stages: the fi rst was rati-

fied in 1987, and the last in 2007. This has not been without rancour. Colleagues from around the
world have clashed noisily at meetings defending their “own” sections, and sometimes national pride
and access to further research funding have influenced voting. Nevertheless, a consensus is emerging

and all the new stages are defined and in place, based on diverse sections such as a road section and
river bank in South China (Hirnantian) and the coast of western Newfoundland (Tremadocian).
Some older names such as Hirnantian and Tremadocian have been retained with slightly different

definitions, whilst some are new, such as Floian and Sandbian, both based on stratotype localities
in Sweden. This new structure is already providing a much more accurate time framework to
describe, analyze and model Ordovician Earth systems (Fig. 2.8).
More information of the work of the Subcommission and on the Ordovician System and its biotas
is available at http://www.blackwellpublishing.com/paleobiology and on the home page of the related
International Geological Correlation Program project 503 “Ordovician palaeogeography and pal-
aeoclimate” linked at http://www.blackwellpublishing.com/paleobiology/.