443
Late
Middle
Ordovician
System
Early
495
Late
Middle
Cambrian
System
Early
543
Earth History, Ch. 13
1
Ch. 13 Review:
Early Paleozoic life & Burgess Shale fauna
• Most animal phyla originated in Cambrian;
Ordovician was a time of diversification within
clades
• Burgess Shale fauna accumulated at the edge of the
continental shelf during middle Cambrian time
• Unusual preservation of soft anatomy is attributed to
rapid burial in deep, oxygen-depleted water
• Significance of the Burgess Shale fauna
Earth History, Ch. 13
2
Cambrian geography and geology
• During Cambrian time, Laurentia was a tropical,
low-lying landmass
• Marine sediments around the margins of Laurentia
formed concentric facies belts
– Nearshore detrital clastic belt
– Shallow water carbonate belt
– Offshore, deeper-water deposits
• Sea level was generally rising during Cambrian
and early Ordovician time, producing a
transgressive stratigraphic sequence
Earth History, Ch. 13
3
Cambrian
sedimentation
patterns
paleoequator
Earth History, Ch. 13
4
Depositional Sequences
Sauk sequence is a
record of transgression
(rising sea level) from
the margins of Laurentia
across the midcontinent
Figure 6-21
(black = nondeposition
or erosion)
Earth History, Ch. 13
5
Sauk transgression
Earth History, Ch. 13
6
Early Cambrian time
Earth History, Ch. 13
7
Middle Cambrian time
Earth History, Ch. 13
8
Taconic
island arc
Late Cambrian time
Earth History, Ch. 13
9
Taconic orogeny
• During Ordovician time, Laurentia remained
tropical, but Gondwanaland migrated toward
the south pole
– Widespread late Ordovician glaciation and drop in sea
level
• Avalonia was a fragment of Gondwanaland that
broke away and nearly collided with Baltica and
Laurentia
– Taconic Orogeny of eastern Laurentia: 1st stage of
Appalachian mountain building
Earth History, Ch. 13
10
Ordovician paleogeography
Time 1
Time 2
Earth History, Ch. 13
Time 3
11
Siberia
Laurentia
Baltica
Taconic arc
Avalonia
Earth History, Ch. 13
12
Siberia
Laurentia
Baltica
Taconic Mtns
Avalonia
Earth History, Ch. 13
13
Taconic orogeny: eastern Laurentia
Accreted Avalonian island arc terranes contain “exotic”
fossils not known in cratonic
Laurentia
Earth History,
Ch. 13
14
Laurentia in Late Ordovician
Earth History, Ch. 13
15
Late Ordovician glaciation
• Evidence for Late Ordovician glaciation
– Worldwide drop in sea level, as determined
by erosional unconformities
– Glacial till, glacial striations, and dropstones
near south Pole (now northern Africa)
– Enrichment of 18O in seawater (16O
preferentially locked up in glacial ice)
• Suggests duration of glaciation was only 0.5 to 1.0
million years
Earth History, Ch. 13
16
Late Ordovician glaciation
Decrease in d18O indicates
melting of glacial ice
Increase in d18O indicates
buildup of glacial ice;
Earth History, Ch. 13
17
Effects of late Ordovician glaciation
• End-Ordovician mass extinction!
• Reduction in generic diversity, but not
elimination of phyla
• Affected groups were brachiopods, trilobites,
corals, bryozoans, acritarchs, graptolites,
nautiloids, conodonts
• Two pulses of extinction
– Tropical, warm-water forms were eliminated during
onset of glaciation (this allowed for geographic
expansion of cool-water biotas)
– Cool-water biotas were eliminated during return to
non-glacial climate!
Earth History, Ch. 13
18
End-Ordovician mass extinction
Earth History, Ch. 13
19
Early Paleozoic
of Iowa
Ordovician rocks crop out
in a wider belt in NE Iowa,
with isolated exposures in northcentral Iowa
Cambrian rocks crop out
along a thin belt adjacent
to the Mississippi River
cross section
Earth History, Ch. 13
20
Iowa cross-section
NW
NE
Precambrian
Earth History, Ch. 13
21
Early Paleozoic of Iowa
Earth History, Ch. 13
22
Early
Paleozoic of
Iowa
Note: No early or
middle Cambrian rocks
in Iowa—Why???
Earth History, Ch. 13
23
Cambrian Jordon Sandstone
Earth History, Ch. 13
24
2006 Discovery:
Winneshiek Lagerstatten and
possible impact structure
Earth History, Ch. 13
25
Earth History, Ch. 13
26
Earth History, Ch. 13
27
2010 NSF grant to fund
additional sample collecting
Earth History, Ch. 13
28
Earth History, Ch. 13
29
Earth History, Ch. 13
30
Two tons of fossiliferous shale
Earth History, Ch. 13
31