middle ordovician system -...
TRANSCRIPT
Earth History, Ch. 13 1
Cambrian
System
Ordovician
System
Early
Early
Middle
Middle
Late
Late
543
495
443
Earth History, Ch. 13 2
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 3
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 4
Cambrian
sedimentation
patterns
paleoequator
Earth History, Ch. 13 5
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 6
Sauk transgression
Earth History, Ch. 13 7
Early Cambrian time
Earth History, Ch. 13 8
Middle Cambrian time
Earth History, Ch. 13 9
Late Cambrian time
Taconic island arc
Earth History, Ch. 13 10
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 11
Ordovician paleogeography
Time 1 Time 2 Time 3
Earth History, Ch. 13 12
Laurentia
Taconic arc
Avalonia
Baltica
Siberia
Earth History, Ch. 13 13
Laurentia
Taconic Mtns Avalonia
Baltica
Siberia
Earth History, Ch. 13 14
Taconic orogeny: eastern Laurentia
Accreted Avalonian island arc terranes contain “exotic”
fossils not known in cratonic Laurentia
Earth History, Ch. 13 15
Laurentia in Late Ordovician
Earth History, Ch. 13 16
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 17
Late Ordovician glaciation
Decrease in d18O indicates
melting of glacial ice
Increase in d18O indicates
buildup of glacial ice;
Earth History, Ch. 13 18
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 19
End-Ordovician mass extinction
Earth History, Ch. 13 20
Early Paleozoic
of Iowa
Cambrian rocks crop out
along a thin belt adjacent
to the Mississippi River
Ordovician rocks crop out
in a wider belt in NE Iowa,
with isolated exposures in north-
central Iowa
cross section
Earth History, Ch. 13 21
Iowa cross-section
Precambrian
NE NW
Earth History, Ch. 13 22
Early Paleozoic of Iowa
Earth History, Ch. 13 23
Early
Paleozoic of
Iowa
Note: No early or
middle Cambrian rocks
in Iowa—Why???
Earth History, Ch. 13 24
Cambrian Jordon Sandstone
Earth History, Ch. 13 25
2006 Discovery:
Winneshiek Lagerstatten and
possible impact structure
Earth History, Ch. 13 26
Earth History, Ch. 13 27
Earth History, Ch. 13 28
2010 NSF grant to fund
additional sample collecting
Earth History, Ch. 13 29
Earth History, Ch. 13 30
Earth History, Ch. 13 31
Two tons of fossiliferous shale