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