Middle Ordovician System -...

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