443 495 543 Late Middle Early Late Middle Early Ordovician System Cambrian System 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: 1 st stage of Appalachian mountain building Earth History, Ch. 13 10
Ordovician paleogeography Time 1 Time 2 Time 3 Earth History, Ch. 13 11
Laurentia Siberia 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 18 O in seawater ( 16 O 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 d 18 O indicates melting of glacial ice Increase in d 18 O 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 cross section Cambrian rocks crop out along a thin belt adjacent to the Mississippi River 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