Exam 3 Flashcards
Percentage of earths history the Precambrian represents
86%
First crustal record
3.8-4.4 BY ago
Precambrian outcrops
1) Cores of mountain ranges
2) Deep Gorges (Grand Canyon)
3) Shield Areas
Shield Area
- The shield is the core of a continent
- Precambrian continental core is exposed in a convex-shaped layer on the earths surface
- Overlain by the platform
Platform
Flat strata that initially overlays the shield
Craton
Shield + Platform
Dating methods that can’t be used for the precambrian
Biostratigraphy can’t be used as there is no record of precambrian life
Cambrian vs. Precambrian
Separated by the existence of Trilobites (Olenellus)
Canadian Shield
North America, contains records of key Precambrian eras and events
Paleoarchaean Volcanics
~3.5 BY ago pillow lava was present, indicating that oceans were present
Mesoarchean
< 3.0 BY ago Quartz (SiO2) and pebbles of Granite are present, indicating that continental crust had to be differentiated by this point
Siderian
- < 2.5 BY ago the first Stromatolites were recorded, which were formed by Cyanobacteria
- ~2.2-2.5 BY ago the Gowganda Formation in Canada indicates glacial deposition
Stenian
~1.2 BY ago the North American Rift began
- Keweenawan Rift complex
Cryogenian
~700 MY ago the ice-house Snowball Earth was recorded by an abundance of glacial till
Earliest Evidence of Precambrian Life
- 3.4-3.5 BY ago
- Warrawoona Grove (NW Australia)
- Onverwatched and Fig Tree Groups (South Africa)
- Prokaryotic Cyanobacteria
Great Oxygenation Event
1.8-2.0 BY ago the Ozone layer formed
Eukaryotic Cells
1.2-1.75 BY ago Acritarchs with a differentiated nucleus were present
First Metazoans
- 600 MY ago more complex life with differentiated functional cellular material were present
- Rawnsly Quartzite within the Ediacaran Hill of Southern Australia
- Ediacaran Assemblage
Vendian (Ediacaran) History
- Techtonic Framework: Rifting
- Aulacogens
- Timing of rifting (~500-600 mya)
Aulacogens
- Rifting occurs in rift triple-junctions
- Aulacogens are failed rifts
Cambrian History
- North America and craton structure
- Transcontinental Arch
Arches and Basins
- Form through the stress of moving the crust
- Basins are where most of earths sediment accumulates
Transcontinental Arch
- Extended from Great Lakes to Northeastern Arizona
- Most of the sedimentary record is to the flanks of the arch
Cratonic Sequences
Large-scale stratagraphic unit representing a major transgressive-regressive episode and bounded by craton-wide unconformities
Sloss’ 1963 Cratonic Sequences
- Tejas = upper Cretaceous to upper Quaternary
- Zuni = lower Jurassic to upper Cretaceous
- Absaroka = upper Mississippian to lower Jurassic
- Kaskaskia = lower Devonian to upper Mississippian
- Tippecanoe - lower Ordovician to lower Devonian
- Sauk = Cambrian to lower Ordovician
Sauk Cratonic Sequence
- Mature quartz sandstone
- Base of well rounded quartz topped with limestone
- Transition to carbonate deposition