Geology of Illinois

Purpose of Geology Field Trip - learn about local geology, goal is to understand Earth’s geologic history through field examination of earth materials (rock, sediment, + soil)

I) Regional Geology of North America (North America Geologic map, Global map of Geologic Provinces)

Active continental margin on Pacific coastline; abundant earthquakes, volcanoes, + mountains due to convergent plate boundary

Passive continental margin
on Atlantic + Gulf of Mexico coastlines; few earthquakes + no volcanoes + mountains, far from tectonic plate boundary

Two Major Orogenic (Mountain) Belts
Appalachian Orogeny - along eastern North America (+ south-central USA); formed in Paleozoic Era (~545 - ~250 million years ago)

Cordilleran Orogeny - along ~all of western North America; formed in Mesozoic + Cenozoic Era (250 million years ago - today)

Craton (continental interior) = relatively old area of continent that has been tectonically stable (little major faulting or folding) for long period of time.
Cratonic (Precambrian) Shield
= igneous + metamorphic rocks of Precambrian age (>545 million years ago), "basement" rocks, e.g., middle of Canada.
Cratonic Platform - relatively thin (typically 1 - 2 km thick) cover of sedimentary rock (mostly Paleozoic + Mesozoic age) on top of Precambrian igneous + metamorphic rocks, e.g., middle of USA. Craton has numerous basins (gentle downwarping of crust) + domes (upwarping of crust)
 
II) Geologic History of Illinois

Precambrian basement = silicic igneous rocks + metamorphic rocks (1.4 - 1.5 billion years ago) possibly formed at convergent tectonic plate boundary

Major unconformity =

Initial failed rift in Late Precambrian

During most of Paleozoic Era, deposition of mostly marine sedimentary rocks in Illinois basin (large embayment open to ocean towards south); many episodes of advance + retreat of shallow ocean water from south (marine transgressions - high sea level + continents flooded + marine regressions - low sea level + erosion) Geologic age of most common bedrock at surface in IL?

Cyclothem - sequence of several sedimentary rock types (sandstone, mudstone, coal, shale, limestone) that represent rising sea level and then falling sea level; characteristic of Late Paleozoic Midcontinent geology.

Illinois basin is spoon-shaped in cross-section with thickest accumulation of rock in southern Illinois; next largest structure is La Salle Deformation Belt, consists mainly of monocline (one-sided fold produced by fault in basement rocks
Marine sedimentation ended at end of Paleozoic Era with uplift of Pascola Arch

Another unconformity on top of Paleozoic bedrock surface

Quaternary Period (link #2) involves many advances + retreats of continental glaciers (thick sheets of moving ice), resulting in blanket of sediment over nearly entire state.  Glacial ice was thickest in Hudson Bay area of Canada + spread to cover most of Canada + much of northern USA.  During Quaternary (last 2 m.y. of Earth history) there were ~10 major advances (time of cooler temperatures termed glacial episode) + ~10 major retreats (time of warmer temperatures termed interglacial episode) of continental glaciers.  In Illinois most of glacial deposits are from either Illinois Episode advance (300,000 - 170,000 yrs ago), which cover nearly entire state, or Wisconsin Episode advance (75,000 - 12,500 yrs ago), which cover only northeastern IL.  During interglacial episodes, climate was warm, resulting in soil formation.  Glacial deposits = till (unsorted sediment, i.e., mud, silt, sand, gravel + boulders, deposited by glacial ice), outwash (sorted sediment, i.e., sand + gravel, deposited by meltwater streams), lake sediments (layers of silt + mud deposited in lakes formed by meltwater), + loess (wind-blown silt).