Geologic and Rock Cycles, Fossils and Unconformities

3 key events: deposition, uplift, erosion The Geologic Cycle 3 key events: deposition, uplift, erosion Sedimentation of beds A-D beneath the sea Uplift above sea level and exposure of D to erosion Continual erosion strips D away completely and exposes C to erosion Subsidence below the sea and sedimentation of E over C; erosion surface of C preserved as an unconformity

Deposition or Sedimentation

Uplift

Erosion

Erosion by wind Erosion by water

Geologic Cycle Cycle repeats over and over through geologic time Forms Strata: layers of rock

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The rock cycle explains how one type of rock can be transformed into another in nature.

Index Fossils Use fossil layers to date rocks: index fossils

Fossils Titanites

What is a Fossil? Any evidence of past life, including remains, traces, imprints as well as life history artifacts. Examples of artifacts include fossilized bird's nests, bee hives, etc

Mastodon at Wheaton College

How Fossils are Formed About 67 million years ago -- A Tyrannosaurus rex died and its body was quickly covered by riverbed sand and mud. Over time, pressure and some remineralization turned its bones to fossils.

Tully Monster; IL State Fossil The Fossil Record All of the fossils that have existed throughout life’s history, whether they have been found or not. Tully Monster; IL State Fossil

Fossils Usually found in sedimentary rock Sometimes in metamorphic rock These are often distorted, hard to interpret Upper layers - younger Deeper layers – older

Principle of Fossil Succession There is a unique, non-repeating pattern (history) of fossils through stratigraphic time. All rocks containing fossils of the same species were deposited during the duration of that species on Earth.

Correlation by fossils Correlation by fossils. Certain index fossils are keys to matching sedimentary strata in widely separated outcrops

Used in conjunction with radioisotope techniques to age rock layers

Unconformities Unconformity: contact between layers of rock that should not touch; represents a gap in the fossil/geological record Layers of sandstone over pink granite

Hutton's unconformity at Siccar Point Berwickshire, Scotland This was the very first (1789) example of an unconformity

Hutton's unconformity

Ordovician/Silurian unconformity in New York state

Nonconformity, Grand Canyon, Arizona Nonconformity, Grand Canyon, Arizona. Nonconformities are types of unconformities in which sedimentary rock depositionally overlies metamorphic or intrusive igneous rock. In this photo, Cambrian sandstone overlies Proterozoic schist, with an age greater than 1.7 Ga--which means that the erosional surface between the two represents more than a billion years of geologic time!

Grey dolomite 500 myo Reddish rhyolite 1.5 byo Description: Precambrian/Cambrian unconformity at the site of the Taum Sauk powerplant about 90 miles south of St. Louis, MO. The upper angled layer layer of grey dolomite is approximately 500 million years old. The reddish rhyolite on which the dolomite rests is approximately 1.5 billion years old. There is a billion years of geologic history missing in this picture. Precambrian/Cambrian unconformity near St. Louis – 1 billion years is missing

Unconformities Disconformity: gap in geological record Caused by long period of erosion between episodes of sedimentation Or by faulting and uplift Fossils in the two layers may be very different

Unconformities Angular unconformity: younger strata overlie an erosion surface on tilted or folded rocks. Implies a specific sequence of events. Nonconformity: contact with overlying sedimentary rocks on top of an erosion surface of plutonic or metamorphic rocks. Implies long-lived erosion prior to burial and re-deposition.

Angular Unconformity Nonconformity

Upper Carboniferous and new red sandstones of the Triassic in Portugal – spectacular angular unconformity

Angular Nonconformity

Angular Unconformity Nonconformity

Great Lakes Lost Interval Mesozoic Era Very recent glacial sediments above ancient bedrock Due to long period of erosion of uplifted sediments Big gap in fossil record