Concepts and Principles Geologic Time

I. Relative Time vs. Absolute Time A._____________________ Does not determine the number years involved but is concerned with the sequence of events (rock layers, erosion, structures formed, etc.) B.______________________: Determines the actual age of the rock in years Relative Age Absolute Age

II. PRINCIPLES USED TO DETERMINE RELATIVE AGE

A. ____________________________ 1.Sedimentary rock layers, or strata, were originally deposited as relatively horizontal sheets of sediment. 2.Strata that do not retain their original horizontality have been displaced by movements of Earth’s crust. Principle of Original Horizontality

Sediment is deposited in essential horizontal layers –Therefore, a sequence of sedimentary rock layers that is steeply inclined from horizontal must have been tilted after deposition and lithification

B. _____________________________ 1.Sedimentary rock layers or lava flows extend laterally (geographically) in all directions until they thin to their termination or ends of their basins of deposition. 2.Another term for this is that they “pinch out. Principle of Lateral Continuity

Gradual Terminations –A rock unit becomes progressively thinner until it pinches out –or where it splits into thinner units each of which pinches out, called intertonging where a rock unit changes by lateral gradation as its composition and/or texture becomes increasingly different

C. ______________________________ Any feature (fracture, fault, intrusive mass) that cuts across a body of sediment or rock is younger than the body of sediment or rock that it cuts across. Principle of Cross-Cutting Relationships

North shore of Lake Superior, Ontario Canada A dark-colored dike has intruded into older light colored granite. Cross-cutting Relationships The dike is younger than the granite.

Templin Highway, Castaic, California A small fault displaces tilted beds. Cross-cutting Relationships The fault is younger than the beds.

Folds

In an undisturbed succession of sedimentary rock layers, –the oldest layer is at the bottom –and the youngest layer is at the top D. ___________________________ Principle of Superposition

Illustration of the principles of superposition and original horizontality Principle of Superposition Superposition: The youngest –rocks are at the top –of the outcrop –and the oldest rocks are at the bottom

Overturned Folds Can cause older rocks to be located above younger rocks.

Thrust Faults Can push older rocks above younger rocks.

Chief Mountain, Glacier National Park, Montana Erosional remanant of a thrust fault Cross section shows older Precambrian rocks thrust over younger Cretaceous rocks

Inclusions or fragments in a rock are older than the rock itself E. ____________________ Principle of Inclusions

Light-colored granite –in northern Wisconsin –showing basalt inclusions (dark) Principle of Inclusions Which rock is older? –Basalt, because the granite includes it

Determining the relative ages of lava flows, sills and associated sedimentary rocks –uses alteration by heat –and inclusions Age of Lava Flows, Sills –A lava flow forms in sequence with the sedimentary layers. Rocks below the lava will have signs of heating but not the rocks above. The rocks above may have lava inclusions.

–A sill will heat the rocks above and below. Sill –The sill might also have inclusions of the rocks above and below, –but neither of these rocks will have inclusions of the sill.

F. __________________ 1.Surfaces that represent a gap in the geologic record. 2.Like pages in a book, they are surfaces on which sediment was not deposited for a period of time, or surface on which erosion has occurred. 3.Unconformities can range in size from bedding planes (surfaces between strata) to continent- wide surfaces Unconformities

The Sequence of Events in the Formation of an Unconformity Siccar Point, Scotland

Hutton’s Unconformity Siccar Point Scottland Near Edinburgh I don’t know who this guy is!

5. Types of Unconformities

a) _________________ An erosional surface that separates older igneous or metamorphic (nonsedimentary) rock from younger overlying sedimentary strata. Note the absence of contact metamorphism in the rocks immediately above the igneous or metamorphic rocks. Nonconformities

Sequence of Events in the Formation of a Nonconformity

Nonconformity

b) ________________________ An erosional surface that separates tilted or folded strata from overlying beds of different attitude. This implies that an area has undergone uplift, and that the uplift was accompanied by either folding or tilting with erosion of the strata prior to later subsidence and continued deposition. Angular Unconformities

The Formation of an Unconformity An erosional surface buried by younger sedimentary rocks

Grand Canyon

c) __________________ An erosional surface that separate essentially parallel sedimentary strata. These are probably the most common types of unconformities and are often the most difficult to recognize. The is especially so when the rock types are similar above and below the erosion surface. Disconformities

Disconformity

Disconformity

Disconformity Along a roadcut in Gladeville, TN

III. Correlation A. SIMILARITY OF ROCK TYPES 1. __________________________________ Being able to trace physically the course of a rock unit. 2. “Walking the Outcrop” Determining the age relationships between rock units or geologic events in separate areas Lithostratigraphic Correlation

Correlation of lithostratigraphic units such as formations –traces rocks laterally across gaps

Correlation Using Lines Drawn to Points of Equivalence Disconformities

Fossils

1. ____________________ a)Short lived species b)Easily identifiable c)Wide geographic distribution Index (Guide) Fossils

The brachiopod Lingula –is not useful because, –although it is easily identified –and has a wide geographic extent, –it has too large a geologic range The brachiopod Atrypa –and trilobite Paradoxides –are well suited –for time-stratigraphic correlation, –because of their short ranges They are guide fossils Index (Guide) Fossils

2. ______________________ Several different fossil species in a rock layer All species existed when the sediment was deposited. Use a fossil “range chart.” Fossil Assemblage

Fossil Ranges

Fossil Assemblages