Module 4: Sedimentary Rocks, Weathering and Geologic Time Study Guide

relative time

("chronostatic") subdivisions of the Earth's geology in a specific order based upon relative age relationships, subdivisions are given names, most of which can be recognized globally, usually on the basis of fossils

CROSS BEDDING

- Consists of sets of beds that are inclined relative to one another. The beds are inclined in the direction that the wind or water was moving at the time of deposition. Boundaries between sets of cross beds usually represent an erosional surface. Cross bedding is very common in beach deposits, sand dunes, and river deposited sediment. Individual beds within cross-bedded strata are useful indicators of current direction and tops and bottoms. Note how the beds become asymptotic to the lower boundary on which they were deposited.

What are sedimentary structures?

-BEDDING(layering) -LAMINAE- small scale layers -STRATA- large scale layers -BEDDING PLANES OR CONTACTS -CROSS BEDDING -GRADED BEDDING

Of the three main types of sedimentary rocks, what are: sandstone, fossiliferous limestone, evaporites, shale, chert

-Clastic: Sandstone, Shale -Chemical: Fossiliferous Limestone, Evaporites, Chert

List in order of smallest to biggest the different sediment sizes

-Clay -Silt -Sand -Gravel

Clastic EX

-Conglomerate(rounded) -Breccia(angular) -Sand stone -Silt stone -Clay stone -Mud stone -Shale

Organic Sedimentary EX

-Limestone -Coal -Dolostone -Travertine -Chalk -Jasper -Agate -Diatomite -Gypsum Rock

Chemical Sedimentary EX

-chert -dolomites -flint -rock salt -iron ore -Fossiliferous Limestone

What are the three main types of sedimentary rocks?

1. CLASTIC 2. ORGANIC SEDIMENTARY 3. CHEMICAL SEDIMENTARY

Mesozoic Era

245 mya to 66 mya Age of Dinosaurs

PreCambrian

4.5 billion years ago (bya) to 545 million years ago (mya)

Paleozoic Era

545 mya to 245 mya Age of Trilobites (Cambrian) Age of Fish (Devonian) Age of Amphibians (Permian)

Cenozoic Era

66 mya to present Age of Mammals

How does fossil type change with each of these major periods?

?

UNCONFORMITY

An unconformity is a surface of non-deposition or erosion separating younger and older rock layers. Because no rocks are being preserved during the period of non-deposition or erosion, unconformities represent a break in the record of geologic time.

GRADED BEDDING

As current velocity decreases, the larger or more dense particles are deposited first, followed by smaller particles. This results in bedding showing a decrease in grain size from the bottom of the bed to the top of the bed. This gives us a method for determining tops and bottoms of beds, since reverse grading will not be expected unless deposition occurs under unusual circumstances. Note that reverse graded bedding cannot occur as current velocity increases, because each layer will simply be removed as the current achieves a velocity high enough to carry sediment of a particular size.

What is the difference between breccia and conglomerate?

Breccia-angular fragments Conglomerate-rounded fragments

What are index fossils?

Certain fossils can be used to help define the relative time of a rock unit. If an organism existed for a relatively short period of time and had a wide geographic distribution, then it can provide an index as to the age of the rocks in which it is preserved

Explain spheroidal weathering. Granites consist of predominantly feldspar and quartz. What do these two minerals weather to?

Chemical weathering along joints in rock - corners have largest surface area and are the first to be broken down - end up with rounded rocks ; decomposed granite = quartz sand and clay

What kind of bedding is preserved in desert environments?

Cross Bedding

What is desert varnish? How long does it take to form?

Desert varnish is a dark coating on rocks found in arid regions ; coating is composed of fine-grained clay minerals of black manganese oxide and red iron oxide ; thousands of years are required to form a complete coat of manganese-rich desert varnish

Explain differential weathering. How would this explain mesas?

Different rock types erode to give different slopes (more resistant layer (cliff forming layer): sandstone, limestone in dry climate ; less resistant layer (slope forming): typically shale) ; we see cliffs at the edge of resistant rock layers and a more inclines slope which flattens out at the bottom, cliff retreat (the rock in the upper cliff face repeatedly breaks away along vertical fractures) produces mesas

What causes exfoliation? What types of rocks undergo this process? What is created by exfoliation?

Exfoliation or pressure relief - overlying material removed above a pluton - rock expands and breaks into onion like layers called exfoliation sheets ; slabs of rock separate and fall off into what's called domes

Large round granite boulders are created by movement and rounding of the corners (T/F)

F

Limestone is preserved in what depositional environment?

Marine environments (shallow marine)

What occurs at boundaries of Paleozoic and Mesozoic and Mesozoic and Cenozoic?

Mass extinction

BEDDING

One of the most obvious features of sedimentary rocks and sediment is the layered structure which they exhibit. The layers are evident because of differences in mineralogy, clast size, degree of sorting, or color of the different layers. In rocks, these differences may be made more prominent by the differences in resistance to weathering or color changes brought out by weathering.

What is found at the base of cliffs undergoing frost wedging?

Rocks fall from frost wedging piles up in a talus slope at base of cliff

In what time period (Precambrian, Paleozoic, Mesozoic, Cenozoic) would you find stromatolite fossils

Trilobite fossils paleozoic era Dinosaur fossils mesozoic era

What causes the red color in rocks?

Weathering oxide prove color to the desert landscape

A sequence of strata with limestone at the bottom and clastic sedimentary rock above indicates a _______ in sea level, called a ___________

change; uplift/subsidence of continent

What is limestone made of? Mineral name and chemical formula

composed primarily of calcium carbonate (CaCO3 ) in the form of the mineral calcite. It most commonly forms in clear, warm, shallow marine waters. It is usually an organic sedimentary rock that forms from the accumulation of shell, coral, algal, and fecal debris. It can also be a chemical sedimentary rock formed by the precipitation of calcium carbonate from lake or ocean water.

Angular unconformities

contact in which younger strata overlie an erosion surface on tilted or folded layered rock

Chemical weathering

involves a chemical change in at least some of the minerals within a rock EXAMPLE: solution (carbonation), hydrolysis, oxidation

physical or mechanical weathering

involves physically breaking rocks into fragments without changing the chemical make-up of the minerals within it EXAMPLE: frost wedging or freeze-thaw

Graded bedding is produced by ____________ on the margin of the deep ocean

largerm dense particles being deposited first, followed by smaller particles

Conformable

layers of rock that have been deposited without any interruption, no gaps in time, no missing record due to erosion, non-deposition, etc, rocks of different composition that have no apparent distinct contact

Which is resistant to weathering in dry (arid) environments? Shale or limestone?

limestone

absolute time

numerical ages in "millions of years" or some other measurement, commonly obtained via radiometric dating methods performed on appropriate rock types

What mineral is resistant to chemical weathering?

quartz

What is generally more resistant to weathering? Shale or sandstone?

sandstone

What type of rock covers roughly 75% of the continents?

sedimentary rocks

Disconformity

specific type of gap wherein the layers above and below an erosional boundary have the same orientation (often horizontal). Given that rock layers above and below an unconformity surface have the same orientation (strike and dip), disconformities may be difficult to recognize. However, as with all unconformities, disconformities involve a significant time gap-- typically on the order of tens of millions of years!

Nonconformity

specific type of unconformity characterized by much older igneous or metamorphic rocks below, overlain by younger sedimentary rocks above an unconformity surface. The older igneous or metamorphic rocks indicate active igneous and/or tectonic period, followed by uplift and erosion. The overlying sedimentary rocks indicate a time of relative subsidence and deposition of sedimentary layers in a basin

BEDDING PLANES OR CONTACTS

surfaces separating two different rock types or rocks of different ages

What is the pattern of sedimentary rock deposits in a transgression? a regression?

transgression- relative rise in sea level recorded in the sedimentary rock record regression- relative decline in sea level recorded in the sedimentary rock record