GY 112 Lecture Notes D. Haywick (2006) 1

GY 112 Lecture Notes Rock Review

Lecture Goals: A) Recap of rock types B) Recap of the rock cycle C) Sedimentary rocks: their role in earth history

Textbook reference: Levin 7th edition (2003), Chapter 2; Levin 8th edition (2006), Chapter 4 (For those of you who have already completed GY 111, you will find your lecture and lab notes of use for this part of GY 112) A) Recap of rock types To cut to the chase, there are 3 main divisions of rocks, but in GY 112, the sedimentary rocks are by far the most important. The reason? They are commonly fossiliferous (meaning that they host fossils). First off, here are the 3 main rock divisions:

Igneous (formed from molten rock) Sedimentary (formed from particulate material)

Metamorphic (alteration of parent rocks through heat, pressure and fluids)

Each of these rock groups have subdivisions. As I said earlier, it is the sedimentary rocks that are the most important group in GY 112. Let’s start with them: Sedimentary rocks are classified according to two different parameters: 1) Grain size (see chart to left) and composition. The grain size refers to the size of the

particles that make up the rocks. These are the “things” that were initially deposited before the sediment became “lithified”, which just means conversion into “rock”. There are 4 major size divisions of sediment that you need to remember: 1) gravel (the largest division; grains ≥ 2.0 mm), 2) sand (grains <2.00 mm but ≥ 0.63 mm), 3) silt (grains <0.63 mm but ≥ 0.004 mm) and clay (grains ≤ 0.004 mm). Obviously, you can’t see the really small silt and clay grains. Sedimentologists (those really cool and wonderful scientists like Dr. Haywick that study sedimentary rocks), frequently have to use microscopes and scanning electron microscopes to study the smallest sedimentary particles.

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As far as composition is concerned, sedimentary rocks are classified into four sub-divisions: 1) Clastic (also known as siliciclastic); rocks contained particles rich in silica-rich

minerals such as quartz, feldspar and clay minerals. Examples include quartz arenite1 sandstone, arkose sandstone, greywacke sandstone, shale, claystone, siltstone and mudstone.

2) Bioclastic; rocks containing remains of once living creatures (aka beasties). Examples include limestone (carbonate shells and skeletal elements), chalk (microscopic carbonate shells and body parts) and biogenic chert (microscopic silica body parts).

3) Chemical; rocks containing chemically precipitated minerals. Examples include the evaporite minerals halite, gypsum, dolostone and anhydrite, as well as travertine (cave precipitates composed of calcite) and oolitic limestone.

4) Organic; rocks containing the remains of plants (i.e., the coals). Examples include peat, lignite, bituminous coal and anthracite.

Please note that many of these sedimentary rocks may also be fossiliferous. For example, a siltstone containing shells can be named a fossiliferous siltstone. A fossil-bearing quartz arenite sandstone can be named a fossiliferous quartz arenite. You should also be familiar with the following sedimentary figure. Apart from grain size, it also summarizes grain shape and sorting, concepts that we will consider in the lab.

1 Note: You need to know the names of all of the rocks that are highlighted red.

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This is a summary chart of the sedimentary rocks from the GY 111 lab manual. It is included here to provide completeness to this lecture. Turn to the last page for a list of the sedimentary rocks that you will see in GY 112.

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The other two rock divisions are included here for completeness of your notes, but you aren’t going to see many of them in GY 112 (only 3): Igneous rocks; two sub-divisions. 1) Extrusive (also known as volcanic) if they are erupted at the Earth’s surface from a

volcano. Examples include basalt (mafic); andesite (intermediate) and rhyolite (felsic).

2) Intrusive (also known as plutonic) if they cool within the interior of the Earth. Examples include dunite and peridotite (ultramafic), gabbro (mafic), diorite (intermediate) and granite (felsic).

Metamorphic rocks; three sub-divisions. 1) Foliated; rocks containing a distinct arrangement of minerals (including rock

cleavage). Examples include slate, phyllite, schist, gneiss and amphibolite. 2) Non-foliated; rocks lacking foliation. Examples include marble, quartzite and

hornfels. 3) Cataclastic; rocks pretty much busted up through faulting. Examples include

mylonites. B) The rock cycle One of the most important things that you will learn in introductory geology class (e.g., GY 111) is the rock cycle. This is the fundamental concept that relates all of the rock divisions into one tight little package. Each rock type can be converted into any other rock type through different physical processes. For example, igneous rocks are converted into sedimentary rocks through weathering followed by lithification. Sedimentary rocks can be converted into metamorphic rocks through heat and pressure. Metamorphic rocks can be converted into igneous rocks through melting. You get the idea. The best way of explaining the rock cycle is through a diagram like the one pictured to the right.

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C) Sedimentary rocks; keys to Earth history As I indicated previously, I am a sedimentologist which means that I study sedimentary rocks. I do this because I like them. I am terribly biased in favor of them (as any of my students will attest to). One of the reasons why I like sedimentary rocks is their usefulness in interpreting Earth history. They are useful because they frequently preserve all sorts of information regarding the depositional environment of their formation (the so called paleoenvironment). They preserve information about current speeds, current directions, water/air temperatures, climate, and the age and tectonic setting of the materials being deposited. Geologists use the term paleocurrent when they are referring to these ancient depositional currents, or paleoclimate if they are describing the climate millions of years ago. They also employ terms like paleotectonic setting if they are dealing with the ancient tectonic environment of the rocks being studied. How do sedimentary rocks do all this? Paleocurrents are determined on the basis of sedimentary structures. When currents flow (air or water), sand particles are moved along the Earth’s surface as bed load. The sand frequently develops ripples and other types of sedimentary structures like cross-stratification. The direction of the currents responsible for the cross-stratification can be easily measured in outcrops. This gives you the paleocurrent direction. The paleocurrent speed can be estimated on the basis of the sediment size and the type of sedimentary structures. This is discussed further in GY 344, one of the core courses to which we subject our geology majors.

Paleoclimate is determined primarily through examination of rock suites (groups of similarly-aged rocks collected from a specific area). For example, rocks formed in a humid area will preserve a record of wet conditions. They may contain abundant organic material (coals), or they may record constant water current activity (cross-bedded sandstone). In contrast, arid regions tend to produce red bed type rocks (red shales, hematite stained sandstone), or in some cases, evaporite minerals such as banded gypsum or halite. Paleotectonic settings are also determined on the basis of rock suites. A sequence of rocks that contains both volcanic rocks and sedimentary rocks probably came from a tectonically active area. Now I know I said that we weren’t going to look at many igneous rocks in this class, but you will have to remember a couple in order to resolve paleotectonic environments. Divergent plate boundaries tend to produce basalts so if you

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encounter basalt (a finely crystalline generally black rock; see image to the left from the Drexel University web site), you suspect a divergent plate paleotectonic setting. In contrast, a convergent place paleotectonic setting would produce granite, a coarsely crystalline, generally pink rock (pictured to the right), as well as sedimentary rocks like arkose, breccia and red shale. Beware: Complications can occur. For example, if divergent tectonics

(e.g., rifting) occurs in the middle of an ocean basin, you would get pillow basalts associated with deep marine sedimentary rocks like chalk, greywacke and black shale. Were it to occur within a continental setting, those basalts will be associated with granites, arkose and breccias. There is one other paleotectonic setting that should be mentioned here. It is the setting when you have no active tectonics (e.g., no volcanoes, earthquakes etc.. The Gulf Coast is like this today. Passive continental margins are those coastlines that have no active tectonics. The rocks formed here included quartz arenite sandstone, limestone, green shales etc. In this type of tectonic setting, no igneous rocks can form. Age control. One of the most useful things that sedimentary rocks do is to provide information about geological age. This is primarily because they can contain fossils. Fossils are studied by paleontologists and good ones (like Dr. Clark) can determine what they are and when they lived. If you find a specific “beastie” in a sedimentary rock, and if you know the age limits of that beastie, then you automatically know the age of the rock. There are other ways to date rocks, and we will get to them over the next couple of days.

Important terms/concepts from today’s lecture (Google any terms that you are not familiar with)

Grain size (gravel, sand, silt, clay)

Sedimentary Rocks; Siliciclastic, Biochemical, Chemical, Organic, Evaporite Depositional Environment (AKA Paleoenvironment)

Rock Cycle Weathering Lithification

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Pressure Paleocurrent Paleoclimate

Paleotectonic Setting (divergent, convergent, passive continental margin) Fossil

Paleontology Sedimentology

This is the complete list of rocks that you will encounter in the Lab component of GY 112

Sedimentary Rocks

Quartz arenite lithic sandstone

greywacke arkose

conglomerate breccia

red shale black shale green shale siltstone*

fossiliferous limestone oolite (oolitic limestone)

non-fossiliferous limestone dolostone

coal amber halite

Igneous Rocks

Granite* Basalt*

Metamorphic Rocks

Schist*

*-you will not see these rocks on lab exams in GY 112