Chapter 14 Sed Rocks ZZ

8/22/2019 Chapter 14 Sed Rocks ZZ

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Chapter 14: Sediment and Sedimentary Rocks

Sedimentary rocks are those rocks that form from sediment that has been mechanicallydeposited by a fluid (e.g., wind, water, or ice) or chemically deposited (precipitated) fromsolution. As I said at the end of Chapter 12 (weathering), the end products of mechanical andchemical weathering are the raw material needed to create a sedimentary rock.

Sedimentary rocks fall into two broad categories, detrital rocks and chemical rocks. Detritalsedimentary rocks consist of cemented sediment grains that represent fragments of pre-existingrocks. Chemical rocks are deposited by the chemical process of precipitation from solution.

Sedimentary rocks form in the following way: weathering of preexisting rocks creates rockfragments, clay minerals, iron oxides, and dissolved compounds in solution. These weatheringproducts are then eroded and transported to somewhere else. Then, rock fragments aredeposited to create a detrital sediment, or chemical compounds are precipitated to create achemical sediment. The sediment (whether detrital or chemical) is then lithified (i.e., turned intoa sedimentary rock) via two processes, including compaction and cementation. To summarize,sedimentary rocks are created via the following processes:

weathering erosion/transport deposition/precipitation lithification.

Lets start by making some comments about detrital rocks. As shown in Figure 14.2, rockfragments change as they are eroded and transported. In general, with increasing transportdistance, they become smaller, rounder, and better sorted, as shown here:

Change in sediment characteristics with transport.

GRAIN SIZE DECREASES

ROUNDING INCREASES

SORTING INCREASES

Gravel/sand

si l t /clay

Delta

RiverNearshore

marine

Offshore

marine

Alluvial Fan

Sand

si l t /claySilt/clay Clay/CaCO3/S iO2

Sea level

Non-marine Marine


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Think of sorting as representing grain size similarity. A well-sorted sediment or sedimentaryrock contains grains that are all similar in size. In contrast, a poorly sorted sediment orsedimentary rock contains a wide range of grain sizes, as shown here:

We can get a general sense of how far a river sediment has been transported by making some

basic observations about grain size, degree of rounding, and degree of sorting. This gives us abasic point of comparison between modern sediments and ancient sedimentary rocks.

To understand the environment in which an ancient sedimentary rock formed, we compare thecharacteristics of this ancient rock with the characteristics of modern sediments.

Stated another way, we use present depositional environments as a standard of comparison inunderstanding ancient depositional environments. The whole game with sedimentary rocks is tofigure out as much as we can about the ancient depositional environment in which thesedimentary rock formed. Fundamentally, the only way to do this is to compare ancientsedimentary rock characteristics with those of modern sediments forming in present-day

depositional environments.

Hey, did you notice that were invoking one of geologys big ideas here? For an honoraryA, doremember the name of the principle that the present is the key to the past?

That would be.uniformitarianism! Simply put, we understand ancient depositionalenvironments in terms of modern ones.

Once we get far enough offshore, the only detrital sediments we find are very fine-grained claydeposits. Under the right conditions, chemicals can precipitate directly from sea water or withthe help of small organisms (plankton) living in the water column. In both cases, chemical

deposition (precipitation) can lead to the accumulation of biological sediments and ultimatelybiochemical sedimentary rocks.

Alluvial fans and rivers are two common non-marine depositional environments, whereas marinedepositional environments include nearshore (e.g., continental shelf) and offshore (e.g., abyssalplain) environments.

Heres an image of an alluvial fan in Death Valley:

Poorly sorted Well sorted

Poorly sorted vs. well sorted sediment.


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Note the road for scale along the bottom of the fan. High up, near the head of the fan, thesediments are quite gravelly. Further downstream on the fan surface, near the road, thesediments become more sandy.

Heres a photo of some ancient alluvial fan deposits in the Mecca Hills, southeast of PalmSprings:

I didnt have a hand lens or car keys with me when I took the above picture, so I impro vised interms of showing an object for scale . Note the large fragment sizes, angular shapes, andpoor sorting of the fragments in the above photo. Having been lithified, this deposit has beenconverted into sedimentary rock (conglomerate). However, we can correlate the characteristics

Alluvial Fan, Death Valley, CA

(image courtesy Google Earth)

Alluvial fan deposits, Mecca Hills, CA.

2,000 ft approx.


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of this rock (e.g., grain size, rounding, and sorting) with similar ones seen in modern alluvial fanenvironmentsthats how geologists know this rock formed in an ancientalluvial fanenvironment.

It should be clear to you by now that gravelly detrital sedimentary rocks like the one abovetypically form in an upstream environment, near the base of mountains, because thats wherewere likely to find, large, poorly sorted, angular fragments that havent trave lled very far from

their source (and so havent become very well rounded or sorted).

In contrast, heres a photo of some shale from the Borrego Formation, near Borrego Springs,CA (note my ring for scale).

The above rock is very, very fine-grained, well sorted, and mostly consists mostly of clay. Youdneed a high-powered microscope to see individual sediment grains.

The above rock is very thin-bedded, which is why we call it a shale rather than claystone (shale= thin-bedded claystone). The white, needle-like crystals growing in between bedding planes

(sedimentary layers) are gypsum crystals.

Judging from the fine grain size, this rock formed in a downstream environment, where currentswere weak.

Once again, we can determine the ancient depositional environment of this rock by correlatingits characteristics with modern depositional environments.

Gypsiferous shale, Borrego Formation, near

Borrego Springs, CA.


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In this case, the shale in the above photo was deposited in an ancient lake that last occupiedthe Borrego Valley about a million years ago. Around that time, the climate became more arid(dry), and evaporative conditions resulted in the precipitation of calcium sulfate (i.e., the mineral,gypsum) between bedding planes. The presence of abundant gypsum tells us that this rockformed under evaporative conditions.

Once a sediment is deposited, it becomes hardened, or lithified, by two processes, including

compaction and cementation (Figure 14.6). Common sedimentary cements include calciumcarbonate (calcite, CaCO3), iron oxide (hematite, Fe2O3), and silica (quartz; SiO2). Rememberthat iron oxide, silica, and the chemical building blocks of calcium carbonate (HCO3

- and Ca++)are common chemical weathering products, so heres a great example of the end products ofweathering serving as the raw material for sedimentary rocks.

Your book goes into quite a bit of detail describing various detrital sedimentary rocks, includingconglomerate, breccia, sandstone, and shale. You should familiarize yourself with the overallproperties of these rocks.

Study Table 14.1 (and the summary rock table, below), but dont worry about knowing particle

diameters. Just remember that conglomerate and breccia are coarse-grained, gravellysedimentary rocks. Sandstone is obviously composed of sand-sized fragments. Mudrocksinclude mudstone, siltstone, and shale. Mud in this context just refers to a fine-grained detritalsediment that represents a mixture of silt- and clay-sized particles. Clay in this context refersto very, very fine-grained particles (many of which also happen to be clay minerals, but its alsopossible to have quartz and feldspar grains in the clay-sized range). Shale is simply mudstoneor claystone that is finely layered (laminated) such that it tends to break apart in thin sheets.

Heres a link to an absolutely amazing scanning electron microscope image of a detritalsandstone (magnified millions of times!)basically some rounded quartz grains cementedtogether with calcite: SEM Sandstone Image.

As you might imagine, many detrital sedimentary rocks consist of a mixture of gravel, sand, silt,and clay. Gravel-rich rocks are easy to identify in the field because the grains are clearly visible.But what about finer-grained rocks that may be mixtures of sand, silt, and clay?

Over hundreds of years, geologists have developed a very sophisticated field test for judging therelative amounts of sand-, silt-, and clay-sized particles in detrital sedimentary rocks.the tastetest!

Basically, it works like this: say you walk up to an outcrop and want to get an approximatesense of how much sand vs. silt vs. clay comprises the detrital rock youre studying. Just break

off a small piece, pop it into your mouth, and chew on it a bit! If your rock is crunchy, itsmostly made of sand. If its gritty, its mostly silt, and if it has a wonderfully creamy texture,its mostly clay. So when a geologist says shes hungry for knowledge about detritalsedimentary rocks, you can take her at her literally .

Heres a simple classification chart for detrital sedimentary rocks:
http://www.greenelectron-images.co.uk/sem/sandstone/calcareous_sandstone.htmlhttp://www.greenelectron-images.co.uk/sem/sandstone/calcareous_sandstone.htmlhttp://www.greenelectron-images.co.uk/sem/sandstone/calcareous_sandstone.html


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DETRITAL SEDIMENTARY ROCKS

Particle Size Description Rock Name (link)

Coarse (> 2 mm)

Gravel

Rounded rock or mineral fragments,

typically poorly sorted.Conglomerate

Angular rock or mineral fragments, typically

poorly sorted.Breccia

Medium

(1/16 - 2 mm)

Sand

Quartz grains & minor accessory minerals;

typically well-sorted, rounded grains;

cements of all types common

Quartz Sandstone

At least 25% feldspar Arkose

at least 20% clay minerals, some rock

fragments, quartz, and feldspar; poorly

sorted

Greywacke

Fine

(1/16 - 1/256 mm)

Silt

Composed of mostly silt-sized particles;

gritty texture.Siltstone

Fine to Very Fine

(1/16 to less than 1/256 mm)

Silt and Clay (Mud)

Composed of a mixture of silt- and clay-sized

particles (mud) or clay-sized particles only;

laminated (thin-bedded).

Shale

Fine to Very Fine

(1/16 to < 1/256 mm)

Composed of a mixture of silt- and clay-sized

particles (mud) or clay-sized particles only;non-laminated (not thin-bedded). Mudstone/Claystone

You wont be required to distinguish various types of sandstone; just know that sandstone is adetrital sedimentary rock composed of sand-sized fragments.

Okay, on to the chemical rocks. Common chemical sedimentary rocks include limestone, chert,and evaporates like rock salt and rock gypsum. We distinguish these various chemical rocksmainly on the basis of chemical composition. Limestone is perhaps the most common chemicalsedimentary rock. It consists mostly of the non-silicate mineral, calcite (calcium carbonate),which can form either organically or inorganically.

Although limestone composed of shell material is biochemical, such limestone still mostlyconsists of fragments that range in size from relatively large to microscopic. Coarse-grained,shell-hash limestone is called coquina (Figure 14.20), whereas fine-grained limestonecomposed of the shells of marine microorganisms is called chalk.

Heres a classification chart for chemical sedimentary rocks:
http://en.wikipedia.org/wiki/File:CambrianConglomerate.jpghttp://libraryphoto.cr.usgs.gov/cgi-bin/show_picture.cgi?ID=ID.%20Hamilton,%20W.B.%201315cthttp://en.wikipedia.org/wiki/File:Sandstone(quartz)USGOV.jpghttp://en.wikipedia.org/wiki/File:TreesUpUluru.JPGhttp://en.wikipedia.org/wiki/File:Mineraly.sk_-_kremenec.jpghttp://en.wikipedia.org/wiki/File:SiltstoneUSGOV.jpghttp://en.wikipedia.org/wiki/File:ShaleUSGOV.jpghttp://en.wikipedia.org/wiki/File:ShaleUSGOV.jpghttp://esp.cr.usgs.gov/info/kt/fig9.jpghttp://libraryphoto.cr.usgs.gov/cgi-bin/show_picture.cgi?ID=ID.%20Waage,%20K.%20M.%20%2021http://libraryphoto.cr.usgs.gov/cgi-bin/show_picture.cgi?ID=ID.%20Waage,%20K.%20M.%20%2021http://libraryphoto.cr.usgs.gov/cgi-bin/show_picture.cgi?ID=ID.%20Waage,%20K.%20M.%20%2021http://libraryphoto.cr.usgs.gov/cgi-bin/show_picture.cgi?ID=ID.%20Waage,%20K.%20M.%20%2021http://esp.cr.usgs.gov/info/kt/fig9.jpghttp://en.wikipedia.org/wiki/File:ShaleUSGOV.jpghttp://en.wikipedia.org/wiki/File:SiltstoneUSGOV.jpghttp://en.wikipedia.org/wiki/File:Mineraly.sk_-_kremenec.jpghttp://en.wikipedia.org/wiki/File:TreesUpUluru.JPGhttp://en.wikipedia.org/wiki/File:Sandstone(quartz)USGOV.jpghttp://libraryphoto.cr.usgs.gov/cgi-bin/show_picture.cgi?ID=ID.%20Hamilton,%20W.B.%201315cthttp://en.wikipedia.org/wiki/File:CambrianConglomerate.jpg


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CHEMICAL SEDIMENTARY ROCKS

Origin Composition Texture/Description Rock Name (link) Origin

Inorganic

Chemical

Calcium Carbonate

CaCO3

Crystalline

(interlocking crystals)Limestone

Precipitated directly

from sea water,

surface water (lakes)

or groundwater

(caves/springs).

Calcium Magnesium

Carbonate

CaMg(CO3)2

Crystalline

(interlocking crystals)Dolostone

Alteration of

limestone by

magnesium-rich

solutions.

Silica

SiO2

Microcrystalline

(interlocking

microscopic crystals);

well-cemented

ChertPrecipitation from

silica-rich fluids.

Sodium Chloride

NaCl

Crystalline

(interlocking crystals)Rock Salt

Evaporation of saline

lakes or sea water.

Calcium Sulfate

CaSO4-2H20

Crystalline

(interlocking crystals)Rock Gypsum

Evaporation of saline

lakes or sea water.

Bio-chemical

Calcium Carbonate

CaCO3

Visible shells/shell

fragments, loosely

cemented

Coquina

(limestone)

Cementation of

shells, shell

fragments.

Microscopic marine

organisms, loosely

cemented

Chalk(limestone)Cementation of

skeletal material.

Shell fragments,cemented with calcite

cement

Fossiliferouslimestone

Cementation ofshells, shell

fragments.

Silica

SiO2

Microscopic marine

organisms, well

cemented

ChertCementation of

skeletal material.

Organic Matter Fine-grained CoalCompressed plant

matter.

Most limestones are biochemical and form in warm, shallow marine waters by invertebrates that

construct their shells out of calcium carbonate. You and I do the same thing except that we usecalcium phosphate to build our bones and teeth.

Note that limestone and chert can form both inorganically and biochemically, althoughbiochemical chert and limestone are more common than their inorganic counterparts.Chalk, for example, is a biochemical form of limestone composed of billions of microscopicorganisms (plankton) that float freely in sea water. When these critters die, their shells sink tothe sea bottom, and in some environments accumulate to form chalk.
http://en.wikipedia.org/wiki/File:OrdOutcropTN.JPGhttp://en.wikipedia.org/wiki/File:7092_pieskovna_Dolinka_pri_Hradisti_pod_Vratnom_dolomit.JPGhttp://en.wikipedia.org/wiki/File:ChertUSGOVjpg.jpghttp://upload.wikimedia.org/wikipedia/commons/0/08/Rock_salt_Poland.JPGhttp://libraryphoto.cr.usgs.gov/cgi-bin/show_picture.cgi?ID=ID.%20Burchard,%20E.F.%20%20511http://en.wikipedia.org/wiki/File:CoquinaFlorida.jpghttp://en.wikipedia.org/wiki/File:CoquinaFlorida.jpghttp://en.wikipedia.org/wiki/File:White_cliffs_of_dover_09_2004.jpghttp://en.wikipedia.org/wiki/File:White_cliffs_of_dover_09_2004.jpghttp://en.wikipedia.org/wiki/File:OrdFossilsMN.JPGhttp://en.wikipedia.org/wiki/File:OrdFossilsMN.JPGhttp://en.wikipedia.org/wiki/File:ChertUSGOVjpg.jpghttp://en.wikipedia.org/wiki/File:Coal_mine_Wyoming.jpghttp://en.wikipedia.org/wiki/File:Coal_mine_Wyoming.jpghttp://en.wikipedia.org/wiki/File:ChertUSGOVjpg.jpghttp://en.wikipedia.org/wiki/File:OrdFossilsMN.JPGhttp://en.wikipedia.org/wiki/File:OrdFossilsMN.JPGhttp://en.wikipedia.org/wiki/File:White_cliffs_of_dover_09_2004.jpghttp://en.wikipedia.org/wiki/File:CoquinaFlorida.jpghttp://libraryphoto.cr.usgs.gov/cgi-bin/show_picture.cgi?ID=ID.%20Burchard,%20E.F.%20%20511http://upload.wikimedia.org/wikipedia/commons/0/08/Rock_salt_Poland.JPGhttp://en.wikipedia.org/wiki/File:ChertUSGOVjpg.jpghttp://en.wikipedia.org/wiki/File:7092_pieskovna_Dolinka_pri_Hradisti_pod_Vratnom_dolomit.JPGhttp://en.wikipedia.org/wiki/File:OrdOutcropTN.JPG


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Heres a link to a website where you can see scanning electronic microscope (SEM) images ofone example of such a crittera calcarious planktonic organism called a coccolith: SEM Imageof Coccoliths in Chalk. Check it out!

Dolomite (or more correctly, dolostone) is just magnesium-rich limestone.

In Florida, theres an amazing military fortress made entirely out of coquina (shell hash)

limestone. Although coquina is about the last rock one would normally choose in making amilitary fortress (because its very soft), the one in Florida has held up quite well over thecenturies. Its so soft that cannon balls just get lodged inside the soft, coquina walls! Heres alink if youre curious:Coquina Fortress(scroll down the page to see a close-up photo ofcoquina).

Chert is a fine-grained chemical sedimentary rock formed mostly of pure silica (microcrystallinequartz).

Like limestone, chert can form both inorganically and biochemically. Its probably fair to say thatmost chert forms in deep marine waters.

When planktonic (free floating) organisms that make their shells out of silica die, their bodiessink to the sea floor and accumulate as siliceous ooze. With lithification, a hard, silica-richbiochemical sedimentary rock is produced called chert.

Chert is a common rock type found in ancient subduction zones, because it often gets scrapedoff the subducting plate and plastered onto the non-subducting plate as subduction proceeds.

Heres an SEM image of a common microscopic organism from which chert is madea silica-rich, shelled animal (zooplankton) called a radiolarian: SEM Image of Radiolarian. Chert istypically packed full of these critters!

Evaporites (mostly rock salt and rock gypsum) obviously form in drying, evaporativeenvironments such as playa lakes (Figure 14.24). As such, evaporates are good indicators ofdry environments.

A modern example of inland, playa (evaporative) lake is Searles Dry Lake, in the Mojave Desert,near Ridgecrest, California.

Every year, thousands of folks descend on Searles Dry Lake for the annual, Gem-O-Ramamineral show and also for the chance to collect rare, pink halite (salt) crystals from the lake bed.

I had the opportunity to experienceGem-O-Ramaa few years back.

Heres what the lake bed looks like:
http://www.greenelectron-images.co.uk/sem/coccoliths/chalk-1.htmlhttp://www.greenelectron-images.co.uk/sem/coccoliths/chalk-1.htmlhttp://www.greenelectron-images.co.uk/sem/coccoliths/chalk-1.htmlhttp://www.gly.uga.edu/railsback/BS-SA.htmlhttp://www.gly.uga.edu/railsback/BS-SA.htmlhttp://www.gly.uga.edu/railsback/BS-SA.htmlhttp://www.ucl.ac.uk/GeolSci/micropal/images/radi/radi007.gifhttp://www.ucl.ac.uk/GeolSci/micropal/images/radi/radi007.gifhttp://www1.iwvisp.com/tronagemclub/GEM-O-RAMA.htmhttp://www1.iwvisp.com/tronagemclub/GEM-O-RAMA.htmhttp://www1.iwvisp.com/tronagemclub/GEM-O-RAMA.htmhttp://www1.iwvisp.com/tronagemclub/GEM-O-RAMA.htmhttp://www.ucl.ac.uk/GeolSci/micropal/images/radi/radi007.gifhttp://www.gly.uga.edu/railsback/BS-SA.htmlhttp://www.greenelectron-images.co.uk/sem/coccoliths/chalk-1.htmlhttp://www.greenelectron-images.co.uk/sem/coccoliths/chalk-1.html


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Its a veritable moonscape consisting of a halite crust and hundreds of salty brine pools. Thebest halite crystals are found within the brine pools, underneath overhanging ledgesyes, youhave to wade in, but its worth getting hot, wet, and salty. Heres a photo of some spectacularpink halite crystals I found in one of the brine pools:

The pink color comes from millions of tiny, salt-loving microorganisms that become incorporatedinto the growing halite crystals as they precipitate.

Searles Dry Lake, Trona, CA

Pink halite crystals, Searles Dry Lake, Trona CA.


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Here again, we can understand the past in terms of the present. Anywhere in the geologicrecord we find rock salt deposits, we know that the area was once a playa lake, or perhaps anevaporating sea, because we know from direct observation that playa lakes and evaporatingseas are where extensive salt deposits form in current geologic time.

Coal (lithified plant material) is another very important biochemical sedimentary rock foreconomic reasons. In contrast to evaporates, coal beds form is wet, soggy, swampy

environments. Heres a photo of a large coal mine in Wyoming:

Wyoming coal mine(courtesy Wikipedia;http://en.wikipedia.org/wiki/Coal).

Make sure youre familiar with all of the common sedimentary rock types mentioned above.Basically, youll want to become pretty familiar with Tables 14.1 and 14.2 and the summarycharts above. On the exam, Ill probably ask you to name various examples of both detrital andchemical sedimentary rocks. Or, Ill give you a description of the rock and ask you to name it.

In general, sedimentary rocks contain clues about the depositional environments in which theyformed. What clues? Things like grain size, degree of rounding, degree of sorting, and evenrock composition, not to mention occasional fossils.

Sometimes the sequence of a group of sedimentary rocks tells us something about ancientdepositional environments and how theyve changed with time.

Try the following link for a simple animation that shows how ancient sea level changes can beinferred from sedimentary rock sequences: Transgression and Regression (scroll down to5.2).

Sedimentary rocks also contain other clues about their depositional environmentsclues thatgeologists call sedimentary structures.
http://en.wikipedia.org/wiki/Coalhttp://en.wikipedia.org/wiki/Coalhttp://en.wikipedia.org/wiki/Coalhttp://www2.wwnorton.com/college/geo/egeo/animations/ch5.htmhttp://www2.wwnorton.com/college/geo/egeo/animations/ch5.htmhttp://www2.wwnorton.com/college/geo/egeo/animations/ch5.htmhttp://www2.wwnorton.com/college/geo/egeo/animations/ch5.htmhttp://en.wikipedia.org/wiki/Coal


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Think of a sedimentary structure as any feature within a sediment or sedimentary rock thatdeveloped during or after deposition or precipitation. Heres a partial list of commonsedimentary structures and what they tell us about depositional environment:

Beddinghorizontal layering. This is the most common sedimentary structure of all, and itspresence is usually what alerts us to the fact that were dealing with a sedimentary rock ( Figure14.26).

Interbedded sandstone and siltstone.

Crossbeddinginclined layering; a common structure found in sandstones; indicative of amarine or nonmarine dune environment (Figure 14.28). Crossbeds are inclined downward in a

downstream direction. For an animated version of the process that produces crossbedding, trythisCrossbedding Animation (5.1).

Crossbedded sandstone, Torrey Pines State Park.

Downstream paleocurrent direction
http://www2.wwnorton.com/college/geo/egeo/animations/ch5.htmhttp://www2.wwnorton.com/college/geo/egeo/animations/ch5.htmhttp://www2.wwnorton.com/college/geo/egeo/animations/ch5.htmhttp://www2.wwnorton.com/college/geo/egeo/animations/ch5.htm


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Ripple Marksindicative of fairly rapid current velocities (Figure 14.29); typically form in river,tidal, desert, and delta environments. Heres a good onlinephoto of some ripple marks.

Ripple marks, Anza-Borrego State Park.

Mud Crackspolygonal cracks sometimes preserved in fine-grained sediments as they dry;indicative of a drying environment (e.g., a playa lake or floodplain). Try this link for some onlinephotosof mudcracks.

Mudcracks, Anza-Borrego State Park.
http://en.wikipedia.org/wiki/Image:Ripple_marks_in_Moenkopi_Formation_rock_off_of_Capitol_Reef_Scenic_Drive.jpeghttp://en.wikipedia.org/wiki/Image:Ripple_marks_in_Moenkopi_Formation_rock_off_of_Capitol_Reef_Scenic_Drive.jpeghttp://en.wikipedia.org/wiki/Image:Ripple_marks_in_Moenkopi_Formation_rock_off_of_Capitol_Reef_Scenic_Drive.jpeghttp://geology.about.com/library/bl/images/blmudcracks.htmhttp://geology.about.com/library/bl/images/blmudcracks.htmhttp://geology.about.com/library/bl/images/blmudcracks.htmhttp://geology.about.com/library/bl/images/blmudcracks.htmhttp://geology.about.com/library/bl/images/blmudcracks.htmhttp://en.wikipedia.org/wiki/Image:Ripple_marks_in_Moenkopi_Formation_rock_off_of_Capitol_Reef_Scenic_Drive.jpeg


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Raindrop Imprintssmall depressions created by the force of falling raindrops on muddysediment.

Raindrop imprints, Anza-Borrego State Park.

Imbricationa domino-style stacking of gravel fragments with long axes inclined upward in adownstream direction.

Imbricated conglomerate, Anza-Borrego State Park.

Downstream transport direction


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Fossils (evidence of ancient life) are also an important component of sedimentary rocks, in partbecause sedimentary rocks are generally the only type of rock that contains fossils. Think of thefossils in a sedimentary rock as a sort of sedimentary structure. Fossils tell us a lot about thecharacteristics of ancient life, but they also tell us a lot about ancient depositional environments.Most often, fossils can be used to distinguish marine from nonmarine depositional environmentsand can also tell us something about ancient climates.

Scallop fossils from the San Diego Formation, Tourmaline Beach.

One last section to read carefully is the one entitled, Plate Tectonics and Sedimentary Rocks.Like almost every other aspect of geology, we can relate sedimentary rock types in a generalway to plate tectonic environment. Let me summarize for you:

Thick accumulations of detrital sedimentary rocks can often be correlated with nearby uplift andmountain building processes occurring at convergent plate boundaries.

Detrital sedimentary rocks containing abundant volcanic rock fragments (greywackesandstones, for example) are often associated with subduction zones, where nearby island arcand continental arc volcanoes emit volcanic debris.

Fault-bounded basins with thick sedimentary rock accumulations are also associated withtransform plate boundaries. As your text points out, many of the petroleum reserves inCalifornia formed in sedimentary basins that developed as the San Andreas transform faultsystem developed.

Divergent boundaries often produce diverse groups of sedimentary rock, with coarse-grained,

gravel-rich detrital sedimentary rocks and basaltic volcanic rocks developing along the sides ofa rift valley in the early stages of plate divergence. Evaporites may form out in the middle of thewidening rift valley (Figure 4.42).

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Chapter 14 Sed Rocks ZZ