Description of Sedimentary Rocks in the Field, in Core, and Hand Samples

This laboratory exercise deals with the methods and techniques employed in the macroscopic examination of sedimentary rocks. This is the aspect of observation and description that we as professional geologists are engaged in most of the time throughout our careers. The hand lens is our main tool for observation. Get in the habit of always having your hand lens with you everywhere you go. Many geologists carry the hand lens on a lariat or string hung around their necks. There is a major safety issue in this in that if you are in brushy areas and take a fall you could end up hanging your self.  This risk can be eliminated by carrying the lens in your pocket or at least minimized if the string is of some weak material like cotton cord which will break easily. This of course is not much of an issue in the laboratory or office. A rock hammer and a pocket knife are also useful. If you find yourself without a rock hammer then go to nearest hardware store and purchase a mason’s hammer or a three pound sledge hammer. Do not use a claw hammer or a ball peen hammer. For a pocket knife just a cheep small thing will do best and it does not have to be sharp. If you are traveling and need to acquire a hammer then at the same time purchase a putty knife because it really works nicely in place of a pocket knife.

An important skill that you need to develop is the taking of notes when observing rocks. A good way to learn to do this is to get in the habit of sequentially looking for various features and recording your observations as you proceed. A good sequence of description for sedimentary rocks in the field is:

1. Fieldname

2. Color

3. Bedding or stratification

4. Grain size (conglomerate and sandstone), clay content (mudrocks), crystal size (limestone, dolostone)

5. Induration or cementation (conglomerate, sandstone, and mudrocks)

6. Primary sedimentary structures

7. Primary minerals, grains, allochems

8. Fossils

9. Porosity

10. Secondary features (minerals and structures)

11. Weathering features (parting character)

12. Outcrop character

13. Thickness

Get in the habit of examining and recording the information in your field book in this sequence. To help you remember the sequence write a copy of it on the inside cover of your field book. Please note that sometimes some of the above properties will not be evident in the field; do not make up information. If you do not observe fossils skip it in your description, avoid using the negative i.e. non-fossiliferou, non-porous, etc. 

Fieldname: This is a very general name applied to the rock, names that everyone is familiar with and that there will be little confusion over.   Remember that those who are reading your descriptions will not be able to ask you for your meaning of this or that. These names do not have any modifiers they stand alone. They are primarily used as a quick look at what the layer is. If your field description is accompanied by a graphic log then each of these will have its own unique pattern and/or color. Below is a list of names that any geologist should be familiar with. Names in parentheses represent the names you would use for unconsolidated material.

English	Spanish
Sandstone (sand)	Arenisca (arena)
Conglomerate (gravel)	Conglomerado (grava)
Breccia (rubble)	Breccia
Siltstone (silt)	Limolita (limo)
Shale or mudstone (mud)	Lutita (barro)
Limestone (shells, coral debris, lime mud, carbonate sand)	Caliza (lime mud = lado calcico)
Dolostone or dolomite	Dolomita
Gypsum or gyprock	Yeso
Anhydrite	Anhidrita
Halite or rocksalt	Halita
Ironstone	Mineral de hierro
Phosphorite	Fosforita

Some names like graywacke, arkose, marl and trap have more than one conflicting meaning and should not be used.  Names like puddingstone and lithographic limestone no longer convey their meaning well since most of us have never see a true lithograph or have eaten a real English pudding. The term oolite, though is does still convey some meaning can present problems in that many oolites that you will encounter in the field you will not recognize as being an oolite until you make a thin section of it while others you will call an oolite only to find out later with the aid of a thin section that is was limestone made of pelloids. It is best to simply call it a limestone. The name coquina has a very specific meaning but unfortunately it has been greatly misused in recent times thanks to freshman geology laboratory manuals. For both oolite and coquina call them limestone and the further aspects of your description will bear out the detail lost by misuse of the name.

Note: Sometimes it can be difficult to tell sandstone from a weathered dolostone or even a limestone and the HCl trick you learned in freshman geology is not really a great test. But remember this; the hardness of quartz relative to you rock hammer is what? And the hardness of calcite or dolomite relative to your rock hammer is what? (i.e. quartz will scratch your rock hammer but calcite and dolomite will not). Also sandstone looking dolostone will frequently have really nice euhedral dolomite rhombohedrons so use your hand lens.

Color:  The problem with color is that what is brown to me may not be brown to you. Most field geologists ignore this problem and call it as they see it and assume that the next guy will have no problem. To convey the best information use common well-defined terminology; then green will be green to all, whereas ‘carolina blue’ would be a mystery to someone from Canberra, Australia. To do this you need a standard for comparison, just like buying paint in the paint store you need color chips. In practice we have three choices:

Goldman, M.I. and Merwin, H.E., 1928. Color chart for the field description of sedimentary rocks. National Research Council, Washington, D.C.

Goddard, E. N., Trask, P. D., DeFord, R. K., Rove, O. N., Singewald, J.T., and Overbeck, R.M., 1970. Rock-color chart. Geological Society of America, Boulder, Colorado.

Munsell Soil Color Charts, 1994 Revised Edition, Macbeth Division of Kollmorgan Instruments Corporation.

Goldman and Merwin (1928) is not much used anymore and really difficult to find a copy of but it does pop up in the literature from time to time; especially in soil science reports prior to 1970. The Goddard et al (1970) chart is more commonly known as the Munsell color code in honor of the Munsell Color Company, the folks who make the color sheets from which GSA makes the color charts. This is the chart that most geologist, engineers, and soil scientists now use. The idea is to sort through the chart sheets looking for the color chip which best matches your sample. There is usually supplied with the charts a cardboard mask for you to layover your sample to give it the same view as that of the chips, and it does really help. Names are recorded in your notes both verbally and symbolically [i.e.: moderate reddish brown, (10R4/6) this latter symbol is read "ten R four-over-six." If a sample has multiple colors it is important to name each and describe their relationship to one another. For example: "mottled, pale red (10R6/2) & pale yellowish orange (10YR8/6)." Multiple colors can be: mottled, interbedded, interlaminated, or gradational.

Grain size, sorting, roundness (sandstone, conglomerate and certain types of limestone): Grain size for sandstones to conglomerates follows the same procedure as discussed in Sediment or Sand Observation and Description. Treat this as a general populations of grains, you will add grain type specifics later on. If you are in the field with only simple equipment then estimate the mean size of the population of grains and state if they are:

· well sorted: most of the grains are of the same diameter 

· sorted: there is some variation in diameter but that variation is not great.

· poorly sorted: there is a moderate variation in grain size and the spread in sizes is fair.

· nonsorted: considerable variation in grain size and the spread in sizes is considerable.

With conglomerate you may also be able to make a statement about grain shape, see procedures in A Brief Study of Pebbles or the Observation and Description of Grains.

Clay Content (shale):  This is clay in the particulate sense, not the mineralogical sense. A major component of shale is clay sized quartz, feldspar and dolomite not just clay (the mineral group). Possible terms exists for stating the clay content in a field description:

gritty: you look at it and can see particles of anything (0 to 32% clay content).

loamy: particles not evident by eye but when you place a small amount on your teeth you can sense your teeth grinding the grit. (33 to 65% clay content)

fat or slick: when you place some on your teeth and get no sensation of grittyness (66 to 100% clay content). The sensation is like biting into a fine slab of chocolate.

Be aware of what you are doing when making this test. In the middle of the Canadian Arctic where man and more importantly sheep have never been there should be no problem... but down stream from a septic tank drain you could get more than you bargained for. And trust me this test is not worth the risk of the green apple two step or worse. (PS if you do chose to do this test just keep in mind that the material of the average shale has been through the alimentary canal of some animal at least ten times; think about what you are putting in your mouth!!!)

Crystal Size (limestone, dolostone, gypsum, anhydrite, halite): For sedimentary rocks where grain size is not apparent but crystal size is then characterize the crystal size in your description. You should not be doing this for sandstone, conglomerate, breccia, and shale and any limestone where you can see the size character of the grains. Generally these terms will be used for dolostone, gypsum, anhydrite and other chemical sedimentary rocks. The terms below are those that can be applied, however it is not exactly easy to differentiate this many size categories without a microscope and thin sections. Generally you should not be able to differentiate between cryptocrystalline, aphanocrystalline and very finely crystalline in hand sample. As a reference, good old sugar or salt in the shaker would be medium crystalline. Some old geology reports will actually characterize a dolostone as being 'sucrosic'. The analogy that is being made is that it looks like brown sugar; i.e. it is medium crystalline, has a lot of intercrystalline porosity and is light brown or buff colored.

· cryptocrystalline less than 0.001 mm

· aphanocrystalline 0.001 to 0.004 mm

· very finely crystalline 0.004 to 0.016 mm

· finely crystalline 0.016 to 0.062 mm

· medium crystalline 0.062 to 0.25 mm

· coarsely crystalline 0.25 to 1.0 mm

· very coarsely crystalline 1.0 to 4 mm

· extremely coarsely crystalline greater than 4 mm

Bedding or stratification: This refers to a feature resulting from the deposition of the sediment, a primary sedimentary structure. It should not be confused with "parting" or how the sample is breaking up... the two are not the same and the physical orientation of parting may be oblique to bedding.  Look at the sample, preferably in outcrop, look for repetitive changes in the color or texture, grain size, etc. and determine the "average" thickness of these repetitions. This can be express by using the terms below or using actual physical measurements. When dealing with shale in the field this aspect may not be evident so you might say that the material is "structurless" or just not say anything at all. In the lab with the aid of x-ray radiography one might actually see all kinds of neat bedding features in the structurless shale. Additional modifiers like wavy, parallel or discontinuous might be added to the main terms, however this is not the place for a full discourse on the sedimentary structures present.

• very thinly laminated less than 0.5 mm
• thinly laminated 0.5 to 3.0 mm
• thickly laminated 3 to 10 mm
• very thinly bedded 10 mm to 3 cm
• thinly bedding 3 cm to 10 cm
• medium bedded 10 cm to 30 cm
• thickly bedded 30 cm to 1 m
• very thickly bedded over 1 m

Sometimes you will come across the term massive. This generally refers to a very thick, greater than a few meters, non-bedded layer as in massive sandstone.

Induration or cementation: This refers to the durability of the material....how well it is lithified. This is somewhat like hardness. We say:

· very well indurated: the rock will be good for sharpening a tool, a very hard material, a hammer will ring when you strike it.

· well indurated: hard rock, will need a hammer to work it with moderate difficulty, will dull a knife blade but a knife will mar the rock.

· moderately indurated: easily broken with a hammer which will make a "thunk" sound, easily carved with a knife.

· poorly indurated: when struck with a hammer it will make a "thud" sound and the hammer may also stick into the rock. Sandstones will be easily broken apart just by rubbing their surface. Sometimes the term friable is applied here for the sandstones. Regardless of rock type a swift kick with a field boot will do damage to the rock.

· non-indurated: can be worked with a spoon, think of digging in the mud when you were little boys and girls, will stick to boot when kicked. 

Most limestone, dolostone and anhydrite are indurated and therefore this is usually skipped in their description unless the material is perceived to be moderately indurated, poorly indurated, on non-indurated.

If you have used the sediment names, sand, gravel, mud, silt etc. then being non-indurated is implied and you should skip repeating that the material is non-indurated.

Primary Sedimentary Structures: At this point in your description you cover all other primary sedimentary structures excluding bedding that you noted earlier. This is a point where a lot of detail can be built into your description. But keep in mind what the overall objectives of your description are and how much time you have to elaborate on sedimentary structures. The type of information that you studied in Interpretation of Sedimentary Structures should be recorded here. Also line drawings frequently help to document you observations. Trace fossils should also be reported in this section with the possible exception of borings in body fossils. Those you will report and describe later, see Fossils below.

Primary minerals, grains and alochems: These are the mineral types and grain types that were a part of the sediment at deposition. List in order of abundance; see Sediment or Sand Observation and Description. You must have direct visual evidence of these to be listed. For shale do not expect to always have something to report. If you have larger grains then make an attempt to characterize them in terms of the mineralogy or lithology. With limestone you assume that calcite is present and do not report such; similarly for dolostone, anhydrite, gypsum, and rocksalt. Fossils and the mineralogy of fossils are reported next, not here. If you are describing a conglomerate and have the time or the need here would be where you would report the results of a pebble count. Allochems are grains that formed as a result of processes within the depositional setting. These are ‘new’ mineral aggregates, not fragments of preexisting rocks. These include ooids, pelloids, intraclasts and fossils however fossils are treated next as a separate item.

Fossils:  If you see fossils of any type the unit then is characterized as being fossiliferous. Some geologist will add a modifier to this like sparsely, slightly, very, extremely, etc. These terms are not always useful in that one person's sparsely fossiliferous is another's slightly fossiliferous. You always note any kind of fossil material and you name it to the best of your abilities. We do this by placing this information is parentheses after the word fossiliferous. The sequence of appearance in the list is from the most abundant to the least abundant. You may at this point also and in any additional information relative to the fossils present such as abrasion, articulation, evidence of predation, orientation, etc.

    fossiliferous (trilobite, brachiopod, gastropod)

    fossiliferous (corals-Zaphrentis sp. and  Cannina sp., gastropod)

    fossiliferous (mollusk-Crassostrea virginica greatly abraded, Dinocardium robustum fragments, Donax variabilis mixed articulated and disarticulated)
Porosity: Any visual evidence of porosity is noted, quantified if possible and characterized. Sedimentary rocks will all have some degree of porosity but if you can visually see such with the unaided eye than the samples is described as being porous. If the observed pores represent a significant volume of the rock than make a semi-quantitative estimate of the porosity. Characterize the pore types by listing them in parentheses from most abundant to least.

    porous (few vugs 1 to 5 mm in diameter)

    porous 20% (moldic pores after pelecypods, some fractures)

Secondary minerals: Any mineral present in the rock that was not present at deposition is a secondary mineral. In the unit description these form a list form most abundant to least abundant. These are expressed as adjectives.  If the sample fizzed with the HCl test it is calcareous. If pyrite is there pyritic. Secondary mica, micaceous (be careful with this one your mica just might be gypsum and then your sample would be gypsiferous). If it is at all reddish then the rock is ferruginous. Have fun learning these terms, it helps to have done a lot of geologic reading; you will also find the Glossary of Geology very useful here. Additional information like crystal size or color can be tagged onto each name in the list. 

In some cases, a rock will have a characteristic smell and that should be noted.

Secondary structures: These are structural entities that formed as the result of a post depositional process. These include common things like veins and veinlets, nodules and concretions. There are also cone-in-cone structures, geodes, and boxwork. Doing a lot of reading of geology papers will help to build your vocabulary for doing this.

Weathering entails how the unit is being affected by the elements. What you observe is a function of the type of exposure (quarry wall, old road cut, natural outcrop, trail bed, stream bottom etc. You should note the parting especially for shale. Parting refers to the size of the chunks that the rock is naturally breaking done into. Do not confuse parting and stratification or bedding they are not the same thing. Terms applied for parting include:

• papery parts are less than 0.5 mm thick
• fissile parts are from 0.5 to 1 mm thick
• platy parts are from 1 to 5 mm thick
• flaggy parts are from 5 to 10 mm thick
• slabby parts are from 10 mm to 3 cm thick

The term fissile is sometimes used to describe any shale that is breaking down into parts regardless of the size and non-fissile is used for the shale that is not parting. This is not a good practice. The above terms may also be used with sandstone and limestone if they are truly parting.

Limestone and dolostone sometimes weather to form a really rough surface which one would not like to slide across. For obvious reasons this would be called tear-pants weathering. If you dig into the literature on karst you will come upon a lot of other terms for the surface character of limestone and dolostone.

Outcrop character: refers to how the unit appears in the field. Possible terms include:

• ridge former: it is a resistant unit that form a ridge or cuesta on the landscape.
• ledge former: on a cliff face or steep hillside it formed a ledge or step.
• recess former: applied to a unit the weathers more readily that the adjacent layers in a cliff the forming a recess in the cliff.
• valley former: forms a valley in the landscape.

Thickness: The most important item; do not ever forget it. Record the measured thickness of the unit that you just described. Further characterize it if there is noted lateral variation like wedging out or forming a lens.

    thickness 4.5 meters

    thickness 2 feet and thinning westerly to less than a foot

    thickness 3 feet maximum lenticular

When describing rocks in the field use abbreviations in your field notes and then later that night translate your abbreviations into full verbal notes in your second field copy. ALWAYS have two copies of your field notes, one from the actual in the field observations and the second made shortly there after. If you lose one copy then you will have a back-up copy to save your tail. Also in general practice, the field copy belongs to you and the fine copy belongs to your employer.

Useful abbreviations include:

abnt abundant

ang angular

anhy anhydrite

aprox approximate

arg argillaceous

ark arkose

bar barite

bdg bedding

bf buff

bioturb bioturbated

blk black

Brac Brachiopod

blky blocky

brn brown

calc calcareous

crb carbonaceous

chlor chlorite

cht chert

cly clay

cmt cement

com common

Crin crinoid

ctc contact

dism disseminated

dk dark

dol dolomite

Ech Echinoid

elg elongate

euhed euhedral

Fe ferruginous

fis fissile

fld feldspar

fnt faint

frac fracture

g good

glau gaauconite

gn green

Grap Graptolite

gy gray

gyp gypsum

hd hard

ind indurated

ireg irregualr

kao kaolinite

lam laminated

lig lignite

lt light

m medium

mar maroon

mas massive

Mol Mollusca

mot mottled

mtx matrix

n no, none

nod nodule

p poor

phos phosphate

prim primary

pt part

pyr pyrite

rd round

rthy earthy

s small

scat scattered

sd sand

sec secondary

sel selenite

sft soft

sh shale

sid siderite

sl slight

spl sample

srt sort

ss sandstone

stg stringer

stric structure

surf surface

sz size

thk thick

thn thin

tr trace

v very

vn vein

/ with

w well

wh white

wk weak

xbd cross-bedded

zn zone

More comprehensive lists of abbreviations can be found but there is no law against using your own abbreviations as long as you remember what they mean and you translate them for others. A typical in the field description of a rock in your field notes would look like:

Shale: wht. N9; th.lam.; v.w.ind.; loamy; mudcks; foss.(brac); mica,dolo;fiss; ledg. former; 28 m thk.

This will be translated later that day to read:

Shale: white (N9); thickly laminated; very well indurated; loamy; mudcracks; fossiliferous (brachiopods); micaceous, dolomitic; fissile; ledge former; thickness 28 meters.

***Note the use of punctuation***. In general the major divisions are separated by semicolons, list within a division are separated by comas, and there is a period at the end of the description.

Fieldname [colon] verbal color (Munsell Color) [semicolon] stratification [semicolon] induration [semicolon] clay content [semicolon] primary minerals [open bracket] most [coma] next most [coma] least [closed bracket] [semicolon] fossiliferous [open bracket] most [coma] next most [coma] least [closed bracket] [semicolon] sedimentary structure [semicolon] secondary feature [coma] secondary feature [coma] secondary feature [semicolon] outcrop character [semicolon] thickness [period]

Specific Procedures for Core Handling and Description:

When logging core in the field as it comes out of the well bore you describe it from the top down but if you are back in the laboratory then you describe it from the bottom up (oldest to youngest). If you are at the well site the first thing that you do once the core is removed from the core barrel is wipe off not wash the drilling mud form the core, dry it and mark it so that for all time you or the next person will know how it sequentially fits together. 

The core barrel is a cylindrical tool that sits behind the drill bit. As the bit cuts the core it rides up past the drill bit and into the core barrel. The core barrel has ratchets (or a core catcher) that the core rides up past but will not allow the core to fall back down past. Core barrels are typically 5, 10 or 30 feet long. When the core barrel is brought to the surface the ratchet or catcher is removed and the core slides out the barrel, hopefully at a slow rate. Sometimes you have to beat it out, blow it out, etc. while other times it just drops out right now all 30 feet of it into your hands or on the ground, which is not a desirable situation. Keeping track of a depth that a particular section of core came from is not exactly straight forward. For each core trip you should always query the driller as to the depth at the start of the run and the depth at the bottom of the run and make note of such. Let’s assume we have a 10 foot core barrel which means that it has the capacity to hold 10 feet of core. Put the barrel and bit down the hole and start drilling. Later the driller reports to you that he has now drilled 10 feet and pulls the barrel to the surface. You extract 8 feet of core. What could have happened?

1. Drilled 10 feet, the bit chewed up the uppermost 2 feet so you got 8 feet of core.
2. Drilled 10 feet, you left 2 feet of core stuck to the bottom of the hole so you got 8 feet of core.
3. Drilled 10 feet, 2 feet of core segments got chewed up by the bit or broken up in the barrel so you got 8 feet.
4. Drilled 10 feet, the bit chewed up 3 feet, you picked up 1 foot form the last trip so you got 8 feet.
5. There was already 10 feet of core standing in the hole, the driller drill 10 feet chewing up the new 10 feet and 2 feet of the old trip.
6. Drilled 10 feet, 2 feet fell out of the core barrel while it was on its way to the surface, you get 8 feet the drill bit on the next run gets the other 2.
7. The driller lied!

When the core comes out of the ground, YOU mark the top of it as the depth of the bottom of the previous trip and the bottom of it as the depth reported before the driller came off bottom. You make the same markings on small wooden blocks that you will insert at the top and at the bottom of the core when you place it in the core box/boxes. You take a black and a red permanent felt marking pen and you run a red line and a black line from the top to the bottom of the core with the black always on the right and red on the left. You write on the inside of the lid of the box that you did this. In your notes you note the amount of core recovery for that trip as a percent. Drill 10 feet got 8 feet, had an 80% recovery. Now if some idiot drops the core box and you end up with a pied core you have a good chance of putting it back together again and working out the true depths.

When describing a core have handy a spray bottle so that you can lightly wet the core sample with water. This really brings out the textural detail of the sample and should always be done. You should also run a line of dilute HCl down the line of the core to ascertain where the calcareous intervals are at. If you remove a segment of core from the box make sure that you return it to the same place and in the same orientation. If you remove a sample for additional analysis leave in its place a wooden block that has written on it how much was take, why, and were it might be found at a later date. Use wooden blocks and permanent ink as they will last longer than simple paper and pen notes.

Your description follows generally that given above for outcrop samples. However in place of weathering and outcrop character you note the quality of the core. Ideally a core sample will be a nice cylinder. Many times it is nothing more than a jumble of chips and fragments. Also usually you are not reporting thickness but instead you report depth intervals.

Hand Samples

The description of hand samples follows the same procedure and sequence as for outcrops except exposure character and weathering are usually not reported, but in their place note the size of the sample that you are describing. If your sample is from a measured outcrop or core DO NOT just take the description at the outcrop or that of the core as the description here. Start completely over.

Exercise: You will be given in lab a special collection which you will make a full description of in your lab notebook using abbreviations whenever possible. Make this a complete simulation of what you would do in the field. The evening after lab translate your field notes into a formal sample description without abbreviations. You must follow the punctuation guidelines describe above. This is to be written on paper that is not in your lab notebook. Then scan each page and insert into a Microsoft Word document. Adjust the size so that when you print a copy it is legible. Email to me as an attachment the Microsoft Word document. When scanning do not exceed 300 dpi resolution and scan as a TEXT or line drawing, not as a black and with photo and color photo. You may have to play with your scans and Microsoft Word document to get it under 5 mb, the current email file size limit. If you are so skilled and you have the resources you may create a PDF document instead of a Microsoft Word document. For those who do not have a scanner at home, there is one in the Cart Lab and each of the campus computer labs should have one. Remember when you email this to me you email subject line is:

GLY 312

And the name of the file will be in the form:

ABCDE05.xxx

Where the ABCDE are the first five letters of your last name, the 05 denotes this lab exercise and the .xxx refers to the file type.

Appendix A: Color Charts (Warning, do not use the charts as illustrated below and the colors are not true but modified as computer screens and printers can not duplicate true colors.)

Notes: