Description of Carbonate Rocks

There are, like in sandstone, dozens of classifications for the carbonate rocks. It is not necessary to learn then all. Here you will explore first the general terminology one may use within the field environment where one only has a hand lens, rock hammer, and possibly a bottle of dilute hydrochloric acid. Then we will examine a classification scheme that can, with some difficulty, be applied in the field, but which is better suited for lab examination of polished slabs. Finally you will explore a classification which requires the application of the petrographic microscope.

Field Terminology for the Carbonate Rocks

Two primary terms apply here limestone and dolostone. If you think the rock is at least 50% calcite it is limestone and if you think that it is at least 50% dolomite call it a dolostone (some folks still call such a dolomite but get used to using dolostone instead for the rock name). To tell the two apart you can use the acid test you learned in Physical Geology....calcite fizzes readily and dolomite not so readily. But keep in mind on hot rock dolomite will fizz like crazy and cold dense calcite will react like dolomite. Also it is important to use a fresh surface in order to avoid having the caliche crust (calcite) on dolostone making you think that you have a limestone at hand. (What is caliche? How does caliche form? HOMEWORK!!!!) Another problem you may encounter is that a cold dolostone with a sucrossic (sugar like) texture will not fizz and will look to you just like sandstone. Remember quartz will scratch the steel or your pocketknife or hammer and dolomite will not. Use of a hand lense will usually help to solve this. A past director of the United States Geological Survey had a problem with telling sucrossic dolostone from a buff colored sandstone.You can use the stain Alizarin red-S in the field, it really works quite nicely. Take a fresh caliche free surface, wash it with the dye mixture and then flush it gently with clean water. Examine the stained surface with your hand lens. Calcite will be pink and dolomite will be clear just like in thin sections. Do not trust your observations after the surface dries. The potassium ferricyanide blue stain does not work well here though some workers claim to have had good results. You may apply to the term limestone or dolostone a number of adjectives to more completely name the rock:

Dolomitic: has the mineral dolomite in it (no we do not have dolomitic dolostone!!) Calcareous: has the mineral calcite in it (do not use with limestone)

Argillaceous: has clay present in it.

Arenaceous: has sand size detrital grains

Fossiliferous: dead plants and animals

Oolitic: oolites present (but you have to be certain of this; use your hand lens)

Pisolitic: contains pisolites (What are pisolites? How do they form? HOMEWORK)

Cherty: chert nodules present

Brecciated: the rock is broken in situ

There are other terms, read the Glossary on the next rainy day!!The full field description of carbonate rocks follows the same procedures discussed in the rock description lab.

The Classification of Dunham (1962) with Modifications

If you are back in the office and have at hand a cut slab you can observe detail that you would not generally see in the field. The slab could be highly polished but if it is not just wet it with water. Staining it with alizarin red S would be useful. Dunhams (1962) classification works real well here for putting a name on the rock. You have seven basic terms to use plus a few later modifications from Dunham’s original work.Mudstone: the rock is as you see it mostly made of carbonate mud or cryptocrystalline carbonate matrix. Grains (fossils, ooids, etc.) will be less than 10 % of the rock. Sometimes you will be wrong in your call here; sometimes cloudy calcite spar (crystals) looks like carbonate mud other times mud looks like spar...but call it as you see it.Wackestone: grains make up more than 10% of the rock but the grains are "mud supported" they float in the mud matrix.Packstone: Lots of sandbox sized grains with mud between them but the grains are grain supported.Grainstone: Sandbox sized grains with spar between them, little or no mud.Floatstone: 10% or more of the grains are greater than 2mm in diameter and mud in the matrix (like a packstone)Rudstone: 10% or more of the grains are greater than 2 mm in diameter and spar is between the grains (like grainstone).Boundstone: "Original components organically bound during deposition" think of colonial corals and stromatolitesEmbry and Klovan (1971) working in Devonian reefs of British Columbia added three subdivisions to the Boundstones. These are now widely used as a modification to Dunham.Bafflestone: "organism acted as baffles"Bindstone: "organisms encrusting and binding"Framestone "organisms building a rigid framework"

Folk's Classification of Carbonate Rocks

The foundation of Folk’s classification is the relative amounts of (1) allochems, (2) calcite cement or 'spar", and (3) microcrystalline to cryptocrystalline calcite matrix or 'micrite. These amounts are best ascertained with the petrographic microscope and generally with the application of modal analysis but with some skill and practice satisfactory results can be obtained with good quality hand samples. Allochems are sepatated into four types, intraclasts, ooids, pellets, and bioclasts.

Think of intraclasts as being 'intra-formational rock fragments'. They can be of any size but generally will range above 2 mm in diameter and less than a few centimeters in diameter. Compositionally they usually consist of microcrystalline to cryptocrystalline calcite but they can also contain within them other allochems such as small bioclasts like ostracods. Their shape is variable and frequently they will be rounded.Ooids are carbonate grains which are usually small less than 1 mm in diameter spherical grains which posses a series of concentric laminations when view in cross section. One must recognize the concentric laminations in order to apply the ooid name. Those ooids, which have diameters greater than a few millimeters, are generally referred to by carbonate petrologists as pisolites.Pellets are microcrystalline to cryptocrystalline calcite or aragonite grains that are probably of fecal origins. They are in most cases less than 2 mm in maximum diameter and elongate or roller in external shape.Bioclasts are the calcite or aragonite tests of organisms that lived contemporaneously with the depositional processes which formed the sediment. There are a wide variety of types of bioclasts each with its own unique texture and shape. Excluded from the bioclasts are any fossils that are not carbonate in nature, teeth, bones, and conodonts. Also excluded are any bioclasts, which were 'fossils' at the time of deposition. Theses would be considered to be detritus or rock fragments (sedimentary rock fragments SRF or Folk's sandstone classification).In practice some problems result from the above-idealized characterizations of these allochems. Early (pre-depositional to syn-depositional) diagenetic processes can render ooids and bioclasts indistinguishable from intraclasts. This is called micritization. Furthermore it is almost impossible to say with confidence that a pellet is of fecal origins unless one witnessed the event. It could just as easily have been an intraclast. For these reasons carbonate petrologists usually when working with Folk's classification call any microcrocrystalline to cryptocrystalline carbonate grain under 2 mm in diameter a pelloid and treat pelloid as being synonymous with Folk's pellet. Similarly any microcrystalline to cryptocrystalline carbonat grain over 2mm in diameter and regardless of its true origins is referred to as an intraclasts.Folk' scheme consists of five classes of carbonate rocks which are designated in geologic shorthand by the Roman numerals I, II, III, IV, V. Class I and II are limestones (calcite and aragonite rocks), partially dolomitized limestones (dolomite <50%), and primary dolomites (their existance is of debate) which have greater than 10% by volume carbonate allochems. Class I rocks have mesocrystaline (easy to see individual crystals) calcite know as spar between the allochems. The Class II rocks have micrite (microcrystalline to crypocrystalline calcite) filling the space between the allochems. Class III rocks all have less than 10% by volume carbonate allochems. Class IV rocks are all "undisturbed bioherm rocks" which are almost synonymous with Dunham's boundstones. The 'bafflestone' of Kloven and Emory's modified Dunham's classification does not fit well in this class. Class V rocks consists entirely of replacement dolomite.Class I and II rocks are further subdivided by the relative percentage of the four different types of allochems and the size range of the allochems. Dealing with size of the allochem first, if the population of allochems is generally greater than 2 mm in diameter than the root name follows that of Grabau or rudite while if they are less than 2 mm in diameter the root name follows the Grabau term arenite. However in the actual name giving the 'aren' portion of arenite is deleated. Class I rocks have spar between the allochems and hence are named the sparites and sparrudite. Class I rocks have micrite between the allochems and are thus named the micrites and micrudites.Each of these four terms must have a prefix that denotes the actual allochem content of the rock. If more than 25% by volume of the allochems (not the rock) are intraclasts then the prefix 'intra' is applied as in intrasparite, intrasparrudite, intramicrite, and intramicrudite. In the pure sense of folk's classification all four rocks could exists however in practice since one is forced to attributes all microcrystalline to cryptocrystalline calcite grains under 2 mm in diameter to the pellets or pelloids the terms intrasparite and intramicrite are not used.If the rock has 25% or less intraclasts but more than 25% by volume ooids the prefix 'oo' is applied as in oosparite, oosparrudite, oomicrite, oomicrudite. Since ooids greater then 2 mm are uncommon and generally called pisolites the terms oosparrudite and oomicrudite are rarely used. Furthermore an oomicrite is

itself a rare rock type as the process that cause the formation of ooids generally excludes the formation of a lime mud or microcrystalline to cryptocrystalline calcite matrix.If the rock has 25% or less intraclasts, 25% or less ooids, and 67% or greater bioclasts that the prefix 'bio' is applied as in biosparite, biosparrudite, biomicrite, biomicrudite. If the rock has 25% or less intraclasts, 25% or less ooids, and 67% or more pellets (pelloids) the prefix ' pel' is applied as in pelsparite, pelsparrudite, pelmicrite, and pelmicrudite. However, since in practice pellets over 2mm in diameter were taken with the intraclasts, one would not have use for the names pelsparrudite and pelmicrudite. If the rock has 25% or less intraclasts, 25% or less ooids, and has pellets and bioclasts in roughly the same amounts (33% to 67%) the prefix 'biopel' is applied as in biopelsparite, biopelsparrudite, biopelmicrite, and biopelmicrudite. The 'bio' always proceeds the 'pel' regardless of which is the most abundant.The class III rocks contain no more than 10% by volume allochems the remainder of their volume being mostly micrite (microcrystalline to cryptocrystalline calcite) hence these are the micrites. If there allochems and the dominant allochems are intraclasts the rock is called an intraclast-bearing micrite. If the dominant allochems are ooids it is an oolite-bearing micrite. If bioclasts are the dominate allochem then the rock is a fossiliferous micrite. If pellets (pelloids) dominate then pelletiferous micrite. If there is less than 1% allochems in the rock it is simply a micrite. Some micrite (rock name) will have patchy areas of calcite spar. These special micrites are called dismicrite. And if the rock is composed of microcrystalline to cryptocrystaline dolomite Folk applies the name dolomicrite. This latter name is now in disuse in favor of aphanocyrstalline dolomite, a rock which falls under Folk's class V.The class IV rocks are the biolithites and they lack further subdivision though such would be useful. If one is dealing with biolithites it would be advisable to switch to Dunham's classification. The bafflestones of the modified Dunham classification do not fit into Folk's biolithites very well and in most cases the terms biomicrudite or fossiliferous micrite would be applied to them.Class V rocks are the 'replacement dolomites'. Which means that the rock is mostly dolomite and that you have applied alizarin red-s to the rock to verify this. All class V rocks are called dolomite by Folk or dolostone by more recent workers. All have an adjective placed before the name that expresses the typical size range of the dolomite crystals.

Aphanocrystalline Under 0.0039 mm

Very finely crystalline 0.0039 to 0.0156 mm

Finely crystalline 0.0156 to 0.0625 mm

Medium crystalline 0.0625 to 0.25 mm

Coarsely crystalline 0.25 to 1.00 mm

Very coarsely crystalline 1.00 to 4.00 mm

Extremely coarsely crystalline Over 4.00 mm

If the rock contains what are called 'allochem ghosts' then an additional adjective is applied depending upon the dominant allochem ghost present; intraclastic, oolitic, biogenic, or pellet. These ghosts are the images of the original allochems that have now become replaced by the secondary mineral dolomite. This will become clearer in a later lab exercise.If the rock of any class is observed to have 10% or more quartz sand, silt or clay as terrugenous detritus grains then the adjectives areanaceous, silty, or argillaceous are applied respectively. Likewise significant amounts (10% or more) of glauconite or phospohorite will yield a related adjective (note these are primary components of the sediment and not authigenetic components).

Diagenesis in the Carbonate Rocks

Objective: In this laboratory exercise you will observe and describe features of diagenesis that are commonly seen in thin sections of the carbonate rocks. These features include cementation, dissolution, mineral replacement, and fracturing among others.

Please thoroughly study the following:

Diagenesis in rocks is generally taken as any process that acts upon sediment or rock that alters it chemical, physical, or textural character. With the carbonate rocks we also extend this a bit to include processes, which also act upon the allochems prior to deposition in sediment. One of the earliest processes is the post-mortem micro-boring of bioclasts by algae and fungi. This weakens the test for later mechanical breakage, forms a passageway into the grain for chemically reacting solutions, and provides a space for the emplacement of other materials. Many bioclasts in thin section view appear to have a halo of micrite around their outer margins; this is a micrite halo, which is the combination of the micro-boring process and later infill of the borings with other material such a cryptocrystalline calcite.

Micro-boring in a mollusk bioclasts. Thin section has been stained with alizarin red-s and was impregnated with blue dyed epoxy to enhance porosity. Neuse Formation (Late Pleistocene) Snows Cut, New Hanover county, North Carolina. Plain light.

Partial infilling of intrapartical porosity of an echinoderm bioclasts with glauconite (rusty red colored area in center of grain). No evidence of permineralization in this grain. If this grain were to become permineralized the presence of the glauconite would help to retain the original features of the grain. Modern, Onslow Bay, North Carolina. Polarized light with the 1st order red plate inserted.

Unstable minerals like aragonite are frequently dissolved leaving behind a moldic pore in the shape of the original allochem. The pores than can be later infilled by various authigenic minerals (calcite be the most common but also gypsum, anhydrite, quartz and dolomite). Sometimes the dissolving material is immediately re-precipitated in adjacent pore spaces. Molds do not have to be of an allochem for even authigenic material can be later dissolved forming a moldic pore. If dissolution results not in a cavity of recognizable form then it is best to refer to it as a solution vug.

Moldic pore after a mollusk bioclasts (pelecypod). Pelecypods produced aragonite so the aragonite was dissolved leaving behind only calcite. Castle Hayne Limestone (Eocene), southeastern North Carolina. Plain light.

Any kind of pore space can be filled or as we will say ‘reduced’ by some filling material. If the fill is an authigenic precipitate like calcite then one can further specify the nature of the fill as being (1) isopachous, meaning it forms a layer of near uniform thickness around the pore, (2) blocky, referring to the mineral as forming a mosaic of crystals within the pore with no special relationship between crystal size and shape and pore wall, or (3) poikilotopic, meaning one single large crystal has engulfed the entire pore and adjacent pores and more than one allochem. If the material filling the pore was not totally filling or reducing the pore we say that such-and-such partially reduces the porosity. If more than one item is filling the pore then we list from the oldest to youngest using the principal of superposition.

Complete reduction of moldic porosity after a mollusk bioclasts by isopachous bladed calcite. The bioclasts was probably initially aragonite; the material for the formation of the calcite could have been derived from the dissolution of similar bioclasts nearby. Castle Hayne Limestone (Eocene), southeastern North Carolina. Polarized light.

Moldic porosity after a mollusk (pelecypod) and porosity reduction by isopachous bladed calcite cement. The parallel lines in the center of the figure are all that remains of the pelecypod. These lines are the micrite envelope that formed post-mortem on the surfaces of the shell due to micro borings. Castle Hayne Limestone (Eocene), southeastern North Carolina. Polarized light.

Moldic porosity after bioclasts (mollusk) with moldic porosity and interparticle porosity partially reduced by isopachous bladed calcite cement. Note that the size of the calcite cement crystals outside of the molds is much larger than those within. This suggests that the interparticle cement was forming at the same time that the mollusk bioclasts was dissolving. Polarized light with the 1sr order red plate inserted.

Moldic porosity after bioclasts and interpartical porosity partially reduced by isopachous bladed calcite cement which was later partially dissolved. This interesting rock is now entirely calcite spar cement and pore space. Bioclasts were most likely all aragonitic. They were dissolved with the dissolved material being reprecipitated as calcite cement. Loss of the crystal points suggests a later dissolution of these. Polarized light.

Solution porosity completely reduced by anhydrite. The rock is otherwise dolomite of replacement origins (note the ghost textural features, especially the pisolite in the upper right area. Grayburg Formation (Permian), Ector County, Texas. Polarized light.

Intergranular porosity completely reduced by poikilotopic anhydrite cement. Allochems are dolomitized pelloids. Grayburg Formtion (Permian), Ector County, Texas. Polarized light.

Complete reduction of moldic porosity after dolomite euhedra by blocky calcite. The rhombohedral shape of this feature is the clue that it was originally dolomite. The rock is now entirely calcite. There is a three step paragenesis here: (1) formation of the dolomite euhedra, (2) dissolution of the euhedra, and (3) precipitation of calcite in the moldic pore. Polarized light with 1st order red plate inserted.

In mineral replacements as secondary mineral (metasome) comes to occupy the special position of a former mineral (paleosome). The process involves two simultaneous chemical reactions, one dissolution of the paleosome and the other precipitation of the metasome, that proceed at the same volumetric rate. This is a very special condition that leads to a very specific type of fabric. In a replacement all material not involved in the dissolution-precipitation reactions maintains it special position and is transferred from being within the paleosome to the metasome. The consequence of this is that the some aspects of the original rock textural fabric is retained as ghost images in the replaced portions. Should dissolution volumetrically exceed the precipitation of the secondary then one simply has a moldic pore that is reduced by some type of cement, much like some of the above. If precipitation of the secondary proceeds faster than the primary is dissolved then the ‘force of crystallization’ does mechanical work on the rock fracturing it, twinning mineral grains, or simply plowing its way through the rock fabric. If you doubt that a growing crystal can do any significant work then think about what freezing ice can do. In both of these situations ghost images of the original fabric are not formed.

Dolomitization of calcite. Here large dolomite crystals have replaced foraminifer and echinoderm bioclasts. The textural features that you see are ghost images of the original fabric of the allochems. The sample is entirely dolomite. Madison Group (Mississippian) Whiterocks River Canyon, Utah. Plain light.

Partial dolomitization of calcite. Ooids have been replaced with dolomite euhedra. Note the retention of the original textural character of the ooids within the dolomite. In this sample the dolomite has been largely selective for the ooids, a common feature of dolomitization. Thin section has been stained with alizarin red-s. Cambro-Ordovician, Spitzburgen.

Dolomitization of calcite where dolomite has partially replaced ooids and the sparry cement between the ooids. Note the ghost texture of both preserved in the dolomite. Two step paragenesis here, (1) interpartical porosity completely reduced by blocky calcite and (2) partial replacement of the calcite by dolomite euhedra. Madison Group (Mississippian), Utah. Thin section stained with alizarin red-s. Plain light.

Dolomite euhedra in limestone. This is possibly of replacement origin but we cannot tell for sure because no ghost texture is evident in the dolomite. The dolomite could be a displacement dolomite. Sample has been stained with alizarin red-s. Plain light.

Calcitization of dolomite. Here a dolostone, which appears to have had a xenotopic fabric (common euhedral crystals) has been entirely replaced with calcite. Note the rhombohedral shapes still evident. Davenport Breccia (Devonian), eastern Iowa. Sample has been stained with alizarin red-s. Plain light.

Calcitization of aragonite. Here a mollusk bioclasts, which was originally aragonite, is now calcite. The textural features that you see, the bars, are a ghost image of the original micro-texture of the bioclasts. The fine dark mosaic of lines is the crystal boundaries of the calcite crystals. Castle Hayne Limestone (Eocene), southeastern North Carolina. Plain light.

Partial Silicification of calcite with the development of radially fibrous or botryoidal quartz also referred to as chalcedony. Eocene, Aiken, South Carolina. Polarized light.

Silicification of calcite. Here an oolitic limestone has been completely replaced by quartz. Fabric, which is evident, is entirely a ghost image of the original rock fabric. The rock is now a chert. Polarized light.

Silicification of calcite with complete replacement of the limestone fabric with quartz. Note the ghost images of the foraminifers. Madison Group (Mississippian), Little Brushy Creek, Utah. Plain light.

Grains or crystals being mechanically forced against adjacent crystals can result in dissolution of one or both. The result is that one grain may embay another or that they may form a common zig-zag like boundary called a micro-styolite. On opposite sides of this boundary may be the same mineral. The one being dissolved is for some reason slightly more soluble than the other. This may be due to trace element chemistry, crystal lattice orientation, or crystal lattice imperfections. Sometimes one observes on a macroscopic scale large zig-zag lines cutting the rock, these we call styolites.

Pressure solution. The echinoderm bioclasts right of center has embayed the echinoderm bioclasts immediately above and below it. Also most other grain boundaries are sutured or micro-styolites. This sample lacks intragranual material (porosity, micrite, spar). Thin section was stained with alizarin red-s. Madison Group (Mississippian), Utah. Plain light.

Rocks can deform in a brittle manner and fracture with the fracture pore being later filled with some form of cement. Generally the cement will have crystals that nucleate on the fracture wall and grow into the opening. Sometimes one observes an apparent fracture that is completely reduced by a prismatic or fibrous mineral that is oriented long axis normal to the wall. Here the force of crystallization of the ‘filling’ material may be the actual cause of the opening of the fracture.

Expect multiple sets of fractures and look for displacements from one side to the other of the fracture. Also look for what gets fractured especially in terms of other features of diagenesis. Apply the principal of cross-cutting relationships.

Fracturing with fracture porosity completely reduced by isopachous saddle dolomite. Saddle dolomite is characterized by having a warped crystal lattice so that when the microscope stage is rotated there is a sweeping extinction. Saddle dolomite is a void filling dolomite. Here a fracture in a dolostone opens (note the dark sediment fill on the floor of the opening). Then the dolomite precipitates in the open space. Polarized light with 1st order red plate inserted.

Fracturing with fracture porosity completely reduced by equant calcite. Two sets of fractures here; the east-west set preceded the north-south. The north-south fracture exhibits dextral trans-tensional displacement (match the shape of the fracture walls). Four step paragenesis evident; two stages of fracture formation and two stages of infilling with calcite. Polarized light with 1st order red plate inserted

Paleoenvironment Analysis of Carbonate Rocks

In this laboratory exercise you will evaluate the environment of deposition of carbonate rock samples by applying the principal of Uniformitarianism and the environmental ranges of modern carbonate producing fauna and flora to the of fauna and flora which you will observed in rock samples.

Environmental Habitat of Modern Carbonate Producers

In the 1960’s and early 70’s Phillip Heckel of the University of Iowa made an exhaustive study of the literature on the habitat of modern marine carbonate producing organisms. Heckel summarized habitat in terms of four parameters: water salinity, water depth, substrate, and water turbidity. The charts below represent simplified versions of Heckel’s charts, they also represent what was known at that period of time in regards to marine organism habitat. Obviously in the 30 years hence new information has come to light however the charts below are a good starting point for the student in understanding just what fossils can and cannot tell about the depositonal condition of the sediments in which they were found. For a more complete evaluation one should first thoroughly review the paper of Heckel (1971) and then expect to spend some time researching the literature since 1970.

Salinity refers to the total amount of dissolved solids in the water; the sum of all the cations, anions, and neutral species. Sodium is the dominant cation while chloride is the dominant anion in marine derived waters. Water in the open ocean is referred to as “normal marine” and has a salinity of about 34 to 35 parts per thousand (ppt) dissolved species. Hypersaline water has had the dissolved constituents concentrated generally by evaporation while brackish has had the marine water diluted by an influx of fresh or terrestrial water.

Water Depth refers to just that, the depth of the water, however one cannot speak of such in terms of meters or feet. Supratidal refers to that portion of the marine system, which is above the height of the high tide. Intertidal refers to that portion of the marine system, which is elevation wise above low tide but below high tide. Subtidal refers to all marine system areas below the elevation of low tide. Shallow subtidal generally refers to the depth found on continental shelves; slope subtidal refers to the continental slope rise areas, while deep subtidal generally refers to abyssal depths. “Fresh water” signifies non-marine systems found on landmasses. Algae, due to the requirement of needing sun light live within the “photic zone,” a subdivision of the shallow subtidal area. Most algae thrive in water depths less than 50 feet (12 meters).

Firmness of Substrate refers to the type of material that the organism prefers to live upon or within. “Hard” substrate is durable, rock hard like material. It could be rock, it could be a submarine hard ground, or it could be the shell of another organism. “Firm” refers to stiff mud or sandy bottoms. “Soft” refers to very plastic gooey muddy bottoms. Organisms, which move about without regard to bottom conditions, are referred to as “mobile.” Care must be taken when evaluating those organisms, which prefer a hard substrate for they could have been living upon a shell within a soft substrate or even upon a mobile object.

Turbidity refers to the amount of suspended sediment within the water column. Organisms like corals, which are filter feeders are very intolerant to suspended sediment. They therefore favor “clear water” which is largely free of such material. Turbid water will have noticeable amounts of sediment; think of this in terms of clouded water or think of it in terms of what divers call visibility. Consider water in which one cannot see one’s hand in front of their face as being very turbid. “Rapid deposition” refers to areas where there is a considerable sediment input generally form a terrestrial source such as in the area of a river delta.

Before we can jump into the determination of the environment of deposition of a particular rock there are a few other things to consider. First and foremost the above charts represents the habitat preferences of modern carbonate producing organisms, the key word is modern. We apply the principal of uniformity (uniformitarinaism) here but we must remember that the present is not exactly like the past. Physical conditions have changed: more oxygen in the atmosphere and hydrosphere today than the distant past, possibly different ratios of dissolved ions in the marine waters, etc. Furthermore evolution may have had an impact for we can never be sure if the modern preference of an organism was the same as it had in the past. We assume that it was.

We must also look to the ranges of organisms through time.  Barnacles were not always on the surface of the Earth and the trilobites have long ago disappeared. For extinct groups like trilobites one might get an idea of their preferred habitat by two avenues of approach:

1. Assume that since trilobites were arthropods then their environmental preferences were similar to the modern arthropods such as ostracodes, barnacles and decapods. 

2. Look to what occurs in a general sense with the trilobites and draw your inferences from the environmental preferences of those organisms.

The next problem to deal with is the quality of the data itself. A particular sediment at deposition will have within it an array of fauna (animals) and flora (plants). Some of these will produce fossilizable material such as calcite or aragonite and thus have a chance (small chance!!!) of being preserved as fossils in the sediment. Those that do not produce such material will only in very exceptional circumstances become fossils. Those that produce aragonite have less of a chance to be preserves than those that produce calcite, hence assume that the fauna and flora of a collection is always biased in favor of the calcite producers and that all the others may have been lost. Therefore it is important to understand what material each type of organism produces. Always remember that what you see is not a complete representation of what was there at deposition. Also the absence of a particular organism cannot be taken as indicative of a particular environmental condition. For example if a particular fossil collection lacks any algae one cannot draw the conclusion that the strata was deposited below the photic zone.

In further regard to the quality of the data the presence of a particular fossil test may itself provide no information on the environmental conditions of the stratum in which it was found. Fossils, which are found in situ, are in their growth position within the stratum. These provide the most reliable information as they lived there and died there. Most fossils that you will encounter in the rocks have been experienced post-mortem transport; they lived somewhere else, died, and were carried to their final resting place where you encountered them. We classify these fossils as being “ indigeneous.” We assume that they were not transported far but we understand that they have been transported and we understand that they may miss lead us in our interpretations. We still us them. Some idea of transport distance can be gained by studying their abrasion. Well-abraded fossils are less trustworthy than pristine fossils. Organisms, which have been transported so far that they are no longer anyway near where they lived, are referred to as “exotic.” An oak tree, which gets washed out into the open ocean during a flood is exotic in the marine environment. Consider cephalopods to always be exotic since upon death they tend to float great distances. Planktonic forams likewise, for they live in the upper part of the water column then die and fall to the sea floor, a totally different place than where they live. The most problematic are the “remenae” fossils. These are not bioclasts but lithoclasts, material reworked form a much older stratigraphic unit. Ordovician brachiopods within Mississippian limestone are remenae. These are difficult to spot unless you are really good with your paleontology. They provide no information of environment of deposition but they do speak toward paleogeography at the time of deposition so they should not be totally ignored.

One other thing to keep in mind is that the collection of fossils, which you make from a stratum or observe in a thin section, does not represent an absolute point in time. The collection represents those conditions, which occurred in that locality during the interval of the depositional process. Susan Kidwell and Daniel Bosence published an excellent paper on this. The concept is referred to by then as “time-averaging.” If you make your collection from the entire formation than your analysis is time-averaged for the entire duration of the deposition of the formation. If the collection is from a single bed (better idea!) then it is time-averaged for the duration of the deposition of that bed. Remember deposition is not making a true recording of the history of the area. Once sediment is deposited infauna (organism living within the sediment) are rearranging it and mixing it, thus time-averaging it.

Faunal and Floral Analysis of a Stratum

The 1st step is to observe you material, bed, hand sample or thin section, for its fossil content. We identify everything down to the most detailed level that we can given our individual skills at taxonomy. If all you can say is it is a brachiopod fine, if you can recognize it as a Productid, better, and if you can put a genus and species name on it great. If we note two brachiopods we do regardless if cannot say anything further. For each taxon (type of fossil we encounter and at what ever level of recognition) we characterize its taphonomy; (is it in growth position? Is it abraded? Is it greatly abraded? Is it the nucleus of an ooid?)

The 2nd step is to go to the above environmental range charts and figure out the conditions of deposition. The result must not be in conflict with any member of the collection. If you have a collection that consists of ostracodes and corals then the inferred salinity would not be ‘fresh to hypersaline’ as corals are restricted to ‘normal marine.’ The corals delineate the salinity range. In writing we say “the presence of corals implies normal marine salinity.” Call it as the data indicates. If all there is to the fauna and flora are gastropods then the salinity could have ranged from fresh to hypersaline, the turbidity could have ranged from clear water to rapid deposition, the substrate could have been hard, or firm or soft. The water depth could have been terrestrial or supratidal, intertidal, subtidal shallow or even deep.