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:

Assume that since trilobites were arthropods then their environmental preferences were similar to the modern arthropods such as ostracodes, barnacles and decapods.
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 1^st 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? details very inportant)

The 2^nd 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.

Exercise#1: Examine as many of the samples as time permits and determine the conditions of deposition for each. Write a formal diagnosis of each sample while you have the sample in front of you. The formal diagnosis contains three parts: (1) your name, date and sample identification number; (2) descriptions of each taxon, and (3) the evaluation of the conditions of deposition. You are showing me what you have learned form several previous lab exercises. Turn in each written diagnosis before you start the next sample.

Paleoenvironment Analysis of Carbonate Rocks

Environmental Habitat of Modern Carbonate Producers

Faunal and Floral Analysis of a Stratum

Further Reading for Serious Students: