Biochemical Sedimentary Rocks - Carbonates and Chert

1. BIOCHEMICAL SEDIMENTARY ROCKS Prepared by Dr. F. Clark Department of Earth and Atmospheric Sciences, University of Alberta August 06

2. INTRODUCTION The two principle groups of biochemical sedimentary rocks are carbonates and siliceous sediments called chert. We will say more about chert in the presentation on chemical sedimentary rocks, for reasons that will be evident there. The bulk of carbonate sedimentary rocks consist in large part of materials that are either directly metabolized or secreted as solid carbonate minerals by organisms as shells or other hard parts, or else their precipitation from saturated marine or other waters is aided by the influence of organisms on water chemistry. It is important to note, however, that not all carbonate production is organically influenced.

3. Calcite.This common carbonate mineral, with the formula CaCO3, reacts strongly with dilute hydrochloric acid. The other polymorph of CaCO3, with the same chemical formula but different crystal structure, is called aragonite, and is often the form in which this material is first precipitated. It reacts [fizzes/effervesces] with HCl as well, and is generally not distinguished in hand specimen.

4. Dolomite.This common carbonate mineral has Mg as well as Ca, with the formula CaMg(CO3)2. It will also react with HCl, but weakly. As seen in the crystals above, dolomite has rhombic cleavage, the same as calcite. However, it is slightly harder than calcite (3 ½ to 4 instead of 3), and only reacts weakly with HCl when powdered. The colour difference between crystals of the two is not consistent.

5. CARBONATE CONSTITUENTS There may be three basic constituents in a carbonate rock. The first, and often most obvious, consists of carbonate grains, of which there are several types (some will be illustrated). They generally, but not always, consist of several smaller crystals of calcite or aragonite. The second constituent is micrite, or microcrystalline calcite, which is generally but a few microns in size. It is often referred to as lime mud, which would be carbonate material less than 1/16 mm. The third constituent is sparry calcite, a fairly coarsely crystalline and clear (in thin section) form. As a gross simplification, the latter two occur in spaces between carbonate grains.

6. Carbonate Grains – Skeletal Grains Both these samples consist almost entirely of skeletal grains, or fossil shell material. The left sample has large fragments and intact shells of the pelecypod mollusc Coquina, whereas the right sample is chalk, comprising an almost pure collection of coccoliths, tiny calcareous plates measuring only 5-10 microns (.005 to .010 mm). These are from a group of planktonic algae called coccolithophores.

7. More Skeletal Grains In the Paleozoic (that interval of time between 544 and 250 million years ago), among the most common invertebrate groups, and thus types of skeletal grains, were brachiopods [green arrows]. In the right sample, the wide, straight hinge line of many specimens [light blue arrows] is the only part of the shell clear of the lime mud matrix. Purple arrows point to crinoids [next slide].

8. More Paleozoic Skeletal Grains The prominent invertebrate fossils in the left sample are bryozoans [blue arrows], colonial organisms whose individuals inhabited the tiny openings, now filled with yellowish lime mud, in the branched skeletons. On the right, purple arrows point to small circular discs called ossicles that once comprised the stalk attaching the body of these crinoids [a group of echinoderms] to the sea floor.

9. Carbonate Grains –Peloids.These are structureless aggregates of lime mud, most commonly invertebrate fecal pellets. In this core sample, blue arrows point to peloids, the green arrow is parallel to lamination [best seen just below], the purple arrow points to a stylolite [discussed later], and the yellow star highlights an area where light acid etching has cleaned up “fuzz” created by the rock saw.

10. Carbonate Grains –Ooids.Ooids are concentrically- laminated grains, the laminae consisting of tiny crystals. This sample, from a saline lake deposit, may be called an oolite. The ooids [blue arrows] are sand-sized [between 1/16 and 2 mm], giving the surface of the rock a pebbled look similar to a basketball or football. They are usually formed in shallow, agitated environments.

11. Silicified Oolite.In this oolitic rock [ooid grainstone], all the original carbonate has been replaced by silica, preserving original details. Carbonates are unstable, and often undergo changes after deposition. All such changes, short of metamorphism, comprise diagenesis. This sample presently is siliceous, but correct interpretation depends on recognizing that it was deposited as a carbonate.

12. Weathering of Carbonates - 1 Both views are of an oolite from the Cayman Islands. The left view shows the fresh surface, with its bright cream colour and pebbled texture, whereas the right view shows the fresh surface toward the left, and the darker, grey weathered surface toward the right, with heavy corrosion pits [green arrows]. The irregular contact between the two surfaces is highlighted in light blue arrows.

13. Weathering of Carbonates - 2 This sample of coquina [skeletal grainstone] shows the fresh surface on the left, with spaces between the pelecypod mollusc shells [yellow arrows] filled with sparry calcite cement. The weathered surface on the right shows that the sparry cement has been selectively dissolved or leached by corrosive meteoric [fresh] waters, exposing the pelecypods in relief.

14. Diagenesis –Recrystal-lization.The instability of carbonate minerals means that when they are buried, they tend to recrystallize. When subjected to stress, a common response of minerals is to grow as fewer, larger crystals, to minimize stress at grain boundaries. This carbonate is presently characterized by coarse, sparry calcite, with significant pores [light blue arrows] developed during crystal growth.

15. Recrystal-lization 2.The coarse individual crystals can easily be seen [dark blue arrows]; this process tends to obliterate details. One survivor of the recrystallization process is the ribbed shell of a brachiopod [yellow arrow]. One common positive effect of recrystallization is the development of pore spaces [light blue arrows] as atoms are rearranged. Porosity enables a rock to hold fluids.

16. Diagenesis –Stylolites.Another effect of stress at grain boundaries is that grains may dissolve by pressure solution. Carbonate may be soluble, but clay and organics generally are not, and these insoluble residues are concentrated at the solution front as stylolites [purple arrow]. The amount of material lost can be gauged by the offset of the distinctive laminae [stars] across the stylolite.

17. Stylolites 2.Samples rich in lime mud are very vulnerable to pressure solution. In this sample, several stylolites are developed [purple arrows]. A greenish layer [green arrow] of unknown original thickness has been almost completely occluded or pinched out by this process. The fossils highlighted by blue arrows are examples of Amphipora, a common stromatoporoid in quiet waters of the Devonian of Western Canada.

18. DOLOMITE –PRIMARY SEDIMENT? There is some debate about whether or not there is any true primary dolomite, that is, precipitated directly from saturated hypersaline [excess salinity] waters. What is clear is that much dolomite, and in the case of the Western Canada Sedimentary Basin, most dolomite, forms as a replacement product, whereby Mg is added to calcium carbonate precursor grains, and thus limestone rocks, to produce dolomite crystals in dolostones. Just as is the case with recrystallization, the process of dolomitization tends to obliterate original textures, such that interpretation of the rocks becomes problematic.

19. DOLOMITE – BIOCHEMICAL? Some introductory texts will take the observation that there may be no true primary dolomite and suggest that this means that dolomite and dolostones are chemical sedimentary rocks, formed by inorganic diagenetic processes. However, the reality is that most dolostones have biochemical carbonate precursor rocks. Thus the interpretation of most dolostones only makes sense if one considers them from a biochemical perspective. For this reason, sedimentology texts will almost invariably place dolostones in the biochemical camp.

20. Dolomitization – The Tyndall Formation The Tyndall Formation is quarried for building stone, and adorns many buildings in Western Canada, including the Tory Building and Telus Centre at the U of A. The left view shows the bland weathered surface, whereas the right view shows the fresh surface. The lighter material is limestone, and the darker colour is dolostone mottling [purple arrows]. The fossil is Receptaculites, possibly a fossil alga.

21. Control of Dolomiti-zation.Dolomitization of the Tyndall Formation has occurred next to burrows [purple arrows]. These ancient burrows were formed in the sediment, before the sediment was indurated/lithified. For whatever reason, the Mg-rich fluids moved through, and out from, the burrows into the surrounding limestone. Note the sunflower pattern to the Receptaculites skeleton.

22. Dolomitization and Dollars Both core samples are from oil exploration wells, and illustrate the Devonian Leduc Formation [left] and Grosmont Formation [right]. Both are fully dolomitized limestones with porosity [blue arrows] that enables them to be prolific hydrocarbon producers. The cylindrical plug [pink star] removed from the Leduc core was analyzed for its petrophysical properties [reservoir characteristics].