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LIMESTONES
1. INTRODUCTION 1.1 Something like about one-fifth of all sedimentary rocks are carbonate rocks. The two main kinds of carbonate rocks, limestones and dolostones, together with sandstones and shales, are what might be called the “big four” of sedimentary rock types. I’m reluctant to try to guess what percentage of all sedimentary rocks those “big four” account for, but the figure must be in the upper nineties. Moreover, carbonate rocks are economically important because together with sandstones they constitute reservoirs for most of the world’s petroleum and gas reserves (and let’s not forget that they are the source of all of the world’s portland cement—not a jazzy, exciting resource, but a very important one for our modern civilization).
1.2 Up until about fifty years ago, the petrologic study of carbonate rocks lagged far behind that of siliciclastic rocks. Since that time, however, there has been great progress, as it has become realized that to a great extent carbonate rocks can be treated as clastic deposits analogous to sandstones and shales (the main exception being reef limestones). Great progress has also been made in recent years on the geochemistry of carbonate precipitation, the role of organisms in carbonate deposition, and the diagenesis of carbonate sediments.
1.3 Carbonate sediments are often described as chemically precipitated. In one sense, that’s true: they are formed by precipitation of one or another carbonate mineral in various sedimentary environments. But don’t let the term “chemically precipitated” fool you: they don’t form in the same way that rock candy does from a sugar solution on your windowsill. Some carbonate sediments are indeed precipitated directly from seawater, in the form of fine crystals in the water column, which then settle to the seafloor, or as successive spherical shells deposited around a nucleus particle in a warm, shallow marine environment. Most, however, are biochemically precipitated, in the tissues of organisms, mainly marine invertebrates of various phyla.
1.4 The title of this chapter could have been “carbonate rocks”, but it seems somewhat more natural to restrict it to limestones. You will learn that dolomite is almost invariably a secondary rather than a primary sedimentary mineral, meaning that carbonate sediments don’t start their lives as dolomite. Much dolomite is precipitated very early, however, at shallow depths in the originally calcium carbonate sediment, although much is also precipitated late, after deep burial has produced solid limestone. Material on dolostone (a carbonate rock consisting mainly of the mineral dolomite) is postponed until the later chapter on diagenesis.
2. CARBONATE MINERALS 2.1 About sixty minerals have the carbonate ion in their composition. But there are only three really important carbonate minerals: calcite , aragonite , and dolomite. (In the parlance of mineralogy, the first two are said to be polymorphs .) And aragonite is unimportant in ancient rocks, because it reverts to calcite with time. Other sedimentary carbonates of non-negligible importance are magnesite (magnesium carbonate) and siderite (ferrous iron carbonate). Dolomites containing some percentage of Fe2+^ are called ferroan dolomite. The middle member of the range between dolomite and hypothetical (Fe, Ca)(CO 3 ) 2 is called ankerite. There are also a few significant carbonate evaporite minerals (trona, natron) we won't consider here.
2.2 Figure 5-1 is a composition triangle showing the range of carbonate minerals stable at the low temperatures at or near the Earth’s surface. It’s in terms of the three divalent positive ions, Ca2+^ (0.99 Å), Mg2+^ (0.66 Å), and Fe2+^ (0. Å), that are abundant and of about the right size to fit into carbonate structures. (Remember that one angstrom is equal to 10-10^ meters, or 0.1 nanometers.)
2.3 The two most important minerals are calcite , CaCO (^) 3, and dolomite , Ca(Mg,Fe)(CO 3 )2. Crystals of calcite and dolomite have rhombohedral symmetry. Think in terms of the simple crystal structure of halite, NaCl, in which effectively spherical Na+^ and Cl-^ ions alternate along each of three mutually perpendicular directions to form a cubic structure with each Na+^ placed between six Cl-^ ions and vice versa. The structure of calcite is similar, with Ca2+^ and CO 3 2- ions alternating in three directions. Ca2+^ is about as big as Na+, but CO 3 2-^ is larger and has an effectively triangular shape and so takes up more space than the Cl- ions. The CO 3 2-^ ions have to be cocked at an angle to fit between the six nearest Ca2+^ ions. So the whole array can be thought of as squeezed along one of the inner diagonals of the cube until the three lines of alternating ions meet at about 106° instead of at 90°. This results in rhombohedral rather than cubic symmetry.
2.4 In dolomite, about half of the Ca 2+^ ions are replaced by Mg 2+^ (and Fe2+) ions. If these ions were of the same size, there could be unlimited substitution of one for the other, and there would be complete isomorphism between calcite and magnesite. But the effective radius of Ca 2+^ is 36% larger than that of Mg 2+, so the presence of more than a few percent Mg 2+^ in the calcite lattice would cause so great a distortion that the structure would be unstable. But it turns out that Ca 2+ and Mg 2+^ can be present in about equal numbers, alternating regularly between the CO 3 2-^ ions along each of the three directions, thus forming separate sheets of Ca2+ and Mg 2+^ in the structure; that’s the mineral dolomite. This results in slightly different angles and different symmetry.
Paleozoic corals (rugose corals, tabulate corals): probably calcite, by good preservation of structures brachiopods, bryozoans, foraminifera : calcite then and now echinoderms : calcite then and now; large single-crystal skeletal components, very durable trilobites : probably calcite, by good preservation of structures mollusks : mostly aragonite, sometimes partly or entirely calcite
3.2 At present, most of the open surface waters of the oceans, except at high latitudes, are about saturated or even supersaturated with respect to CaCO 3 , so there must be at least a broad inorganic control on precipitation of CaCO 3. Figure 5-2 shows the degree of saturation of aragonite and calcite with depth for both the Atlantic Ocean and the Pacific Ocean. It's not known how long things have been this way, but judging from the overall similarity of present deposits and past deposits, the situation has probably been the same at least back into the Precambrian. In the Precambrian, the saturation of seawater with calcium car- bonate may have been the highest of all time, because of much higher levels of CO 2 in the atmosphere.
3.3 By river contributions of dissolved load from rock weathering on the continents, the doubling time of Ca2+^ in the oceans should be about a million years. (By the doubling time is meant the time it would take for the concentration of a given substance to increase by a factor of two if no other process is at work to remove the substance at the same time.) Obviously something must be taking it out at about the same rate. The same is true for Mg2+^ ion, but the doubling time is
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Figure 5-2: Degree of saturation with respect to calcite and aragonite with depth in the Atlantic arid Pacific Oceans
ATLANTIC OCEAN PACIFIC OCEAN
ATLANTIC OCEAN PACIFIC OCEAN
DEPTH Km
DEPTH Km
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.5 .6 .8 1.0 1.5 2.0 3.0 4.0 5.0 7.
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.3 .4 .5 .6 .8 1.0 1.5 2.0 3.0 4.0 5.
DEGREE OF SATURATION, CALCITE DEGREE OF SATURATION, ARAGONITE
Figure by MIT OCW.
about an order of magnitude greater, something like twenty million years. In the case of calcium ion, the obvious process by which it’s removed from solution is precipitation of carbonate minerals. For magnesium ion, however, the removal process is less obvious—because, as you will see presently, dolomite is being precipitated as a primary sedimentary mineral almost not at all in the modern oceans. It’s known that Mg is removed from seawater very efficiently by reaction of the seawater with freshly crystallized basalt at and near mid-ocean ridges. Also, in shallow-water carbonate-producing environments the magnesium in solution is buried with the calcium carbonate sediments, where, with time, during shallow-burial diagenesis, it dolomitizes the calcium carbonate minerals—about which you will learn more in the later chapter on diagenesis.
3.4 As for the entire ocean bottom, about one gram of CaCO 3 is deposited per square centimeter of ocean bottom per 1000 years on the average , on the basis of balance considerations. Over large areas, however, no carbonate is being deposited at all, whereas in other places the rates of accumulation are far greater than the average.
3.5 The rate of deposition of CaCO 3 is much greater in shallow ocean than in the deep ocean, but the volume of newly deposited shallow-water CaCO 3 is far smaller than that of deep- sea CaCO 3. But this is true only for the present time: planktonic carbonate-secreting organisms evolved late in geologic history, in the Mesozoic, so the volume of shallow-water CaCO 3 must have been much greater in the Precambrian and the Paleozoic than now (on the reasonable assumption that the rate of supply of dissolved calcium ion was about the same).
3.6 Another point worth noting here is that, from the standpoint of long-term geologic history, deep-sea storage of carbonate is only temporary : sea-floor spreading and subduction of oceanic lithosphere at continental margins provides a very satisfying way of reincorporating deep-sea carbonate deposits into the geological record of the continents, albeit usually in unrecognizable form. Before the plate-tectonics revolution in the 1960s, geologists believed that what was put into the deep ocean stayed there forever!
4. GEOCHEMISTRY OF CARBONATE PRECIPITATION 4.1 Precipitation of carbonate in natural waters is more complicated than that of, say, halides or sulfates, because of the dissolution of carbon dioxide in natural waters. Here are the reactions that are relevant to carbonate precipitation:
CO 2 (gas) + H 2 O ⇔ CO 2 (aqueous solution) + H 2 O
CO 2 + H 2 O ⇔ H 2 CO 3 (carbonic acid, about 1%)
H 2 CO 3 ⇔ H+^ + HCO 3 -^ K = 4.3 x 10-
and
anything that decreases the concentration of dissolved CO 2 tends to cause precipitation of calcium carbonate.
The two most important effects are:
temperature: as the water temperature increases, the equilibrium solubility of CO 2 decreases, so as sea water is warmed there is a tendency for CO 2 to be released back into the atmosphere and for CaCO 3 to be precipitated.
photosynthesis: in photosynthesis, plants take up CO 2 from the environment and fix it in organic compounds in their tissues, thereby releasing oxygen.
4.3 Another important factor is this: carbonate-secreting marine invertebrates live in greatest numbers in the warm shallow parts of the oceans. These organisms can secrete CaCO 3 even from water that is not saturated in CaCO 3 , but they do it best and most abundantly where the water is saturated.
4.4 So it makes sense that most of the CaCO 3 precipitated in the oceans today is in warm, shallow water , where the water is warmed, so that the concentra- tion of dissolved CO 2 is lowered and saturation with respect to CaCO 3 is thus enhanced, and where aquatic plants (largely algae) flourish.
4.5 In summary, there is a broad inorganic control on carbonate precipitation in the oceans, but the specific controls have to do with local water temperature and photosynthesis.
4.6 So far in this section we have addressed only the precipitation or dissolution of the calcium carbonate minerals. How about the mineral dolomite? To deal with precipitation of dolomite, we have to think about undersaturation and oversaturation (also called supersaturation ). Suppose you put a piece of calcite in a beaker of distilled water, and for the sake of simplicity you arrange that no carbon dioxide is dissolved in the water. You know what will happen: the calcite dissolves slowly, and, as it does, the concentrations of dissolved Ca2+^ ions and CO 3 2-^ ions increase The solution is said to be undersaturated with respect to calcite. Eventually (and it would take months), the reaction reaches equilibrium, whereupon the concentrations of the Ca2+^ ions and the CO 3 2-^ ions reach constant values. If, however, we somehow pump Ca2+^ ions into the solution, we drive the reaction toward precipitation of calcite. The solution is said to be oversaturated.
4.7 We can think separately about the saturation state of calcite (or aragonite) in seawater and about the saturation state of dolomite in seawater. The conventional way of doing that is to define a quantity variously called the solution quotient or the mass action quotient or the ion activity product (the last designated IAP), which has exactly the same form as the equilibrium constant but the concentrations are those that exist at a given time in the solution, whether the
solution is in equilibrium (the state of saturation) or undersaturated or oversaturated. If the solution is oversaturated, then the IAP is greater than the equilibrium constant K; if the solution is undersaturated, then the IAP is less than the equilibrium constant K.
4.8 It turns out that in warm, shallow seawater (the part of the oceans that is most relevant to precipitation of carbonate minerals, as you saw above), all three of the important carbonate minerals—calcite, aragonite, and dolomite—are in a state of oversaturation : that is, the situation should be conducive to precipitation. You have seen that calcite or aragonite are indeed precipitated, but dolomite is not. Yet, perhaps surprisingly, the ratio IAP/K for the three minerals is as follows:
5.2 Carbonate deposits are most abundant between about 30° N and 30° S. (They are not restricted to that zone, though: the topic of cold-water carbonates is an active area of research nowadays.) The low-latitude regions are where the surface waters tend to be saturated with respect to CaCO 3 , and where the water is warm enough the year round for carbonate-secreting organisms to flourish. But this picture is obviously complicated in detail, by patterns of ocean currents, nutrient supply, and dilution by siliciclastic sediments.
5.3 There are three main classes of modern marine carbonate sediments :
- calcareous oozes in the deep ocean
- carbonate buildups
- calcareous sand and mud on platforms (shelf and ramp carbonates) 6. CALCAREOUS OOZE 6.1 More than a third of the present deep ocean bottom is covered with sediment containing more than 30% CaCO 3. The carbonate in these sediments is in the form of tiny shells or tests of various carbonate-secreting planktonic organisms that live in the warm shallow waters above. Such deposits are called ooze ; there is calcareous ooze and siliceous ooze , depending upon what the organisms secrete.
6.2 The most abundant kind of calcareous ooze is foraminiferal ooze. Foraminifera are single-celled protozans. In the oceans of today there are about thirty species of foraminifera (forams, for short) belonging to two families, Globigerinidae and Globorotaliidae. These are the only two of more than fifty foram families that are adapted to a planktonic rather than a benthic mode of life. But these planktonic forams grow in enormous numbers in the warm shallow waters of the ocean. At subdividing time the protoplasm of each foram subdivides into zoospores that swarm out to develop into new forams, leaving the empty test to sink to the bottom. So very little organic matter goes down with the tests. Figure 5-3: (left) Brasier, M.D., 1980, Microfossils: George Allen & Unwin, 193 p. (Figure 13.27g, p. 116). (center) Brasier, M.D., 1980, Microfossils: George Allen & Unwin, 193 p. (Figure 13.27e, p. 116) (right) Boersma, A., 1978, Foraminifera, in Haq, B.U., and Boersma, A., Introduction to Marine Micropaleontology: Elsevier, 376 p. (Appendix I, p. 69-75) shows what some planktonic forams look like.
6.3 Because of the greater solubility of CaCO 3 in the colder deeper waters, which are derived from cold polar regions and thus contain more dissolved CO 2 , below a certain depth the foram tests are completely dissolved before they have a chance to be covered by more tests. Figure 5-4 is a graph of CaCO 3 content of bottom sediments vs. depth.
6.4 Calcareous oozes are abundant down to about 4000 m, but below that they become much less abundant; there is very little carbonate below about 4500 m. Oceanographers call the depth between 4000 and 4500 m where carbonate becomes less abundant the carbonate compensation depth , or CCD; it’s actually a narrow zone rather than a single depth.
6.5 The oceans are undersaturated with respect to CaCO 3 below about 500 m everywhere; the reason carbonate is present down to thousands of meters is basi- cally a time-lag effect. Presumably the CCD has to do with what happens on the bottom, not what happens on the way down , because it doesn’t take long for the tests to get to the bottom, but they sit there exposed to the water for a long time.
6.6 The distribution of calcareous ooze is very irregular. Calcareous ooze is present mainly on high areas in low latitudes, and it’s present to greater depths where surface productivity is high. There’s not much under areas of low productivity, where nutrient concentrations in the surface waters are low, or at high latitudes, where surface waters are unfavorable.
6.7 Forams are the major constituent of calcareous ooze, but not the only one. Two other kinds of carbonate-secreting planktonic organisms also contribute to calcareous ooze:
- a minor group of tiny gastropods, called pteropods and heteropods (mostly the former); these are larger than forams but they secrete aragonite, so they are not found below about 3500 m;
- two kinds of calcareous algae, called coccoliths and rhabdoliths (mostly the former).
- the subtidal open shelf and shelf margin , characterized by in-place accumulations of carbonate sands, carbonate muds, and reefs;
- the shoreline , where sediments are transported from the open shelf onto beaches and tidal flats; and
- the slope and basin , where shelf-edge sediments are transported seaward, often by mass movements, and redeposited at depth.
7.1.3 The shallow-water carbonate factory is very sensitive to sea-level change. At various times in Earth history, sea level has fluctuated, on time scales of tens of thousands to hundreds of thousands of years, and with magnitudes ranging from meters to hundreds of meters. Most of the major carbonate-secreting organisms flourish when the water is shallow. If sea-level rise is slow, and the concomitant increase in water depth is slow, the carbonate factory can keep up production; this is called keep-up mode. But if sea level increases fast enough, and water depth thereby increases fast enough, the carbonate factory has a strong tendency to shut down. This is called give-up mode.
7.2 Regional Geometry
7.2.1 The term platform carbonate is in common use for all accumulations of carbonate sediments in tectonically stable shallow-water environments. This includes reefs, but I’ll deal with them later. Because in areas of high productivity carbonate sedimentation can be so rapid, carbonates tend to mold their own environment , even on the scale of entire shelves. So the regional bathymetry of carbonate areas tends to be more varied than that of siliciclastic shelves , which are more familiar to most geologists. Below is an outline of the regional geometry of shallow-water carbonate bodies. Refer to Figures 5-7, 5-8, 5-9, and 5-10.
7.3.2 The Bahama Islands are located on several submerged shallow carbonate platforms that lie just off the North American continental shelf. The banks cover 60,000 square miles, but the land area of the islands is only about 4400 square miles. Water depths over most of the banks are less than ten meters! There are several large islands and thousands of very small islands called cays. The exposed land surface is very pure Pleistocene limestone with little soil development. In places elevations are over a hundred meters; these are fossil subaerial dune ridges of oolitic sediment. Keep in mind that the entire surface area of the banks was exposed during the low stands of sea level during the Pleistocene.
7.3.3 The largest of the Bahama Banks, Great Bahama Bank, is split by two troughs, Tongue of the Ocean and Exuma Sound, which extend down to deep ocean depths, and each of the banks is separated from its neighbors by deep-water channels. The shelf margins are well defined. The upper few hundred meters of the slope is very steep, greater than the angle of repose; this is probably controlled by reef development in the past.
7.3.4 The Bahamas lie in the belt of northeast trade winds. This governs the position of islands on the windward margins of the platforms. Hurricanes are common, and they do important geological work. Throughgoing currents are warmed as they pass over the banks, leading to supersaturation with respect to calcium carbonate.
7.3.5 Bahama reefs play a minor part in the carbonate sediment picture today, presumably because of the Pleistocene history of fluctuating sea level. But deep borings show that they were much more important in the past. Reef construction is most important along the outer edges of the banks facing deep ocean water, and they are much better developed on the windward sides than on the leeward sides.
7.3.6 Great deposits of pure calcium carbonate sand and mud are forming and accumulating on the banks. With stable sea level, the carbonate sediment is transported to deep water as fast as it is produced, but slow crustal subsidence has caused about 5 km of carbonate sediment to have been deposited since at least the early Cretaceous , so the banks date from not long after the opening of the Atlantic.
7.3.7 All of the various kinds of shelf-carbonate constituents described in the next section are found on the Bahama Banks, most of them in abundance. Areas of ooids and of aragonite muds in particular are shown on the sketch map in Figure 5-12.
Ooids
Ooids are nearly spherical grains consisting of a grain of calcareous or noncalcareous material serving as a nucleus around which successive layers or shells of calcium carbonate are precipitated or accreted while the particle is moved in flowing water that is supersaturated with respect to calcium carbonate Figure 5- 13: (left): Tucker, M., 1991, Sedimentary Petrology; An Introduction to the Origin of Sedimentary Rocks: Blackwell, 260 p. (Figure 4.2, p. 110). (right): Williams, H., Turner, F.J., and Gilbert, C.M., 1982, Petrography; An Introduction to the Study of Rocks in Thin Section: W.H., Freeman, 626 p. (Figure 14-8, p. 385). Size ranges mostly from a fraction of a millimeter to about 2 mm— although, especially in the Neoproterozoic, they can be on a centimeter scale. If the coating is thin relative to the size of the nucleus, the ooids are called superficial ooids or coated grains****. Commonly there are two kinds of concentric spherical shells in the ooid structure: tangentially oriented aragonite needles, and non-oriented cryptocrystalline aragonite. Ooids tend to contain organic matter in the form of algal mucilaginous matter; this is seen as dark brown areas in thin section. Ooids can also show radial magnesian calcite. This seems to result from purely inorganic precipitation. Ooids are common in the ancient and are known but not common in modern carbonate sediments. The processes of ooid growth are still a subject of discussion.
Pellets
Pellets are rounded grains of very fine-grained aragonite and calcite , a few tenths of a millimeter to about a millimeter in size. They are usually elliptical or ovoid in shape, but they may be broken to form beehive-shaped grains. Some are clearly fecal pellets excreted by such organisms as worms, gastropods, mollusks, and crustaceans. These are soft and friable at first, but they soon become well cemented. Other pellets seem to be formed by cementation and rounding of friable irregular aggregates of aragonite silt. Because it is usually difficult or impossible to distinguish among the various possible processes that form such objects, the term peloid is in common use.
Intraclasts
Intraclasts are fragments of carbonate sediment , usually fine-grained, that was deposited and then later ripped up by strong currents to be redeposited with other carbonate sediment. The derivation of the word implies that the ripping up took place within the environment of carbonate deposition, geologically soon after the depositional of the sediment; don’t confuse these with carbonate rock fragments in a largely siliciclastic conglomerate. The stage of cementation varies considerably. Commonly the intraclasts are tabular, reflecting breakage of semi- consolidated sediment along stratification planes.
Carbonate Mud
Carbonate mud consisting mostly of needle-shaped aragonite crystals is common in areas with weak currents. Some of this carbonate mud is produced by abrasion of larger grains, but most seems to be precipitated directly from seawater. The nature of this precipitation has been controversial: is it purely inorganic, or is it caused by algae? (You can imagine precipitation of aragonite next to the bodies of photosynthesizing algae, because the CO 2 content of the water right next to their bodies is decreased, which favors carbonate precipitation.) The answer seems to be that both processes operate, but that algal precipitation is generally much more important than inorganic precipitation.
8. REEFS 8.1 Introduction 8.1.1 A reef, rising above the sea floor, is an entity of its own making—a sedimentary system within itself. The numerous, large calcium-carbonate- secreting organisms stand upon the remains of their ancestors and are surrounded and often buried by the skeletal remains of the many small organisms that once lived on, beneath, and between them (Figure 5-14).
8.1.2 At present, far more shallow-water carbonate sediment is produced in, or in connection with, reefs than by any other means. Judging from the presence of reefs in the stratigraphic record, reefs have been important sites of carbonate sedimentation from the Archean. Throughout geologic history, they have
