BIOCHEMICAL BIOCHEMICAL SEDIMENTARY ROCKSSEDIMENTARY ROCKS

Prepared by Dr. F. ClarkPrepared by Dr. F. ClarkDepartment of Earth and AtmosphericDepartment of Earth and Atmospheric

Sciences, University of AlbertaSciences, University of AlbertaAugust 06August 06

INTRODUCTIONINTRODUCTION

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

Calcite.Calcite.This common This common carbonate carbonate mineral, with mineral, with the formula the formula CaCOCaCO33, , reacts reacts strongly with strongly with dilute dilute hydrochloric hydrochloric acid.acid.

The other polymorph of CaCOThe other polymorph of CaCO33, with the same chemical formula but , with the same chemical formula but different crystal structure, is called aragonite, and is often the form in different crystal structure, is called aragonite, and is often the form in which this material is first precipitated. It reacts [fizzes/effervesces] which this material is first precipitated. It reacts [fizzes/effervesces] with HCl as well, and is generally not distinguished in hand specimen.with HCl as well, and is generally not distinguished in hand specimen.

DolomiteDolomite..This common This common carbonate carbonate mineral has mineral has Mg as well as Mg as well as Ca, with the Ca, with the formula formula CaMg(COCaMg(CO33))22. . It will also It will also react with react with HCl, but HCl, but weakly.weakly.As seen in the crystals above, dolomite has rhombic cleavage, the 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 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 instead of 3), and only reacts weakly with HCl when powdered. The colour difference between crystals of the two is not consistent.colour difference between crystals of the two is not consistent.

CARBONATE CONSTITUENTSCARBONATE CONSTITUENTS

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

Carbonate Grains – Skeletal GrainsCarbonate Grains – Skeletal Grains

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

More Skeletal GrainsMore Skeletal Grains

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

More Paleozoic Skeletal GrainsMore Paleozoic Skeletal Grains

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

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

CarbonatCarbonate Grains e Grains ––Ooids.Ooids.Ooids are Ooids are concentricallconcentrically- laminated y- laminated grains, the grains, the laminae laminae consisting of consisting of tiny crystals.tiny crystals.This sample, from a saline lake deposit, may be called an oolite. The 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 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 the surface of the rock a pebbled look similar to a basketball or football. They are usually formed in shallow, agitated environments. football. They are usually formed in shallow, agitated environments.

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

Weathering of Carbonates - 1Weathering of Carbonates - 1

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

Weathering of Carbonates - 2Weathering of Carbonates - 2

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

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

Recrystal-Recrystal-lization 2.lization 2.The coarse The coarse individual individual crystals can crystals can easily be easily be seen [dark seen [dark blue arrows]; blue arrows]; this process this process tends to tends to obliterate obliterate details.details.

One survivor of the recrystallization process is the ribbed shell of a One survivor of the recrystallization process is the ribbed shell of a brachiopod [yellow arrow]. One common positive effect of brachiopod [yellow arrow]. One common positive effect of recrystallization is the development of pore spaces [light blue arrows] recrystallization is the development of pore spaces [light blue arrows] as atoms are rearranged. Porosity enables a rock to hold fluids. as atoms are rearranged. Porosity enables a rock to hold fluids.

Diagenesis Diagenesis ––Stylolites.Stylolites.Another Another effect of effect of stress at stress at grain grain boundaries is boundaries is that grains that grains may dissolve may dissolve by pressure by pressure solution.solution.

Carbonate may be soluble, but clay and organics generally are not, Carbonate may be soluble, but clay and organics generally are not, and these insoluble residues are concentrated at the solution front as and these insoluble residues are concentrated at the solution front as stylolites [purple arrow]. The amount of material lost can be gauged stylolites [purple arrow]. The amount of material lost can be gauged by the offset of the distinctive laminae [stars] across the stylolite.by the offset of the distinctive laminae [stars] across the stylolite.

Stylolites Stylolites 2.2.Samples rich Samples rich in lime mud in lime mud are very are very vulnerable to vulnerable to pressure pressure solution. In solution. In this sample, this sample, several several stylolites are stylolites are developed developed [purple [purple arrows].arrows].A greenish layer [green arrow] of unknown original thickness has been A greenish layer [green arrow] of unknown original thickness has been almost completely occluded or pinched out by this process. The fossils almost completely occluded or pinched out by this process. The fossils highlighted by blue arrows are examples of highlighted by blue arrows are examples of AmphiporaAmphipora, a common , a common stromatoporoid in quiet waters of the Devonian of Western Canada.stromatoporoid in quiet waters of the Devonian of Western Canada.

DOLOMITE –PRIMARY SEDIMENT?DOLOMITE –PRIMARY SEDIMENT?

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

DOLOMITE – BIOCHEMICAL?DOLOMITE – BIOCHEMICAL?

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

Dolomitization – The Tyndall FormationDolomitization – The Tyndall Formation

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

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

Dolomitization and DollarsDolomitization and Dollars

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