Chalk is affected by different diagenetic processes in comparison with other carbonate rocks. Commonly observed lithification processes are pressure-solution and reprecipitation and neomorphism. The first process is divisible into: (a) spot-welding or welding; (b) overgrowth; (c) stylolitization and cementation. Further, pressure-solution and reprecipitation is dependent on (a) geostatic pressure and (b) the ratio of Mg/Ca in the pore fluid. Overgrowth of calcite cement is controlled by the size and shape of the biogenic crystals. Neomorphism is also important in diagenesis. Neomorphic process dominates when pressure-solution process is ineffective. Organic and insoluble residue content dictate neomorphism. In conclusion, pressure-solution or neomorphism or a combination of both accounts for various modes of lithification observed in chalk

Kirumakki, Nagaraja

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Transactions of the Nebraska Academy of 
Sciences and Affiliated Societies Nebraska Academy of Sciences 
1977 
Some Aspects of Lithification In Chalk 
Nagaraja Kirumakki 
University of Nebraska-Lincoln 
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Kirumakki, Nagaraja, "Some Aspects of Lithification In Chalk" (1977). Transactions of the Nebraska 
Academy of Sciences and Affiliated Societies. 449. 
https://digitalcommons.unl.edu/tnas/449 
This Article is brought to you for free and open access by the Nebraska Academy of Sciences at 
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SOME ASPECTS OF LITHIFICATION IN CHALK 
NAGARAJA KIRUMAKKI 
Department of Geology 
University of Nebraska-Lincoln 68588 
Chalk is affected by different diagenetic processes in compari-
son with other carbonate rocks. Commonly observed lithification 
processes are pressure-solution and reprecipitation and neomorphism. 
The fIrst process is divisible into: (a) spot-welding or welding; (b) over-
growth; (c) stylolitization and cementation. Further, pressure-llolution 
and reprecipitation is dependent on (a) geostatic pressure and (b) the 
ratio of Mg/Ca in the pore fluid. Overgrowth of calcite cement is con-
trolled by the size and shape of the biogenic crystals. Neomorphism is 
also important in diagenesis. Neomorphic process dominates when 
pressure-llolution process is ineffective. Organic and insoluble residue 
content dictate neomorphism. In conclusion, pressure-llolution or 
neomorphism or a combination of both accounts for various modes of 
lithification observed in chalk. 
t t t 
The initial reports of the deep sea drilling project 
(JOIDES) have yielded new data and new impetus to studies 
of pelagic and deep water carbonate sediments. A further aid 
to the understanding of depositional and diagenetic environ-
ments of the pelagic sediments has been given by the recent 
discoveries of oil fields under the North Sea where the Creta-
ceous chalk is a major petroleum reservoir. Further, detailed 
observations with the scanning electron microscope have 
disclosed different processes of lithification, such as dissolu-
tion and precipitation. Application of these new ideas to the 
study of chalk has been an interesting revelation to many 
petrographers. 
Chalk is a pelagic sediment of biogenic origin. The Glos-
sary of Geology (I 973 :117) defmes chalk as "soft, pure, 
earthy, fine-textured, usually white to light gray or buff lime-
stone of marine origin, consisting almost wholly (90-99%) 
of calcite, formed mainly by shallow-water accumulation of 
calcareous tests of floating micro-organisms (chiefly fora-
minifers) and of comminuted remains of calcareous algae 
(such as coccoliths and rhabdoliths) set in a structureless 
matrix of very finely crystalline calcite (some of which may 
have been chemically precipitated) and marked by a porous, 
somewhat friable, and unindurated or slightly coherent char-
acter." 
At the present time, pelagic sedimentation is virtually 
confined to ocean basins. In the Mesozoic era, on the other 
hand, pelagic deposits, including chalk, were laid down on the 
continental basement. In Late Cretaceous time there were 
widespread continental seaways, and practically every con-
tinent was affected by this world-wide marine transgression. 
The best known and most widely distributed deposits of these 
seaways are chalks of the Cenomanian-Maastrichtian age. The 
chalks of western Europe are of particular interest here be-
cause of their economic importance and their contribution 
to progress in chalk petrography. 
Lithification includes all diagenetic processes which 
serve to convert unconsolidated sediment to coherent rock. 
The diagenetic alteration of chalk has long been a subject of 
controversy in petrography. For example, the lithification of 
the Irish Chalk has been variously attributed to the presence 
of aragonite in the original sediment and to metamorphism 
by overlying basalts (Bathurst, 1975 :401408). Similarly, 
major hypotheses offered to explain radical differences in 
hardness of Cretaceous chalk in Great Britain all appear 
inadequate (Scholle, 1974:178). In this paper, an attempt is 
made to summarize the new data and examine the mechan-
isms proposed by various authors to explain patterns of chalk 
diagenesis. 
Unlike most regions of shallow water carbonate deposi-
tion where the major carbonate component is aragonite (e.g., 
the Arabian Gulf), the bulk of chalk mud is totally biogenic 
in origin, consisting mostly of stable, low-Mg calcite. Conse-
quently, the mechanisms of chalk diagenesis have to be ex-
plained by processes other than simple compaction, leaching, 
solution (fresh-water influx), and cementation. Common and 
widely observed processes are pressure-solution and repreci-
pitation and neomorphism. The mechanism of pressure-
solution and reprecipitation is divisible into: (a) spot-welding 
or welding; (b) selective overgrowth on some calcite grains; 
and (c) stylolitization and cementation. The process of repre-
cipitation is usually local and occurs as calcite cement. Solu-
tion-precipitation is dependent on two factors: the depth of 
burial and pore-water chemistry. 
Schlanger (1974) concludes that lithification generally 
increases with age and the depth of burial. The history of 
compaction starts with dewatering and is followed by in-
creased packing, spot-welding, subsequently by dissolution-
precipitation (selective overgrowth cement) and stylolitiza-
tion and extensive cementation. Interesting advances in 
theoretical understanding of pressure-solution have come 
about with Neugebauer's (1974) analysis of the solution in 
the chalk. His main thesis is that chalk remains soft and unce-
mented because Mg-rich pore fluids, which are supersaturated 
229 
with respect to low-Mg calcite, inhibit pressure-solution and 
reprecipitation until considerable overloads are reached. When 
this inhibiting factor (supersaturation) is removed, pressure-
solution and cementation can proceed at an accelerated pace. 
During diagenesis, the Mg/Ca ratio of the pore-fluid gener-
ally decreases considerably (Neugebauer, 1974: 156), which, 
along with the inevitable rise in geostatic pressure, leads to 
extensive pressure-solution: and culminates in stylolitization 
and cementation. Neugebauer constructed a model, illus-
trating the relationship among such factors as overburden, 
dissolution of nannofossils and cement fabric, and the ratio 
of MgfCa in the pore solution. His results compared well with 
deep-sea drilling data, and he concluded that an overburden 
of at least 20004000 m is required for the complete lithi-
fication of the chalk by pressure-solution. 
It is very interesting to note where the precipitation of 
cement in chalks takes place. Cement in a typical chalk occurs 
as pore-fllling, sparry calcite and also as overgrowths on 
nannofossils. Neugebauer (1974: 168) observed that different 
group of fossils (coccoliths, foraminifera, Inoceramus, etc.) 
are coated with different amounts of overgrowth cement. He 
noted that size and geometrical shape of the host grain control 
the form of calcite overgrowths. Foraminifera acquire smaller 
overgrowths than macrofossils but larger overgrowths than 
coccoliths. Neugebauer (1974:172) points out further that 
coccolithic elements, unlike other biogenic crystals of the 
same size, are somewhat protected from dissolution because 
of their stable form. Adelseck (1975 :969-970) demonstrates 
that dissolution is further dependent on the thickness of the 
coccolith plates and that within the forminifera, the individual 
species resistance is dependent on the pore size and pore 
density of the test. 
Wise and Kelt (1972) demonstrated the cementation of 
an Oligocene, South Atlantic chalk by calcite overgrowths on 
coccoliths and discoasters. Scholle (1974) attributed cementa-
tion of English chalk to overgrowth on coccoliths, while 
Mapstone (1975) believed that the cement in the Danian and 
Maastrichitian chalk from the Tor Field (North Sea) was 
derived from pressure-solution of calcite grains (mostly indi-
vidual coccolithophorids). Similarly, J~rgensen (1975) favored 
the consolidation of the soft sediment to white chalk in the 
Danish region over pressure-solution. 
Scholle (1976:719) states that small variations in initial 
grain size, faunal composition, or clay content can lead to 
significant stratum-to-stratum variations in lithification. J~r­
gensen (1975:312) noted in Danish chalk that, though argil-
laceous chalk and pure chalk were subjected to the same 
lithostatic pressure, the lack of lithification in argillaceous 
chalk was due to the fact that very little cement was pro-
duced at the clay/carbonate grain contact. The hard grounds 
that are seen in the Cretaceous chalk of Europe and in the 
chalk and shaly chalk strata of the Greenhorn Formation 
(Upper Cretaceous) in the western interior U.S. are examples 
of this type of cementation. Hard grounds are layers of hard, 
230 
well cemented chalk in a sequence of porous, poorly ce-
mented, soft~halk strata; they are believed to be the rem-
nants of hardened sea floors. 
Neomorphism plays a key role in hardening a loose, 
noncoherent, carbonate mud to chalk wherein pressure-
solution processes are subordinate. Neomorphism as used by 
Folk (1965 :20-21). includes all transformations between one 
mineral and its polymorph. The newly formed crystals can be 
of differing sizes. The growth of carbonate mud to sparry 
calcite is controlled by insoluble residue and organic content. 
The Upper Cretaceous Greenhorn.chalk is a good example of 
lithification by the growth of calcite grains (Kirumakki, 1976: 
47; Hattin, 1976:83-86). 
The conclusion is inescapable: Variations in the char-
acter of chalks are related to different diagenetic histories. 
The occurrence of spot-welding at point contacts of calcite 
grains, the presence of extensive overgrowth cement, and the 
abundance of stylolites all testify to the idea that pressure-
solution is a possible mechanism of cementation. The various 
modes of lithification found in chalks can, to a large extent, 
be attributed to: (1) cementation by pressure-solution, and 
(2) independent neomorphism, or (3) some combination of 
the above processes. 
ACKNOWLEDGMENTS 
Professors P.R Krutak and D.G. Kersey gave many 
helpful suggestions and comments for improvement. Profes-
sor R Smith critically reviewed the manuscript. My thanks 
to them for their help and time. 
REFERENCES 
Adelseck, C.H., Jf. 1975. Test morphology and crystal size. 
Abs. Geol. Soc. of Amer. pp. 696-670. 
Bathurst, RG.C. 1975. Carbonate sediments and their dia-
genesis. Developments in Sedimentology. Elsevier 
Scientific Publishing Company. 658 pp. 
Folk, RL. 1965. Some aspects of recrystallization in ancient 
limestones. Soc. of Econ. Paleontol. and Mineralogists. 
13:1448. 
Glossary of Geology. 1973. Washington, D.C., American 
Geological Institute. 857 pp. 
Hattin, D.E. 1976. Stratigraphy and depositional environ-
ment of Greenhorn Limestone (Upper Cretaceous) of 
Kansas. Bull. Kansas Geol. Surv. 209 :1-128. 
J~rgensen, N.S. 1975. Mg/Sr distribution and diagenesis of 
Maastrichtian white chalk and Danian bryozoan lime-
stone from Jylland, Denmark. Bull. Geol. Soc. of Den-
mark. 24(34):299-325. 
Kirumakki, N.S. 1976. Lithification in Greenhorn Chalk of 
eastern Nebraska. Abs. Proc. Nebr. Acad. of Sci. 47. 
Mapstone, N.B. 1975. Diagenetic history of a North Sea chalk. 
Sedimentology. 22:601-614. 
Neugebauer, J. 1974. Some aspects of cementation in chalk. 
Internation'l. Assoc. ofSedimentol. 1 :149-176. 
Schlanger, O.S. and R.G. Douglas. 1974. The pelagic ooze-
chalk-limestone transition and its implications for 
marine stratigraphy. Internat'l. Assoc. of Sedimentol. 
1 :117-148. 
Scholle, P.A. 1974. Diagenesis of Upper Cretaceous chalk 
from England, northern Ireland, and the North Sea. 
Internat'l. Assoc. of Sedimentol. 1 :177-210. 
__ . 1976. Diagenetic patterns of chalks. Bull. Amer. 
Assoc. of Petroleum Geolog. 60(4):719. 
Wise, S.W., Jr. and K.R. Kelts. 1972. Inferred diagenetic his-
tory of a weakly-silicified deep-sea chalk. Trans. Gulf 
Coast Assoc. of Geol. Soc. 22:177-203. 
231 



Some Aspects of Lithification In Chalk

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Transactions of the Nebraska Academy of Sciences and Affiliated Societies Nebraska Academy of Sciences 1977 Some Aspects of Lithification In Chalk Nagaraja Kirumakki University of Nebraska-Lincoln Follow this and additional works at: https://digitalcommons.unl.edu/tnas Kirumakki, Nagaraja, "Some Aspects of Lithification In Chalk" (1977). Transactions of the Nebraska Academy of Sciences and Affiliated Societies. 449. https://digitalcommons.unl.edu/tnas/449 This Article is brought to you for free and open access by the Nebraska Academy of Sciences at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Transactions of the Nebraska Academy of Sciences and Affiliated Societies by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. SOME ASPECTS OF LITHIFICATION IN CHALK NAGARAJA KIRUMAKKI Department of Geology University of Nebraska-Lincoln 68588 Chalk is affected by different diagenetic processes in compari-son with other carbonate rocks. Commonly observed lithification processes are pressure-solution and reprecipitation and neomorphism. The fIrst process is divisible into: (a) spot-welding or welding; (b) over-growth; (c) stylolitization and cementation. Further, pressure-llolution and reprecipitation is dependent on (a) geostatic pressure and (b) the ratio of Mg/Ca in the pore fluid. Overgrowth of calcite cement is con-trolled by the size and shape of the biogenic crystals. Neomorphism is also important in diagenesis. Neomorphic process dominates when pressure-llolution process is ineffective. Organic and insoluble residue content dictate neomorphism. In conclusion, pressure-llolution or neomorphism or a combination of both accounts for various modes of lithification observed in chalk. t t t The initial reports of the deep sea drilling project (JOIDES) have yielded new data and new impetus to studies of pelagic and deep water carbonate sediments. A further aid to the understanding of depositional and diagenetic environ-ments of the pelagic sediments has been given by the recent discoveries of oil fields under the North Sea where the Creta-ceous chalk is a major petroleum reservoir. Further, detailed observations with the scanning electron microscope have disclosed different processes of lithification, such as dissolu-tion and precipitation. Application of these new ideas to the study of chalk has been an interesting revelation to many petrographers. Chalk is a pelagic sediment of biogenic origin. The Glos-sary of Geology (I 973 :117) defmes chalk as "soft, pure, earthy, fine-textured, usually white to light gray or buff lime-stone of marine origin, consisting almost wholly (90-99%) of calcite, formed mainly by shallow-water accumulation of calcareous tests of floating micro-organisms (chiefly fora-minifers) and of comminuted remains of calcareous algae (such as coccoliths and rhabdoliths) set in a structureless matrix of very finely crystalline calcite (some of which may have been chemically precipitated) and marked by a porous, somewhat friable, and unindurated or slightly coherent char-acter." At the present time, pelagic sedimentation is virtually confined to ocean basins. In the Mesozoic era, on the other hand, pelagic deposits, including chalk, were laid down on the continental basement. In Late Cretaceous time there were widespread continental seaways, and practically every con-tinent was affected by this world-wide marine transgression. The best known and most widely distributed deposits of these seaways are chalks of the Cenomanian-Maastrichtian age. The chalks of western Europe are of particular interest here be-cause of their economic importance and their contribution to progress in chalk petrography. Lithification includes all diagenetic processes which serve to convert unconsolidated sediment to coherent rock. The diagenetic alteration of chalk has long been a subject of controversy in petrography. For example, the lithification of the Irish Chalk has been variously attributed to the presence of aragonite in the original sediment and to metamorphism by overlying basalts (Bathurst, 1975 :401408). Similarly, major hypotheses offered to explain radical differences in hardness of Cretaceous chalk in Great Britain all appear inadequate (Scholle, 1974:178). In this paper, an attempt is made to summarize the new data and examine the mechan-isms proposed by various authors to explain patterns of chalk diagenesis. Unlike most regions of shallow water carbonate deposi-tion where the major carbonate component is aragonite (e.g., the Arabian Gulf), the bulk of chalk mud is totally biogenic in origin, consisting mostly of stable, low-Mg calcite. Conse-quently, the mechanisms of chalk diagenesis have to be ex-plained by processes other than simple compaction, leaching, solution (fresh-water influx), and cementation. Common and widely observed processes are pressure-solution and repreci-pitation and neomorphism. The mechanism of pressure-solution and reprecipitation is divisible into: (a) spot-welding or welding; (b) selective overgrowth on some calcite grains; and (c) stylolitization and cementation. The process of repre-cipitation is usually local and occurs as calcite cement. Solu-tion-precipitation is dependent on two factors: the depth of burial and pore-water chemistry. Schlanger (1974) concludes that lithification generally increases with age and the depth of burial. The history of compaction starts with dewatering and is followed by in-creased packing, spot-welding, subsequently by dissolution-precipitation (selective overgrowth cement) and stylolitiza-tion and extensive cementation. Interesting advances in theoretical understanding of pressure-solution have come about with Neugebauer's (1974) analysis of the solution in the chalk. His main thesis is that chalk remains soft and unce-mented because Mg-rich pore fluids, which are supersaturated 229 with respect to low-Mg calcite, inhibit pressure-solution and reprecipitation until considerable overloads are reached. When this inhibiting factor (supersaturation) is removed, pressure-solution and cementation can proceed at an accelerated pace. During diagenesis, the Mg/Ca ratio of the pore-fluid gener-ally decreases considerably (Neugebauer, 1974: 156), which, along with the inevitable rise in geostatic pressure, leads to extensive pressure-solution: and culminates in stylolitization and cementation. Neugebauer constructed a model, illus-trating the relationship among such factors as overburden, dissolution of nannofossils and cement fabric, and the ratio of MgfCa in the pore solution. His results compared well with deep-sea drilling data, and he concluded that an overburden of at least 20004000 m is required for the complete lithi-fication of the chalk by pressure-solution. It is very interesting to note where the precipitation of cement in chalks takes place. Cement in a typical chalk occurs as pore-fllling, sparry calcite and also as overgrowths on nannofossils. Neugebauer (1974: 168) observed that different group of fossils (coccoliths, foraminifera, Inoceramus, etc.) are coated with different amounts of overgrowth cement. He noted that size and geometrical shape of the host grain control the form of calcite overgrowths. Foraminifera acquire smaller overgrowths than macrofossils but larger overgrowths than coccoliths. Neugebauer (1974:172) points out further that coccolithic elements, unlike other biogenic crystals of the same size, are somewhat protected from dissolution because of their stable form. Adelseck (1975 :969-970) demonstrates that dissolution is further dependent on the thickness of the coccolith plates and that within the forminifera, the individual species resistance is dependent on the pore size and pore density of the test. Wise and Kelt (1972) demonstrated the cementation of an Oligocene, South Atlantic chalk by calcite overgrowths on coccoliths and discoasters. Scholle (1974) attributed cementa-tion of English chalk to overgrowth on coccoliths, while Mapstone (1975) believed that the cement in the Danian and Maastrichitian chalk from the Tor Field (North Sea) was derived from pressure-solution of calcite grains (mostly indi-vidual coccolithophorids). Similarly, J~rgensen (1975) favored the consolidation of the soft sediment to white chalk in the Danish region over pressure-solution. Scholle (1976:719) states that small variations in initial grain size, faunal composition, or clay content can lead to significant stratum-to-stratum variations in lithification. J~r­gensen (1975:312) noted in Danish chalk that, though argil-laceous chalk and pure chalk were subjected to the same lithostatic pressure, the lack of lithification in argillaceous chalk was due to the fact that very little cement was pro-duced at the clay/carbonate grain contact. The hard grounds that are seen in the Cretaceous chalk of Europe and in the chalk and shaly chalk strata of the Greenhorn Formation (Upper Cretaceous) in the western interior U.S. are examples of this type of cementation. Hard grounds are layers of hard, 230 well cemented chalk in a sequence of porous, poorly ce-mented, soft~halk strata; they are believed to be the rem-nants of hardened sea floors. Neomorphism plays a key role in hardening a loose, noncoherent, carbonate mud to chalk wherein pressure-solution processes are subordinate. Neomorphism as used by Folk (1965 :20-21). includes all transformations between one mineral and its polymorph. The newly formed crystals can be of differing sizes. The growth of carbonate mud to sparry calcite is controlled by insoluble residue and organic content. The Upper Cretaceous Greenhorn.chalk is a good example of lithification by the growth of calcite grains (Kirumakki, 1976: 47; Hattin, 1976:83-86). The conclusion is inescapable: Variations in the char-acter of chalks are related to different diagenetic histories. The occurrence of spot-welding at point contacts of calcite grains, the presence of extensive overgrowth cement, and the abundance of stylolites all testify to the idea that pressure-solution is a possible mechanism of cementation. The various modes of lithification found in chalks can, to a large extent, be attributed to: (1) cementation by pressure-solution, and (2) independent neomorphism, or (3) some combination of the above processes. ACKNOWLEDGMENTS Professors P.R Krutak and D.G. Kersey gave many helpful suggestions and comments for improvement. Profes-sor R Smith critically reviewed the manuscript. My thanks to them for their help and time. REFERENCES Adelseck, C.H., Jf. 1975. Test morphology and crystal size. Abs. Geol. Soc. of Amer. pp. 696-670. Bathurst, RG.C. 1975. Carbonate sediments and their dia-genesis. Developments in Sedimentology. Elsevier Scientific Publishing Company. 658 pp. Folk, RL. 1965. Some aspects of recrystallization in ancient limestones. Soc. of Econ. Paleontol. and Mineralogists. 13:1448. Glossary of Geology. 1973. Washington, D.C., American Geological Institute. 857 pp. Hattin, D.E. 1976. Stratigraphy and depositional environ-ment of Greenhorn Limestone (Upper Cretaceous) of Kansas. Bull. Kansas Geol. Surv. 209 :1-128. J~rgensen, N.S. 1975. Mg/Sr distribution and diagenesis of Maastrichtian white chalk and Danian bryozoan lime-stone from Jylland, Denmark. Bull. Geol. Soc. of Den-mark. 24(34):299-325. Kirumakki, N.S. 1976. Lithification in Greenhorn Chalk of eastern Nebraska. Abs. Proc. Nebr. Acad. of Sci. 47. Mapstone, N.B. 1975. Diagenetic history of a North Sea chalk. Sedimentology. 22:601-614. Neugebauer, J. 1974. Some aspects of cementation in chalk. Internation'l. Assoc. ofSedimentol. 1 :149-176. Schlanger, O.S. and R.G. Douglas. 1974. The pelagic ooze-chalk-limestone transition and its implications for marine stratigraphy. Internat'l. Assoc. of Sedimentol. 1 :117-148. Scholle, P.A. 1974. Diagenesis of Upper Cretaceous chalk from England, northern Ireland, and the North Sea. Internat'l. Assoc. of Sedimentol. 1 :177-210. __ . 1976. Diagenetic patterns of chalks. Bull. Amer. Assoc. of Petroleum Geolog. 60(4):719. Wise, S.W., Jr. and K.R. Kelts. 1972. Inferred diagenetic his-tory of a weakly-silicified deep-sea chalk. Trans. Gulf Coast Assoc. of Geol. Soc. 22:177-203. 231 

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