FACIES The Formation of Micritic Limestones and the ...

FACIES

34 159-176 PI. 42-44

8 Figs.

1 Tab.

ERLANGEN 1996

The Formation of Micritic Limestones and the Development of Limestone-Marl Alternations in the Silurian of Gotland, Sweden Axel Munnecke and Christian Samtleben, Kiel KEYWORDS: LIMESTONE-MARLALTERNAnONS - MICRITIC LIMESTONES - ARAGONITE - DlAGENESIS -MICRITE - MICRDSPAR - GarlAND - SILURIAN

CONTENTS 1 1.1

[ntroduction Micrite 1.2 Limestone-marl alternations 1.3 Open questions 2 Geologic selting 3 Methods 4 Observations 4.1 Limestone-marl alternations 4.2 Limestones 4.3 Marls 5 Results and Discussion 5.1 Formation of micritic limestones 5.2 Development of limestone-mar] alternations 6 Conclusions References

SUMMARY Micritic limestone-mar! alternations make up the major part of the Silurian strata on GOlland (Sweden). Their position on the stable Baltic Shield protccted them from deep burial and teclOnic suess and allowed the preservation of early slages of burial diagenesis, including lithification. In the micrilic limestones certnin characteristics have been preserved (e.g., pitted microspar crystals, sharp bQcnls

X

X

nrimarilv HMC-co

" urimarilv aluonitic COmDOneoU

X

X

orllanic microfossils

X

(uo""m~'l""

X ("""""",led)

ichnofossils

X funcomnactedl

X (""....,led)

micrite

X

X

microsoar

X

sparite

X (as matrix ~ ¥rainsto,~~ and as fillin In fossils

X (within fossils)

teni"enous matter

X

X

dolomite

X

X

Dyrite

X

X

(onlv SteWcems)

X

cOffioactioo ohenomena

X

pressure solution

X (roue)

carbonate cootenl (") an samples (0=476)

ex:;::e Lower Visby Beds (0",,63) exa le Ho"klint Beds (0- 168)

62 - 97 le 78) 62 - 91 le 7;l 64-95(e78

14 - 74 re 39) 16 - 2S (e 22) 37 -74(051\

Tab. I. Compositions and di.genetic charllCleOslics of limestones -.nd mll1ls in the Silurian of Gol1and.

This means Lhat the original carbonale mods on GoLland IlOl first aller 10 micrite and later to microspar by aggrading neomorphism. The fonnation of ffitcrosparcryStals obviously is a one·slep process: they show Lhe primary grain siz.e and shape of the cements lilhifying the origidid

nal aragonite-dominated carbonate mud. The 'SANDBERGcurve' (SANDBERG 1983). distinguishing belween calcitic

and aragonitic episodes in the Phanerozoic (using results of investigations on ooids and cements). does not seem to be applicable to the mineralogical composition of carbonate muds. According to the 'SA/Io'OBERo·curve' the Silurian lies near the Paleozoic calcite maximum. During subsequent stages of diagenesis the micrite cryst.aJs. fragile microfossils, nannofossils, as well as thin shells recrystallize into larger crystals by Slight aggrading neomorphism of the larger microspar crystals (pI. 44/6-8; Fig. 6). This neomorphism blurs the clues 10 primary structures and early diagenetic processes. Due to modest overburden pressure and absence of tectonic stress, the limestone diagenesis on Gotland stopped shanty after lith ification, so me early stages ofdiagenesis are preserved (F;g.6).

5.2 Development 0( limestone-mar! alternations

What happens to the the marls during the Iithification of the limestones? On the one hand they show a significant reduction in thickness but on me other hand almost no signs of pressure solution (Tab. 1). Also, the calcitic components down to the size of nannospheres lack disso-

lution (PI. 43/2). Cements do IlOlOCCur in the marts with the exception of fillings in some fossils. Because no primary aragonitic bioclasts. for example mol1uscs. are found in the marls (Tab. I), it is assumed, mat they represent those partS of me original sediment in which aragonite has been removed. The pitted structure of me microspar crystals of the limestones points to a primary aragonile-dominated mud. It is presumed that the precursor sediment of the marlsconsisted toa large degree ofaragonite, too, because sedimentary structures sometimes cross the boundaries between limestones and marls (Fig. 5), indicating that the same sediment can alter to limestone as well as to mar!. It is likely that solution of aragonite in me marls made carbonale available for the cementation of me limestones. Generally, HMC with more than 12%mol Mg-content is more soluble than aragonite (W ALlER 1985). However. as observed in younger sediments (Tertiary-Quatemary), aragonite is the most unstable calcium carbonate mineral in diagenesis because HMC·componenlS usually release their magnesium prior to the solution of aragonite. thus decreasing their solubility (FRIEDMAN 1964. WlNI..A/Io'D 1968. TOWE &. H£MWJEN 1976). Because on Gotland components which are corn posed primarily ofHMC (e.g.•ainoids) are found in limestones and marls (Tab. 1). it is assumed that aragonilt: was the only carbonate mineral involved in the processes of early diagenetic carbonate redistribution. Field observations on Gotland show that the same precursor sediment can alter to limestone or marl (see above). Although obviously coarser layers are preferen· tially cemented it cannOl be supposed that diagenesis principally enhanced primary differences of sediment

172

,

seaDoor

.. .

'

.'

'.'

sedimentation :.

. ' ,.' .

.

\

'" -~------------------~:~... -~;;;.::....:.

F="='l -----------------

compaction or the marts

o eVe

liviog . organisms

limestone

"''''

layer

~

• '*

marl diffusional t"""",,"

Fig. 7. DeveJopmentof rhythmic limestone-marl alternations caused by aragonite dissolution and calcite predpi\.ation within the ASZ (aragonite solution zone) and further completion of the marlstones due to continuous sedimentary overburden. The dissolved carbonate is transported by diffusion. Occasionally, regularly arranged layers of limes lOne nodules replace well bedded limestones.

material because nOl every shell layer is cemented (Fig. 5). In addition, lhe slight angle discordance between shell layer and limestone beds (Fig. 5) can only be explained by

a diagenetic origin of the alternation. Obviously, the diageneticaJ redistribution of carbonate is the dominant process in the development of limestone-marl alternations. This process is only influenced by primary differences in material. Based on our observations, we propose the following hypothesis for the development of limestone-marI ailernations in the Silurian ofGOlland: in the original sort muds vertical changes in pore water chemistry, mainly prodUCed by microbial decomposition of organic "material, are presumed, resulting in vertical geochemical gradiems. Decreasing carbonate saturation within the sediment column caused aragonite to be unstable and to be dissolved at a certain depth, producing a dissolution-zone roughly parallel to the seanoor, in the following called 'ASZ' (aragonite solution zone). Within this zone aragonite was dissolved, transported, and preCipitated as calcite (microspar) in regions where aragonite was still stable (Fig. 7). As the amount of pore water is by far not large enough for lhe transportation of the dissolved carbonate by pore water flow (ENOS & SAWATSKY 1981), the transport mechanism must be a diffusion process, as it is assumed for Lower Jurassic deposits in Great Britain by RAISWEtL (1988b). Because of the symmetrical structure of limestone beds or

nodules with highest carbonate contents in their central parts, the transport of the carbonate ions by diffusion must have been in upward as well as in downward direction (Fig. 7). The parts of the sediment depleted in aragonite became marls, lhose wilh precipitation of calcite were fonned to limestones. Although primary differences in sediment composition (e.g., carbonate content, bioclasts, organic matter, grain size, porosity) can serve as centers of cementation, they are not essential for the developmem of limestone-mar! alternations: the processes of aragonitedissolution and calcite-precipitation must have been effective in homogeneoussedimenlS, too, because the maximal distance ofadiffusional transpon is limited by geochcmical gradients (RAISWELL 1987, 1988b). Our assumption is that the regularity ofthc resulting limCSLOne-marl alternation is substantially innuenCed by the homogeneity of the primary sediment: homogeneous sediments led to regular alternations. Notwithstanding the facl that in numerous shallow marine sequences on Gotland irregularly distributed scdimcnt malcrials are common which strongly innuenced the diagenctical processes and resulted in more irregular limestone-marl alternations. By cementation, lhe limestone beds evaded furthercompaction; lhedissolution of their aragonilic components presumably took place in a later stage, this caused theempty-pit-fabric of the microspar crystals. The diagenesis of the micrilic limestones was nearly completed just below lhe ASZ. The marls, on the

173 Fig. 8. Schematicprcsc:ntationofthe tr&nsfonnalionofhomogencoussort5ediments (porosity about SQIl,) with different portions of aragonite, calcite. Uld clay into allernating sequences of solid carlxmlle rocks. The cubonate content of the rocks is outlined in circles (white = lime, black = clay/terrigenous material).

,do,

.. other hand, which already underwent a volume decrease

by aragonite depletion, lacked cement and became more and more compacted due to increasing sedimentary overburden (Fig. 7). They represent a compacted residual sediment depleted in aragonite. While Ihe rdaljv~ thick· nesses of alternating Iimeslone and marl beds depend mostly on the primary aragonite content (see below), the absobue thicknesses probably are influenced or even de· lemlined by the geochemical gradients. The nature of the presumed geochemical gradients is

not clear.In recent sedimenlS the sltOngestchanges in pore water chemistry are observed within lIle uppennost '2 mof the sedimenLary column. A sudden increase in carbonate saturation is found in the lOne of anaerobic methane oxidation (CANFlf.LD & RA1SWELL 199 I). Above and below this zone carbonate saturation of pore water is considerably lower. sometimes it is even undersaturated with respect to aragonite (CANFlELD & RAISWEU. 199 I). However, an explanation of fossil limestone-marl alternations by comparison with recent gcochemical pore water processes is questionable because there are no modem analogues for such alternations. Nevertheless. rhythmic precipitations produced at a moving reaction front (Liesegang band formation) are common phenomena in recent geochemical processes (DEI! 1986, JACOB et al. 1994, ORTOLEVA 1994). lbe question concerning the sediment depth of the ASZ arises. Ourobservations point to a position just below the seanoor:

a) Easy to compact organic microfossils. such as acritarehs, generally are flattened in the marls (PI. 43/8). wheras they are excellently preserved in limestones (pIA 3/ 7, cf. MUNNECKE & SERVAlS subm.). If lithification of the limestones would have taken placeseveral hundred meters

below the seanoor their organic microfossils should show signs of compaction. b) Fragile calcitic components in the limestones (e.g., thin-branched bryozoans) do not exhibit any signs of bending. breaking, or pressure solution. likewise pointing to an early Iithification. c) Micritic nodules overgrown by stromalOpOroids, which areobservcd occasionally. and numerous hardgrounds in the Silurian of Gotland indicate exposition of the lilhification front to seawater. Also in similar sequences of other regions incrustation of micritic nodules by sessile organisms or hardgrounds are observed and taken as indication of an exhumation of early lilhified sediment (V01GT 1968, KE.~NEl)Y & KUNGER 1972, BAIRD 1976. JONES et al. 1979. LINDSTIl.OM 1979. MOI..l.ER & KVINGAN 1988). This could hardly be possible with a deep positioning of the lithification front. Our observations indicate that the aragonite content of the soft sediment is the most imponant factor for the

development of limestone-marl alternations. The more aragonite is available in the precursor sedimem the higher must be lhe amount of limestone related to marl in the resulting alternation (Fig. 8). The carbonate content of the mar/ is delermined by the ratio or calcitic to terrigenous material in the primary mud because aragonite is completely removed. Consequently, pure aragonitic muds alter to bedded limestones because those parts oflhe primary sediment in which aragonite dissolution took place are entirely dissolved, leaving a bedding plane (Fig. 8). The carbonate content of lhe limestones depends as well on the amount ofcarbonate minerals in the soft sediment as on its porosity (Fig. 8).

174 6 CONCLUSIONS The Silurian carbonalcs on Gotland represent an exlraordinary case among Palaeozoic carbonaLCS because of their exceptionally good state of preservation. 'Extraordinary case' does not refer to sedimentary fades, but to the diagenesis that came LO an end shortly after earlyIithification. Early diagenetic characteriSlics well preserved on Gotland

have been altered by later phases of diagenesis in most carbonates of other regions, even if lhey are considerably younger. Our observations allow interpretalions concerning the formalion of micritic limestones and (he development of JimesLOne-marl alternations: a) MicriLic limestones originated as carbonate mud consisting of varying portions of calcitic, aragonitic and

terrigcnous material. The calcitic panconsists of microspar, miCTite, bioc1asLS, microfossils and spheroidal nanno-organisms ('nannospheres'), presumably tests of planktonic algae. Disintegrated nannospheres obviously produced a major pan of lhe fine-grained material (micriLC). The piued crystals of microspar are inferred to be cemented aragonite needle mud; they show the primary size and shape of the cements, which poikilotopicly lithified the original mud. b) The differentiation ofaragoniLC-containi ng sedimCnlS into limestones and mar!s occurred during early burial diagenesis, that is, without heavy overburden. The most important factor is not pressure solution but aragonite dissolution and calcite precipitation within a diagenetical zone close below the seafloor. These early diagenetical processes result in more or less regular limestone-marl alternations. The higher the content of aragonite in the primary sediment, the higher is the limcstone/marl-ratio in the resulting limestone-mar! alternation. c) Regular limestone-mar! alternations can result from relatively homogeneous carbonate muds. Primary depositional differences are not necessary for their development, bUl have an influence on the formation of the sequences. At least for a pan of the limestone-mar! alternations on Gorland a diagenetic origin is concluded, although in most alternations a posilive proof is hardly possible because of the parallelism of sedimentary and diagenetic structures. d) Although the question about the source of the aragonitic needles - organic or inorganic - cannot be answered yet, it is concluded that in the Silurian aragonite-containing muds played an important role in formation and diagcnesis of marine carbonates. The 'SANDBERG-Curve' (SANDBERG 1983) does not seem to be applicable to mineralogical composition ofcarbonate muds. According to the 'SANDBERG-curve' the Silurian lies near the Palaeozoic calcite maximum. Although the processes of aragonite dissolution and calcite precipitation seem to be general in early diagenesis of carbonate muds, we do not conclude that limestone-

marl alternations are generally produced solely by diagenesis. But one should be able to exclude the possibility of a diagenelic formation before inLCrpreting such sequences as results of externally controlled sedimentary processes and claim, for example, climatic (Milankovitch-) cycles as possible causes.

ACKNOWLEDGMENTS The authors are grateful to Dagmar Waldhoffor help in sample preparation and to Vte Schuldr for photographical assistance. Oliver Greeff helped in the translation of this paper. Priska Schafer(Kiel) and Andre Freiwald (Bremen) made constructive remarks. John J. G. Reijmer (Kiel) and Randolph P. Steinen (Connecticut) improved an early draft of wis manuscript. We appreciate the positive crilique and valuable suggestions of Wemer Ricken (KOln). Some improvements are based on critical remarks of Bruce H. Wilkinson (Ann Arbor). The investigation was supponed by we DeulSChe Forschungsgemeinschaft (DFG).

REFERENCES B"'III:O, G. C. (1976): Coul encrusted concretions: a k.ey lO recognition of a 'shale on shale' erosion surface. - Lethaia 9, 293·302,9 Figs., Oslo B...THuRsT, R. G. C. (1970): Problems oflithificalion in carbonate muds. - Geologist's Association Proceedings 81, 429440, London -- (1975): Carbonale Sediments and their Diagenesis. - Developments in Sedimentology. 2nd ed., 658p., 359 Figs., 24 Tables, Amsterdam (Elsevier) B...TIlURST, R. G. C. (1980): Lithificalion ofcarbonate sediments. - Science Progress 66, 451-471, 3 Figs., Oxford -- (1987): Dill.genelically enhanced bedding in argillaceous platfonn limestones: stratified cemenlation and selective compaction.-Sedimemology34, 749-778, 28 Figs., I Table, Oxford -- (1991): Pressure·dissolution and limestone bedding: the innuence of stratified cementation. -In: ElNsl!Ll!. G., RICKEN, W. &. Sl!ILACIH!Il., A. (cds.): Cycles and Evenu; in Sllaligfllphy. - 450-463,6 Figs., Berlin (Springer) (1993): Microfabrics in carbonalediagenesis: a critical look at forty yeus in research. - In: RF2AK. R. &. L",volE. D. L. (eds.): Carbonale Microfabrics. -3· 14, New York (Springer) BF.lI'JISDORF, H. &. KNlTTat. H. (1986): Oiagenetic layering and lamination. - Mincilungen aus dem Geologisch·Palliomologischen InSlilul der Univcrsitlil Hamburg 60, 267 -273, 5 Figs., Hamburg Bvl!ll.s, C. W. &. STASKO. L. E. (1978): Trace fossils and sedimentologic interpretation - McGregor MembecofPlalleville Formation (Ordov icbm) of Wisconsin. - Journal ofSwimen· tary Petrology 48,1303-1309,5 Figs., Tuba CA,NI'I&J), D. E. &. RA,ISwe...L, R. (1991): Carbonate precipilation and dissolution· its relevance to fossil preservation. - In: A1..USON, A. &. BRIOGS. D. E. G. (eds.): Taphonomy. Releas· ing the Data Loeked in the Fossil Record.-411-453. I1 Figs., 2 Tables, New York (Plenum Press) CONICLlO, M. & JAMES, N. (1985): Calcified algae as sedimenl contributors (0 early Paleozoic limestones: evidence from deep-waler sediments of the Cow Head Group, Western Newfoundland. - Journal of SedimenLary Petrology 5515, 746-754,11 Figs.. Tuba D!:.E, G. T. (1986): PalternS produced by precipilalion. al a moving reaction front. - Physical Review Letters 5713, 275· 278,4 Figs., I Table, New York

175

DIX. G. R. &: MUUJNlI. H. T. (1988): R.pid buri.1 di'lenesis or dcep.W'lCr carbonates; E:\um. Sound. Bahamas. - Geolol)' 16. 680-683. 3 Figs.• Boulder [)u,.-KAM. R. 1. (1962); Classific.tion or carbonlle rocks accordin, to depositionallurure. - American Association or Petroleum GeoIoSisu Memoir I. 108-121. 7 Pis.• 1 Table. Tuls. EaK ..... OT. G. (1968): Experimental compaclion or carbonate sedimenu. - In: MOUD:. G . .t:. FREDMA."'l. G. M. (eds.); Recent Developments in Carbonate Sedimentolosy. - 58-65. 8 Fi,s.• I Table. Berlin (Springer) EotiIt. W. (1982): Diasenelic redistribulion or carbonate•• proceu in ronning limMtone-marl aItemalions (Devoni.n and C.rbonirerous. Rheinisches Schie£ersebir,e. W. Germany). - In: ElNsllLE. G. &:. SEJL\ClIEJt. A. (eds.): Cyclic and Event Stratific.tion. - 98-112. 12 Figs.• Berlin (Springer) EINSEtfl. G. (1982a): General remarD .bout the n.ture. occurrence and recognition or cyclic sequences (pcriodites). -In; ElNsPJ..P.. G. &. SHD.ACllEJt. A. (eds.); Cyclic and Event Stralific.lion. _ 3-7.1 Fig .• Berlin (Springer) .. (I982b): Limestone-marl cycles (pcriodites): diagnosis. significance. c.uses·. review. -In: tlNSFJ.F.. G. &. SF.IL"'CIlF.R. A. (eds.): Cyclic and Evem Str.tific.tion. _ 8-53, 14 FiIS., 2 Tables, Berlin (Springer) EL~Sf.l..'" G. &t RICKI!."'l. W. (1991); Limeslone·marlaliemations - .noverview. -In: EINSEtE. G .• RICKE."'l. W. &t SEILACHEIl.. A. (eds.): Cycles and Events in Stutiguphy. - 23-47. 10 Figs.. I T.ble. Berlin (Springu) EINsa.." G .• RICK£N. W_ a:. SElUl.OIER. A. (1991): Cycles and events in strlligraphy - basic conccpu and temts. - In; E!NSEUl. G.• RKXEN. W. cl SElUl.CIIl!lt. A_ (eds.): Cydes and Evenu in Straligraphy. - 1-19. 8 Figs.• Berlin (Springer) E.vos. P. &: S... w... nKY. L H. (1981): Pore networks in Holocene carbonate sedimenl$. - Journal or Sedimentary Petrolol)' 51(3).961-985.20 Figs.. 4 Tables. Tulsa FtsOlfJI. A. G., HOSlO. S. cl GAAIlISOS. R. E. (1967): Electron MicrolraphsorLimestones and their Nannorossils. 141 p., 94 FiIS.• Princeton (Univ. Press) FLOCJEL. E. (1967): Elektronenmikroskopische Untersuchungen an mikritischen Kalken. - Geologische Rundschau 56. 341358.2 Pis.. 2 Figs.. 1 T.ble. Stullgart FOLK. R. L. (1959); Practical petrographic classific.lion or limeSlones. - American Associ.tion or Petroleum Geo 10 gists Bulletin 43/1.1-38.41 Figs.• 2 T.bles, Tuba (1965): Some aspects or recrystalli7.ltion in ancient limenones. -In: PR... y. L. C. & MURR...Y. R. C.(eds.): Dolomitiution • nd limestone diagenesis. - SEPM Special Publicalion 13. 14-48.14 Figs.. 7 Tables. Tuba (1974): The natural history of crystalline calcium carbonate: efrect or magnesium content and salinity. - Journal or Sedimentary Petrology 44/1. 40-53. 9 Figs.• I Table. Tulsa FREDMAN. G. M. (1964); Early di.genesis and lithification in carbonate sediments. - Journal or Sedimentary Petrology 34/ 4.777-813.53 Fi,s .• Tub. (1975); The making and unmaking or limestones or lhe downs and ups of porosity. - Journal or Sedimentary Petrol· ogy 45n, 379-398. 22 Figs .. I T.ble. Tuba FkYKM....~, P. (1989): Carbonate ramp r.cies or the Klinteberg Form.lion, Wenlock·Ludlow transition on Gotland, Sweden. - SverileS Geololiska Under~kningSerie C 820, 1-79. Stockholm GAATh"EII, S. (1977): N.nnorossils and biostratilraphy; an over· view. _ Euth·Sdence Reviews 13. 227·250, 3 Figs.. Amsteru

d~

GINS8Ul10, R. N. (1957); Early di.genesis and Iithification or sh.llow-w.tercarbonate sediments in South Aorida. - SEPM Special Publication 13, 80-99. 18 Figs.• Tub. GRONOEJ.. J. &t ROsU'".Jl.. H. J. (1963): Zur Entstehung der obc:rdevonischen KalkknollcngestcineTIlllringens. -Geologie 12.1009-1038.23 Figs.• 5 Tables, Berlin HIIJ..LAM. A. (1964): Origin orthc limestone-shale rhylhm in the

Blue Liu of England; a composite theory. - Journal or GeololY 72. 157-169. 6 FiIS.• I Table. Chic'IO (1986): Origin or minor limestone-shale cyclM: climatically induced or di.genetic?- Geolol)' 14. 609-612, 4 FiIS.• Boul-

""

HAu..£Y, R. B. (1987): Burial dialenMis or carbon.te rocks.ColoradoSchool orMinesQuarterly82, 1-15, 16 Figs.. Golden HAAl.A..... o. B.• Au.lsn.ONO. L L.,Cox, A. V.• CaAlO. L. E.• SMml. A. G. &t Swnt, D. G. (1990): A geoloCic time ICale.- 263p.• Cambridce (Univ. Pren) Hale. J. E. (1921 ): GouIands SilurstrlligraJi.- Svsiges Geologish Under~kninl Suie C 305. I-lOO. 12 Fill., Stockholm HEOE, J. E. (1960): The Silurian orGolland. -In: R8CJN£U.. G. cl H.E.oE. J. E. (eds.): The Lower Paleozoic or Scani•. - 44-87. 2 Figs.. Intern.lional Geological CongrMI. Guide to e;l;CUfsions A22 and CI7 HENNtNOSMOEN. G. (1974): A comment. Oricin or limestone nodules in the Lower Palaeoz.oicorthe Oslo Region. - Norsk Geologisk TidSlkrirt 54, 401--412. 3 Fill.• Oslo HONJO. S. (1969): Study or fine grained carbonlle m.trh: sedimentation and di.genesis or 'micrite'. _ Paleontological So· ciely or hpan Speci.1 Paper 14, 67·82, 2 T.bles. Tokyo ILUES. H. (1949); Ober die erdgeschichtliche Bedeutung der Konkretionen. _ Zeitschrirl der Deuuchen Geologischen Gesellsch.rt 101. 95·98. Hannover J",008, K. H.• DIETlUCll. S. a:. K111lO. H.-J. (1994); Selr·orlaniza. lion or mineral r.bries. -In; KlI.l1HL. J. H. (cd.); Fracllls and Dynamic Systems in Geoscience. - 259-268. 6 FiIS.. Berlin (Sprin;u) JEI'l'S5O!'i', L, Vu..... V. cl MA.-.."NIX. P. (1994): Silurian conodont· based correl.tions between Golland (S..... eden) and Saarema.a (Estonia). _ Geololical Magazine 131n. 201·218. 5 Figs.• London JO/'iI1$, B., O!.DEJlSII.....".. A. E. &: NAA80~"SE. G. M. (1979): Nalure and orilin of rubbly limestone in the Upper Silurian Read Bay Formation or Arctic Canada. - Sedimentary Geology 24. 227·252, I PI., 12 FiIS., I T.ble. Amsterdam KE...·~';"F.DY, W. J. &t KI.ISGER. W. J. (1972): Hiatus concretions.nd hardgrounds horizons in the Cretaceous orZululand (South Mric.). - P.laeontolol)' 15/4. 539·549. 3 Pis., 3 Figs.• Kf.~T, E. (1936): The rormation or the hydraulic limestones or the Lowcr Lias. -Geological M.guine 73. 476-478. 2 FiIS .• London L"'SEMI, Z. &. S....'1DIlEIIG. P. A. (1983): Recognition or original mineralogy in micrites (abstract). - American Association or Pelroleum Gcologists Bulletin 67. 499·500, Tub• a:. .. (1984): Transrormation or ar.gonite.domin.ted lime muds to microcrystalline limestones. -Geology 12.420-423. I Fi; .. Boulder L"sE.\lI. Z. &. S...SDIlEIIO. P. A. (1993): Microbbric .nd composition.1 clues 10 dominant mud mineralogy or micrite plecursors. -In: RF.1....K. R. &t LAvOJE, D. L (cds.): Carbonate Micror.brics. _ 173·185,6 FiIS., 2 Tables. New York (Sprinler) LASF_\lI, Z.. SA.~DIlEIIG. P. A. &. BO...lIoM....~. M. R. (1990): New microlextural criterion ror dirrelenti.tion or compaclion and early cenlenl.ltion in fine-grained limestonM. - Geology 18, 370-373.2 Fils., Boulder LAc~uo, S. &: BASSF.TT. M. G. (1981); Gotland: the an.tomy or a Silurian carbonale platrorm. - Episodes 2. 23·27. 10 Figs., Ott....... U.UFaD. S. et. Jl'J'I'Ssos. L. (1976): Silific.tion and bentonitcs in the Silurian orGotland. - Geologiska Forenin,ens i Stockholm FOrhlndlingar 98. 31-44.4 Fi,s., Stockhobn LINDS1ltOM. M. (1979): Diagenesis or Lower Ordovician hardgrounds in Sweden. - Geologica et Palaeontologica 13. 9-30.3 Pis.. 5 Figs. Marburg MA,IITlN$SON. A. (1967): The succession and correlation orOSlncode f.unas in the Silurian of Gotland. - Geologiska Fijreningens i Slockholm Forhandlingar 89, 350·386, 2 Figs., Stockhobn

176 MOi...l..EJI, N. K. &. KVINOAN. K. (1988): The genesis of nodular limestones in theOrdovician and Silurian oflheOslo Region. - Sedimentology 35, 405-420, 8 Figs.• 1 Table, Oxford MOSHIER. S. O. (1989); Microporosity in micritic limestones; a review. ~ Sedimentary Geology 63. 191-213, 15 Figs.• Amsterdam MU'HNECICB. A. &. SRJl.VAlS, T.: Scanning eleclron microscopy of polished, slightly etched rock surfaces: a method (0 observe palynomotphs inSilU. - 5 Pis., 3 Figs., submitted to Palynology. Dallas NOBU!, 1. A. &. Ho'Wl'll.S, K. D. M. (1974): Early marine IiLhificalion

of the nodular limestones in the Silurian of New Brunswick. - Sedimentology 21, 597-609, 5 Figs.• I Table, Oxford ORTOlJ!.VA, P. (1994): Gcochemical Self-Organization. - Oxford

Monographs on Geology and Geophysics 23, 411p., 205 Figs.• Oxford

ORTOU!v A, P., 0EwERs, T. &. S AtIER, B. (1993): Modeling diagenetic bedding, sty!olites. concretions and other mechanochemical structures. -In: RRZAK, R. & LAvOIE, D. L. (cds.): Carbonate Microfabrics. - 291-300,6 Figs., New York (Springer) PRAY, L. C. (1960): Compaclion in calcilutites (abstract). Bulletin of the Geological Society of America 71. 1946, Boulder R.o.1SWELL. R. (1987): Non-steady state microbiological diagenesis and the origin of concretions and nodular limestones. - In: MAltSHAU.. J. D. (ed): Diagenesis of Sedimentary Sequences. - Geological Society Special Publication 36. 41-54. 6 Figs., London (19881): Chemical model for the origin of limestone-shale cycles by anaerobic methane oxidation. - Geology 16, 641644.2 Figs.• I Table, Boulder (1988b): Evidence for surface reaction-conlIolled growth of carbonate concretions in shales. - Sedimentology 35, 571575, 2 Figs.• Oxford RUEN, W. (1986): Diagenetic bedding - a model for marllimestone alternations. - Lecture Notes in Earth Sciences 6, 1-210,94 Figs.• 19 Tables. Berlin (1992): A volwne and mass approach tocarbonatediagenesis: the role of compaction and cementation. - In: WOlF, K. H. & CHIUNOARI.o.N, G. V. (eds.): Diagenesis, Ill. - 291-316. 10 Figs.• Amsterdam (Elsevier) Rt