1
1 New Perspectives in Microfacies
A rapid evolution in our understanding of carbonaterocks was triggered in the mid- to late 50’s by the dis-covery of carbonate reservoirs in various parts of theworld, followed by intensive research of recent car-bonates. Modern and ancient carbonate environments,diagenetic processes and facies models were studiedbetween about 1955 and 1965. During the late 60’s andthe 70’s microfacies became an essential part of faciesanalysis and paleoenvironmental interpretation of lime-stones. The increasing importance of limestones anddolomites as reservoir rocks and the use of thin-sec-tion fossils in subdividing carbonate platforms gavesubstantial impetus to the progress of microfacies re-search. The 1980’s were characterized by an increasedapplication of geochemical techniques with the objectof developing predictive models for carbonate diagen-esis and porosity. The subsequent period of time hasseen the rapid development of sequence stratigraphywhich now plays a major role in the characterizationof the geometry and reservoir potential of carbonaterocks.
as 1927 by the use of microscopic features of lime-stones for oil exploration in Texas.
Hovelaque and Kilian (1900) published the first il-lustrated volume of thin-section photographs of car-bonates. This approach was continued by the ‘Interna-tional Sedimentary Petrographical Series’ initiated byCuvillier at the Third International World PetroleumConference in Paris in 1951. The volumes publishedwithin this series as well as other comprehensive mono-graphs containing numerous plates with thin-sectionphotographs (see Sect. 10.4) contributed substantiallyto the rapid adoption of the microfacies approach.During the past decades microfacies has become anestablished part of the study of carbonate rocks. How-ever, many authors use microfacies criteria in describ-ing and classifying limestones but fail to explore thesignificance of these criteria for the interpretation ofdepositional and diagenetic histories of carbonate rocks.Part 2 and 3 of this book demonstrate the great poten-tial of microfacies studies in basin analysis and appliedcarbonate geology.
1.1 The Microfacies Concept
As originally defined by Brown (1943) and againindependently by Cuvillier (1952) the term ‘micro-facies’ referred only to petrographic and paleontologi-cal criteria studied in thin-sections. Today, however,microfacies is regarded as the total of all sedimento-logical and paleontological data which can be de-scribed and classified from thin sections, peels, pol-ished slabs or rock samples.
Field geology, including mapping and profiling, is aprerequisite for successful microfacies analysis. Theimportance of combined field work and thin-sectionstudies was already emphasized in the earliest thin-sec-tion based investigations of carbonate rocks directedtowards genetic interpretations of limestones (Peters1863; Gümbel 1873; Sorby 1879; Hantken 1884). Thepractical application of thin-section criteria of lime-stones was demonstrated by Udden and Waite as early
1.2 New Perspectives
New perspectives in the study and application ofmicrofacies have resulted from the application of newtechniques, from the availability of new comparativedata on modern and ancient carbonates, from new dataconcerning the origin and distribution patterns of car-bonate grains and the dominant biological control oncarbonate sedimentation, from new concepts on defin-ing facies models, and from new ideas concerning thecourse of carbonate sedimentation through geologicaltime.
New techniques. Whilst microscopy is essential inthe study of carbonate rocks there are many additionaltechniques which should also be utilized to maximizethe information offered by microfacies analysis (Tucker1988). Cathodoluminescence microscopy, fluid inclu-sion microscopy, scanning electron microscopy as well
E. Flügel, Microfacies of Carbonate Rocks, 2nd ed.,
DOI 10.1007/978-3-642-03796-2_1, Springer-Verlag Berlin Heidelberg 2010
2New Perspectives in Microfacies
Fig. 1.1. Limestones as paleoclimatic proxies. Shelf limestones formed in tropical warm-water and in non-tropical cool-water environments differ in the association of specific grain types (see Sect. 12.2). The sample is a skeletal grainstoneconsisting predominantly of bryozoans (B) and benthic foraminifera (F). Gastropods (G) are rare. Black grains are mainlyred algae. The skeletons of many bryozoan fragments have lost their structure due to the attacks of microborers and havebeen transformed into microcrystalline grains. The sediment is only poorly lithified as shown by the high open porosity.Marine carbonate cement occurs as thin rims of fibrous calcite crystals (arrows). The grain association as well as the weakcementation are characteristic of cool-water carbonates. The sample comes from Oligocene deposits of Leg 133 of the OceanDrilling Program which dealt with the geological evolution of the Great Barrier Reef Province off northeastern Australia.Courtesy of T. Brachert (Mainz).
as stable isotope geochemistry have become commontools in sedimentological studies (Chap.3). Chemicalanalysis and X-ray techniques are well-established rou-tine methods used to determine major, minor and traceelement concentrations and the composition of carbon-ate rocks. The combination of these techniques withmicrofacies studies offers new possibilities for the in-terpretation of diagenetic pathways of carbonates(Chap. 7).
Quantification of thin-section criteria, based on com-puter-assisted frequency analysis and using multivari-ate methods, has become an essential prerequisite forthe categorization of microfacies types, the evaluationof the sedimentary budget of basin fills and for com-puter simulations having the potential to constrain quan-titative sedimentary models (see Sect. 6.3.2).Lacustrine and terrestrial carbonates. Despite theincreasing economic importance of non-marine sedi-ments, many students interested in microfacies aresomewhat hesitant in applying microfacies analysis to
non-marine carbonate rocks. This attitude may reflectclassification problems related to the poverty in char-acteristic non-marine microfacies types as comparedwith the wealth of types known from marine environ-ments. This hesitancy is, however, only partly justi-fied: there exist succinct and informative reviews ofmodern and ancient fresh and saltwater lake deposits,travertines and pedogenic carbonates (see Sect. 2.4.1and Sect. 15.1 to Sect. 15.4). Ancient calcretes, char-acterized by a set of distinctivemicrofacies criteria, arevery important indicators of subaerial exposure and arepotentially very useful for defining the nature of se-quence boundaries and recognizing changes in paleo-climate. The same is true for paleokarst which is easilyrecognized from microfacies features.
New comparative data of modern and ancient sedi-ments. Modern carbonate depositional systems onlypartly reflect the wide range of settings and environ-ments established during the earth’s history becausebiological, geological and chemical controls have
New Perspectives in Microfacies3
Fig. 1.2. Limestones as potential reservoir rocks. The sample has been impregnated with blue-dye-resin that fills open poresbetween and within skeletal grains (miliolid foraminifera, MF; bryozoans, B). Primary inter- and intra-particle porosity hasbeen somewhat reduced by syntaxial overgrowth cements (O) on echinoderm fragments, dogtooth and granular cementswithin foraminiferal shells. Highly porous limestones may be good reservoir rocks depending on their permeability, and thegeometry and distribution of pores. Tertiary: Paris Basin, France.
changed with time. Modern carbonates, however, dem-onstrate how organisms are involved in carbonate pro-duction and which physical, biological and chemicalprocesses may be recorded in ancient carbonate rocks.Current microfacies interpretations are strongly influ-enced by new data both on shallow- and deep-marinesedimentation (see Chap. 2).
Carbonate platforms and ramps. Facies interpreta-tions for ancient shallow-marine carbonates have beenheavily dependent on comparisons with sedimentationpatterns of the Great Bahama Bank, the Florida shelfand parts of the Persian Gulf. Whilst ‘platform carbon-ates’ have previously been regarded as the normal typeof shallow-marine sedimentation, case studies of an-cient shallow-marine carbonates together with the in-vestigation of modern examples now indicate that gen-tly sloping carbonate ramps were also of major impor-tance during the Phanerozoic. Ramps and platforms dif-fer in their geometry, depositional depths and the dis-tribution patterns of facies zones: microfacies reflectthese differences (Sect. 14.3). Platform and ramp car-bonates are controlled by variations in biogenic pro-duction as well as by fluctuations in both sea level andin accommodation and sedimentation rates. Microfacies
may reflect short-term environmental changes and high-frequency sea-level fluctuations as well as long-termpatterns in the formation of carbonate buildups (Sect.15.6). Platform-basin relations are recorded in alloch-thonous carbonates formed in shallow-water environ-ments and deposited on the slope and within the basins(Sect. 15.7.5). The reconstruction of ‘vanished’ shal-low-marine depositional environments is a very prom-ising tool for microfacies studies (Sect. 16.3).Warm-water, temperate-water and cold-water car-bonates. The paradigm of a predominantly warm-wa-ter origin for ‘sun-born’ shallow-marine carbonates hasbecome obsolete as a result of current investigationsof temperate, boreal, subarctic and even polar shelf car-bonates (Sect. 2.4.4.3; Sect. 16.4). High-latitude ma-rine carbonate production, cold-water organic reefs,deep-marine seep and vent communities (Sect. 16.5)as well as high bioclastic sedimentation rates in coldocean waters are factors which need to be consideredin the environmental interpretation of ancient carbon-ates. Increasing numbers of ancient examples of thesetypes of carbonates are being recognized.
The ability to distinguish ancient ‘tropical’ warm-water, and ‘non-tropical’ temperate and cold-water car-
4A First Glance at a Thin Section
bonates and to recognize ‘warm-mode’ and ‘cold-mode’paleolatitudes are improving rapidly.
Carbonate grains. Increasingly new more informa-tion is available concerning the biological and non-bio-logical controls on the origin of carbonate grains, thesite of deposition of carbonate grains, and on the dis-tribution and dominance of grain types during the Phan-erozoic. Characteristic grain type associations and thespecific composition of grain types (e.g. ooids, oncoids,pisoids) provide hints on paleolatitudes and environ-mental controls for both marine and non-marine car-bonate deposition (Sect. 4.2; Sect. 12.2).Plate 1 A First Glance at a Thin Section
Bioerosion. The biological destruction of carbonatesubstrates by borers, raspers and crushers is currentlybeing intensively studied. Important objectives of thesestudies, which are also significant in microfacies analy-sis, are the application of boring morphology and bor-ing patterns for paleobathymetric reconstructions andthe estimation of the quantitative contribution of bio-eroders for the marine carbonate budget (Sect. 9.3).Microbial carbonates: Carbonate deposits producedor localized by microbial communities are known frommarine, marginal-marine, freshwater and terrestrialenvironments. The role of bacteria and cyanobacteria
The plate exhibits a thin section of a Mesozoic limestone from the Austrian Alps. Although the first appearancemay be puzzling, a thorough investigation of the microfacies criteria reveals an amazing history for this rock.
1The thin section shows fossils and dark-colored areas. The fossils correspond to organisms once living in a reef andcontributing to the formation of a reef limestone. The dark-colored areas represent various types of fine-grained sediment.The reef limestone contains calcareous sponges, serpulid worms (SW), tube-like microproblematica (Microtubus , M),and bivalves. Sponges and encrusting Microtubus and serpulids formed an organic framework. The sediment within theframework (S1) consists of tiny, densely packed microcrystalline particles (peloids) and some ostracods and foraminifera.Both elements can only be seen under greater magnification. The sponges are represented by non-chambered inozoans(IS), chambered sphinctozoans (SS) and chaetetid sponges (CS). Microtubus occurs encrusted on and within sponges, andis concentrated near the periphery of the framework, but the outermost part is a microbial crust (MC) formed by bacteri-ally induced carbonate precipitation. The surface of the microbial crust outlines a distinct relief. The eye-catching largebivalve shell (BI) was heavily attacked by boring organisms including boring bivalves (BB) and microborers drilling tinyholes (MB). The bivalves are infilled with dark calcareous sediment (S2) different in composition from the sedimentwithin the organic reef framework. The conspicuous circular section might represent a pelagic fossil (H, Heterastridium ). The reddish fine-grained sediment (S3) yields densely packed skeletal grains including sponge spicules and pelagic shelldebris as well as some radiolarians and pelagic ostracods.
This sediment and the reef limestones are covered by a thin Fe/Mn crust (arrows) forming small microbialmicrostromatolites. The overlying sediment S4 contains (white) skeletal elements of crinoids, mostly parts of arms asso-ciated with pelagic shell debris. Radiolarians, small ammonites and pelagic algal cysts are too small to be visible on theplate. Interpretation: Microtubus indicates a Late Triassic (Norian to Rhaetian) age for the reef limestone. The composition ofthe reef-building assemblage characterizes reefs formed in protected low-energy settings. Environmental change is indi-cated by the peripheral microbial crust covering the reef fabric. The reef limestone and the microbial crust were transectedby a fissure that remained open for some time as indicated by the calcite tapestry. Later on the fissure was infilled by theopen-marine pelagic sediment S3. The Fe/Mn crust covering both the surface of the microbial crust and the fissure fillmarks a discontinuity surface corresponding to a hardground and indicating a gap in the sedimentary record. The microfaciesof sediment S4 reflects the renewed onset of pelagic sedimentation. The sediments S3 and S4 are Liassic in age. The timehidden in the pelagic sedimentary record, however, can not be determined from the thin-section data.
Solution took place within the reef limestone as shown by solution voids (V) within the originally aragonitic sponges.In contrast primary calcitic fossils, e.g. Microtubus , are well preserved. These voids and the cement type of the calcitetapestry within the fissure point to freshwater diagenesis, an interpretation that also is supported by stable isotope data.The small-scale microfacies data of the thin section reflect the large-scale developments in sedimentation at the Triassic-Jurassic boundary in the Northern Calcareous Alps: Platform and reef development – platform destruction related tosubaerial exposure and drowning – transition to open-marine pelagic sedimentation.
The sample comes from the Tropfbruch quarry in Adnet near Salzburg, Austria, where Rhaetian reef limestones (Fig.8.2) are overlain by Early Jurassic carbonates. See also the polished palte of this outcrop on page XXIII.
The story behind this thin section is rather difficult, but the following chapters will answer to all of your questionsand give the information needed.
Plate 1: A First Glance at a Thin Section5
6in the precipitation of fine-grained carbonate and inthe formation of microbialites is becoming increasinglywell understood. Laboratory experiments simulatingbacterially-controlled carbonate precipitation, the rec-ognition of bacterially constructed carbonate crystalsand grains and the study of modern biofilms combineto indicate the importance of microbes in the forma-tion of many items included within the list of micro-facies criteria (e.g. micrite, peloids, ooids, oncoids, mi-crite envelopes, stromatactis, carbonate crusts; Sect.9.1). Combined studies of microfacies and geobio-chemical data are necessary in order to understand con-structive microbial controls (e.g. biogenic crusts andmud mounds), destructive processes (e.g. boring mi-crobes) and carbonate cementation.
Carbonate sequences and cycles. The recognitionand interpretation of cyclic patterns is a major objec-tive of modern carbonate sedimentology. Becausecyclicity is reflected by systematic changes in biota,grain composition, texture and early diagenetic crite-ria, microfacies studies are able to contribute to a bet-ter understanding of short-term depositional variations(Sect. 16.1.1). The three major prerequisites for se-quence stratigraphy – a sound paleoenvironmental in-terpretation, an estimate of synchronicity, and the dif-ferentiation between regional and local effects – maybe vigorously supported by microfacies studies whichalso improve the interpretation of sequence boundarieswith regard to origin, timing and location (Sect. 16.1.2).New concepts in defining facies models. The valueof ‘models’ in facies analysis depends largely on thesignificance attributed to a sedimentary model by theauthor. The majority of models suggested for the dif-ferent modes of carbonate sedimentation are based oncomparative studies between modern and ancient car-bonates. These ‘conceptual models’ remain importanttools because they facilitate the attribution of charac-teristic sedimentological or paleontological data to spe-cific facies belts (Chap. 14). However, a too rigoroususe of conceptual models for one’s own facies studiesmay result in significant errors if the more descriptive,static character of the model is not critically consid-ered.
‘Dynamic models’, concentrating on processes andcontrols and computer simulations dealing with the ma-jor controlling factors of carbonate buildups, provide apromising platform for the discussion of potential stepsin the development of sedimentary bodies (e.g. carbon-ate platforms), organic structures (e.g. reefs) or indi-vidual sequences (e.g. cycles). The evaluation of com-puter-generated facies models should be strongly de-
A First Glance at a Thin Section
pendent on microfacies and paleontological criteria be-cause only these criteria deliver the paleoenvironmen-tal information that can verify, refine or disprove theo-retical concepts.
Secular changes during the Phanerozoic. There isconsiderable evidence for temporal and secular fluc-tuations in major facies types, the dominant types ofcarbonate cements, the mineralogy of skeletal grainsand probably also in diagenesis and carbonate geochem-istry. Some of these fluctuations can be explained byglobal changes in biological factors (e.g. role of car-bonate plankton, nutrients), others by global changesin climate and oceanography. Microfacies criteria forcarbonate rocks provide evidence for changes throughtime as shown by distinct differences in the composi-tion of Phanerozoic bioclastic sands, types of calcare-ous ooids, composition and size of oncoids, open-spacestructures etc. (Sect. 16.7). The recognition of thesechanges has a major impact on depositional and diage-netic models for ancient carbonates.
Applied microfacies. Carbonate rocks contain morethan 50% of the world’s oil and gas reserves, are im-portant hosts to ore deposits and form very importantraw materials. Depositional facies and diagenetic pat-terns determine the physical and chemical propertiesof these limestones and dolomites which control theirreservoir potential (Chap. 17) and their technologicalattributes (Chap. 18). Microfacies studies assist in un-derstanding the origin and history of these characteris-tics.
Basics: New perspectives in microfacies
Brown, J.S. (1943): Suggested use of the word microfacies.– Economic Geology, 38, p. 325
Carozzi, A.V. (1989): Carbonate rock depositional models.A microfacies approach. – 604 pp., Englewood Cliffs(Prentice Hall)
Cuvillier, J. (1952): Le notion de ‘microfacies’ et ses appli-cations. – VIII Congreso Nazionale di Metano e Petroleo,sect. I, 1-7
Cuvillier, J. (1962): Étude et utilisation rationelle de micro-facies. – Revue de Micropaléontologie, 4, 3-6
Flügel, E. (1982): Microfacies analysis of limestones. –633pp., Berlin (Springer)
Spence, G.H., Tucker, M. (1999): Modelling carbonatemicrofacies in the context of high-frequency dynamic rela-tive sea-level and environmental changes. – Journal ofSedimentary Research, 69, 947-961
Udden, J.A., Waite, V.V. (1927): Some microscopic charac-teristics of the Bend and Ellenburger limestones. – TexasUniversity Bulletin, 27, 8 pp.
Wilson, J.L. (1975): Carbonate facies in geologic history. –471 pp., Berlin (Springer)Further reading: K002