New Perspectives in Microfacies - Microfacies of Carbonate Rocks

Geology Reference

In-Depth Information

in the precipitation of fine-grained carbonate and in

the formation of microbialites is becoming increasingly

well understood. Laboratory experiments simulating

bacterially-controlled carbonate precipitation, the rec-

ognition of bacterially constructed carbonate crystals

and grains and the study of modern biofilms combine

to 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 and

mud mounds), destructive processes (e.g. boring mi-

crobes) and carbonate cementation.

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 is

considerable evidence for temporal and secular fluc-

tuations in major facies types, the dominant types of

carbonate cements, the mineralogy of skeletal grains

and probably also in diagenesis and carbonate geochem-

istry. Some of these fluctuations can be explained by

global changes in biological factors (e.g. role of car-

bonate plankton, nutrients), others by global changes

in climate and oceanography. Microfacies criteria for

carbonate rocks provide evidence for changes through

time as shown by distinct differences in the composi-

tion of Phanerozoic bioclastic sands, types of calcare-

ous ooids, composition and size of oncoids, open-space

structures etc. (Sect. 16.7). The recognition of these

changes has a major impact on depositional and diage-

netic models for ancient carbonates.

Carbonate sequences and cycles . The recognition

and interpretation of cyclic patterns is a major objec-

tive of modern carbonate sedimentology. Because

cyclicity 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 - may

be vigorously supported by microfacies studies which

also improve the interpretation of sequence boundaries

with regard to origin, timing and location (Sect. 16.1.2).

Applied microfacies. Carbonate rocks contain more

than 50% of the world's oil and gas reserves, are im-

portant hosts to ore deposits and form very important

raw materials. Depositional facies and diagenetic pat-

terns determine the physical and chemical properties

of these limestones and dolomites which control their

reservoir potential (Chap. 17) and their technological

attributes (Chap. 18). Microfacies studies assist in un-

derstanding the origin and history of these characteris-

tics.

New concepts in defining facies models. The value

of 'models' in facies analysis depends largely on the

significance attributed to a sedimentary model by the

author. The majority of models suggested for the dif-

ferent modes of carbonate sedimentation are based on

comparative studies between modern and ancient car-

bonates. These 'conceptual models' remain important

tools because they facilitate the attribution of charac-

teristic sedimentological or paleontological data to spe-

cific facies belts (Chap. 14). However, a too rigorous

use of conceptual models for one's own facies studies

may result in significant errors if the more descriptive,

static character of the model is not critically consid-

ered.

'Dynamic models', concentrating on processes and

controls and computer simulations dealing with the ma-

jor controlling factors of carbonate buildups, provide a

promising platform for the discussion of potential steps

in 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-

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. -

633 pp., Berlin (Springer)

Spence, G.H., Tucker, M. (1999): Modelling carbonate

microfacies in the context of high-frequency dynamic rela-

tive sea-level and environmental changes. - Journal of

Sedimentary Research, 69 , 947-961

Udden, J.A., Waite, V.V. (1927): Some microscopic charac-

teristics of the Bend and Ellenburger limestones. - Texas

University Bulletin, 27 , 8 pp.

Wilson, J.L. (1975): Carbonate facies in geologic history. -

471 pp., Berlin (Springer)