Depositional Models, Facies Zones and Standard Microfacies - Microfacies of Carbonate Rocks - page 674

Forereef Backreef

Wave base

Reef core

I-f II-f III-f IV-f IV / V V IV-b III-b II-b I-b

A Water quiet water intermittently slightly moderately strongly moderately slightly intermittently intermittently

energy below agitated agitated agitated agitated water agitated agitated agitated agitated

wave base water water water water water water water

Turbulence negligible episodic moderate strong strong strong moderate episodic episodic

Rounding original poor moderate good to very good to moderate good to moderate poor to original

fossil shapes to good to good moderate moderate to good good fossil shapes

Sorting angular frag matrix: good, matrix: good to matrix: moderate good to matrix: matrix: good angular frag

ments if fossils: poor moderate, moderate fossils: poor moderate moderate fossils: poor ments if

broken to good fossils: mod fossils: mod to good broken

erate to good erate to good

B Texture packstones, packstones, floatstones, rudstones, rudstones, bindstones, rudstones, floatstones, packstones, mudstones,

grainstones, grainstones, grainstones, baffle bindstones, frame bafflestones grainstones, grainstones wackestones

mudstones rare rare stones rare stones, rare rud

floatstones rudstones grainstones rudstones stones

C Amount of -> -> -> primary micrite decreasing -> -> -> fibrous calcite, -> -> -> primary micrite increasing -> -> ->

micrite sparite, micrite

D

Porosity intraparticle intraparticle, interparticle, shelter shelter shelter shelter shelter intraparticle, intraparticle,

types interparticle shelter voids, voids, voids, voids, voids, voids, interparticle, fenestral

intraparticle growth interparticle growth growth intraparticle, fenestral

framework, framework, framework, interparticle

interparticle interparticle interparticle

Pore size small medium medium large to medium large to medium to small

to small to small medium to small medium small

Porosity below 10% up to 1% up to 15% up to 10% up to 25% up to 10% up to 40% up to 10% up to 20% up to 30%

mostly < 5%

E Fossil calcispheres, small large tabulate crinoids, bulbous + Stachyodes, Amphipora, algae algae

assemblage foraminifera crinoids crinoids corals, tabular massive Girvanella, Parathuram (fenestral (fenestral

Stachyodes, stromato stromato Renalcis mina, pores) laminites)

brachiopods, poroids poroids, ostracods

crinoids, micrite en

Girvanella, velopes,

Renalcis syntaxial

rim cement

F Indicator crinoids / goniatites / brachiopods crinoids, Solenopora, Amphipora / calcispheres | Parathurammina /

fossils Keega Vermiporella / encrusting Sphaerocodium

Fig. 14.9. Facies model for Middle and Late Devonian reef carbonates after Machel and Hunter (1994). The model differentiates
five water energy zones with increasing water energy from I to V, designated as either f (forereef) or b (backreef). Use of the
model requires an estimation of frequencies, differentiation of growth forms and some knowledge of fossils. Part A estimates
water energy by noting rounding and sorting. Part B refers to the limestone texture. Part C records the amount of micrite.
Part D lists the amount, size and the predominant type of porosity. Part E lists assemblages that are common in the different
facies zones and have their distributional maxima within the zone(s). Part F lists 'indicator fossils' which are usually found
only in backreef, forereef, or reef core settings. The criteria used in distinguishing facies zones can be observed in outcrops,
cores as well as in large thin sections. See Fig. 14.8 to see how the model is applied.

Fig. 14.10. The location of reef growth and the development of the reef zones depend on the submarine topography. In the
case of a more gently dipping slope, maximum reef growth is located deeper than in the example shown in Fig. 14.9. The
shallow forereef zone IV-f is missing or only incompletely developed. After Machel and Hunter (1994).