ON THE DIFFICULTIESOF STRATIGRAPHIC HIATUSES

INTERPRETATIONBernard Mamet & Alain Préat

University of Brussels

MAMET B & PREAT A (2003) Sur les difficultés d’interprétation des hiatu stratigraphiques

(exemple tiré de la transition dévono-carbonifère, Bassin de Dinant).

Geologica Belgica 6/1-2:49-65, 6figs

THE AVESNOIS ‘BASIN’vs

THE DINANT BASIN

• a case study?• with possible consequence(s)…

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Van Steenwinkel, 1988

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THE PROBLEM ??? A biostratigraphic hiatus at the D-C transition

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Conodont guides defining theD-C/b ARE NOT recorded inthe Dinant and Avesnois basins

studied interval

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1-2: Epinette+Etroeungt+Avesnelles+Pont d’Arcole+Grives

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STRATIGRAPHYOF AVESNOIS

and of BOCAHUTQUARRY

(Mansy et al.,1988)

studiedinterval

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SEDIMENTOLOGY‘bed/bed analysis’

• ± 350 samples (100m) AVESNES/HELPE• 120 samples (40m) ANSEREMME

7 MICROFACIESFAMILIES

1mm

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MICROFACIES

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mf1 mf2

‘bac

kgro

und’

mf2=

mf1+Td

250µ 1mm

distaltempestite

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mf3

mf3=

mf2+T

mf4

mf4=

mf3+Tp

proximaltempestite

250µ

100µ

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mf5 mf6

250µ

1mm

FairWeatherWaveBase

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mf7

250µ

100µ

1mm

semi-restrictedplatform

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EVOLUTION

SÉDI

ME N T.

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REG

TRANS

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AVESNES-SUR-HELPE

• thick and ‘continuous’ profile• RAMP + PLATFORM settings (SWB-FWWB)• relative sea level fluctuations• erosive event near the ‘D/C b’

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EVOLUTION

SÉDI

ME N T.

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ANSEREMME• thinner and ‘discontinuous’ profile• RAMP + PLATFORM settings (SWB-FWWB)• strong salinity fluctuations• important erosion near the ‘D/C b’

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MICROBRECCIA ANALYSIS

microbreccia do not contain ‘pre-Hastière’ sediments/rocks (Etroeungt-Avesnelles)

BUT CONTAIN ONLY

Hastière sediments/rocks(reworked littoral lagoonal environments)

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INTERPRETATION?

biostratigraphy sequence stratigraphy D/C boundary

HIATUSES

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CORRÉLATION

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SEQUENCE STRATIGRAPHYVan Steenwinkel, 1993

SB1 SB1

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CONCLUSIONorigin of the hiatuses?

1 steady subsidence?2 facies constancy?3 sea level fluctuations?…. ? nb paraconformity as a rule

1’ numerous parasequences2’salinity and energy fluctuations3’ they surely exist BUT no detailed correlations are possible (Van Steenwinkel

1993 and this work….)

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CONCLUSIONorigin of the hiatuses?

SO WHAT?

BLOCK-FAULTINGcan match all the observations

• non deposition? (pre-Hastière)• erosion of an unknown

lower Hastière part

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bear in mind that…*the profiles are in a same basin, near each other*the succession is ‘apparently’ the same*the microfacies, the microfaunal/floral assem- blages and the lithologies are also the same …

these similarities are fallaciousdespite similar eustatic fluctuations

HALF OF THE SERIES ARE MISSING

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HASTIERE LIMESTONEcm-dm microsequences [18m at Ansermme]

• (1) proximal crinoidal tempestites• (2) ‘false’ peloidal grainstones with

calcisphaerids, Bisphaera, brachiopods, crinoids[open lagoon]

• (3) ‘false’ grainstones with very abundant ostracods and Earlandia minima + a few

Quasiumbella [lagoon]• (4) packstones-’false’ grainstones with micritized grains and ‘cut in bits’ lumps

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HASTIERE LIMESTONE

the (micro)parasequences are not

bathymetric BUT record

strong salinity fluctuations

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HASTIERE LIMESTONE

facies (1) to (4) evaporitic diagenesis[calcite pseudomorphs, idiotopic dolomite…] UNSTABLE ENVIRONMENTS WITH STRONG SALINITY FLUCTUATIONS semi-open to restricted with reflux brines

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HASTIERE LIMESTONE(upper part)

• normal salinity with the photic zone of an openmarine environment

• return of plurilocular foraminifers, of girvanella…..

• same evolution at Avesnes AND Anseremme

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