Boussaha, Myriam

Moreau, Julien

Nielsen, Lars

Stemmerik, Lars

Thibault, Nicolas Rudolph

English

Copenhagen University Research Information System

u n i ve r s i t y  o f  co pe n h ag e n  Københavns UniversitetMechanical and chemical processes affecting the chalk during burial, insights fromcombined reflection seismics, well data and field workMoreau, Julien; Boussaha, Myriam; Thibault, Nicolas Rudolph; Nielsen, Lars; Stemmerik,LarsPublished in:Geophysical Research AbstractsPublication date:2014Document versionPublisher's PDF, also known as Version of recordCitation for published version (APA):Moreau, J., Boussaha, M., Thibault, N. R., Nielsen, L., & Stemmerik, L. (2014). Mechanical and chemicalprocesses affecting the chalk during burial, insights from combined reflection seismics, well data and field work.Geophysical Research Abstracts, 16, [1890].Download date: 03. Feb. 2020Geophysical Research AbstractsVol. 16, EGU2014-1890, 2014EGU General Assembly 2014© Author(s) 2014. CC Attribution 3.0 License.Mechanical and chemical processes affecting the chalk during burial,insights from combined reflection seismics, well data and field workJulien Moreau (1), Myriam Boussaha (1), Lars Nielsen (1), Nicolas Thibault (1), and Lars Stemmerik (2)(1) University of Copenhagen, IGN, Geology, Copenhagen, Denmark (julien.moreau@geo.ku.dk), (2) National HistoryMuseum of Denmark, CopenhagenThe chalk must undergo several phases of grain reorganisation and chemical reactions during its diagenetic evo-lution from a carbonaceous ooze to a sedimentary rock. Some of these transformations could be observed onstructures from the kilometre- to the micrometre-scale with seismic reflection and cores analyses, respectively.However, few sites allow to combine all the different scale of observation for chalk diagenesis. Onshore and off-shore high resolution seismics, two fully cored >350 m wells with wireline logging tools and very high qualityexposures from a coastal cliff and a quarry form such an exceptional dataset in the Stevns peninsula area, east-ern Danish Basin (Denmark). The studied chalk interval in the area is of Maastrichtian to Danian age. The chalkhas been divided in 4 lithofacies, chalk-marl alternations, white chalk, white chalk with flint layers and bryozoanchalk. Advanced stratigraphic works have been performed with astronomical calibration based on stable isotopestratigraphy, wireline logs as well as several palaeontological proxies and detailed sedimentological analysis.Since a couple of decades, a specific kind of fractures has been described in the Chalk of Denmark, the so-calledhairline fractures. They have recently been interpreted as compaction bands associated with the pore collapse ofthe chalk. We have observed these fractures on the field and on the cores in specific intervals. At depth, these frac-tures are in genetic relation with the formation of some stylolithes. The pressure-solution allows the formation ofcarbonate seams in the hairline fractures. At larger scale, on the field are observed faults which are sealed with flintprecipitations. They slightly offset (<1 m) strata underlined by flint bands. On the onshore and offshore seismicreflection profiles, numerous strata-bound faults form noisy intervals as well as amplitude anomalies. Their normaloffsets are less than 25 m. Their branching patterns, and their restriction to certain stratigraphic intervals (mainlywhite chalk) is comparable to the observations made on the cores and on the field.We consider that all these features observed at different scales record different diagenetic phases responsible of thetransformation of a soft ooze into a rock. It is suggested that after deposition of nanofossil ooze, the water startsto escapes and the ooze compacts into a granular sediment. This phenomenon is associated with the strata boundfaults. Later on, the flint starts to precipitate along the strata but also the faults. The pore space continuously reduceswith burial and the compaction bands form. Ultimately, stylolithes appear and the remobilised carbonates seal theremaining pore space preferentially along the fractures (the compaction bands). The link between these differentfeatures has been realised thanks to the simultaneous analyses of large-scale geophysical data and small-scale coreand field geological observations, providing a better understanding of the complex processes of lithification ofcarbonates.

Mechanical and chemical processes affecting the chalk during burial, insights from combined reflection seismics, well data and field work

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Mechanical and chemical processes affecting the chalk during burial, insights from combined reflection seismics, well data and field work

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