1 1 GEOLOGICAL SURVEY OF DENMARK AND GREENLAND BULLETIN 25 • 2011 Upper Cretaceous chalk facies and depositional history recorded in the Mona-1 core, Mona Ridge, Danish North Sea Kresten Anderskouv and Finn Surlyk GEOLOGICAL SURVEY OF DENMARK AND GREENLAND DANISH MINISTRY OF CLIMATE, ENERGY AND BUILDING 2 2 Geological Survey of Denmark and Greenland Bulletin 25 Key words Upper Cretaceous chalk, Danish North Sea, Mona-1, facies analysis Cover Detail of a shear-deformed, matrix-supported chalk conglomerate representing a deformed debris-f low deposit (Mona-1 polished core, 10890 ft). Such chalk deposits are typical of the Upper Cretaceous succession in the Mona-1 well and exemplify two important processes of redeposition: f luid-like deformation during debris f low and plastic deformation during slumping or the final stages of debris-f low deposition. Photograph courtesy of Wintershall Noordzee B.V. Chief editor of this series: Adam A. Garde Editorial board of this series: John A. Korstgård, Department of Geoscience, Aarhus University; Minik Rosing, Geological Museum, University of Copenhagen; Finn Surlyk, Department of Geography and Geology, University of Copenhagen Scientific editor of this volume: Jon R. Ineson Editorial secretaries: Jane Holst and Esben W. Glendal Referees: Frans van Buchem (DK) and Maurice E. Tucker (UK) Illustrations: Jette Halskov, Britta Munch and Christian Hagen Digital photographic work: Benny M. Schark Graphic production: Kristian A. Rasmussen Printers: Rosendahls . Schultz Grafisk A/S, Albertslund, Denmark Manuscript received: 6 December 2010 Final version approved: 19 October 2011 Printed: 29 December 2011 ISSN 1604-8156 I S BN 9 78 - 8 7-78 71-327- 8 Citation of the name of this series It is recommended that the name of this series is cited in full, viz. Geological Survey of Denmark and Greenland Bulletin . If abbreviation of this volume is necessary, the following form is suggested: Geol. Surv. Den. Green. Bull. 25, 60 pp. Available from Geological Survey of Denmark and Greenland (GEUS) Øster Voldgade 10, DK-1350 Copenhagen K, Denmark Phone: +45 38 14 20 00, fax: +45 38 14 20 50, e-mail: [email protected] or at www.geus.dk/publications/bull © De Nationale Geologiske Undersøgelser for Danmark og Grønland (GEUS), 2011 For the full text of the GEUS copyright clause, please refer to www.geus.dk/publications/bull 3 3 Contents Abstract ..................................................................................................... Introduction ............................................................................................... Setting and stratigraphy ................................................................................. Materials and methods ................................................................................... Facies analysis ............................................................................................. Definitions and terminology ........................................................................... Facies 1: Bioturbated chalk ............................................................................ Facies 2: Bioturbated marly chalk ..................................................................... Facies 3: Laminated chalk .............................................................................. Facies 4: Laminated marl ................................................................................ Facies 5: Graded silty chalk ............................................................................. Facies 6: Graded silty chalk with packstone lamination ............................................ Facies 7: Graded packstone and wackestone .......................................................... Facies 8: Non-graded packstone and wackestone .................................................... Facies 9: Structureless chalk ............................................................................. Facies 10: Structureless matrix-supported chalk conglomerate ..................................... Facies 11: Clast-supported chalk conglomerate ..................................................... Facies 12: Shear-deformed bioturbated chalk ........................................................ Facies 13: Shear-deformed matrix-supported chalk conglomerate ................................ Facies 14: Shear-banded chalk ......................................................................... Slumps .................................................................................................... Genetically related facies ............................................................................... Facies associations and stratigraphic development ................................................. Facies associations ....................................................................................... Facies association 1 (pelagic chalk) ................................................................. 4 4 5 5 Anderskouv, K. & Surlyk, F. 2011: Upper Cretaceous chalk facies and depo - sitional history recorded in the Mona-1 core, Mona Ridge, Danish North Sea. Geological Survey of Denmark and Greenland Bulletin 25, 60 pp. The 331 m long core from the Mona-1 well in the Danish North Sea spans almost the entire Upper Cretaceous Chalk Group but only about 10% of Late Cretaceous time is represented. The succession comprises 14 facies representing pelagic deposition, turbidity flow, and mass-transport processes, including mudflow, debris flow, and slumping. Pelagic deposits vary mainly in terms of the concentration of siliciclastic material, the trace-fossil assemblage, and the presence or ab - sence of primary sedimentary structures. Pelagic sedimentation was probably punctuated by the deposition of thin turbidites, and the resultant deposits were thoroughly bioturbated if deposited during normal oxygenation at the sea floor. Periodic benthic dysoxia resulted in the preservation of primary structures, as represented by laminated chalk which consists of thin pelagic laminae alternating with thin turbidites. In addition to the thin turbidites in the laminated chalk, four dif - ferent turbidite facies are interpreted as representing high- to low-energy flows. Clast-supported chalk conglomerates have previously not been differentiated from other turbidites, but are here interpreted to be directly related to the down-slope evolution of debris flows. Debris flows are rep - resented by matrix-supported conglomerates, which form one of the most common facies in the succession. High-concentration, gravity-driven suspension flows passed into dilute visco-plastic flows during the final stages of deposition and resulted in the deposition of structureless chalks. Limited shear deformation produced distinct quasi-facies from which the precursor facies can be deduced, whereas intense or continued shear deformation produced a shear-banded quasi-facies from which the precursor facies cannot be deduced in all cases. A series of major slump packages (14–18 in total) are interpreted, forming over 40% of the succession; debrites appear to be the most common precursor facies involved in slumping. The vertical succession of facies records an earliest Cenomanian facies shift from dominantly siliciclastic to chalk deposition. The Cenomanian – late Campanian period was dominated by erosion or sediment by-pass with minor associated mass-transport deposits preserved. Basin fill - ing by pelagites and turbidites prevailed in the late Campanian, whereas Maastrichtian pelagic deposition was interrupted by increasingly frequent and voluminous mass-transport events. Authors’ address Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: [email protected], [email protected] Abstract