6533b7d8fe1ef96bd126ab4c

RESEARCH PRODUCT

Stratigraphy and stage boundaries in reference sections of the Upper Cretaceous Chalk in the east of the Paris Basin: the “Craie 700” Provins boreholes

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Abstract The Provins boreholes (Poigny: Craie 701, and Sainte-Colombe: Craie 702) drilled in 1999 as part of the “Craie 700” project initiated by the Paris Basin Geologists' Association provide reference sections for the Upper Cretaceous of the eastern part of the Paris Basin. The lithology of the Cretaceous drilled at Poigny and Sainte-Colombe is presented in two logs displaying major litho-events that provide lines of correlation. Bio-events are expressed by the appearance/disappearance of index species of fossils. Foraminifera give a biozonation close to that defined by previous work on the Anglo-Paris Basin. Some of these bio-events provide a second set of lines useful for correlation. From the results of these lithobiostratigraphic studies, stage boundaries from the Upper Albian to the Campanian have been located with a maximum uncertainty of a few metres. The main results are as follows: (1) below the Tertiary cover, the sedimentation of the Chalk seems to have been continuous: no major hiatuses have been identified from the Campanian down to the Cenomanian; (2) the Upper Albian was reached at the base of the Sainte-Colombe Borehole; (3) in this borehole, the stages have approximately the following thicknesses: Upper Albian “Vraconian” (Vraconnien) (13 m), Cenomanian (58 m), Turonian (180 m), Coniacian (80 m), Santonian (70 m), Campanian (217 m; about 265 m at Poigny); (4) the Chalk succession is 40–50 m thicker at Poigny than at Sainte-Colombe; (5) lines of correlation based on litho- and bio-events and stage boundaries are sub-parallel; (6) a 17-m-thick dolomitic body occurring only in the Sainte-Colombe Borehole is located at the transition between the lower and upper parts of the Campanian; (7) clay-rich beds corresponding to kaolinite-bentonites used for long-distance correlations are recognized in the Turonian and are very accurate marker beds. Three tectonic phases are evident in the two boreholes. The pre-Ilsede phase is discrete in the Middle Turonian. The early Ilsede phase begins in the lower part of the Coniacian. The main Ilsede phase reduces the thickness of the middle part of the Coniacian (foraminiferal biozone S/b thin or absent). The Wernigerode phase seems to have reduced the thickness of the upper part of the Santonian (?lack of crinoid zones). The development of a thick dolomitic body in the Sainte-Colombe Borehole, and the presence of bored surfaces bounding the same stratigraphic interval in the Poigny Borehole, may be related to the Peine phase, active between the early and late Campanian.