Anatomy of a Condensed Section: The Lower Cenomanian Glaucony-Rich Deposits of Cap Blanc-Nez (Boulonnais, Northern France)
Alessandro Amorosi, Maria Carla Centineo, 2000. "Anatomy of a Condensed Section: The Lower Cenomanian Glaucony-Rich Deposits of Cap Blanc-Nez (Boulonnais, Northern France)", Marine Authigenesis: From Global to Microbial, Craig R. Glenn, Liliane Prévôt-Lucas, Jacques Lucas
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Detailed examination of the lowermost Cenomanian, glaucony–rich deposits of Cap Blatic–Nez (Northern France) reveals a hierarchy of depositional units within a stratigraphically condensed succession, providing a fundamental tool for its sequence stratigraphic interpretation.
Condensation is obvious in the highly bioturbated basal glaucony–rich unit (Strouanne Formation), where glaucony is predominantly autochtho nous, highly evolved (˂ 8% K20), and generally constitutes 60–90% of the sand fraction. Glaucony concentration decreases markedly upwards (to less than 10%) in the overlying rhythmically bedded marls and chalks (Petit Blanc–Nez Formation), where the green grains are allochthonous and comparatively less evolved (6.0–6.5% K20). Glaucony distribution shows a vertical cyclic pattern within the condensed interval. Maxima of glau cony concentrations are invariably recorded at intensively burrowed omission surfaces, interpreted as bases of either parasequences or parasequence sets.
The characterization of glaucony attributes indicates that a prolonged break in sedimentation, on the order of 0.5–1 Ma, occurred in the study area during the Early Cenomanian. The highly condensed nature of the basal glaucony–rich deposits suggests that either one or two of the third–order Lower Cenomanian depositional sequences recognized in the coeval, less condensed successions of Western Europe are missing at Cap Blanc–Nez. Glaucony maturity in the overlying marl–chalk couplets is consistent with interpretation of these deposits as Milankovitch–scalc cycles.
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Marine Authigenesis: From Global to Microbial
This volume is a collection of 33 state-of-the-art papers focusing on various aspects of authigenic and diagenetic marine minerals and related global elemental cycling. The commingling of the various studies of authigenic minerals in this volume, including the most recent advances in knowledge concerning the occurrence and origins of phosphorites, glauconites, dolomites, siderites, manganese-iron associations, barites, ironstones, and other marine chemical sediments/sedimentary rocks of early authigenic/diagenetic origin, is partly the result of the increasing awareness that there are many overlaps, even direct co-associations, between different authigenic minerals, both in time, space, and genesis. Taken together, this compilation represents a holistic approach towards marine authigenesis that considers the integrated whole more than the simple sum of its parts.