The Mid-Cenomanian Event in shallow marine environments: Influence on carbonate producers and depositional sequences (northern Aquitaine Basin, France)

Abstract

a b s t r a c tThe Mid-Cenomanian Event was a positive carbon-isotope (d 13 C) excursion recorded in hemipelagic basins of the western Tethyan Sea, North to Tropical Atlantic Ocean, and Japan. It is thought of as a prelude to the Oceanic Anoxic Event 2. However, the Mid-Cenomanian Event has never been studied in detail in shallow marine platform deposits and it is not known how it relates to carbonate production and stratigraphic geometry. To better understand how this carbon cycle disruption influenced the neritic biological communities in shallow carbonates during the Cenomanian, a facies, geochemical, diagenetic, and sequence stratigraphic study of the northern Aquitaine platform has been conducted. Seventy-six d 13 C and d 18 O measurements have been made on micrite, rudists, and diagenetic cements. Fifteen sedimentary facies have been arranged into four depositional environments. Three third-order sequences (C B , C C , C D ) are defined from late early Cenomanian to early late Cenomanian and are well correlated with eustatic cycles in European basins. Two peaks of the Mid-Cenomanian Event (MCE1a, þ1.2‰, and MCE1b, þ1.7‰) have been identified for the first time in shallow marine carbonates.Analysis of diagenetic blocky calcite cements suggests that diagenesis did not affect the d 13 C of micrite, which can be discussed in terms of the initial signal. The Mid-Cenomanian Event was synchronous with a turnover in neritic carbonate producers marking a transition from photozoan to heterozoan facies. This facies change resulted from the establishment of mesotrophic to eutrophic conditions at the early/midCenomanian transition, reflecting a clear connection between the Mid-Cenomanian Event and neritic biological communities. Depositional geometry and carbonate production varied with d 13 C during the Mid-Cenomanian Event on the Aquitaine platform. When d 13 C values were between 2.5‰ and 3‰, the geometry was a flat platform with a high carbonate sedimentation rate leading to the formation of sandbars and rudist bioherms (Accommodation/Sedimentation ratio less than 1, A/S < 1). When the d 13 C value exceeded 3‰, a carbonate demise occurred and clays and marls were deposited in the lower offshore environment (A/S >> 1). The general carbonate demise affecting the northern Aquitaine platform during the mid-Cenomanian can be explained by both a eustatic sea-level rise and the establishment of eutrophic conditions. The coincidence of the Mid-Cenomanian Event with both (1) the occurrence of mesotrophic to eutrophic conditions marked by carbonate producer turnover from photozoan to heterozoan facies and (2) the transgressive cycles, suggests that eustatic sea-level rise leading to high trophic conditions could explain this positive d 13 C excursion in the Atlantic and western Tethyan domain. During the mid-Cenomanian, carbon cycle perturbations largely controlled the neritic biological communities on shallow carbonate platforms in a part of the western Tethyan domain.

Cretaceous (Hauterivian–Cenomanian) palaeoceanographic conditions in southeastern Tethys (Matruh Basin, Egypt): Implications for the Cretaceous climate of northeastern Gondwana

“…The northern Tethyan margin of SW France records a lower Cenomanian 3rd order depositional regressive sequence (C2) and a middle Cenomanian 3rd order transgressive depositional sequence (C3; Andrieu et al, 2015). These sequences bear a striking similarity to the lower Cenomanian 2nd order regressive sequence (lower PF-3) and the middle Cenomanian 3rd order transgressive sequence (middle PF-3) in the present study.…”

The complex diagenetic history of discontinuities in shallow‐marine carbonate rocks: New insights from high‐resolution ion microprobe investigation of δ18O and δ13C of early cements

“…Their relatively high δ 18 O (−2·5 to −1·0‰) indicates that no transformation occurred during burial or early exposure. Moreover, their δ 13 C values would have been close to the marine carbon isotope ratio from 1 to 4‰ (Andrieu et al ., ; Navarro‐Ramirez et al ., ) if stabilization had occurred in marine water, which is not the case as δ 13 C values of micritic cements range from −20·0 to −0·1‰ (median: −13·2‰). The high δ 18 O and very low δ 13 C values of these micritic products argue for precipitation of low‐magnesium calcite in marine water during deposition.…”

Extensional reactivation of the Penninic Frontal Thrust 3 Ma ago as evidenced by U-Pb dating on calcite in fault zone cataclasite

“…Results are presented in Table 1. The protocol is described in detail by Andrieu et al (2015). Several mg (~1mm 3 ) of sample for each calcite generation were collected using a Dremel 4000 with a 3.2 mm head.…”