The latest Jurassic of the Boulonnais cliffs (N-France, Strait of Dover) corresponds to a homoclinal ramp-type depositional environment, where sedimentation was clastic-dominated ...(marlstones-sandstones). The Tithonian marlstone formations exhibit two or three carbonate beds in the vicinity of Wimereux city (3 beds at Wimereux-North and 2 beds at Wimereux-South). The beds are made up with quite pure, fine-grained (microspar) carbonate in sharp contrast with the background sedimentation. This paper presents a sedimentological and geochemical study of these limestone beds, aiming to determine whether they are of diagenetic origin and hence whether they could have acted as permeability/migration barriers during hydrocarbon maturation/migration. Mineralogical, chemical, and C and O stable isotope data allow us to infer that the beds formed during synsedimentary early diagenesis at the sediment–water interface (or close to it) as the result of a rise of alkalinity induced by bacterial sulfate reduction. The rise of alkalinity was not counter-balanced by the accumulation of H2S released by sulfate reduction and carbonate ion supersaturation was rapidly reached, causing the formation of laterally-continuous limestone beds. Conditions prone to bacterial sulfate reduction developed episodically at the sediment–water interface as the result of spills of anoxic pore waters onto the seafloor. These spills were probably released by synsedimentary fault movements. Such continuous limestone beds being formed under rather common conditions during the earliest stage of diagenesis of shale deposits must be more frequent in the geological record than hitherto identified.
► Diagenetic limestone beds can form directly at the sediment–water interface through bacterial sulfate reduction. ► They cannot be distinguished from sedimentary limestone beds without the help of C and O isotope data. ► Sulfate reduction was induced by spills of anoxic waters on the sea floor released by synsedimentary fault movements.
Dedolomitization is a common diagenetic process in shallow burial environments and is often associated with sulphates in mixed carbonate‐evaporite successions. In these settings, elevated Ca2+/Mg2+ ...ratios necessary for dedolomitization result from the dissolution of sulphate phases by the incursion of undersaturated groundwater. Reported dedolomite textures from other studies are varied, but the most prevalent is a rhombic texture interpreted to result from the partial to complete pseudomorphic replacement of secondary dolomite rhombs formed in the burial diagenetic realm. In this study of primary cryptocrystalline to finely crystalline dolomicrites in the Prairie Evaporite Formation of north‐eastern Alberta, dedolomitization has resulted in sutured to loosely packed mosaics of dedolomite that range from subhedral to distinctly euhedral (rhombic) crystal fabrics; however, no prior aggrading neomorphism producing dolomite rhombs is evident in the precursor dolomicrites. Non‐pseudomorphic dedolomitization of the dolomicrites results in textures that include rhombic dedolomite crystals with cloudy cores comprising remnant dolomicrite and clear rims. These textures are similar to those observed in the pseudomorphic dedolomitization of secondary dolomite rhombs. The Prairie Evaporite Formation of north‐eastern Alberta has experienced extensive karstification near the erosional margin of the sedimentary succession. Dedolomitization of dolomicrites occurs in marker beds within the Prairie Evaporite succession associated with evaporite karstification. Along with stratigraphic and petrographic considerations, stable isotope results support the interpretation of a shallow dedolomitization event influenced by meteoric waters derived from the basin margin. Negative δ18O and low δ13C values (averages of −13·6‰VPDB and 0·5‰VPDB, respectively) of the dedolomite, compared with those of the primary dolomicrite (averages of −6·0‰VPDB and 1·2‰VPDB, respectively), point to isotopically light diagenetic fluids. These results show that rhombic dedolomite textures can form through shallow, non‐pseudomorphic dedolomitization of dolomicrites by meteoric fluids in the presence of sulphates, with resulting textures that are similar to the pseudomorphic dedolomitization of secondary dolomite rhombs.
•High total organic carbon (up to 12.3 wt%) Mesoproterozoic shale unit.•Fe speciation, δ15N, TOC, and element abundances suggest a stratified water column.•Sulphurisation may have played an important ...role in organic matter preservation.•Monte Carlo results do not necessitate extremely high organic carbon burial rates.
The latest Mesoproterozoic Arctic Bay Formation (Borden Basin, Nunavut, Canada) is up to ∼1130 m-thick and contains a significant proportion of unusually organic-rich black shale (up to 12.3 wt% total organic carbon). Insofar as increased biological productivity is related to organic matter burial, this organic-rich succession is seemingly incongruent with the low biological productivity world hypothesised for much of the Proterozoic. To better understand the conditions leading to development of this organic-rich unit, we explore the redox geochemistry of the Arctic Bay Formation using a multi-proxy approach (nitrogen isotopes, iron speciation, total organic carbon, total sulphur, and trace metal abundances). Redox proxy data support a stratified water column, with oxic surface waters underlain by intermittently euxinic waters, which are in turn underlain by persistently ferruginous deeper waters. The highly alkaline, restricted marine basin in which the Arctic Bay Formation was deposited may have allowed for rapid sequestration of highly reactive iron in carbonate minerals, resulting in an ‘excess’ of sulphur that resulted in sulphurisation of organic matter. Estimates for organic matter burial rates during deposition of the Arctic Bay Formation suggest that they were perhaps ∼5–6 times mid-Proterozoic average values (although there are permissible scenarios in which it was extremely productive), underscoring that such organic-rich sedimentary rocks could be produced in a low productivity world.
