Controls on organic accumulation in late Jurassic shales of northwestern Europe as inferred from trace-metal geochemistry

Abstract

In the Kimmeridge Clay Formation of the Wessex-Weald Basin, five organic-matter-rich intervals (or ORIs), dated from Kimmeridgian-Tithonian times, can be correlated from distal depositional environments in Dorset and Yorkshire (UK) to the proximal environments in Boulonnais, northern France. The ORIs are superimposed on a meter-scale cyclic distribution of organic matter (OM), referred to as primary cyclicity, which is commonly interpreted to result from Milankovitch climate forcing. The present work addresses the distribution of redox-sensitive and/or sulfide-forming trace metals and selected major elements (Si, Al and Fe) in Kimmeridge Clay shales from the Cleveland Basin (Yorkshire) and the Boulonnais cliffs with two objectives: 1) to determine whether the ORIs formed in similar paleoenvironments, and 2) to identify the mechanism(s) of OM accumulation. High-resolution geochemical data from primary cycles in the Yorkshire boreholes (Marton and Ebberstone boreholes), were studied and the results are then applied with lower resolution sampling at the ORI scale in the Flixton borehole and Boulonnais cliff. Good correlations are found between total organic carbon (TOC) vs Cu/Al and Ni/Al, but relationships between TOC and Mo/Al, V/Al and U/Al are more complex. Cu and Ni enrichment is interpreted to have resulted from passive accumulation with OM in an oxygen-deficient basinal setting, which prevented the subsequent loss of Cu and Ni from the sediment. Mo and V were significantly enriched only in sediments where considerable amounts of OM (TOC>7 %) accumulated, the result of strongly reducing conditions and OM burial. At the scale of the Flixton ORIs, the samples with the highest Mo and V concentrations also show relative Fe enrichment, suggesting pyrite formation in the water column (combination of euxinic conditions and presumably low sedimentation rates). Samples from all ORIs were slightly enriched in Si relative to Al, interpreted as reflecting decreased sediment flux during transgressive and early-highstand systems tracts. The data show that in some ORIs, OM accumulation proceeded while productivity was not particularly high and sediments were not experiencing strong anoxia. In other ORIs, OM accumulation was accompanied by widespread anoxia and possibly euxinic conditions in distal settings. Though somewhat different from each other, the ORIs have all developed during episodes of reduced terrigenous supply (transgressive episodes). The common feature linking these contrasted episodes of enhanced OM storage (ORIs) must be the conjunction of productivity coupled with a decrease in the dilution effect by the land-derived supply, in a depositional environment prone to water stratification and, therefore, favorable to OM preservation and accumulation.

Small-scaled lateral variations of an organic-rich formation in a ramp-type depositional environment (the Late Jurassic of the Boulonnais, France): impact of the clastic supply

“…has some of the characteristics of a potential petroleum source rock, as well as its lateral, time-equivalent counterpart of the Kimmeridge Clay Fm. in UK Tribovillard et al, 2005). However, the Argiles de Châtillon Fm.…”

Basinal restriction, black shales, Re‐Os dating, and the Early Toarcian (Jurassic) oceanic anoxic event

“…The regression slopes have a range of slopes, but many are low. The rsRe/TOC and rsOs/TOC for the Kimmeridge Clay (wheatleyensis Zone of Dorset) are particularly low and seem extreme, while the rsMo/TOC for the Kimmeridge Clay (hudlestoni and wheatleyensis Zones of the Cleveland Basin [see Tribovillard et al, 1994Tribovillard et al, , 2004b is around 3, which is low but not as extreme as the 0.5 for the Toarcian sediments of the Cleveland Basin. We interpret all of these the low slopes to be diagnostic of water mass restriction.…”

“…[7] Molybdenum is removed from euxinic waters by reduction to the thiomolybdate complexes (MoO x S 4-x 2À ) in the presence of free H 2 S and subsequent sequestration in either sedimenting organic matter, sedimenting pyrite formed in the water column, or both [Helz et al, 1996;Erickson and Helz, 2000;Vorlicek and Helz, 2002;Tribovillard et al, 2004aTribovillard et al, , 2006Tribovillard et al, , 2008. Because of this removal, the renewal time i.e., degree of restriction, of the subpycnoclinal water mass in modern, euxinic, marine basins, can be estimated from the slope of the regression line between Mo and TOC (rsMo/TOC hereinafter, to distinguish it from raw Mo/TOC ratios) in a crossplot of TOC against Mo in sediments [ Algeo and Lyons, 2006].…”

Trace Elements in Marine Sediments from the Oxfordian (Late Jurassic): Implications for Seawater Chemistry, Erosional Processes, Changes in Oceanic Circulation and More

“…For example, in the case of U and V enrichment without Mo enrichment, suboxic/anoxic depositional without free H 2 S can be inferred. Conversely, sediments exhibiting concurrent enrichments in U, V and Mo reflect euxinic conditions at the sediment-water interface or in the water column [18,23,55,56].…”