Disseminated ‘jigsaw piece’ dolomite in Upper Jurassic shelf sandstones, Central North Sea: an example of cement growth during bioturbation?

Abstract

Unusual textural and chemical characteristics of disseminated dolomite inUpper Jurassic shelf sediments of the North Sea have provided the basis for a proposed new interpretation of early diagenetic dolomite authigenesis in highly bioturbated marine sandstones. The dolomite is present throughout the Franklin Sandstone Formation of the Franklin and Elgin Fields as discrete, non-ferroan, generally unzoned, subhedral to highly anhedral`jigsaw piece' crystals. These are of a similar size to the detrital silicate grains and typically account for »5% of the rock volume. The dolomite crystals are never seen to form polycrystalline aggregates or concretions, or ever to envelop the adjacent silicate grains. They are uniformly dispersed throughout the sandstones, irrespective of detrital grain size or clay content. Dolomite authigenesis predated all the other signi®cant diagenetic events visible in thin section. The dolomite is overgrown by late diagenetic ankerite, and bulk samples display stable isotope compositions that lie on a mixing trend between these components. Extrapolation of this trend suggests that the dolomite has nearmarine d18 O values and low, positive d 13C values. The unusual textural and chemical characteristics of this dolomite can all be reconciled if it formed in the near-surface zone of active bioturbation. Sea water provided a plentiful reservoir of Mg and a pore¯uid of regionally consistent d 18 O. Labile bioclastic debris (e.g. aragonite, Mg-calcite) supplied isotopically positive carbon to the pore¯uids during shallow-burial dissolution. Such dissolution took place in response to the ambient`calcite sea' conditions, but may have been catalysed by organic matter oxidation reactions. Bioturbation not only ensured that the dissolving carbonate was dispersed throughout the sandstones, but also prohibited coalescence of the dolomite crystals and consequent cementation of the grain framework. Continued exchange of Mg 2+ and Ca 2+ with the seawater reservoir maintained a suf®cient Mg/Ca ratio for dolomite (rather than calcite) to form. Irregular crystal shapes resulted from dissolution, of both the dolomite and the enclosed ®ne calcitic shell debris, before ankerite precipitation during deep-burial diagenesis.

Systematic sequence-scale controls on carbonate cementation in a siliciclastic sedimentary basin: Examples from Upper Cretaceous shallow marine deposits of Utah and Coloradao, USA

“…10). Such trends have been reported for dolomite cements in other systems (e.g., Irwin et al, 1977;Klein et al, 1999;Hendry et al, 2000;Hesse et al, 2004). There have been two mechanisms put forward to explain such trends.…”

“…The decrease in d 18 O results from increasing temperatures of precipitation with decreasing d 13 C invoked to represent increased supply of bicarbonate from decarboxylation reactions at depth. The second mechanism invokes the trend to be the result of two distinct mineral phases precipitated at different times during burial but present in differing proportions (e.g., Hendry et al, 2000). Both cases could theoretically result in decreased d 13 C and d 18 O towards cement body edges, assuming outward growth from the centre.…”

Extensive carbonate cementation of fluvial sandstones: An integrated outcrop and petrographic analysis from the Upper Cretaceous, Book Cliffs, Utah

“…10). Such trends have been reported for dolomite cements in other sedimentary systems (e.g., Irwin et al, 1977;Klein et al, 1999;Hendry et al, 2000;Hesse et al, 2004;Taylor and Machent, 2010). Such a co-variant trend could have been the result of the precipitation of a mineral cement phase that evolved isotopically in composition during burial in response to changes in temperature and carbon sources (Mozley and Burns, 1993;Klein et al, 1999;Hesse et al, 2004).…”

“…There is evidence for growth zonation in SEM observations, reflected in variations in Fe-content of cement. Alternatively, the co-variant trend may be the result of a mixture of two distinct mineral phases that precipitated at different times in the rock and are present at different relative concentrations in the samples (Hendry et al, 2000). There is evidence from SEM observations for more than one phase of dolomite precipitation in many of the samples (Fig.…”

Oil charge preserves exceptional porosity in deeply buried, overpressured, sandstones: Central North Sea, UK

“…Illite clay is more evenly distributed as measured by XRD. Early dolomite cements have almost constant abundance as these predate the oil charge (Hendry et al 2000b). Horizontal lines are 2 SE; numbers show data quantity for each point.…”