Abstract

Spheroidal (cannon ball) calcite-cemented concretions, some of gigantic size, were studied from three Cretaceous shelf sandstone units: Ferron, Frontier, and Second Frontier. The concretions have diameters between 2 cm and 6 m; those larger than 40 cm in diameter in the Ferron and Frontier have septarian structure. Rare concretions in the Second Frontier with cone-in-cone also have septaria. The pattern of carbon and oxygen isotopic data and intergranular volume (IGV) data across concretions indicate that concretion growth was complex; not all concretions grew progressively from their centers toward their edges. In many concretions displacive fascicular-optic high-Mg calcite (FOC) precipitated simultaneously throughout the concretions (pervasive growth pattern), although the earliest cementation was concentrated in and near their centers. Remaining pores were subsequently filled by poikilotopic calcite of lower Mg content. Displacive FOC cement in the bodies of concretions and in cone-in-cone structure, and the large IGV values, indicate that these carbonate phases formed at burial depths no greater than tens of meters. IGV values indicate that spar cement continued to depths of at least 50 m and possibly deeper. The association of organic matter and microbial structures, or microbial byproducts, with displacive FOC and cone-in-cone calcite raises the possibility that biogenic processes were responsible for the apparently rapid growth of the displacive calcite.

The oxygen isotope composition of pore waters was fairly uniform (in the range of δ18O of −4 to −6‰ SMOW) during 80% of concretion growth but became even more depleted in 18O during the last stage of growth and during septarian fracture filling, ranging into values as depleted as −14‰ SMOW. δ13C in calcite is negative (−4 to −18‰ V-PDB) but variable, reflecting a range of microbial processes under both oxidizing and reducing conditions. Earlier calcites fall mostly in the range of −10 to −16‰ V-PDB; relatively later calcite, in veins and on concretion margins, is more enriched in 13C. Fe and Mn concentrations in calcites have a rough positive correlation with higher δ13C values. The concentration of Mg, derived from seawater and altered volcanic rock fragments, covaries positively with the degree of meteoric influence: lower Mg contents correlate positively with relatively depleted δ18O values. 87Sr/86Sr values are typical of Sr derived from coeval seawater or marine skeletal debris.

Growth of concretions 4 to 6 m in diameter, apparently in < 5 My, requires some spatial localization of growth sites by a self-organization process, and probably a large supply of carbonate shells in the original sediment. Elongation of larger concretions in the plane of bedding suggests that advective rather than diffusive supply of cement components dominated late in concretion growth history. Concretions formed close to the tops of parasequence boundaries.

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