Calcite-Cemented Concretions in Cretaceous Sandstone, Wyoming and Utah, U.S.A.

摘要: 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 6 m; those larger than 40 in diameter the Ferron Frontier septarian structure. Rare with cone-in-cone also septaria. pattern carbon oxygen isotopic data intergranular volume (IGV) across indicate that concretion growth was complex; not all grew progressively their centers toward edges. In many displacive fascicular-optic high-Mg calcite (FOC) precipitated simultaneously throughout (pervasive pattern), although earliest cementation concentrated near centers. Remaining pores subsequently filled by poikilotopic lower Mg content. Displacive FOC cement bodies structure, large IGV values, these carbonate phases formed at burial depths no greater tens meters. values spar continued to least 50 m possibly deeper. association organic matter microbial structures, or byproducts, raises possibility biogenic processes responsible for apparently rapid calcite. isotope composition pore waters fairly uniform (in range 18O -4 -6‰ SMOW) during 80% but became even more depleted last stage fracture filling, ranging into as -14‰ SMOW. 13C is negative (-4 -18‰ V-PDB) variable, reflecting a under both oxidizing reducing conditions. Earlier calcites fall mostly -10 -16‰ V-PDB; relatively later calcite, veins on margins, enriched 13C. Fe Mn concentrations rough positive correlation higher values. concentration Mg, derived seawater altered volcanic rock fragments, covaries positively degree meteoric influence: contents correlate 87Sr/86Sr are typical Sr coeval marine skeletal debris. Growth 4 diameter, < 5 My, requires spatial localization sites self-organization process, probably supply shells original sediment. Elongation plane bedding suggests advective rather diffusive components dominated late history. Concretions close tops parasequence boundaries.