ABSTRACT

Compositional and textural characteristics of 13 calcite-cemented concretions are compared with those in adjacent but essentially uncemented host sandstones to test the belief that concretions better preserve original detrital compositions than do host rocks. Sandstones sampled are from five upper Eocene to Pliocene clastic units deposited in a piggyback setting and from one Miocene unit in the foreland basin of the northern Apennines.

The intergranular volume of the sandstones indicates that in younger piggy-back units calcite cement was introduced after maximum burial or nearly so, whereas in older and more deeply buried piggy-back and foreland units minor compaction continued in host sandstones after concretion growth stopped.

The concretions differ little from their host sandstones in bulk framework composition, abundance of detrital carbonate grains, and chemistry of feldspars. Detrital grains in concretions, however, underwent a six-fold greater degree of calcite replacement than detrital grains in host samples. Four of six formations have no difference in relative abundance of heavy minerals between concretions and host sandstones. The two units showing significant differences in the heavy-mineral content are those that underwent deeper burial and higher temperatures, as shown by independent evidence. In the Ranzano Formation epidote is up to three times more abundant in concretions than in host samples, and other unstable species are present only in the concretions. In the Marnoso Arenacea Formation ho nblende is twice as abundant in the concretions than in host sandstones. In these two units pore-occluding calcite cement in concretions probably prevented contact with corrosive formation water, whereas the host sandstones lost significant fractions of these unstable species.

Thus, our data indicate that calcite-cemented concretions do not essarily preserve unstable grains more readily than host sandstones, especially if cementation occurs late in the burial history of the sandstones. In the examined formations the main factors controlling the capability of concretions to preserve unstable framework grains seem to be (1) the types of unstable grains, (2) their susceptibility to dissolution by interstitial fluids or replacement by calcite, (3) burial depth and temperature during and after concretion development, and (4) time. Correct provenance reconstructions of sandstone units containing concretions must be preceded by assessment of any diagenetic alteration affecting the framework grains of both concretions aM host rocks.