ABSTRACT

In Cretaceous shales and limy shales of the Magdalena Valley of Colombia, South America, occur many carbonate concretions. These concretions commonly contain such marine organisms as ammonites. Occasionally, one finds the full body of a fish, like the specimen in the accompanying photographs. The occurrence of carbonate concretions in central basin shale facies is common not only throughout the Cretaceous of the lengthy Andean geosyncline but in similar facies throughout the world. The occurrence of fish in a full or nearly full-bodied state of preservation in concretions is also quite widely known.

The manner of occurrence of most carbonate concretions in shales clearly indicates that their origin is syngenetic or early diagenetic. They were formed very soon after deposition, when the enveloping muds were still in a very plastic, unlithified state. For instance, the muds filled in alongside the concretions and, as a result of later compaction, they now "flow" around them. Also, the occurrence of fish and various other kinds of organisms in a full-bodied, uncompressed state indicates that their calcification and the enveloping concretion development were early enough to protect the carcass both from extreme decomposition and from the compressive effect of many thousands of pounds of subsequent overburden pressure.

The author points out that the central basin deposition environments in which most carbonate concretion-bearing shales were deposited was rather generally unfavorable, or of limited favorability, to carbonate deposition. Such environments are stagnant, hence usually charged with carbon-dioxide, and the degree of alkalinity (pH) is too low for calcium carbonate deposition. It is suggested, and some evidence is cited, that the rapid calcification of the organic core and the accumulation of the enveloping carbonate concretion in these generally unfavorable realms result from the creation of a favorable environment in the immediate vicinity of the organic material. An alkalinity adequate for rapid carbonate deposition from calcium-ion-rich waters may be locally created by the ammonia that evolves rapidly as soon as proteinaceous (nitrogen-bearing) organic matter begins to decompose.

Similarly, the limited decomposition, mainly anaerobic, of the organic matter that usually impregnates the deeper basinal muds may temporarily create an alkalinity adequate to cause calcium carbonate to be released from the lime-bearing solutions permeating the muds. This may explain the common calcareous character of shales in basins where pure limestones are largely restricted to the aerated shallow flank, shelf or lesser bottom-high areas.