Contrasting Diagenetic Histories of Concretions vs. Host Rocks, Lion Mountain Member, Riley Formation (Upper Cambrian), Texas

E. F. McBride

White, elliptical, calcite-cemented concretion nuclei up to 1 m long contrast markedly in color, composition, and diagenetic history from more glauconite-rich concretion rinds and from dark-green glaucarenite host rocks. Concretion nuclei are loosely packed deposits of trilobite carapaces and minor quartz and glauconite that have intergranular volumes of 58%. The nuclei are shell-lag deposits that were cemented by calcite at the sea floor or after burial of a few meters. Concretion rinds, composed of subequal amounts of quartz and compactionally deformed glauconite, have an intergranular volume of only 32% and minor quartz overgrowths that preceded pore-occluding calcite cement. The rinds underwent burial for several million years to tens of millions of years to depths of several hundred meters before they were cemented. The host rock is predominately glauconite with very minor quartz and calcite cement. Strontium isotopic ratios of host-rock calcite cement are variable (0.7084 to 0.7093), but the lowest value suggests precipitation during the Middle Ordovician. In the absence of significant amounts of carbonate cement, the host rock underwent complete dissolution of trilobite carapaces and maximum compaction with total loss of porosity through squashing of glauconite grains. Maximum burial during this stage was completed by the end of Ordovician time.

Carbon and oxygen isotopic values of calcite from 16 samples show no differences between concretion nuclei, rinds, and host rocks. ^dgr¹³C ranges from 2.22 to 2.85 (PDB) and probably reflects derivation of carbon from trilobite carapaces and other skeletal grains. ^dgr¹⁸O ranges from -4.6 to -6.2 (PDB) and reflects either the original composition of the cement and trilobites or the modification of original grains by meteoric-dominated ground water.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.