GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 341-12
Presentation Time: 9:00 AM-6:30 PM


ANDERSON, Joshua G., Geology Department, Pomona College, 185 E. Sixth St., Claremont, CA 91711 and GAINES, Robert R., Geology, Pomona College, 185 East Sixth Street, Claremont, CA 91711,

Carbonate concretions provide critical records of biogeochemical processes in ancient marine sediments, and also hold importance for interpreting the long-term evolution of oceanic chemistry over geologic time. The early Paleozoic is of special interest as a time of rapid transitions in Earth’s biosphere and surface chemistry, yet concretions from this interval remain poorly studied compared to more recent examples. This study focuses on middle Ordovician strata of the Glenogle Formation in the southeastern Canadian Rockies, which comprises a thick succession of organic rich graptolitic mudrocks. Concretionary bodies were found to occur in multiple discrete intervals of 0.4 – 3.0 m in thickness, separated by much larger (often >10m) intervals that lack concretions. Carbonate concretions spanning a wide array of sizes (10’s of cm) and shapes (ovoid to tabular) are present, and are comprised of calcite with subordinate pyrite that occurs dispersed throughout the concretionary framework as well as in fissures of later diagenetic origin. These concretions exhibit a primarlily displacive growth habit, but replaced bedding is also observed in some cases. The calcite bodies appear to represent typical concretion growth promoted by organic processes, however, they occur in close stratigraphic proximity (<1m) to smaller authigenic mineral masses whose growth appears to have been promoted by inorganic forcing. These ovoid mineral aggregates range in size from 1-6 cm and are comprised of a barite-quartz-pyrite-calcite mineral assemblage, with barite appearing to comprise the majority of the aggregate framework and calcite occurring primarily as a late phase fracture-fill. These aggregates exhibit a clearly displacive growth habit, invoking precipitation during early diagenesis. Importantly, the presence of abundant barite as nodules within the mudrocks appears to require episodes of evaporative concentration of seawater in this productive deep-water basin on the middle Ordovician passive margin. Geochemical data allow for detailed reconstruction of pore water biogeochemistry during episodes of authigenesis driven by alternately organic and inorganic processes.