DIAGENETIC IRON OXIDE CONCRETIONS ASSOCIATED WITH VERTEBRATE FOSSILS IN THE UPPER TRIASSIC CHINLE FORMATION OF NORTHERN NEW MEXICO: IMPLICATIONS FOR PALEOENVIRONMENTAL AND DIAGENETIC HISTORY

Diagenetic, spheroidal, iron oxide concretions are commonly associated with fossil vertebrate and charcoal specimens in the Hayden Quarry at Ghost Ranch in the Upper Triassic Chinle Formation of the Chama Basin of northern New Mexico. The iron oxide concretions are most common in channel deposits of poorly-sorted, intraformation granule conglomerates, and occur both dispersed throughout the sediment and encasing fossil wood, charcoal, and the articular ends of bones. The layers with the highest amount of vertebrate bone material are associated with the highest abundances of the concretions, and in these layers, 20-25% of bones are associated with concretions. These vertebrate-rich layers comprise roughly 10% of each channel’s stratigraphic thickness.

Measurements of 175 concretions from two paleochannels at the Hayden Quarry indicate roughly equant dimensions and mean principal axes of 27.8 mm x 22.0 mm x 17.8 mm, with standard deviations of 5.6-8.8 mm. Petrography, QEMSCAN and XRD analyses indicate sequential precipitates of calcite, pyrite (with alteration to goethite), and gypsum comprising the concretion cements. The iron oxide concretions rarely exhibit concentric layering or rinds. Many concretions have subhedral to euhedral isometric pyrite crystals dispersed throughout, likely associated with anaerobic sulfate reducing bacteria. Pyrite nucleated preferentially around vertebrate and plant matter because of their porous nature of original bone and bacterial decay, and organic compounds that might have attracted ferrous or ferric iron. Pyrite subsequently oxidized to goethite, and was lastly crosscut by blocky gypsum. These iron oxide concretions are important records of the diagenetic history and paleoenvironment, with initial subaqueous reducing conditions and multiple subsequent events of oxidation.