New Mexico Geological Society Annual Spring Meeting — Abstracts

Although the source of silica for sedimentary chert is often attributed to siliceous organisms such as sponge spicules or diatoms, a postulated alternative silica source is eolian, quartz-rich dust. As noted by Cecil , late Paleozoic sedimentary chert is almost always associated with strata deposited under arid paleoclimates and almost always contains detrital quartz silt and sand suspended in a chert matrix. The detrital quartz grains (£ 20 mm) generally contain evidence of the dissolution (corrosion) of a disordered crystalline lattice induced by eolian abrasion.

Across much of New Mexico, the Gray Mesa Formation is a distinctive interval of ledge- and cliff-forming marine limestone, usually 100-200 m thick, in the middle of the Pennsylvanian succession. Most of the limestones of the Gray Mesa Formation are lime mudstones and fossiliferous wackestones and packstones that are characteristic of marine shelf environments. At many outcrops, the Gray Mesa Formation can be divided into three members (ascending): Elephant Butte Member, chert-rich Whiskey Canyon Member and Garcia Member.

We sampled limestones of the Whiskey Canyon Member at the Crest of Montezuma (also called Montezuma Ridge or Mountain) just northeast of the village of Placitas in southeastern Sandoval County, New Mexico. Fusulinids and conodonts from the strata we sampled indicate an early Desmoinesian age. All samples contained abundant chert, both massively bedded and nodular. The chert and limestone display abundant angular detrital quartz (£ 20 mm) and fine sand grains with corroded surfaces interpreted to have formed by the corrosion of a disordered crystalline lattice caused by eolian abrasion. The original angular quartz grains thus show morphological characteristics of eolian dust deposited in a marine environment. Microcrystalline quartz (chert) was re-precipitated following dissolution of all or part of the detrital quartz grains. Consequently, dissolution of the eolo-marine dust provided the source of silica for the enclosed chert. Homogenous detrital zircon U-Pb ages in the limestone suggest a local source of siliciclastic input, whereas a broad distribution of ages in the chert beds implies an expanded source region, consistent with sediment transport by wind. Therefore, we propose that chert in the Gray Mesa Formation in central New Mexico was derived from the dissolution of eolian dust. The presence of large amounts of eolo-marine dust in the Gray Mesa limestones indicates that the climatic changes accompanying the Atokan-Desmoinesian transition in central Pangea, indicated by changes in wetland floras and geological features, were widespread across the tropical region that stretched across central and western Pangea.

References:

1. Cecil, C.B., 2004, Eolian dust and the origin of sedimentary chert: U. S. Geological Survey, Open-File Report 2004-1098.
2. Cecil, C.B., 2015, Paleoclimate and the origin of Paleozoic chert: Time to re-examine the origins of chert in the rock record: The Sedimentary Record, v. 13, p. 4–10.
3. Cecil, C.B., Hemingway, B.S., and Dulong, F.T., 2018, The chemistry of eolian quartz dust and the origin of chert: Journal of Sedimentary Research, v. 88, p. 743–752.