LEDGER, E. B., and M. C. CROCKER, Stephen F. Austin State Univ., Department of Geology, Nacogoches, TX; SCOTT BEATY, Illinois Geological Survey, Carbondale, IL; M. W. ROWE, Texas A&M Univ., College Station, TX; and MOSES ATTREP, Jr., Los Alamos National Laboratory, Los Alamos, NM

ABSTRACT: Origin of the Arkansas/Caballos Novacultite Based on Rare Earth Element Count

Current ideas on the silica source for the Arkansas/Caballos Novaculite (ACN) range from recrystallization of a radiolarian chert to direct chemical precipitation of silica gel by inorganic means. New rare earth element (REE) data indicate a rock composed of silica from both sources. Previous petrographic studies of ACN thin sections reveal an average of a few percent radiolarian tests with a few samples showing up to 25 percent. Our calculations based on Ce anomalies in modern radiolarian tests agree with the earlier work in that the ACN averages only a few percent radiolaria, with a few samples giving higher values. Thus, most of the silica content of the ACN must be accounted for by inorganic silica.

Rare earth element plots of ACN normalized to North American Shale Composite (NASC) reveal patterns typical of terrigenous clastic material containing REE-bearing heavy minerals, but with very low total REE contents.

On the other hand, true radiolarian bedded cherts usually give very different plots with strong negative Ce anomalies reflecting seawater REE composition. Thus, the REE content suggests that the ACN is composed of a small amount of continental clastic detritus, a usually minor radiolarian component, and mostly chemically precipitated silica.

The source of the silica is probably hot spring activity on the sea floor along a spreading center and associated transform faults. Although pH variation cannot cause silica precipitation in a normal marine environment, silica solubility is strongly enhanced in high temperature waters. Silica-saturated water from submarine hot springs could precipitate silica as it cooled by mixing with normal seawater. Unusually low continent elevations during the Devonian and favorable tectonic conditions allowed the accumulation of this rare rock type.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.