TRACE ELEMENT EVIDENCE OF A VOLCANIC SOURCE FOR LOWER MISSISSIPPIAN CHERT AND NOVACULITE, SOUTHERN MIDCONTINENT

The Lower Mississippian succession in the southern Ozark region of Arkansas, Oklahoma and Missouri, comprises an unconformity-bounded, third order transgressive-regressive carbonate cycle represented by an essentially chert-free transgressive interval succeeded by a chert-bearing, maximum flooding and highstand-regressive sequence. The maximum flooding interval comprises calisiltites with penecontemporaneous, poorly fossiliferous, nodular, black chert that disrupts the bedding, and exhibits compaction phenomena. The succeeding highstand-regressive sequence is represented by carbonate grainstones and packstones with later diagenetic, bedded, white to light gray or light brown chert that follows the bedding, replicates the host carbonate facies, and exhibits an obvious replacement fabric. This entire sequence is isochronous with the upper portion of the Arkansas Novaculite deposited as siliceous sediment in an bathyal oceanic setting, and subsequently orogenically deformed as part of the core area of the Ouachita Mountains in Oklahoma and Arkansas. Previous explanations of the silica source for these occurrences has involved remobilization of the hard parts of siliceous microorganisms, although a minority opinion has cited a volcanic origin for some or all of the silica.

Samples taken from each of the three siliceous lithologies were examined by energy dispersive x-ray for minor element concentrations, and compared to the elemental distributions in sand-size volcanic ash shards recovered as insoluble residues from the carbonate intervals. Silica comprised approximately 85 wt %-92 wt % of each lithology. All three lithologies exhibited a consistent contribution from aluminum, comprising nearly 2 wt %, followed by magnesium, sodium, potassium and calcium, in decreasing order of abundance, usually between .22 wt %-.60 wt %. The distribution of these elements was consistent among the three lithologies, and the insoluble residues, suggesting a similar silica source regardless of depositional circumstance. Elemental composition and concentrations suggests that the silica was derived from a granitoid igneous source, mostly likely ash falls, reflecting volcanism associated with the Ouachita orogeny.