EXPLOSIVE BASALTIC PHREATOMAGMATIC VOLCANISM DURING EARLY CAMBRIAN RIFTING IN SOUTHERN OKLAHOMA: EVIDENCE FROM QUARRY EXPOSURES IN THE ARBUCKLE MOUNTAINS

Southern Oklahoma and adjacent parts of Texas contain a major bimodal igneous assemblage emplaced during Early Cambrian rifting associated with opening of Iapetus. The uppermost main igneous unit within the rift is the Carlton Rhyolite, which extends for ~40,000 km² in the subsurface. Based on data from basement wells and limited outcrops, the rhyolites were largely emplaced nonexplosively as a series of stacked lava flows, and they are intercalated with basaltic lavas in the subsurface. Here we report results of recent work on an unusual series of chaotic, polymict igneous breccias that occur within the rhyolite in the Arbuckle Mountains of southern Oklahoma. The breccias record a markedly different style of volcanism than has generally been recognized in the Cambrian volcanic field. Price et al. (1998) suggested that the breccias were produced by explosive subsurface magma-water interactions; our work supports and elaborates on this interpretation.

The breccias are exposed within an active quarry in the western part of the Arbuckles and appear to form a discordant mass ≥ 1 km across. They are matrix- to clast-supported, typically lack layering, and contain angular clasts up to > 1 m across of different textural types of microgranite, rhyolite, diabase and basalt. The breccia matrix partly comprises intermixed felsic and mafic lithic and crystal debris ranging in size down to irresolvable dust, but variably vesicular, originally glassy sideromelane and tachylite basalt pyroclasts with angular to fluidal shapes are also present. The characteristics of these juvenile pyroclasts point to phreatomagmatic disruption of basalt magma driven by explosive release of magmatic volatiles combined with heating of external water to steam. We infer that the breccias fill the diatreme feeder conduit of a basaltic maar-type volcano excavated within the rhyolite sequence and generated when rising mafic magma intruded groundwater-rich parts of the rhyolite volcanic pile, leading to violent subsurface explosions. Work by Puckett et al. (2012) shows that basaltic phreatomagmatic pyroclastic deposits are interbedded with lavas in the subsurface northwest of the Arbuckle outcrops, suggesting that phreatomagmatic processes may have played an important role in the evolution of parts of the southern Oklahoma volcanic field.