A NEWLY DISCOVERED EOCENE(?) BASALTIC PHREATOMAGMATIC VENT COMPLEX IN BIG BEND NATIONAL PARK, TRANS-PECOS TEXAS

Within Big Bend National Park, a prominent hill occurs just east of the park road between Exhibit Ridge and Hannold Hill. According to mapping by previous workers, fluvial strata of the Hannold Hill Fm crop out on the lower flanks of the hill and are overlain by Canoe Fm, with small mafic intrusions cutting the latter unit. Both formations are Eocene, but the Canoe Fm contains abundant felsic tuff; similar tuffs are not present in the underlying strata. Our recent work shows that the hill consists of a basaltic vent complex up to ~300 m wide with a roughly elliptical outline and a chaotic internal structure; no intact Canoe Fm is present. Large parts of the vent are filled by masses of basaltic lapillistone and lapilli-tuff (Facies A) comprising angular to fluidal, moderately vesicular, originally glassy pyroclasts now altered to palagonite and intermixed with terrigenous sand grains and disaggregated Canoe tuff. These characteristics indicate a phreatomagmatic origin for the pyroclastic debris. Facies A shows steeply dipping contacts against the Hannold Hill Fm and, at higher levels, surrounds tabular rafts of Canoe felsic tuff that are up to 60 m long and show variable dips. In some rafts, the tuff is depositionally overlain by basaltic agglomerate and lapilli-tuff (Facies B) up to 5 m thick. Facies B contains scoriaceous basaltic bombs and lapilli thoroughly intermixed with terrigenous mud, recording ejection of juvenile pyroclasts together with unlithified sediment. This facies is inferred to represent layers of phreatomagmatic ejecta that accumulated on the rim of the vent and then slumped inward during continued explosive activity. Bodies of coherent basalt up to 80 m across intrude the vent fill and connect with smaller tongues of basalt that penetrate Facies A in an intricate manner, recording continued injection of magma as explosive activity ceased.

We infer that the vent developed when phreatomagmatic explosions were generated as rising basaltic magma encountered unlithified, groundwater-rich sediment at shallow levels in the Eocene succession. The vent formed either during or after accumulation of the Canoe Fm and may be the same age as recently discovered Eocene phreatomagmatic vents farther north in the Big Bend area. Ar/Ar dating on basalt from the vent is in progress in order to test this hypothesis.