Journal of Volcanology and Geothermal Research 181 (2009) 155–172 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores Nonexplosive and explosive magma/wet-sediment interaction during emplacement of Eocene intrusions into Cretaceous to Eocene strata, Trans-Pecos igneous province, West Texas Kenneth S. Befus a,⁎, Richard E. Hanson a, Daniel P. Miggins b, John A. Breyer a, Arthur B. Busbey a a Department of Geology, Texas Christian University, Box 298830, Fort Worth, TX 76129, USA b U.S. Geological Survey, Denver Federal Center, Box 25046, Denver, CO 80225, USA article info abstract Article history: Eocene intrusion of alkaline basaltic to trachyandesitic magmas into unlithified, Upper Cretaceous Received 16 June 2008 (Maastrichtian) to Eocene fluvial strata in part of the Trans-Pecos igneous province in West Texas produced Accepted 22 December 2008 an array of features recording both nonexplosive and explosive magma/wet-sediment interaction. Intrusive Available online 13 January 2009 complexes with 40Ar/39Ar dates of ~47–46 Ma consist of coherent basalt, peperite, and disrupted sediment. Two of the complexes cutting Cretaceous strata contain masses of conglomerate derived from Eocene fluvial Keywords: deposits that, at the onset of intrusive activity, would have been N400–500 m above the present level of phreatomagmatism peperite exposure. These intrusive complexes are inferred to be remnants of diatremes that fed maar volcanoes during diatreme an early stage of magmatism in this part of the Trans-Pecos province. Disrupted Cretaceous strata along Trans-Pecos Texas diatreme margins record collapse of conduit walls during and after subsurface phreatomagmatic explosions. 40Ar/39Ar geochronology Eocene conglomerate slumped downward from higher levels during vent excavation. Coherent to pillowed basaltic intrusions emplaced at the close of explosive activity formed peperite within the conglomerate, within disrupted Cretaceous strata in the conduit walls, and within inferred remnants of the phreatomag- matic slurry that filled the vents during explosive volcanism. A younger series of intrusions with 40Ar/39Ar dates of ~42 Ma underwent nonexplosive interaction with Upper Cretaceous to Paleocene mud and sand. Dikes and sills show fluidal, billowed, quenched margins against the host strata, recording development of surface instabilities between magma and groundwater-rich sediment. Accentuation of billowed margins resulted in propagation of intrusive pillows into the adjacent sediment. More intense disruption and mingling of quenched magma with sediment locally produced fluidal and blocky peperite, but sufficient volumes of pore fluid were not heated rapidly enough to generate phreatomagmatic explosions. This work suggests that Trans-Pecos Texas may be an important locale for the study of subvolcanic phreato- magmatic processes and associated phenomena. Eocene intrusions in the study area underwent complex interactions with wet sediment at shallow levels beneath the surface in strata as old as Maastrichtian, which must have remained unlithified and rich in pore water for ~20–25 Ma. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Little previous work has been carried out on physical interactions between these intrusions and their host sedimentary sequences. Trans-Pecos Texas contains large volumes of Cenozoic igneous Here we describe mafic to intermediate Eocene intrusions that rock emplaced during arc magmatism and subsequent Basin-and- exhibit a spectrum of features formed when magma penetrated Range extension (Fig. 1; Henry et al., 1991; White et al., 2006). unlithified fluvial strata ranging in age from Cretaceous to Eocene. Widespread felsic extrusive units in the province have received most Nonexplosive interaction between magma and sediment rich in pore attention from volcanologists, but mafic to intermediate rocks are water led to formation of intrusive pillows and different types of also common and include abundant hypabyssal intrusions (e.g., peperite. Explosive interactions produced complex diatremes Maxwell et al., 1967; Henry and McDowell, 1986; Henry et al., 1989). inferred to represent the conduits of small phreatomagmatic volcanoes now eroded away. Such features are of interest because they shed light on processes occurring in the subsurface prior to and during explosive hydrovolcanism, at levels not accessible by direct ⁎ Corresponding author. observation of active maars and tuff rings. Also, they may provide E-mail address: [email protected] (K.S. Befus). constraints on paleohydrological and paleoclimatic conditions 0377-0273/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2008.12.017 156 K.S. Befus et al. / Journal of Volcanology and Geothermal Research 181 (2009) 155–172 Fig. 1. Extent of Cenozoic igneous rocks in Trans-Pecos Texas, modified from Barker (1977). Study area indicated by star. Area of Big Bend National Park is indicated by dark grey shading (green in color version). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) during the evolution of sedimentary and volcanic sequences in non- ~48–46 Ma (Henry and McDowell, 1986; Henry et al., 1986, 1989; marine depositional settings. Miggins et al., 2006). These early phases of magmatism preceded emplacement of voluminous felsic rocks throughout the province 2. Geological framework beginning at ~38 Ma (Henry and McDowell, 1986). Henry et al. (1991) inferred that subduction-related magmatism within the province gave Initial stages of Cenozoic igneous activity in the Trans-Pecos way to Basin-and-Range extensional magmatism at ~31 Ma. More province define the eastern edge of the Cordilleran magmatic arc recently, White et al. (2006) suggested that much of the Cenozoic (Barker, 1987; Price et al., 1987; Gilmer et al., 2003). A felsic stock in magmatism prior to ~27 Ma in Trans-Pecos Texas was related to roll- the western part of the province was emplaced in the Paleocene at back or break-off of the subducted slab, followed by lithosphere ~64 Ma (Gilmer et al., 2003), but the oldest known Cenozoic igneous delamination during initial stages of continental rifting. Igneous rocks rocks elsewhere in the region are Eocene and have isotopic dates of emplaced throughout the Cenozoic in the Trans-Pecos province, Fig. 2. Geologic map of study area. General geology after Horton (2006). Thickness of dikes not to scale. Boundary of Big Bend National Park is indicated; rest of area lies within privately owned Pitcock Rosillos Ranch. K.S. Befus et al. / Journal of Volcanology and Geothermal Research 181 (2009) 155–172 157 including those emplaced during early stages of arc-related magma- Table 1 tism, are characterized by alkaline, partly silica-undersaturated Chemical analyses and CIPW norms of dikes (A9b, A10a, D10), Cottonwood Wash intrusion (C15), and other discordant intrusions (A2, B6, C8) compositions (Barker, 1987; Price et al., 1987). In this paper we focus on Eocene hypabyssal intrusions present on Sample the southern flank of the Rosillos Mountains in the southeastern part A9b A10a D10 C15 A2 B6 C8 of the Trans-Pecos province, adjacent to and partly within Big Bend SiO2 48.28 48.26 56.01 53.16 45.62 48.16 41.14 National Park (Fig. 2). The study area is partly covered by Plio– TiO2 2.32 2.29 1.64 2.14 3.94 2.95 4.40 Pleistocene alluvial fan deposits shed from the Oligocene Rosillos Al2O3 18.54 18.50 17.37 17.55 17.00 18.56 18.72 FeO 11.52 11.83 8.53 9.77 11.67 11.47 15.18 Mountains quartz syenite laccolith. Cretaceous and Paleocene fluvial MnO 0.28 0.25 0.15 0.19 0.26 0.15 0.53 strata exposed beneath the fan deposits are assigned to the Aguja, MgO 3.18 4.46 1.90 2.43 4.87 3.68 4.37 Javelina, and Black Peaks Formations (Fig. 3; Horton, 2006), following CaO 8.89 7.11 4.74 6.14 10.83 9.42 10.02 the terminology of Maxwell et al. (1967) and Lehman (1988, 1991). Na2O 5.78 6.07 5.32 5.00 3.29 4.00 3.80 The Aguja Formation contains restricted outcrops of near-vent K2O 0.11 0.15 3.50 2.59 1.10 1.08 0.26 P2O5 1.10 1.09 0.83 1.02 1.42 0.52 1.58 Cretaceous intraplate basaltic phreatomagmatic deposits (Breyer LOI 7.27 7.24 1.54 2.21 3.59 2.48 7.92 et al., 2007), but there is no other evidence for local volcanism Suma 91.05 90.98 97.42 95.74 95.52 95.65 89.60 until the Eocene. The Javelina Formation and succeeding units Zr 323 319 480 325 197 202 201 were deposited within the intermontane Tornillo basin, which was Nb 48 46 60 47 33 25 35 Y38414338352733 delimited by regions that underwent uplift during latest Cretaceous Q ––––––– to Paleogene Laramide deformation (Lehman, 1991). Vertebrate or 0.65 0.89 20.68 15.31 6.50 6.38 1.54 fossils generally provide good age control on stratigraphic units ab 36.22 37.64 45.02 41.79 24.39 28.62 19.80 within the Tornillo basin (Fig. 3; Lehman,1985; Schiebout et al.,1987; an 24.32 22.79 13.18 17.79 28.37 29.50 33.25 – Lehman, 1991; Lehman and Coulson, 2002). ne 6.87 7.44 0.28 1.87 2.83 6.69 fi di 10.69 4.51 4.18 1.10 13.29 11.59 5.19 Ma c to intermediate intrusions in the study area include a series hy ––1.08 ––– – of east–northeast-trending, generally subvertical dikes (Fig. 2), as well ol 13.80 18.84 10.08 12.34 13.05 12.94 19.54 as less common sills and inclined sheets. Two discordant intrusive mt 3.36 3.32 2.38 3.10 5.71 4.27 6.39 masses with complex internal makeup occur in the southwestern part il 4.41 4.35 3.11 4.06 7.48 5.60 8.36 ap 2.54 2.52 1.93 2.36 3.30 1.21 3.65 of the study area (“western intrusive complexes” in Fig.