<<

Available online at www.sciencedirect.com

Journal of Volcanology and Geothermal Research 173 (2008) 245–264 www.elsevier.com/locate/jvolgeores

Cretaceous basaltic phreatomagmatic volcanism in West Texas: Maar complex at Peña Mountain, Big Bend National Park ⁎ K.S. Befus a, , R.E. Hanson a, T.M. Lehman b, W.R. Griffin c

a Department of Geology, Texas Christian University, Box 298830, Fort Worth, TX 76129, USA b Department of Geosciences, Texas Tech University, Box 41053, Lubbock, TX 79409, USA c Department of Geosciences, University of Texas at Dallas, Box 830688, Richardson, Texas 75083, USA Received 23 March 2007; accepted 25 January 2008 Available online 10 March 2008

Abstract

A structurally complex succession of basaltic pyroclastic deposits produced from overlapping phreatomagmatic volcanoes occurs within Upper Cretaceous floodplain deposits in the Aguja Formation in Big Bend National Park, West Texas. Together with similar basaltic deposits recently documented elsewhere in the Aguja Formation, these rocks provide evidence for an episode of phreatomagmatic volcanism that predates onset of arc magmatism in the region in the Paleogene. At Peña Mountain, the pyroclastic deposits are ≥70 m thick and consist dominantly of tabular beds of lapillistone and lapilli containing angular to fluidal pyroclasts of altered intermixed with abundant accidental terrigenous detritus derived from underlying Aguja sediments. Tephra characteristics indicate derivation from phreatomagmatic explosions involving fine- scale interaction between and sediment in the shallow subsurface. Deposition occurred by pyroclastic fall and base-surge processes in near-vent settings; most base-surge deposits lack tractional sedimentary structures and are inferred to have formed by suspension sedimentation from rapidly decelerating surges. Complexly deformed pyroclastic strata beneath a distinct truncation surface within the succession record construction and collapse of an initial , followed by a shift in the location of the conduit and excavation of another maar crater into Aguja strata nearby. Preserved portions of the margin of this second crater are defined by a zone of intense soft-sediment disruption of pyroclastic and nonvolcanic strata. U–Pb isotopic analyses of zircon grains from three basaltic bombs in the succession reveal the presence of abundant xenocrysts, in some cases with ages N1.0 Ga. The youngest concordant analyses for all three samples yield a weighted mean age of 76.9±1.2 Ma, consistent with the presence of Late Campanian vertebrate in the upper Aguja Formation. We infer that the volcanism is related to the intraplate Balcones igneous province, which was emplaced in the same time frame in a marine setting farther east. © 2008 Elsevier B.V. All rights reserved.

Keywords: phreatomagmatism; Aguja Formation; Trans-Pecos Texas; Balcones igneous province; Cretaceous volcanism

1. Introduction strata in the eastern part of the Trans-Pecos province. These strata significantly predate the onset of arc volcanism in the The Trans-Pecos province of Texas is well known in the region, leading Breyer et al. (2007) to suggest that the near-vent literature for the extensive Cenozoic igneous rocks present in basaltic deposits represent a westward extension of the the region (Fig. 1), which were emplaced during arc magmatism intraplate Balcones igneous province, which comprises a large and subsequent Basin-and-Range extension (Price et al., 1987; number of igneous centers active in the Late Cretaceous farther Henry et al., 1991; White et al., 2006). Recently, Breyer et al. to the east (Fig. 2). Here we report a second, better exposed and (2007) documented restricted outcrops of near-vent basaltic more complex succession of proximal Upper Cretaceous phreatomagmatic pyroclastic deposits within Upper Cretaceous basaltic pyroclastic deposits in Trans-Pecos Texas, at Peña Mountain in Big Bend National Park. The succession provides the opportunity to examine in detail the style of eruptions and ⁎ Corresponding author. the nature of source vents associated with a hitherto unknown E-mail address: [email protected] (K.S. Befus). Cretaceous volcanic episode in the Trans-Pecos region.

0377-0273/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2008.01.021 246 K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264

2. Geological framework

Voluminous magmatism occurred in the Trans-Pecos province primarily at 48–16 Ma (Henry et al., 1991), although one plutonic complex in the western part of the province has yielded isotopic ages as old as 64 Ma (Gilmer et al., 2003). Igneous rocks emplaced during the early part of this time frame define the easternmost extent of the Cordilleran magmatic arc, which swept eastward in the Cretaceous and Paleogene. Arc volcanism was widespread in and western in the Cretaceous (Fig. 2), but the arc front did not reach the Trans-Pecos region until the Paleogene (Damon et al., 1981; Henry et al., 1991). Prior to the work of Breyer et al. (2007), the only known Cretaceous volcanic rocks in the region were distal felsic tuffs presumably derived Fig. 2. Regional context of Late Cretaceous volcanism in Texas, modified from from arc volcanoes farther west (Lehman et al., 2006). Breyer et al. (2007). Position of Ouachita structural front is from Thomas et al. Both the basaltic pyroclastic deposits reported by Breyer et al. (1989). Location of volcanoes in Balcones igneous province is from Luttrell (2007) and those described herein occur in the Upper Cretaceous (1977). Leading edge of magmatic arc in Mexico is from Damon et al. (1981). Aguja Formation, within the upper shale member (Fig. 3), an Late Campanian shoreline is from Atchley et al. (2004). informally named unit at the top of the formation that is exposed near and within Big Bend National Park (Lehman, 1985, 1989, are exposed in the fault block and comprise interbedded 1991). The Aguja Formation has a maximum thickness of pyroclastic fall and base-surge deposits containing a high ∼300 m. Lower parts of the unit consist dominantly of marine to proportion of terrigenous detritus intermixed with juvenile paralic and mudstone derived from source regions basaltic tephra. Impact sags occur beneath coarse basaltic farther west. The upper shale member was deposited at the end of pyroclasts and beneath blocks of sedimentary rock up to 1 m a major regression associated with final progradation of the across derived from underlying strata. The pyroclastic deposits strandline eastward across the region. The lower part of the are inferred to have accumulated in a near-vent setting on the member accumulated in deltaic coastal marsh and coastal flood- flanks of a small maar or tuff ring; the source vent is not exposed. plain environments, whereas the upper part was deposited in an The deposits are gradationally overlain by fossiliferous strata inland floodplain (Lehman, 1985, 1986). Abundant paleosol that appear to have accumulated in a small or pond and horizons within the upper part of the member record alternating contain abundant turtle fossils. The assemblage of turtle genera humid and semi-arid conditions (Lehman, 1989; Atchley et al., present is found elsewhere in the Big Bend region only in the 2004). In situ tree trunks suggest that the floodplains at upper shale member of the Aguja Formation (Tomlinson, 1997). times supported tropical evergreen forests (Lehman and Wheeler, 2001). 3. Basaltic phreatomagmatic deposits at Peña Mountain The basaltic pyroclastic deposits described by Breyer et al. (2007) occur in a small, fault-bounded block ∼250 m across 3.1. General features within a structurally complicated area on the privately owned Pitcock Rosillos Ranch, adjacent to the northern part of Big Peña Mountain is a prominent ridge in the southwestern part Bend National Park. Approximately 15 m of pyroclastic strata of Big Bend National Park (Fig. 4), ∼40 km southwest of the

Fig. 1. Locations of near-vent Cretaceous basaltic pyroclastic deposits in Trans- Pecos Texas (A=Pitcock Rosillos Ranch; B=Peña Mountain). Extent of Fig. 3. Stratigraphic framework of Aguja Formation in Big Bend region. Cenozoic igneous rocks is from Barker (1977); area of Big Bend National Park Thicknesses are not to scale. Modified from Lehman (1985) and Lehman and is indicated. Coulson (2002); time scale is from Gradstein et al. (2004). K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264 247

Fig. 4. Geological map of Cretaceous basaltic pyroclastic deposits on Peña Mountain, from Lehman (unpublished mapping) and Befus (2006). Location is shown in Fig. 1. pyroclastic deposits reported by Breyer et al. (2007). The ridge is These nonvolcanic fluvial strata include banded purple and grey capped by an Oligocene syenodiorite intrusion inferred by overbank mudstones exhibiting paleosol horizons, with less Carman (1994) to be a thick . The western part of the sill common channel or sheet up to several meters thick. intrudes terrigenous strata within the upper shale member of the Lag deposits within the channels contain abundant pebbles of Aguja Formation. The eastern part of the sill intrudes previously reworked pedogenic carbonate derived from of paleosol undescribed basaltic pyroclastic deposits ≥70 m thick that are horizons. On the eastern side of the mountain, the pyroclastic exposed on the northern and southern flanks of the mountain. beds form an eastward-thinning tongue underlain and overlain Contact metamorphism is only noticeable within a few meters of by fluvial strata of the upper shale member. the intrusive contact, and there is no evidence that emplacement The lower part of the pyroclastic succession on the northern of the sill caused significant deformation of the country rocks. side of Peña Mountain shows abnormally steep dips (up to 80°) The pyroclastic deposits conformably overlie typical inland and is separated from overlying, more gently dipping floodplain facies in the upper part of the upper shale member. pyroclastic strata by a distinct truncation surface (Figs. 4–6). 248 K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264

3.2. Tephra composition and characteristics

Grain-size designations and terminology of pyroclastic deposits used here in general follow White and Houghton (2006), although we retain use of the term “lapillistone” for deposits consisting dominantly of lapilli (Fisher, 1961). We also follow White and Houghton (2006) in applying pyroclastic terminology to sedimentary detritus ejected explosively during phreatomagmatic eruptions. Additional descriptive details to those given here may be found in Befus (2006). Pyroclastic deposits at Peña Mountain comprise interbedded lapilli tuff, lapillistone, and tuff. Most lapilli are ≤3cmin length, but outsized basaltic and sedimentary clasts (described more fully below) make up to 15% of some beds. Alteration of the tephra generally imparts a light yellow-grey to grey-brown color to the deposits. Armored lapilli are present, in which basaltic or sedimentary pyroclasts are coated with a layer of ash several millimeters thick (Fig. 7A), but accretionary lapilli have not been observed. Fluidal bombs and angular, subequant juvenile clasts of basalt are up to 1 m in length (Fig. 7B). Bombs contain ≤30% vesicles and in some cases show cauliflower- type margins, typical of bombs produced from phreatomag- matic eruptions (Lorenz, 1973). In thin section, the basalt contains euhedral 1–2 mm in length, which are almost completely replaced by carbonate and yellow-brown (smectitic?) clay. Coarser basaltic clasts generally have an intersertal or pilotaxitic groundmass, with microlites partly replaced by clay ±carbonate. Basaltic ash and finer lapilli (≤3 cm) typically contain abundant glass, which is altered to extremely fine- grained clay and zeolites (?) but retains the light-brown, transparent appearance of sideromelane. Coarser grained carbonate also replaces the glass in places. , a typical initial alteration product of sideromelane (e.g., Hay and Iijima, 1968), has been recognized only in minor amounts but may have been obscured by subsequent alteration. Basaltic ash and finer lapilli containing altered sideromelane have fluidal to blocky, angular shapes (Fig. 8), with margins of angular particles being defined by curviplanar fractures. Vesicularity is typically in the range 0–30%, but sparse sco- riaceous clasts are intermixed with poorly vesicular pyroclasts in some beds. Angular particles of poorly vesicular tachylite, in which plagioclase microlites are contained in a mesostasis of nearly opaque glass (Fig. 8D), are present in minor amounts (b2–3%). Fig. 5. Measured section of lower and upper pyroclastic sequences on the The basaltic tephra is intermixed with abundant accidental northern side of Peña Mountain. Location of base of section is shown in Fig. 4. sedimentary detritus, including angular to subrounded, medium Coarse clasts are shown to scale; bedding in lapilli tuff is partly schematic. Thin sand- to silt-sized monocrystalline , plagioclase, and tuff interbeds between coarser layers are not shown. orthoclase grains (Fig. 8) and medium to fine sand-sized lithic grains of felsic and intermediate ; grains of Light-grey, planar-bedded terrigenous to partly calcareous mud- polycrystalline quartz and mica-quartz schist are present in stone and siltstone form a discontinuous layer ≤2 m thick minor amounts. Lapillistones typically contain ≤15% of these deposited on the truncation surface. Fossil bivalves, gastropods, accidental grains. Lapilli tuffs and tuffs contain up to ∼80% and charophyte algae indicate a lacustrine origin for this unit. accidental grains, and in many of these beds the coarser Complex structural relations discussed more fully below indi- pyroclasts are set within an altered matrix comprising fine to cate that part of a crater margin is exposed on the southern side very fine basaltic ash intermixed with a high proportion of of the mountain. terrigenous mud (Fig. 8A). We have not attempted to estimate K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264 249

Fig. 6. View looking southwest at truncation surface (dashed) separating pyroclastic sequences on the northern side of Peña Mountain. the percentage of sediment in this finer fraction because of the layer on the southern side of the mountain (Fig. 4). A single difficulty in discerning terrigenous mud from altered basaltic ash available analysis for the Pitcock Rosillos Ranch basaltic in thin section. Sand grains of quartz and felsic volcanic rock deposits is also shown for comparison. The major-element also commonly occur as xenocrysts and xenoliths within the analyses are difficult to interpret because of the high degree of basaltic pyroclasts (Fig. 8B). alteration, which is reflected in the high CaO and LOI values. Angular to subrounded clasts ≤1 m across of light-grey The samples show significant scatter on a total alkalis versus mudstone (Fig. 9A, B) and grey to brown sandstone and silica diagram (Fig. 10), but the three Peña Mountain samples conglomerate are common within the succession. The conglom- cluster tightly in the field for subalkaline basalt in the Zr/TiO2 erate contains reworked pedogenic carbonate nodules, and some versus Nb/Y diagram. The latter result is considered more distinctive sandstone blocks contain abundant oyster shells. reliable because it is based on trace elements that are resistant to Some mudstone clasts were only partly lithified when they were disturbance during low-temperature alteration (Winchester and erupted and were flattened and disrupted upon impact, with Floyd, 1977). The sample from the Pitcock Rosillos Ranch, in partly detached portions being connected by thin, plastically contrast, plots in the field for alkaline basalt. deformed necks. One unusually large slab of mudstone 3 m long contains abundant subangular to fluidal bodies of basalt ≤10 cm 3.2.1. Interpretation in length. The fluidal basalt bodies have highly irregular margins The general low vesicularity of the basaltic tephra indicates and show quench fragmentation and fine-scale peperitic that explosive release of magmatic volatiles played only a intermixing with the mud in the ejected slab (Fig. 9C). limited role in eruptive behavior and that much of the impetus Chemical analyses of three basalt bombs are given in Table 1. for explosions came from conversion of external water to steam. Sample B4 was collected from the lower pyroclastic sequence Abundant sideromelane in the juvenile pyroclasts is consistent below the truncation surface on the northern side of Peña with rapid quenching of magma in contact with water during Mountain. Samples C29 and C30 come from a single bomb-rich phreatomagmatic interactions (e.g., Fisher and Schmincke,

Fig. 7. A. Lapilli-tuff and lapillistone layers. AL=armored lapillus in lapilli-tuff bed. Pencil points to erosional scour at base of bed, which cuts into underlying tuff and is filled with coarse lag deposit. Openwork lapillistone is visible in lower part of view. B. Irregular basaltic bomb, which fractured upon impact. Arrow points to pencil for scale. Bomb is exposed on bedding plane within lapilli tuff; loose pieces weathered from bomb are visible in foreground. 250 K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264

Fig. 8. Photomicrographs of pyroclastic deposits (plane light). Ol=altered olivine phenocrysts; V=vesicles (filled with clay in some cases). A. Tuff containing dark- colored shards of altered sideromelane intermixed with quartz and sand grains and set within a matrix rich in terrigenous mud. B. Part of a vesicular basaltic lapillus consisting of altered sideromelane and containing a polycrystalline quartz xenocryst (Q). Cc=calcite cement. C. Altered sideromelane shards (S) with angular to fluidal shapes, intermixed with abundant quartz and feldspar sand grains in tuff; interstices between grains are filled with terrigenous mud. D. Lapilli tuff, with vesicular, altered sideromelane lapilli exhibiting fluidal margins (outlined). Matrix between coarser clasts contains quartz and feldspar sand grains and interstitial mud, partly replaced by carbonate. Note angular tachylite grain (T).

1984). Sparse tachylite grains are inferred to have been derived 3.3. Depositional features from magma batches that had cooled more slowly in the conduit prior to explosive disruption. Intermixture of sideromelane and Pyroclastic deposits at Peña Mountain consist dominantly of tachylite clasts, as well as the wide range in vesicularity shown tabular beds of lapilli tuff and lapillistone ≤30 cm thick that by pyroclasts within a single deposit, probably results from are laterally continuous on the scale of individual outcrops explosive disruption of heterogeneous batches of magma in the (Fig. 11A). Representative measured sections are shown in conduit, coupled with recycling of clasts during repeated Figs. 5, 12, and 13. Lapillistones are relatively well sorted, with eruptions (Houghton and Smith, 1993). openwork texture (Fig. 7A), and are cemented by carbonate. Abundant sand- and mud-sized terrigenous detritus inter- Lapilli tuffs either lack finer ash particles and show openwork mixed with the juvenile basaltic tephra, as well as the large texture or contain abundant fine ash and are matrix-supported. number of coarser accidental sedimentary clasts, suggest that Out-sized clasts in some cases show well-defined impact phreatomagmatic explosions were generated when rising sags (Figs. 9B, 11B), indicating emplacement along ballistic magma encountered -rich strata. Many of the trajectories. mudstone clasts show evidence of being poorly consolidated Lapillistone and lapilli-tuff beds are generally either massive at the time of eruption. Xenocrysts of sand grains within the and structureless (Fig. 9B) or show normal or inverse grading. basaltic pyroclasts imply intimate, fine-scale magma-sediment Some beds exhibit internal planar bedding defined by diffuse interaction prior to or during eruption. Similar xenocrysts within changes in particle size or proportion of matrix. Tabular lapilli basaltic pyroclasts are known from other phreatomagmatic tend to be oriented parallel to bedding, and discontinuous trains of settings where magma has interacted extensively with unlithified lapilli occur in otherwise massive parts of beds. Some beds have sands (e.g., Hanson and Elliot, 1996; Lorenz et al., 2002; Ross basal erosional scour features up to 10 cm deep (Figs. 7A, 12). and White, 2006). Peperitic textures within one large mudstone Low-angle cross-stratification or undulatory or wavy bedding are block are inferred to record precursory stages in magma/wet- present in a small percentage (b10%) of beds (Figs. 12 and 14), sediment interaction in the subsurface that led to explosive particularly in their upper parts, and record development of broad, phreatomagmatism (cf. White, 1991, 1996). low-amplitude bedforms. Dips on stoss and lee sides of these K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264 251

Fig. 9. A. Accidental mudstone clasts within lapilli-tuff beds, with knife (circled) resting on largest clast; other mudstone clasts are arrowed. B. Massive lapilli tuffs separated by dark- tuff layers. Light-grey mudstone and dark-brown basalt clasts are outlined. Note impact sag (dashed) beneath one basalt clast. Hammer for scale. C. Part of a light-grey mudstone clast 3 m long exhibiting peperitic intermixing with fluidal basalt bodies contained in clast. Margin of mudstone clast against enclosing lapilli-tuff is outlined. Pencil (arrowed) for scale. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) dune-like structures are typically ≤15°. Crests are smooth and enclosing beds are rotated back to horizontal, the trunk is rounded, and are draped by overlying layers. Individual bedforms vertical. A fragment of wood lies on a bedding plane within show progressive upward growth from planar bases. the enclosing pyroclastic strata (Fig. 15), but it is unclear Interbeds of tuff 1–25 cm thick commonly separate the whether this fragment was derived from the larger trunk. coarser layers (Fig. 9B) and are massive or show diffuse planar Sparse, noncarbonized fossil wood fragments and parts of logs bedding and lamination, with less common low-angle cross- ≤30 cm long are also present in other parts of the pyroclastic stratification. Some tuff interbeds show swirled or contorted succession on both the northern and southern sides of the laminae recording soft-sediment deformation of cohesive ash mountain. not obviously related to impact sags. The lower part of the succession on the southern side of Peña 3.3.1. Interpretation Mountain contains unusually thick (4–9m),massiveto The range of depositional features is consistent with em- diffusely planar-bedded lapillistone and lapilli-tuff layers that placement of tephra from base surges and by direct fallout from exhibit basal erosional scour features (Fig. 13). Basaltic bombs eruption columns during pulsatory phreatomagmatic eruptions. and blocks and accidental blocks of sandstone and mudstone Erosional scour features, undulatory bedding, and/or low-angle are abundant in these layers and include the 3-m-long clast of cross-stratification in some lapillistone and lapilli-tuff beds mudstone shown in Fig. 9C. indicate lateral transport of tephra by turbulent, decelerating A fossil trunk of an araucarioid conifer 1.2 m tall (Fig. 15)is base surges (Moore, 1967; Fisher and Waters, 1970; Waters and enclosed by the stratigraphically lowest pyroclastic beds on the Fisher, 1971; Crowe and Fisher, 1973; Druitt, 1998; Wohletz, northwestern side of the mountain (locality PM 1.2 in Wheeler 1998). Zones of inverse grading within some beds suggest and Lehman, 2005). The trunk shows excellent preservation of development of high-particle-concentration traction carpets in microscopic textures (see Plate I-D in Wheeler and Lehman, the basal parts of surges (Sohn, 1997). Beds exhibiting typical 2005), with no evidence of carbonization. Its roots are not features of base-surge deposits include those enclosing the visible, but the lowest exposed portion of the trunk is ∼15 cm upright fossil conifer trunk shown in Fig. 15. We infer that the above the contact with Aguja floodplain mudstone. When the tree was killed by relatively weak, low-temperature surges that 252 K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264

Table 1 magmatic sequences (e.g., Sohn and Chough, 1989; White, Chemical analyses of basaltic bombs from Peña Mountain (B4, C29, and C30) 1991; Németh and White, 2003). and basaltic block from Pitcock Rosillos Ranch (A19c) Sorting in phreatomagmatic deposits is controlled in part by Sample the wetness of the tephra during eruption and deposition (e.g., B4 C29 C30 A19c Self et al., 1980; Ross, 1986; Sohn and Chough, 1989; Valentine

SiO2 46.61 48.65 54.77 43.41 and Fisher, 2000). Fines-depleted beds with openwork textures TiO2 1.48 1.55 1.61 3.54 in the Peña Mountain succession are inferred to have been Al2O3 19.35 18.15 20.93 17.66 produced from high-temperature parts of eruption columns FeO 7.30 2.59 3.84 14.29 containing dry, superheated steam, which allowed efficient MnO 0.13 0.10 0.10 0.24 MgO 3.74 1.82 3.33 4.83 winnowing of finer particles during deposition by either CaO 15.34 19.29 7.53 10.16 pyroclastic fall or base-surge processes (e.g., Druitt, 1992; Na2O 4.23 5.61 5.71 5.12 Colella and Hiscott, 1997). Cohesion of wet tephra emplaced at K2O 0.31 1.60 1.54 0.06 temperatures below the boiling point of water prevents efficient P2O5 1.50 0.64 0.64 0.70 sorting and likely explains formation of ash-rich, matrix- LOI 10.54 12.50 5.93 9.76 Sum a 88.00 85.90 92.31 86.64 supported lapilli-tuff beds within the succession (cf. Allen et al., Zr 100 79 107 178 1996). That some of the tephra was moist during or after Nb 10 9 11 35 deposition is proven by the occurrence of armored lapilli, soft- Y56363624sediment deformation of fine-grained layers, and numerous Major elements in wt.% (normalized to 100% on volatile-free basis), trace elements impact sags within the succession. The presence of noncarbo- in ppm. Major elements analyzed by XRF on fused glass beads with lithium nized wood is also consistent with low emplacement tempera- tetraborate flux. Trace elements analyzed by ICP-MS. Analyses were carried out at tures for some deposits. These inferred variations in temperature the State University Geoanalytical Laboratory, Pullman, Washington. a Total before normalization. and wetness between different units within the succession imply fluctuations in the proportions of magma and water (or wet were unable to knock it down. There is no evidence that ash was plastered against the tree, but such a feature might be difficult to recognize in ancient, lithified deposits. Little or no reworking of tephra appears to have occurred by wind or running water after initial deposition. The geometry of preserved bedforms is consistent with tractional sedimentation under conditions of high lateral bed shear stress at the base of rapidly moving pyroclastic density currents, and contrasts markedly with tractional structures found in typical eolian or fluvial deposits (e.g., Waters and Fisher, 1971; Smith and Katzman, 1991; Valentine and Fisher, 2000). Tabular, laterally continuous, massive to diffusely planar- bedded lapillistone and lapilli-tuff beds lacking basal scour features or tractional bedforms either are of pyroclastic fall origin or were deposited from high-concentration, deflating base surges in which rapid suspension sedimentation prevented tractional reworking of particles. Base-surge deposits of this type are well known in phreatomagmatic settings (Sohn and Chough, 1989; Chough and Sohn, 1990; Lajoie et al., 1992; Allen et al., 1996; Colella and Hiscott, 1997; Vazquez and Ort, 2006). The unusually thick, massive to diffusely bedded, bomb- and block-rich units in the lower part of the section shown in Fig. 13 must have been emplaced, at least in part, from turbulent pyroclastic density currents, as shown by the presence of erosional scour features at the bases of the units. These beds are inferred to have been deposited from heavily overladen base surges close to the vent, probably combined with ballistic fallout of tephra. The high proportion of blocks and bombs makes these beds similar to the explosion breccias described by Wohletz and Sheridan (1983), which are interpreted to record Fig. 10. Analyses of basaltic pyroclasts plotted on standard diagrams. A. Total initial conduit-clearing explosive eruptions during construction alkalis versus silica diagram of Le Bas et al. (1986);Bta=basaltic of phreatomagmatic volcanoes. Analogous deposits have also ; TB=tephrite and basanite; Trb=. B. Zr/TiO2 versus been described in the literature from other near-vent phreato- Nb/Y diagram of Winchester and Floyd (1977), revised by Pearce (1996). K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264 253

Fig. 11. A. Steeply dipping lapilli-tuff and lapillistone beds on the northern side of Peña Mountain, beneath truncation surface. Arrow points to notebook for scale. B. Asymmetric impact sag beneath block of Aguja conglomerate. Arrow points to knife for scale. sediment) undergoing explosive interactions in the conduit evidence for faulting along the contact. Strata within the upper (Sheridan and Wohletz, 1983; Sohn, 1996; White, 1996). sequence dip relatively gently to the southeast, consistent with Accretionary lapilli, which are common in many phreato- regional attitudes of Upper Cretaceous strata in the area. magmatic deposits (Schumacher and Schmincke, 1991), are On the southern side of Peña Mountain, a zone up to 60 m lacking in the present case. Reasons for their absence are across of intense structural disruption of original bedding unclear. In cases where the tephra is inferred to have been moist separates less disturbed pyroclastic beds from typical non- during transport and deposition, excess water may have volcanic Aguja strata to the east and south. The disrupted zone prevented formation or preservation of well-defined accretion- has an overall arcuate trend (Fig. 17A), but its margins are ary lapilli (e.g., Walker, 1981; Ross, 1986; Schumacher and locally highly irregular (Fig. 17B). The outer part of the Schmincke, 1995; Martin and Németh, 2005). Leat and disrupted zone affects nonvolcanic Aguja sandstones and Thompson (1988) have suggested that a high proportion of mudstones, in which individual beds generally can only be fine-grained accidental sedimentary detritus ejected during traced a few meters and are deformed by numerous soft- phreatomagmatic eruptions may also inhibit formation of sediment faults and small-scale folds. Locally, more resistant accretionary lapilli. sandstone units can be traced for tens of meters within the disrupted zone and help to define its overall structure. Bed- 3.4. Structural relations ding in one such unit progressively steepens and then be- comes overturned toward the contact with the pyroclastic rocks The well-defined truncation surface on the northern side of (Fig. 17A). Peña Mountain separates the pyroclastic strata into two different The inner part of the disrupted zone consists of pyroclastic sequences with similar depositional features but different strata that show comparable depositional features to other parts structural geometry. Bedding attitudes beneath the truncation of the pyroclastic succession but are intensely deformed in the surface vary markedly over distances of tens to hundreds of same manner as the adjacent nonvolcanic Aguja strata. The meters, with dips ranging up to 80° (Fig. 4). The structural contact between the nonvolcanic strata and the disrupted complexity is partly caused by large folds with irregular trends, pyroclastic beds is defined in places by a subvertical, soft- and partly by variable degrees of offset along high-angle sediment fault zone that abruptly truncates bedding on both syndepositional normal faults. In places, these faults define sides. Sandstone beds are dragged down into the fault zone and small grabens a few meters across (Fig. 5). The amount of offset form detached, meter-scale masses strung out along it. Masses decreases upward along individual faults, some of which are of nonvolcanic Aguja strata are also enclosed by disrupted filled by zones of disaggregated pyroclastic material that lacks pyroclastic strata inward from the fault. In some cases, steeply evidence of brittle cataclasis and is inferred to have been tilted pyroclastic layers rest depositionally on more strongly injected in a soft-sediment state. All of these faults terminate deformed nonvolcanic strata, indicating that some disruption of against the truncation surface (Fig. 16). A longer fault cutting Aguja strata occurred prior to deposition of the pyroclastic beds. the western part of the succession on the northern side of the The zone of disrupted pyroclastic beds is in abrupt contact mountain (Fig. 4) displaces the truncation surface and may be with a sequence ≥65 m thick of less disturbed pyroclastic strata unrelated to the Cretaceous volcanism. (Fig. 13). Bedding trends within the less disturbed pyroclastic The base of the upper sequence above the truncation surface sequence generally are subparallel to the contact with the is partly defined by the laterally discontinuous lacustrine unit disrupted zone but in places are truncated by it (Fig. 17A, B). shown in Fig. 5. Where that unit is lacking, the lowest Locally near the contact, the less disturbed strata dip into the pyroclastic beds in the upper sequence lie on the tilted lower disrupted zone, but they typically dip 25–60° to the northwest, sequence with marked angular discordance (Fig. 6); there is no with dips tending to increase upward in the sequence. 254 K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264

because neither the pyroclastic strata above the truncation surface nor laterally equivalent, nonvolcanic parts of the Aguja Formation show comparable effects. We infer that the sequence underwent tilting and soft-sediment deformation during slump- ing of unstable rim deposits into a developing crater to the south. Similar truncation surfaces resulting from collapse of parts of small volcanoes have been described by Sohn and Park (2005) in Pleistocene tuff rings and tuff cones in Korea.

Fig. 12. Detailed measured section of representative pyroclastic beds on northern side of Peña Mountain. Location is shown in Fig. 4. Coarse clasts are shown to scale.

3.4.1. Interpretation An interpretative cross section of the pyroclastic deposits exposed at Peña Mountain is given in Fig. 18. Detailed interpretation is rendered difficult by the fact that only small parts of the succession are preserved, whereas other parts are obscured by the younger syenodiorite sill. We have therefore not attempted to calculate original volumes of the deposits or source vents. An early stage in the development of the succession is recorded by the sequence beneath the truncation surface on the Fig. 13. Measured section of less disturbed pyroclastic strata on southern side of northern side of Peña Mountain. The structural complexity Peña Mountain. Location of base of section is shown in Figs. 4 and 17B. Coarse within this sequence is not the result of tectonic deformation clasts are generally shown schematically, as are beds in upper 35 m. K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264 255

In order to complement the existing paleontological con- straints on the age of the upper shale member, U–Pb isotopic analyses were conducted on zircon separated from the same three basaltic bombs within the Peña Mountain pyroclastic succession that yielded the chemical analyses shown in Table 1. Zircon was extracted from the samples using standard crushing and -separation techniques at the University of Texas, Dallas. Hand-picked zircon grains were analyzed using the sensitive high mass resolution ion microprobe with reverse geometry (SHRIMP-RG) operated by the U.S. Geological Survey and Stanford University. Analytical techniques are given in Appendix A, and results are shown in Table 2. Analyzed zircon grains in all three samples are typically euhedral, with dipyramidal terminations and concentric zona- tion typical of igneous zircon; inherited cores were not observed (Fig. 19). Most grains yielded concordant or nearly concordant analyses representing a wide range in crystallization ages, in some cases N1.0 Ga (Fig. 20A, C, E). These old grains are clearly xenocrysts. The two youngest concordant analyses for sample B4 (Fig. 20B) have overlapping 2σ error ellipses and yield a weighted mean age of 77.9±1.4 Ma, within error of the age for the youngest concordant analysis (76.8±2.0 Ma) for sample C29 (Fig. 20D). The four youngest concordant analyses σ Fig. 14. Lapillistone, lapilli-tuff, and tuff layers near location of fossil conifer for sample C30 also have overlapping 2 error ellipses and trunk on northwestern side of Peña Mountain. Massive lapillistone beds in upper yield a weighted mean age of 76.3±2.1 Ma. An older part of view are separated by thin tuff layers. Lapilli tuff and tuff in middle part population of concordant, overlapping analyses from the same of view show low-angle cross-stratification and wavy bedding. Low-angle sample (Fig. 20F) yields a weighted mean age of 96.7±1.3 Ma. cross-stratification (dashed) is also visible in lapillistone at base of view. Pencil Interpretation of the age results for the analyzed zircon grains (arrowed) for scale. is rendered problematic by the abundance of xenocrystic grains of various ages in the samples, which have similar Th/U ratios On the southern side of the mountain, the disrupted zone and appear similar under cathodoluminescence to the grains separating the pyroclastic deposits from typical nonvolcanic yielding the youngest concordant analyses. The sources of these Aguja strata to the southeast and south (Fig. 17A, B) is inferred xenocrystic zircon grains are unclear. Older components in the to represent a partly preserved crater margin. Collapse of the zircon population may have been incorporated into the basaltic crater margin caused disruption of poorly consolidated Aguja magma during its transit through Precambrian and Paleozoic strata and adjacent pyroclastic beds. Piecemeal collapse is crust inferred to underlie the region at depth (e.g., James and recorded by highly irregular, subvertical contacts within parts of Henry, 1993). An alternate possibility is that at least some of the the disrupted zone (Fig. 17B). Less disturbed pyroclastic strata xenocrystic grains were incorporated during interaction between in contact with the disrupted zone accumulated at higher levels basaltic magma and Cretaceous terrigenous sediment that and underwent tilting during downward subsidence as the crater preceded or accompanied explosive phreatomagmatic eruptions. expanded laterally.

4. Age of the pyroclastic deposits

The lower part of the upper shale member of the Aguja Formation contains a diverse assemblage of fossil mammals and reptiles (including dinosaurs) that indicate a Late Campanian age (Rowe et al., 1992; Sankey, 2001). Fossils are generally less abundant and diagnostic in the upper part of the upper shale member (Lehman, 1985), where the pyroclastic deposits described herein occur, and that part of the member could extend into the Early Maastrichtian. The overlying Javelina Formation contains Maastrichtian vertebrate fossils (Lehman, 1985; Lehman and Coulson, 2002), and a distal felsic tuff present within the middle of that unit and inferred to be derived Fig. 15. Fossil conifer trunk (labeled) within pyroclastic strata on northwestern from the Cordilleran arc farther west has yielded a U–Pb mon- side of Peña Mountain (Fig. 4). Head of hammer to right of trunk is parallel to azite age of 69.0±0.9 Ma (Lehman et al., 2006). bedding. A fragment of wood lying on a bedding plane is also labeled. 256 K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264

Fig. 16. Soft-sediment fault zone (outlined) filled with disaggregated lapilli and ash within lower pyroclastic sequence beneath truncation surface (dashed) on northern side of Peña Mountain. Pencils (arrowed) for scale.

The youngest concordant analyses for all three Peña Mountain magma and external water (or water-rich sediments) during samples have overlapping 2σ error limits and form a distinct explosive volcanism. There is no sedimentological evidence for population separate from the older analyses. When considered large bodies of standing water in this part of the Big Bend together, the youngest concordant analyses yield a weighted region at the onset of Cretaceous phreatomagmatic activity. As mean age of 76.9±1.2 Ma (Fig. 21). One interpretation is that in many other documented examples of explosive phreatomag- these youngest grains are also xenocrysts, which would require matic volcanism in nonmarine settings, the most likely source the Peña Mountain basaltic pyroclastic deposits to be younger for the required volumes of water appears to be groundwater than ∼77 Ma. Another interpretation, which we prefer, is that the within underlying units. The high proportion of accidental weighted mean age for the distinct population of youngest zircon sedimentary detritus in the form of loose sand grains and grains represents the crystallization age of the basalt. abundant mud intermixed with the Peña Mountain pyroclastic U–Pb isotopic analyses obtained in the same laboratory were deposits points to generation of explosions within unlithified, reported by Breyer et al. (2007) for zircon extracted from a groundwater-rich sediments at shallow levels in the subsurface juvenile basaltic block in the Cretaceous pyroclastic sequence (e.g., Leat and Thompson, 1988; White, 1991; Hanson and on the Pitcock Rosillos Ranch. These data also revealed the Elliot, 1996; Lorenz, 2003; McClintock and White, 2006; Ross presence of numerous xenocrysts. Accidental blocks of fine- and White, 2006). Types and relative proportions of these grained felsic -vitric tuff contained within the basaltic accidental grains are similar to those found in sandstones in deposits are one possible source for the zircon xenocrysts. The underlying parts of the Aguja Formation, which contain a high youngest concordant analyses from the sample yielded a proportion of detritus derived from the Cretaceous weighted mean age of 72.6±1.5 Ma, which was inferred to farther west (Lehman, 1985, 1991; Bohanan, 1987). Most represent the crystallization age of the basalt. Both this result coarser accidental sedimentary clasts in the Peña Mountain and our new data for the Peña Mountain deposits are consistent pyroclastic deposits can be matched with lithologies within with the available paleontological constraints and with the time floodplain deposits of the upper shale member, and it is likely scale of Ogg et al. (2004), who place the upper boundary of the that subsurface phreatomagmatic explosions were initiated at Campanian at 70.6±0.6 Ma. Considered together, the paleon- shallow levels in that unit. The upper shale member is tological and geochronological evidence indicates that basaltic composed dominantly of impermeable mudstones, with a volcanism at Peña Mountain and farther to the northeast in the subordinate amount of sandstones that could have provided area of the Pitcock Rosillos Ranch occurred in a relatively adequate flow of groundwater. Following White (1991, 1996), narrow time frame within the Late Cretaceous. we infer that water-rich mud rather than free water acted as a major coolant during explosive subsurface fuel-coolant inter- 5. Discussion actions. Peperitic textures in the erupted mudstone slab shown in Fig. 9C document initial stages in these processes. 5.1. Role of subsurface phreatomagmatic explosive activity The base of the upper shale member is exposed ∼1.5 km west of Peña Mountain, and stratigraphic sections given in Textural characteristics of the tephra in the Peña Mountain Lehman (1985) indicate that the nonvolcanic part of the pyroclastic deposits indicate interaction between basaltic member beneath the pyroclastic succession is 100–140 m thick K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264 257

would require correcting present stratigraphic thicknesses for compaction. Distinctive sandstone blocks rich in oyster fossils must have been derived from the marine Rattlesnake Mountain Sandstone Member of the Aguja Formation, which is the only unit within the Cretaceous stratigraphic sequence in the region that contains large numbers of oyster fossils (Lehman, 1985). This unit is estimated to occur ∼20 m beneath the base of the upper shale member in the Peña Mountain area, based on projection of thickness trends in the Aguja Formation. The locus of explosive activity may have migrated downward as ground- water was exhausted at shallow depths, as is inferred for many subaerial phreatomagmatic volcanoes (Lorenz, 1986). How- ever, we have not observed consistent vertical variations in composition of accidental sedimentary debris within the Peña Mountain pyroclastic succession that would provide direct support for such a model.

5.2. Nature and evolution of source vents

Abundant coarse ballistic clasts within the Peña Mountain pyroclastic deposits indicate accumulation in near-vent settings, which is consistent with the general scarcity of tractional current structures in base-surge deposits within the succession. Several studies of lateral facies relations in outflow surge deposits from phreatomagmatic volcanoes have shown that massive beds are common in proximal settings (typically within a few hundred meters of the vent) and transform downcurrent to beds showing increasing evidence for tractional sedimentation, recording progressive dilution of the surges and decrease in rate of fallout of particles from suspension (e.g., Sohn and Chough, 1989; Chough and Sohn, 1990; Lajoie et al., 1992; Vazquez and Ort, 2006). Preservation of the upright fossil conifer trunk within surge deposits on the northwestern side of Peña Mountain is somewhat surprising given the proximal setting and suggests rapid lateral variations in surge energy during pulsatory phreatomagmatic explosions. The gross morphology and internal depositional features of the pyroclastic sequences above and below the truncation surface on the northern side of Peña Mountain are consistent with deposition in overlapping tephra aprons rimming tuff rings or maars, the most common types of subaerial monogenetic phreatomagmatic volcanoes (Wohletz and Sheridan, 1983; Cas and Wright, 1987). Field relations on the southern side of the mountain (Fig. 17A, B) reveal the presence of a typical maar volcano, in which the crater floor was below the ground surface and was excavated into pre-existing strata (Lorenz, 1973, 1986), with downward subsidence of rim deposits into the developing crater. The Peña Mountain pyroclastic deposits provide a case study Fig. 17. Tape-and-compass maps showing characteristic features developed for recognition of ancient, partly preserved maar sequences in along contact between pyroclastic deposits and nonvolcanic Aguja strata on the stratigraphic record and illustrate the structural complica- southeastern side of Peña Mountain. Locations are shown in Fig. 4. A. Arcuate part of contact, with zones of strongly disrupted strata. B. Part of contact tions that may result from synvolcanic collapse or subsidence of showing highly irregular geometry. Note difference in scale between figures. parts of the volcanoes. Similar collapse of small phreatomag- matic volcanoes has been well documented in other studies in the area of concern here. This estimate provides some (e.g., Gutmann, 1976; Sohn and Park, 2005) and may result indication of possible depths at which subsurface phreatomag- from instability of the substrate, coupled with removal of matic explosions occurred, although a more rigorous estimate support due to lateral expansion of crater margins. Sohn and 258 K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264

Fig. 18. Interpretative cross section of pyroclastic rocks at Peña Mountain, along line A–A′ in Fig. 4. No vertical exaggeration.

Park (2005) have pointed out that such collapse processes are Late Campanian (to possibly Early Maastrichtian) age for the likely to be common where the substrate consists of poorly pyroclastic deposits at both localities. Monogenetic volcanoes of consolidated, mechanically weak sediments. In the present case, the type described here commonly occur in clusters (Connor and volcano collapse appears to have resulted from construction of Conway, 2000), making it likely that additional examples of the Late Cretaceous maar-type vent complexes within and upon same age remain to be discovered in the region. unconsolidated floodplain sediments in the upper shale member Our new data from Peña Mountain are consistent with the of the Aguja Formation. suggestion of Breyer et al. (2007) that Cretaceous basaltic One possible scenario for the evolution of the Peña Mountain pyroclastic deposits in the Big Bend region represent a western pyroclastic succession is shown in Fig. 22, in which initial extension of the intraplate Balcones igneous province. As formation of a maar or tuff ring was followed by collapse of the presently defined in the literature (Barker et al., 1987; Byerly, crater margins and large-scale slumping to generate the 1991; Ewing, 2004), the Balcones province comprises several deformed sequence beneath the truncation surface on the hundred individual small volcanoes and/or associated intrusions, northern side of Peña Mountain. Erosion of the slumped beds which occur both in outcrop and in the subsurface and are aligned was followed by deposition of the overlying lacustrine unit along the frontal zone of the largely buried, Late Paleozoic during a period of volcanic quiescence. The exposed succession Ouachitaorogenicbelt(Fig. 2). Rock compositions are to the south represents a higher structural level than the dominantly melilite and nephelinite, with less truncation surface to the north and is inferred to record abundant , , and basanite (Spencer, 1969; lateral migration of the conduit during an episode of renewed Barker et al., 1987). Different volcanic centers or intrusions have explosive volcanism. Subsidence of pyroclastic deposits formed yielded 40Ar/39Ar ages of ∼82–72 Ma (Miggins et al., 2004; during this second episode juxtaposed them with disrupted Griffin et al., 2005), as well as a more limited number of U–Pb strata along the new crater margin. Pyroclastic beds overlying zircon ages of ∼86–77 Ma (Griffin et al., 2005). The overall age the truncation surface on the northern side of the mountain may range of the province is best constrained by stratigraphic relations equate with part of the succession exposed to the south, as between the volcanoes and adjacent fossiliferous Upper Cretac- implied in Fig. 22, but could also have originated from another eous carbonate units. This biostratigraphic evidence shows that nearby vent. igneous activity spanned the Cenomanian through Maastrichtian (i.e., much of the Late Cretaceous), with the largest percentage of 5.3. Regional considerations the magmatism occurring in the Campanian (Byerly, 1991; Ewing, 2004). The similarity in timing between the peak in Upper Cretaceous near-vent basaltic successions erupted Balcones igneous activity and basaltic volcanism in the Big Bend from small phreatomagmatic volcanoes have now been region farther to the west suggests a direct relation. documented from two separate localities in the Big Bend region In the main part of the Balcones province, volcanism of West Texas. Although U–Pb zircon geochronological data for occurred in marine environments and constructed subaqueous the deposits can be interpreted in various ways, it should be to emergent Surtseyan tuff cones, in which explosive phreato- stressed that the available paleontological evidence indicates a magmatism was driven by interaction of ultramafic and Table 2 – SHRIMP-RG U–Pb isotopic analyses of zircon in basaltic bombs from Peña Mountain 207 204 204 207 Pb-corrected 204 Pb-corrected 204 Pb-corrected 232 238 206 238 σ 207 206 σ 238 206 207 206 207 235 Grain Comm. UTh232 Th/238 U 206 Pb/238 U 1σ 207 Pb/206 Pb 1σ Total % Total % 238 U/206 Pb % 207 Pb/206 Pb % 207 Pb/235 U% Error corr. 206 (ppm) (ppm) age error age error 238 206 error 207 206 error error error error coeff.a No. 206 Pb (ppm) (ppm) age error age error 238 U/206 Pb error 207 Pb/206 Pb error error error error coeff.a (%) (Ma) (Ma) (Ma) (Ma) B4-1 0.09 912 381 0.43 77.2 0.9 140 51 82.91 1.2 0.0483 2.2 82.84 1.2 0.0488 2.2 0.08 2.5 0.473 B4-2 −0.34 950 539 0.59 74.1 0.9 −157 73 86.85 1.2 0.0448 2.2 87.02 1.2 0.0432 2.9 0.07 3.2 0.367

B4-3 0.12 442 133 0.31 78.6 1.2 180 71 81.41 1.5 0.0485 3.1 81.29 1.5 0.0497 3.0 0.08 3.4 0.444 245 (2008) 173 Research Geothermal and Volcanology of Journal / al. et Befus K.S. B4-4 0.01 471 433 0.95 388.1 4.2 399 28 16.11 1.1 0.0545 1.3 16.11 1.1 0.0547 1.3 0.47 1.7 0.651 B4-5 0.05 299 62 0.21 1209.7 12.8 1147 16 4.86 1.1 0.0784 0.7 4.86 1.1 0.0780 0.8 2.21 1.3 0.812 B4-6 0.16 247 135 0.56 412.2 4.9 444 63 15.10 1.2 0.0572 1.7 15.13 1.2 0.0558 2.8 0.51 3.1 0.396 B4-7 0.00 423 224 0.55 358.6 4.1 377 32 17.47 1.2 0.0541 1.4 17.47 1.2 0.0541 1.4 0.43 1.8 0.641 B4-8 0.30 262 201 0.79 378.7 4.5 280 67 16.52 1.2 0.0545 1.7 16.57 1.2 0.0519 2.9 0.43 3.2 0.383 B4-9 0.00 621 105 0.18 94.1 1.2 89 60 68.01 1.3 0.0478 2.5 68.01 1.3 0.0478 2.5 0.10 2.8 0.446 B4-10 0.36 253 134 0.55 557.9 6.3 433 58 11.06 1.2 0.0586 1.4 11.10 1.2 0.0555 2.6 0.69 2.9 0.408 B4-11 0.01 327 19 0.06 1389.9 26.0 1433 12 4.15 1.9 0.0905 0.6 4.15 1.9 0.0904 0.6 3.01 2.0 0.954 B4-12 0.18 152 65 0.44 513.3 6.6 491 52 12.05 1.3 0.0586 1.9 12.07 1.3 0.0570 2.4 0.65 2.7 0.486 B4-13 0.00 330 191 0.60 87.4 1.3 87 82 73.24 1.5 0.0478 3.4 73.24 1.5 0.0478 3.4 0.09 3.7 0.389 B4-14 1.01 313 68 0.23 146.3 2.0 −317 249 43.55 1.4 0.0495 2.8 43.99 1.4 0.0405 9.7 0.13 9.8 0.144 C29-1 0.04 811 79 0.10 816.1 8.2 931 31 7.37 1.0 0.0705 1.5 7.38 1.0 0.0701 1.5 1.31 1.8 0.559 C29-2 0.13 325 166 0.53 1408.6 14.5 1621 12 4.04 1.1 0.1009 0.6 4.04 1.1 0.0998 0.7 3.40 1.2 0.852 C29-3 0.16 121 58 0.50 1685.4 20.5 1616 19 3.36 1.2 0.1009 0.9 3.36 1.2 0.0996 1.0 4.08 1.6 0.771 C29-4 0.03 369 156 0.44 587.9 6.2 564 25 10.48 1.1 0.0592 1.1 10.48 1.1 0.0589 1.1 0.77 1.6 0.687 C29-5 0.00 355 176 0.51 88.1 1.3 −40 83 72.89 1.4 0.0453 3.4 72.89 1.4 0.0453 3.4 0.09 3.7 0.384 C29-6 0.00 517 244 0.49 76.6 1.0 95 69 83.60 1.3 0.0479 2.9 83.60 1.3 0.0479 2.9 0.08 3.2 0.412 C29-7 0.00 116 39 0.35 947.2 12.3 1042 28 6.29 1.3 0.0740 1.4 6.29 1.3 0.0740 1.4 1.62 1.9 0.695 C29-8 0.00 336 834 2.56 457.5 5.0 421 30 13.61 1.1 0.0552 1.3 13.61 1.1 0.0552 1.3 0.56 1.7 0.640 C29-9 0.07 418 223 0.55 249.9 2.8 260 44 25.28 1.1 0.0520 1.7 25.29 1.1 0.0514 1.9 0.28 2.2 0.513 C29-10 0.49 294 128 0.45 273.9 3.3 76 112 23.03 1.2 0.0518 2.1 23.15 1.2 0.0475 4.7 0.28 4.9 0.256 C29-11 −0.06 1017 462 0.47 85.8 1.0 −46 52 74.89 1.1 0.0447 2.1 74.84 1.1 0.0452 2.2 0.08 2.4 0.464 C29-12 0.05 361 127 0.36 437.3 4.8 438 33 14.24 1.1 0.0560 1.4 14.25 1.1 0.0556 1.5 0.54 1.8 0.606 C30-1 0.03 1024 544 0.55 392.3 3.8 342 20 15.95 1.0 0.0536 0.8 15.96 1.0 0.0533 0.9 0.46 1.3 0.743 C30-2 0.48 517 167 0.33 74.9 1.0 −151 135 85.58 1.3 0.0475 2.9 85.99 1.3 0.0433 5.4 0.07 5.6 0.236 C30-3 0.00 116 59 0.53 1056.9 13.3 1020 26 5.62 1.3 0.0732 1.3 5.62 1.3 0.0732 1.3 1.80 1.8 0.709 C30-4 0.32 237 187 0.82 96.9 1.5 147 127 65.77 1.6 0.0518 3.7 65.98 1.6 0.0490 5.4 0.10 5.6 0.279 –

C30-5 0.18 561 363 0.67 77.6 1.6 45 85 82.51 2.1 0.0485 2.7 82.66 2.1 0.0469 3.6 0.08 4.1 0.502 264 C30-6 0.00 311 119 0.39 95.1 1.4 150 81 67.22 1.5 0.0490 3.5 67.22 1.5 0.0490 3.5 0.10 3.8 0.387 C30-7 0.62 231 88 0.39 100.5 1.6 −227 199 63.69 1.6 0.0475 3.8 64.09 1.6 0.0420 7.9 0.09 8.1 0.197 C30-8 0.45 165 92 0.58 199.9 3.0 −5 161 31.77 1.5 0.0499 3.2 31.91 1.5 0.0460 6.7 0.20 6.8 0.223 C30-9 0.79 196 106 0.56 95.0 1.6 −159 291 67.17 1.7 0.0501 4.2 67.70 1.8 0.0431 11.7 0.09 11.9 0.149 C30-10 0.00 359 165 0.47 76.3 1.1 152 90 83.84 1.5 0.0491 3.8 83.84 1.5 0.0491 3.8 0.08 4.1 0.355 C30-11 0.00 251 148 0.61 96.4 1.5 163 85 66.29 1.5 0.0493 3.6 66.29 1.5 0.0493 3.6 0.10 3.9 0.388 C30-12 1.07 218 166 0.79 274.8 3.6 −226 228 22.97 1.3 0.0514 2.3 23.22 1.4 0.0420 9.0 0.25 1.4 0.150 C30-13 0.00 671 266 0.41 133.9 1.5 84 46 47.71 1.1 0.0477 1.9 47.71 1.1 0.0477 1.9 0.14 1.1 0.504 C30-14 −1.28 163 79 0.50 76.9 1.5 673 259 82.98 1.9 0.0509 5.2 81.93 2.1 0.0620 12.1 0.10 2.1 0.170 C30-15 3.11 343 97 0.29 107.3 1.5 −540 546 58.45 1.3 0.0648 2.6 60.32 1.6 0.0372 20.3 0.09 1.6 0.078 aError correlation coefficient. 259 260 K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264

the first author by the U.S. Department of the Interior. The manuscript benefited from thorough reviews by Ian Skilling and an anonymous reviewer.

Appendix A

U–Pb zircon isotopic analyses were performed in 2005 by W.R. Griffin at the SHRIMP-RG facility co-operated by the U.S. Geological Survey and Stanford University. Zircon grains were mounted in epoxy, ground and polished to a 1 μm finish to reveal quasi-equatorial sections through the grains, and imaged via cathodoluminescence on a JEOL 5600LLV scanning electron microscope and via transmitted light on an optical microscope prior to and following analysis. Grain mounts were washed with a saturated EDTA solution, rinsed in distilled water, dried in a vacuum oven, and coated with gold. The primary ion beam of the SHRIMP-RG instrument was first rastered for 120 s over the analytical spot to remove a small region of the gold coat and any surface contamination. The beam was then focused for an analysis time of 12 min, which typically produced an ablation spot 20–40 μm wide and 1– μ Fig.19. Cathodoluminescence images of zircon grains yielding youngest con- 2 m deep. Secondary ions were generated from the target spot 2− cordant analyses for samples B4, C29, and C30. Circles indicate analyzed spots. with an O primary ion beam at 4–6 nA. Nine peaks were measured sequentially for each zircon spot. Concentration data for unknown zircon grains were calibrated against the R33 with sea water (Ewing and Caran, 1982; Ewing, 2004). zircon standard [419 Ma, quartz diorite of Braintree Complex, The style of volcanism is different in the inferred extension of Vermont (Black et al., 2004)]. the province in West Texas, where the vents were located in Data reduction followed the methods described by Williams low-relief floodplain settings far west of the Late Cretaceous (1997) and Ireland and Williams (2003) and was performed by shoreline (Fig. 2). This environment favored formation of maar- J. Wooden at the USGS/Stanford lab using SQUID 1.02 type volcanoes fed by explosive interactions between uprising software (Ludwig, 2001). Correction for common lead was basaltic magma and groundwater-rich sediment in the shallow performed by fitting a model age for the sample, based on the subsurface. The resulting pyroclastic deposits are characterized uncorrected data, to the lead evolution model of Stacey and by intermixture of juvenile basaltic pyroclasts with acciden- Kramers (1975) and applying that to the uncorrected data. tal terrigenous debris derived from underlying parts of the Analysis points were plotted and ages calculated using Isoplot/ Cretaceous sequence. Ex 3.0 (Ludwig, 2000). Based on limited trace-element data presented herein, the Peña Mountain subalkaline are compositionally distinct References from the alkaline basaltic deposits present on the Pitcock Rosillos Ranch. They also contrast markedly with the main part of the Allen, S.R., Bryner, V.F., Smith, I.M., Ballance, P.F., 1996. Facies analysis of Balcones province to the east, where only alkaline compositions pyroclastic deposits within basaltic tuff-rings of the volcanic are known and melilite are abundant (Spencer, 1969; field, . New Zealand Journal of Geology and Geophysics 39, Barker et al., 1987). More detailed geochemical studies are 309–327. needed on the West Texas Cretaceous basalts, which could Atchley, S.C., Nordt, L.C., Dworkin, S.I., 2004. Eustatic control on alluvial sequence stratigraphy: a possible example from the Cretaceous–Tertiary provide useful insights into along-strike variations in magma transition of the Tornillo Basin, Big Bend National Park, West Texas, U.S.A. compositions and petrogenesis within the Balcones province. Journal of Sedimentary Research 74, 391–404. Barker, D.S., 1977. Northern Trans-Pecos magmatic province: introduction and Acknowledgments comparison with the Kenya Rift. Geological Society of America Bulletin 88, 1421–1427. Barker, D.S., Mitchell, R.H., McKay, D., 1987. Late Cretaceous nephelinite to We thank Roy Pitcock for allowing the first two authors to phonolite magmatism in the Balcones province, Texas. In: Morris, E.M., stay on his ranch during part of the field work. We are indebted Pasteris, J.D. (Eds.), Mantle Metasomatism and Alkaline Volcanism. to Joe Wooden for assistance with acquisition and interpretation Geological Society of America Special Paper, vol. 215, pp. 293–304. of the U–Pb zircon geochronological data. We also thank Trey Black, L.P.,Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J.W., Valley, J.W., Hargrove for his help in working with the zircon images, and Mundil, R., Campbell, I.H., Korsch, R.J., Williams, I.S., Foudoulis, C., 2004. Improved 206Pb/238U microprobe geochronology by the monitoring of a John Breyer for assistance with parts of the field work. Art trace-element-related matrix effect; SHRIMP, ID-TIMS, ELA-ICP-MS and Busbey gave valuable help in figure preparation. Permission to oxygen isotope documentation for a series of zircon standards. Chemical carry out field work in Big Bend National Park was granted to Geology 205, 115–140. K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264 261

Fig. 20. U–Pb zircon isotopic analyses plotted on standard concordia diagrams (A, C, E) and Tera-Wasserburg diagrams (B, D, F). Analyses are shown as 2σ error ellipses or as solid dots where ellipse is too small to show at scale of diagram. Labels for individual analyses correspond to those in Table 2. Ages on concordia are shown in Ma. A. Analyses yielding ages N300 Ma for sample B4. B. Analyses yielding youngest ages for B4. C. Analyses yielding ages N400 Ma for sample C29. D. Analyses yielding youngest ages for C29. E. Analyses yielding ages N140 Ma for sample C30. F. Analyses yielding youngest ages for C30. MSWD=mean square of weighted deviates.

Befus, K.S., 2006. Late Cretaceous and Eocene phreatomagmatic volcanism and Breyer, J.A., Busbey, A.B., Hanson, R.E., Griffin, W.R., Hargrove, U.S., magma-sediment interaction in the Big Bend area of West Texas. MS Thesis, Bergman, S.C., Befus, K.S., 2007. Evidence for Late Cretaceous volcanism Texas Christian University. in Trans-Pecos Texas. Journal of Geology 115, 243–251. Bohanan, J.P., 1987. Sedimentology and petrography of deltaic facies in Byerly, G.R., 1991. Igneous activity. In: Salvador, A. (Ed.), The Geology of the Aguja Formation, Brewster County, Texas. MS Thesis, Texas Tech North America, Volume J: The Gulf of Mexico Basin. Geological Society of University. America, pp. 91–108. 262 K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264

Fig. 21. Concordia diagram for youngest concordant analyses for all three samples from Peña Mountain, shown as 2σ error ellipses. Heavy black ellipse is weighted mean age for all analyses. MSWD=mean square of weighted deviates.

Carman, M.F., 1994. Mechanisms of differentiation in shallow mafic alkaline intrusions, as illustrated in the Big Bend area, western Texas. Journal of Volcanology and Geothermal Research 61, 1–44. Cas, R.A.F., Wright, J.V., 1987. Volcanic Successions: Modern and Ancient. Allen & Unwin, London. 528 pp. Chough, S.K., Sohn, Y.K., 1990. Depositional mechanics and sequences of base surges, Songaksan tuff ring, Cheju Island, Korea. Sedimentology 37, 1115–1135. Colella, A., Hiscott, R.N., 1997. Pyroclastic surges of the Pleistocene Monte Guardia sequence (Lipari Island, Italy): depositional processes. Sedimentol- ogy 44, 47–66. Connor, C.B., Conway, F.M., 2000. Basaltic volcanic fields. In: Sigurdsson, H., Houghton, B.F., McNutt, S.R., Rymer, H., Stix, J. (Eds.), Encyclopedia of Volcanoes. Academic Press, San Diego, pp. 331–343. Fig. 22. Model for evolution of overlapping maar volcanoes at Peña Mountain. Crowe, B.M., Fisher, R.V., 1973. Sedimentary structures in base-surge deposits North is to left in each case. A. Formation of first maar, deposits of which are with special reference to cross-bedding, Ubehebe craters, Death Valley, now partly exposed on northern side of mountain. B. Collapse of part of crater . Geological Society of America Bulletin 84, 663–682. margin, erosional truncation, and deposition of lacustrine beds. C. Formation of Damon, P.E., Shafiqullah, M., Clark, K.F., 1981. Age trends of igneous activity new maar slightly south of previous one. Outflow deposits cover truncation in relation to metallogenesis in the southern Cordillera. In: Dickinson, W.R., surface to north. D. Collapse of parts of new crater margin, and subsidence of Payne, W.D. (Eds.), Relations of Tectonics to Deposits in the Southern pyroclastic strata into crater. Cordillera. Arizona Geological Society Digest, vol. 14, pp. 137–154. Druitt, T.H., 1992. Emplacement of the 18 May 1980 lateral blast deposit ENE Griffin, W.R., Foland, K.A., Bergman, S.C., Leybourne, M.I., 2005. New of Mount St. Helens, Washington. Bulletin of Volcanology 54, 554–572. geochemical and geochronological constraints on the Balcones igneous Druitt, T.H., 1998. Pyroclastic density currents. In: Gilbert, J.S., Sparks, R.S.J. province, Texas. Geological Society of America Abstracts with Programs (Eds.), The Physics of Explosive Volcanic Eruptions. Geological Society of 37 (3), 9. London Special Publication, vol. 145, pp. 145–182. Gutmann, J.T., 1976. Geology of Crater Elegante, Sonora, Mexico. Geological Ewing, T.E., 2004. Review of the geology of the Uvalde area. In: Ewing, T.E. Society of America Bulletin 87, 1718–1729. (Ed.), Volcanoes, Asphalt, Tectonics and Groundwater in the Uvalde Area, Hanson, R.E., Elliot, D.H., 1996. Rift-related Jurassic basaltic phreatomagmatic Southwest Texas. South Texas Geological Society Guidebook, vol. 2004-1, volcanism in the central Transantarctic Mountains: precursory stage to pp. 1–23. flood-basalt effusion. Bulletin of Volcanology 58, 327–347. Ewing, T.E., Caran, S.C., 1982. Late Cretaceous volcanism in south and central Hay, R.L., Iijima, A., 1968. Nature and origin of palagonite tuffs of the Honolulu Texas—stratigraphic, structural, and seismic models. Gulf Coast Associa- Group on Oahu, Hawaii. In: Coats, R.R., Hay, R.L., Anderson, C.A. (Eds.), tion of Geological Societies Transactions 32, 137–145. Studies in Volcanology. Geological Society of America Memoir, vol. 116, Fisher, R.V., 1961. Proposed classification of volcaniclastic sediments and pp. 331–376. rocks. Geological Society of America Bulletin 72, 1409–1414. Henry, C.D., Price, J.G., James, E.W., 1991. Mid-Cenozoic stress evolution and Fisher, R.V., Schmincke, H.-U., 1984. Pyroclastic Rocks. Springer-Verlag, magmatism in the Southern Cordillera, Texas and Mexico: transition Berlin. 472 pp. from continental arc to intraplate extension. Journal of Geophysical Research Fisher, R.V., Waters, A.C., 1970. Base surge bed forms in maar volcanoes. 96, 13,545–13, 560. American Journal of Science 268, 157–180. Houghton, B.F., Smith, R.T., 1993. Recycling of magmatic clasts during Gilmer, A.K., Kyle, J.R., Connelly, J.N., Mathur, R.D., Henry, C.D., 2003. explosive eruptions: estimating the true juvenile content of phreatomagmatic Extension of Laramide magmatism in southwestern North America into volcanic deposits. Bulletin of Volcanology 55, 414–420. Trans-Pecos Texas. Geology 31, 447–450. Ireland, T.R., Williams, I.S., 2003. Considerations in zircon geochronology Gradstein, F.M., Ogg, J.G., Smith, A.G. (Eds.), 2004. A Geologic Time Scale by SIMS. In: Hanchar, J.M., Hoskin, P.W.O. (Eds.), Zircon. Reviews in 2004. Cambridge University Press, Cambridge. 589 pp. Mineralogy and Geochemistry, vol. 53, pp. 215–241. K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264 263

James, E.W., Henry, C.D., 1993. Southeastern extent of the North American Ogg, J.G., Agterberg, F.P., Gradstein, F.M., 2004. The Cretaceous period. In: craton in Texas and Chihuahua as revealed by Pb isotopes. Geological Gradstein, F.M., Ogg, J.G., Smith, A.G. (Eds.), A Geologic Time Scale Society of America Bulletin 105, 116–126. 2004. Cambridge University Press, Cambridge, pp. 344–383. Lajoie, J., Lanzafame, G., Rossi, P.L., Tranne, C.A., 1992. Lateral facies Pearce, J.A., 1996. A user's guide to basalt discrimination diagrams. In: Bailes, variations in hydromagmatic pyroclastic deposits at Linosa, Italy. Journal of A.H., Christiansen, E.H., Galley, A.G., Jenner, G.A., Keith, J.D., Kerrich, Volcanology and Geothermal Research 54, 135–143. R., Lentz, D.R., Lesher, C.M., Lucas, S.B., Ludden, J.N., Pearce, J.A., Leat, P.T., Thompson, R.N., 1988. Miocene hydrovolcanism in NW Colorado, Peloquin, S.A., Stern, R.A., Stone, W.E., Syme, E.C., Swinden, H.S., USA, fueled by explosive mixing of basic magma and wet unconsolidated Wyman, D.A. (Eds.), Trace Element Geochemistry of Volcanic Rocks; sediment. Bulletin of Volcanology 50, 229–243. Applications for Massive Sulfide Exploration. Short Course Notes— Le Bas, M.J., Le Maitre, R.W., Streckeisen, A., Zanettin, B.A., 1986. Chemical Geological Association of , vol. 12, pp. 79–113. classification of volcanic rocks based on the total alkali silica diagram. Price, J.G., Henry, C.D., Barker, D.S., Parker, D.F., 1987. Alkalic rocks of Journal of Petrology 27, 745–750. contrasting tectonic settings in Trans-Pecos Texas. In: Morris, E.M., Lehman, T.M., 1985. Stratigraphy, sedimentology, and paleontology of Upper Pasteris, J.D. (Eds.), Mantle Metasomatism and Alkaline Magmatism. Cretaceous (Campanian–Maastrichtian) sedimentary rocks in Trans-Pecos Geological Society of America Special Paper, vol. 215, pp. 335–346. Texas. PhD Thesis, University of Texas at Austin. Ross, G.M., 1986. Eruptive style and construction of shallow marine mafic tuff Lehman, T.M., 1986. Late Cretaceous sedimentation in Trans-Pecos Texas. In: cones in the Narakay Volcanic Complex (Proterozoic, Hornby Bay Group, Pausé, P.H., Spears, R.G. (Eds.), Geology of the Big Bend Area and Solitario Northwest Territories, Canada). Journal of Volcanology and Geothermal Dome, Texas. West Texas Geological Society 1986 Field Trip Guidebook, Research 27, 265–297. vol. 86–82, pp. 105–110. Ross, P.-S., White, J.D.L., 2006. Debris jets in continental phreatomagmatic Lehman, T.M., 1989. Upper Cretaceous (Maastrichtian) paleosols in Trans- volcanoes: A field study of their subterranean deposits in the Coombs Hills Pecos Texas. Geological Society of America Bulletin 101, 188–203. vent complex, Antarctica. Journal of Volcanology and Geothermal Research Lehman, T.M., 1991. Sedimentation and tectonism in the Laramide Tornillo 149, 62–84. Basin of West Texas. Sedimentary Geology 75, 9–28. Rowe, T., Cifelli, R.L., Lehman, T.M., Weil, A., 1992. The Campanian Terlingua Lehman, T.M., Coulson, A.B., 2002. A juvenile specimen of the sauropod local fauna, with a summary of other vertebrates from the Aguja Formation, dinosaur Alamosaurus sanjaunensis from the Upper Cretaceous of Big Bend Trans-Pecos Texas. Journal of Vertebrate Paleontology 12, 472–493. National Park, Texas. Journal of Paleontology 76, 156–172. Sankey, J.T., 2001. Late Campanian southern dinosaurs, Aguja Formation, Big Lehman, T.M., Wheeler, E.A., 2001. A fossil dicotyledonous woodland/forest Bend, Texas. Journal of Paleontology 75, 208–215. from the Upper Cretaceous of Big Bend National Park, Texas. Palaios 16, Schumacher, R., Schmincke, H.-U., 1991. Internal structure and occurrence 102–108. of accretionary lapilli—a case study at Volcano. Bulletin of Lehman, T.M., McDowell, F.W., Connelly, J.N., 2006. First isotopic (U–Pb) age Volcanology 53, 612–634. for the Late Cretaceous Alamosaurus vertebrate fauna of West Texas, and its Schumacher, R., Schmincke, H.-U., 1995. Models for the origin of accretionary significance as a link between two faunal provinces. Journal of Vertebrate lapilli. Bulletin of Volcanology 56, 626–639. Paleontology 26, 922–928. Self, S., Kienle, J., Huot, J.-P., 1980. Ukinrek Maars, Alaska, II. Deposits and Lorenz, V., 1973. On the formation of maars. Bulletin Volcanologique 37, formation of the 1977 craters. Journal of Volcanology and Geothermal 183–204. Research 7, 39–65. Lorenz, V., 1986. On the growth of maars and diatremes and its relevance to the Sheridan, M.F., Wohletz, K.H., 1983. Hydrovolcanism: basic considerations and formation of tuff rings. Bulletin of Volcanology 48, 265–274. review. Journal of Volcanology and Geothermal Research 17, 1–29. Lorenz, V., 2003. Maar-diatreme volcanoes, their formation, and their setting in Smith, G.A., Katzman, D., 1991. Discrimination of eolian and pyroclastic-surge hard-rock or soft-rock environments. GeoLines 15, 72–83. processes in the generation of cross-bedded tuffs, Jemez Mountains volcanic Lorenz, V., Zimanowski, B., Buettner, R., 2002. On the formation of deep-seated field, New Mexico. Geology 19, 465–468. subterranean peperite-like magma-sediment mixtures. Journal of Volcanology Sohn, Y.K., 1996. Hydrovolcanic processes forming basaltic tuff rings and and Geothermal Research 114, 107–118. cones on Cheju Island, Korea. Geological Society of America Bulletin 108, Ludwig, K.R., 2000. Isoplot/Ex 3.0: A geological toolkit for Microsoft Excel. 1119–1211. Berkeley Geochronology Center Special Publication 1. Sohn, Y.K., 1997. On traction-carpet sedimentation. Journal of Sedimentary Ludwig, K.R., 2001. 2001 SQUID 1.02 Add-In for Excel™. Berkeley Research 67, 502–509. Geochronology Center Special Publication 2. Sohn, Y.K., Chough, S.K., 1989. Depositional processes of the Suwolbong tuff Luttrell, P.E., 1977. Carbonate facies distribution and diagenesis associated with ring, Cheju Island (Korea). Sedimentology 36, 837–855. volcanic cones; Anacacho (Upper Cretaceous), Elaine Field, Sohn, Y.K., Park, K.H., 2005. Composite tuff ring/cone complexes in Jeju Dimmit County, Texas. In: Bebout, D.G., Loucks, R.G. (Eds.), Cretaceous Island, Korea: possible consequences of substrate collapse and vent Carbonates of Texas and Mexico: Applications to Subsurface Exploration. migration. Journal of Volcanology and Geothermal Research 141, 157–175. University of Texas at Austin, Bureau of Economic Geology, Report of Spencer, A.B., 1969. Alkalic igneous rocks of the Balcones province, Texas. Investigations, vol. 89, pp. 260–285. Journal of Petrology 10, 272–306. Martin, U., Németh, K., 2005. Eruptive and depositional history of a Pliocene tuff Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope ring that developed in a fluvio-lacustrine basin: Kissomlyó volcano (western evolution by a two-stage model. Earth and Planetary Science Letters 26, Hungary). Journal of Volcanology and Geothermal Research 147, 342–356. 207–221. McClintock, M., White, J.D.L., 2006. Large phreatomagmatic vent complex at Thomas, W.A., Viele, G.W., Arbenz, J.K., Nicholas, R.L., Denison, R.E., Coombs Hills, Antarctica: wet, explosive initiation of volcanism Muehlberger, W.R., Tauvers, P.R., 1989. Tectonic map of the Ouachita in the Ferrar-Karoo LIP. Bulletin of Volcanology 68, 215–239. orogen. In: Hatcher Jr., R.D., Thomas, W.A., Viele, G.W. (Eds.), The Miggins, D.P., Blome, C.D., Smith, D.V., 2004. Preliminary 40Ar/39Ar Geology of North America, Volume F-2: The Appalachian-Ouachita Orogen geochronology of igneous intrusions from Uvalde County, Texas: defining in the United States. Geological Society of America. Plate 9. a more precise eruption history for the southern Balcones Volcanic Province. Tomlinson, S.L., 1997. Late Cretaceous and Early Tertiary turtles from the Big U.S. Geological Survey, Open File Report 2004–1031, 31 pp. Bend region, Brewster County, Texas. PhD Thesis. Texas Tech University. Moore, J.G., 1967. Base surge in recent volcanic eruptions. Bulletin Volcanologique Valentine, G.A., Fisher, R.V., 2000. Pyroclastic surges and blasts. In: 30, 337–363. Sigurdsson, H., Houghton, B.F., McNutt, S.R., Rymer, H., Stix, J. (Eds.), Németh, K., White, J.D.L., 2003. Reconstructing eruption processes of a Encyclopedia of Volcanoes. Academic Press, San Diego, pp. 571–580. Miocene monogenetic from vent remnants: Waipiata Volcanic Vazquez, J.A., Ort, M.H., 2006. Facies variation of eruption units produced by Field, South Island, New Zealand. Journal of Volcanology and Geothermal the passage of single pyroclastic surge currents, Hopi Buttes volcanic field, Research 124, 1–21. USA. Journal of Volcanology and Geothermal Research 154, 222–236. 264 K.S. Befus et al. / Journal of Volcanology and Geothermal Research 173 (2008) 245–264

Walker, G.P.L., 1981. Characteristics of two phreatoplinian ashes, and their White, J.C., Benker, S.C., Ren, M., Urbanczyk, K.M., Corrick, D.W., 2006. water-flushed origin. Journal of Volcanology and Geothermal Research 9, Petrogenesis and tectonic setting of the peralkaline Pine Canyon , 395–407. Trans-Pecos Texas, USA. Lithos 91, 74–94. Waters, A.C., Fisher, R.V., 1971. Base surges and their deposits: Capelinhos and Williams, I.S., 1997. U–Th–Pb geochronology by ion microprobe: not just ages Taal volcanoes. Journal of Geophysical Research 76, 5596–5614. but histories. Society of Economic Geologists Reviews in Economic Wheeler, E.A., Lehman, T.M., 2005. Upper Cretaceous–Paleocene conifer woods Geology 7, 1–35. from Big Bend National Park, Texas. Palaeogeography, Palaeoclimatology, Winchester, J.A., Floyd, P.A., 1977. Geochemical discrimination of different Palaeoecology 226, 233–258. magma series and their differentiation products using immobile elements. White, J.D.L., 1991. Maar-diatreme phreatomagmatism at Hopi Buttes, Navajo Chemical Geology 20, 325–343. Nation (Arizona), USA. Bulletin of Volcanology 53, 239–258. Wohletz, K.H., 1998. Pyroclastic surges and compressible two-phase flow. In: White, J.D.L., 1996. Impure coolants and interaction dynamics of phreatomagmatic Freundt, A., Rosi, M. (Eds.), From Magma to Tephra. Elsevier, Amsterdam, eruptions. Journal of Volcanology and Geothermal Research 74, 155–170. pp. 247–312. White, J.D.L., Houghton, B.F., 2006. Primary volcaniclastic rocks. Geology 34, Wohletz, K.H., Sheridan, M.F., 1983. Hydrovolcanic explosions II. Evolution of 677–680. basaltic tuff rings and tuff cones. American Journal of Science 283, 385–413.