Miocene hydrovolcanism in NW Colorado, USA, fuelled by explosive mixing of basic magma and wet unconsolidated sediment

Buil Volcanol (1988) 50:229-243 Voliä~n~ology © Springer-Verlag 1988

Miocene hydrovolcanism in NW Colorado, USA, fuelled by explosive mixing of basic magma and wet unconsolidated sediment

PT Leat and RN Thompson

Department of Geology, University of Durham, South Road, Durham DH1 3LE, UK

Abstract. The Yampa and Elkhead Mountains volcanic fields were erupted into sediment-filled fault basins during Miocene crustal extension in NW Colorado. Post-Miocene uplift and erosion has exposed alkali basalt lavas, pyroclastic deposits, volcanic necks and dykes which record hydrovolcanic and strombolian phenomena at different erosion depths. The occurrence of these different phenomena was related to the degree of lithifica- tion of the rocks through which the magmas rose. Hydrovolcanic interactions only occurred where rising basaltic magma encountered wet, porous, non-lithified sediments of the 600 m thick Mio- cene Brown's Park Formation. The interactions were fuelled by groundwater in these sediments: there was probably no standing surface water. Dykes intruded into the sediments have pillowed sides, and local swirled inclusions of sediment that were injected while fluidized in steam from heated pore water. Volcanic necks in the sediments consist of basaltic tuff, sediment blocks and separated grains derived from the sediments, lithic blocks (mostly derived from a conglomerate forming the local base of the Brown's Park For- mation), and dykes composed of disaggregated sediment. The necks are cut by contemporaneous basalt dykes. Hydrovolcanic pyroclastic deposits formed tuff cones up to 100 m thick consisting of bedded air-fall, pyroclastic surge, and massive, poorly sorted deposits (MPSDs). All these contain sub-equal volumes of basaltic tuff and disaggregated sediment grains from the Brown's Park For- mation. Possible explosive and effusive modes of formation for the MPSDs are discussed. Contem- poraneous strombolian scoria deposits overlie lithified Cretaceous sedimentary rocks or thick basalt lavas. Volcanic necks intruded into the Creta- ceous rocks consist of basalt clasts (some with spindle-shape), lithic clasts, and megacrysts de-

rived from the magma, and are cut by basalt dykes. Rarely, strombolian deposits are interbedded with hydrovolcanic pyroclastic deposits, re- cording changes in eruption behaviour during one eruption. The hydrovolcanic eruptions occurred by interaction of magma with groundwater in the Brown's Park sediments. The explosive interactions disaggregated the sediment. Such direct di- gestion of sediment by the magma in the vents would probably not have released enough water to maintain a water/magma mass ratio sufficient for hydrovolcanic explosions to produce the tuff cones. Probably, additional water (perhaps 76% of the total) was derived by flow through the permeable sediments (especially the basal conglomerate to the formation), and into the vents.

Introduction

There are many detailed descriptions of hydrovolcanism at monogenetic centres, the tuff rings and cones that are their characteristic eruptive products, and the agglomerate- and tuff-filled pipes that fed the eruptions (e. g. Williams 1936; Francis 1970; Waters and Fisher 1971; Heiken 1971; For- syth and Chisholm 1977; Self et al. 1980; Delaney and Pollard 1981; Frazzeta et al. 1983; Leys 1983; Wohletz and Sheridan 1983; Lorenz 1984; Fisher and Schmincke 1984; Houghton and Hackett 1984; Houghton and Schmincke 1986). The processes of explosive magma-water interaction have also been the subject of recent theoretical and experimental studies (Sheridan and Wohletz 1981; Wohletz and McQueen 1984; Wohletz 1986; Ko-

Offprint requests to: PT Leat

230 Leat and Thompson: Hydrovolcanism in NW Colorado

kelaar 1986). These investigations have led to a concept in hydrovolcanism, whereby the style of eruption and nature of ejected products ('wet' or 'dry') are linked to the magma/water ratio at the site of the explosions (Sheridan and Wohletz 1981, 1983; Wohletz and Sheridan 1983), and the confining pressure and style of the magma/water interactions (Kokelaar 1986). In this paper we describe Miocene hydrovolcanic centres where the eruptions seem to have been 'wet' but, nevertheless, the only source of water appears to have been intergranular fluid in unconsolidated and permeable clastic sediments. In contact with up- welling dykes of basic magma, these sediments were disaggregated by the expansion of their pore water and thereafter mixed energetically with it and the resulting peperite was erupted as hydrovolcanic tephra. We consider this phenomenon to be the eruptive equivalent of the non-eruptive fluidization of unconsolidated sediments around hypabyssal intrusions described by Kokelaar (1982).

Geological setting

This paper is concerned with the Elkhead Moun- tains and Yampa area in NW Colorado (Fig. 1). It is underlain by Cretaceous and Miocene sediments, bounded to the East by the structurally- high Proterozoic Park Range. The region is at the northern end of the zone of elongate Tertiary basins and associated Neogene normal faults within the Southern Rocky Mountains that Tweto (1979a) identified as the northward extension of the Rio Grande Rift system of New Mexico. Faulting began at about 28 Ma and sediments ac- cumulated in the basins between about 25 Ma and 10 Ma, when the area was subject to both overall and differential uplift and erosion (Izett 1975; Larson et al. 1975; Tweto 1979a; Snyder 1980). Radiometric ages of the igneous rocks in the Elk- head Mountains and Yampa Valley cluster around 10 Ma (Marvin et al. 1974; C. W. Naeser in Izett 1975). The basic members of this igneous suite are in contact with both lithified Cretaceous and non-lithified Miocene sediments at present exposure levels. A significant point, developed below, is that there were two styles of volcanic activity. Strombolian eruptions occurred in areas of outcrop of Cretaceous rocks, whereas hydrovolcanic eruptions occurred in areas of outcrop of the non-lithified Miocene sediments. Dykes, volcanic necks and pyroclastic deposits of both styles are present, owing to differential erosion. Howev-

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er, no single locality exposes a complete vertical section from volcanic neck to pyroclastic deposits. Out descriptions are therefore taken from numer- ous localities (shown in Figs. 1, 2 and described in a lõcality list available from the authors upon re- quest).

The Miocene sediments belong to the Brown's Park Formation (for the most part) and the North Park Formation (locally). They are described in detail by Kucera (1962), Buffler (1967), Seger- strom and Young (1972), Izett (1975), Larson et al. (1975), Tweto (1976) and Snyder (1980). Both For« mations consist mainly of well-sorted, poorly ce- mented or non-lithified siltstones and fine- grained sandstones. The Brown's Park Formation has a present maximum thickness in the Elkhead- Yampa region of about 600 m (Izett 1975; Tweto 1976). The base of the formation consists of up to 100 m of conglomerate (Buffler 1967; Larson et al. 1975). River-channel sandstones occur locally in the formations, but the bulk of the sediments

Leat and Thompson: Hydrovolcanism in NW Colorado 231

are aeolian. Some of these minimally-consolidated deposits are typical dune sands. Locally in the Elkhead Mountains and predominantly in the Yampa area the fine-grained sediments show massive bedding or faint low-angled dune bedding. These are probably loess-like deposits of wind-redistributed flood-plain sediments (Kueera 1962; Buffler 1967; Izett 1975, and personal com- munication 1985; Tweto 1976; Snyder 1980). This aeolian facies is involved in most examples of hydrovolcanism we describe below. Rounding of the grains varies from poor to excellent. Although most samples are predominantly composed of quartz, this is locally subordinate to fragments of feldspar, amphibole, and colourless glass shards. This volcaniclastic debris is probably windblown silicic ash from plinian eruptions far to the west of NW Colorado (Izett 1975). The Formations contain a vertebrate fossil fauna including species of rhinocerous, camel, horse and antelope diagnostic of a semi-arid climate, probably resembling the present floor of the East Africa rift (Izett 1975; Larson et al. 1975; Snyder 1980). Although a few examples of lacustrine deposits have been recorded in the Brown's Park Formation of both the Elkhead Mountains and the Yampa area (Ku- cera 1962; Buffler 1967), we stress that we have found no signs of these in the vicinities of the vol- eaniclastic localities discussed in this report. The hydrovolcanic deposits are interstratified with the Miocene sedimentary Formations, but published data are insufficient to determine their exact stra- tigraphic position. Bedding contracts between the sediments and pyroclastic deposits were nowhere observed by us.

In the Elkhead Mountains, basic and interme- diate lava flows cap many peaks. Pyroclastic sequences underlie the lavas or cap the summits, but no volcanic necks are exposed. The area is crossed by a NW-SE trending mafic dyke swarm.

In the Yampa area (Fig. 2) prominent volcanic necks form crags up to 80 m high. Pyroclastic sequences are widespread in the vicinity of the necks. A few hills between the Yampa Valley and the Park Range are capped by lavas (Fig. 2). Many dykes of the NW-SE trending swarm running through this area intersect the necks or are eut by them.

In the Elkhead Mountains, the compositions of the igneous rocks range from shoshonitic basalt and minette to andesite, dacite and rhyolite (Christensen 1942; Buffler 1967; Leat et al. 1988). Only phenomena relating to shoshonitic basalt compositions are discussed here, as pyroclastic

Fig. 2. Geological map of the Yampa area modified from Tweto (1976). Quaternary superficial deposits are not shown. The probable vents within the area marked "No access" were not examined by us. The three necks marked by ? near Phipps- burg were mapped as such by Kucera (1962) and Tweto (1976), but not identified during re-investigation of the localities by US

deposits derived from the other magma compositions are minor. The Yampa igneous rocks range from alkali basalt to potassic basanite (Leat et al. 1988). For brevity, all these marie rocks are called basalt in this paper.

Dykes; their interaction with the Brown's Park formation sediments

In the Elkhead Mountains the thicker dykes (<6 m wide) are continuous laterally for distances of up to 8 km. The dykes around Yampa ( < 5 m wide) extend up to 4 km in areas underlain by Cretaceous sandstones and shales. Within the Brown's Park sediments around Yampa the dykes tend to be discontinuous or aligned in en-echelon segments. Their surfaces are often wrinkly or ro- pey, indicating that the sediment deformed read- ily as the magma penetrated it. Figure 3 shows an extreme example where the dyke surface is almost pillowed.


Fig. 3. Near-vertical dyke, ca. 0.2 m thick intruded into the Brown's Park Formation sediments (eroding away). Dyke has prominent pillow-like walls (locality 24)

Fig. 4. Block cut from a dyke showing brecciation and incorporation of sediment (white) into basalt (black). The basalt is chilled against the pockets and streaks of sediment (locality 39). Length of photograph: 15 cm

Fig. 5. Photomicrograph of part of block shown in Fig. 4, showing intimate mixing of weakly vesicular basalt (black) and fine-grained sediment. The basalt-sediment contact varies from sharp to gradational, and micro-pillows of basar intrude the sediment. Width of photograph: 3 c m

In the vicinity of volcanic necks, the dykes commonly pass laterally over distances of about 10-20 m from tabular form into a confused mixture of sediment and brecciated, chilled magma (e.g. locality 2).

Figures 4 and 5 illustrate basalt-sediment (fine-grained sand) relationships in a dyke (locality 39). The chilled margin is at the base of the block in Fig. 4 and the white streaks are sediment. Immediately adjacent to the margin there is an irregular and discontinuous but broadly planar zone of sediment-filled fissures in the basalt. The

basalt is locally chilled adjacent to these fissures. Further into the dyke (top of the block in Fig. 4), the bodies of sediment are still broadly tabular and parallel to the margin, but in detail have swirling cuspate contacts with the surrounding basalt. Figure 5 shows these relationships in a thin section, showing chilling of the basalt against the incorporated sediment, and the resemblance of the resulting texture to a miniature pillowed outcrop of basalt within acid melt in a net-veined complex (cf. Blake et al. 1965). But the leucocratic component in Fig. 5 is not acid magma; it is a


quartz-rich sand which shows no sign of melting. Our interpretation of this magma-sediment interaction is similar to that suggested by Kokelaar (1982) and Walker and Francis (1987) for several British localities. As the dyke intersected non- lithified Brown's Park Formation sediments and chilled against them, heat from the magma vapou- rized interstitial water in the sediment. The vapour's expansion disaggregated the sediment, and the highly mobile mixture of vapour and sediment was capable of energetic penetration of joints in the rapidly-chilled outer margin of the dyke. Pulses of the mobile vapour/sediment mix then proceeded to mingle with still-fluid basalt magma in the interior of the dyke. The dyke represents a frozen example of the bulk interaction of magma and wet sediment described by Kokelaar (1986), although no explosions apparently occurred, probably because of confining pressure.

Volcanie neeks

The distribution of the volcanic necks in the Yampa area is indicated in Fig. 2. We found no convincing volcanic necks in the Elkhead Moun- tains. Many of the Yampa volcanic necks form prominent topographic features 200 m across at the base and up to 80 m high, as in the group 4 km NNE of Yampa (Fig. 2). Others are smaller features situated where many intersecting dykes cut an associated volcanic breccia, or where a single dyke locally becomes significantly thickened and filled by volcanic breccia. The range of volcanic rocks so produced is similar to those described from the Navajo region of the Colorado Plateau by Williams (1936) and around Ship Rock, New Me×ico, by Delaney and Pollard (1981). Most of the features here called volcanic necks cannot be proved by us to have fed surface eruptions, but breccias in some contain spindle bombs (e.g. locality 54). Such bombs were probably explosively expelled from vents, failed to clear crater rims, and incorporated into sub-surface vent-filling breccias.

The necks are complex structures. Most consist of a volcanic breccia (herein called the neck breccia) intruded by basalt dykes 0.3-5 m thick. The neck breccias consist of various proportions of the following components in approximately de- creasing abundance: (i) basaltic scoria; (ii) disaggregated sand and silt grains, presumably derived from the Brown's Park Formation sediments into which many of the necks were emplaced; (iii) lithic blocks up to 1 m across of locally-derived up-

per-crust; (iv) blocks of non-lithified sediment, presumabty derived from the Brown's Park For- mation; (v) sediment dykes consisting of material derived from the Brown's Park Formation sediments, and containing sparse basaltic material; (vi) a megacryst assemblage (plagioclase, pyroxene, amphibole, biotite, Fe-Ti oxides), also found in Yampa dykes, lavas and pyroclastic deposits.

There are two groups of necks which correlate with the nature of the country rock into which they are emplaced. Group 1 necks are within shales and lithified sandstones of Cretaceous age, or where the Brown's Park sediments apparently form only a thin cover over Cretaceous rocks. They are predominantly composed of basar clasts. Their lithology and distribution with re- spect to underlying sedimentary formations is comparable to those of the strombolian pyroclastic scoria deposits of the Yampa area (see below). Group 2 necks are within thick Brown's Park For- mation sediments. Their matrix consists of well- mixed basalt clasts and disaggregated sediment. Their lithology and distribution is comparable to the hydrovolcanic deposits of the area. We therefore postulate that Group 1 necks were associated with strombolian eruptions, whereas Group 2 necks were hydrovolcanic.

In detail, Group 1 necks consist of heck breccias cut by basalt dykes (localities 26, 27, 28, 32, 54). The breccias are chaotic, poorly sorted and consist of: (a) basalt clasts -- many of these are vesiculated, and some have shapes of spindle- bombs; (b) rounded to angular lithic blocks from local upper crust; (c) sparse megacrysts. Aggluti- nation of the breccias is variable. Most of the basalt dykes associated with the necks are near-vertical an appear to die out within 20 m from the necks, but others are laterally persistent (Fig. 2). Commonly, dykes running in different directions cross-cut at necks. Most dykes cut the heck breccias while others are truncated by them. Probably the dykes at one neck red a single monogenetic strombolian eruption, the magma being forced to follow more than one route through the very last part of its ascent, but some dykes may never have red surface eruption.

The necks of Group 2 (localities 19, 23, 24, 35, 36) consist of basalt dykes and neck breccias (Fig. 6) containing all the lithological components listed above (i-vi). The bulk of each neck breccia is a matfix consisting of an intimate mixture of approximately equal amounts of basalt fragments and disaggregated sediment grains derived from the Brown's Park Formation. This matrix is similar to that of hydrovolcanic pyroclastic dep•sits


Fig. 6. Group 2 (hydrovolcanic) neck ca. 30 m wide at base. Large rounded blocks up to 6 m across of sediment derived from the Brown's Park Formation are set in the neck breccia. The neck is cut by several thin, irregular basalt dykes (locality 24)

in the Yampa area (see below), and also to peperite deposits described from contacts of intrusions and wet sediment (Kokelaar 1982). Most of the basalt fragments are 1 mm-3 cm across, poorly vesicular and angular. Other basalt fragments (> 2 cm across) are more vesicular and rounded. Some large basalt clasts (>10 cm) are rounded and resemble lava pillows; some have bread-crust surfaces. In places, fine matrix, with few large clasts, is in irregular, sharp-to-gradational contact with coarser parts of the heck breccias containing targer basalt clasts and lithic blocks. Many of these matrix patches appear to form upwardly in- trusive flame-like structures ca. 5-50 cm across which give the impression of having been fluidized (cf. Kokelaar 1982).

Most of the basalt dykes associated with the hydrovolcanic necks are near-vertical, but others form inclined sheets where they cut the Brown's Park Formation sediments and the margins of the necks. Some dykes are truncated by the necks, and others cut the neck breccias. The impression is gained of a series of basalt dyke intrusions during the volcanic activity at each neck. The dykes that cut the necks change orientation and progressively lose their identity towards the cores of the necks, forming a series of irregular pillows.

The blocks of non-lithified sediment, believed to have been derived from the Brown's Park For- mation are up to 6 m across (Fig. 6). They are strongly rounded and ellipsoidal, and are themselves invaded by irregular flame-shaped protru- sions of the fine-grained matrix of the neck breccia. They are likely to have been torn from the vent walls and, once incorporated into the vent, were apparently rapidly destroyed by surface abrasion and sub-division of blocks by injection of the matrix of the breccia. This view is consistent with the absence of coarse Brown's Park sediment blocks from hydrovolcanic pyroclastic deposits of the Yampa area (see below). S imilar blocks of sediment in hydrovolcanic necks have been described from the Carboniferous of Fife, Scotland (Francis 1970; Forsyth and Chisholm 1977). The dykes of sediment derived from the Brown's Park Formation are up to 6 m wide and cut the entire (ca. 80 m) exposed height of individual necks (localities 23, 26). The sediment dykes show sharp but irregular contacts with the host neck breccia, and contain very sparse basalt fragments. They are likely to have been formed by explosive mobilization of sediment from the walls of the volcanic necks. Upwardly directed emplacement was probably achieved as a mixture of steam and disaggregated sediment (cf. Kokelaar 1982). Preserved sediment dykes cu~ all other rocks in the volcanic necks, and taust have been emplaced at the end of volcanic activity at each heck.

Pyroclastic deposits

Pyroclastic deposits are exposed mostly along the east side of Yampa valley and in Stagecoach valley (Fig. 2). Exposure is generally good, and the deposits form undulating topography and crags up to 60 m high. Because of post-Miocene erosion, original volcanic topography is nowhere preserved. The pyroclastic deposits are of two types: strombolian deposits and hydrovolcanic deposits.

Strombolian deposits

These a rea minor facies in the area, occurring at localities 1, 3, 7, 8, 9, 26, 43, 53. They consist of non-consolidated or weakly agglutinated black or red basaltic scoria. The deposits are well-sorted and consist of vesicular basalt clasts, including abundant spindle bombs. They show weak bed-

Leat and Thornpson: Hydrovolcanism in NW Colorado 235

ding with apparent dips between near-horizontal and ca. 30 °. A strombolian (or possibly fire-foun- taining) mechanism of formation (cf. Wilson and Head 1981) can be inferred. The deposits are commonly interbedded with lava flows. Mostly, scoria deposits overlie either Cretaceous rock (where the Brown's Park Formation sediments were not present at eruption) or a thick sequence of lava flows (localities 1, 3, 7, 9, 53). At only one place (locality 8) were strombolian deposits observed within the same eruptive sequence as hydrovolcanic deposits, overlying the Brown's Park Formation: the eruption there was dominantly hydrovolcanic but transient strombolian activity deposited two interbedded scoria horizons (cf. Woh- letz 1986; Houghton and Schmincke 1986; Cas and Wright 1987).

Hydrovoleanic deposits

These dominant pyroclastic deposits are well-exposed at localities 2, 3, 6, 11, 13, 14, 15, 31, 37, 42, 48. They always õverlie, or are interbedded with, the Brown's Park Formation sediments, although a basal contact of the deposits on the sediments was nowhere observed. At one locality (11), they overlie a basalt lava flow. They are well-bedded, generally fine-grained, indurated, grey-yellow deposits. The matrix consists of sub-equal proportions of quartz-rich sand and fragmented basalt. Angular and sub-rounded clasts of basalt are up to ca. 0.2 m across. Some blocks having vesicular cores and dense rinds appear to be bombs, though spindle shapes were not observed. The deposits contain less than 5% strongly-rounded to suban- gular blocks of presumed PreCambrian rock. Most of the deposits contain megacrysts of plagioclase (up to 2 cm long), pyroxene, Fe-Ti oxides, amphibole and biotite, presumably derived from the basaltic magma during fragmentation: many dykes and lavas in the Yampa area contain such megacrysts. No large blocks of the Brown's Park Formation were observed in the deposits, in contrast to their presence in some volcanic necks.

The deposits are interpreted as pyroclastic, as suggested by Buffler (1967), rather than reworked (Kucera 1962), because they contain abundant bomb sags and locally developed antidune structures, indicating deposition from pyroclastic surges. Sedimentary structures indicative of deposition from running water, such as high-angle cross bedding or cross-sections of water cut channels, are rare. The deposits are all thoroughly li-

thified and therefore cannot be studied by stand- ard granulometric methods. The main agent of ce- mentation was intergranular deposition of car- bonate, but palagonitization of glassy basalt fragments also contributed to the process.

On a gross scale, some of the deposits are approximately horizontally bedded, but many dip variably from gentle (< 10 °) to vertical (Fig. 7), and even locally overturned. Tectonic movements since the Miocene cannot account for such steep dips as most Cretaceous and all Tertiary sedimentary strata in the region have near-horizontal bedding (e.g. Larson et al. 1975; Tweto 1976). Where bedding is steep, strike angles vary, and there are abundant angular unconformities, most of which die out along strike distances of ca. 30 m. Small- scale soft-sedment slump structures are abundant.

Three main types of beds were identified; (i) massive, poorly sorted deposits (herein called MPSDs); (ii) well-bedded, well-sorted, laterally continuous beds, interpreted as air-fall deposits; (iii) generally fine-grained, laterally discontinuous beds showing low-angle cross bedding, interpreted as pyroclastic surge deposits. The air-fall and pyroclastic surge deposits are commonly intimately interbedded, and we were unable to determine the likely mode of deposition for many individual beds (cf. Walker 1984). Nevertheless, an approximate estimate of relative proportions at one well-exposed locality (13) (50% MPSD, 40% air-fall, 10% surge) is probably representative of the area as a whole.

Massive, poorly sorted deposits (MPSDs). These are problematic deposits whose origin is debata- ble. They form internally homogeneous, structure- less beds, usually 0.5-15 m thick (Fig. 7) at localities 3, 13, 15, 31, 48. Many are interbedded with other MPSDs to form flow-on-flow sequences; others are interbedded with air-fall and surge deposits. Their main constituent is a matrix consisting of approximately equal amounts of quartz- rich fine sand and basaltic fragments smaller than 1 mm. Blocks of basalt and PreCambrian rock range up to about 20 cm across. Abundant basalt clasts, 0.5-2.0 cm across, are commonly angular and weakly vesicular. Their surfaces are broken and cut through vesicles, except in some large clasts which have a relatively smooth rind. The average maximum grain size of the MPSDs (as measured, for example, by the mean diameter of the five largest clasts in a l m 2 outcrop) varies from bed to bed in a succession, but is always greater than that of interbedded air-fall and pyro-

236 Leat and Thompson: Hydrovo]canism in NW Colorado

Fig. 7. Hydrovolcanic deposits ca. 60 m thick consisting of tabular MPSDs and interbedded air-falt and pyroclastic surge deposits. Note the bedding is near vertical at the base, but dips at ca. 30 ° at the top of the succession (locality 13)

clastic surge deposits. No pipes, pod-like groups of clasts, water- or gas-escape structures, internal stratification features such as basal layers or clast- enriched layers were observed in the MPSDs. There is a weak normal grading of basalt clasts in some thicker beds. They do not have reworked tops.

Most of the MPSDs are tabular, with constant thickness over at least 25 m; others fill U-shaped gullies about 2-4 m deep and about 3-7 m wide (localities 13, 15), and are erosional into underlying beds (cf. Fisher 1977). Slopes of the gully walls range from about 30 ° to near-vertical, bot- toms are semi-circular, but it is difficult to interpret gully shape from two-dimensional exposure. Some gullies are composite in that they were filled, and subsequently re-excavated and refilled. Pyroclastic surge and air-fall deposits also form a part of the material filling the gullies. Our preferred interpretation is that the U-shaped gullies were eroded by a kind of mass flow subsequently deposited as MSPDs, which form the major part of the infilling deposits. This is consistent with the known ability of some lahars to erode poorly consolidated deposits such as alluvial deposits (e.g. Janda et al. 1981; Lowe et al. 1986). However, we cannot rule out the possibility that the gullies

were eroded by pyroclastic surges (Fisher 1977; Leys 1983). The MPSDs have many of the features of lahars and mudflows (cf. Fisher and Schmincke 1984), but these words have genetic implications which may not be appropriate for these deposits. Possible mechanisms of formation of the MPSDs are discussed betow.

Air-fall deposits. Deposits interpreted as air-fall are well-sorted and range from massive beds up to 0.5 m thick to finely stratified, fine-grained deposits. The beds are laterally continuous in thickness for distances of over 20 m. The deposits consist dominantly of basaltic clasts rarely over 2 cm across and disaggregated sand. Grading of basalt clasts in individual beds is normal. Where the air- fall deposits are thick, they are disturbed by many deep bomb-sags due to bombs and blocks up to ca. 0.2 m in diameter.

Pyroclastic sur9e deposits. These were identified by their laterally discontinuous nature and low- angle cross bedding. They form internally cross- stratified bedsets up to about 5 cm thick which probably represent products of single surge events (Walker 1984). Cross-bedding within the bedsets is picked out by sharp changes in grain size be-

Leat and Thornpson: Hydrovolcanism in NW Colorado 237

tween individual beds > 2-4 mm thick. Sorting in individual beds is good. Few bedsets contain basaltic fragment over 1 cm in diameter. The surge deposits are intimately associated with air-fall deposits, as observed in comparable deposits by Walker (1984). Pyroclastic surge deposits locally dominate sections over 1 m thick. Antidune structures like those illustrated by Schmincke et al. (1973, Fig. 4b, c) were identified at localities 3, 48.

Formation of the tephra

In the Yampa area, there is good evidence that many of the pyroclastic deposits originated in hydrovolcanic eruptions, and that the vents of these eruptions are locally preserved as hydrovolcanie necks. Many related dykes also show evidence of magma interactions with wet sediment. The geo- graphical association of these pyroclastic deposits and intrusions in the Yampa area is so close (Fig. 2) that we think it is reasonable to conclude that they are different parts of the same hydrovolcanic phenomenon. The area therefore provides an excellent opportunity to study the process of formation of hydrovolvanic tephra at a range of erosion levels in a single volcanic field.

The hydrovolcanic necks intrude non-lithified fine-grained Brown's Park sediments. Disaggre- gated sediment grains from this Formation contri- bute some 50% by volume to the hydrovolcanic necks, most of the rest being fragmented basalt. Basalt dykes interacted with the Brown's Park sediments (cf. Figs. 4, 5), but because the dykes are thin (up to 5 m), incorporation of sediment into dykes cannot have been a major process in the formation of the voluminous sediment-basalt mix- tures in the necks and pyroclastic deposits. The bulk of the incorporation of sediment into the basalt is more likely to have occurred at the necks. These are up to 200 m across, which suggests that they expanded outwards by ingesting the surrounding sediment. Probably, expansion of the neck occurred partly by slumping of sediment into the necks (cf. Lorenz 1985). Nevertheless, the targe sedimentary dykes in the necks imply a further process of upward-directed mobilization of the wall rocks.

We emphasize that fluviatile sediments are rare in the Brown's Park Formation, and that we found no lacustrine deposits within it. Water to fuel the hydrovolcanic eruptions therefore cannot have been supplied from lakes. Nevertheless, the 13rown's Park Formation sediments are porous

and permeable, and capable of holding a large amount of interstitial water. The water table during the Miocene is likely to have been close to the surface as replenishment of groundwater could have occurred by run-off from the higher Park Range to the east (Fig. 1). Furthermore, the Mio- cene fauna in the Brown's Park Formation (Lar- son et al. 1975) include species which are likely to have required daily drinking water. This leads us to suggest that the watet to fuel the hydrovolcanic eruptions was the abundant groundwater within the sediments.

Evidence for mass incorporation of Brown's Park Formation sediment in the volcanic necks comes from: (1) the presence of blocks of the sediment, which apparently become rapidly mixed into the matrix of the neck breccia; (2) sediment dykes intruded at a late stage into the necks; (3) the high proportion of disaggregated sediment in the necks and pyroclastic deposits. An even higher proportion (up to 95%) of disaggregated sediment grains was noted by Heiken and Woh- letz (1985) in the hydrovolcanic deposits of the basaltic Kilbourne Hole tuff ring, New Mexico. In that case, sand and silt were derived from the underlying Santa Fe group of sediments which, in the vicinity of Kilbourne Hole are largely non- lithified and strongly resemble the Brown's Park Formation. It is likely that the hydrovolcanic explosions at Yampa resulted from mixing of basalt magma with a sediment-water mixture rather than by water alone. Our preferred mechanism is similar to the one proposed for surtseyan hydrovolcanic eruptions by Kokelaar (1983), except that magma is envisaged to explosively interact with a slurry of groundwater and sediment in the neck (along with previously fragmented basalt), instead of a slurry of seawater and fragmented basalt. The vesicularity of scoria in the Yampa hydrovolcanic deposits and necks suggests that magma vesicula- tion occurred before quenching by water, imply- ing shallow hydrovolcanic interaction. Basalt clast formation was probably a result of magma/ wet sediment bulk interaction explosivity (Koke- laar 1986).

Source of lithic blocks

A variety of lithic blocks occurs in the hydrovolcanic pyroclastic deposits and necks, and the strombolian necks in the Yampa area. The lithic blocks potentially provide information on the way in which the magma traversed the crust and the depth of hydravolcanic explosions (cf. Sheridan


and Wohletz 1983; Lorenz 1985). Most of the lithic blocks are of PreCambrian upper-crustal litho- logies (granite, schist, amphibolite), with less common samples of presumed Cretaceous sandstones. The PreCambrian blocks are mostly rounded and could have been derived either from dyke walls in the PreCambrian crust, or from a conglomerate which forms the local base of the Brown's Park Formation. This conglomerate is 70-100 m thick beneath the Yampa area, according to Larson et al. (1975, Fig. 5). Some of the Pre- Cambrian blocks must have been derived directly from PreCambrian crust because some lithic clast- bearing dykes were emplaced into Cretaceous rocks and therefore never intersected the Miocene conglomerate. These clasts are relatively small, poorly rounded and lithologically different from most clasts in the necks (locality 18). Probably, most of the abundant strongly rounded, large Pre- Cambrian lithic clasts in the necks and pyroclastic deposits were derived from the Miocene conglomerate. Some direct evidence for this comes from PreCambrian blocks up to 0.2 m across in sediment dykes in necks. These sediment dykes represent fluidized batches of Brown's Park sediment which did not mix significantly with magma (locality 23). This suggests that hydrovolcanic interactions took place between magma and ground- watet in both the conglomerate and fine-grained facies of the Brown's Park Formation, and over a range of depths below each volcano.

Style of eruption

In this section, we discuss some aspects of the nature of the hydrovolcanic eruptions and deposits, and comment on the likely original structure of the volcanoes.

Evidence for 'wet' eruptions

Products of hydromagmatic eruptions -- and in particular pyroclastic surges -- can be divided into 'dry' and 'wet' types according to whether or not condensed water occurs in the erupted mixture of gas and pyroclasts (Sheridan and Wohletz 1981, 1983; Frazzeta et al. 1983; Walker 1984). The 'wetness' of surges is thought to depend largely on the initial water/magma mass ratio in the vent, and the types probably form a continuous gradational series (Sheridan and Wohletz 1981; Walker 1984). 'Dry' hydrovolcanic deposits are characterized by air-fall deposits interbedded

with cross-bedded and planar pyroclastic surge deposits. 'Wet' deposits are characterized by mudflow and sheetwash deposits interbedded with air- fall and surge deposits, and massive surge beds may be present (Sheridan and Wohletz 1983). Ac- cretionary lapilli, bedding slumps, vesiculated tuffs and induration of the deposits are also characteristic. 'Wet' surges are capable of plastering vertical layers onto trees, buildings and cliff faces (e,g. Waters and Fisher 1971; Cas and Wright 1987).

In the Yampa pyroclastic deposits, the presence of massive deposits (MPSDs) resembling mudflows, bedding slumps, and the induration of the deposits indicates that 'wet' eruptions were dominant. The absence of welI-shaped accretion- ary lapilli, a diagnostic feature of "wet' eruptions, may be due to the abundant grains of disaggregated sediment in the clouds that transported the Yampa deposits. Possibly coated lapilli are present, but not easily observed, because of the induration of the deposits. We conclude that the Yampa eruptions produced wet eruptions, and that the water/magma mass ratio was high.

Volcanic form

The two main kinds of hydrovolcanic features are tuff cones and tuff rings (Wohletz and Sheridan 1983). Tuff rings are formed by phreatic air-fall breccias and thin air-fall and surge beds (mostly from 'dry' eruptions) with a maximum thickness of ca. 50 m or less, and having low maximum dips (3°-12°). Tuff cones are formed additionally of massive beds, including mudflows; the eruptions are mostly 'wet', the maximum thickness of the deposits is ca. 100-350 m and maximum dips are high (24°-30 °).

The maximum observed section through hydrovolcanic deposits in the Yampa area is ca. 100 m. The evidence for major slumping of the Yampa deposits (see below) precludes use of observed dips to interpret Original volcanic form. But the abundance of massive beds (MPSDs), evidence for 'wet' eruptions and the thickness of the deposits, indicate that these were originally tuff cones.

Origin of the massive, poorly sorted deposits (MDSDs)

There are several possibte mechanisms to account for the origin of these deposits in the hydrovolcanic pyroclastic deposits.


VENT / ~ ~

I 5'0 \1 250

VENT

2so / »óo

©

400 500

Fig. 8 A-C. Idealized sketch sections showing structure of a strombolian cinder cone and a hypothetical hydrovolcanic tuff cone. A Half-section of a cinder cone showing the initial cone (1) wherein the scoria remain at the site of initial air-fall de- positon and the secondary cone (2) wherein scoria are redeposited in talus slides (after McGetchin et al. 1974). B Analogous half-section of a tuff cone showing initial cone (1), and secondary cone (2) wherein tephra are redeposited by mass flows (arrows). Horizontal scale equals vertical scale and is the same for A and B. The horizontal scale in distance from the centre of the vent is shown in metres. Note the larger size of the initial tuff cone relative to the initial cinder cone. The dimensions of the tuff cone are based on data presented by Wohletz and Sheridan (1983). C Hypothetical section through a distal por- tion of the tuff cone in B, showing the initial cone dominated by air-fall and surge deposits (dashed ornament) and the secondary cone composed of interbedded air-fall and surge deposits and mass flows (triangles)

(i) Remobilization ofwet tephra. The geometric development of tuff cones, growing from addition of wer air-fall and surge deposits during an eruption, is analogous in a general way to strombolian cinder cone development (McGetchin et al. 1974). Strombolian eruptions deposit scoria thickly very close to the vent, and thickness of scoria de- creases rapidly away (Fig. 8). During an initial pe- riod of cinder cone growth, scoria clasts remain where they fall. But the proximal deposits will soon exceed their maximum angle of rest. There- after, virtually all non-agglutinated scoria on a cinder cone will be incorporated into active talus moving away from the crater rim on both the internal and external slopes of the cone (McGetchin et al. 1974). Hydrovolcanic eruptions deposit a maximum thickness of tephra further from the vent than strombolian eruptions (Fig. 8; Wohletz and Sheridan 1983). It follows that, compared to

strombolian cones, hydrovolcanic cones must ,achieve a greater volume before the point is reached when the dip of the deposits exceeds their maximum angle of repose. This effect is only partly counterbalanced by the relatively low maximum angle of repose of wet hydrovolcanic tephra (ca. 240-30 °, Wohletz and Sheridan 1983). Once this point has been reached, deposits around the rim of the tuff cones will be remobil- ized. This might occur by catastrophic slumping of sectors of the tuff cone, or by shedding of upper layers. In the latter case (Fig. 8), the mass flows so generated may deposit massive, poorly sorted deposits (MPSDs) on the flanks of the cones, interbedded with air-fall and surge layers, as observed at Yampa. Figure 8 suggests that mudflows should be absent from the initial phase of growth of tuff cones, and be common in the later phases. We cannot demonstrate such a pro- gression at Yampa, since complete vertical sections through tuff cones are not exposed.

(ii) Eruption column collapse. The generation of pyroclastic flows, including some of small volume, by collapse of eruption columns or from dense eruption clouds that barely clear crater rims is well documented (e. g. Sparks et al. 1978; Row- ley et al. 1981). The dense, collapsed cloud experi- ences rapid 'deflation' upon falling to the ground, involving expulsion of much of the gas, and subsequent transport by mass flow (Sparks et al. 1978; Walker 1985). Pyroclastic flows probably formed by this mechanism are interbedded with air-fall and surge deposits in some hydromagmatic volcanoes (Frazzeta et al. 1983 ; Fisher et al. 1983), and collapse of eruption columns might be promoted by condensed water in hydrovolcanic eruption clouds. Waters and Fisher (1971) described collapse of hydrovolcanic clouds at Cape- linhos, Azores, which generated base surge deposits. If a collapsing gas/pyroclast mixture con- tained abundant condensed water, so that all grain surfaces were wet, the situation might arise that, upon deflation, or during subsequent mass flow, condensed water might at some point become the effective continuous medium of the flow, and the body would subsequently behave as a mudflow.

(iii) Effusion from the vent. The hydrovolcanic neck breccias at Yampa are lithologically similar to the MPSDs in the area. When active, the necks consisted of a slurry of fragmental basalt, disaggregated sediment, and probably both steam and condensed water (cf. Kokelaar 1983). In pfinciple,


this slurry would have been capable of direct effusion to form wet mass flows, without explosive action. Non-explosive expulsion of the slurry from the neck could have occurred following an in- crease in the rate of intrusion of basalt into the neck, or slumping of vent walls. Possibly effusive mass flows from narrow vents could clear very low sectors of crater walls and flow away from the crater.

(iv) En masse deposition from jetted slurries. Ex- plosive expulsion of the vent-filling slurry as a si- deways-directed fountain could deposit tephra as massive, poorly sorted beds with only limited movement across the ground (Kokelaar 1986, pp. 284-5).

In the Yampa area and in the Elkhead Moun- tains we could find no evidence for initial tuff cone facies, wherein MPSDs are absent (cf. Fig. 8), and we suspect that MPSDs are commonly represented amongst the first-erupted tephra in several of the tuff cones. This suggests that at least some of the MPSDs are primary deposits generated by mechanisms ii-iv.

Large-scale slumping

Slumped and tilted bedding and locally developed unconformities are common features of well-bedded hydrovolcanic deposits in tuff cones and of the interiors of diatremes, these features form by: (1) unconformable deposition of in- wardly dipping tephra on crater walls overlying outwardly dipping beds (e.g. Heiken 1971: Koke- [aar 1983); (2) slumping of over-steepened wer tephra to form small folds and local unconformities (Leys 1983: Kokelaar 1983); (3) large-scale slumping of pyroclastic deposits (and underlying country rock) into vents. This process could occur either by rotation as landslides, so that dips away from the vent are progressively steepened (cf. Wohletz and Sheridan 1983, Fig. 1), or by pro- gressive steepening of beds towards the vent (cf. Forsyth and Chisholm 1977; Lorenz 1985, Fig. 7).

In the Yampa area, small-scale slumps and unconformities which we attribute to type 2 processes (above) are locally well developed. Some localities (2, 5, 13, 15, 31, 37, 48) show steep dips (up to vertical or even overturned) along > 100 m of strike (Fig. 7). We attribute their formation to type 3 processes.

A B

TUFF CONE

B R o w W s . : ' ' " " . / " . . . P , ù K .)~,~ 5oom

• - - - I , to l ~

~ o . . o ù I~r'l . . . . . iNoN- urùwJEo) '

" . -2___2 - - 2 _ _ , . , ,

A ù ~ < , o / o o / / ~ ' / , ù ó ~ a / ~ (LITHIFIED)

Fig. 9. Sketch of a vertical cross-section showing the proposed relationship between the Yampa hydrovolcanic necks and tuff cones. A assuming no major slumping; B illustrating a possi- bie slumping mechanism broadly consistent with models proposed by Francis (1970) and Wohletz and Sheridan (1983). Hydrovolcanic interactions are initiated when the basalt dyke (black) encounters the conglomerate and fine-grained sediment facies of the Brown's Park Formation. Solid arrows indicate likely flow of interstitial water through the sediment during the eruption

Model for hydrovolcanism

Figure 9 shows our preferred model for the hydrovolcanic activity in the Yampa area. The thickness of the Brown's Park Formation is drawn at 600 m, the approximate thickness of the Forma- tion in the area (Izett 1975; Tweto 1976), although it is locally thinner. The basal conglomerate is 70 m thick (Larson et al. 1975). Kokelaar (1982) discussed the phase relationships for water as a function of pressure and temperature, and pointed out that hydrovolcanic explosions are un- likely at pressures above the critical point, which occurs at 221 bars in pure water. This pressure is equivalent to a depth of 1.1 km beneath wet unconsolidated Brown's Park sediments, assuming a density of 2000 kg m -3 for the wet sediment. Hy- drovolcanic explosions might, therefore, have occurred at all depths in the Brown's Park including at its base; the basal conglomerate could have been involved in the hydrovolcanic interactions, allowing transport of clasts from the conglomerate into the necks and tephra.

Once explosive interactions between a dyke with constant magma supply and groundwater become sustained, a mixture of fragmented basalt,


disaggregated sediment and water as vapour and / or liquid is erupted, and a discrete volcanic neck taust develop. In order for hydrovolcanic explosions to continue, basalt newly injected into the heck taust receive a fresh supply of water. Watet could be derived from either: (a) interstitial water; released when the porous sediment is in- gested into the neck; (b) a flow of groundwater through the permeable sediment towards and into the neck; (c) the return of previously erupted condensed watet into the neck in the form of mass flows (probably a process of minor importance to the water budged in the neck). We now further consider the first two possible sources for the water.

Earlier, we summarized evidence to suggest that the sediments in the walls of the hydrovolcanic necks had been incorporated into the necks themselves. This process resulted in the pyroclastic deposits and neck breccias containing roughly equal volumes of fragmented basalt and disaggregated sediment grains. The amount of water con- tained in the interstices of the sediment that is released by this interaction can be calculated, and the ratio of mass of released water to the mass of the magma is equal to the ratio of the porosity of the sediment to the density of the magma (in gm cm-3). For a porosity of 0.2 and a magma density of 2.8 gm cm -3, the water /magma ratio is 0.07. If a water /magma mass ratio of at least 0.3 is needed to maintain 'wet' hydrovolcanic eruptions, such at those at Yampa (Sheridan and Wohletz 1983) -- the actual ratio for 'wet' eruptions might be 1-30 (Kokelaar 1986) -- a maximum of 24% of the total watet budget is estimated to have been derived from direct incorporation of interstitial water during mixing of the magma and sediment.

The bulk of the rest (76%) of the water may have been derived by the flow of groundwater towards the eruption vent through the permeable Brown's Park sediment. It is possible roughly to test whether this amount of water flow is feasible by estimating the likely magmatic mass eruption rates and then calculating the amount of water flow through aquifers needed to maintain hydrovolcanic eruption. The volume of the Yampa tuff cones can be estimated to have been roughly 0.05 km 3. Allowing for 50% of disaggregated sediment in the tuff cones, and for vesicularity, for a magma density of 2800km m -3, the mass of magma represented by each cone is approximately 5.6x 10 ~° kg. Magmatic mass eruption rates were therefore around 10 » kg s -~, if the eruptions lasted 10 days, which is probably a min-

imum duration for the type of eruption (cf. Kienle et al. 1980). If the necks are considered as fissures of length 200 m, the average mass eruption rates per unit length of fissure were 500 kg s - ~ m - 1 ; a reasonable value for a strombolian eruption (Wil- son and Head 1981) or a hydrovolcanic equivalent. Assuming a water /magma mass ratio of at least 0.3 for the 'wet' Yampa hydrovolcanic explosions (cf. Sheridan and Wohletz 1983), and that 76% of this water flowed through the sediment toward the necks, the mass flow of water, per unit length of fissure was 114 kg s -~ m -~. This value of mass flow of water is equal to 2 VYpwP (Walker et al. 1984) where Vis the velocity of flow of water through the interstices of the sediment, Y is the vertical thickness of the aquifer, Pw is the density of water (1000 kg m-3), and P is the porosity of the sediment (0.2). If the total thickness of the Brown's Park Formation acted as an aquifer (Y=600 m), then the value for V is 4.8 x 10-4 m s - ~. If the only efficient aquifer was the basal conglomerate (Y= 70 m) V equals 4 .1x10 -3 ms -1. According to Walker et al. (1984) possible values for V in gravels and sandstones range from 10 -~ ms -~ to 10 - 6 ms -~. Since our estimates of values for V during the Yampa hydrovolcanism lie well within this range, it is possible that most of the water needed to drive the hydrovolcanic eruptions (for any reasonable water /magma ratio) was derived by flow of groundwater through the permeable Brown's Park sediments, or perhaps mostly through the basal conglomerate, and into the necks (Fig. 9).

We recognize that there are many problems with this kind of treatment of hydrovolcanism. Hydromagmatic explosions may occur for water/ magma mass ratios between 0.1 and 30 (Sheridan and Wohletz 1983; Kokelaar 1986), and vents in- crease in size during an eruption. Nevertheless, we think that the ability of sufficient groundwater to gain entry into the vent by flow through permeable sediment in order to maintain a water / magma mass ratio required for hydrovolcanic activity is critically dependent on the magmatic mass eruption rate, which may vary over several orders of magnitude during the course of a single eruption (e.g. Wilson and Head 1981). It follows that, given a constant supply of groundwater during one eruption, hydrovolcanic activity may temporarily be replaced by strombolian or fire- fountain behaviour, if the magmatic mass eruption rate temporarily increases, so that the water / magma mass ratio falls. We suggest that this process provides a good explanation for many of the occurrences of strombolian scoria deposits inter-


bedded with hydrovolcanic deposits in tuff cones (e.g. the Elkhead Mountains, locality 8; also Self et al. 1980; Sheridan and Wohletz 1983; Lorenz 1984; Houghton and Schmincke 1986; Cas and Wright 1987). The process also goes some way to explain the close association of different styles of hydrovolcanic eruptions (e.g. transitions from 'wet' to 'dry' eruptions Kokelaar 1986; Verwoerd and Chevallier 1987).

Acknowledgements. This work was funded by the Natural En- vironment Research Council (UK) in the form of a grant (No. GR3/5299) to R. N. Thompson. We thank the following land- owners for allowing access: in the Yampa area; Earl and Kirk Crowner, Tore Gregory, Don Hinkle, Francis Moore, Jim Pa- trick, Ray Pedersen, Louis Rossi, J. B. Willcockson; in the Elk- head Mountains; Dr. H. Hubbard, F. Marsh, A. K. Salisbury, R. Stanton. We are grateful to Gilbert Williams for advice on access to the outcrops in the Elkhead Mountains, and to the Yackey family and G. Williams for hospitality. We thank Dr. D. L. Peck, Director U. S. Geological Survey, for his support of the project, and Drs. G. A. Izett, P. W. Lipman and C. S. V. Barclay and the U.S.G.S., Denver, for logistical support and substantial helpful advice on all aspects of the work. We thank Dr. M. A. Morrison for useful discussions. We thank P. Koke- laar and B. F. Houghton for detailed and helpful reviews of the paper, and also P. W. Lipman, J. White and R. F. Fisher for further constructive comments. We thank C. A. Blair and C. A. Halloway for typing.

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Received October lõ, 1987/Accepted February 8, 1988

Documents

Miocene hydrovolcanism in NW Colorado, USA, fuelled by explosive mixing of basic magma and wet unconsolidated sediment