Evolution of a Plio-Pleistocene volcanogenic-alluvial fan: The Puye Formation, , New Mexico

DAMON B. WARESBACK ) _ ^ , , rr . . _ , , ,. ^ B N TURBEVILLE* I Department of Geology, University of Texas at Arlington, Arlington, Texas 76019

ABSTRACT complete with closely spaced tephrostrati- of lava domes (Williams, 1932; Eppler and oth- graphic control, and it records the growth ers, 1987; Rose, 1987), summit spines (Bullard, The Plio-Pleistocene Puye Formation, and partial denudation of high-standing, 1976; Williams and McBirney, 1979), or over- north-central New Mexico, contains >15 km3 northeastern Jemez Mountains volcanoes. steepened lava flows and coulees (Rose and oth- of volcaniclastic alluvial sediments deposited ers, 1976). in response to rift-margin volcanism asso- INTRODUCTION The close relationship between primary- ciated with the later evolution of the Tschi- pyroclastic and reworked deposits is well dis- coma volcanic center, northeastern Jemez Volcanic domes associated with andesitic to played in the Puye Formation of north-central Mountains. Stratigraphic and sedimentologic dacitic stratovolcanoes commonly grow beneath New Mexico. This coarse-grained, volcaniclastic- evidence indicates that aggradation of Puye a cover of blocky talus that is sporadically shed alluvial sequence formed in response to the deposits was nearly continuous and contem- from the edifice (Williams, 1932; Rose, 1972; growth of a broad volcanic complex on the poraneous with graben development and es- Eppler and others, 1987). In active volcanic ter- western margin of the Española basin, central tablishment of the as the ranes, especially adjacent to the steep-sided con- (Fig. 1; Kelley, 1956, 1979; through-flowing axial drainage system in this structional flanks of composite cones, debris Manley, 1979). Puye deposits offer the oppor- region. Much of the Puye sediments de- avalanches and volcanic mudflows are prevalent tunity to study volcaniclastic depositional proc- bouched into an adjacent subsiding depocen- as integral features of explosive eruptions esses related to high-standing stratovolcanoes in ter, resulting in near-complete preservation of (Fisher, 1960; Ui, 1983; Cas and Wright, 1987). a semiarid, intracontinental rift setting. This en- this wedge-shaped volcanogenic fan. These processes contribute significant volumes vironment contrasts sharply with the more Primary-pyroclastic and reworked facies in of coarse-grained material to fluvial systems that commonly described humid-region, arc-adjacent the Puye Formation exhibit distinctive 5- to redistribute volcaniclastic sediments away from alluvial plain and back-arc apron sequences po- 30-m scale cyclicity directly related to vol- the volcano (Davies and others, 1978; Stanley, sitioned near continental margins (Van Houten, canic activity in the northeastern Jemez high- 1978; Smith, 1987a). Sediments that subse- 1976; Kuenzi and others, 1979; Vessel and Da- lands. Reworked-pyroclastic and conglomer- quently debouch into adjacent depocenters can vies, 1981; Mathisen and Vondra, 1983). atic sequences deposited by high-energy develop cyclical conglomeratic-fan sequences Detailed analysis of Puye lithofacies provides braided streams, sediment-charged sheet- that are the distal lithofacies of lava-dome talus a model of dynamic sedimentation related to the floods, sediment-gravity flows, and in shal- successions. growth and denudation of late volcanic low lakes reflect markedly increased sediment Explosive eruptions from stratocones are domes. Primary-pyroclastic deposits in the Puye loads during and following explosive erup- commonly recorded by pyroclastic deposits in- reflect periods of explosive activity that accom- tions. Individual sequences change markedly terbedded in coarse-grained, marginal volcani- panied the growth and partial collapse of indi- in character with increased distance from the clastic sequences (Koch and McLean, 1975; vidual lava domes. Reworked facies in this source area. Analysis of these abrupt lateral Mathisen and Vondra, 1983; Fritz and Harrison, sequence subsequently developed by reworking and vertical variations reveals that Puye sed- 1985). These pyroclastic facies are generally di- of volcanic sediments during inter-eruption pe- imentation was controlled by the influences agnostic of specific types of explosive activity riods of lava-dome erosion. of setniarid climate and syndepositional tec- (compare with Bloomfield and Valastro, 1977; tonism, although the associations that most Scott, 1987). The recognition of cyclic conglom- GEOLOGIC SETTING characterize the fan resulted largely from ex- eratic-tuff sequences greatly simplifies interpre- plosive volcanism. Cessation of fan growth is tation of explosive eruption history and provides The Puye Formation (Manley, 1976; Wares- attributed to waning sediment supply as a re- a basis for the evaluation of alluvial-sediment back, 1986) is one of several to Holo- sult of volcanic quiescence and to the onset of accumulation rates and volcanic fan-building cene alluvial sequences exposed along the basinwide pedimentation associated with processes. Two types of pyroclastic activity most central Rio Grande rift. The Puye alluvial fan downcutting of the ancestral Rio Grande. commonly accompany volcanic dome devel- (-4.0 to 1.7 ± 0.1 Ma; Turbeville, 1986; The Puye fan therefore provides a detailed opment (Newhall and Melson, 1983): (1) erup- T. Spell, unpub. data) constitutes the exposed record of rift-basin alluvial sedimentation, tions that produce lithic-rich pumice-fall depos- upper portion of the Puye Formation (Griggs, its and (Fisher and Heiken, 1982; 1964) and contains >15 km3 of coarse-grained Heiken and Wohletz, 1987) and (2) block-and- volcaniclastic sediments. Much of this material •Present address: Department of Geological Sci- ash avalanches and pyroclastic flows produced ences, University of Texas at Austin, Austin, Texas was shed from the northeastern part of the 78713-7909. Turbeville is corresponding author. by the gravitational collapse of unstable portions Tschicoma volcanic "center" (7.5 to 2.0 Ma;

Geological Society of America Bulletin, v. 102, p. 298-314,10 figs., 1 table, March 1990.

298

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Singer and Kudo, 1986; Baldridge and others, 1987), a series of spatially and temporally over- lapping composite volcanoes, lava flows, and dome complexes in the northeastern Jemez Mountains volcanic field (Fig. 1; Bailey and others, 1969; Gardner and others, 1986). The Puye fan is confined to the northern part of the deeply incised Pajarito Plateau in the east- ern portion of the Jemez Mountains (Figs. 1,2). Here the fan partly overlies coarse-grained braided alluvium of the Totavi formation (an- cestral Rio Grande axial stream gravels), re- cently revised by Waresback (1986). The Totavi formation also includes finer-grained flood-plain deposits (siltstones, claystones) that are inti- mately interbedded throughout the Puye (lower and upper lake sequences; Fig. 2). The Puye fan crops out in a 200-km2 tract adjacent to the eastern Tschicoma outcrop area and extends eastward into the Española basin. Late Pliocene expansion of the Puye was con- trolled by pre-existing topographic barriers and confined on the north by a volcanic construct of late Miocene Lobato lavas (Fig. 1; Bald- ridge and Vaniman, 1985). The fan thins east- ward along depositional dip from >140 m in proximal exposures to ~65 m in cliffs formed where the fan toe was truncated by incision of the Rio Grande (Fig. 2). The Puye thins south- ward to <30 m and abruptly wedges out against Pliocene basaltic lava flows of the Cerros del Rio volcanic field (Fig. 1; Aubele, 1979). This gross morphology is attributed to a combination of simple geometric depositional thinning, grad- ual hinge-line subsidence along the northern Pa- jarito fault zone (Fig. 1; Manley, 1979; Gardner and Goff, 1984), and postemplacement fan-toe erosion and escarpment retreat. Outside of their graben confinement, Puye deposits had very low preservation potential due to constant regrading and redistribution (Manley, 1976). Remnants of Pliocene conglomeratic sequences do exist to the northwest (Smith and others, 1970), although these deposits are not considered part of the Puye system. The Puye fan is particularly suited for study- ing alluvial sedimentation in an arid, intracon- tinental volcanic setting. Puye deposits are well exposed in continuous dip sections formed by downcutting of east-flowing tributaries to the Rio Grande. Numerous north-south-trending canyons provide complete three-dimensional exposure, and the distal-fan segment is exposed along cliffs that form the prominent Puye Es- Figure 1. Tectonic map of the Española basin and Jemez Mountains volcanic field, central carpment (Fig. 1). This nearly continuous expo- Rio Grande rift, New Mexico (after Manley, 1979). The study area (after Smith and others, sure of deposits from proximal through distal- 1970) and the surface extent of the Tschicoma Formation (TF), El Rechuelos Rhyolite domes fan facies is perhaps the most complete yet (ER), Puye Formation (PF), Lobato Basalt (LB), northern Cerros del Rio basalt field (CRB), reported for this type of succession. In addition, and deposits (SFG) are shown below. The overlapping Valles calderas (VC) the approximately horizontal attitude of most and Toledo Embayment (TE; Self and others, 1986) are also included. deposits in the fan, minimal offsets by local fault-

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Totavi PUYE FAN formation Cerros del Rio

- - i, -- V-w« kip l^-i/ZLV '^-I'J^-JI Jemez ^V-rJ' Mountains ~ v Volcanic Pile Santa Fe Group Sediments Figure 2. Schematic cross section of the Pajarito Plateau, showing relationships between the Puye alluvial fan, the northeastern Jemez volcanic pile to the west, and Santa Fe Group sediments (Manley, 1979) and Cerros del Rio to the east. Note especially the interfingering between Puye fanglomeratic deposits and lacustrine/fluvial sediments of the newly revised Totavi formation (ancestral Rio Grande deposits; Waresback, 1986). The vertical scale has been exaggerated for detail.

ing, and lack of deep (>5 m) incision and can- of this alluvial system suggests that the surround- placed southwest of the fan was obliterated yon infilling resulted in almost constant deposit ing environments were not entirely suitable for (along with Tschicoma lava flows and domes) thicknesses commonly traceable over several the preservation of primary pumice and ash by later episodes of caldera collapse in the cen- kilometers. Preservation of Puye deposits owed deposits (for example, north, south, or east of the tral Jemez Mountains (Baldridge and Vaniman, much to the emplacement of a protective cover Puye fan; Fig. 1). that had been em- 1986; Self and others, 1986). of Lower (Fig. 2) -1.45 to 1.51 Ma (Izett and others, 1981; ^»R \ STREAM-CHANNEST L T. Spell, unpub. data). Figure 3. (A) Approximate DEPOSITS SHEETFLOOD 33% The Puye fan provides an excellent sampling volume percentages of Puye fan DEPOSITS of lava lithologies from the eastern Tschicoma lithofacies from measured sec- 23% massif. Volcaniclastic material in the fan was tions (Table 1; Fig. 5). (B) Lat- derived largely from several presently denuded eral (down-dip) variability in the AIRFALL 2.2% dacite, rhyodacite, and rhyolite lava domes (in- proportions of fan lithofacies; CLAST-POOR IGNIMBRITES 2.8% DEBRIS-FLOW cluding the El Rechuelos Rhyolite domes, Fig. volume percentages also vary CLAST-RICH DEPOSITS S-FLOW 1; Loeffler and others, 1988). Lithologic evi- considerably with position in 24% DEPOSITS dence of a lava-dome provenance for most Puye vertical successions (see text). A OS15 % conglomerates includes high proportions of ex- tremely porphyritic and commonly banded lava NW Kilometers From Source SE clasts, blocky-vesicular and obsidian clasts, and 18 23 26 a highly variable suite of lava lithologies com- ,i..... pared to lava flows currently exposed in the SFP 90- Tschicoma highlands (Turbeville, 1986; Bald- B gggsg~ ~ ridge and others, 1987). ocypOOOt 80 ÉM The Puye also records a history (3.5-1.9 Ma) 70- SCD of explosive activity from these domes which a) h60 produced abundant dacitic and rhyodacitic E Sí i 50- y 3 tephra (Turbeville and others, 1989). Much of CD O .-¿s-,--- -40 this tephra was distributed as widespread airfall > : O V \ s blankets across the surface of the growing fan. 30- ¡8 .-. H':' Localized preservation of tephra deposits in this - CRDF - 20 setting depended on the rapid accumulation of JI-^b protective cover, either as epiclastic debris or 10- î\ additional pyroclastic material. The paucity of IBLOCK - AND- ASH coeval pyroclastic sequences beyond the limits FLOW DEPOSITS AIRFALLS&IGNIMBRITES

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TABLE 1. LITHOFACIES AND SEDIMENTARY STRUCTURES OF TOTAVI FLUVIAl^LACUSTRINE DEPOSITS AND PUYE PUYE LITHOFACIES AND DEPOSIT VOLCANICLASTIC DEPOSITS CHARACTERISTICS Facies Lithofacies Sedimentary structures Interpretation The lithofacies classification used in this study code

is based on the system introduced by Miall Gcsu Gravel, clast Imbrication, Clast-rich debris (1977) and expanded by Mathisen and Vondra supported, nonstratified; grading flow; hyperconcentrated minor sand, silt, flood flow (1983), Fritz and Harrison (1985), Waresback or clay lenses usually (1986), and Smith (1987a). This scheme (Table as a capping Gcsh Gravel, clast Imbrication, Sheetflood; stream 1) includes both primary and reworked volcani- supported, interbeds horizontal flood (for example, clastic deposits, and it covers material deposited of sand, silt, or clay stratification longitudinal bar) lenses and/or filling over a wide range of sediment/water ratios. Five interstices major lithofacies groups are recognized in the Gest Gravel, stratified Trough cross-beds Channel fills Puye (Fig. 3A), the proportions of each varying Gcsp Gravel, stratified Planar cross-beds Transverse bars both laterally and vertically in the fan (Fig. 3B). Gmsu Gravel, matrix Oriented clasts Debris flow- supported, nonstratified, parallel to flow; mudflow ungraded fluvially reworked Clast-Supported Conglomerates (Gcs) top Gmsi Gravel, matrix Debris flow- supported, nonstratified, inverse grading; mudflow Clast-supported conglomerates are the domi- graded basal shear zone nant lithofacies type in the Puye (56 vol.% of Gmsn Gravel, matrix Clast-poor debris supported, nonstratified, normal grading; flow with low-yield total deposits, Fig. 3). These deposits are subdi- graded lacks laminar strength (mudflow) vided into two major conglomeratic types that shear zone Sh Sand, fine to Horizontal Sheetflood, bar top differ in external structure (that is, gross geome- very coarse; lamination (lower- and upper-flow try, nature of lower bounding surfaces) and in- commonly pebbly regime) ternal characteristics (that is, grading, stratifica- St Sand, medium to Trough cross-beds Dunes (lower-flow very coarse; regime) tion, grain-size distribution). Lithofacies Gcsh pebbly

includes clast-supported deposits with horizontal Sp Sand, medium to Planar cross-beds Transverse bars very coarse; (lower-flow regime) stratification, commonly associated with gravel- pebbly bar accretion and low sediment concentrations Sr Sand, very fine Ripple laminated Lacustrine deposits in turbulent stream-channel and sheetflood Fsc Silt, mud Fine lamination Lacustrine deposits flows (Miall, 1977). Lithotype Gcsu, in contrast, Vab Basaltic ash, Fine lamination, Airfall tephra denotes extremely poor sorting, lack of internal commonly graded trough cross-beds, stratification, and variably developed grading. ripples Vas Silicic ash; Horizontal Airfall pumice Channel Deposits (Gcsu, Gcsh, rare Gcst, lapillistones lamination, variable grading Gcsp). Clast-supported, channelized deposits Vbr Volcanic breccia; Commonly inversely Primary pyroclastic (33% of total deposits) display a wide range in ash matrix with graded flows (ignimbrite, grain size, geometry, deposit thickness, and in- mooolithologic, block-and-ash flows) angular clasts ternal structures, and they vary considerably with position in the fan wedge (Figs. 3B, 4, 5). In proximal and midfan exposures, these exhibit broad channel-form geometries with concave- tion by fully turbulent, traction-dominated dis- ever, are subordinate to coarser-grained litho- upward, erosional lower contacts in transverse persions (Miall, 1981). Deeply incised channels, facies in the fan (13%, Fig. 3) with only 4% orientations, and planar to slightly undulating however, are rare throughout the Puye, reflect- occurring as bar-top sands interbedded with basal surfaces parallel to paleoflow (Fig. 4A). ing the strongly aggradational nature of these conglomerates and ~9% as sheet-forming depos- Contacts between adjacent units are markedly high-energy, sediment-laden streams. These its. Lithofacies Gcsh and Gcsu (Table 1) form erosional, although planar contacts occur spo- conditions developed primarily in response to the bulk of the associations in proximal and me- radically. Individual channel sequences range rising base levels of the trunk stream (ancestral dial conglomeratic sequences (Figs. 5A, 5B) from 30 cm to >6 m thick (avg 1.75 m) and Rio Grande) and very high sediment yield in with rare occurrences of deposits exhibiting become thicker and coarser grained upward in response to explosive eruptions. Aggrading base trough cross-stratification (Gcst). the fan (Figs. 5 A, 5B) but thin abruptly downfan levels and an over-all lack of perennial discharge Distally, Puye stream-channel deposits be- (Figs. 4B, 4C, 5C). Proximal deposits are coarse in this region (see below) favored sourceward come much thinner (10 cm to 3.5 m; avg 85 cm; grained, very poorly sorted (a = 2.25 to 3.18; migration of the intersection point of deposition Fig. 4B), better sorted (o = 1.69 to 2.07), and Waresback, 1986), exhibit both normal and re- and apparently inhibited deep incision of the fan finer grained. Distal deposits form laterally ex- verse grading, and are dominated by angular to surface (compare with Blissenbach, 1954; tensive, lenticular bodies with high width/depth subrounded cobbles and pebbles with subordi- Hooke, 1967). ratios (for example, some distal channels with nate sand and mud. Boulders are common (Figs. Some stream-channel conglomerates are apparent widths > 100 m have depths generally 4A, 4C), suggesting periodic high-velocity capped by pebbly sandstones (St, Sp; Table 1), <1 m). Contacts between individual beds are discharge. reflecting shallowing of flow and consequent typically erosional but range from gradational The characteristic geometry, clast-supported deposition during falling flood stages. These to planar and sharp (Figs. 4B, 4C). Deposits framework, coarse grain size, poor sorting, and deposits form discontinuous lensoid bodies with are most commonly polymodal and normally rarity of well-developed horizontal stratification predominantly horizontal or low-angle stratifi- graded with upward decreases in pebbles and in these deposits suggest stream-channel deposi- cation (Fig. 4D). Sandstones in general, how- increases in the proportions of sand and minor

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Figure 4. Lower-fan and mid-fan sequences: (A) Inversely graded, boulder-rich debris-flow deposit (crdf) overlying coarse-grained, stream- channel conglomerates (sc) above reworked, clast-poor debris-flow (cpdf) deposits. Note the flat base and fine-grained basal zone of the boulder-rich unit, and stratification developed into channel margin (lower right). (B) Low-angle trough cross-bedding (arrows) developed in sheetflood (sf) and distal braided-stream (dbs) deposits overlying clast-rich debris-flow (crdf) deposits (scale = 1.5 m). (C) Thick sequence of sheetflood (SF) and braided-stream deposits in the distal-fan facies, interbedded with pumice-fall (pf) and debris-flow (cpdf) units; note the common occurrence of outsized boulders (arrow; person for scale in lower center). (D) Stacked pumiceous, clast-poor debris-flow (cpdf) deposits with anomalous clast concentrations at flow boundaries (arrows). Note the channel gravels (sc) developed into the reworked top of the mudflow sequence and the flat-based, inversely graded, debris-flow (crdf) deposit above.

mud; outsized clasts (boulders and cobbles) are filled and open-framework fabrics) or sandstone and b-axes oriented transverse to flow, and are generally confined to the lower parts of deposits. lenses in many poorly imbricated Puye con- commonly gradational laterally and vertically to These features suggest progressive decreases in glomerates suggests rapid deposition from high matrix-supported conglomerates. flow depth and competency during falling flood sediment-concentration dispersions, common in Sheet Deposits (Gcsh, Sh). Sheet-like stages. In contrast to proximal deposits, stratifi- regions where extremely high sediment yield oc- sandy-pebble conglomerates and pebbly sand- cation in these conglomerates is better devel- curs in response to explosive eruptions. Smith stones (23% of total lithofacies) form laterally oped and expressed as low-angle to flat-lying (1986) applied the term "hyperconcentrated- continuous bodies throughout the Puye (Fig. laminations in which there is clear segregation flood flow" to such high-discharge sediment 4C). Proximally, these deposits are confined to between coarse- and fine-grained sediment frac- flows with sediment/water ratios intermediate the lower parts of the over-all succession but in tions (Fig. 4B). Lithofacies Gcsh and minor Gcst between cohesive debris flows and normal medial and distal exposures are abundant include the bulk of these deposits (Table 1). stream-channel flow. The resulting deposits throughout the sequence (Fig. 5). Sheet deposits The absence of well-developed horizontal (Gcsu) lack traction-induced stratification or range in thickness from 10 cm to 3 m (avg < 1 stratification (for example, alternating matrix- sorting, are typically ungraded, exhibit both a- m) and extend laterally (transverse and parallel

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T3 BLOCK-AND-ASH STREAM CHANNEL FLOW 2

BLOCK-AND-ASH FLOW 13 0 CPDF CRDF CRDF STREAM CHANNEL CRDF BLOCK-AND-ASH FLOW STREAM CHANNEL

STREAM CHANNEL

STREAM CHANNEL

7 0- SHEETFLOOD

BLOCK-AND-ASH FLOW CRDF SHEETFLOOD 1 STREAM CHANNEL I I I I 60- CISISQ PCB Grain Size : »-i CO 0 . SHEETFLOOD 0 E SHEETFL00D STREAM CHANNEL 50- Vas

OJO CALIENTE SANDSTONE 'I II I I CISISQ PCB CISISQ PCB Grain Size Grain Size

Figure 5. Representative stratigraphy of Puye depositional megasequences in proximal (A, 0-8 km), medial (B, 8-16 km), and distal (C, 8-24 km) exposures. Lithofacies codes refer to Table 1 and are accompanied by interpreted depositional mechanisms. An ignimbrite in the interval 40-55 m of the medial fades (B) was dated as 2.53 ± 0.1 Ma; and a pumice-fall deposit that caps the fan, as 1.75 ± 0.08 Ma (see text).

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ER TUFF i CRDF 110 AXIAL CPDF STREAM CHANNEL SHEETFLOOD BRAIDED STREAM

SHEETFLOOD

CPDF

BRAIDED STREAM

BRAIDED STREAM

CPDF

SHEETFLOOD SHEETFLOOD

SHEETFLOOD LACUSTRINE

CRDF SHEETFLOOD CPDF

CPDF STREAM CHANNEL

CRDF ' I I I I I I I ^^ C IS I S G PCB STREAM CHANNÈL Grain Size

STREAM CHANNEL formation Figure 5. (Continued). Ti'l I I CISISQ PCB Grain Size features suggest deposition from high-energy, flows (compare with Lowe, 1979). These aque- sediment-charged sheetflows on broad, low- ously fluidized sediment-gravity flows produced to flow) for several hundred meters with only relief depositional surfaces (Miall, 1977). The a spectrum of debris-flow deposit types, ranging slight thinning. Lower contacts are largely ero- pervasive parallel stratification, common occur- in coarse-particle content from clast-rich sional, although the depth of individual scours rence of scattered outsized cobbles, and the (CRDF; Figs. 4A, 6A) to clast-poor (CPDF or rarely exceeds 15 cm, and most are essentially over-all paucity of fines indicate deposition en- "mudflow"; Fig. 4D). The relative proportions flat. tirely under upper-flow regime conditions and textural variability of these deposits are Stacked sand-gravel couplets 1-5 cm thick (Smith, 1987b). generally pronounced between Puye fan facies, are common features in these sequences and and transitions between ideal end-member types suggest deposition under pulsating flow condi- Matrix-Supported Conglomerates (Gms) appear to be more the rule than the exception. tions. These deposits are similar in grain size, Clast-Rich Deposits (Gmsu, Gmsi). This sorting, structure, and thickness to braided- For matrix-supported deposits, the code Gms group consists of sandy-cobble and boulder con- stream deposits with which they are commonly (Table 1) is adopted and includes an indication glomerates that occur throughout the Puye (15% interbedded, and they are distinguished with dif- of deposit grading as inverse, normal, or un- total facies) but are most prevalent proximally ficulty on the basis of having (1) laterally exten- graded (i, n, or u, respectively). These conglom- and in upper parts of the succession (Figs. 5A, sive, sheet-like geometries (Fig. 4C); (2) absence erates are a diverse and volumetrically signifi- 5B). Deposits range from 20 cm to >4 m thick of deep scours; (3) laterally persistent, highly cant lithofacies in the Puye (Fig. 3) and suggest (avg 1.2 m); form laterally continuous, tabular uniform horizontal stratification; and (4) a lack deposition en masse from a broad continuum of bodies parallel to flow (Figs. 4D, 6A); and of apparent size grading at the unit level. These gravity-driven, cohesive, and noncohesive mass commonly persist downfan for several kilome-

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O) c "5 - j y XJ BANDELIER TUFF 2 LACUSTRINE

Gcsu STREAM CHANNEL MAFIC LAVAS

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< I I I I I I I z CISiSQ PCB o Grain Size AXIAL STREAM CHANNEL Figure 5. (Continued).

Totavi formation textural features that suggest both subaerial and Grain Size of the matrix rather than turbulence (Fisher, subaqeuous emplacement. In contrast to the 1971; Enos, 1977). Reverse grading in these clast-rich members, CPDF deposits generally deposits suggests that dispersive pressures during lack basal-shear zones or well-developed reverse ters. In some cases, lenticular transverse cross flow were sufficiently high for vertical grain-size grading, and locally they exhibit more abrupt sections reflect the tendency for debris flows to segregation, especially in the coarse fractions, decreases in maximum particle size downfan. have infilled and conformed to broad channel and provided adequate support for large clasts CPDF deposits also lack traction structures and geometries (Fig. 4A). Clast-rich deposits are un- above the base of the bed (Figs. 4A, 6A). The consist mainly of subrounded clasts (generally stratified and strongly polymodal, with particles ability of sediment-gravity flows to transport pebbles) supported by a finer-grained matrix that range in size from clay to boulders >3.5 m very large amounts of material for considerable without strongly oriented fabrics (Figs. 4D, 7A). in diameter (Figs. 6A, 6B). Individual clasts vary distances over fairly gentle slopes is well dis- Most deposits are polymodal and texturally clas- from angular to subrounded and display increas- played by Puye deposits and has been docu- sified as muddy-sandy pebble conglomerates ing roundness with clast size. The coarser- mented in other settings (Rodine and Johnson, and pebbly-muddy sandstones. grained conglomerates commonly contain re- 1976; Lawson, 1980). The apparently high mo- Subaerially emplaced CPDF deposits form versely graded basal ("shear") layers a few bility of many Puye flows must have relied laterally continuous, sheetlike bodies commonly centimeters thick (Figs. 4A, 4D), exhibit over-all mainly on changing rheology during flow, the interbedded with, and often truncated by, clast- reverse coarse-tail grading (Figs. 4D, 6A), and more viscous (clast-rich) flows predominating in supported sheetflood or braided-stream conglom- are clast-supported in parts. Deposits are very proximal exposures. Rows that reached distal erates (Fig. 4D). Most beds are massive (Gmsu), poorly sorted (a<£ = 2.83 to 3.34), the coarsest parts of the fan (Figs. 4D, 6C, 6D) produced or commonly coarse-tail normally graded members being among the more poorly sorted clast-poor mudstones that offer marked textural (Gmsn), and range from 20 cm to >3 m thick deposits exposed in the fan (Waresback, 1986). contrasts to proximal analogues. (avg 1.0 m). Basal contacts are sharp and planar, The absence of traction-induced stratification, Matrix-Rich Deposits (Gmsu, Gmsn). and lack erosional (scour) features. Where commonly non-erosive bases, and local a-axis Matrix-rich deposits (24% of total lithofacies) stacked units occur, flow boundaries are typi- parallel fabrics suggest that these coarse sed- predominate in outcrops of the mid- and distal cally obscure (Figs. 6C, 7A), although persistent iments were transported under predominantly fan exposures where they constitute as much as lenses of rounded gravels (Fig. 8) are generally laminar-flow regimes where the primary 50% of some successions (Figs. 5B, 5C). These diagnostic of reworking between emplacement sediment-support mechanism was the strength deposits exhibit a wide variety of grain size and events.

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Figure 6. Upper-fan sequences. (A) Clast-rich, nearly monolithologic debris-flow deposits in the medial fan; note the sharp contact of the flat lower surface, nearly clast-supported fabric, and large boulder (>3.5-m diam.) under D. Waresback. (B) Monolithologic block-and-ash sequences interbedded with coarse-grained debris-flow deposits in proximal exposures of the upper Puye fan (person for scale in lower center). (C) Coarse-grained sequence of debris-flow and stream-channel conglomerates; note the over-all coarsening-upward trends and ash-filled, U-shaped channel (arrow; person sitting to right of arrow for scale). (D) Subaqueously emplaced mudflow (CPDF) deposits in the upper lacustrine sequence (Fig. 2); note the sharp change in grain size above basaltic ash bed Vab3 (Fig. 5C).

Some CPDF deposits contain coarser clasts the base. Similar boulder concentrations have erosion, however, these deposits probably ex- randomly dispersed through the body of the been interpreted to develop where large clasts tended much farther east into the Española basin deposit (Fig. 7A). The typical absence of reverse are essentially "bulldozed" along the lower beds (Fig. 1). These deposits are distinguished from grading and pronounced coarse-particle angular- by flow (Ui, 1983). Many of these coarser- their subaerially emplaced counterparts on the ity in these deposits suggest that the main grained CPDF deposits are correlative with basis of (1) marked reductions in maximum and sediment-support mechanism was the high yield CRDF deposits upfan (Figs. 5, 7), possibly sug- mean particle size, (2) compositional variations strength of a cohesive matrix with little or no gesting extreme examples of the transitions in as a result of mixing with flood-basin fines and contribution from dispersive pressures asso- flow rheology mentioned above and described micaceous lake-bottom muds, and (3) associa- ciated with particle-particle interaction (Lind- in more detail below. tion with thick sequences of interbedded pumi- say, 1968; Naylor, 1980). This implies that An increase in the predominance of lacustrine ceous lake deposits (Fig. 7C). Most deposits vertical segregation and settling commonly deposits (Sr, Fsc; Table 1) in distal-fan expo- lack internal stratification and are typically un- ensued without restriction during, or imme- sures suggests that shallow lakes often inundated graded and devoid of clasts larger than small diately following, emplacement (compare with large portions of the fan toe during Puye devel- pebbles. Significant mixing of these marginally Broscoe and Thompson, 1969; Nemec and opment (Figs. 2, 5C). Subaqueously emplaced derived with Española basin Steel, 1984; Shultz, 1984). Significant increases mudflows in the distal Puye are a minor fan lake-bottom muds is evident by abundant dis- in yield strength, however, are indicated in some facies preserved locally in the upper part of the seminated matrix muscovite that was most deposits where large clasts are supported above succession (Fig. 6D). Prior to fan dissection and probably derived from weathering of metagra-

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Figure 7. Pyroclastic facies. (A) Pumiceous apron in the lower fan, consisting of stacked ashy mudflow (cpdf) deposits interbedded with sheetflood (sf) and stream-channel (sc) deposits, and overlain by coarse debris-flow (crdf) deposits. Pumiceous mudflow units grade laterally to primary pumice-falls (pf). (B) Puye ignimbrite (between vertical arrows = 3.5 m) interbedded with ashy debris-flow deposits and coarse stream-channel conglomerates in the medial-fan facies. Horizontal arrows point to individual pumice-fall deposits (see Fig. 5). (C) Subaque- ously emplaced airfall ash in the upper Puye interbedded with coarse-grained, normally graded debris-flow (crdf) and stream-channel deposits, in turn overlying lacustrine sediments (LS). Above the ash, debris-flow deposits grade laterally (-100 m upfan) to inversely graded units (see text). (D) 1.5-m-thick rhyolite pumice fall (from a source in the central Jemez Mountains; see text) in the uppermost part of the Puye fan. The unit, dated as 1.75 ± 0.08 Ma, overlies calcic soils and is overlain by 2-4 m of coarse-grained stream-channel conglomerates (sc).

nitic terranes in the Sangre de Cristo Mountains who applied the terms "pumice-flow deposit" ponents in Puye mudstones and conglomerates east of the basin (Fig. 1). Incorporation of these and "ash-flow deposit," respectively, to distin- (Figs. 4C, 6C, 7A). Pumice and scoria falls in fines by mixing is thus interpreted to be partly guish between coarse- and fine-grained pumi- the fan (Vas, Vab) form an integral part of cycli- responsible for the otherwise anomalous over-all ceous tuffs (as the term "ignimbrite" carries no cal sequences that also contain ignimbrites (Vbr) abundance of mud in these sequences (Wares- indication of grain size; Walker, 1983). Block- and mass-flow conglomerates related to the back, 1986). and-ash flow deposits, on the other hand, are same eruptive episodes (Fig. 8). Similarly, confined to proximal exposures (Fig. 5A) and pyroclastic-fall deposits contain abundant acces- Pyroclastic Facies consist mainly of dense, nonvesicular lava sory-lithic clasts similar to clast types in Puye blocks set in an ash matrix with subordinate conglomerates and to lavas exposed in the east- Primary pyroclastic facies (for example, Vas, amounts of poorly vesicular pumice (Fig. 6B). ern Jemez highlands (Turbeville and others, Vbr; Table 1) include clast-supported dacitic, Volumetrically, primary tephra constitute 1989). rhyodacitic, and rhyolitic pumice falls (Figs. 5,7) only 5% of the Puye fan (Fig. 3A), although the Tephra deposits in the Puye were informally and poorly sorted ignimbrites and block-and-ash significance of explosive eruptions to the sedi- divided into three stratigraphic groups according deposits (Figs. 6B, 7B). Where relevant, we use mentary budget is strongly apparent in the to distinctive changes in the dominant juvenile the terminology of Wright and others (1984) abundance of pumice and ash as detrital com- clast type, environment of deposition, and sus-

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crude bedding waning traction Stream Stream current waning traction Flow current Flow crude bedding — markedly erosiohal Debris last-enriched matrix-supported Stacked Flows / sharp, Stream Debris non-erosive base Flow clast-supported Intersurge Flows matrix-supported fluidal intersurge Stacked Block-and- i'^âl- monolithologic Debris monolithologic gffigangular clasts Flows subrounded clasts Ash Flows matrix Ignimbrite ashy matrix Pumice Fall Pumice Fall

DISTAL Figure 8. Idealized, eruption-related depositional sequences of proximal, medial, and distal Puye fan facies, detailing cycles in- terpreted for eruptive and inter-eruptive phases of sedimentation (compare with Fig. 5 and see text for details).

Stream/ erosive contact Debris Fl. pected sources (Turbeville, 1986). Tephra in the basal shear zone lower part of the fan includes several widespread Stream pumice-fall and thin ash-flow deposits (Fig. 7B), Flow in addition to abundant tephra redeposited as sharp base pumiceous CPDF deposits (Fig. 7A). Mudflow The middle part of the fan sequence (40- to 50-m interval, Fig. 5B) contains a lithic-rich, nonwelded ignimbrite (the "Puye ignimbrite"; Fig. 7B; dated as 2.53 ± 0.1 Ma by K/Ar analy- Sheetflow waning traction sis of plagioclase and hornblende separates; current Turbeville and Self, 1988) and coarse pumice- isolated flow deposits with large (>70 cm in diameter), Stacked megaclasts subrounded pumice blocks. Associated with the sheetflow ignimbrite sequence are several coarse-grained, Pumiceous intersurge lithic-rich pumice falls and fine-grained capping ash falls. -normally graded Mudflows Tephra deposits in the upper parts of the fan matrix-enriched include several pumice falls (Figs. 5, 7B), abundant lake-deposited pumice and ash (Fig. Pumice Fall 7C), and thick sequences of water-lain basaltic ash (Fig. 6D). Exposures of individual units are

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considerably more restricted than lower group Proximal facies dominated by coarse-grained of these sequences are attributed to the diver- deposits and differ markedly with respect to ignimbrites and chaotic block-and-ash sequences gence of flow (and consequent decreases in mineralogy, lithic types, and grain-size charac- (Fig. 6B) commonly grade downfan to coarse, competency) upon exiting the confines of apical teristics (Turbeville, 1986). Two rhyolite pum- but better-sorted, CRDF deposits (Figs. 4A, 6A, channels and merging onto lower-relief deposi- ice deposits at the very top of the sequence (Figs. 7B). These associations in turn locally grade lat- tional surfaces. 5B, 7D) differ from other Puye tephra units and erally (and vertically) to hyperconcentrated A marked downslope increase in the propor- are conjectured to have been derived from flood-flow deposits (Fig. 7A) and eventually to tion of sheetflood deposits is also typical of the source vents currently buried beneath the Valles sheetflood- and stream-channel-dominated fa- Puye (Figs. 4C, 8). These deposits volumetri- calderas (Fig. 1; Turbeville and Self, 1988). cies interbedded with ash-rich CPDF deposits cally predominate in outcrop over distal Only one exposure of each unit has been found, (Figs. 4B, 4D, 6C). Downfan thinning and fin- braided-stream deposits and become approxi- but it is apparent from deposit characteristics ing trends in these sequences reflect both the mately proportional to CPDF deposits (Figs. (Turbeville and others, 1989) that they were greater availability of coarse-grained talus in 5B, 5C). Progressive downfan decreases in the once widespread plinian deposits. proximal exposures and the rapid sedimentation thickness of sheetflood deposits accompany im- The northwesternmost exposures of Puye of coarser clasts where local fan topography proved sorting, better-developed horizontal tephra deposits (Figs. 1, 5A) are clearly prox- changed abruptly. As a result of these transi- stratification (Fig. 4C), and concurrent decreases imal. This is supported by the presence of tions, the abundance of a distinctive clast type in in maximum and mean particle sizes and parti- (1) greater thicknesses and over-all coarseness of proximal deposits becomes less apparent down- cle angularity (Waresback, 1986). Increases in pumice falls, (2) thick accumulations of coarse, fan, as these clasts were mixed with other Tschi- the frequency of cross-stratal, planar bedsets and lithic-dominated ignimbrites and ashy CRDF coma lithologies (Turbeville and others, 1989) horizontally laminated sands and mud offer dis- deposits (Figs. 5A, 6B), and (3) a sequence of and spread radially into thinner horizons. tinctive contrasts to the more massive (structure- nearly monolithologic block-and-ash flow de- Debris-flow deposits and lithic-rich ignim- less) proximal deposits (compare Figs. 4B and posits in the upper part of the section (Fig. 6B). brites in the Puye commonly exhibit pro- 4C). The predominance of sheetflood over distal The primary deposits and recognizable erup- nounced lateral variability, especially where the braided-stream deposits suggests rapid vertical tion sequences suggest that a minimum of 15 moving flows apparently entered streams or aggradation and infilling of shallow channels small-magnitude explosive eruptions (0.1 to 1.0 lakes in the inundated portions of the fan (Figs. and consequent localized lateral shifts in the km3 each), and 2 or more episodes of gravita- 6D, 7C, 8). Lake sediments interbedded in these locus of deposition. Sheetflood deposits there- tional lava-dome collapse, occurred in the north- sequences contain abundant pumice and ash, fore reflect interchannel and intrachannel depo- eastern Tschicoma highlands in late Pliocene and they constitute as much as 70% of the ex- sition, considered to have been the dominant time (Turbeville and others, 1989). At least posed Puye deposits in distal outcrops (Fig. 5C). mode of sediment emplacement in the distal three explosive eruptions, possibly from the El Transitions from inversely graded, clast-rich (16-24 km) fan. Rechuelos Rhyolite domes (Fig. 1), are repre- deposits to fine-grained (largely ungraded or A progressive downfan decrease in the me- sented in the upper part of the fan, and two normally graded), matrix-dominated deposits dian grain size of subaerially emplaced pumice plinian eruptions from sources in the central part occur in as short a lateral distance as 200 m. falls was related to southeasterly dispersal of of the Jemez field. In addition, several phreato- These abrupt transitions suggest marked down- tephra from source vents in the Tschicoma high- magmatic basaltic eruptions from the Cerros del fan matrix-enrichment of the coarser mass flows lands (Turbeville and others, 1989). The highly Rio volcanic field are recorded in scoriaceous in sand, silt, and clay fractions, possibly caused unsystematic lateral variations observed in the lake sediments in the upper-Puye sequence (Fig. by additional entrainment of water and mixing thicknesses and number of primary deposits, 6D; Fisher and others, 1984). with lake-bottom muds. Significant dilution by however, was due to extensive localized rework- water in such cases would have resulted in de- ing of the initially widespread tephra layers. LATERAL FACIES VARIATIONS creased flow competency and markedly de- Pumice falls in proximal exposures are com- creased effective viscosities of the moving flows monly separated by several meters of conglom- Abrupt downslope facies transitions are char- (Hampton, 1979; Lowe, 1979; Fisher, 1983). erates and mudstones (Figs. 5A, 8), whereas acteristic of alluvial systems where sedimenta- Rapid sedimentation of coarse clasts would have distally these units become more closely spaced tion occurs in immediate response to significant in turn been promoted as basal-shear stresses (Figs. 5B, 5C) and eventually converge to form changes in local relief and subsequent diver- suddenly decreased below the yield strength of thick sequences of primary and reworked pum- gence of channelized flow (Miall, 1970, 1981; the flows (Fisher, 1971; Enos, 1977; Postma, ice and ash (Fig. 7A). The poorer preservation Rust and Koster, 1984). This is in marked con- 1986). of loosely consolidated tephra in the distal-fan trast to downstream trends in braided rivers and In the proximal, apical portions of the fan segments is attributed to lower vertical-aggrada- broad alluvial plains that require considerable (Figs. 1, 2), stream-channel deposits constitute tion rates of protective conglomeratic (or ignim- transport distances to develop characteristic fa- as much as 50% of certain sequences (Figs. 5 A, brite) sequences on the lower relief (distal) cies (Miall, 1977; Rust, 1978). The Puye fan 6C, 8). These deposits, however, decrease in depositional slopes. exhibits marked variations in the type and pro- abundance progressively downfan and are re- portion of lithofacies as a function of proximity placed volumetrically by better-sorted, thinner, STRATIGRAPHIC RELATIONSHIPS to source area and position in the vertical suc- and finer-grained distal braided-stream and AND CYCLIC FACIES VARIATIONS cession (Figs. 3B, 5). An idealized model for sheetflood deposits (Figs. 4B, 4C, 5B). The these variations (Fig. 8) shows several character- abundance of stream-channel deposits upfan is Small-Scale Vertical Trends istics considered typical of arid-region alluvial due in part to the effects of high source-area sedimentation (Bull, 1972; Nilsen, 1982; Mack relief in promoting runoff and channelized flow Cyclic sedimentation in the Puye fan is evi- and Rasmussen, 1984). at the apex of the fan. Distal thinning and fining dent on several scales, ranging from a few meters

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in thickness to sequences >30 m (discussed below). The stacking of distinctive, eruption- related depositional sequences on the 5- to 30- Proximal Facies m scale (Figs. 5, 8) characterized much of the fan's development. These facies are the result of tephra and volcanic-debris flows generated dur- Distal Facies ing explosive eruptions, and fluvial reworking during inter-eruptive periods (compare with Davies and others, 1978; Kuenzi and others, Figure 9. Comparison of maximum particle- 1979). size variations (average of 10 largest clasts ob- Cyclic sequences in proximal exposures (Fig. served per unit) between proximal and medial 8) consist of one or more tephra deposits over- Puye megasequences. Differences in coarsening lain by pumice- and ash-rich debris in the lower and thickening trends between the two sections portions of the fan and by very coarse-grained, are due to the thick, upper, block-and-ash flow block-and-ash flow deposits toward the top of sequences in proximal exposures and the pre- the section (Figs. 5A, 6B). Between explosive ponderance of lake deposits and flood-basin fines events, abundant unconsolidated lava and py- in medial exposures (see text). roclastic talus were redistributed as clast-rich, ashy debris flows emplaced in stacked assem- blages (Fig. 8, medial sequences). The combina- tion of (1) steep and unvegetated slopes covered by abundant unconsolidated debris and (2) epi- sodic rainfall following eruptions must have greatly helped to initiate these coarse-grained its (Figs. 4C, 5C, 6D). Individual CPDF depos- mass flows. Some units are separated by thin, its are separated by fine-grained, braided-stream clast-supported deposits of apparently less vis- and intermittent sheetflood conglomerates (Figs. cous, traction-dominated flow, commonly asso- 4D, 5C). Inter-eruptive depositional sequences ciated with the waning stages of debris-flow are considerably more complex than in prox- deposition (Fisher, 1971; Mack and Rasmussen, imal exposures and suggest the common avul- 1984; Nemec and Steel, 1984). sion of shallow braided streams in response to Clast-supported conglomerates also common- rapid channel infilling by streamflow gravels ly developed above individual coarse-grained and mudflows (compare with Broscoe and mass-flow sequences (Figs. 4D, 8), where the Thompson, 1969; Lombard and others, 1981). upper parts of debris-flow deposits were partly 1976; Lawson, 1980). Thick sequences of alter- Stacked sheetflood conglomerates in these se- regraded by shallow braided streams. Coarsen- nating clast-rich and matrix-dominated conglom- quences reflect subsequent interchannel deposi- ing" or fining-upward trends in these fluvial con- erates developed as debris flows episodically tion in the abandoned fan segments (Figs. 4B, glomerates reflect individual flood events where- infilled stream channels following eruptions 4C). CRDF and coarser-grained stream-channel by sedimentation was locally controlled by rapid (Figs. 4A, 6A). Renewed incision then occurred deposits are absent in most of the distal fan ex- downslope variations in discharge and sediment as the system adjusted to changes in fan topog- posures except toward the top of the over-all loads (Allen, 1981; Rust and Koster, 1984). raphy and fluctuations in local base level. succession (Figs. 5, 6C), where there is indica- Where these conglomeratic units are superim- Upward decreases in the proportions of CRDF tion of abrupt fan-wide progradation. posed, individual flood cycles are distinguished deposits in some sequences (Fig. 5B) suggest by subtle differences in structural and textural progressively longer periods required for condi- Depositional Megasequences characteristics (Figs. 6C, 8). This stacking of tions of gravitational instability to develop on streamflow gravels reflects a trend toward sys- the proximal fan slopes. This is partly attributed A single large-scale, coarsening- and thicken- tem equilibrium during periods of relative vol- to the reduced availability of coarse-grained ing-upward sequence in the Puye fan is evident canic inactivity. The pervasive sheet-like geome- material during periods of volcanic quiescence,, in proximal exposures where the fan wedge is try of many of these conglomerates, their very when sediment supply depended solely on lava- thickest (Figs. 5A, 9). This megasequence best poor sorting, common normal grading, and hor- dome erosion. Those sequences dominated by reflects the nearly continuous emplacement of izontal bedding, however, suggest intermittent stacked mass-flow deposits (Figs. 5, 7A) must Puye deposits between about 4.0 and 1.7 Ma. periods of aggradation during unconfined super- therefore have originated largely as a conse- The large volumes of material shed during the critical flow due to markedly increased sediment quence of the frequency and intensity of explo- denudation (erosional and large-scale collapse) loads (compare with Smith, 1987b; T. Runkel, sive eruptions. This cyclicity is also apparent in of high-standing lava domes must have greatly unpub. data). corresponding variations in the thickness and increased the competency and capacity of Farther eastward, cyclic Puye sequences spacing of associated pyroclastic-fall deposits in sediment-gravity flows and fluvial systems as the (Figs. 5B, 8) consist of one or more airfall units these intervals (Figs. 5B, 7B). fan gradually prograded basinward over fine- overlain (and hence preserved) by ashy debris- At the fan toe (distal facies; Fig. 8), primary grained flood-basin sediments (Fig. 2). Similar flow deposits (Fig. 7B), attesting to the com- pyroclastic deposits, when present, are thin (few progradational behavior has been reported from monly non-erosive nature of mass-flow em- centimeters) and commonly interbedded with arid-region alluvial systems (Hooke, 1967; placement (compare with Rodine and Johnson, stacked pumiceous CPDF and sheetflood depos- Miall, 1977; Steel and others, 1977) and is ap-

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parent in the Puye by an over-all upward de- LATE PLIOCENE JEMEZ flow deposits and interbedded tuffs characterize crease in sheetflood gravels and CPDF deposits VOLCANISM AND GROWTH much of the upper fan in proximal and medial that dominate the lower parts of the succession OF THE PUYE FAN exposures (Figs. 5A, 5B, 6A, 7B) and grade dis- (Fig. 5). The Puye megasequence is capped in tally to intervals dominated by pumiceous sheet- proximal and medial exposures by stacked Evolution of the Puye fan from late Pliocene flood and distal braided-stream deposits (Figs. stream-channel deposits that reflect sedimenta- to the present is generalized in Figure 10. Explo- 4C, 8). Pumice compositions of the upper-group tion during volcanic inactivity following the sive activity is initially represented in the fan by tephra are more silicic than units below (Turbe- culmination of explosive eruptions recorded in coarse-grained pumice falls and pumiceous ville and others, 1989), and fall deposits contain the Puye (Fig. 6C). This sequence also suggests CPDF deposits (Fig. 5B). Debris flows gener- a variety of cognate and accessory lithic types the initiation of fanhead entrenchment as a con- ated during these eruptions apparently choked not observed in deposits of the lower group: sequence of waning sediment supply, yet con- the local axial drainage (Cummans, 1981), re- abundant obsidian, highly porphyritic pumi- trasts sharply with nondepositional inter-erupt- sulting in the formation of an extensive lake at ceous material, and distinctive banded clasts with ion periods of significant duration, reported the fan toe (lower lake sequence; Fig. 2). Lake apatite phenocrysts (Turbeville, 1986). These from more widespread volcaniclastic aprons sediments contain abundant tephra and grade tuffs are commonly water-laid or extensively (Walton, 1979; Smith, 1987b). laterally to stacked pumiceous CPDF and sheet- reworked in medial and distal exposures (Fig. Coarsening- and thickening-upward sequences flood deposits (Fig. 7 A) that form a prominent, 7C), attesting to the dominance of fluvial and are not restricted to any specific physiographic 2- to 10-m-thick apron around the perimeter of lacustrine systems as the Puye fan matured (Fig. segment of the Puye fan. The megasequence the Puye fan at this horizon (Figs. 5B, 5C). This 10C). Contemporaneous explosive eruption of described above, however, is incomplete in sequence suggests the sporadic remobilization of felsic magma from the Tschicoma and El Re- other exposures due to the intervention of flood- large portions of a once thick but poorly consol- chuelos domes, and basaltic activity from the basin fines deposited during onlap and inunda- idated sequence of airfall tephra, portions of northern Cerros del Rio vents (Figs. 1, 2), re- tion of the midfan slope by lake water which are exposed upfan. The emplacement and sulted in the coeval subaqueous emplacement of impounded behind Cerros del Rio basalt flows preservation of these mudflows are attributed to copious amounts of silicic and scoriaceous (Fig. 2). The over-all succession in these areas the comparatively low relief on the fan at that tephra (Figs. 5C, 6C, 6D). consists of at least two coarsening- and thicken- time (Fig. 10A). This geometry inhibited signifi- In proximal exposures, two block-and-ash ing-upward sequences (Figs. 5B, 5C, 9). In the cant incision by drainage and enhanced the ac- flow sequences in the upper fan (Figs. 5A, 6B) lower 70 m of the fan, these sequences record cumulation of deposits considered typical of suggest eruptions associated with gravitational the progradation of coarse-grained streamflow high rates of aggradation (Allen, 1981; Miall, collapse from portions of a distinctive horn- and CRDF conglomerates over finer-grained, 1981; Nemec and Steel, 1984). blende-rhyodacite lava dome that has not yet braided-stream, sheetflood, and CPDF deposits Several successive sequences in the 10- to 45- been identified in surviving Tschicoma expo- (Figs. 4A, 7 A, 7B). Sequences in the 70 to 110+ m interval of the lower fan (Fig. 5) are attributed sures (Baldridge and others, 1987; Turbeville m interval consist primarily of stacked deposits to periods of explosive activity and subsequently and others, 1989). These episodes are also of subaqueously emplaced CRDF and CPDF accelerated sedimentation rates. Each sequence apparent distally in the upper parts of the deposits (Figs. 5C, 6D, 7C) that grade upfan to consists of closely spaced pumice falls, thin ash- Puye, where coarse-grained, near-monolitholog- subaerially emplaced debris-flow deposits and flow deposits, and pumiceous and ashy CPDF ic (hornblende-rhyodacite) CRDF deposits are locally to hyperconcentrated flood-flow con- deposits (Fig. 7B) that grade laterally and verti- stacked upon one another with little evidence of glomerates (Fig. 7B). cally to primary tephra. Pumiceous deposits are significant reworking between emplacement In medial exposures, the upper Puye sequence separated from one another by several meters of events (Figs. 5, 6A, 7B). exhibits rapid vertical transition from coarse commonly ash-rich sands, gravels, and con- Inundation of the mid-fan and fan toe during clastics to fine-grained mudstone (Figs. 5C, 6D), glomerates deposited by braided streams, sheet- the latter stages of Puye development created a marking initial deposition in the encroaching floods, and debris flows (Fig. 8). long-lived, lacustrine-fan delta (Fig. 10D) dom- lacustrine environment. These fine-grained units Explosive eruptions in the Tschicoma high- inated by the subaqueous emplacement of lake are in turn overlain by stacked conglomeratic lands continued as the Puye fan prograded east- sediments, tephra, basaltic lavas, and sediment- mudstones that occur as uniform, laterally con- ward into the Española basin (Fig. 10B). The gravity flows (upper lake sequence; Figs. 2,6D, tinuous, sheet-like beds ranging from 20 cm to resulting sequences are accompanied by in- 7C). Fan growth began to wane as this lake was 2.6 m thick (75 cm avg; Fig. 6C). Periodic shal- creases in the proportions of coarse debris-flow drained following the culmination of volcanism lowing and emergence in this region are sug- and pumice-flow deposits (Figs. 5, 4A) that in the Tschicoma highlands and re-establishment gested by clast-supported braided-stream con- were partly regraded upon emplacement (Fig. of the modern (through-going) Rio Grande sys- glomerates, and thinly stratified, granular 4D). CRDF deposits become thicker and more tem. Truncation of the fan toe in the early Pleis- sandstones formed by current winnowing along abundant upward in the fan and eventually pre- tocene eventually led to fanhead entrenchment the upper surfaces of stacked mudstones. These dominate over CPDF and sheetflood deposits and the onset of fan dissection (Fig. 10E). fluctuations in lake levels were probably the re- (Fig. 5). These trends reflect increases in source- Although conglomeratic sedimentation on the sult of variations in both discharge of the ances- area relief and proximal fan slope, and they sug- Puye surface is considered to have continued tral Rio Grande and the rates of sediment supply gest an over-all increase in the amount of coarse intermittently until emplacement of the Lower from the basin margins. The thick sequence of material made available to mass flows by remo- Bandelier Tuff (ca. 1.5 Ma), a reasonable upper channel-fill gravels capping the over-all section bilization of proximal lava-dome talus. This ac- limit for fan progradation is constrained by a (Figs. 5, 6C) records the eventual draining of tivity culminated with eruption and emplace- rhyolite plinian deposit in the uppermost part of this lake and the onset of fan dissection as the ment of the lithic-rich Puye ignimbrite (Figs. 5, the Puye Formation (Figs. 5B, 7D). This deposit Rio Grande incised obstructing Cerros del Rio 7B) and associated deposits at ca. 2.5 Ma. has recently been dated by 40Ar/39Ar analysis basaltic lava dams (Fig. 2). Thick sequences of coarse-grained, debris- of single sanidine crystals (T. Spell, unpub. data)

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as 1.75 ± 0.08 Ma. These data support previous Smith (1979) based on relative stratigraphy. The lution, however, is beyond the scope of the pres- interpretations that this fall deposit was formed previously reported K/Ar (bulk sanidine sepa- ent study. during the plinian phase of eruptions that also rate) date of 2.84 ± 0.07 Ma for this ignimbrite produced the San Diego Canyon ignimbrite "B" (Self and others, 1986) is here refuted in favor of DISCUSSION AND CONCLUSIONS emplaced southwest of the Valles calderas new data that suggest a significant xenocrystic (Turbeville and Self, 1988). The latter unit has feldspar component in these tephra. Further Puye fan sedimentation was controlled largely also been redated by the 40Ar/39Ar method as speculation on the significance of this revision to by an allocyclic mechanism related to the fre- 1.78 ± 0.02 Ma, consistent with an estimate by current interpretations of Jemez Mountains evo- quency and intensity of individual eruptions and to the amount of coarse detritus supplied to the fan from the northeastern Tschicoma highlands. ^ 4.0 m.y. Ancestral Puye deposits exhibit features typical of arid- Rio Grande region alluvial fans (Blissenbach, 1954; Hooke, Lobato Basalt 1967; Bull, 1972) and also of widespread vol- Tschicoma canic alluvial plains (Walton, 1986; Smith, •omes 1987b; T. Runkle, unpub. data). These features include (1) a characteristic wedge-shaped geom-

1.8 MILLION

Incident Alluvial Fan B 3.0 m.y

Puye PRESENT Alluvial Fan fotavi

Pajarito

Plateau Rjo Grande

2.0 m.y,

Cerros del Rio Basalts

Figure 10. Evolution of the Puye alluvial fan and Española basin shown schematically from mid-Pliocene to present, (a) Basin paieogeog- raphy ~4 Ma at the initiation of the Rio Grande as the through-flowing axial drainage system in this area, (b) Growth of the Puye fan at 3 Ma resulted in eastward migration of the ancestral Rio Grande as fanglomerates prograded basinward across the flood plain, (c) Episodic damming of the axial drainage by Cerros del Rio basalt flows at -2 Ma (see Fig. 2) created shallow lakes that inundated the Puye fan toe during the latter stages of fan development, (d) Española basin paleogeography at 1.8 Ma, showing contemporaneous Puye fan progradation and culminative Tschicoma volcanism, following damming events (by Cerros del Rio basalts) that led to establishment of an upper lacustrine-fan delta (see text), (e) Present-day geomorphology showing the highly dissected eastern Pajarito plateau and western part of the Española basin.

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etry and arcuate outcrop pattern; (2) abrupt The sensitivity of the fan environment (due to ACKNOWLEDGMENTS transitions from sequences dominated by coarse- proximity to source) to periodic fault adjustment grained, debris-flow deposits to successions with is often shown by sedimentary cyclicity in the We gratefully acknowledge support by the abundant fine-grained flood-basin and lacustrine fan pile (Steel and others, 1977; Mack and New Mexico Bureau of Mines and Mineral Re- sediments; (3) downfan decreases in the thick- Rasmussen, 1984; Blair and Bilodeau, 1988). sources, Associated Western Universities ness, median grain size, and over-all particle an- The Puye fan shows evidence of having been (AWU), Inc., and colleagues in the Earth and gularity of conglomerates; and (4) apparently emplaced during a period of gradual subsidence Space Sciences Division of Los Alamos Na- continuous deposition throughout most of the in the Española basin, which contrasts sharply to tional Laboratories. We thank the U.S. Forestry sequence (as opposed to alternating episodes of the more episodic late Miocene fault activity Service for their frequent assistance and the San large-scale aggradation and degradation; Wal- that apparently accompanied deposition of the Ildefonso Pueblo for land access. Jack Flannery, ton, 1979). Preservation of these sequences was nearby Cochiti Formation (Gardner and others, Dave Kuentz, John Wolff, Steve Self, David greatly enhanced by gradual subsidence in the 1986). The over-all wedge-shaped geometry of Vaniman, Grant Heiken, John Hawley, Kim receiving (Española) basin (Manley, 1979; the Puye fan, common "telescoping" of pyro- Manley, Dave Dethier, and Jim Aldrich pro- Dethier and Martin, 1984) and by a semi-arid clastic and debris-flow deposits (Waresback, vided moral support and greatly appreciated in- climate that has been interpreted for this area in 1986), minor growth faults developed in the dis- sights into these complicated sequences. We are late Pliocene and early Pleistocene time (Kelley, tal portions of the fan, and significant offsets of grateful to Terry Spell, State University of New 1979; Dethier and others, 1988). the proximal fan along the Pajarito fault (Man- York at Albany, for permission to cite unpub- 40 39 Climate is important to alluvial-fan develop- ley, 1979; Dethier and Demsey, 1984; Gardner lished Ar/ Ar data acquired (April 1989) on ment, as fan sedimentation is controlled by the and Goff, 1984), support our suggestion that the a VG Isotopes 1200S system in the argon facility frequency of precipitation that in turn strongly Puye fan prograded eastward into a gradually of T. M. Harrison. A special thanks is extended influences the rates of erosion and sediment subsiding depocenter. to John G. McPherson for his assistance. Early yield (Melton, 1965; Winder, 1965). Coarse- Although syndepositional subsidence and versions of this manuscript greatly benefited grained conglomeratic sequences similar to climate are interpreted to have been responsible from comments by D. S. Barker, Tony Runkel, those described above have also been well doc- for the thickness, geometry, and preservation of G. A. Smith, Greg Mack, and W. Fritz. umented in temperate climatic settings charac- Puye deposits, the sedimentary cyclicity exhib- terized by ephemeral drainage and periodically ited in the fan was largely due to the influence REFERENCES CITED

high discharge (Smith, 1987b). In areas of more of explosive eruptions. Vertical trends on the 5- Allen, P. A., 1981, Sediments and processes on a small stream-flow dominated, extensive perennial discharge and plentiful alluvial fan, Shetland Islands: Sedimentary Geology, v. 29, to 30-m scale and those in megasequence (30 to p. 31-66. ground-stabilizing vegetation, however, the in- > 140 m) reflect the combined influences of late Aubele, J. C., 1979, The Cerros del Rio volcanic field, in New Mexico Geolog- ical Society, 30th Field Conference, Guidebook: Santa Fe Country, itiation of mass flows and sheetfloods is likely to Pliocene activity from two adjacent volcanic p. 243-252. be greatly inhibited (Bull, 1972), and conglom- Bailey, R. A., Smith, R. L., and Ross, C. S., 1969, Stratigraphic nomenclature centers, a northeastern Tschicoma dome com- of volcanic rocks in the Jemez Mountains, New Mexico: U.S. Geologi- eratic sequences that develop are susceptible to plex and vents in the northern part of the Cerros cal Survey Bulletin I274-P, p. 1-19. Batdridge, W. S., and Vaniman, D. T., 1985, Field and petrologic studies of the continuous regrading (Winder, 1965; Nilsen, del Rio basalt field. Tschicoma domes steadily Lobato Basalt, Jemez Mountains [abs.], in Heiken, G., compiler, Pro- ceedings of the Workshop on Recent Research in the Valles Caldera, 1982). That the Puye fan developed under supplied abundant volcaniclastic sediments di- Los Alamos, NM, USA, Oct. 15-18, 1984: LANL Report LA-10339- largely semiarid conditions is supported by an rectly to the prograding fan system, whereas the C, p. 5-6. 1986, Spatial and temporal patterns of Neogene volcanism in the north- over-all lack of plant remains in pyroclastic and Cerros lavas greatly influenced sedimentation eastern Jemez volcanic Held, New Mexico [abs.]: EOS (American Geophysical Union Transactions), v. 67, no. 44, p. 1253. CPDF deposits, by the occurrence of calcic soil from the basin margins by enhancing lake de- Baldridge, W. S, Vaniman, D. T., and ShaSqullah, M., 1987, Late Miocene to horizons throughout the sequence, and by the velopment and consequently affecting local base Pliocene development of the northern Jemez volcanic field, New Mex- ico [abs.]: EOS (American Geophysical Union Transactions), v. 68, commonly well-preserved nature of thin ash level (Figs. 2, 10D). Entrenchment of distribu- no. 44, p. 1532. Blair, T. C., and Bilodeau, W. L., 1988, Development of tectonic cyclothems in deposits that would not have survived under tary streams heading in the Tschicoma highlands rift, pull-apart, and foreland basins: Sedimentary response to episodic wetter climatic conditions. The development of was subsequently inhibited by these processes, tectonism: Geology, v. 16, p. 517-520. Blissenbach, E., 1954, Geology of alluvial fans in semi-arid regions: Geological high-energy ephemeral processes across the and aggradation on the fan surface was effec- Society of America Bulletin, v. 65, p. 175-190. growing fan surface enhanced the initiation of Bloomfleld, K., and Valastro, S., Jr., 1977, Late Quaternary tephrochronology tively enhanced. of Nevado de Toluca volcano, central Mexico: Overseas Geology and sheetfloods and coarse-grained mass flows. The Mineral Resources, v. 46, p. 1-16. Puye fan deposits exhibit over-all coarsening- Broscoe, A. J., and Thompson, S., 1969, Observations on an alpine mudflow, high silt/clay ratios of most Puye debris-flow upward depositional trends related to post-4 Ma Steele Creek, Yukon: Canadian Journal of Earth Sciences, v. 6, deposits (Waresback, 1986) are typical of p. 219-229. tectonic activity along this portion of the Rio Bull, W. B., 1972, Recognition of alluvial fan deposits in the stratigraphic coarse-grained volcanic mass flows in regions record, in Rigby, J. B., and Hamblin, W. K., eds., Recognition of Grande rift. In this respect, the fan is an integral ancient sedimentary environments: Society of Economic Paleontologists where clay production is inhibited by rapid dep- stratigraphic feature of this major intracontinen- and Mineralogists Special Publication 16, p. 63-83. osition of unweathered debris (Hooke, 1967), Bullard, F. M., 1976, Volcanoes of the Earth: Austin, Texas, University of tal depression and is contemporaneous with Texas Press, 579 p. and where the over-all rates of chemical Cas, R.A.F., and Wright, J. V., 1987, Volcanic successions: Modern and an- other alluvial sequences exposed along the rift. cient: London, England, Allen and Unwin, 528 p. weathering are low (Dethier and Demsey, 1984; Volcanogenic-alluvial facies as completely pre- Cummans, J., 1981, Chronology of mudflows in the South Fork Toutle River Harrington and Aldrich, 1984). following the May 18 eruption: U.S. Geological Survey Professional served as the Puye Formation, however, are rare Paper 1250, p. 479-486. Davies, D. K, Vessel, R. K„ Mites, R. C, Foley, M. G„ and Bonis, S. B„ 1978, Tectonic setting has long been considered the in the stratigraphic record, suggesting that such Fluvial transport and downstream sediment modifications in an active volcanic region, in Miall, A. D., ed., Fluvial sedimentology: Canadian most important allocyclic variable controlling sequences have limited preservation potential Society of Petroleum Geologists Memoir 5, p. 61-84. and that the Puye is therefore unusual in this Dethier, D. P., and Demsey, K. A., 1984, Erosionai history and soil develop- alluvial-fan development, as differential move- ment on Quaternary surfaces, northwest Espafiola Basin, New Mexico: ment between source area and adjacent depo- regard. We suggest that the development of New Mexico Geological Society, 35th Annual Field Conference Guide- book: Rio Grande Rift: Northern New Mexico, p. 227-235. center (basin) governs both the preservation Puye Formation lithofacies adequately reflects Dethier, D. P., and Martin, B., 1984, Geology and structure along part of the northeast Jemez Mountains, New Mexico: New Mexico Geological potential and accumulation rates of the clastic sedimentation in a specific (semiarid, conti- Society, 35th Annual Field Conference Guidebook: Rio Grande Rift wedge (Bull, 1972; Miall, 1981; Nilsen, 1982). nental-rift-related) volcanogenic setting. Northern New Mexico, p. 145-150.

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Dethier, D. P., Harrington, C. D., and Aldrich, M. J., 1988, Late Cenozoic rates Lombard, R. E., Miles, M. B., Nelsen, L. M., Kresh, D. L., and Carpenter, P. J., Smith, G. A., 1986, Coarse-grained non-marine volcaniclastic sediments: Ter- of erosion in the western Española basin, New Mexico: Evidence from 1981, The impact of mudflows of May 18 on the lower Toutle and minology and depositional processes: Geological Society of America geologic dating of erosion surfaces: Geological Society of America Bul- Coultz Rivers: U.S. Geological Survey Professional Paper 1250, Bulletin, v. 97, p. 1-10. letin, v. 100, p. 928-937. p. 693-700. 1987a, The influence of explosive volcanism on fluvial sedimentation: Enos, P., 1977, Flow regimes in debris flows: Sedimentology, v. 24, Lowe, D. R., 1979, Sediment gravity flows: Their classification and some The Deschutes Formation (Neogene) in central Oregon, Journal of p. 133-142. problems of application to natural flows and deposits: Society of Eco- Sedimentary Petrology, v. 57, p. 613-629. EppJer, D. B., Fiak, J., and Fletcher, R., 1987, Rheologie properties and nomic Paleontologists and Mineralogists Special Publication 27, Smith, G. A., 1987b, Sedimentology of volcanism-mduced aggradation in flu- kinematics of emplacement of the Chaos Jumbles rockfáll avalanche, p. 75-82. vial basins: Examples from the Pacific Northwest, U.S.A., in Ethridge, Lassen Volcanic National Park, California: Journal of Geophysical Re- Mack, G. H., and Rasmussen, K. A., 1984, Alluvial fan sedimentation of the F. G., Flores, R. M., and Harvey, M. D., eds., Developments in fluvial search, v. 92, p. 3623-3633. Cutler Formation (Permo-Pennsylvanian) near Gateway, Colorado: sedimentology: Society of Economic Paleontologists and Mineralogists Fisher, R. V., 1960, Criteria for recognition of laharic breccias, southern Cas- Geological Society of America Bulletin, v. 95, p. 475-482. Special Paper 39, p. 217-228. cade Mountains, Washington: Geological Society of America Bulletin, Manley, K., 1976, The late Cenozoic history of the Española basin, New Smith, R. L., 1979, Ash-flow magmatism: Geological Society of America Spe- v. 71, p. 127-132. Mexico [Ph.D. thesis}: Boulder, Colorado, University of Colorado, cial Paper 180, p. 5-27. 1971, Features of coarse-grained, high-concentration fluids and their 171 p. Smith, R. L., Bailey, R. A., and Ross, C. S., 1970, Geologic map of the Jemez deposits: Journal of Sedimentary Petrology, v. 41, p. 916-927. 1979, Stratigraphy and structure of the Española basin, New Mexico, in Mountains, New Mexico: U.S. Geological Survey Map 1-571. 1983, Flow transformations in sediment gravity flows: Geology, v. 11, Riecker, R. E., éd., Rio Grande rift: Tectonics and magmatism: Wash- Stanley, D. J., 1978, Pumice gravels in the Riviere Claire, Martinique: Selective p. 273-274. ington, D.C., American Geophysical Union, p. 71-87. sorting by fluvial processes: Sedimentary Geology, v. 21, p. 161-168. Fisher, R. V., and Heiken, G., 1982, Mt. Pelee, Martinique: May 8 and 20, Mathisen, M. E., and Vondra, C. F., 1983, The fluvial and pyroclastic deposits Steele, R. J., Maehle, S., Nilsen, H., Roe, S. L., and Spinnager, A., 1977, pyroclastic flows and surges: Journal of Volcanology and Geothermal of the Cagayan basin, Northern Luzon, Philippines—An example of Coarsening-upward cycles in the alluvium of Horaelen Basin (Devo- Research, v. 13, p. 339-371. non-marine volcaniclastic sedimentation in an interarc basin: Sedimen- nian), Norway—Sedimentary response to tectonic events: Geological Fisher, R. V., Heiken, G. H., Purtyman, W. D., and Wohletz, K. H., 1984, tology, v. 30, p. 369-392. Society of America Bulletin, v. 88, p. 8222-8236. Cerros del Rio, New Mexico, maars [abs.]: EOS (American Geophysi- Melton, M. A., 1965, The geomorphic and paleoclimatic significance of alluvial Turbeville, B. N., 1986, Mineralogy, fades analysis, and interpretation of felsic cal Union Transactions), v. 65, no. 45, p. 1145. deposits in southern Arizona: Journal of Geology, v. 73, p. 1-38. pyroclastic deposits of the Puye Formation, Jemez Mountains, New Fritz, W. J., and Harrison, S., 1985, Early Tertiary volcaniclastic deposits of the Miall, A. D., 1970, Devonian alluvial fans, Prince of Wales Island, Arctic Mexico [M.S. thesis]: Arlington, Texas, University of Texas, 427 p. northern Rocky Mountains, in Flores, R. M., and Kaplan, S. S., eds., Canada: Journal of Sedimentary Petrology, v. 40, p. 556-571. Turbeville, B. N., and Self, S., 1988, San Diego Canyon ignimbrites: Pre- Cenozoic paleogeography of west-central U.S.: Society of Economic 1977, A review of the braided river depositional environment: Earth Bandelier Tuff ignimbrite explosive rhyolitic volcanism in the Jemez Paleontologists and Mineralogists, Rocky Mountain Section, 3rd Pa- Science Reviews, v. 13, p. 1-62. Mountains, New Mexico: Journal of Geophysical Research, v. 93, leogeography Symposium, Proceedings, Denver, Colorado, p. 383-402. 1981, ed., Sedimentation and tectonics in alluvial basins: Geological p. 6148-6156. Gardner, J. N., and Goff, F., 1984, Potassium-argon dates from the Jemez Association of Canada Special Paper 23,272 p. Turbeville, B. N., Waresback, D. B., and Self, S., 1989, Lava dome growth and volcanic field: Implications for tectonic activity in the north-central Rio Naylor, M. A., 1980, The origin of inverse grading in muddy debris flow explosive volcanism in the Jemez Mountains, New Mexico: Evidence Grande rift: New Mexico Geological Society, 35th Annual Field Con- deposits—A review: Journal of Sedimentary Petrology, v. 50, from the Plio-Pleistocene Puye alluvial fan: Journal of Volcanology and ference, Guidebook: Rio Grande rift: Northern New Mexico, p. 75-82. p. 1111-1116. Geothermal Research, v. 36, p. 267-291. Gardner, J. N., Goff, F., Garcia, S., and Hagan, R. G, 1986, Stratigraphic Nemec, W., and Steel, R. J., 1984, Alluvial and coastal conglomerates: Their Ui, T., 1983, Volcanic dry avalanche deposits—Identification and comparison relations and lithologic variations in the Jemez Mountains volcanic significant features and some comments on gravelly mass-flow deposits, with nonvolcanic debris stream deposits: Journal of Volcanology and field, New Mexico: Journal of Geophysical Research, v. 91, in Koster, E. H., and Steel, R. J., eds., Sedimentology of gravels and Geothermal Research, v. 18, p. 135-150. p. 1763-1779. conglomerates: Canadian Society of Petroleum Geologists Memoir 10, Van Houten, F. B., 1976, Late Cenozoic volcaniclastic deposits, Andean fore- Griggs, R. L., 1964, Geology and groundwater resources of the Los Alamos p. 1-31. deep, Colombia: Geological Sodety of America Bulletin, v. 87, area, New Mexico: U.S. Geological Survey Water-Supply Paper 1753, Newhall, C. G., and Melson, W. G., 1983, Explosive activity associated with p. 481-495. 107 p. the growth of volcanic domes: Journal of Volcanology and Geothermal Vessel, R. K., and Davies, D. K., 1981, Non-marine sedimentation in an active Hampton, M. A., 1979, Buoyancy in debris flows: Journal of Sedimentary Research, v. 17, p. 111-131. fore-arc basin: Sodety of Economic Paleontologists and Mineralogists Petrology, v. 49, p. 753-758. Nilsen, T. H.. 1982, Alluvial fan deposits, in SchoUe, P. A., and Spearing, D., Special Publication 31, p. 31-45. Harrington, C. D., and Aldrich, M. J., Jr., 1984, Development and deformation eds., Sandstone depositional environments: American Association of Walker, G.P.L., 1983, Ignimbrite types and ignimbrite problems: Journal of of Quaternary surfaces on the northeastern flank of the Jemez Moun- Petroleum Geologists Memoir 31, p. 49-86. Volcanology and Geothermal Research, v. 17, p. 65-88. tains: New Mexico Geological Society, 35th Annual Field Conference, Postma, G., 1986, Classification for sediment gravity-flow deposits based on Walton, A. W., 1979, Volcanic sediment apron in the Tascotal Formation Guidebook: Rio Grande rift: Northern New Mexico, p. 235-240. flow conditions during sedimentation: Geology, v. 14, p. 291-294. (Oligocene?), Trans-Pecos Texas: Journal of Sedimentary Petrology, Heiken, G., and Wohletz, K., 1987, Tephra deposits associated with silicic Rodine, J. D., and Johnson, A. M., 1976, The ability of debris, heavily freight- v. 49, p. 303-314. domes and lava flows: Geological Society of America Special Paper ed with coarse clastic material, to flow on gentle slopes: Sedimentology, 1986, Effect of Oligocene volcanism on sedimentation in the Trans- 212, p. 55-76. v. 23, p. 213-234. Pecos volcanic field of Texas: Geological Sodety of America Bulletin, Hooke, R. LeB., 1967, Processes on arid-region alluvial fans: Journal of Geol- Rose, W. I., Jr., 1972, Santiaguito volcanic dome, Guatemala: Geological v. 97, p. 1192-1207. ogy, v. 75, p. 438-460. Society of America Bulletin, v. 83, p. 1413-1434. Waresback, D., 1986, The Puye Formation, New Mexico: Analysis of a conti- Izett, G. A., Obradovich, J. D., Naesser, C. W., and Cebula, G. T., 1981, Rose, W. I., Jr., 1987, Volcanic activity at Santiaguito volcano, 1976-1984, nental rift-filling volcaniclastic alluvial fan sequence [M.S. thesis]: Potassium-argon and fission-track zircon ages of Cerro Toledo rhyolite Geological Society of America Special Paper 212, p. 17-28. Arlington, Texas, University of Texas, 225 p. tephra in the Jemez Mountains, New Mexico: U.S. Geological Survey Rose, W. I., Jr., Pearson, T., and Bonis, S., 1976, Nuée ardente eruption from Williams, H., 1932, The history and characters of volcanic domes: University of Professional Paper 199-D, 43 p. the foot of a dacite lava flow, Santiaguito volcano, Guatemala: Bulletin California Publications in Geological Sdences, v. 21, p. 51-146. Kelley, V. C., 1956, The Rio Grande depression from Taos to Santa Fe: New Volcanologique, v. 40, p. 23-38. Williams, H., and McBimey, A. R., 1979, Volcanology: San Francisco, Cali- Mexico Geological Society, 7th Field Conference, Guidebook, Rust, B. R., 1978, Depositional models for braided alluvium, in Miall, A., ed., fornia, Freeman, Cooper & Co., 397 p. p. 109-114. Fluvial sedimentology: Canadian Society of Petroleum Geologists Mem- Winder, C. G., 1965, Alluvial cone construction by alpine mudflow in a humid 1979, Geomorphology of the Española basin: New Mexico Geological oir 5, p. 605-625. temperate region: Canadian Journal of Earth Sdences, v. 2, p. 270-277. Society, 30th Field Conference, Guidebook: Santa Fe Country, Rust, B. R., and Koster, E. H„ 1984, Coarse alluvial deposits, in Walker, R. G., Wright, J. V., Roobol, M. J., Smith, A. L., Sparks, R.S.J., Brazier, S. A., Rose, p. 281-288. ed., Fades models: Geological Association of Canada Reprint Series 1 W. I., Jr., and Sigurdsson, H., 1984, Late Quaternary explosive silicic Koch, A. J., and McLean, H., 1975, Pleistocene tephra and ash flow deposits in (2nd edition), p. 53-70. volcanism on St. Luda, West Indies: Geological Magazine, v. 121, the volcanic highlands of Guatemala: Geological Society of America Scott, W. E., 1987, Holocene rhyodacite eruptions on the flanks of South Sister p. 1-15. Bulletin, v. 84, p. 2957-2980. volcano, Oregon: Geological Society of America Special Paper 212, Kuenzi, D. W., Horst, O. H., and McGehee, R. V., 1979, Effect of volcanic p. 35-54. activity on fluvial-deltaic sedimentation in a modem arc-trench gap, Self, S., Goff, F., Gardner, J. N., Wright, J. V., and Kite, W. M., 1986, southern Guatemala: Geological Society of America Bulletin, v. 90, Explosive rhyolitic volcanism in the Jemez Mountains: Vent locations, p. 827-838. caldera development and relation to regional structure: Journal of Geo- Lawson, D. E., 1980, Mobilization, movement and deposition of active subaer- physical Research, v. 91, p. 1779-1799. ial sediment gravity flows, Matan uska Glacier, Alaska: Journal of Shultz, A. W.( 1984, Subaerial debris-flow deposition in the upper Paleozoic Geology, v. 90, p. 279-300. Cutler Formation, western Colorado: Journal of Sedimentary Petrol- Lindsay, J. F., 1968, The development of clast fabric in mudflows: Journal of ogy, v. 54, p. 759-772. Sedimentary Petrology, v. 38, p. 1242-1253. Singer, B. S., and Kudo, A. M., 1986, Assimilation-fractional crystallization of MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 1,1988 Loeffler, B. M., Vaniman, D. T., Baldridge, W. S., and Shafiqutlah, M., 1988, rocks in the northwestern Jemez Mountains volcanic REVISED MANUSCRIPT RECEIVED JULY 10,1989 Neogene rhyolites of the northern Jemez volcanic field, New Mexico: field, New Mexico: Contributions to Mineralogy and Petrology, v. 94, MANUSCRIPT ACCEPTED JULY 19,1989 Journal of Geophysical Research, v. 93, p. 6157-6168. p. 374-386. FINAL ILLUSTRATIONS ACCEPTED NOVEMBER 27,1989

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