Origin of the Joya Honda maar, San Luis Potosí, México

Jotiof

~01candogy

andgeothermalmearch

ELSEVIER

Journal of Volcanology

and Geothermal

Research 74 (1996)

I- 18

Origin of the Joya Honda maar, San Luis Potosi, Mkxico Jo& Jorge Aranda-G6mez aT*, James F. Luhr b a Estacirin Regional de1 Centro, Institute de Geologia, Uniuersidad National Autbnoma de Mixico, Guanajuato, Gto., 36000, Mt!xico b Department of Mineral Sciences, NHB-119. Smithsonian Institution, Received 18 January

1996; accepted

Washington, DC, USA

19 June 1996

Abstract Joya Honda is a Quatemary maar of unusual type from the Mexican Basin and Range Province. Its _ 300-m-deep crater is excavated in Cretaceous limestones. The surrounding tephra deposit, which in places is > 100 m thick, begins with a series of weakly indurated pyroclastic-surge and -fall layers that we interpret as dry-surge deposits. These are overlain by the main sequence of strongly indurated, massive tuff breccias that we interpret as wet-surge deposits. Joya Honda formed subaerially from the interaction of groundwater with rapidly ascending intraplate-type basanitic magma carrying peridotitic mantle xenoliths. Local aquifer characteristics controlled the style of eruption and the nature of the deposits. Groundwater in the limestone-hosted aquifer beneath Joya Honda was apparently contained within solution-enhanced fractures. At the onset of the eruption, magma began to interact with a moderate amount of groundwater, producing the dry-surge deposits, which are typical of deposits found at many maars and tuff rings. As the eruption continued, the crater grew and the hydromagmatic blasts fractured the limestones around the explosion foci. A marked increase in the water/magma ratio of the system followed when a large fracture or a portion of the limestone with enhanced secondary permeability was intersected by the expanding crater. Subsequent phreatomagmatic explosions occurred in a system with groundwater flow rates several orders of magnitude larger than in the initial dry-surge stage. At the maar rim these wet eruptions led to the emplacement of massive tuff breccias through a combination of fallout, surges and mudflows. These steeply dipping tuff breccias are similar to deposits found at many tuff cones. Juvenile clasts in the near-vent deposits show marked upward increases in both hydration (palagonitization) and vesicularity. The increased palagonitization with height in the section appears to be a consequence of the overall increased wetness of the eruption with time, correlating with greater carbonate cementation and lithification in the upper part of the deposit. The transition toward higher vesicularity is interpreted as evidence of a gradual reduction in the confining pressure for the ascending magma prior to explosive fragmentation, perhaps related to unroofing during progressive excavation of the overlying maar crater. Thus, Joya Honda does not support maar-formation models that invoke downward displacement of explosion foci, caused by formation of a cone of depression in the aquifer, in order to maintain the confining pressure for the hydromagmatic blasts. Keywords:

maars; phreatomagmatism;

Mexico; pyroclastic

surges; volcanic breccia; San Luis Potosi, Mexico; aquifers

1. Introduction Maar-type

* Corresponding author. 0377-0273/96/$15.00 Copyright PII SO377-0273(96)00044-3

phreatomagmatic 0 1996 Elsevier Science B.V. All rights reserved.

volcanoes or phreatic

are activity

produced (Waters

by and

2

JJ. Arundu-Gbmez,

J.F. Luhr/Journal

of Volcunolog,v and Geothermal Research 74 (19%) I-IX

Fisher, 1970), and can be divided into three different morphological types (Cas and Wright, 1988): (I) maars (sensu stricro); (2) tuff rings; and (3) tuff cones. Maars are relatively large craters, up to 3 km in diameter, partially to completely surrounded by low ramparts of bedded ejecta that decrease rapidly in thickness away from the rim. Maar craters have steep to nearly vertical walls, and by definition country rock is exposed beneath the pyroclastic deposits (Lorenz, 1973, 1986). Tuff rings and tuff cones are constructional landforms that rise above the pre-volcanic surface. Their crater floors lie on or above the pre-eruption level, and their walls expose only volcanic material. Distinction between tuff rings and tuff cones is based both on morphology and on the nature of associated pyroclastic deposits. The crater diameter to cone basal diameter ratio tends to be larger in tuff rings than in tuff cones. The pyroelastic blankets around tuff rings consist of poorly indurated, thinly bedded surge and tephra-fall deposits, typically, but not always produced by “dry” surges (Sheridan and Wohletz, 1983; Chough and Sohn, 1990). Tuff rings have outward dips less than 12”, which are reflected by the gentle slopes of the outer ramparts. Tuff cones are steep-sided landforms constructed from massive, thickly bedded, highly indurated pyroclastic material with bedding angles up to 30”, interpreted by Wohletz and Sheridan (1983) and Sheridan and Wohletz (1983) to have formed by “wet” surges. Inward dipping beds along the crater walls are common in both tuff rings and tuff cones, but are rare or absent in maar craters (Ollier, 1967; Lorenz, 1973).

Although there are clear morphological contrasts among the three different varieties of maar-type volcanoes, only Tut distinctive pyroclastic sequences are formed by hydromagmatic activity. Maar craters (s.s.1 can be associated with either type, but the thinly bedded, poorly indurated, shallowly dipping sequences of tuff rings are much more common than the massive, strongly indurated, steeply dipping sequences of tuff cones (Wohletz and Sheridan, 19831. According to the generalized model of Wohletz and Sheridan (1983) the strikingly different stratigraphic sequences found around tuff rings and tuff cones are controlled by: (1) the water/magma ratio, with “ wetter’ ’ eruptions favoring the formation of tuff-cone over tuff-ring sequences; and (2) the depth of water-magma interaction: shallower for tuff cones and deeper for tuff rings and maars. For maar craters, the initial water/magma interaction is sufficiently deep to start excavating the crater into the underlying country rock. The considerable diameter and depth attained by some maars has been ascribed to downward displacement of the explosion foci, due to temporary creation of cones of depression in the aquifer, and to collapse of the wall rock into the diatreme root zone (Lorenz, 1986). This paper focuses on Joya Honda maar, a crater excavated into Cretaceous limestones during middle Pleistocene times. Its proximal pyroclastic sequence begins with a thin (< 9 ml tuff-ring-like tephra deposit, and is crowned by a thick (up to 90 ml tuff-cone-like sequence. We contrast the general characteristics of groundwater flow in porous-media

Fig. 1. (a) Joya Honda (JH) is located in San Luis Potosi (outlined), 35 km northeast of the state’s capital (SLP). (b) Generalized geologic map of San Luis Potosi State showing the location of other phreatomagmatic volcanoes in the area. Maars excavated in limestones and surrounded by wet-surge deposits (tuff-cone-like sequences) are: Joya Honda (I); Xalapasco de Santo Domingo (4); El Banco (5); Joya de Los Contreras (6); and Joya Prieta (7). The Laguna de Los Palau (3) and Pozo de1 Carmen (2) maars were excavated in gravel deposits. where the aquifer was probably of the porous-media type, and they are surrounded by dry-surge deposits (tuff-ring-like sequences). (c) Generalized geologic map of the Joya Honda region. The map unit labeled limrstonrvand shales includes the older (Aptian-Turonian) limestones of the Tamaulipas, Cuesta de1 Cura and Abra formations, and the limestones interlayered with shales of the Soyatal and Indidura formations. The younger marine sediments (Coniacian) are sandstones and shales of the Grdenas Formation. All Laramide-age folds in the area tend to be overturned to the east, The axes of the major folds were drawn in the areas where the oldest (anticlinorium) and youngest (synclinorium) rocks occur. The Quaternary (Table 1) basanites are divided into lavas and pyroclastic rocks. Note the displacement of the major structures northeast of Joya Honda, and the occurrence of lineaments, which are clearly seen on satellite images in the maar area. The nature of the faults (whether normal or strike-slip displacement) is unknown (modified after Labarthe-Hemandez et al., 1982). The gravel distribution in the southeastern corner of the map was taken from Detenal (1982). Volcanic centers: CV = Cerro Verde cinder cone: JJf = Joya Honda maar; J = Joyuela tuff cone; PC = Pozo de1 Carmen maar: LP = Laguna de 10s Palau maar. Towns: VH = Villa Hidalgo; Ll = Llano Novela; A = Armadillo.

J.J. Aranda-Gdmez, J.F. Luhr/ Journal of Volcanology and Geothemtal Research 74 (1996) l-18

(a) _

\

WN

TrenddLsamklefokJs

QUATERNARY Alluvium Basanite (1: pyroclasts, 2: lava81 Gravels

OLlGOCENE Volcanic rocks

CRETACEOUS Sandstones and shales Limestones and shales

0

Maar

*

Cinder cone Fault

e-f+ *

Fold axes Towns

3

4

J.J. Aranda-Gbmez, .I.F. Luhr / Journal of Volcanology and Geothermal Reseurch 74 ( 19961 I - 18

aquifers, contained in unconsolidated sediments, with those in fracture-controlled limestone aquifers. We believe that the fracture-controlled limestone aquifer beneath Joya Honda explains its seeming paradox: a dominantly tuff-cone-like pyroclastic sequence surrounding a large maar crater excavated - 220 m below the pre-volcanic surface.

Table 1 K-Ar Ages for groundmass separates from Quatemary basanites of the Ventura and Santo Domingo volcanic fields, San Luis Potosi, Mtxico

SLP-23 1.383

4.06073 2.631 X IO- ”

2.3

1.10_+0.21

2. Regional setting

SLP-34 1.884 SLP-34 1.884

3.92X33 4.715X 10 ” 3.92833 4.620X IO-”

12.4 12.2

I .44 + 0.08

Ln Joya SLP-5 SLP-5 SLP-5

1.99696 1.314X IO-” 1.99696 1.352~ IO-” 4.0319X 1,289x10-”

7.8 8.0 14.2

0.45 + 0.06 0.46+0.01 0.44 + 0.04

2.88538 6.501 x IO-” 2.88538 6.121 X IO-”

3.1 2.9

0.35io.14 0.33+0.12

Joya Honda lies 35 km northeast of the city San Luis Potosi in central Mtxico (Fig. la), near the southern end of the Basin and Range Province. It was formed at about 1.1 Ma (Table 1) during eruption of intraplate-type basanitic magma that carried mantle and deep crustal xenoliths to the surface. The crater was excavated (Fig. lc and Fig. 2) in the Cuesta de1 Cura (Albian-Cenomanian) and Tamaulipas Formations (Aptian), which are thinly bedded calcareous mudstones with variably abundant chert bands/lenses and minor shale partings. The limestones in this area locally contain thick beds of calcareous debris breccia. Regional stratigraphic studies reveal that the Mesozoic sequence around Joya Honda represents the shelf and foreslope facies of a large calcareous platform located toward the east (i.e., the Valles-San Luis Potosi Platform: Carrillo-Bravo, 1971). The Mesozoic sequence in the area was folded during the early Tertiary Laramide Orogeny. Joya Honda lies at the intersection of the overturned anticlinorium crest of Sierra de1 Coro with a N60”E-trending fault zone (Fig. lc) of late Tertiary or Quaternary age (Aranda-G6mez and Labarthe-Hernhndez, 1975). Close to Joya Honda are four other maars excavated in limestones (Fig. lb): Joya Prieta, Joya de Los Contreras, El Banco and Xalapasco de Santo Domingo (Labarthe-HernLndez, 1978; ArandaG6mez, 1982; Luhr et al., 1989). All of them are surrounded by strongly indurated tuff breccias (wetsurge deposits) similar to those at Joya Honda. In contrast, the nearby Laguna de Los Palau and POZO de1 Carmen maars (Fig. lb and c) are surrounded by poorly indurated, thinly bedded pyroclastic sequences (dry-surge deposits). These two maars were excavated in gravels resting atop the Mesozoic limestones.

Sample K+ Weight (No.) (wt.%,) (g)

“‘Ar * (mol/g)

%‘“Ar * Age+ Itr (Ma)

La Joya Honda

1.41kO.07

de 10s Contreras

1.683 1.683 1.683

Cerro el Apaste

SLP-IO 1.070 SLP-10 1.070

Analyses performed at Berkeley Geochronology Center. Decay constants: A, + A,, = 0.581 X lo-‘” yrF’; 4 = 4.962~ IO-“’ yr-‘; h= 5.543X lo-‘” yr-‘; and ‘“K/Klola, = 1.167~ 10eJ. “‘Ar * refers to radiogenic component. Sample locations given in appendix of Luhr et al. (1989).

Detailed descriptions of Joya Honda, its geologic setting, and the petrology of the lavas and scoriae have been published elsewhere (Labarthe-HernLndez, 1978; Luhr et al., 1989; Pier et al., 1989; ArandaG6mez et al., 1993). In the following sections we briefly describe the crater and its associated tephra deposits. We demonstrate that Joya Honda is a maar S~FZSU stricto (Ollier, 1967; Lorenz, 1973, 1986; Cas and Wright, 1988), and that its associated deposits show a transition in emplacement mechanisms over time, from early dry surges to later and dominant wet surges.

3. Local geology: Joya Honda The large Joya Honda crater has nearly vertical walls (Fig. 3). Its form is elliptical, with axes 1100 and 850 m long, oriented N60”E and N30”W, respectively. Maximum relief from the floor to the top of the rim is approximately 300 m. The maar-related tephra sequence, described below, rests unconformably atop folded and faulted Cretaceous lime-

J. J. Aranda-Gdmez, J. F. Luhr / Journal of Volcanology and Geothermal Research 74 (I 996) I - I8

QUATERNARY /__zi Alluvium m

Talus deposit

m]

Basanitic lavas

zj

Tuff-breccia

n

Tuff

CRETACEOUS r?

Attitude of near-vent tephra

e Fig. 2. Detailed geologic (tuff breccias).

Limestones and shales

map of the Joya Honda crater area. Note the steep outward dips of the Joya Honda near-vent

stones. The pyroclastic deposit is thickest (N 120 ml in the northern wall of the maar and thins rapidly around the crater. It is absent in the southern part of the crater, where the maar wall, 110 m high, exposes only limestones (Figs. 2 and 3). The depth of the excavated crater, measured from the highest point on the pre-volcanic surface to the bottom of the maar is N 220 m. A stratigraphic section through Joya Honda’s near-vent tephra sequence is shown in Fig. 4, with sixteen intervals distinguished. Descriptions of these intervals are given in Table 2. The folded Mesozoic sediments are unconformably overlain by a thin (0.3

NW

N25W

wet-surge

deposits

ml soil horizon (Interval 1: I- 11, which in turn is covered by a 1.6-m-thick scoria-fall deposit (I-2). Resting atop this tephra is a paleosoil with abundant clasts of limestone (I-3). This indicates that an explosive eruption took place in the same area thousands of years before the maar-forming eruption. The maar-related pyroclastic sequence begins with unit I-4, a basanite-rich surge deposit with large clasts of limestone (up to 20 cm), and extends upward for N 50 m through soil-capped tuff-breccia unit I-16. The lower 8.4 m of the sequence, from I-4 to I-10, consists of tan-colored, generally well sorted and bedded tuffs with conspicuous sedimentary

SE

SW

2000

N80E

YE

maal 1800

Diafreme’

Cretaceous I km

limesto’ne Ikm

Fig. 3. Schematic cross sections of Joya Honda. The large maar crater shows outcrops of pre-maar rocks in its walls. The heavy dashed line represents the pre-volcanic surface. Joya Honda was excavated in the southern flank of a hill,

6

J.J. Aranda-Gdmez,

J.F. Luhr/ Journal of Volcanology and Geothetmal

Research 74 (19961 l-18

53.67

blocks and boulders

49.96

? .P506

ia_

Limestone

w

Llthic breccia

@I

Matrix-supporled

blocks and boulders with gravel-sled

clasts

tuff breccia

@

Massive

E

Finely laminated,

m

Finely laminated

tuff with sandwave

m

Finely laminated

tuffwiih

h.i

Pebble

~

Contoned beddlng due to impact ballistlc clast or to load cast

[@A

Tuff with abundant

B@ @

t”ff planar-bedded

trains in massive

Sasanitic Ballisiic

wavy

of lapilli

horizons

clast and impact

pB

m Erosional channel in surge layers 614 Soil horiion with verilcal columnar

(Z.$ 24.47

r!%j E

Structureless Tuff breccla.

, @J SLPJOO

Limestone Sample

beds

bedding

tuff-breccia

accretionary

scorla-fall

tuff

pints

soil horizon with limestone partially covered

cl&s

by soil

and then location

3.30

1.90 37.670.00

Fig. 4. Stratigraphic section measured in the eastern wall of Joya Honda. White accidental clasts in the tuff breccias of Joya Honda are mostly limestone and chert with a very small amount of propylitized andesites. Compare overall nature of the pyroclastic deposits around Joya Honda with the tuff-cone sequence of Wohletz and Sheridan (1983). The numbered intervals correspond to the intervals described in Table 2. Grain-size scales at the top of each column show visual estimates of the median clast sizes.

structures such as channels, antidunes, accretionary lapilli, impact pits, and load casts. These are interpreted as dry-surge deposits. Units I-l 1 and I- 12 form a transition to the tuff breccias in the upper part of the sequence; I-l 1 (1.6 m) is similar to the upper tuff breccias, whereas the overlying I-l 2 (1.5 m> is typical of the lower dry-surge deposits. The upper 39.1 m of the sequence, from I-13 to I- 16, representing 78% of the measured section, is an accumulation of highly indurated, massive to diffusely bedded, heterolithologic tuff breccias (Figs. 4 and 5), with clasts of juvenile basanite and accidental

limestone, chert. propylitized andesite. peridotite and feldspathic granulite. The tuff breccias are divided into two members based on an overall color change from light brown at the base (I-13) to reddish brown at the top (I-161, and a transition from diffuse bedding at the top of I-13 to more distinct bedding in the coarser grained tuff-breccias of I-14 to I- 16. The reddish brown color of I-14 to I-16 is related to the presence of abundant, highly vesiculated and oxidized palagonite clasts. The pyroclastic sequence related to Joya Honda covered an irregular pre-volcanic surface, and pri-

J.J. Arandu-GBmez, J.F. Luhr/Joumal

of Volcanology and Geothermal Research 74 (1996) I-18

Fig. 5. Highly indurated heterolithologic tuff-breccia emplaced by wet surges around Joya Honda Note the ill-defined stratification and the steep bedding angle.

mary dips near the vent may locally be up to 30” (Fig. 2). We use the term wet-surge to describe these mechanisms deposits, although their emplacement may have been similar to those of high-density, non-expanded mudflows. Vent-margin fallout also probably contributed to their accumulation. Less than 1 km northeast of the crater center the near-vent tuff breccias grade into well sorted, unconsolidated scoria-fall deposits. Three kilometers from the crater center, the tephra grade to a medium- to coarsegrained basanitic tuff with conspicuous mantle bedding.

4. Petrography

of near-vent

Joya Honda samples

Bulk rock samples were collected from dry-surge deposits of the lower sequence at 1-4, I-5a, I-5b, I-6a, I-6b, I-8 and I-12, and from wet-surge deposits of the upper sequence at I-l 3a and I-14a (Table 2; Fig.

7

4). The specimens are mainly formed by variable amounts of basanite and accidental limestone (Table 3). Minor constituents are chert (Fig. 6a and c) and rare clasts of sandstone derived from the Mesozoic marine sequence, xenocrysts acquired from comminution of the upper mantle and lower crustal xenoliths, and rare propylitized andesite from the mid-Tertiary volcanic rocks of the region. Significantly, cuspate shards and splintery fragments of glass are absent. As a consequence of the many clast lithologies present, the mineralogy of the rocks is quite diverse and unusual. The juvenile basanitic clasts include the primary paragenesis: olivine + titanaugite + opaques (titanomagnetite and/or ilmenite) and a set of accidental minerals derived mainly from xenoliths of spine1 lherzolite and feldspathic granulites. Olivine is the most common xenocryst in the basanite clasts as well as in the whole-rock tuff samples (Fig. 6a and c), along with less common orthopyroxene, chrome diopside, and spinel. In the juvenile clasts each phase shows characteristic reaction rims or coronae at the contact with the transporting basanite (Fig. 6d). In general, samples from the dry-surge deposits (I-4 to I-8 and I-12) are fine grained and the sedimentary and volcanic clasts “float” in a matrix made of finely comminuted limestone (Fig. 6b), which appears as a cryptocrystalline aggregate under the microscope. Juvenile clasts of the dry-surge deposits are non-vesiculated and unaltered (Fig. 6a and b). The lowermost sample (from I-4) is completely devoid of matrix and is composed of very well sorted, subrounded clasts of basanite and limestone with an average diameter of 0.5 to 1 mm (Fig. 6a). This rock has an open framework and the clasts are weakly cemented by calcite that forms thin coatings around the clasts (Fig. 6a). The overlying wet-surge deposits (tuff breccias) are poorly sorted, with large clasts, up to 40 cm long, set in a tuffaceous matrix, The juvenile clasts become progressively more vesiculated and palagonitized upsection (Fig. 6c and d). Among the basanite clasts, the highest degrees of vesicularity and alteration are found in the uppermost sample (I-14a), where some fragments of “frothy” palagonite (Fig. 6f) coexist with moderately vesiculated clasts of palagonite and rare fragments of non-hydrated basanitic glass (Fig. 6d and e). Also common in this

J.J. Aranda-Gdmez, J.F. Luhr/ Journal

8

of Volcanology and Geothermal Research 74 (1996) l-18

Table 2 Stratigraphic Interval

section of the pyroclastic Meters

0.

I. 2a.

0.3 0.3

2b.

1.3

3a.

1.0

3b. 3c. 4.

0.3 0.1 0.2

5a.

0.1

5b.

0.06

6a. 6b.

0.3 0.04

6c. 7.

0.05 0.04 0.45

8.

0.5

9a.

1.0

9b. 9c.

0.03 2.4

9d. 9e.

0.3 1.5

lOa.

0.5

lob.

0.7

Il.

1.6

12.

1.5

13a.

3.0

6d.

-

sequence exposed on the eastern wall of Joya Honda maar

Description Cuesta de1 Cura Limestone (Cretaceousl Soil horizon. Contains 5-10% tabular fragments of limestone and chert, 5-40 cm long. Caliche cemented. Relatively coarse-grained basanitic scoria-fall deposit, with 5-10% of clasts > 20 cm in diameter. The visually estimated median grain size in the deposit is 1.5-3 cm. A sequence of 7 scoria-fall beds, each lo-20 cm thick, of relatively well sorted basanitic Scotia, with a median grain size of 0.5-l cm. Maximum clast diameter is 2 cm. A buff-colored, caliche-cemented soil horizon with _ 40% subangular to subrounded fragments of limestone, up to 20 cm in diameter. Median clast size is 10 cm. The cementing carbonate has finely laminated structures in a few places. Columnar-jointed, salmon-pink soil horizon with abundant clasts of limestone up to 15 cm in diameter. Buff-colored. fine-grained soil horizon with limestone fragments up to 4 cm in diameter. Coarse-grained. well bedded, medium-gray basanite-rich surge deposit with rare ballistic fragments of limestone up to 20 cm in diameter. Large clasts did not produce bed sags. The base of this horizon shows conspicuous wavy laminations. Sample SLP-500. Surge deposit, very fine-grained, greenish gray. distinctly laminated, with low-angle cross bedding and impact pits. Accretionary lapilli bed. The size of accretionary lapilli diminishes gradually from 5 to 2 mm toward the top. Sample SLP-501 was collected at the contact between I-5a and I-5b. Tan-colored, very fine-grained, finely laminated surge deposit, Cross bedded sets are 2 cm thick. Ash-fall bed. The base is very uneven. It produced soft-sediment deformation in the underlying surge unit, Sample SLP-502 was collected at the contact between I-6a and I-6b. Same as 5b. 5 cm thick. Same as 5b. Its top was deformed by the weight of the overlying deposit, Massive to diffusely bedded surge deposit. Bedding is marked by fine trails of basanitic ash. Small fragments of limestone (to 2 cm) are present near the base. These become coarser grained (to 9 cm) and angular to subangular toward the top. Relatively coarse-grained (0.5- 1.O cm) surge deposit, composed of limestone > chert > basanite. Channels I-2 m wide and 0.2-0.5 m deep with internal cross stratification. Sample SLP-503. Very finely laminated surge deposit with abundant accretionary lapilli. It shows low-angle cross stratification. with bed sets 65 cm thick. At the base of the interval is a very fine-grained, pink-colored, massive bed, 4 cm thick. Erosional channels, 2.5 m wide and up to 55 cm deep are common; they are filled with very fine to fine gravel: limestone clasts reach 8 cm. A small break in the surge sequence, marked by a thin bed ( - 3 cm thick) of very fine gravel ( 5 1 cm). Same as 9a. It becomes coarser grained toward the top. The pebbles occur as discontinuous trains. most of them are nearly 2 cm in diameter. Their maximum size in the uppermost meter of the interval is 6 cm. Abundant accretionary lapilli are present throughout the interval. A friable, fine-grained, massive, buff-colored. poorly indurated tuff. Similar to 8d, but with a fining-upward sequence. At the base it contains nearly 10% limestone clasts with a median diameter of 2 cm. Accretionary lapilli up to 5 mm in diameter are abundant. A poorly cemented, fine-grained tuff. In some places large ballistic clasts produced symmetrical bomb sags in the underlying bed. Massive to weakly laminated, very fine-grained surge deposit with abundant accretionary lapilli. It shows marked thickness variations related to load casts and impact pits in the upper surface of the interval. Accretionary lapilli are abundant. Very poorly sorted, fine to medium gravel (l-2 cm) with 15% large (to 55 cm) angular blocks of limestone. In a few places weak, diffuse planar bedding is evident. We interpreted this as a lithic-fall deposit (‘?I Very fine-grained surge deposit. It is massive and contains abundant accretionary lapilli near the base. In the central part are normally graded beds 15-20 cm thick. The upper part shows diffuse planar cross bedding. Sample SLP-504. Tuff-breccia with diffuse, large-scale beds, 40 cm to 1.0 m thick, which give the deposit an overall layered appearance. In places these beds show internal parallel laminations l-2 cm thick, marked by small changes in grain size or lithology. The deposit is formed by subangular clasts of limestone (60%) and basanite (40%) with a median diameter of l-l.5 cm. The largest clasts observed were 6 cm across. A few spinel-lherzolite xenoliths, 2 cm in diameter, were observed. Sample SLP-505.

J.J. Aranda-Gbmez, J.F. Luhr/Joumal

of Volcanology and Geothenal

9

Research 74 (1996) I-18

Table 2 (continued1 13b.

4.6

13c.

2.3

14a.

3.2

14b.

6.4

14c.

3.6

15.

12.8

16.

3.2

Similar to I-13a. The amount of coherent spinel-Iherzolite xenoliths is greater. The largest mantle xenolith observed was 12 cm in diameter. The size (up to 15 cm) and abundance of limestone clasts is also greater than for I-13a. Tuff-breccia, the median clast size is 3 cm. The largest block of limestone observed is 40 cm. Bedding is defined by small changes in grain size. Mantle xenoliths are common. Tuff-breccia, with median clast size of 0.5 cm. Basanite increases to 70% and is bright orange to brown, intensely oxidized, scoriaceous palagonite. The abundance and size of clasts of limestone decrease compared to 1-13~. The largest limestone clast observed in I-14a is 3 cm in diameter. Sample SLP-506. Same as I-14a, but mantle xenoliths are more abundant. Median clast size increases to 2 cm. A few dense clasts of basanite were observed, up to 35 cm in diameter. The amount and size of limestone clasts gradually increases upward. The largest block observed was 50 cm in diameter. Mantle xenoliths are up to 12 cm across. Large ballistic clasts did not produce bomb sags in the underlying deposits. Median clast size decreases to 0.5 cm. Oxidized Scotia increases to 40%, in contrast to the 20% observed in the upper portion of 1-14~. A few isolated fragments of limestone reach 15 cm diameter. At the top of the interval scoria content decreases to 20%. Tuff-breccia, poorly exposed, covered by soil and caliche.

sample are rounded, slightly vesiculated palagonite fragments with internal jigsaw cracks (Fig. 6e1, characteristic of vitric hydroclastic shards (Fisher and Schmincke, 1984). Glass in samples from I-13a and I-14a is bright orange to red, and opaque minerals appear partially to completely oxidized.

5. The role of groundwater in the formation of La Joya Honda Inasmuch as there is compelling evidence of water-magma interaction in the near-vent tephra deposits of Joya Honda, and the volcano did not occupy the site of a lake at the time of the eruption, the local source of water for the hydromagmatic activity must have been an aquifer contained in the pre-maar rocks. The paradox presented by Joya Honda, then, is the association of a large crater, deeply excavated into competent limestones (commonly regarded as material with limited groundwater transmissivity), and its tuff cone-like tephra deposit, which by conventional interpretation indicates a large water/magma ratio and shallow-water-magma interaction, commonly, but not necessarily with a body of standing water above the vent (e.g., Leat and Thompson, 1988; Godchaux et al., 1992). We propose that the Joya Honda paradox can be explained by: (1) recognizing the radically different behavior

of groundwater in fracture-controlled aquifers compared to those dominated by matrix porosity; and (2) the potential “open-system” nature of fracture-controlled aquifers, where the confining pressure for the hydrovolcanic explosions might be given only by the weight of the water column in the open fractures and/or solution conduits (Fig. 7a). In contrast, the confining pressure for expanding heated vapor in porous-media aquifers is given by the rupture strength of the surrounding rocks and the weight of the

Table 3 Clast and matrix abundances

in the Joya Honda whole-rock

tuff

samples a Interval: Sample (SLP-1: Accidental fragments Juvenile clasts ’ Matrix

I-4 500

I-5 501

I-6 502

I-8 503

I-12 504

I-13 505

I-14 506

b 25.6 14.8 17.3 40.1 18.0 31.3 22.8 42.8 17.4 21.7 19.6 13.8 41.5 48.0 31.5 d 67.8 61.0 38.5 68.2 27.2 29.2

a All modes determined by counting > 500 points on a single thin section. b Refers to upper crustal xenoliths, mainly (98-99%) limestone fragments derived from units exposed at the surface. All samples contain small amounts (l-28) of sedimentary chert from the same source. ’ Include juvenile basanite fragments and xenocrysts derived from deep-seated inclusions (spine1 lherzolite and feldspathic granulites). d Open-framework sample with virtually no matrix. 13.6% void spaces and 17.9% calcite cement coating the clasts.

IO

J.J. Aranda-Gbmez, J.F. Luhr/Joumal

of Volcanology and Geothermal Research 74 (1996) I-18

Fig. 6. Plane light photomicrographs of thin sections from whole-rock samples of Joya Honda tuffs and tuff breccias. Each photo is 3.2 mm across. Mineral and rock abbreviations: 0 = olivine; C = calcite; Op = orthopyroxene; CH = chert; Ls = limestone: Ba = basanite (unaltered); Pa = palagonite. (a) Open-framework, well sorted surge bed (I-4). Most clasts of both limestone and non-vesiculated and glassy basanite are subrounded and covered by a thin coating of calcite cement, (b) Matrix-supported, finely laminated heterolithologic tuff (I- 12). Fragments are subrounded to subangular, and the lamination in the rock is defined by changes in the grain size of the clasts. (cl Palagonitized clasts in tuff breccia (I-13a). Note the presence of an olivine xenocryst in one of the basanite fragments. (dl Palagonitized clast with jigsaw cracks in tuff breccia (I-14a). The orthopyroxene xenocryst is surrounded by a reaction rim. (el Composite clast of basanite in tuff breccia (I-l4a). An unaltered, rounded, non-vesiculated clast is enveloped by vesiculated and palagonitized glass. (f) Clast of basanite with frothy structure in tuff breccia (I-14al.

J.J. Aranda-Gbmez, J. F. Luhr / Journal of Volcanology and Geothermal Research 74 (1996) 1-l 8

11

OPEN SYSTEM

W. T.

. Ascending magma

Ascending magma (a)

Fracture-controlled aquifer

CLOSED SYSTEMS Y

Y

3--

Y

X

-_t_ 200300

.

W.

T.

m

Aquifer 1

--/--/--/--/--/--/--/ \1\1\1\1\1\1\1

;:;:;:B;ediobk;;: 4 Ascending magma

(b) Case I: Unconfined porous-media aquifer

~

:.

-‘-,,-.-‘-,‘-L--,.W.T.

:,.

:.

!

lO()_j50m ~-~-~-~-~-~-~-~_~-~_~_ -----------v---------m

:...

Upper aquifer - - . - ---_ Aqultard_-_

/

Lower aquifer

--/--/--/--/--/--/--/ \1\1\1\1\1\1\1

4;:;:;:B;ediobk;;:

Ascending magma (c)

Case II: Sealed, porous-media aquifer

systems for vapor expansion after Fig. 7. Highly idealized, fracture-controlled and porous-media aquifers seen as “open” and “closed” magma-water interaction. Prior to the hydromagmatic activity, pressure is hydrostatic in the unconfined aquifers (both fracture-controlled and porous-media) depicted in (a)-(c). Pressure is lithostatic in sealed porous-media aquifers, such as the lower one shown in cc). We argue that pressure is also effectively lithostatic in unconfined porous-media aquifers in (b)-(c) during the instantaneous expansion of vapor heated by water-magma interaction. After water-magma contact, vapor expansion must work against the weight of the overlying sediments (b) or the lithostatic pressure and yield strength of the sealing rock in the confined porous-media aquifer (c). Depending on local geometry, fracture-controlled aquifers may act as open or partly open systems, and the confining pressure for the expansion of the gases might be hydrostatic. Note that fractures in the open system may act like the muzzle of a gun, directing the trajectories of the ejected materials.

12

J.J. Amnda-G&nez, J.F. Luhr/

Journal of Volcanology und Geothermal Research 74 (19%) 1-18

overlying rock column (Fig. 7b). Prior to contact with the magma, water pressure in most porous-media aquifers is likely to be hydrostatic (Fig. 7b). Of particular importance is whether or not the water in the system can be vaporized (i.e., whether the hydrostatic pressure is below or above the critical pressure of water). Because the dramatic vapor pressure increase associated with hydromagmatic activity occurs in a very short period of time (a fraction of a second: Dobran and Papale, 1993), the steam overpressure can not be gradually released through the interconnected pores in neighboring areas of the aquifer, where the water is still under hydrostatic pressure. Therefore, once the water is vaporized, the expanding gases must work to (1) disrupt and/or fragment the surrounding rocks and (2) to “lift” and eject the country rock and juvenile fragments from the explosion site. We conclude that in porous-media aquifers the confining pressure for the explosion is essentially lithostatic (Fig. 7b) or slightly larger, if the aquifer is capped by competent solid rock (Fig. 7c). 5.1. The nature of the Joya Honda aquqer Large sinkholes and other surficial morphologic features that are regarded as diagnostic of karst terrains (Ford and Williams, 1989) are scarce in the calcareous mudstones around Joya Honda (ArandaGomez and Labarthe-Hernandez, 1975). The Cuesta de1 Cura and Tamaulipas limestones are thinly bedded, somewhat rich in clay, and have variably abundant chert. These characteristics appear to have inhibited the development of large solution features in the area. This contrasts with the cleaner platform carbonates (El Abra Formation) and evaporites (Guaxcama Formation) to the east, where some extremely large karst basins (poljes) have been reported (Wenzens, 1973) and abundant sinkholes are present (e.g., De Cserna and Bello-Barradas, 1963). Minor debris breccias, formed by clasts derived from the cleaner reef limestones, are interbedded with the dominant calcareous mudstones. Selective solution of these could have improved the secondary hydraulic conductivity beneath the maar, producing a heterogeneous and anisotropic aquifer (Ford and Williams, 1989). However, it seems that a large, well integrated, karst-type aquifer, with large dissolution

conduits did not exist beneath Joya Honda in the mid-Pleistocene. Groundwater circulation in the aquifer underneath Joya Honda was probably controlled by a network of fractures, somewhat enlarged by solution, related to the intersection of regional fracture sets (e.g., Lattman and Parizek, 1964) in the hinge area of the Sierra de1 Coro anticlinorium and a cross-cutting fault zone (Fig. lc). However, the existence of a few larger, solution-enhanced conduits along the debris-breccia beds cannot be ruled out. Large springs and producing wells are not now present in the limestones around Joya Honda. The groundwater exploited in the nearby Villa Hidalgo and San Luis Potosi hydrological basins comes from aquifers hosted in valley-filling alluvium overlying the limestones (V.J. Martinez-Ruiz, pers. commun., 1992). Marked fluctuations in the depth of the water table, due to climatic changes and/or tectonic movements during the late Tertiary and/or the Quaternary have been called upon to explain the multi-episodic development of the karst in Sierra de Guadalcazar (Wenzens, 1973; Torres-Hernandez, 199 l), 3.5 km northeast of Joya Honda. On the eastern slope of Sierra de Alvarez (Fig. lc), 25 km southeast of Joya Honda, the Cretaceous marine sediments are covered by unconsolidated gravels (Aranda-Gomez and Labarthe-Hernandez, 1975). The gravels, in turn, are overlain in places by xenolith-bearing alkali basalts, similar to those that formed Joya Honda. Similar gravels are exposed in the walls of the Laguna de 10s Palau Maar (Fig. lc), 30 km southeast of Joya Honda (Aranda-Gomez, 1982). Fossils of bison, sabertooth cats, and mammoths (?), collected in gravels that directly underlie alkali basalts near Pozo de1 Carmen (Fig. lc), indicate a savanna-like environment (I. Ferrusquia, pers. commun., 1993). Thus, the climate during middle Pleistocene times was considerably wetter than the present 330 mm/yr of precipitation (Garcia-Miranda, 1988). In the area between Armadillo and Llano Novela (Fig. Ic), the gravels form a flat surface that is now being actively eroded. This observation indicates regional uplift and/or lowering of base level. Either the decrease in rainfall or regional uplift could have caused a substantial drop in the water table of the aquifer beneath Joya Honda since the middle Pleistocene. Consequently, the only evidence for the existence of the Joya Honda aquifer is the maar itself, and no direct

J.J. Aranda-Gdmez, J.F. Luhr/Joumal

hydrological information can be used to evaluate the characteristics of the mid-Pleistocene aquifer. Thus, our analysis is based on an evaluation of general characteristics of groundwater flow through a series of interconnected tubes and channels, characteristic of fissured (Singhal and Singhal, 1990) or conduitcontrolled aquifers (Llopis-Llado, 1970; Trudgill, 1985; Ford and Williams, 1989). 5.2. Groundwater flow in fracture-controlled porous-media aqu!fers: general characteristics

13

of Volcanology and Geothenal Research 74 (1996) l-18

and

In the following discussion we contrast groundwater flow in porous-media or continuous aquifers with flow in fracture-controlled or discontinuous aquifers, keeping in mind that these two types represent endmembers in a spectrum of groundwater behavior. Groundwater flow in porous media, where water occupies a continuum of interconnected voids, and moves slowly in a laminar fashion, obeys Darcy’s law. In fracture systems, by contrast, water can move at high velocities (Williams, 1984; Pinneker and Pissarsky, 1984) and in turbulent fashion, even at relatively small hydraulic gradients (Ford and Williams, 1989; Domenico and Schwartz, 1990; Field, 1990; Marin et al., 1990). Solid limestone tends to be almost impermeable (Freeze and Cherry, 19791, with very low intrinsic permeability (10m3 to low5 darcy) and hydraulic conductivity (lop8 to 1O-5 ems-‘1. Secondary porosity, induced by fracturing and development of solution conduits, may cause dramatic increases in both properties. Solution enlargment of a fissure network may produce, in extreme cases, as much as lo6 increase in the hydraulic conductivity (Smith et al., 1976). In many porous-media aquifers, such as those in well-sorted sand and gravel deposits, the hydraulic conductivity (K) is assumed to be independent of position within the formation (i.e., the aquifers may be regarded as homogeneous and isotropic). In contrast, fracture-controlled aquifers in limestones are anisotropic as well as heterogeneous because solution-enhanced secondary permeability is greatest near the surface, which causes hydraulic conductivity to diminish with depth. Likewise, the fracture density in rocks may vary widely, depending on local structural or lithologic conditions. Storativity and water yield in fracture-controlled aquifers

may be erratic, with output peaks following rainfall events (Trudgill, 1985; Marin et al., 1990). Compared to porous-media aquifers, fracture-controlled aquifers have little storage capability and their specific retention may approach zero. In a simplistic way, fracture-controlled systems may be regarded more like an underground fluvial network, which during and immediately after the rainy season may be full and even overflowing through springs, but which may be almost completely drained at the end of the dry season. Porous-media aquifers tend to have a larger storativity and are, consequently, a more reliable source of water for an extended period of time, but discharge at any time may be considerably smaller compared to a fracture-controlled aquifer due to its smaller hydraulic conductivity. 5.3. The hydromagmatic-rnagmatic cones to cinder cones

spectrum:

tufs

As pointed out earlier, other Quatemary maars (Table 1) are found in the vicinity of Joya Honda. Four of them are similar to Joya Honda in being excavated in Mesozoic limestones and surrounded by indurated wet-surge deposits. The other two, Laguna de Los Palau and Pozo de1 Carmen (Fig. 21, are excavated in gravel deposits above the limestones and surrounded by dry-surge deposits. Therefore, the relationship between the maar-related pyroclastic sequence and the nature of the underlying aquifer postulated for Joya Honda might hold for other maar volcanoes. Maars surrounded by tuff-cone sequences of wet-surge deposits may be preferentially located in areas with underlying fracture-controlled aquifers that produce relatively high water yields to the explosion foci. Maars surrounded by tuff-ring sequences of dry-surge deposits, in contrast, may correlate with underlying porous-media aquifers that produce relatively low water yields to the explosion foci. Other examples of tuff-ring maars underlain by porous-media aquifers include Crater Elegante (Gutmann, 19761, Potrillo Maar (Reeves and De Hon, 19651, Kilbourne Hole (Reiche, 19401, and the Valle de Santiago maars (Ordoaez, 1900; J. Randall, pers. commun., 1992). The existence of tuff-cone sequences of wet-surge deposits developed on top of porous-media aquifers, such as in the Yampa and Elkhead Mountains vol-

14

J.J. Aranda-GAmez, J.F. Luhr/ Journal of Volcanology and Geothermal Research 74

canic fields (Lest and Thompson, 1988), indicates that in some porous-media aquifers the flow rate of water into the vent may be sufficiently great to equal that in fracture-controlled systems. On the other hand, fracture-controlled aquifers may not be able to supply large amounts of water to the volcanic vent. For example, if magma intrudes into an area of reduced permeability and hydraulic conductivity, a maar surmounted by a tuff ring may form (e.g., the basal sequence of Joya Honda). Alternatively, if the fracture-controlled aquifer is virtually empty during the dry season, or if the magma fails to intercept a hydrologically important fracture within the rock mass, a cinder cone may form, such as Cerro Verde cinder cone, which erupted 14 km north-northeast of Joya Honda atop the same Cretaceous limestones and presumably the same fracture-controlled aquifer (Fig. 1~1.

6. Discussion Water-magma interactions during hydromagmatic eruptions depend on the rate of magma ascent, confining pressure on the magma prior to and during interaction with external water, the nature of the water supply, and the strength of the rock or elastic deposit hosting the aquifer. Petrographic observations indicate that the juvenile clasts become progressively more vesiculated and hydrated (palagonitized) upward in the Joya Honda sequence, corresponding to the transition from early dry-surge deposits to later wet-surge deposits. No information is available about possible primary variations in magmatic volatile contents with time during the eruption. Since this magma carried large peridotite xenoliths from the mantle, it must have risen rapidly to the surface (Spera, 1984). Accordingly, we assume that it was a single magma batch without significant variability in volatile contents. Rapid ascent also precludes degassing of the early erupted clasts as an explanation of their poor vesicularity. Consequently, we infer that the increased vesiculation of the juvenile clasts with height in the Joya Honda maar deposit reflects decreased confining pressure on the magma just prior to explosive fragmentation. At the start of the eruption, lithostatic pressure may have been sufficient to keep magmatic

CIY96) I-18

water and other volatiles confined within the melt up to the moment of explosive magma-groundwater interaction. As the eruption progressed, unroofing of the magma associated with excavation of the maar crater may have lowered lithostatic pressure on the magma to the point that vesiculation was progressively more advanced prior to explosive fragmentation. The upsection increase in palagonitization of juvenile clasts probably retlects post-eruption hydration associated with enhanced cementation and lithification in the progressively wetter tuff breccias at the top of the eruptive sequence. The abundance of upper crustal (limestone) xenoliths at Joya Honda and the sedimentary and petrographic characteristics of its deposits (Table 3; Figs. 4-6) suggest that the sequence originated from a series of hydromagmatic explosions at relatively shallow crustal levels (Valentine and Groves, 19961. Assuming that the water/magma interaction occurred within the range of hydrostatic pressures relevant to such eruptions (20-30 bar, Lorenz, 19861, we consider two limiting cases. In the first end-member, a constant and abundant source of water is available throughout the volcanic activity, such as in the formation of a tuff cone, where sometimes a shallow body of standing water lies above the vent area (Godchaux et al., 19921. In this case, the episodic nature of the deposits is mainly related to fluctuations in the magma supply rate (Lest and Thompson, 1988). In the second end-member, a steady flow of magma nears the surface and intersects an aquifer. In this case. periodic flow of groundwater toward the magma and the consequent hydromagmatic eruptions are controlled by the aquifer characteristics. For Joya Honda it is not possible to know whether magma reached the critical depth (Lorenz, 19861 for hydrovolcanic explosions in the form of pulses or as steady flow. In our analysis we consider the second case of constant magmatic ascent rate, in order to evaluate the potential influence of groundwater flow in a porous-media versus a fracture-controlled aquifer on the development of maar-type volcanoes. Hydromagmatic eruptions, which rapidly vaporize large amounts of water, have the net effect of a large and almost instantaneous withdrawal of groundwater within a finite volume of the aquifer. Therefore, if hydromagmatic activity occurs more or less continuously throughout a period of several days (Miiller

J.J. Aranda-Gbmez, J.F. Luhr/Journal

of Volcanology and Geothermal Research 74 (1996) I-18

and Veyl, 1957; Kienle et al., 19801, lowering of the water table beneath the vent can be expected (Lorenz, 19861, similar to a cone of depression formed when a pumping test is performed on an aquifer. The most important factors in the formation of cones of depression are time, hydraulic conductivity (K) and discharge ((2). The amount of drawdown in an aquifer is directly proportional to Q and inversely proportional to K. By comparison with the mechanism of geyser formation (Williams and McBimey, 19791, we assume that all the water contained in a finite volume of unknown dimensions in the aquifer below the vent is instantaneously consumed by a hydromagmatic explosion. After the explosion, the upper part of the vent is choked by crater-rim collapse breccias (Lorenz, 1986) and by fall-back material, and the conditions for a new hydromagmatic blast start building again (Sheridan and Wohletz, 1983). The time between explosions is a function of the rates at which magma and groundwater can converge on the vent. In a porous-media aquifer, with a low to moderate hydraulic conductivity, water may not flow fast enough to the vent area, despite the existence of abundant groundwater in the rest of the aquifer. Thus, conditions for a purely magmatic eruption may be reached and Strombolian explosions may occur. In fracture-controlled aquifers, in which secondary permeability may locally be very large, water flow is controlled by “pipe” systems, and water velocity may be very high, on the order of kilometers per day (compared to meters per day or centimeters per day in a porous-media aquifer). Therefore, the time between phreatomagmatic blasts could be considerably shorter and the total energy output of the volcanic event may be consumed through a closely spaced series of approximately uniform explosions (Wohletz and Sheridan, 1983). The resultant pyroclastic deposits will be monotonous, with only minor breaks, marked by subtle changes in the average grain size and/or the proportion of juvenile to accidental material: like the tuff breccias in the upper part of the Joya Honda sequence. Most tuff cones do not expose basement rocks in their walls and consequently are not true maars. However, Joya Honda crater, which is associated with a tuff-cone-like sequence dominated by wetsurge deposits, was excavated > 220 m deep into limestones. This can be explained by water/magma

15

interaction (the explosion foci) well below the ground surface, within the competent limestones. The Joya Honda aquifer was fracture-controlled and the system was “open” (Fig. 7a) or partially open, with the confining pressure for the initial water vaporization given only by the hydrostatic pressure in the aquifer. Therefore, the pressure of 20-30 bar proposed by Lorenz (1986) could have been attained more than 200-300 m below the surface (depending on the depth to the water table: “X” in Fig. 7a). Once the water vaporized, the energy of the system was spent through a series of phreatomagmatic blasts that were directed towards the north, probably by the preferred orientation of the fissure system, as indicated by the distribution of the tephra deposits (Fig. 2). Beginning with the hydromagmatic disruption of non-vesiculated magma represented by unit 1-4, phreatomagmatic blasts occurred at considerable depth and with the intervention of relatively small amounts of groundwater, as attested by the non-vesiculated and unaltered nature of the juvenile clasts in units I-4 to I-8 (Fig. 6a and b). The early part of the eruption was comparable to dynamite blasts in open drill holes (without tamping and/or stemming); one part of the explosion energy was dissipated through the open fissures and another part produced a fractured chamber (Peele, 1941) around the focus of the eruption. These early detonations caused: (11 an increase of the secondary permeability around the point of magma/water interaction, which gradually allowed major water influx to the system; and (2) excavation of the early maar crater, unroofing that led to decreased lithostatic pressure on the ascending magma and increased vesiculation. These changes are registered in the rocks by the increase in vesicular&y of juvenile fragments in the upper part of the tuff. Finally, the growing crater intersected a major fracture in the aquifer or a zone within the limestones with larger secondary porosity and permeability and the water/magma ratio in the system increased dramatically, reaching the condition for the formation wet surges and emplacement of the tuff-breccias (I-l 1, I-l 3 to I-16). This greater inflow of water is indicated not only by the indurated and massive nature of the upper tephra deposit (Table 2; Figs. 4 and 51, but also by the strong increase in palagonite in the tuff breccias. Thus, to explain the remarkable depth of Joya Honda there is no need to call upon a

I6

J.J. Aranda-Gbmez, J.F. Luhr/ Journal of Volcanology and Geothermal Research 74 (1996) I-18

gradual deepening of the explosion foci due to formation of a cone of depression in the aquifer. The upsection increase in vesiculation of the basanite clasts indicates a decrease in confining pressure for the ascending magma with time. Whether or not a large diatreme underlies Joya Honda, as suggested by the models of Lorenz (1973, 1986) is unknown. The near-vent tephra deposit of Joya Honda has other remarkable characteristics. The roundness in the clasts was probably caused by mechanical abrasion during the eruption because both the quenched basanite and the accidental limestone clasts show it. Many of the fragments appear to have fallen back into the vent, to be repeatedly involved in phreatomagmatic explosions; rare composite palagonite clasts (Fig. 6e) and the variable roundness of the fragments in most samples support this interpretation. In the case of the limestones, roundness may had been caused in part by chemical attack by weak acidic solutions formed by combination of the volcanic gases with the water. The dissolved material was likely re-deposited as cement (Fig. 6a and c>.

7. Summary

and conclusions

Joya Honda has the morphological characteristics of a true maar. Although the near-vent eruptive sequence begins with dry-surge deposits, the upper 3/4 of the sequence consists of very well-indurated tuff breccias (Figs. 4 and 5) with ill-defined bedding and primary dips up to 30” (Fig. 2), inferred to have formed from wet surges. This contrasts with the pyroclastic sequences around other maars such as Crater Elegante (Gutmann, 1976) and La Breiia-El Jagiiey (Aranda-Gomez et al., 1992a,b), which consist entirely of dry-surge deposits with primary dips less than 14”. Joya Honda was formed in middle Pleistocene times by hydromagmatic eruptions of basanitic magmas that carried large mantle xenoliths to the surface and, consequently, must have travelled rapidly through the crust from the upper mantle, without forming high-level magma reservoirs. In attempting to explain the unusual characteristics of Joya Honda it is significant that the country rocks are limestones, and that the maar is located at the intersection between the hinge zone of a regional fold and a cross-cutting fault system (Fig. la>. These

structural features may have led to a local increase of secondary permeability in the limestones due to the presence of at least two fracture sets: one parallel to the axial plane of the fold and the other related to the cross-cutting system. Groundwater flow in a porous medium is radically different from that in an aquifer controlled by fissures or solution conduits. Even though there is no direct evidence for the nature of the mid-Pleistocene ( _ 1.l Ma) aquifer beneath Joya Honda besides the existence of the maar itself, we deduce from the nature of the country rock, the absence of large sinkholes in the immediate surroundings of the crater, and the structural setting of the maar, that the Joya Honda aquifer was dominantly controlled by fractures in the limestones. The similarity of the upper Joya Honda tuff breccias to wet surge deposits that typify tuff-cone sequences can be explained by the large discharge rates achievable in a fracture-controlled aquifer, which allows a constant and abundant water supply to the vent. The deep Joya Honda crater was developed by hydromagmatic explosions well below (> 200-300 m) the pre-maar surface because the confining pressure for the expanding vapor was controlled only by the hydrostatic pressure in the aquifer and the water table depth (Fig. 7a). The Joya Honda tephra sequence shows upward increases in the vesicularity and intensity of palagonitization in the juvenile clasts. In samples collected from the early fine-grained tuffs, interpreted as drysurge deposits, the basanite is for the most part non-vesiculated and unaltered (I-4 to I- 10, I-12). The very well-indurated wet-surge deposits from the upper part of eruptive sequence (I- 11, I- I3 to I- 16) contain abundant clasts of palagonite, some of them highly vesiculated. We interpret this as evidence for a significant increase in the water/magma ratio at the level of their explosive interaction, and for a gradual decline in the confining pressure for the ascending magma just prior to explosive disruption, perhaps as a consequence of unroofing related to maar crater development.

Acknowledgements

by

Early versions of this manuscript were reviewed Karen Prestegaard, Juan Randall, Guillermo

J.J. Aranda-Gbmez, J.F. Luhr/Joumal

of Volcanology and Georhenal

Labarthe, Michael Sheridan, Neil Summer, Martha Godchaux and Gerard0 Aguirre, all of whom made valuable comments that improved our treatment. Critical reviews by James White, Grant Heiken, and Greg Valentine were especially helpful. Juan Torn& Vgzquez prepared oversized thin sections of the samples for petrographic study.

References Aranda-Gomez, J.J., 1982. Ultramafic and high grade metamorphic xenoliths from central Mexico. Ph.D. Diss., Univ. Oregon, Eugene, OR, 236 pp. Aranda-G6mez. J.J. and Labarthe-Hemandez, G., 1975. Estudio Geol6gico de la Hoja Villa Hidalgo, S.L.P. Univ. A&n. San Luis Potosi, Inst. Geol., Foll. Tic., 53: 35-58. Aranda-G6mez, J.J. Luhr, J.F. and Pier, J.G., 1992a. The La Brena-El Jagiiey maar complex, Durango, Mexico. I. Geological evolution. Bull. Volcanol., 54: 393-404. Aranda-Gomez, J.J., Luhr, J.F. and Pier, J.G., 1992b. On the formation of La Brefia Crater; a discussion to the paper “A new type of maar volcano from the State of Durango-The El Jagiiey-La Breba complex reinterpreted” by Eric R. Swanson. Univ. Nat. Auton. Mtx. Inst. Geol. Rev., 9-2: 204-210. Aranda-G6mez. J.J., Luhr, J.F. and Pier, .I., 1993. Geologia de 10s volcanes cuatemarios portadores de xenolitos provenientes de1 manto y de la base de la corteza de1 Estado de San Luis Potosi. Inst. Geol., Univ. Nat. Audn. Mtx. Bol., 106: l-22. Carrillo-Bravo, J., 1971. La Plataforma Valles-San Luis Potosi. Bol. Asoc. Mex. Geol. Pet., 23: l-6: l-102. Cas, R.A.F. and Wright, J.V., 1988. Volcanic Successions Modem and Ancient. Unwin Hyman, London, 528 pp. Chough, S.K. and Sohn, Y.K., 1990. Depositional mechanisms and sequences of base surges, Songaksan tuff ring, Cheju Island, Korea. Sedimentology, 37: 1115-l 135. De Csema, E. and Bello-Banadas, A., 1963. Geologia de la parte central de la Sierra de Alvarez, Municipio de Zaragoza, Edo. de San Luis Potosi. Univ. Nat. Au&. Mtx. Inst. Geol., Bol., 71: 23-63. Detenal, 1982. Carta geologica S. L. Potosi F14-4, Mexico. Secretaria de Programacidn y Presupuesto, Escala 1:250,000. Dobran, F. and Papale, P., 1993. Magma-water interaction in closed systems and application to lava tunnels and volcanic conduits. J. Geophys. Res., 98tB8): 14,041-14,058. Domenico, P.A. and Schwartz, F.W., 1990. Physical and Chemical Hydrology. Wiley, New York, NY, 824 pp. Field, M.S., 1990. Transport of chemical contaminants in karst terranes: outline and summary. In: ES. Simpson and J.M. Sharp Jr. (Editors), Selected papers on Hydrogeology from the 28th Int. Geol. Congr., Washington, DC, July 9-19, 1989. Heinz Heise, Hannover, pp. 17-27. Fisher, R.V. and Schmincke, H.-U., 1984. Pyroclastic Rocks. Springer, Berlin, 472 pp.

Research 74 (19961 I-18

17

Ford, D.C. and Williams, P.W., 1989. Karst Geomotphology and Hydrology. Unwin Hyman, London, 601 pp. Freeze, R.A. and Cherry, J.A., 1979. Groundwater. Prentice-Hall, Englewood Cliffs, NJ, 604 pp. Garcia-Miranda, E., 1988. Moditicaciones al sistema de clasificaci6n de KGppen. Author, Mexico, 4th ed., 217 pp. Godchaux, M.M., Bonnichsen, B. and Jenks, M.D., 1992. Types of phreatomagmatic volcanoes in the western Snake River Plain, Idaho, USA. J. Volcanol. Geotherm. Res., 52: l-25. Gutmann, J.F., 1976. Geology of Crater Elegante, Sonora, Mexico. Geol. Sot. Am. Bull., 87: 1718-1729. Kienle, J., Kyle, P.R., Self, S., Motyka, R.J. and Lorenz, V., 1980. Ukinrek maars, Alaska, I. April 1977 eruption sequence, petrology and tectonic setting. J. Volcanol. Geotherm. Res., 7: 1 l-37. Labarthe-Hemandez, G., 1978. Algunos xalapascos en el Estado de San Luis Potosi. Univ. Autbn. San Luis Potosi, Inst. Geol. Metal., Foll. Ttc. 58, 17 pp. Labarthe-Hemandez, G., Tristan, M. and Aranda-Gbmez, J.J., 1982. Revisi6n estratigdfica de1 Cenozoico de la parte central de San Luis Potosi: Univ. Autbn. San Luis Potosi, Inst. Geol. Metal., Foll. TCc. 85, 208 pp. Lattman, L.H. and Parizek, R.R., 1964. Relationship between fracture traces and the occurrence of groundwater in carbonate rocks. J. Hydrol., 2: 73-91. Leat, P.T. and Thompson, R.N., 1988. Miocene hydrovolcanism in NW Colorado, USA, fueled by explosive mixing of basic magma and wet unconsolidated sediment. Bull. Volcanol., 50: 229-243. Llopis-Llad6, N., 1970. Fundamentos de Hidrogeologia Carstica (Introducci6n a la Geoespeleologia). Blume, Madrid, 269 pp. Lorenz, V., 1973. On the formation of maars. Bull. Volcanol., 37: 183-204. Lorenz, V., 1986. On the growth of maars. Bull. Volcanol., 48: 265-274. Luhr, J.F., Aranda-Gbmez, J.J. and Pier, J.G., 1989. SpinelIherzolite-bearing, Quatemary volcanic centers in San Luis Potosi, Mexico. I. Geology, mineralogy and petrology. J. Geophys. Res., 94(B6): 7916-7940. Marin, L.E., Perry, EC., Pope, K.O., Duller, C.E., Booth, C.J. and Villasuso, M., 1990. Hurricane Gilbert: Its effects on the aquifer in northern Yucatan, Mexico. In: E.S. Simpson and J.M. Sharp Jr. (Editors), Selected papers on Hydrogeology from the 28th Int. Geol. Congr., Washington, DC, July 9-19, 1989. Heinz Heise, Hannover, pp. 11 l-127. Miller, J.B. and Veyl, G., 1957. The birth of Nilahue, a new maar type volcano at Rihinahue, Chile. 20th Int. Geol. Congr., Mexico City, Rep. Sect., I: 375-396. Ollier, CD., 1967. Maars, their characteristics, varieties and definition. Bull. Volcanol., 31: 45-73. Ordoiiez, E., 1900. Les volcans du Valle de Santiago. Mem. Sot. Cient. Antonio Alzate, 14: 299-326. Peele, R., 1941. Mining Engineer’s Handbook. Wiley, New York, NY, pp. 5-11. Pier. J.G., Podosek, F., Luhr, J.F., Brannon, I. and Aranda-G6mez, J.J., 1989. Spinel-lherzolite-bearing Quatemary volcanic cen-

18

J.J. Aranda-Gbmez, J.F. Luhr / Journal of Volcanology and Geothermal Research 74 Cl996) I-18

ters in San Luis Potosi, Mexico. Il. Sr and Nd isotopic systematics. .I. Geophys. Res., 94(B6): 7941-7951. Pinneker, E.V. and Pissarsky, B.I., 1984. Karst of the southwestem Baikal region. In: A. Burger and L. Dubertret (Editors), Hydrogeology of Karstic Terrains, Case Histories. lnt. Assoc. Hydrogeol., 1: 238-239 (abstract). Reeves Jr., C.C. and De Hon, R.A., 1965. Geology of Potrillo Maar, New Mexico and northern Chihuahua, Mexico. Am. J. Sci., 263: 401-409. Reiche, P., 1940. The origin of Kilboume Hole. New Mexico. Am. J. Sci., 238: 212-235. Sheridan, M.F. and Wohletz, K.H., 1983. Hydrovolcanism: Basic considerations and review. J. Volcanol. Geotherm. Res., 17: l-29. Singhal, B.B.S. and Singhal, D.C., 1990. Evaluation of aquifer parameters and well characteristics in fractured rock formations in Kamataka, India. In: E.S. Simpson and J.M. Sharp Jr. (Editors), Selected Papers on Hydrogeology from the 28th lnt. Geol. Congr. Washington, DC, July 9-19. Heinz Heise, Hannover, pp. 35 l-362. Smith, D.I., Atkinson, T.C. and Drew, D.P., 1976. The hydrology of limestone terrains, In: T.D. Ford and C.H.D. Cullingford (Editors), The Science of Speleology. Academic Press, London, pp. 179-212. Spera, F.J., 1984. Carbon dioxide in petrogenesis III. Role of volatiles in the ascent of alkaline magma with special refer-

ence to xenolith-bearing mafic magmas. Contrib. Mineral. Petrol., 88: 217-232. Tones-Hemandea, J.R., 1991. Evolucidn estructural de la Sierra de Guadalcazar, Estado de San Luis Potosi (resumen). Convencidn sobre la Evolution Geologica de Mexico. Mem. Univ. Nat. Auton. M&x. Inst. Geol., pp. 218-220. Trudgill, S., 1985. Limestone Geomorphology. Longman, London, 196 pp. Valentine, G.A. and Groves, K.R.. 1996. Entrainment of country rock during basaltic eruptions of the Lucero Volcanic Field, New Mexico. J. Geol.. 104: 71-90. Waters. A.C. and Fisher, R.V.. 197. Maar volcanoes. Proc. 2nd Columbia River Basalt Symp.. Cheney, WA, Eastern Washington State College Press, pp. 157- 170. Wenzens, G., 1973. lnvestigaciones geomorfol6gicas en la region clratica del norte de San Luis Potosi y sur de Nuevo Leon. Sot. Geol. Mex. Bol.. 34(1-2): 71-79. Williams, H. and McBimey, A.R.. 1979. Volcanology. Freeman, San Francisco, CA. 397 pp. Williams, P.W.. 1984. Karst hydrology of the Takaka Valley and the source of New Zealand’s largest spring. In: A. Burger and L. Dubertret (Editors), Hydrogeology of Karstic Terrains. Case Histories. lnt. Assoc. Hydrogeol.. 1: 65-69 (abstract). Wohletz, K.H. and Sheridan, M.F.. 1983. Hydrovolcanic explosions. Il. Evolution of basaltic tuff rings and tuff cones. Am. .I. Sci.. 283: 385-413.