JOACHIM D. MEYER AJ;SJS'i 'C1x1' JJH OFEN80H of GEOLOGY 'L'l"Hlxe L"?Oyer8l'l'y
GEOLOGY OF THE AHUACHAPAN AREA WESTERN EL SALVADOR, CENTRAL AMERICA JOACHIM D. MEYER AJ;SJS'I 'c1X1' JJH OFEN80H OF GEOLOGY 'l'L"hLXE L"?OYER8l'l'Y
CONTENTS Page ABSTRACT------··------19 5 l. INTRODUCTION------______. ______19 5 II. GEOGRAPHY_ ------______19 7 Ill. STRATIGRAPHY ______------__ 19 8 FOLDED PLATE: Geologic Map ______opposite page 2 00 IV. LOCAL STRATIGRAPHY______200 V. STRUCTURAL GEOLOGY______------______2 08 VI. LOCAL STRUCTURE______. 210 VII. GEOLOGIC HISTORY------·______213 REFERENCES CITED ______214
ABSTRACT l. INTRODUCTION The Ahuachapan area lies in Western El El Salvador, the smallest and most popul Salvador between the town of Ahuachapan ous country in Central America, has nu and the Guatemalan border. It is within an merous hot springs and fumaroles, associated E-W trending graben which transgresses with active and recently active volcanoes, but the republic lacks the usual sources of the entire republic and which is believed to energy: coal, oil and gas. In 1955, the have formed from the collapse of a geanti Servicio Geologico N acional de El Salvador cline at the end of the Pliocene. Quaternary initiated geological investigations in various volcanism along the margins and within the parts of the country to locate favorable areas graben largely filled it with volcanic debris, to develop natural steam as a source of consisting of flat-lying tuffs, agglomerates energy. One of the most promising areas and lavas. The sequence in the area, be is near Ahuachapan in western El Salvador. ginning with the oldest, is: ancient agglo This report is based on the detailed study merate, laminar andesite, massive andesite, of an area near Ahuachapan where previous blue ignimbrite, gray agglomerate, lower work had revealed the possibility of de brown tuff, pink ignimbrite, gray ignim veloping economic supplies of natural steam. brite, pumice and upper brown tuff. The The field work was done during the spring thickness of these rocks attains a maximum of 1960 in cooperation with the Servicio of 42 5 meters. Geologico N acional. The strata are disrupted by a number of Location northeast-trending normal faults. Volcanic The Abuachapan area, comprising about activity is still in progress at various centers 120 square kilometers, is located in south in the region. western El Salvador between the town of
EDITORIAL COMMITTEE FOR THIS PAPER: FRED M . BuLLARD, Department of Geology, The University of Texas, Austin, Texas
GABRIEL DENG01 Instituto Centroamericano de Investigacion y Tecnologia Indus trial, Guatemala GROVER E. MURRAY, The President, Texas Technological College, Lubbock, Texas
195 198 T11 lcrne Studies in Geo lo gy Vol. 5
Encucntros, where the maximum relief is as Awai g- . * The 1nost significant factor of ~00 meters from the plain (elevation 500 the climate is the torrential rainfall during meters) to the bottom of the valley (eleva the summer which has been measured at a tion 200 meters). The deeply incised river rate as high as 12 .5 em per minute. Con indicates the youth of the valleys. The plain tinued rains of such volume lasting for two docs not terminate abruptly at the valley, or three days are called "temporals" and but continues in two terrace levels, the have great erosive power. Consequently, the loosely consolidated tuffs and ash de origins of which arc discussed in the section posits are quickly reworked making their on
28) assigned these strata to the Albian glomerates, lavas, indurated scoria and ash Stage. deposir-s which are interbedded with fluvia Tertiary rocks tile and lacustrine sedimentary rocks. There arc numerous fossil soil horizons included DLir~ ( 1960a, p. 14) assigned the young in the sequence. e~t unlt of the 11etapan sequence, which drscordantly overlies the two older units to Quaternary rocks the lower Miocene. It consists of d;nse andesitic tuff- p ebble conglomera~es, red Eruption products from the southern quartz-pebble conglomerates, reddish fine chain of volcanoes are mostly Pleistocene grained SJ.ndstones and reddish-gray Jime age. The pyroclastics, such as scoria, pumice, s:one~pebble conglomerates, and according tuffs and welded ruffs are generally dacitic, ro ?tutcn and Gealey ( 1949, p. 1740), is but the lavas are generally basaltic. These denved at least in part from erosion of the rocks, which came from a chain of vol underlying beds as it contains pebbles of canoes in places more than 2000 meters reworked, fossiliferous Albian Limes·one. high, are spread over the entire interior The Metapan section is overlain discordant region. Of the more than fifty volcanoes ly by a thick sequence of acidic tuffs which in this chain, the 1110st important are Con ch.agua, S:m Mig/uel, Usulutcin, Tecapa, San resemb~e the rocks of the Chalatenanao Formmion which crops out in the Hondur~n Vrccnte, Boqueron, Santa Ana, Izalco, La guna Verde and Apaneca. Almost all of B~rder Mountains and is probably upper these have well-preserved symmetrical cones. Mrocene Jge. Di.irr ( 1960a, p. 13) believes Interbedded with the volcanic rocks are thin that these acidic tuffs are the extrusive facies cf granitic-dioritic plutons which in f~uv~atile an~1 lacustrine deposits inclm1ing lignite and dratomite; fossil soils occur with truded the acid tuffs and the Metapan se in the sequence. quence. Along the contacts limestone was In alignment with the southern chain of a] ter~d to lime-silicate rock and shaly sand strato-volcanoes are a number of basaltic stone was altered to hornfels and mcta vc)lcanic domes or lava don1es such as San quarrzite. The acidic tuffs also are met:l. Jacinto Mountain near San Salvador. Other morphosed, and comparison of these mcw features connected with the southern yoJ morphosed tuffs with the intrusive grano canic chain are the explosion-collapse cal diorite supports the conclusion that both deras now occupied by Lake Ilopango and c~t_De frorTt the same parent magma. Highly Lake Coatepeque. Great quantities of daci srl1ceous intrusives of sin1ilar age crop out ti.c pumice were erupted and deposited by between the towns San Isidro and San a ir-borne sl1owers and mud flows (lahars) Miguel. They are considered a late acid as far as 25 km from the vent ( \V'illiam<> phase of the intrusions in the Metapan and Meyer-Abich, 1955, p. 19). The yoJ area. canocs Izalco, Boquer6n, and San 1vfiguel Pliocene volcanic rocks of the Balsam of the southern chain have all eru ptcd b va Formation are the most wide-spread in El in historic times. Salvador. They are present in the BalsarTt The most abundant fossils in the republic Mountains, in the Jucuaran Mountains, the are Quaternary, including remains of Pleis Sierra de Tacuba and between the more tocene horses, camels, giant sloths, bisons, recent volcanic complexes of the northern mammoths, sabre-tooth cats and mastodons volcanic chain. They ~are mainly interbedded ( Stirton and Gealey, 1949, p. 17·1.2). One tuffs, ctgglomerates and lavas which are gen very unusual find is human footprints erally basaltic in the lower part of the sec pressed inro water-soaked tuff and dated by tion and andesitic in the upper part. Grebe ( 1956, p. 55) as 1000-1500 years Overlying the Balsam Formation are old. erup::ion products from the northern vol Archcological dating of potsherds found canic chain which includes the broad straw beneath 3 meters of pumice from Boqueron volcanic edifices of Cacaguatigue, Tecoma established an age of 2000 years (Williams tepe, Guazapa, and Capullo. These extinct and Mcycr-Abich, 195 5, p. 23) for the and largely eroded volcanoes reached the pumice eruption. The last volcanic activity height cf their activity during the late Plio at Boquer6n was in 1917 when a series of cene, producing tuffs, welded tuffs, ag- cinder- cones were formed near the summit 198 Tl!lctne St ~tdies in Geo lo gy Vol. 5 No.4 Geolog ')' of tbe A I:JIIacbap cin Arect 199
Encuc:ntros, where the maximum relief is as Awaig- . * The most significant factor of 28 ) assigned these strata to the A lb ian glomerates, lavas, indurated scoria and ash 100 meters from the plain (elevation 500 the clim ate is the torrential rainfall during Stage. deposi~"s which are interbedded with fluvia meters) to the bottom of the valley (eleva the summer which has been measured at a T ertiary rocks tile and lacustrine sedimentary rocks. There rate as high as 12. 5 em per minute. Con arc numerous fossil soil horizons included tion 200 meters). The deeply incised river Di.irr ( 1960a, p. 14) assigned the young tinued rains of such volume lasting for two in the sequence. indicates the youth of the valleys. The plain es t unit of the .i\1etapan sequence, which or three days are called "temporals" and docs not terminate abruptly at the valley, discordantly overlies the two older units, to have great erosive power. Consequently, Quaternary rocks but continues in two terrace levels, the the lower Miocene. It consists of dense the loosely consolidated tuffs and ash de Eruption products from the southern origins of which arc discussed in the section andesitic tuff- p ebble c o n g l omera~es, red posits are quickly reworked making their chain of volcanoes are mostly Pleistocene on geologic history. quartz-pebble conglom erates, reddish fine iden ti fica tion m ore difficult. age. The pyroclastics, such as scoria, pumice, grai ned s ~wd s ton es and reddish-gray lime Other prominent land forms include the App roximately 80% of the area has been ruffs and welded cuffs are generally dacitic, s::o ne-pcbble conglomerates, and according ruins of two extinct volcanoes. The volcano cleared for cultivation with even the steep but the lavas are generally basaltic. These to Stirtcn and G ealey ( 1949, p . 1740), is Las Chinama<:>, located in the northern slopes of volcanoes and river valleys uti rocks, which came from a chain of vol derived at least in p art from erosion of the corner of the area, remains as a large hill, lized. The uncultivated patches are rolling canoes in places more than 2000 meters underl ying beds as it contains pebbles of the crater having eroded away. The only grasslands, scattered clumps of small trees, high, are spread over the entire interior reworked, fossiliferous Albian Limeseo ne. thorny underbrush and agaves. Along the region Of the more than fifty volcanoes indication of its origin are four lava flows The Metap an section is overlain discordant banks of stream s the trees and underbrush in this chain, the most important arc Con which appear to have come from the llill. ly by a thick sequence of acidic tu ffs which are so dense that they form an almost im chagua, S:w Miguel, Usuluran, Tecapa, S,ll1 The other volcano i.:; situated 2.5 km from resemb!e the rocks of the Chalatcnango penetrable forest. Vicente, Boquer6n, Santa Ana, Izalco, La Lo'> Encuentros and displays a shallnw bowl Formation w hich crop s out in the Honduran guna Verde and Apaneca. Almost all of shaped crater partly encircled by five peri Border M ountains and is probabl y upper these have well-preserved symmetrical cones. pheral faults at a distance of 500 meters III. STRATIGRAPHY Miocene age. Di_irr ( 1960a, p. 13) believes Interbedded with the volcanic rocks are thin from the crater. R EGJONAL STRATIGRAPHY that these acidic tuffs are the extrusive fluviatile and lacustrine deposits including The highest elevation measured is 759 G eologically, El Salvador is a young coun facies c f granitic-dioritic plu ro ns whi ch in I ignite and diatomite; fossil soils occur with meters at a poinr 500 meter<> southwest of try. Approximately one fourth of the out truded the ac id ru ffs and the Metapan se in the sequence. the Lagun,l del Llano; the lowest is 200 cropping rocks are of Quaternary age. Most q uence. Along the contacts limestone was In alignment with the southern chain of meters at Los Encuentros. of the remaining rocks are Tertiary ( prin a1r ered to lime-silicate rock and shaly sand erato-volcanoes are a number of bas,lltic cipally Pliocene ) and all are of volcanic stone was altered to hornfels and meta \ c1lcanic domes or lava domes such as S.m Drainage origin. The only non-volcanic deposits pres q uartzite. The acidic tuffs also are meta Jacinto Mountain near San Salvador. Other The ma 10r rivers in the area are the R 1o ent are about 200 square kilometers of Cre morphosed, and comparison of these meta features connected with the southern vol Paz ,111d ·the Rio Molino. T he R io P az taceous sedimentary rocks in northwestern m orphosed tuffs w ith the intrusive grano canic chain are the explosion-collapse cal originates near Lake Ayarza, 40 km south El Salvador (see Fig. 2). diorite supports the conclusion that both deras now occupied by Lake Ilop~mgo and cast of Guatemala City and discharges inro ca me from the sam e p arent m agma. H ighly Lake Coarepeque. Great quantities of (hci the Pacific Ocean. The Rio M olino ori.P-:i Cretaceous rocks siliceous intrusives of similar age crop out ric pumice were erupted and deposited by natcs on the slopes of the Cerro las Ninfas Cretaceous m arine sedimentary rocks crop between the towns SJ.n Isidro and San air-b:Hne s11owers and mud flmv<; (blurs) and fhms into rhe Rio Paz Clt los Encuen out near Metapan in northwestern El Sal M iguel. They are considered a late acid as far as 25 km from the vent ( \"'{Tilliams tros. With the excep6on of the Rio Paz vador. These rocks comprise the "Metapan phase of the intrusions in the Metapan and l\1cyer-Abich, 1955, p. 19). The ,·ol and the Rio Molino, which are fed to some sequence" which includes three divisions. area. canocs Izalco, Boquer6n, and S,ln l\figuel extent by springs, the streams are inter The two lower units are of Cretaceous age, Pliocene volcanic rocks of the Balsam of the southern chain have all erupted bva mittent. During the rainy season ( June but the upper one is believed (Di_irr, 1960a) Formation are the m ost w ide-spread in El in historic times. September) all of the creeks carry w ater, to be Miocene. Salvador. They are present in the Balsam The most abundanc fossils in the republic and the rivers swell u nder the daily torren The lowest unit consists essentially of a M ountains, in the Jucuaran M ountains, the are Quaternary, including remains of Plc:is tial, tropical rains. Nearly all creeks flow sequence of multi-colored, unfossiliferous, Sierra de T acuba and between the more rocene horses, camels, giant sloths, bisons, in a northwestward direction into the Rio shaly sandstones and quartz-pebble con recent volcanic complexes of the northern mammoths, sabre-tooth cars and mastodons Paz. Generally, consequent streams follow glomerates which are slightly metamor volcanic chain. They ~a r e m ainly interbedded (Stirton and Gealey, 1949, p. 17,12). One the slope on the inclined fault blocks until phosed at some localities. Concordantly tuffs, 2.gglomerates and lavas w hich arc gen very unusual find is human footprints they are deflected at a fault escarpment. overlying these clastic rocks are light-gray, erally basaltic in the lower p art of the sec pressed into water-soaked tuff and dated by They then follow the fault until they are massive limestones and dark-colored, flaggy ticm and andesitic in the upper part. Grebe (1956, p . 55) as 1000-1 500 years captured by a headward-eroding stream dis limestones conra1n111g T oucasia sp. and Overlying the Balsam Formation arc old. secting the fault block. This trellis p attern miliolid Foraminifera. Mi_illereid ( 1939, p . e ru p~ i o n products from the northern vol Archeological dating of potsherds found is especially apparent in the central portion canic chain which includes the broad srraro beneath 3 meters of pumice from Boquen)n * Aw-climate producing savannas, dry eswblished an age of 2000 years ( \Xlilli.1 ms of the map area. winter season; a-highest mean monthly volcanic edifices of Cacaguatigue, T ecom a and Mcycr-Abich, 195 5, p. 23) for the temperature, above 22 o C; i-extreme an tep e, Guazapa, and Capullo. These extinct Climate and V egetation nual m ean temperature difference, less than and largely eroded volcanoes reached the pumice eruption The last volcanic ,l~ti \·iry 5o C; g- temperatl.ue variation, type Gan at Boquer6n was in 1917 when a scnes of The climate of this portion of El s ~llvador height c£ their activity during the late Plio ges, hottest before the solstice, wet season ci ndcr- cones were formed near the summit is described b y K oppen ( 1931, p. 122-1 28 ) during the ummer. cene, producing tuffs, welded tuffs, Clg - 200 T ulane St11dies in Geo logy Vol. 5
Age Rock units Dominant rock type Thickness
Recent Alluvium reworked pyroclastics - lYPe dacitic tuffs, welded Pleistocene San Salvador tuffs and pumice, - - basaltic lavas 2,500 m Upper Type rhyolitic to andesitic Pliocene Guazapa tuffs, welded tuffs, agglomerates and lavas l, 500 m
Lower Type andesitic to basaltic and Middle Balsam agglomerates, endurated Pliocene tuffs and lavas 1,500 m
Upper Type rhyolitic to dacitic Miocene Chalatenango light-colored endurated tuffs 500 m
~ ~ ~ unconf ...... reddish-violet sand- Lower stones, quartz-pebble Miocene conglomerates and lime- 400 m stone-pebble conglomerates ~-..._..- Type, ~ ~ ~ r...... unconf. '-·" Upper Meta pan Cretaceous gray limestones 100 m (Albian)
Lower red shaly sandstone, Cretaceous quartz-pebble conglom- erates 350 m
Figure 2. Stratigraphic column of E l Salvado1· (modified from Di:i.1T, 1960a) . and a lava flow spread down the flanks of IV. LOCAL STRATIGRAPHY the mountain. Introduction Fluviatile and lacustrine deposits of pum ice and ash of PleistOcene and R ecent age The most recent geologic map of El Sal are found in the bottom of the river valleys, vad or, published by the Servicio Geologico in local depressions of the Interior Valley, N acional in 1960, indicates that all Otttcrops the Central graben and the Coastal Plain west of Abuachapan belong to the upper where they cover an area of approximately most member of the G uazapa Formation 3500 sq. km. T he reworked pyroclastics are which is late Pliocene to early Pleistocene. interbedded wi th thin deposits of lignite, This unit is described as "volcanic material carbonaceous shale, diatomite and fresh in tectonic depressions". From detailed ob water limestone. servations in the fa ll of 1959 Seeger dis- No.4 Geology of the Almachapc!n Area 201 tinguished ( 1960a, p. 49) a number of oldest unit exposed in the Ahuachapan area. stratigraphic units and, beginning with the The Pleistocene and Recent are not separ oldest, assigned the following temporary ated in the map area because the volcanic field nan1es: aglo1nerados antiguos, bori rocks are almost devoid of fossils and no zonte 1'0jo) mulesitct lmnina1'1 andesite£ maciza. absolute age determinations have been made. tobas fundidas antigttas) aglomerc~dos de The only reported fossil from the area is a colo1· g1ris. toba inferior de color cafe, tobas i\ 1Iegatherimn femur which was found ap fundidas jovenes and toba superior de color proximately 2 km west of the village of las cafe. These field nan1es are retained in this Chinamas. Di.irr (personal communication, report with three exceptions. The horizonte 1960) considers it to be Pleistocene to post rojo was abandoned because, during the glacial age. course of the field work, this unit proved to Ancient Agglomerate be a red clay formed by alteration of the un The ancient agglomerate is the oldest ex derlying aglome1'ados antiguos. Di.irr (personal posed rock unit within the Central graben of communication, 1960) bdieves that thermal western El Salvador. It consis:s of medium waters which percolated up through the an gray, angular, dense, andesitic blocks ranging cient agglomerate in some localities altered in size from 2 tO 8 em which are imbedded it to red clay. Previously Seeger (personal in a gray, sand-size tuff rightly compacted communication, 1960) believed the red clay and cemented into dense rock. The only to be correlative with the red clay which minerals recognizable in band specimens are m:1rks the Pliocene-Pleistocene boundary in light-colored, subhedral to anhedral plagio the Sierra de Tacuba, but after thorough clase phenocrysts, many of which seem to investigation, the red clay horizon proved be in an advanced stage of corrosion. They to be discontinuous. Seeger's name tobas comprise 15 to 25 c; of the andesite blocks. f;mdidas cmti gi!CtS was changed to blue Red clay which crops out as a number of ignimbrite. Further, his tobcts fmzdidc!S thick, wedge-shaped stringers below the con jo venes was subdivided into two units, the tact of the laminar andesite is uppermost lower, pink ignimbrite, and the upper, gray in this unit. The red clay is believed to be ignimbrite. Finally, the toba superior de an alteration product deposited by thermal color cafe was subdivided intO two units of waters, which percolated up through the equal rank, the upper brown tuff and the ancient agglomerate tO the overlying ~m pLtmice. The units, as used by Seeger, and pervious laminar andesite. This concluswn as revised for this report are summarized is based largely on remnants of altered below: blocks contained within the red clay. Spanish ;mits English 11nits The Rio Paz valley is a deep trench cut through the stratigraphic sequence of the toba s~tperi01' de color I upper brown tuff graben. The ancient agglomerate crops out (pumice cafe intermittently along the river banks and to bas fmzdidas jo venes I gray ignimbrite 200 meters north -of the base of section {pink ignimbrite MS 5 (see geologic map ) reaches its great toba inferior de color lower brown tuff est exposed thickness of 15 meters. The cafe total thickness of the ancient agglomerate aglomerados de color gray agglomerate cannot be measured in the Ahuachapan area grtS as the lower contact is not exposed in the tobas fmzdidas blue ignimbrite graben. However, Seeger ( 1960a, p. 50) ctnti g11c1S measured a thickness of more than 150 meters in the Sierra de Tacuba, where the massive andesite anclesita ma cizct lower contact is exposed. andesita lmnincw laminar andesite horizonte rojo / laminar Andesite aglome1rados cmtig11os)( ancient abobalomerate The laminar andesite overlies the ancient
aolrlomcrate.bb It is a dense, blue-gray . to . All units of the sequence are Quaternary slightly violet, fine-grained to apbamnc age. as the red clay in the Sierra de Tacuba, rock. In hand specimens anhedral to sub which marks the Tertiary-Quaternary bound hedral grains of plagioclase ( 0.5-1.0 mm) ary, underlies the ancient agglomerate, the and some smaller ( 0.2-0.5 mm) black gra111s 202 T11lane St11dies i1~ Geolog')' Vol. 5
of ferro-magnesian minerals are visible. black crystals of hornblende ( 0.2-0.5 mm in
Petrographic analysis reveals that the dark diameter). The fe1dsoar-'- of the massive minerals include 1 r:f magnetite (grains 0.2 andesite weathers considerably faster than mm in diameter), 3-5 S'{, hornblende (slight that in the laminar andesit~, leaving an ly corroded prismatic crystals ), and 1-2 % outer crust of porm1s, friable, bright laven hypersthene (similarly altered ). Plagioclase, der rock. The ferromagnesian minerals are which makes up 25 Cf of the rock was iden little altered. Under microscopic examina tified as andesine ( Ab;; ~- An-t. !i ) ; thus, the tion the composition is sin1i1ar to the lami rock is classified as a hornblende-hyper nar andesite: magnetite, in small grains, sthene.: andesite. The matrix is cryptocrystal 2 % ; hypersthene, partly altered to chlorite, line to glassy. The phenocrysts lie at ran 5 c;, ; hornblende phenocrysts, almost un dom, but small microlites in the matrix ex altered, 3cr ; and, pbgioclase 35-40S'f . The hibit good flow structure. plagioclase occurs in well-zoned crystals The laminar andesite is distinguished which range in composition from medium from the overlying massive andesite by the andesine in the cores to basic oligoclase presence of closely spaced ( 2-8 em ) hori near the borders. Thus, the massive andesite zontal joints which are best developed in also is classified as a hypersthene-hornblende the region of the Las Chinamas volcano. andesite. It is termed massive as the hori Orhcr joints are vertical, but no well-de zontal JOints are considerably less pro fined joint pattern was recognized. Hori nounced than in the underlying laminar zontal and vertical joints are also well de andesite, and it is practically free of ver ycJoped in exposures along the Rio Paz, tical joints. northwest of the Laguna Seca, forming The outcrops are, like the laminar ande cubical, angular cobbles. site, limited to the Rio Paz valley, the low The laminar andesite is exposed near est part of the Rio Molino valley and the water level along the Rio Paz valley, except mouths of tributary valleys. The maximum in the northern part of the map area where thickness of approximately 100 meters was it is covered by Las Chinamas lava, gray measured in the Rio Paz valley 2 km up agglomerate, talus and river gravel of a wide stream from Los Encuentros, the junction flood plain. It can also be traced for a short of the Rio Paz with the Rio Molino. distance up the Rio Molino valley. It at tains its greatest thickness of approximately Blue Ignimbrite 50 meters at a point 2 km downstream from The blue ignimbrite overlies the massive the base of the second measured section, andesite. It consists of a matrix of blue MS 2, on the Guatemalan side of the Rio gray to violet-gray 2.sh- and dust-si ze vol Paz valley. Since the lower contact is not canic ejecta. Contained within the matrix e;.;posed at this locality, the 50 meters rep are black to dark-gray spindle-shaped inclu resents only a partial thickness. At localities sions of dust and glassy material which where the lower contact is exposed the vary in size from 2 mm to 6 em in diameter. total thickness ranges from 15 to 20 meters. The sinuous inclusions are well-aligned but Outcrops of laminar andesite are also known the lineation curves slightly in each boulder from the Sierra de Tacuba, where it caps or cobble. The fabric might be compared the mountains over large areas. Seeger to that of an augen-gneiss. Small druses of ( 1960a, p. 50) states that this unit and the quartz, although rare, are present. overlying massive andesite are among the The rock contains traces of cubical and most widespread deposits in the region of rounded magnetite grains scattered through Ahuachapan. out. Approximately 1% are augite crystals, some of which display perfect octagonal Massive Andesite sections normal to the c-axis. A similar Over1 y ing the laminar andesite is the amount of hornblende also exhibits fairly massive andesite, a slightly less tough, pink well-preserved crystals. The much larger ish-gray to lavender rock. It consists of a plagioclase grains, andesine (Abn1 -An:\ H), fine-grained to aphanitic matrix containing occur as broken fragments sprinkled with white, anhedral plagioclase phenocrysts (2-3 inclusions. Some of rhe larger grains are miD in diameter), some of which exhibit zoned and show undulose extinction, which albite twinning. less conspicuous are small supports the conclusion that the unit was No. 4 Geology of the Ahuachapc!n Area 203
Figure 3. Bombs of black scoria in gray agglomerate in the loop of the Chinamas road, 1. 4 km northwest of the village Las Chinamas. The bombs attain a maximum size of 50 em. still hot and plastic when deposited. The Gray Agglomerate microlitic m atrix is m ade up largely of glass In all five sections measured the gray shards exhibiting a pronounced fl ow strtlC agglomerate consists of many different ture and an advanced degree of welding. layers which can be distinguished only for On this basis the validity of the fi eld term short distances, as the macroscopic character "ignimbrite" was confirmed. Mineralogi of this tmit varies considerably from place cally, the blue ignimbrite is a hornblende to place. In general, the unit consists of augite andesite. bombs and blocks of black scoria (5-SO SC The blue ignimbrite crops out as small of the rock) imbedded in a loosely com scattered patches high along the R io Paz pacted matrix of dark-gray to light-gray vol and Rio Molir!o valleys. Evidence that it canic ash. The largest scoria bombs (Fig. 3) was largely destroyed by erosion was found are found around the volcano las Chinamas at localities MS 2 and MS 3 (see map) . where they attain 50 em in diameter. The The largest and most easily accessible out vesicles are large in the center of the bombs crop is located 2 km southwest of the la Fip;ure 4. The low <:- r bl'O\Vn h1ff near the village of Los Toles. At the lower 1·igbt 1s a fresh expO!::U1'E-' of the t uff ; in the upper left half of the photograph the material is dc.nker due to weath ering . The spheroidal-type weathe1·ing is typical fo1· this unit. mas. The joints (Fig. 3) are healed by a lap ill i-size particles of obsidian and pumice. yellowish-white siliceous material. The color and composition varies both The gray agglomerate has a greater out laterally and verticall y. There are no bedcl ing crop area than the older units. Ir is exposed planes and the material is only slightly com along most of the R io Molino valley, where pacted. The rarity or absence of soil hori it forms steep slopes even though the mate zons aids in distinguishing rhis unit from rial is loosely compacted. Similarly, along the upper brown tuff. the Rio Paz there are outcrops of gray ag In general, outcrops of the lower brown glomerate forming the steep v::dley slopes ruff (Fig. 4) are restricted to an area west and some of the terraces at the edge of the of fau lt No. 14 (see geologic map ), where plateau. they cap the hills and form the rop of the South of Los T oles the gray agglomerate western part of the plateau. The thickness forms the base of the section, bur the total generally varies between 10 and 30 meters. thickness is not exposed. West and north Seeger ( 1960b, p. J 83-184) in the core de of the Laguna Seca it overlies the massive scriptions of wells in Play6n de Ahuacbapan andesite and around the volcano Las China reports a thickness of 30-SO meters. Other mas outcrops can be traced up the deep well clara from Play6n de Salitre indicate vall eys which dissect the plateau. Seeger that this unit thickens ro the south. ( 1960a. p. 53) reports outcrops of various Pink Ignimbrite kinds of agglomerate layers from Tacu ba to San L()renzo which seem to correspond with This welded tuff unit varies widely in its the gray agglomerate from the Ahuachapan macroscopic appearance. Commonly the our area. He reported thicknesses of as n1uch crops consist of pink, more or less earthy, as 150 meters near Tacuba. In the map area nonbedcled volcanic ash. Contained in this the unit attains its greatest thickness of 116 matrix are fl at, sometimes sinuous inclu meters at locality MS 1 (see geologic map). sions ( 2 mm-5 em in diameter) of the same composition as the matrix, but made con Lower Brown Tuff spicuous by their reddish-brown color. In The lower brown tuff consists of pale some outcrops the inclusions are well yellow to light brown sand and dust-size aligned while in others no orientation is volcanic ejecta with traces of sand and apparent. Lu·ge rounded inclusions of purn- No.4 Geolog)' of tbe AlJIIctchctpcln Area 205 ice bombs (average diameter 7 em) are rare. The above descriptions applies gen erally to ::tll outcrops south of the villao-e ~ o~ El Tigre. North of this village, and esp~ oally around the laguna Seca, rocks in every outcrop differ in color, hardness and size of inclusions. The color varies from light brown to light gray to a dull red. The ~~ inclusions are the same color as the matrix ' but of a darker shade. A petrographic analysis of one of the harder welded tuffs showed: magnetite grains, 2%: plagioclase, 15 S'f; and traces of hypersthene. The plagio clase, which was identified as andesine (Ab:;.-; - An-t:;), contains many inclusions, and the crystal outlines are corroded. The large sinuous inclusions contain welded glass shards, but the matrix does not. Flow struc ture is present only in the inclusions. In the lagL111a Seca area it may be possible, with more detailed field work, to recognize a se quence of superposed deposits of volcanic showers or avalanches. All rocks arc devoid of fossils, and age relations are based on superposition alone. A diagran1 illustrating how deposition and erosion of three vol canic glowing avalanche deposits can con fuse stratigraphic work is given in Fig. 5. Units 2 and 3 are hard welded tuffs like the pink ignimbrite from the laguna Seca, ~ but unit 1 is a soft, friable and easily eroded Figure 5. Diagram sho\Yin,g· six succes tuff like the lower brown tuff. No bedding sive stages of deposition and erosion of planes are recognized in any of these units. tluee glowing cloud deposits. Small scattered outcrops of pink ignim brite occur in the upper Rio Molino valley. the rock surface is wet they become dark South of Palo Pique the thickest section of gray in color :111d stand out prominently. pink ignimbrite (20 meters) was measured. The rock is composed of: andesine The largest exposures are west of fault No. ( AB:; 4 - An u;), 2 0%; euhedral hornblende 14 on the fault escarpment and capping p 11enocrysts, 2-3 r~; hypersthene, 2-3 r'(;.; and most of the bills. Due to its compact nature traces of magnetite grains and augite crystal the ignimbrite forms ledges above the soft fragments. The matrix is cryptocrystalline lower brown tuff. A typical outcrop con to glassy with slight indication of flow sists of large blocks and boulders which on structure. A few welded glass shards are weathering become dark brown to gray. recognizable. The larger lens-shaped inclu sions arc outlined by dark borders but other Gray Ignimbrite \Vise arc indistinguishable from the matrix. Above the pink ignimbrite is a blue-gray In the northern part of the Ahuachapan to Jight-gray indurated volcanic ash deposit area outcrops of the gray ignimbrite are which is named the gray ignimbrite. The I imited to the bottoms of deep gulches color does not change appreciably on weath which cut through the overlying pumice ering. The rock has a hacldy tO conchoidal and the upper brown tuff. Along the fault fracture. There are many flat, lens-shaped escarpment of fault No. 4 only scattered inclusions with pronounced alignment. The patches of gray ignimbrite are present. The inclusions are of the same color and mate outcrops along the Rio Molino arc the only rial as the matrix. On a dry surface the in ones in which the thickness can be meas r: Jusions are difficult tO discern, but when ured (maximum, 20 meters). The outcrops 206 Tulane St~tdies in Geology Vol. 5 Fi are not bedded bLlt they all contain complex out, so that the upper brown tuff lies di systems of joints. The extension of fault rectly on the gray agglomerate. No. 11 marks the western limit of the gray ignimbrite. Whether the unit was deposited Upper Brown Tuff beyond this line and has subsequently been The most widely outcropping pyroclastic removed by erosion, remains in question. unit in the map area is the upper brown tuff. It is a lithic tuff consisting of yellow Pumice ish-brown to rust-brown volcanic ash and Above the gray ignimbrite is a relatively particles of porous rock. Less than 1% of thin layer of unstratified air-borne pumice, the particles are of lapilli size. The upper which is composed entirely of lapilli and brown tuff differs little from the lower blocks. Most of the fragments are yellowish brown tuff, although the latter is. commonly white, but some, and especially the large slightly lighter in color. The outcrops are blocks ( 10-15 cn1 in diameter), are bright devoid of bedding planes with the excep pink. Thin interbedded, brown tuff layers tion of soil horizons, and it is difficult in (average thickness 3 cn1) reflect the topog 1nost instances to determine whether the raphy at the time of each eruption and af tuff is in place or has been redeposited. As ford an example of how air-borne pyro the tuff is easily eroded, it is spread over clastic material is deposited and eroded the outcrops of older units especially along (Fig. 6) . fault escarprnents, covering these units con1- Pumice outcrops are found in road cuts, pletely or concealing their true thicknesses. ravines, along faults, high on the slopes of The thickness of the upper brown tuff the Rio Molino valley, and in the western ranges from 1-2 meters in the north and corner of the map area, where the overlying west of the map area to 20-30 n1eters near upper brown tuff is thin or has been re the town of Ahuachapan. Seeger ( 1960a, moved by erosion. The greatest thickness of p. 55 ) reports a maximum thickness of 150 pumice ( 15 meters) was measured 2 km n1eters near the town of Ataco. The upper west of Ahuachapan in a tributary valley brown tuff is characterized by the great of the Rio Molino. Northwest of the vil abundance of soil horizons. These are n1ost lage of Las Chinamas the pumice pinches easily recognized in road cuts, attain thick- No.4 Geology of the Ahttachap!m Area 207 nesses of 1-2 meters and are marked by a eastern slope are covered by cobbles and darker brown color and the presence of boulders of the eastern flow. This is a small, interwoven root canals (diameter 0.5 medium gray to dark-gray, vesicular lava mm) . Tbe root canals are lined or filled which weathers yellowish-brown. The rock with a yellowish-white siliceous substance. is not as resistant as the lava of the southern Rarely one finds charred remains of rootlets flow due largely to its vesicular character. in the canals. In hand specimens it resembles the massive andesite. It contains relatively large ( 1-2 El Gringo Lava mm in diameter), subhedral plagioclases Probably the oldest Java in the map area with albite twinning, and small, black, is the scattered remnants of a flow on the slender needles of a ferro-magnesian min slopes of El Gringo volcano in the western eral. Thin sections show sodic labradorite corner of tbe map area. The lava is a light ( Ab4s- An.!~ ), 45%; hornblende, 5%; gray to medium-gray, fine-grained, hard hypersthene as clear, elongate, prismatic rock con::aining small anhedral untwinned crystals, 5 %; and cubical and octahedral plagioclase pbenocrysts. Scattered through magnetite grains, 1%. There is no olivine. out the rock are small brown, rust-like par The matrix consists of a non-trachytic mix tiel es. Thin sections show plagioclase ( sodic ture of plagioclase microlites and glass; thus labradorite, Ab-1s- An.-;~), 35%; hypers tbe rock is a hyalopilitic hypersthene-horn thene, partly altered to cblorite through blende tholeite. mamn1illary etching, 59(;.; hornblende, 3%; The northern and the northeas tern flows small, round grains of magnetite, 1%; and are identical in macroscopic appearance, traces of ilmenite. Since olivine does not and it is possible that they are one and the occur, the rock is a hypersthene-hornblende same flow with the intervening area con tbolcite. The matrix is cryptocrystalline to cealed by a cover of gray agglomerate. The glassy with little or no alignment of the rock crops out as patches of cobbles and microlites or phenocrysts. boulders of dark-gray to black, vesicular lava on the northern slope and extends as far as La Chinamas lavas the alluvial plain of the Rio Paz. The The Las Chinamas volcano, located 3 km patches continue on the other side of the northwest of the village of the same name, river into Guatemala. At the northern base is a very old, eroded cone. Remnants of of the volcano boulders of green and red four lava flows, described as northern, vesicular lava are mixed with the dark-gray southern, eastern and northeastern, diverge lava, but it is questionable whether they from a common center near the top of the were derived from the Las Chinamas vol cone. cano, or whether they are part of a fluvial The southern flow is a black to dark deposit of the Rio Paz. Microscopically, the a-ray dense fine-bo-rained rock with a mero- two lava flows are very similar. The mag bcrystalline ' texture.' It is composed of: an- netite ( 2%) and the hornblende ( 5% ) desine phenocrysts with many small inclu content are the same in both flows. The sions, 30%; hornblende, 3%; hypersthene northeastern flow contains 3% hypersthene as small broken fragments, 3%; and small while the northern flow contains 6%. The rounded crystals of magnetite, 2%. The plao-ioclase content is about the same (25%) dark-a-ray to brown grmmdmass is crypto exc~pt that the northeastern flow contains crystalline to glassy with no evidence of calcic andesine ( Ab:-;~ -An -1x), and the flow structure. On the basis of the composi northern flow, sodic labradorite ( Ab-tn tion of the plagioclase ( Ab:-;.-;- An-1:-;) the An:-; 1 ). Theoretically, therefore, the north rock is classified as a hypersthene-horn eastern flow should be called an andesite blende andesite. The southern flow crops and the northern flow a basalt, however, out on the south slope of the volcano, and more sections must be examined before this two laro-e isolated patches are exposed in distinction can be verified. the can~on near the base of section MS .4 (see a-eo logic map ) . The remnants of th1s Em palizada Lava flow ~onsisr of large boulders imbedded in A lon a tongue of lava, most likely from gray agglomerate. the volc~10 Empalizada (Fig. 7), occupi~s The top of the volcanic cone and the the Rio Molino Valley. Outcrops of sohd 208 T11lcme Studies 112 Geolog_1 Vol. 5 Figure 7. Volcano Empalizada (highest peak on left, 1380 meters) looking south from. the intersection of the road to Las Chinamas and fault No. L Fault No. 1 c1·osses the pho tograph from the lowe1· left corner to the mi::ldle of ~h~., rig·ht edg~ . The out~rops at. the lower right m·e pumic:e overlain by upper Lrown tu tf. 'I he moun tams on the nght honzon are part of the Siena de T acuba. , ==- lava bedrock occur as far as the village Palo fied as a hyalopilitic augtte hypersthene Pique, but below this point there are only tholeite. large boulders of lava. It is difficult to de Alluvium termine where the lava flow ends, as the Significant deposits of alluvium are found front is greatly eroded and boulders fill the only in the bottom of the Rio Paz valley stream bed for a considerable distance. A where the material consists largely of sample from a bedrock outcrop between pebbles, cobbles and boulders of igneous fault No. 5 and fault No. 4 ( Planta de Luz) rock. Low energy banks show floodplains contains dark gray plagioclase grains ( 0.1- composed of water-borne tuffaceous mate 0.5 em in diameter) with good albite twin rial. ning. A few grains exhibit labradorescence. V. STRUCTURAL GEOLOGY There are also slightly smaller phenocrysts ~)f brown hypersthene and black horn Regional Structure blende. Most of the lava is a dense and In El S::dvador there are three principal dark-gray to dark-brown rock. Near the vil systems of linear tectonic elements which lage of Palo Pique there are black aphanitic, trend WNW, NNW and ENE. The WNW non-vesicular lava boulders mixed with dark sys tem is the main one and it delineates the gray, aphanitic lava containing large vesicles. major geologic-morphologic provinces of El Another type of lava is medium-gray, fine Salvador. grained and contains many white plagioclase The WNW system consists of five tec grains (average diameter 2 mm) . The tonic lines, three of which cross the republic latter was examined petrographically and (Fig. 8) with considerable vertical dis found to contain: labradorite ( Ab:w -Anf;,1) placement. The most northern one (axis 1) in large euhedral crystals, 3 5%; euhedral forms the southern boundary of the Hon crystals of hypersthene, 3 91-; magnetite duran Border Mountains province and rep grains, 3%; and traces of fresh augite frag resents downfaulting to the south of about ments. The matrix is glass. Taking the tex 1000 meters ( DL_irr, 1960a, p. 15). The sec ture and the absence of olivine into con ond (axis 2) is marked by a chain of ex sideration, the Em palizada lava is classi- tinct volcanoes such as Guazapa and Caca- No. 4 Geo logy of the Ahuachapan Area 209 meters below sea level) almost to sea level. These cones can be regarded as volcanoes in stat11 nascenti (Di.irr, 1960a, p. 16). Ob servatio?s indicate that the tectonic activity started 111 the north and progressed toward :he south. Today, axis 3 has already passed 2 lts peak of activity, and axes 4 and 5 are 3 now entering the active stage of tectonism. The NNE system is less pronounced and does not have n:uch effect on the morphol 4 ogy of the regwn. Nevertheless, it seems that large horizontal dislocations have oc 5 curred along this system. Di.irr considers them contemporaneous with the discloca tions on the WNW axes. The youthful belt of Pacific volcanoes crosses N icara o-ua in 5 the N W direction, and is deflected bto the Fig-ure 8. Axes of the tectonic W NW WNW across El Salvador. Each volcanic system. (from Dihr, 1960a, p . 26) chain terminates where it intersects a tec tonic element of the NNE system. Only in guatique which were built from lava and a few places is the NNE system accom tephra extruding from the fault. This to panied by volcanism which in each case pographic rise forms the southern boundary formed later than that of the WNW system. of the Interior Valley province. The In terior Valley is thus a graben similar to T he youngest tectonic system in El Sal the Central Valley. T he Interior Mountain vador, along which all volcanic eruptions in province is ba s i c~ ll y a horst which is bor historic time (since the Spanish conquest) dered by the fault of axis 2 in the north and have occurred, is the NNW system. It con another fault in the south which is part of sists of fa ul ts and rifts within the volcanic the Central graben. The northern fault of edifices, along which the effusive products the Central graben is very irregular and are expelled. T his system intersects the progresses as a series of parallel and en main volcanic chain (WNW system, axis echelon faults. The abse nce of seismic 3) at a high angle and is responsible for tremors along this line indicates that it is the spacing of the volcanic centers along no longer active. The southern boundary of the main axis, a fact which was recognized the Central graben is formed by axis 3, the by Dollfuss and Montserrat in 1868. For most prominent of all tectonic elements in more examples and evidence the reader is El Salvador. It continues into Guatemala to referred to Durr (1960a) and Schulz (1960). the west and into Nicaragua to the east. The blocks which border the Central horst The highest and younges t volcanoes of El and the two grabens are slightly inclined. Salvador, many of which are still active, are The northern block dips to the north and situated along this zone. Shallow earth the southern block dips into the Pacific at quake foci , situated on this axis at dep ths an angle of 2 o to 3 o . It appears that this of about 10 km ( Schulz, 1960, p. 33), in part of El Salvador was raised as a WNW dicate that tectonic movements are still con trending geanticline, the crest of which sub tinuing. The fourth axis lies in the Pacific sequently collapsed to form the Central and Ocean some 25 km from the El Salvador Interior grabens. Underneath the vast pile coast and is characterized by very pro of Quaternary volcanics there are believed nounced seismic activity, and foci lying at to be eroded remnants of a late Tertiary depths up to 100 km. The fifth axis is lo volcanic belt (Bullard, 195 7, p. 3 58). cated still farther south and forms the M id A succession of marine terraces along dle American trench, a submarine graben parts of the Pacific coast indicates recent which runs parallel to the coast and is also uplifts of the geanticline. The fact that the the locus of much seismic activity. W ithin terraces are not continuous indicates that the trench there exist large cones which the uplift did not occur uniformly, but that rise from the bottom of the trench ( 3000 various blocks moved up at different rates, 210 TzdaJZ e St11dies in Geo logy Vol. 5 Figure 9. The Sierra de Tacuba, the southe1·n upthrown block of the Central graben, from the southern bank of the Laguna del Llano. which is further evidence for the existence massive and laminar andesites and the an of NNW -rift system. cient agglomerate in the Ahuachapan area. The structural conditions related to the The oldest unit exposed near the edge of tbe escarpments is a thick sequence of red Central graben of western El Salvador have clay, which according to Di1rr (personal a direct bearing on the structure of the communication), marks the close of the Ahuachapan area which is located within Pliocene epoch. the Central graben. The northern fault scarp On a map of the volcanoes in the graben of the graben consists of a number of and along its borders, definite alignments paralle1 and en echeln n, northward-dipping of volanic centers can be recognized. Along fault blocks which form a discontinuous a NNE line, passing the towns of San Lo mountain chain and pass juc:;t north of the renzo and Ahuachapan, lie the volcanoes towns of Santa Ana; Chalchuapa and San Empalizada, Salitre, San Lorenzo and Chin Lorenzo The southern fault escarDn"!ent, the go. A NNW line can be drawn through Sierra de Tacuba ( Fig. 9), is likewise an the volcanoes Santa Ana, El Pozo, Laguna inclined fault block, dipping gently to the Seca and Chingo. The most conspicuous south at an an gle of 2-3 o . Its escarpment volcanic chain is the Apaneca arch (Fig. is more continuous than that of the north 10 ) , a row of volcanoes 15 00-2 000 meters ern fault. It strikes in general N o W 60 high forming an arc along the southern and passes 2-3 km south of the towns upthrown block. Juay{la, Apaneca and Ataco. South of Ta cuba its direction changes to N 4 5 o W, and the fault continues on this bearing toward VI. LOCAL STRUCTURE the Rio Paz. The elevation of the southern The stratigraphic units in the Ahuachapan fault escarpn1 ent decreases from 150D meters area are flat-lying, lens and wedge-shaped near Ataco to 800 meters near the Rio Paz. rock bodies. The thickness of each unit The throw for each of the fault-s bounding changes greatly over short distances. The the graben is estimated at 800-1000 meters only appreciable deformation which has dis (Seeger, personal communication, 1960). turbed the beds is faulting. These normal T he exposed rocks on tbe upthrown blocks fault~ show up well on aerial photographs; on either side of the graben are Pleistocene but, 111 the field, fault criteria such as fault andesites and agglomerates similar to the planes, slickensides, and fault breccia, are No.4 Geology of the Ahuachapan A rea 211 soon destroyed by erosion. Exceptions are the Ahuachapan region at an angle of N the laguna del llano, the largest lake in the 2 5o W (Fig. 10, black arrow) . If this area, formed against fault No. 4, and the force is considered the principal stress axis intermittent laguna Seca, situated at the in a stress ellipsoid, the direction of maxi~ intersection of faults No. 15, 16 and 17. mum shearing stress would coincide ap The escarpment of fault No. 14 has been proximately with the graben-bounding faults dissected by small, intermittent streams and and the faults of the NNE system. It is displays five rounded, but still recognizable, possible that the faults bounding the graben triangular spurs. Two sets of faults may be are oblique slip faults, even though there distinguished, a north-trending set and a is no noticeable horizontal component of northeast-trending set. The dip of the fault net slip. At this stage there should be vol planes is difficult to determine as the fault canoes along all tectonic lines as they all lines are covered by debris. Presumably, represent lines of weakness. The graben is however, uncemented and only slightly com crossed by tecronic lines of the NNE system pacted sedimentary rocks will fracture at an dividing it into blocks which were moved angle near 60 o, similar tO the experimental by the initial stresses from N 25 o W. Block faults obtained in sandbox experiments. On ABCD was displaced north-northeastward, fault No. 4, just west of the laguna del and the blocks on either side moved south~ llano, the pumice is drag folded and dis southwestward. As the NNE lines meet the rupted by many small secondary faults. The NNW line (southern boundary fault of the attitude of these small faults is parallel to graben ) at slightly acute angles, certain tec the attitude of the major fault (N 30 ° E, tonic lines are subjected to tension and dip 60 o) . Secondary faults on this kind can others to compression. Block ABCD tended be seen near many of the rna jor faults of tO move north along direction X, perpen the area. They are especially apparent in the dicularly away from the foot wall of fault lower section of the upper brown tuff which DC, but was deflected by the acute angle contains narrow stringers of white pumice ADC, and moved in direction Y. Thus, line that make excellent 1narker beds. AD, a strikeslip linear, was sub jected to Most of the indurated units in the strati tension along the resultant Z and became a graphic sequence are cut by joints, but no gravity fault. Magma rose along this line definite pattern could be detected. A num~ of weakness to form the volcanoes Empali ber of - north-northeast striking, vertical zada, Salitre, San Lorenzo and Chingo. The joints are noted in the gray agglomerate curva ture of the Apaneca arch (D-C) may 1 km northeast of the volcano las China be due to the near contemporaneous tee~ mas. They are healed with dirty white conic processes along th e NNE and WNW siliceous n;aterial and possibly formed as a systems as block ABCD moved northward result of expansion of the volcanic edifice with friction along the sides forming· the preceding an eruption. WNW curve. T lie tectonic line of the Now that the structural conditions of the NNW system (A-C) supports the volcanoes map area and its immediate surroundings Cerro Verde, Santa Ana, El Pozo, Laguna have been described, the mechanism respon Seca, a small unnamed volcano and Chingo. sible for the various tecronic lines can be It may be considered as an extension frac considered. Dl.irr ( 1960a, p. 264-265 ) has ture which manifests itself not only in the proposed a hypothesis to explain the pres A-C line, but also in many small faults near ence of volcanoes along the lines A-D, A-C San Lorenzo and Salitre. Moreover, Schulz and D-C and their absence along the lines (personal communication, 1960) has de A-B, E-D and C-G (Fi~. 10). The Laramide tected recent seismic disturbances alon~ the fold belt of Guatemala describes a curve, same line. Li nes E-D , C-G and A-B are the convex side pointing toward western El Jines of compression, and along those lines Salvador with a bearing of N 25 o W volcanism is absent. (Eardley, 1951, p. 595). As in other folded The volcanoes las Chinamas and El mountain belts like the Alps, a high welt Gringo are older than most of the others. exerts a force on the lower area perpendicu They are largelv eroded and buried by their lar tO the long dimension of the welt. own debris and that of younger volcanoes, Therefore, it can be postulated that the and as they do not belong to a chain of vol Guatemalan fold belt exerted a force on canoes, it is impossible tO determine to 212 T11lmze Studies 111 Geology VoL 5 A \ I F~~~ ~A~~ I ~ u I d B ...... 0 LAGUNA IECA 0 0 c Figure 10. Sketch of p1·ominent volcanic chains in western El Salvado1·. A hypothe-· tical stress-sb·ain diagram (inset) illustrates the relative movement of fault blocks and ex plains the p1·esence and absence of volcanoes along certain tectonic lines (from Diirr, 1960a). which tectonic line they are related. The all features such as the symmetrical cone, volcan.o Las Chinamas is a large rounded craters and barren lava flows, leaving only hill just cast of the international bridge large scoria bombs and scattered remnants across the Rio Paz. Erosion has obliterated of four lava flows on the slopes of the hill. No.4 Geology of the Ahttachapdn Area 213 By middle Pleistocene time volcanism also began along the south-bounding fault of the Central graben and the northern row of vol C;lnoes probably declined in activity. The volcanoes :1.long the Central graben, Santa Ana, Boqueron~ San Vicente, Usulutan and San Miguel, continued their activity into Figure 11. Block diagram showing the Recent time, flooding the graben with lava bowl-shaped crater of El Gringo volcano flows, nuees cwde17te.r, and ash and pumice and the peripheral fault scarps. showers. Many historic eruptions and fre quent earthquakes demonstrate that tecton The volcano El Gringo is located 2.5 km ism still is active along this line. The cul northeast of Los Encuentros. It is a hill mination of this later phase appears to have capped by a bowl-shaped summit (Fig. 11) been only a few thousand years ago. lined with remnants of lava flows. The El The geologic history of the Ahuacbap~n Gringo lava is the oldest in the Ahauchapan reo-ion is summarized in a series of dta- b . area as indicated by the alteration of hyper grams (Fig. 12 ) illustra ring the successt ve sthene. Partly encompassing the crater are stat'es of graben development and volcan several peripheral faults which may have isn~ Dl.irr (personal communication, 1960) formed through cauldron subsidence after recognizes a thick sequence o~ red clay beds evacuation of the lava. as marking the end of the Plwcene. There- VII. GEOLOGIC HISTORY The geologic history of Central America, including El Salvador, has been discussed in some detail by Eardley ( 1951), Schuchert Early Pleistocene ( 193 5), Imlay ( 1944) and others. In the present discussion conclusions concerning the geologic history are drawn from evi dence observed in the field. The oldest rocks in western El Salvador, except for the Cretaceous (Albian) and Early Pleistocene lower Miocene Metapan, are thick, wide spread Pliocene volcanic rocks which must have been derived from a chain of vol canoes, the cones of which are eroded and buried by their own and later, eruptive products. This volcanic chain was located in the area now occupied by the Central and Interior grabens. Therefore one can postulate that in the late Pliocene, arching Middle Pleistocene of a geanticline striking west-northwest pro duced tension in the upper crust. Rising magma was extruded along the rifts near the crest of the geanticline, and by the close of the Pliocene a long chain of volcanoes had been built. In the early Pleistocene most of this activity ceased, and the crest of the geanticline collapsed into the magn1a chamber forming the Central and Interior grabens. As the border faults reached the magma chamber, which now was under re newed pressure, a new stage of volcanism was initiated along the southern boundary Figure 12. Sketch showing stag·es of of the Interior graben with the building of development of the graben near Ahuacha the Volcanoes Guaza pa and Cacaguatique. pan. 214 Tulane Studies in Geology Vol. 5 fore, the overlying ancient agglomerate area : along the Rio Paz, where boiling wa (small triangles) and the laminar and mas ter issues from the laminar andesite; and, sive andesite (wavy lines) are very early along the Rio Molino, where steam vents Pleistocene age. They were erupted from a cover the hillside. Recent uplift of the volcanic chain which occupied the crest of Ahuachapan area is indicated by the pres the geanticline. Subsidence of the graben ence of terraces along the streams (Fig. 11) . must have occurred following the deposition of these units, as the youngest unit of the REFERENCES CITED sequence on the upthrown blocks is the BULLARD, F. M., 1957, Active volcanoes of massive andesite. By middle Pleistocene the Central America: 20th Internatl. Geol. southern upthrown block was faulted again, Cong., sec. 1, Vulcanologia de Cenozoic, vol. 2, pp. 351-371. and in the graben, volcanoes such as Las DOLLFUSS, AUGUSTE and E. DE MONTSERRAT, Chinamas and El Gringo were built, their 1868, Voyage geologique dans les Republi eruptive products burying the early Pleisto ques de Guatemala et de Salvador: Paris, cene volcanoes. By late Pleistocene El 1 vol. Gringo and Las Chinamas became extinct. DURR, FRITZ, 1956, La situacion actual de la El Gringo probably produced the blue mineria en El Salvador: Anal. Serv. ignimbrite in the initial explosive stage of Geol. N ac. El Salvador, Bol. 2, pp. 68-69. its activity, and lava flows later issued from DuRR, FRITZ, 1960a, El marco geologico: Serv. Geol. N ac. El Salvador, Energ. the vent. The volcano Las Chinamas pro Geoterm., Infor me 1, pp. 8-29. duced the gray agglomerate, which is in DURR, FRITZ, 1960b, La region de Ahuacha dicated by the large bombs of scoria within pan : Serv. Geol. Nac. El Salvador, Energ. this unit (Fig. 3). Moreover, the Chinamas Geoterm., Informe 1, pp. 43-47. lavas, and especially the southern flow, are DURR, FRITZ and GUNTER STOBER, 1956, interbedded with gray agglomerate, and in Sucesion normal de los Estratos de Meta fact the thick sequence of gray agglomerate pan: Anal. Serv. Geol. N ac. El Salvador, nearly buried the cones. New volcanic Bol. 2, pp. 44-54. vents on the A paneca arch blanketed the EARDLEY, A. J., 1951, Structural geology of North America: Harper Brothers, New countryside with brown ash and dust, and York. nzuJes ardentes deposited the pink and gray GREBE, W. H., 1956, Huellas humanas fo ignimbrites. The different kinds of pink siles en la planicie costera de El Salva ignimbrite around the Laguna Seca indicate dor: AnaL Serv. Geol. Nac. El Salvador, several violent ash eruptions. Explosive ac Bol. 2, pp. 55-62. tivity at the Salitre volcanic field produced IMLAY, R. W., 1944, Cretaceous formations the pumice, as indicated by the pink pumice of Central Ame1·ica and Mexico: Amer. A ssoc. Petrol. Geol., Bull., vol. 28, pp. blocks which range from 3-8 em in diameter 1077-1195. near Ahuachapan to 20 em in diameter to KoPPEN, W ALDIMIR, 1931, Grundriss der ward Salitre. It is difficult to determine the Klimakunde: Walter de G1·uyter & Co., origin of the upper brown tuff, but possible Berlin. sources are the Apaneca arch and the San MuLLERRIED, F. K. G., 1939, Investigacio Lorenzo volcano. This tuff is assigned ten nes y exploraciones geogra.fico-geologicas tatively to the Recent. The lava flow from en la porcion nor-oeste de la America Central: Inst. Pan-Am. Geog. Hist., pub. Em palizada along the Rio Molino valley is 38, pp. 1-47. believed to be the same age as the upper SAPPER, KARL, 1952, Los volcanes de la Ame brown tuff. In thin section rnost hyper l'ica Central: Verl. Max Niemeyer, Halle. sthene grains are fresh, euhedral and show SCHUCHERT, CHARLES, 1935, Historical ge little or no corrosion. Today volcanism in ology of the Antillean-Caribbean region: western El Salvador is confined to fumarolic John Wiley and Sons, Inc., London. activity. The deep-seated n1asses of tnagma ScHULZ, RuDOLF, 1960, Actividad sismica are cooling and crystallizing, giving off en El Salvador: Serv. Geol. N ac. El Sal large amounts of juvenile water which vador, Energ. Geoterm., Informe 1, pp. _nixes with infiltrated meteoric water dur 30-34. SEEBACH, KARL VON, 1892, Ueber Vulkane ing its ascent. The water reaches the surface Central Americas: Dieterichsche Ver in thermal springs and boiling n1ud craters. lagsbuchhandlung, Goettingen. Thermal springs and related features are SEEGER, DIETRICH, 1960, Geologia de la re found at two localities in the Ahuachapan gion de Ahuachapan: Serv. Geol. N ac. No.4 Geoto gy of the Ahztachapan Area 215 El Salvador, Energ. Geoterm., Informe 1, WILLIAMS, HOWELL and HELMUT MEYER pp. 48-65. ABICH, 1955, Volcanism in the southern STIRTON, ~· A. and W. K. GEALEY, 1949, part of El Salvador with particular ref Reconnaissance of the geology and ver erence to the collapse basins of Lakes tebulte paleontology of El Salvador Cen Coatepeque and Ilopango: Univ. of Cal h ·al America: Geol. Soc. America, 'Bull., ifornia, Publ. in Geol. Sci., vol. 32, pp. vol. 60, pp. 1731-1753. 1-64. December 29, 1967 REVIEWS GEOHYDROLOGY; MECHANICS OF INCREMENTAL DEFOR~1ATIONS: SAUDI ARABIA: PETROLEUM INDUSTRY; IRAl\J: PETROLEUlvi INDUSTRY· IRAQ; PETROLEUM INDUSTRY , HAMILTON M. JOHNSON FROFE>"801? OP (JfJOf~OU}' I YJJ GEOL>lfl'.':UCS 'J 'l ' C l rt:J l .)..J\ 'E N8f'l'Y GEOHYDROLOGY by Roger J. lVI. de \"'Viest. tion of mass I make use of partial deriva Published by John Wiley & Sons, Inc., tives, but expertness in partial differentia New York, 1965, xv + 366 p., $10.95 tion is neither required nor expected from A bibliography of 345 entries, the de- the reader. Complex variables are not intro tailed table of contents (five pages) divided duced .... Paragraphs containing more ad intO 8 chapters and subdivided into 90 topic vanced material that may be skipped in a headings, 2 fold-in plates, 186 line figures beginner's course are preceded by an orna and phoros, 2 appendices and a 6-page mental line and set in reduced type." double-column index . . . this statistical re This is an excellent book and well ful view gives an indication of the careful and fills its purpose. It derives from material thorough treatment provided here for this presented in lectures in N.S.F. sponsored subject. What is "geohydrology"? In the SUMMER INSTITUTES, so has been care author's tll1dersranding, it is the science of fully and thoughtfully threshed to separate groundwater hydrology as followed by civil and discard the chaff. The explanations are engineers rather than geologists. A com clear and sufficient, the proofs easily fol panion book) co-authored by de Wiest and lowed, and the teaching sequence logical. Stanley Davis, discusses the opposite ap For the good of embryo civil and geological proach. engineers, it is to be hoped that it achieves Much the longest chapter in the book a wide acceptance. deals with the elements of surface hydrol ogy, such as the drainage basin, precipita MECHANICS OF INCREMENTAL DEFORMA tion, evapo-transpiration, runoff, and case TIONS by Maurice A. Biot. Published by studies illustrating these facwrs. Other chap John Wiley & Sons, New York, 1965, ters include ground-water flow, theory of xvii + 504 p., $17.50 ground-water movement, steady state flow, The sub-title of this book gives the key the mechanics of well flow, multiple phase to the material covered much more clearly flow, and numerical and experimental meth than does the title; it is "theory of elasticity ods in ground-water flow. and viscoelasticity of initially stressed solids The opening chapter, a particularly de and fluids, including thermo-dynamic foun lightful and informative history of hydro dations and applications to finite strain." logic studies (although carried t!l1der many The author's objective was to prepare a names in the five n1illenia discussed ) is the work intermediate between the formalistic perfect beginning to get and keep a stu approach of the mathematician and the dent's interest. And this is the approach pragmatic treatment usual with engineers. throughout the book; first and foremost it The result is a rigorous book characterized is a textbook to be used in teaching the by the use of cartesian concepts but not science of the flow of ground-water to in requiring a knowledge of tensor calculus or termediate and advanced stndents. The other more specialized techniques. Since the mathematics, while rigorous, has been care theory is valid for non-elastic media, it has fully selected for the level of the classes in been found to be applicable to such prob volved. As the author pointed out "in the iems as tecronic folding in geodynamics. derivation of the equation for the conserva- Further extension of these methods has