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 <rcolo<ric history. b b iden ti fica ti on more difficult. Other prominent land forms include the Approximately 80% of the area has been ruins of two extinct volcanoes. T he volcano cleared for cultivation with even the steep Las Chinamas, located in the northern slopes of volcanoes and river valleys uti­ corner of the area, remains as a large hill, lized. The uncultivated patches are rolling the crater having eroded away. The only grassland s, scattered clumps of small trees, indication of its origin arc fo ur lava flows thorny underbrush and agaves. Along the which appear to 111ve come fro m the hill. banks of streams the trees and underbrush The other volcano ic:; situated 2. 5 km from are so dense that they form an almost im­ I.os En<.ucntros and displays a shallnw bowl­ penetrabl e forest. shaped crater partly encircled by five peri­ pheral faults at a distance of 500 meters III. STRATIGRAPHY from the crater. R EG IONAL STRATIGRAPHY Tht highest elevation measured is 759 G eologically, El Salvador is a young coun­ meters at a poinr 500 meters southwest of try. Approximately one fourth of the out­ the Laguna del Llano; the lowest is 200 cropping rocks are of Quaternary age. Most meters at Los Encuentros. of the remaining rocks are Tertiary ( prin­ cipally Pliocene ) and all are of volcanic Drainage origin. The only non-volcanic deposits pres­ The ma ior rivers in the area are the R io ent are about 200 square kilometers of Cre­ Paz and the Rio Molino. T he R io Paz taceous sedimentary rocks in northwestern originates near Lake Ayarza, 40 km south­ El Salvador (see Fig. 2 ) . east of Guatemala City and discharges into the Pacific Ocean. T he Rio Molino ori rz i­ Cretaceous rocks natcs on the slopes of the Cerro las Ninfas Cretaceous marine sedimentary rocks crop .. nd flows into the Rio Paz at Los Encuen­ out near Metapan in northwestern El Sal­ tros. With the excep6on of the Rio Paz vador. These rocks comprise the "Metapan and the Rio Molino, which are fed to some sequence" which includes three divisions. extent by springs, the streams are inter­ The two lower units are of Cretaceous age, mittent. During the rainy season ( June­ but the upper one is believed (Di..irr, 1960a) September) all of the creeks carry water, to be Miocene. and the rivers swell u nder the daily torren­ The lowest unit consists essentially of a tial, tropical rains. Nearly all creeks flow sequence of multi-colored, unfossiliferous, in a northwestward direction into the Rio shaly sandstones and quartz-pebble con­ Paz. Generally, consequent streams follow glomerates which are slightly metamor­ the slope on the inclined fault blocks until phosed at some localities. Concordantly they are deflected at a fault escarpment. overlying these clastic rocks are light-gray, T hey then follow the fault until they are massive limestones and dark-colored, flaggy captured by a headward-eroding stream dis­ limestones conta1111ng T oucasia sp. and secting the fault block. This trellis pattern miliolid Foraminifera. Mi..illereid ( 1939, p. is especially apparent in the central portion * Aw-climate producing savannas, dry of the map area. winter season; a-highest mean monthly temperature, above 22 o C; i-extreme an­ Climate and Vegetation nual mean temperature difference, less than 5o C; g- tempe1·ature variation, type Gan­ The climate of this portion of El Salvador ges, hottest before the solstice, wet season is described by Koppen ( 1931, p. 122-1 28 ) during the summer. No.4 Geology of the Ah;tachctpcin Arect 199 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.