The Coastal Plain

GEOLOGY AND OIL RESOURCES OF COLOMBIA; TIlE COASTAL PLAIN. ELFRED BECK. INTRODUCTION. The increasingdemand for petroleum and its productshas directedattention to the unexploredregions of South America. Becauseof its proximity to the United States, Colombia has beenscrutinized and a numberof developmentschemes have been 15utinto operationduring the last few years. Geologicalinvesti- gationshave beenmade by nearly all the large oil companiesand interestseems to begrowing steadily. The writer spent six months in Colombia exploring a large. tract of land near the Caribbean coast. The work was carried on from March to August, I919, for the purposeof making a geologicalmap and report on the so-calledSan Andres concession. This paper includesa detailed accountof the geology in the vicinity of the concession,a reconnaissance report of the remain- ing area, and comparisonswith the geologyof Colombiaas a whole. This was a pioneerexploration, as no previouswork had been done. Only a generaloutline map was availableand little or no information has ever beenpublished concerning the geologyand geographyof this territory. Methods of l•ork.•The country is exceedinglydifficult to investigateonaccount of thetropical climate and densely woode•l regions. H.orses and mules afford the only meansof transpor- tation. Althoughprovision should always be madefor "camp- ing out," it is rarely necessary,for small towns are numerous and shelterunder a palm-leafroof can readily be secured. Hard- shipsare many, and specialcare must be taken in the prevention of disease and sickness. 457 458 ELFRED BECK. In swampyregions or in otherimpassable country. distances to certainpoints were calculatedby first determiningthe distancea crew of canoe-mencould push a canoe in an hour, and then recordingthe dis/•ancetraveled by the watch. The Brunton compasswas usedto determinethe direction. The mule-pacing methodwas advantageousfor generalmaps, where only approxi- mate locationswere called for. A plane table and telescopic alidade were used where accuracywas required and detailed results necessary. Climate and People.--Geographically,Colomb!a is a tropical country,but it has great climatic diversity. The climate is tropical on the coast and low-lying river basins,but owing to the great relief and the curiousconfiguration of the mountainchains; .theclimate becomes a matterof locality. 'At SantaMarta one can go from a hot climateto a cold one in a few hours. It is possibleto reachan elevationof •8,ooo feet on the Sierra Nevada Mountains. Regardlessof the intenseheat of the day, the even- ings are always cool. In general there are two rainy seasons; the so-calledwinters from April to June and Septemberto De- cember;sei•arated by'two periods of dry weather. The average day temperatureof the Magdalenaand Sinu countryis about95 degreesand at night it.may fall to 70 degrees. The total area of Colombia is 46•,ooo squaremiles, and ac- cordingto the censusof •9•2 the populationwas 5,500,000. The peopleare clusteredtogether in small towns; rural districtsare relativelyfew. The coastalinhabitants are a mixed race,prob- ably of Spanish,Indian, and Negro blood. They are a law- abidingpeople with a greatrespect for foreigners. GENERAL FEATURES. Location.--The area discussedin this report lies on the Carib- beancoast of Colombia,South America, and extend• in a strip about 40 to 5ø miles wide from the mouth of the Magdalena River southwestto PuertasEscondido. (See Fig. 30.) Culture.•Roads in this part of Colombiaare narrow and re- semblecow-paths. The only improvementsmade on themcon- GEOLOGY AND OIL RESOURCES OF COLOMBIA. 459 sist in cutting the brush twice a year. If this were not done, the road w.ouldbe closedin lessthan a year'stime. Occasionally a bridge is constructed. The main roads follow either divides or valleys and connectinglines are few. It is interestingto note that town sitesare invariably located' on the outcrop of a sandstoneformation or on the bank of a ranges, drain- streamwhere there is a sandysoil. The larger towns,such as Cartagena,Barranquilla, Sincelejo, Monteria, Tolu, CarmenSan O'nofre,Palmito, Lorica, and SanAndres, have telegraph serv- ice,post offices, and some have electric-lighting plants. The first two mentionedcities are up to date in nearly all modernattain- mentsand havea populationof about50,00o. Topo#raphy.--Theregion may be dividedinto lowlands, along the coastand river valleys, where dismal swamps, lakes and lagoonsare numerous;and into an uplandcountry, which is ruggedand rocky. The altituderanges from sealevel to nearly 460 ELFRED BECK. I,OOO feet at several places on a divide, which trends north- easterly close to the center of the area. The ridges are com- paratively uniform in height and are of prime importancein locatingroads. The characterof the underlyingformations has influencedthe shapeof the hills, the more resistantrocks forming conspicuousoutcrops or "Cerros" (from the Spanish,meaning hills), but the dip of bedsis the determiningfeature. The out- cropping ledges of sandstoneand limestoneincline at sharp angles,forming a seriesof escarpmentswhich trend in a north- easterly direction. Some of the most prominent of these topo- graphic features are: Cerro Tofeme, south of Chima; Cerro Bombo,near Huertas; Cerro Chiquito,north of Sabaneta;Cerro Pato, south of Sabaneta;Cerro Sapo, south of T.oluviejo. In this ruggedterritory erosionis active and deepgulches and valleysare characteristic. The regioneast of the Sincelejoanti- cline or east of the outcrop of the San Antonio sandstonesis characterizedby soft sandstonesand shalesdipping at low angles to the east, which producea gentle rolling plain. The Palmito limestoneforms a seriesof ridges or "Cerros" for a long dis- tanceacross the area. This formation is inclinedat an angle of about 9ø degrees,and on accountof its resistanceto erosionthe adjacent shalebeds have beencut away, leaving the limestonein the form of an outlier. Caves formed by percolating surface waters are common in this limestone. The San Antoni.o sand- stonescœp the major part of the Sincelejoanticline and syncline and form the divide betweenthe Magdalenaand coastaldrainage systems. The tributariesof the Sinu and 'Magdalenarivers drain most of the area. The streamsare intermittent and flow only in the wet season. The water ordinarily standsin pools along a dry bed or in artificial reservoirs,made by throwing dirt damsacross creeks. Sometimesafter heavy rains suchstreams as the arroyo Mucha, arroyo Pechelin,and arroyo San Antonio.become dan- gerousraging t.orrents which are practicallyimpassable. The Sinu and Magdalenarivers are navigablefor severalhun- dredmiles. A gasolinelaunch runs from Cartagena up the Sinu GEOLOGY AND OIL RESOURCES OF COLOMBIA. 461 as far as Monteria, making the trip twice a week. Accommo- dation for passengersis providedin additionto freight facilities. Transportationon the Magdalena is done entirely by large flat- bottom boats. Semi-weeklyservice is maintained betweenall pointsas far as Honda on the Upper Magdalena. Under•7roundI/Vater Resources.--The topography of the country, the amount of vegetation,.and the porosity of the surface are factors that contr. ol the amount of rainfall that enters the surface and gives rise to artesian and spring water. In this area the annual rainfall averages90 inchesand is unequallydis- tributed throughoutthe year. The region is characterizedby sandysoil and. dense forest growth, consequently a large portion of the rainfall entersthe ground. The ground water table is constantlyfluctuating; i.t rises in the wet seasonand sinks during the dry season. Several at- tempts have been made to secure fresh-water w,ells in the cattle regions near the Caribbean coast. Most of-these have been failures. In all the casesexamined by the writer it was found that the wells were dug in clays and shales,therefore yielding little or no water. ß Fro. 3L Sketch showing the occurrenceof water on coast'alplain, near Madre de Dios. If the wellswere deepened,the chancesof strikinga porous or perviouslimestone, a sandstone,or a gravel bed would be good. Fig. 3t illustrateswhy wells near 'Madrede Dios were failures,and shows a possiblemethod for completinga good well. At Sincelejothere occurs an ideal arrangementof the strata for obtainingan artesianwell. Fl.owingwater shouldbe ob- tainedat a depthof •5o-2oo feet, but deepersands would de- velopa strongerflowing well, sincethe hydraulichead increases with depth. A detailedillustration is shownin Fig. 32. 4.62 ELFRED BECK. An unusualoccurrence of a fresh-waterspring is found near Palmito, where the water penetratesa porous limestone (per- colatingdown through bedding planes and crevices)and a sandy shaleand producesthe springat the baseof the hill. FIG. 32. Sketch of the Sincelojo syncline,.illustrating an ideal condition for an artesian well. ß Goodwater wells may readily be obtainedby drilling porous strata. If drilled beneath the lowest fluctuatio• of the water tablethe. supply will bepermanent, thereby relieving the scarcity whichoccurs every year, and at the sametime improve the health conditions. STRATIGRAPHY. A rough sketchof the mountainranges and drainagesystems of Colombiais shownin F•g. 3¸. It will be notedt. hat the moun- tain chainsrun in a northeasterlydirection, producing lines of folding which range from north •5 degreeseast to north 5ø degreeseast. The mountainmasses whose development have probablybeen the mostactive in determiningthe natureof the minor structural featuresin the coastalterritory are the WesternCordillera

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