Abstract

A program of geologic mapping and mineral investigation in Colombia was undertaken cooperatively by the Colombian Instituto Nacional de Investigaciones Geológico-Mineras (formely known as the Inventario Minero Nacional), and the U.S. Geological Survey1 sponsored by the Government of Colombia and the Agency for International Development, U.S. Department of State. The purpose was to study and evaluate mineral resources (excluding of petroleum, coal, emeralds, and alluvial gold) of four selected areas, designated Zonesl to IV, that total abo ut 70.00C km². The work in Zone III, in the Cordillera Oriental, was done from 1965 to 1968.

The northeast-trending of Cordillera Oriental of Colombia swings abruptly to north-northwest in the area of this report and divides around the southern end of the Maracaibo Basin. This section of the Cordillera Oriental is referred to as the Santander Massif.

Radiometric age determinations indica te that the oldest rocks of the Santander Massif are Precambrian age and include high-grade gneiss, schist, and migmatite of the Bucaramanga Formation. These rocks were probably part of the Precambrian Guayana Shield. Low to medium grade metamorphic rocks of Late Precambrian to Ordovician age include phyllite, schist, metasilstone, metasandstone, and marble of the Silgará Formation, a geosynclinal series of considerable extent in the Cordillera Oriental and possibly the Cordillera de Mérida of Venezuela. Orthogneiss ranging from granite to tonalite is widely distributed in the high and medium grade metamorphic rocks oí the central core oí the massif and probably represents rocks of two ages, Precambrian and Ordovician to Early Devonian. Younger orthogneiss and the Silgará are overlain by Middle Devonian beds of the Floresta Formation which show a generally low but varying degree of meta morphism. Phyllites and argilhtes are common, and infrequent marble and other calcareous beds are fossiliferous. Except for recrystallization in limestones oí the Permian Carboniferous Diamante Formation, sedimentary rocks younger than Devonian are unmetamorphosed.

The effects of Precambrian regional dynamothermal metamorphism and plutonism on Precambrian geosynclinal deposits reached the upper amphibolite facies in the Bucaramanga Gneiss. Geosynclinal deposits oí the Silgará Formation were subjected similar conditions in Late Ordovician and Early Silurian time but reached only the green schist or lower amphibolite facies. Orthogneisses generally show a concordance of foliation and lineation with those of neighboring bodies of the Silgará Formation and the Bucaramanga Gneiss rocks, as well as similari ties in grade of metamorphism. Regional dynamothermal metamorphism in Late Permian and Triassic time reached low grade in the Floresta Formation and caused recrystallization of limestone of the Diamante Formation. The Bucaramanga and Silgará metamorphic rocks show eviáence of retrogressive metamorphism with high activity of potassium and water,  but whether this occurred at the time the Floresta was metamorphosed or la ter is not clear.

Batholiths, plutons, and stocks of igneous rocks in the Santander Massif range from diorite to granite. Radioactive age data indicate that most of them belong to a single plutonic interval. These are referred to as the Santander Plutonic Group, and are Jurassic and Jurassic-Triassic. Two suites of this group are pink granite and quartz monzonite, andgray quartz monzonite and granodiorite. Contact relations indicate that the pink and more granitic rocks are younger than the gray and more mafic rocks, but radioactive age data are in conflict with this. Undated plutonic rocks that are not clearly related to the group are assigned to relatively older or younger age positions.

Rhyolite occurs west of the Bucaramanga fault as a small body in one Jocality and as an intrusive sheet with granophyre and intrusive breccias in Triassic sedimentary rocks in another locality. The age is unknown but probably is younger than the Santander Plutonic Group.

Felsic, mafic, and lamprophyric, dikes are common in the batholiths, plutons, and adjacent rocks and most appear to be genetically related to the larger igneous bodies, whereas rarer dikes of dacite porphyry, basalt and diabase are not related. Basalt and diabase dikes are widely scattered aild have been found nearly as high in the section as the Jurassi Cretaceous boundary. Dacite porphyry is the only igneous rock that intrudes rocks of Gretaceous age.

With the uplift that accompanied emplacement of batholiths in Latest Triassic and Jurassic time, erosion of the roof rocks furnished fine-grained redbeds and conglomerates of the Jordán Formation followed by erosion of the batholiths themselves that provided the coarse-grained and conglomeratic arkosic sediments of the Girón Formation in thick accumulations off the flanks of the uplift. This period was followed by marine invasion and sedimentation of the Cretaceous period. In the Magdalena Valley area, Lower Cretaceous sedimentation began with quartz sands of the Tambor Formation and continued with fossiliferous limestone of the Rosa Blanca Formation, black shale of the Paja Formation, fossiliferous limestone, glauconitic sandstone and black shale of the Tablazo Formation, and still more black shale of the Simití Formation. In Late Cretaceous time, calcareous black shale with chert and phosphatic beds in the upper part of the La Luna Formation were deposited during the time of most widespread marine transgression. Thereafter gray shales with limonitic beds of the Umir Formation accumulated as marine conditions wore gradually succeeded by continental deposition with coal beds in latest Cretaceous.

Cretaceous deposition over the area was mostly uniform in character if not in thickness, and remnants of these rocks that have escaped erosión in the massif are similar to the Cretaceous rocks of the Magdalena Valley to the west and the Maracaibo Basin to the east.

Continental conditions prevalied in the Magdalena Valley area through the Tertiary with sandstone and shale containing coal beds in the Paleocene Lisama Formation, followed in the Eocene by thick conglomeratic sandstone of the La Paz Formation and sandstone silstone, and shale of the Esmeraldas Formation, in the Oligocene by shale of the Mugrosa Formation, and shale with coarse conglomeratic sandstone of the Colorado Formation, in the Miocene by still coarser and

thicker sediments of the Real Group, and continuing into the Pliocene and Pleistocene in the Mesa Group. The section of Tertiary rocks in the Colombian part of the Maracaibo Basin is mostly similar in origin and lithologic character but thinner than that in the Magdalena Valley. These rocks were eroded from, or were never deposited in the area that is now the highest part of the massif.

Alpine glaciation occurred on the Santander Massif during the Pleistocene, and widespread terraces in the lower valleys may date from this period. Orogeny is probably at or near its highest leve! at the present time with streams eroding the flanks of the massif at a high rate, aided by deep weathering and landslides.

The Bucaramanga fault, a major fault of regional extent., trends north·northwestward across the area and apparently extends on to the north coast as the Santa Marta fault defining the western boundary of the Santa Marta Mountains. The present investigations indicate a long and complex history for the Bucaramanga fault with earlier lateral displacement, followed by later uplift of the Santander Massif to the east that continues to the present time.

West of the Bucaramanga fault are three areas of rather distinct structural character:

1. A wedge-shaped, dwon-faulted block between the Bucaramanga and Suárez faults is mostly an area of mesas, tilted slightly westward, capped by basal Cretaceous sandstone. At the thin north end of thewedge, Quatemary gravels and mudflows accumulated in the fault-formed basinand now form the dissected terrace on which Bucaramanga, the main city of the region, is located.
2. A plateau belt bordering the mesas west of the Suárez fauli consists mostly of dissected beds, undulating to steeply dipping, of the thick Girón Formation.
3. West of the plateau area ali sedimentary rocks from Jurassic to Tertiary plunge westward into the deep trough of the Nuevo Mundo syncline. This narrow syncline is on the deeper easter side of the geosynclinal area of the Magdalena Valley basin. lt is mostly separated from the shallower part of the geosyncline to the west by the north trending La Salina fault, which places Upper Cretaceous rocks on the east side in contact with Oligocene and Miocene rocks on the west.

In the high country that continues south and east of the metamorphic and igneous rocks of the Santander Massif, two north trending structural basins are separated by the regional Servitá fault. The western basin contains sedimentary rocks ranging from Devonian to Upper Cretaceous and is complexly faulted. Rocks of the eastern basin range from Lower Cretaceous to Eocene and have undergone compressional folding that is more intense toward the west.

Many faults were mapped to the east and west of the Bucaramanga fault, and many more are indicated by lineaments on aerial photographs. Most have trends withina range of north-northeast to north-northwest, mostly parallel to the trend of structure. Only a few major faults cut across this trend. On the east and west flanks of the Santander Massif, belts of sedimentary rocks that include mostly Cretaceous formations have escaped erosion on the downthrown sides of long faults. On the east flank the down· thrown sides are on the west, and on the west flank the downthrown sides are on the east, which suggests either more active uplift of the flank areas or collapse of the central area relate to the flanks.