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Abstract

The world-renowned emerald mines of Muzo, in the Department of Boyacá, are situated within a rather uniform series of black shales, which frequently present local dislocations and irregularities. These disturbances sometimes make it difficult to determine the dip or the true position (normal or inverted) of the beds.

With the aid of rather poorly preserved but nevertheless sufficiently diagnostic fossils, and by comparison with neighboring regions, it was possible to recognize the following stratigraphical sequence:

• Upper Albian ......................... 1,280 m

• Higher Middle Albian .................. 1,240 m

• Lower Middle Albian ................... 600 m

• Lower Albian ......................... 230 m

• Upper Aptian ........................ 1,300 m

• Barremian (and Lower Aptian?) ......... 300 m

• Hauterivian more than ................. 150 m

The sequence from the Hauterivian up to the Lower Albian is developed east of the Minero River, forming there the narrow Muzo anticline, and then rises again toward the river.

West of the Minero River, in contrast, only Middle and Upper Albian beds are found, forming the wide Itoco anticline, in the core of which the emerald mines are located. The contact between the stratigraphical units east and west of the Minero River represents a NNE–SSW-trending (reverse?) fault with approximately 5,000 m of vertical displacement.

It is likely that the region is also crossed by NW–SE-trending minor faults; one of them has been observed in the immediate vicinity of the emerald mines.

At the present stage of our knowledge, it is difficult to evaluate to what extent these stratigraphical and structural conditions are related to the origin of the Muzo emeralds, which occur in Lower Middle Albian shales. However, particular attention should be drawn to the enormous subsidence that this portion of the basin experienced during Lower Middle Albian times and in subsequent stages.