Geological and geotechnical studies were performed for the final engineering design of the hydroelectric project El Cajon in the Santiago River, located 47 km SE of Tepic, Nayarit State. The studies include mapping of 16 km2 (at 1: 1,000 to 1: 100 scales) and underground exploration with 1853 m in 13 bore-holes and geophysical survey. Power house and main facilities are underground and will be located in left river margin.

The Tertiary ignimbrite El Cajon (geotechnical units TicU2 and TicU3) will be cut during underground excavations; such rock unit is intruded by Quaternary diabasic dikes (Qdd) and is cut by 3 main structural systems (faults and joints) and pseudo-stratification. As a result of underground excavations performed in two sector of exploration gallery # 2, it was possible to confirm the continuity of the Crucero-Lagarto Fault (N45°E/50°SE) up to the dikes zone. The latter has a N-S/56-70°E to N40°W/65°NE direction, and due to its position, induces instability conditions, capable to produce block falling in both areas explored. The main instability zones were found on the cave ceiling and on downstream wall.

Diabasic dikes (Qdd) are hard rocks, but when dried, they become soft and release blocks of different size (small to big). They are located in weak areas of ignimbrites TicU2 and TicU3, and intruded along old joint systems, which were reopened. There is groundwater percolation along the dikes that produce a deep and pervasive alteration.

During the exploration process, in accordance with civil engineers staff, it was decided to move both caves (power house and oscillation pit) 50 m down-streams of the original site location; as a consequence, complementary geological explorations were performed. The new power house location was selected to avoid cutting the dike zone and it's only expected to cut 2 small dikes which are parallel to the others but have better mechanical conditions and are only 0,5 m thick. Furthermore, the site where new transformer platform and electric substation and other facilities will be built have a better rock quality.