GEOLOGY OF THE MAZATLAN REGION, SOUTHERN SINALOA STATE, MEXICO

The study area is located along the Pacific Coast and belongs to the Guerrero Terrane. The oldest rocks that crop out, the El Venadillo Formation, are a sequence of interbedded shale and sandstone, with beds mostly between 1 and 15 cm of thickness, but with some sandstone beds up to 40 cm thick. They are marine turbidites with scarce limestone lenses of unknown age. The El Venadillo Formation is cut by intrusive bodies of different ages. The oldest intrusives are gabbros that show magmatic differentiation and vary in composition from gabbro to plagiogranite. They were dated as Early Cretaceous by Henry and Fredrikson (1987). At the eastern and central-northern part of the area are banded rocks that Henry and Fredrikson (1987) define as basement formed by orthogneiss. However, this appears to be two young dioritic intrusive units, one with banding of magmatic origin and containing xenoliths of sandstone from El Venadillo Formation, and the other a protomylonite associated with strike-slip faulting. The relationship between the gabbroic rocks and the dioritic intrusives is unknown. In the northern part of the area there is a unit, strongly affected by contact metamorphism, whose original composition probably was a sequence of volcaniclastics and andesites. At the center of the area there are small outcrops of recrystallized calcareous rocks. Both, the volcaniclastic/volcanic and the recrystallized limestone are found as roof pendants within young, Cenozoic granitoids. Therefore, their stratigraphic position and age remain unknown. Resting unconformably on all the previously described rocks there is a volcanic sequence that belongs to the Sierra Madre Occidental. The El Venadillo Formation and some gabbroic dykes show structures associated with at least two compressional events of deformation. The first produced foliation, thrust faulting, and isoclinal folds. The second event originated chevron folds, kink bands, and wide folding. The granitoids and Cenozoic volcanic rocks are affected only by normal faults and north-south trending strike-slip faults.