Geology of the Comayagua Quadrangle, Honduras, Central America

The Montaña de Comayagua structural belt is a zone trending N. 60° W., more than 130 kilometers long, of exposed earliest Tertiary and older rocks that were complexly deformed during the Laramide orogeny. The Honduras Depression, a discontinuous north-trending graben system, extends from the Pacific coast to the Caribbean. The Comayagua graben, a major segment of the Honduras Depression, intersects the older structural belt in the Comayagua Quadrangle. Paleozoic (?) low-rank metamorphic rocks that record two periods of metamorphism underlie two Mesozoic redbed sequences separated by a carbonate group. Cenozoic volcanic and associated sedimentary rocks nonconformably overlie all older rocks. Red elastic rocks of the Todos Santos Formation accumulated as alluvial fans that filled structural depressions. Minor volcanism and faulting accompanied this deposition. The overlying Yojoa Group of carbonate rocks accumulated during a transgression. Red clastic flood plain and deltaic rocks of the Valle de Angeles Group were derived from a rising region outside the map area (probably to the south). The deformation that terminated Valle de Angeles deposition produced the Montaña de Comayagua structural belt, which is a 20 to 30-kilometer wide, N. 60° W.-trending structural high composed of asymmetrical, N. 70° to 90° W.-trending folds, some of which are cut by reverse faults. The folds and reverse faults may be the consequence of left-lateral shearing. Many of the important mineral deposits in Honduras occur along this structural belt. After this deformation, andesite lava flows and mid-Miocene siliceous ignimbrites, tuffs, and associated volcanic rocks were deposited across the deformed older rocks. Normal faulting along west-northwest, northwest, north, northeast, and east-west trends began during volcanism and continued almost to the present. Approximately 2 kilometers of structural relief resulted from this period of normal faulting in the Comayagua Quadrangle. This episode of normal faulting formed the north-trending Comayagua graben and other grabens comprising the Honduras Depression, as well as similar features elsewhere in Honduras and adjacent parts of Guatemala. The complex pattern of normal faults and grabens seems to be the result of left-lateral simple shear deformation of the northwestern part of the Caribbean plate of lithosphere. This simple shear deformation is a consequence of underthrusting at the Middle America Trench and left-lateral strike-slip movement along the Bartlett Trough fault system. Igneous rocks of various compositions intruded the area in several episodes: one was pre-Mesozoic; others were pre-ignimbrite; and the youngest clearly occurred after the mid-Miocene. The Valle de Comayagua was partially filled with a lake following a major episode of graben faulting. Pediments have been cut on the lake beds since through-flowing drainage was established. Earth resources include ground water, limestone, sand and gravel, dimension stone, and possibly small amounts of silver, lead, zinc, and copper.