Geology of the San Pedro Zacapa quadrangle, Honduras, Central America
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The San Pedro Zacapa quadrangle lies along the valley of the Río Ulúa, just southwest of Lago de Yojoa in west-central Honduras. Most of the area lies within the sierras of northern Central America; volcanic ranges and plateaus encroach upon the quadrangle from the south. No pre-Cretaceous rocks are exposed, but a basement of Paleozoic or older low grade metasediments and metavolcanic rocks is known to underlie this region. Very probably a thick section of Jurassic continental clastic strata is also present below the Cretaceous units. The oldest exposed strata belong to the Albian Atima Formation, carbonate shelf deposits of thick-bedded micrite. Late Cretaceous-early Tertiary redbeds of the Valle de Ángeles Group conformably overlie the Atima limestone. The redbeds have been informally divided into three members: a lower member of coarse continental clastic strata, a middle member of Cenomanian shallow marine limestone, and an upper member of fine-grained redbeds with associated gypsum, limestone conglomerate, and coarse metamorphic-rock conglomerate. The Cenomanian limestone member probably is not correlative with the Esquías limestone (which is thought to be Eocene, and does not crop out in the map area). Previous reports included the Cenomanian limestone within the Atima Formation; however, the extensive redbed section separating the limestones makes it apparent that the upper limestone is a part of the Valle de Ángeles Group. After deposition of the Valle de Ángeles limestone member, the region was subjected to late Cretaceous-early Tertiary deformation which produced normal faults and east-trending, broad, open folds in the Atima Formation and lower members of the Valle de Ángeles Group. Limestone conglomerate was shed into the central part of the quadrangle from upfaulted limestone highlands to the northeast. A quartz microdiorite was emplaced at Cerro Lavanderos. To the southeast more severe deformation resulted in the Montaña de Comayagua structural belt, which trends N. 60° W. across central Honduras. Fine-grained andesitic flow rocks of the Matagalpa Formation were erupted subaerially across eroded Mesozoic strata beginning in Oligocene time. These eruptions were probably related to subduction along the Middle America Trench. Following a quiescent period, voluminous mid-Miocene and Pliocene ignimbrites with associated tuffaceous strata and flows were deposited nonconformably over Matagalpa rocks throughout much of northern Central America. The Zacapa area lies to the north of the main accumulations of Padre Miguel ignimbrites; fluviatile tuffaceous strata comprise the bulk of the Padre Miguel Group in the map area. A 400 meter thick pile of basaltic flows was built up around a local eruptive center along the Ulúa fault system. Late Cenozoic normal faulting, primarily along northwest and northeast trends, has occurred almost continuously since Matagalpa time. The Santa Bárbara graben, a major depression with more than 1500 meters of structural relief, extends from the south-central part of the Zacapa quadrangle for thirty-five kilometers to the northwest. North of the map area the graben is bounded by a large horst block which appears to have been uplifted by a deep-seated intrusion. Important silver and base metal mineralization at El Mochito is associated with this intrusion. During Plio-Pleistocene time the Santa Bárbara graben was partially infilled by up to 200 meters of valley fill conglomerate, sandstone, and siltstone derived primarily from Tertiary volcanic units, but including redbed detritus as well. Present-day streams are removing this fill and down-cutting into older units. Minor Quaternary basalt eruptions occurred at the north end of Lago de Yojoa, and within the map area a small Quaternary (?) eruption issued from a fault along the Horconcitos horst. Small pockets of stibnite and impure gypsum are the only deposits of economic interest in the Zacapa quadrangle.