Geology of the Cretaceous-Tertiary(?) Rocks of the Southwest Quarter of the Monte Guilarte Quadrangle, West-Central Puerto Rico

Abstract

The southwest quarter of the Monte-Guilaerte quadrangle in west central Puerto Rico covers an area of about SO square kilometers. It is located about 8 kilometers south of the southern Puero Rico fault zone. This shear zone is a northwest-southeast trending lineament which divides the central block of the island from the southern block. The oldest rock unit in the area is the Rio Loco Formation which consists mainly of pillowed basaltic lava flows, flow breccias, hyaloclastite deposits, volcaniclastic breccias and conglomerates, and minor calcareous sedimentary rocks and limestone lenses. The rocks are Upper Cretaceous and were deposited subaqueously. The Maricao Basalt, which is in fault contact with the Rio Loco Formation, comprises about 1,300 meters of massive augite-rich volcaniclastic breccias and conglomerates, sandstones, and subordinate calcareous sedimentary rocks, basaltic lava flows, and limestone. The unit is Campanian to Maestrichtian in age. The principal sources of the volcaniclastic material were pre-existing rocks which were exposed subaerialy and were being eroded. The sediments were rapidly buried preventing further reworking. The Yauco Mudstone, which unconformably overlies the Maricao Basalt, consits of about 360 meters of thin- to medium-bedded calcareous volcaniclastic sandstones and siltstones, claystones, limestones, and minor calcareous conglomerates and breccias, and. tuffs. The unit is Campanian to Maestrichtian, but maybe possibly in part as old as Turonian. The principal source of the Yauco volcaniclastic material was pre-existing volcanic and hypabyssal rocks. The sediments were deposited in a shallow-water marine environment which was tectonically unstable; this resulted in widespread accumulations in southwestern Puerto Rico. The Yauco Mudstone is conformably overlain by an unnamed hornblende breccia unit in one part of the area and by the Sabana Grande Formation in another area. The unnamed hornblende breccia unit consists mainly of hornblende-rich volcaniclastic breccias and conglomerates with minor interbeds of calcareous volcaniclastic sedimentary rocks. The unit is Campanian, probably Turonian, to Maestrichtian in age. The source of the sediments was pre-existing rocks which were exposed to the east and/or north. The overall tectonic environment of this unit is similar to that of the Yauco Mudstone. The Sabana Grande Formation consists mainly of amygdaloidal, massive, basaltic, lava flows with minor flow breccias, and pyroxene-rich sandstones. It has been assigned a Turonian to Campanian age but based on paleontological data obtained during this investigation, the unit may be as young as Maestrichtian. The unit was extruded subaerially and since it is con cordant within the Yauco Mudstone it indicates that the basin of Yauco deposition in this area was extremely shallow with periods of exposure above sea level. The entire volcanic-sedimentary sequence was intruded in late Cretaceous to early Tertiary time by augite porphyry and hornblende porphyry bodies. During or after Maestrichtian time the area was deformed into large open folds. Later, faulting became the predominant type of deformation. Vertical faults are apparently more common than other types. The small amount of lava flow, except in the Rio Loco Formation and Sabana Grande Formation, and little pyroclastic material in the sedimentary units strongly indicates that little explosive volcanic activity was going on during deposition of the units. The abundance of volcanic, hypabyssal, and igneous rock fragments in the sediments indicates that the main sources of the sediments were pre-existing rocks. The presence of unpillowed lava flows within the sedimentary units suggests that the sediments were deposited in a shallow water area with periods of exposures above sea level. Paleogeographic evidence indicates that the overall source of the sediments was probably to the north and/or east. The presence of felsic rock fragments in the upper part of the sequence is evidence that felsic source rocks were exposed. Apparently the island arc was mature by Late Cretaceous time with andesite and dacite the dominant rock types.