The Rio San Juan complex includes the largest exposed area of Cretaceous basement rocks exposed in the North Coast Zone of Hispaniola. The northern area of the complex consists of: (1) the Imbert Formation, a Paleocene to lower Eocene series of interbedded sandstone, conglomerate, white tuff, and sedimentary serpentinite; (2) Hicotea schists, pervasively fractured, mafic greenschists with patchily developed glaucophane and lawsonite; (3) Puerca Gorda schists, dominantly mafic, foliated greenschists, also with patchily developed glaucophane and lawsonite; (4) El Guineal schists, fine-grained, dominantly felsic schists; (5) Jagua Clara melange, retrograde eclogite, and garnet blueschist blocks in a highly metasomatically altered and hydrated ultramafic matrix; (6) Arroyo Sabana melange, fine-grained blueschist, marble, altered volcanic and mica-plagioclase blocks in a serpentinite matrix; and (7) Gaspar Hernandez serpentinites. The southern area of the complex is structurally simpler and consists of: (8) Cuaba amphibolites, mafic to felsic gneisses intruded by (9) the Rio Boba Intrusive Suite, gabbros, diorites, and ultramafic cumulates. We suggest that the Hicotea, Puerca Gorda, and El Guineal schists were metamorphosed in a Late Cretaceous subduction zone where they were protruded at depth Fust by the Jagua Clara melange and later by the Arroyo Sabana melange. Serpentinite protrusion also occurred during the Paleocene-Lower Eocene, and when some of these protrusions breached the surface, they were the source of clasts for conglomerate beds in the Imbert Formation. The southern part of the complex was formed either in the Hispaniola magmatic arc or by intrusion of fore-arc gabbroic magmas (the Rio Boba Suite) into fore-arc basement represented by the Cuaba amphibolites. The southern area was juxtaposed against the northern area before the Paleocene, but the mechanism by which this was achieved is unclear. Exposure of the assembled complex had occurred by the Eocene, but the complex was probably covered by clastic sediments during Late Eocene to Miocene times. Both the Cretaceous subduction complex and the mid-Tertiary clastic cover were disrupted and deformed by Neogene, east-west-trending, sinistral, transcurrent movements. Major fault displacements of tens to hundreds of kilometers separated the Puerto Plata area from the Rio San Juan complex, and both areas from southeastern Cuba. Minor displacements occurred within the Rio San Juan complex on sinistral oblique and strike-slip faults. Serpentinite protrusions were probably also reactivated at this time. Transcurrent faulting continues to the present and has resulted in eastward tilting of the Rio San Juan complex and the establishment of its present drainage systems.

Chaotic, allochthonous, submarine gravity slide deposits (olistostromes) have been reported with increasing frequency during the past 15 yrs in stratigraphic columns throughout the world. One such unit, estimated to be several hundred meters thick, is distributed over 90 sq km in the Puerto Plata area of north-central Dominican Republic. The gray, clay-sized, structureless matrix of this unit behaves as a quick-clay under shock, and is composed of kaolinite, quartz, montmorillonite, and illite. Pelagic Foraminifera date the matrix as Paleocene(?) or early Eocene(?). Exotic blocks within the olistostrome include limestone, serpentinite, andesite, pillow volcanics, and tuffaceous rocks. The longest dimensions of the blocks range from 1 cm to 1.5 km, and the known ages range from pre-Paleocene to middle Eocene. Thus, the exotic blocks are older, the same age as, and younger than their enclosing matrix. The matrix probably was once a marine tuffaceous unit of Paleocene-early Eocene age which was deposited rapidly and retained enough water to liquify spontaneously and to begin moving rapidly down a gentle submarine basin slope following an earthquake shock during the late-middle Eocene. Younger more competent overlying rocks were ruptured and incorporated into the moving mass, while older rocks were torn loose from the submarine slope. The entire unit was emplaced essentially instantaneously into a marine sedimentary sequence. Although olistostromes record catastrophic events in the geologic record, they do not necessarily imply a major orogeny. However, most described olistostromes were formed during times of tectonic activity. The middle Eocene was such a time in the Puerto Plata area.