Geology of the Fondo Negro region, Dominican Republic

Detailed geologic mapping of the Fondo Negro-Sierra de Martin Garcia region of the Southwestern Dominican Republic establishes that this area is part of a broad and diffuse Northern Caribbean plate boundary zone (PBZ). Faulting in the study area is predominantly left lateral strike-slip, with secondary compression (thrusting) causing both substantial, and rapid, uplift of the region. Folding associated with the wrench faulting affects rocks of all ages in the field area from Paleocene limestones to Quaternary alluvial fans. New stratigraphic designations are proposed for mappable litho-units in the area, and these are correlated with Hispaniola stratigraphy in general based on biostratigraphic age determinations. Thus a new formation name, the Fondo Negro Formation is proposed for a thick (2650 m. exposed) predominately calcareous siltstone, marl and sandstone sequence which is exposed between the village of Fondo Negro and the Sierra de Martin Garcia where it lies in tectonic contact with Paleocene to early Miocene carbonates. The Fondo Negro Formation contains microfauna belonging to the Tortonian and Messinian intervals. The lowest exposed section of the Fondo Negro Formation contains distinctive, ridge forming, sandy limestones and this unit is described as the Gajo Largo member. The 400m. of predominantly grey calcareous shales apparently conformably overlying the Fondo Negro Formation are redefined as the Bao Formation, also of Messinian age, and conformably above that the Pliocene, Arroyo Blanco Formation, also more specifically defined than previous studies. The Arroyo Blanco Formation comprises sandy marls, and resedimented carbonate shelf debris, as well as coarse sandstones and conglomerates. In the east it is approximately 100m thick, and it varies as a facies transition into evaporites in the northwest of the field area. The evaporite faeies, including massive pure gypsum beds are probably thicker than the facies in the east. The carbonates of the Sierra de Martin Garcia were not mapped in detail, but paleontological determinations for samples collected indicate a broader age range for the carbonates than suggested on existing maps (Blesch, 1967), from Paleocene to Early Miocene. In addition to the primary field mapping, reconnaissance studies and paleontological analysis of samples from the Tavera Basin suggest that it is the earliest basin yet identified related to the initiation of the northern Caribbean PBZ. That is that this basin can be related to strike-slip faulting in probably early Oligocene times and this represents movement along the Cayman Trough. An interpretive cross-section across Hispaniola is presented which suggests that present strike-slip activity is greatest in the Cibao Valley, the Enriquillo Valley and in the northern, offshore areas of Hispaniola, and that the Island is cut by a great number of splay faults. The mountain ranges, the Sierra de Neiba and the Sierra de Bahoruco are compressional structures related to bends in the strike-slip fault system. An attempt is made to integrate this in the perspective of the Cainozoic evolution of the Greater Antilles