323 Neogene paleontology in the northern Dominican Republic: Field surveys, lithology, environment, and age

This is the first in a series of papers on the Neogene fossiliferous sediments exposed along the south flank of the Cibao Vally west of the city of Santiago, Dominican Republic. Part One provides field data and stratigraphic and environmental conclusions as a framework for subsequent, detailed faunal and floral studies. Later refinement and alteration in the conclusions is to be expected.

Field work carried out in 1978, 1979 and 1980 concentrated on the Río Gurabo and Río Cana sections while other rivers as far east as the Río Yaque del Norte were studied in less detail. The work has provided a number of parallel transects through sediments ranging in age from Miocene to middle Pliocene, giving an insight into the development of the south flank of the Cibao trough. The 887 samples collected in the field have been sorted and distributed for macrofossil and microfossil analysis.

The classical subdivision into formations has been found to be difficult to use in the field, because each unit can be reliably differentiated only in its type area. We have stabilized the fourfold subdivision of the Miocene and Pliocene rocks as far as possible in an appendix.

In both Río Cana and Río Gurabo, the sediment sequences start with shallow- water conglomerates, sands and silts and progress to highly calcareous silts in which beds of coral are frequent but in most instances suggest transport on an unstable slope. The Río Gurabo has a deeper-water, more open- marine facies than is seen in the Río Cana. In the Río Cana is found a thick sequence (340 m) of coral- rich limestone. This facies is not seen in the Río Gurabo, where the time equivalents seems to be silts interbedded with much thinner coral debris beds, conglomerates and pebbly sands. In both sections the highest outcrops are of sands and conglomerates, with conspicuous channeling in those of the Río Gurabo. The general tendency is for deeper-water environments to occur upsection in both rivers, but this has to be seen in the context of increasing distance into the trough. The marked instability of the north flank of the Cordillera Central in the Neogene is evident from the sedimentation patterns in the two river sections.

In the Río Yaque del Norte, fossiliferous silts (Baitoa Formation) overlying a thick sequence of conglomerates of the Tabera Group are believed to represent a time interval not seen in the Río Gurabo and Río Cana. A thick limestone interval occurs higher in the sequence, but this appears to be of latest Miocene age, unlike that seen in the Río Cana.

Year published: 1986