Neogene Basin Evolution in the Southwestern Dominican Republic: A Foraminiferal Study.

Abstract

The Azua Basin sequence is a 2500 m thick Miocene Pliocene regressive interval. Limestones and marls of the Sombrerito Formation, the lowermost unit, ranges from lower Miocene to lowermost upper Miocene. The turbidite sequence of the Trinchera Formation spans an interval from uppermost middle Miocene to lower Pliocene. Blue-green, neritic lower Pliocene siltstones of the Quita Coraza Formation overlie the Trinchera in the seaward part of the basin. Shallow-marine strata in the lower part of the Arroyo Blanco Formation range from uppermost Miocene to middle Pliocene. Principal Components Analysis of rich benthic foraminiferal faunas aided in the recognition of species associations. Positive loadings on Factor 1 define a bathyal-neritic boundary zone assemblage: Bolivina minima (0.88), Cassidulina norcrossi var. australis (0.73), and Gyroidina regularis (0.71). Lower bathyal species Pleurostomella alternans ($-$0.64), Bolivina paula ($-$0.62), and Cibicides wuellerstorfi ($-$0.49) show strong negative loadings. Positive loadings on Factor 2 define a middle to upper bathyal assemblage: Oridorsalis umbonatus (0.68), Hoeglundina elegans (0.57), and Uvigerina peregrina (0.51). Factor 3 represents a middle to outer neritic assemblage that includes Angulogerina jamaicensis (0.83), Cassidulina carinata (0.54), and Cibicidoides pachyderma forma sublittoralis (0.51). Factor 4 has heavy loadings on inner neritic species including Haynesina depressula (0.74), and Buliminella elegantissima (0.73). Neritic species associated with Factor 5 include Cancris sagra (0.69) and Bolivina lowmani (0.59). Plots of the factor scores in the stratigraphic sections demonstrate shoaling upward from lower bathyal to inner neritic depths. Cluster analysis reveals seven major sample groupings that correspond almost directly to the faunal zones identified in the Azua Basin by Bermudez (1949). Migration of foraminiferal environments toward the Caribbean during the Miocene and Pliocene is paralleled by time-transgression of lithofacies. The southwestern Dominican Republic evolved in two stages: (1) a tectonically quite open ocean stage before the late Miocene; (2) a subsequent tectonically-active, laterally-constrained clastic basin stage. These events are interpreted to represent rapid filling of a thrust-block bounded "ramp valley" basin by sediments derived from the uplift of the nearby Cordillera Central, coincident with activation of the Hispaniola restraining bend in the late Miocene.