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Calibrating Deposition of a Tropical Mixed System: Cibao Basin, Dominican Republic


The Gurabo Formation (Late Miocene-Early Pliocene) in the Cibao Basin of the Dominican Republic provides an example of a prograded, mixed siliciclastic-carbonate sequence. This sequence can provide a near-modern outcrop analog for tropical mixed system deposition on the shelf margin. The elevation-constrained reconstruction of the sequence shows a lower parasequence (PS1) with a relative minimum sea level rise of 90 m based on a basal pinning point with subaerial exposure and fluvial gravel, up to the top of the parasequence where facies shallow to a few meters water depth. This lower parasequence is likely correlative to the Zanclean Flood event and ranges in age from ~5.7-5.0 Ma. The upper parasequence (PS2) has a total thickness of ~180 m and represents the onset of relatively high sea level in the Early Pliocene. This upper Gurabo unit is dated to roughly 5.0-4.1 Ma. The two Gurabo parasequences show distinct lithologic and faunal trends upsection. The PS1 has a transitional fauna (from heterozoan affinity) to a more photozoan fauna upsection. Typical skeletal grains include abundant benthic foraminifera, ahermatypic corals, molluscs, and bryozoa. The PS2 fauna is photozoan and includes hermatypic corals, benthic foraminifera, molluscs, and coralline red algae. The coralline red algae become dominant near the top of the succession. Several trends in grain composition and size characteristics also define the two parasequences. The PS1 has a mean matrix grain size of silt-sized grains, whereas the PS2 is dominated by mean matrix size of very fine sand. The <63 μm matrix (mud) component tends to decrease upward in both parasequences from ~40% at the base, to <20% near the top. This decrease is likely related to less siliciclastic material as the parasequences shallow and sediment becomes more carbonate rich. This interpretation is supported by the increase in percent carbonate in the matrix at the top of each parasequence. These trends have implications for the distribution of carbonate skeletal grains and the potential for porosity development in tropical mixed systems.