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The Nature of Cyclic Bedding in Mixed Carbonate-Siliciclastic Clinothems, Neogene of the Dominican Republic


Cyclic mixed carbonate-siliciclastic deposits of the Mao Formation (Cibao Basin, Dominican Republic) prograde 6 km basinward in ∼0.5 m.y. during the early Pliocene. The cyclical deposits were formed as upper clinothem forsets during a prolonged transgression and highstand. The Mao Formation comprises a single depositional sequence bounded by subaerial exposure. The cyclic bedding is defined by a depositional couplet consisting of coral and skeletal debris admixed with muddy sediment (mostly silt with some fine sand) to form the ‘limestone’ component of the couplet. The ‘siliciclastic’ component has a greater percentage of muddy sediment and less coral-skeletal debris. The couplets bundle into at least ten high-frequency sequences (HFS). Each HFS consists of a basal massive (1-3 m thick) silty-sand bed with gravel, that transitions upward to a 10-15 m thick unit of coral-mud couplets. The basal silt-sand beds are interpreted as periods of stillstand or falling sea level. Probability curves generated from bed measurements (∼140 m thick section) provide baseline data on the variability of siliciclastic mud and limestone bed thickness. The basal silt-sand beds appear as significant jumps in thickness on cumulative thickness curves. In the overlying coral-mud couplet, limestone beds have a greater mean thickness (28.7 cm) than the mud-rich beds (20.2 cm). Mixed system progradation in the early Pliocene is synchronous with progradation in the pure carbonate system along nearby western Great Bahama Bank. That this synchronicity is consistent suggests an overriding sea level control on progradation of these mixed system clinothems.