Estimating paleotidal constituents from Pliocene “tidal gauges”—an example from the paleo-Orinoco Delta, Trinidad

Abstract

The Neogene Orinoco Delta is one of the typical river deltas where both the records of modern and ancient tidal processes can be studied. A ∼5 m.y./>10-km-thick succession on the island of Trinidad contains remnants of paleo-Orinoco deltaic deposition preserving both tide- and wave-influenced delta lobes within the same time intervals, just like the modern Orinoco Delta that has both wave-dominated and tidal-dominated lobes. The tide-influenced delta lobes and estuaries preserve some spectacular tidal bedding signals, including tidal rhythmites. The tidal record encoded within the tidal rhythmites is preserved well enough that the primary tidal constituents responsible for the tidal currents that deposited the rhythmic facies can be inferred. Lower to upper Pliocene tidal rhythmites were examined in two main paleo-Orinoco sub-environments: (1) estuarine and delta lobe deposits of Morne L’Enfer Formation at Erin Bay, and (2) abandoned tidal channels associated with tide-influenced delta-front deposits from the Telemaque Sandstone Member of the Manzanilla Formation at Matura Bay. Both wave- and river-current signals are also present in most of the study areas. Tidal constituent analysis of unusually well-preserved paleo-Orinoco tidal rhythmites reveals a hierarchy of tidal signals that include semidiurnal, diurnal, fortnightly (neap−spring), monthly (perigee−apogee), semi-yearly, and possibly seasonal and yearly cycles that span thickness intervals ranging from millimeters to meters. The tidal constituents were dominated by, in decreasing importance, M2, S2, and likely K1 rather than O1. The modern tidal data clearly show that K1 is more important in terms of tide-generating potential than O1, as was likely so in the Pliocene. In both the rock and modern records, N2 is more significant than O1, P1, and K2 in terms of tide-generating potential. The comparison between the constituent analysis of the ancient tidal record and the modern tidal measurements reveals their similarities in tidal patterns and constituent types. From this, we deduce that the tidal constituents responsible for the Pliocene Orinoco Delta tides were mixed semidiurnal tidal cycles similar to those found today in Trinidad (Atlantic type of synodically dominated) rather than the Caribbean (tropically dominated) type.