Abstract

The arid Ojo de Liebre area, Baja California, Mexico, has three coastal lagoons. The hypersaline Laguna Ojo de Liebre has brine pans separated from the inner lagoon by a low barrier breached by small channels and extensive salt flats.

The lagoon barrier, approximately 2 miles wide, has numerous high barchan dunes. The lagoon has relatively deep, steep-walled channels and wide intertidal flats; the main channels are directly behind the lagoon barrier. Spartina and Salicornia marsh grow on the inner edges of the intertidal flats and on land bridges between lagoons.

Maximum tidal currents are at least 2.5 knots in the inlets and lower channels and are very turbulent. Tidal delay between the inlet and the inner lagoon is approximately 3 hours. Tidal ranges are 4–9 feet.

Most sediment is fine to very fine well-sorted and slightly skewed sand. The immediate source of the sand is the lagoon barrier, open-ocean beach, and near-shore zone. Less well-sorted sand and pebbles in some channels are attributed to former river deposition. Silt and clay in the inner lagoon and marshes are in part derived from older sediment. The average sand is approximately 95 per cent quartz, 4 per cent dark minerals, and 1 per cent or less Foraminifera and shell. Most samples contain apatite pellets presumed to be authigenic in the near-shore open ocean.

The following foraminiferal assemblages have been differentiated: near-shore open-ocean, dune, lower-lagoon, inner-lagoon, and marsh. Three molluscan assemblages are recognized. Organic production in the lagoon is very high; average fixation is 47.2 mgC/m3/day. There are large living populations of Foraminifera and Mollusca.

Processes of sedimentation include wind delivery of sand from the barrier and distribution by turbulent tidal currents. Most deposition occurs on intertidal flats and in the inner lagoon, due to loss of water turbulence. The channels next to the barrier tend to maintain themselves. Tidal delay is related to the mean depth of the lagoon, roughness of the bottom, and width of the channel.

The Ojo de Liebre barrier is believed to have begun as a fringing beach on a gently sloping coastal plain when sea level was about 40 feet lower than at present. The inlets developed as a result of hydraulic head due to tidal delay. A river supplied abundant sand, and the barrier grew upward as sea level rose. The barrier has prograded seaward about 1 mile in the last 1800 ± 200 years. Land bridges developed and separated the lagoon into three lagoons owing to meeting of tidal currents which were introduced simultaneously through adjacent inlets and which result in reduced current and rapid sediment deposition. This process is observed in Laguna Santo Domingo.

Evidence is given for an earlier lagoon cycle, older than 30,000 years B. P.

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