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42757-B8
Paleogeography and Correlation of Pliocene Basins in the Gulf of California
Field studies in 2007 were organized to collect stratigraphical and paleontological data at Marquer Bay on the west side of Isla Carmen and on the south end of Isla Angel de La Guarda. Basins on these islands are among more than a dozen large Pliocene basins scattered throughout the Gulf of California and our studies aim to provide the first overview on their organization since the pioneering work of C.A. Anderson (1) and J.W. Durham (2) was published more than 50 years ago. The basin at Marquer Bay (3.42 km2 in area) is symmetrical to the Arroyo Blanco basin on the opposite east side of Isla Carman, previously surveyed by Eros et al. (3). As at Arroyo Blanco, the full thickness of strata on Marquer Bay is unknown because basement rocks are not exposed in arroyos that cut through the thickest part of the succession. Strata in sea cliffs on Marquer Bay vary from 17 m to 20 m in thickness. To the north, the succession is enriched approximately 65% by rhodolith debris. Whole rhodoliths with a fruticose morphology are common, but their size does not exceed 4 cm in diameter. Half the 11.5-m thick sequence to the south is formed by massive limestone composed of calcarenite 90% derived from rhodolith debris. There are no whole rhodoliths within the debris. The rhodolith limestone is capped by reef limestone 3.25 m in thickness, considered by Durham (2) to be Pliocene in age. We found Porites corals extensively developed in the middle of the basin at an elevation 70 m above present sea level. The age of this widespread coral capstone remains to be tested by radiometric dating. Roughly 10 m of rhodolith-rich limestone of undisputed Pliocene age has a wide distribution through the Marquer Bay basin. Based on reduction experiments using modern rhodoliths with a diameter of 5 cm to determine the volume of crushed rhodolith debris in manufactured deposits, we extrapolate that a 10-m thick unit would require 400 billion whole rhodoliths to yield a deposit with a volume of 25,650,000 cubic meters. This result assumes the stratum is evenly distributed throughout the basin with an average purity of 75% rhodolith debris. The numbers are huge, but not nearly as great as the rhodolith contribution needed to make limestone beds in the somewhat larger Arroyo Blanco basin (3.87 km2 in area). Multiple units with abundant rhodolith debris occur throughout the 157-m thick succession in the Arroyo Blanco basin, calculated by Eros et al. (3) to fill 60% of the basin by volume. Based on our laboratory extrapolations, 3 trillion whole rhodoliths would be required to undergo reduction in order to fill the same volume. Overall, rhodolth concentrations in the Marquer Bay and Arroyo Blanco basins compare very well with the paleogeography of other basins in the Gulf of California where carbonate sediments were trapped in east or west facing embayments (see paleogeographic model, TOC).
An exception is illustrated by a south-facing lagoon with an area of 50 hectares that filled with coarse rhodolith debris on Isla Coronados during the Late Pleistocene (4). In this case, however, the lagoon was contained on its seaward margin by a fixed barrier formed by islets of andesite rock that trapped distinct over-wash deposits from the open gulf.
Pliocene lagoons on southern Angel de la Guarda covered 20 km2 flanked by andesite highlands on the landward side. The 45-m thick conglomerate described by Anderson (1) is a proximal deposit derived from these highlands. Flanking Pliocene deposits on the seaward side are four low andesite islets that allowed seawater to flow through narrow passages from the open gulf. A passage that leads to a secluded lagoon is flanked by two larger paleoislands that rise 50 m above the surrounding landscape (see Nugget). Instead of rhodolith debris, oyster and pecten deposits are widely preserved where siltstone beds up to 14 m thick abut against andesite paleoshores. Fossil oysters on Angel de la Guarda are identified as Ostrea californica osunai, the same subspecies found by Hertlein (5) on Peninsula Concepción in the Central Gulf Region. The fundamental difference between Pliocene rhodolilth deposits and oyster deposits is that the former occur as wave-transported materials, whereas the latter grew in place within zones where wave activity was restricted by the physical geography.
1. Anderson, C.A., Geological Society of America Memoir (1950) 43 (1), 1-53.
2. Durham, J.W., Geological Society of America Memoir (1950) 43(2), 1-216.
3. Eros, J.M., Johnson, M.E., Backus, D.H., Canadian Journal of Earth Sciences (2006) 43, 1149-1165.
4. Johnson, M.E., López-Pérez, RA., Ranson, C.R., and Ledesma-Vázquez, J., Ciencias Marinas (2007) 33(2), 105-120.
5. Hertlein, L.G., Proceedings of the California Academy of Sciences (1966) 30(14), 265-284.
