42757-B8
Paleogeography and Correlation of Pliocene Basins in the Gulf of California
Field and laboratory studies in 2008 aimed to collect and analyze stratigraphical and palentological data from Paderones Blancos on Isla Cerralvo in the southern Gulf of California. While a previously published source is brief and erroneous as to the location, content, and thicknesses of strata, it correctly identifies calcareous algae as the main constituent of a marker bed (1). Bleached rhodolith debris (from coralline red algae) form a 10-m thick layer that, while faulted, can be traced laterally for approximately 0.75 km (see TOC). Based on thin-section analysis, the rhodolith content in this unit reaches 85%, with the balance coming from other fossils and eroded products of igneous rocks. In this regard, the massive rhodolith bed on Isla Cerravlo compares in purity and depositional features with Pliocene rhodolith beds at Arroyo Blanco and Marquer Bay (Isla Carmen) and Punta Bajo near Loreto. Other fossils associated with the Cerralvo rhodolith bed include the echoinoid Clypeaster bowersi and bivalve Argopecten revellei, which overlap stratigraphically to indicate a middle Pliocene age. Above the rhodolith bed is a very thick succession of prograded siltstone mixed with abundant cobbles and small boulders eroded from igneous rocks. The sequence is capped by reef rock, 7 m thick, with candelabra-shaped corals (Pavona gigantea) typically preserved in growth position. The largest colony observed intact is 0.75 m high with sub-parallel branches 3 cm in diameter. The two carbonate units represent distinct high-stands in sea level separated by intervening clastic strata that reflect either a drawdown in sea level or local island uplift. Systematic study of a dozen Pliocene marine basins throughout the Gulf of California tests the extent to which correlations relate to sea-level changes due to global (eustatic) events, as opposed to more regional tectonic events. Former Pliocene shorelines represented by geological unconformities are widely recognized by the distribution of basal conglomerate and limestone beds against Miocene andesite (dated from 19 Ma to 13 Ma) or older Cretaceous granodiorite (dated from 99 Ma to 78 Ma). Abrupt contacts between igneous and sedimentary rocks together with the uneven landscapes on which they rest help gauge the amount of absolute sea-level change recorded from one district to another along the present gulf axis. The height of 12 columns (see nugget) corresponds directly to the time span in millions of years over which different gulf basins accumulated strata. The actual starting point for initial flooding within the San Nicolás and El Mangle basins (columns 6 & 7) is grounded on radiometric dates for rocks at or near the base of those successions. In all other cases, however, the relative placement of the numbered columns is based on concurrent range zones for fossil pectens and echinoids (2). Some stratigraphic sequences reflect a clear history of global sea-level change, as at Punta Chivato and Santa Rosaliíta (columns 4 & 5), while parts of Isla Carmen (column 10) show prolonged effects of local tectonic subsidence. Overall, the spread of fully marine waters to the Coyote Mountains of southern California (column 1) is linked to the initial rapid rise in global sea level during early Pliocene time. Global sea level at about 4.5 Ma may have exceeded the present datum by as much as 90 m (3). In so much as columns 2, 3, 5 - 9, 11 and 12 are limited to a Middle or Upper Pliocene position, those districts must have been elevated above sea level or closed off from rising waters in early Pliocene time. Because marine life reached as far north as the Coyote Mountains during early Pliocene time, it signifies only that southern districts such as El Mangle (column 7) were bypassed at that time or that Lower Pliocene strata near El Mangle are now hidden offshore. In some regions such as Isla Angel de la Guarda, Punta Chivato, and Santa Rosaliíta, there is little or no sign of post-depositional tilting related to tectonic influences. Sedimentary beds remain flat lying in lagoonal settings or retain evenly radial syndepositional dips of about 6° around island cores. Elsewhere, interference by contemporary tectonics is clearly evident, as at San Francisquito and Punta El Bajo (columns 3 & 8). In so much as the narrow time frame for deposition of Middle Pliocene strata on Isla Monserrat (column 11) came and went without any subsequent addition of Upper Pliocene strata, it appears that the entire island experienced sufficient vertical uplift prior to the Late Pliocene to put the area beyond reach of rising global sea level at the end of the epoch. 1. Hertlein, L.G., Proceedings of the California Academy of Sciences (1966) 30(14), 265-284. 2. Eros, J.M., Johnson, M.E., Backus, D.H., Canadian Journal of Earth Sciences (2006) 43, 1149-1165. 3. Haq, B.U., Hardenbol, J., and Vail, P.R., Science (1987) 235, 1156-1167.
