OPALIZED WOOD FROM THE UPPER PLIOCENE OF BAJA CALIFORNIA SUR: A COASTAL-PLAIN DEPOSIT ON THE GULF OF CALIFORNIA

The precursor of a coastal plain ecosystem dominated by salt-tolerant plants in the genus Atriplex is preserved in a unique deposit stratigaphically bracketed by hardpan clays and an overlying marine carbonate ramp at El Mangle near Loreto on the Gulf of California in Baja California Sur, Mexico. The age of the 2-m-thick deposit is constrained by the index fossil Clypeaster marquerensis, an upper Pliocene echinoid from the capping ramp structure. Recovery of a possible nannofossil (Discoaster brouweri?) from this siliceous deposit also suggests a pre-Pleistocene age. Opalized wood accounts for 20% and silicified clay-size material accounts for 30% by sample weight. The balance (50%) includes friable sand-size detritus, and coarse opal bits. Overall composition is 75% silica; fossil wood is 98% silica. Still active locally, geothermal springs are the likely source for mineralization. In outcrop, fossil wood is visible mostly as twig-size fragments but rare branch-size material also occurs. Viewed by SEM, individual twigs show an inner pith core, xylem vesicles with possible sieve plates, and a sheath of fine phloem structures in exquisite detail that allows comparison with living halophytes. The deposit also features a mix of rare fibers with affinities to cactus wood, possibly washed down slope by runoff, as well as small marine gastropods and salt-water diatoms (Campylodiscus clypeus) washed ashore by tides and waves. Given the size of the deposit, which is traceable up to 8 km across Ensenada El Mangle, a more thorough sampling of the Pliocene ecosystem and comparison with its modern counterpart will be carried out in the future.