Adventures in Baja (cont.) |
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After lunch in the town square, the group explored the Mision San Ignacio
de Kadakaman, built by the Dominicans in 1786. We drove past the impressive
volcano Volcan las Tres Virgenes, which may have erupted as recently
as the mid-18th century.
Bahía Concepción—We visited a beach on the western shore of this beautiful bay, where the water was very warm and clear, perfect for snorkeling among a variety of snails, fish, rays, starfish and more. Our beach was a tombolo, a sand spit connecting the mainland to an island formed by a tilted half-graben. Because the erosion of sediments from the mainland was cut off by the faulted block, the beach was a “biological beach” made up of bits  Beautifully preserved branching corals like this were found in the walls of an arroyo near Punta Chivato where the arroyo cuts through a Pliocene reef deposit. (photo by David K. Smith) |
of weathered shell and coral rather than the
more familiar silica sand beach. Students looked at the unique environment
of a nearby mangrove swamp, and experimented with how trails left by
modern gastropods, bivalves and brittle stars might be similar to trace
fossils found in Precambrian rocks.
Vicinity of Punta Chivato—On the way to Punta Chivato, we stopped to hike a short distance up an arroyo. In the arroyo wall, we could see a cross-section of a Pliocene reef in living position, complete with branching corals, and bivalves and gastropods of all sizes. As we walked further up the arroyo, we saw the sediments change to show evidence of an ancient rocky shoreline, and then arroyo deposits that revealed volcanic rocks moving downstream to the shoreline. Afterwards, at Punta Chivato, Professor Ledesma Vázquez pointed out that we were seeing the gently sloped back side of a half-graben, similar to the one we had seen the previous day. Professors Ledesma Vázquez and Lipps led a few students on a hike up the Pliocene carbonate ramps surrounding a 3.5 million-year-old island of tilted Miocene volcanic rocks. The various facies from nearshore-rocky to offshore-carbonate sands were well exposed. These deposits were also examined by the entire group on the rocky headland at Punta Chivato. Near Loreto—Stopping at a locality in the Loreto Basin, Professor Ledesma Vázquez explained how several lines of evidence point to a new interpretation of the formation of the Bahía Concepción region. Farther south, fossil-bearing outcrops showed evidence of an underwater landslide of Pliocene age, with fossilized pectens, gastropods, barnacles, oysters, clams, and trace fossils (possibly shrimp burrows). |
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