Geologic Setting
Pinacate sits prominently in the sand sea northeast of the head of the Gulf of California (Mar de Cortez), its black oval obvious here on the right and in this first space photograph of the Earth made from a V-2 rocket in 1945. Google Earth shows both access (turn the roads on) and geologic detail. The above NASA image from low earth orbit has been mapped to identify important geographic features (1/12). Baja California and the adjacent Pacific Ocean are drifting toward the left (northwest) relative to North America. The Gulf of California and the desert at its head hide the spreading centers and transform faults of an active plate boundary. Pinacate is solidly on continental crust and not part of plate-tectonic craziness. NASA STS092-703-81.
Pinacate is part of the Southern Basin and Range physiographic province characterized by isolated, sub-parallel mountain ranges separated by broad desert basins. Physiography such as this is the result of crustal extension normal to the axes of the ranges, ductile stretching at depth and brittle fracture at the surface. This orogeny occurred in mid-Tertiary times 10-15 Ma. Detritus was eroded from the mountians, accumulated in adjacent basins and was removed by the local river system in accordance with an energy budget controlled by climate and slope.
The story of Pinacate geology began when the boundary between the Pacific and North America plates jumped inland five to six million years ago. Baja California, attached to the Pacific plate, began drifting northwestward. The Gulf of California opened both as a structural basin, a locus of seafloor spreading, but more important as a huge sink for sediment. The new "base-level" increased the gradient and the energies of the local rivers. These rivers excavated the detritus that had accumulated around the ranges and carried it into the opening Gulf in what I call "the Great Denudation."
The Great Denudation created "superposed drainages" all over the region, inconsequential drainage ways cut deep into the rocks or that run all the way across mountain ranges. They aren't real streams in this arid environment but indications of once-upon-a-time runoff. These features are best explained as remnants of drainage patterns that had become established on detritus that once covered the mountains. When the Gulf opened to provide both a severely lowered base-level and a sink for the sediment, that detritus was removed and the rivers cut down into whatever bedrock they excavated. This is also called antecedent or superimposed drainage.