2013 Rendezvous Trip Report - East Mojave Geology Tour

EAST MOJAVE GEOLOGY TOUR - March 2, 2013    written by trip Leader Bill Neill

From Baker, 6 vehicles drove about 170 miles through and bordering Mojave National Preserve, and viewed five rock types that represent the geological history of the East Mojave.  Besides my wife Gwenn, participants were:  Bob Jacoby; Steve & Debbie Marschke; Bruce Barnett and family; Terry Ogden; Gary Preston; Bob Peltzman & Anne Landon.  While heading south on Kelbaker Road, our first stop was at the toe of a basalt sheet that flowed as molten lava down from the cinder cones.  The basalt is the youngest volcanic rock in the East Mojave, around 10,000 years old, and it rests on the oldest formation in the desert, metamorphic granitic rock called gneiss that’s about 1.4 billion years old.The older rock forms the ancient “Precambrian basement” of North America, extending from the San Gabriel Mountains eastward and northeastward across the continent to the Appalachian Mountains and Canadian Shield.

 The Precambrian basement developed before North America existed as a discrete continent, by agglomeration of volcanic island arcs; and it is absent from the rest of California, because most of California is relatively young, geologically.  The basalt of the Cinder Cones is similar in composition to oceanic crust — dark, heavy and iron-rich -- and was generated by deep fracturing of the continental crust.  Other nearby occurrences are Amboy Crater and Pisgah Crater to the south and southwest.  While climbing up to the basalt layer, our webchick Debbie discovered two small cavities where inch-long cores had been removed for paleomagnetic study, which led to a discussion of the Earth’s magnetic field and of how radiometric age dating of geomagnetic reversals is applied to measuring the rates of sea-floor spreading.Before leaving the cinder cone areas, we detoured 5 miles off Kelbaker Road to visit the Lava Tube, which is entered by metal stairs but is not otherwise developed or maintained.The second oldest rock type in the East Mojave is limestone of Paleozoic age, roughly 250 million to 500 million years old, deposited when the most advanced life forms were fish and amphibians.  Much of the Providence Mountains is formed of limestone, from which the Mitchell Caverns were dissolved on the east side.  We could not easily access limestone on the west side, so we were fortunate to find limestone outcrops next to Kelbaker Road several miles north of Kelso Depot.This limestone was deposited in a shallow tropical sea on a slowly subsiding continental margin, similar to the Bahamas and Yucatan coast today.  Paleozoic limestone is also exposed near Lucerne Valley and Victorville, where it’s mined for cement, and it extends eastward to the Grand Canyon.After lunch at the Kelso Depot Visitor Center, we briefly visited the Kelso Dunes which are located north of the Granite Mountains.  As their name indicates, these mountains are formed of granitic rock about 70 to 150 million years old, of Mesozoic age when dinosaurs ruled the Earth.  Granitic rocks are relatively light in weight, light in color, rich in silica and iron-poor.  These rocks compose most of the Sierra Nevada and coastal mountains of Southern California.  These granitic rocks formed in the roots of volcanoes, by slow crystallization of magma chambers beneath a linear chain of volcanoes at the edge of the continent.  The continental margin of California was an active subduction zone, like the Andes now on the west coast of South America, where oceanic crust slowly sinks beneath the continent at a deep trench.  Marine sandstone and shale deposited in the Mesozoic oceanic trench either melted to form the volcanoes or later rose back to the surface to form the Coast Ranges north of Santa Barbara, after oceanic plate subduction ceased and was replaced by lateral plate motion of the San Andreas Fault.While at Kelso Dunes, we discussed how during Mesozoic time similar desert sand dunes covered most of Utah and northern Arizona, in the rain shadow of the coastal volcanoes.  During later burial, vast thick layers of windblown sand were converted to the Navajo and Entrada Sandstones now exposed on the Colorado Plateau at Zion, Capitol Reef, Canyonlands and Arches. After leaving Kelso Dunes, we attempted to reach Clipper Valley by a gas pipeline road, but progress was too slow so we returned to Kelbaker Road and headed south to I-40, thus getting a good view of the jumbled monoliths at the east end of the Granite Mountains.Our last stop was at the Hole-in-the-Wall Visitor Center, where we viewed multiple layers of soft and hard volcanic rock erupted about 14 million years ago from the nearby Wood Mountains caldera.  This volcanic activity may have been produced by early crustal extension in the Basin and Range Province between the Sierra Nevada and Wasatch Range of central Utah. Although the resistant layers that cap mesas and buttes resemble the basaltic lava flows near the cinder cones, this lighter colored rock is silica-rich, iron-poor, and more viscous when molten, and it was all deposited from dense clouds of volcanic ash.  Repeatedly one or more volcanoes erupted explosively, like Mount Mazama to form Crater Lake, and like Mt. Saint Helens in 1980; and if the ash was cooled when time it landed, it formed soft, erodible layers of volcanic tuff and pumice; but if the ash still remained extremely hot, then it compressed and welded into hard solid rock devoid of gas bubbles.In the late afternoon, our caravan headed down Cedar Canyon to Cima, then back to I-15 through the Joshua tree forest of Cima Dome, underlain by decomposed granite from Mesozoic intrusions.