Geology of The Death Valley Area - A Carbonate Shelf Forms

A Carbonate Shelf Forms

A carbonate shelf started to develop over the sandy mudflats early in Paleozoic time. Sediment accumulated on the new but slowly subsiding continental shelf all through the Paleozoic and into the Early Mesozoic. Erosion had so subdued nearby parts of the continent that rivers ran clear, no longer supplying abundant sand and silt to the continental shelf. At the time, the Death Valley area was within ten or twenty degrees of the Paleozoic equator. So the combination of a warm sunlit climate and clear mud-free waters promoted prolific production of biotic (from life) carbonates. Thick beds of carbonate-rich sediments were periodically interrupted by periods of emergence, creating the (in order of deposition);

• Carrara Formation,
• Bonanza King Formation,
• Nopah Formation, and the
• Pogonip Group.

These sediments were lithified into limestone and dolomite after they were buried and compacted by yet more sediment. Thickest of these units is the dolomitic Bonanza King Formation, which forms the dark and light banded lower slopes of Pyramid Peak and the gorges of Titus and Grotto Canyons.

An intervening period occurred in the Mid Ordovician (about 450 Ma) when a sheet of quartz-rich sand blanketed a large part of the continent after the above-mentioned units were laid down. The sand later hardened into sandstone and later still metamorphosed into the 400-foot (100 m) thick Eureka Quartzite. This great white band of Ordovician rock stands out on the summit of Pyramid Peak, near the Racetrack, and high on the east shoulder of Tucki Mountain. No American source is known for the Eureka sand, which once blanketed a 150,000 square miles (390,000 km2) belt from California to Alberta. It may have been swept southward by longshore currents from an eroding sandstone terrain in Canada.

Deposition of carbonate sediments resumed and continued into the Triassic. Four formations were deposited during this time (from oldest to youngest);

• Ely Springs Dolomite,
• Hidden Valley Dolomite,
• Lost Burro Formation, and the
• Tin Mountain Limestone.

The other period of interruption occurred between 350 and 250 Ma when sporadic pulses of mud swept southward into the Death Valley region during the erosion of highlands in north-central Nevada.

Although details of geography varied during this immense interval of time, a north-northeasterly trending coastline generally ran from Arizona up through Utah. A marine carbonate platform only tens of feet deep but more than 100 miles (160 km) wide stretched westward to a fringing rim of offshore reefs. Lime-rich mud and sand eroded by storm waves from the reefs and the platform collected on the quieter ocean floor at depths of 100 feet (30 m) or so. The Death Valley area's carbonates appear to represent all three environments (down-slope basin, reef, and back-reef platform) owing to movement through time of the reef-line itself.

All told these eight formations and one group are 20,000 feet (6,100 m) thick and are buried below much of the Cottonwood, Funeral, Grapevine, and Panamint ranges. Good outcrops can be seen in the southern Funeral Mountains outside the park and in Butte Valley within park borders. The Eureka Quartzite appears as a relatively thin, nearly white band with the grayish Pogonip Group below and the almost black Ely Springs Dolomite above. All strata are often vertically displaced by normal faulting.