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Geology of the National Parks - Death Valley


click for timescale Death Valley National Park through time

Illustration on the way
Late Precambrian Noonday Formation scoured in Mosaic Canyon by periodic flow.

Glaciers in the Tropics?: Late Precambrian time

The Kingston Peak Formation (prominent near Wildrose, Harrisburg Flats, and Butte Valley) contains thick conglomerate beds of pebbles and boulders in a sandy-muddy matrix (figs. 46 and 47). Above and below it are dolomite formations with algal and current features common in warm shallow waters and tidal flats. (Dolomite is forming today on arid mudflats fringing the Persian Gulf.) But boulders rarely move on flat surfaces, and unsorted mixtures of boulders with mud and sand reflect rapid downslope transport, a far cry from the winnowing of large and small particles accomplished by waves and currents on tidal flats. We thus have a contradiction and another enigma.

A few solutions come to mind, but none is wholly satisfactory. (1 ) Erosion of nearby uplifts might temporarily have permitted alluvial fans to spread offshore onto the dolomite platform. (2) During opening of the rift which was to become the Pacific Ocean, the offshore area profoundly deepened, promoting great submarine landslides. (3) Glaciers (or shelf-ice such as fringes the Antarctic today) might have moved the bouldery Kingston Peak debris onto the marine shelf. Each mechanism can explain the conglomerates, but there is no agreement on which was dominant. Conceivably, all three might have coincided, but our picture of that remote time remains clouded.

Migmatite boulder
Large boulders of gneiss and schist (high-grade metamorphic rock) cover alluvial fans along Highway 178 along the western flank of the Black Mountains. This boulder is a migmatite, a highly deformed metamorphic rock that has light-colored granitic intrusions (called aplite). Rocks like these form deep in the Earth's crust.

Scratched boulders in the Kingston Peak Formation do, however, resemble striated stones in modern glaciers. And occasional isolated boulders found in beds of mudstone or limestone on Tucki Mountain are reminiscent of dropstones from melting icebergs, settling onto muddy bottoms far from shore.

Only three periods of glacial advance from polar regions are well-documented: (1) the Quaternary Ice age of the past one-million years; (2) a late Paleozoic Ice-age, 250 million years ago; and (3) a late Precambrian Ice-age, known on most continents and roughly contemporaneous with the Kingston Peak Formation! If the Death Valley area truly had Precambrian glaciers, how then do we explain the warm- climate dolomites adjacent to the glacial deposits? One very speculative hypothesis makes an interesting story: By late Precambrian time, the atmosphere's ozone layer had for the first time become an adequate screen against ultraviolet radiation, permitting invasion of shallow open waters by marine organisms. An enormously expanded population of photosynthesizing marine plants might then have fixed so much carbonate in shallow-water dolomites to have reduced significantly the C02 in the atmosphere. Since atmospheric C02 is known to create a "greenhouse effect" promoting warmer climates, its abrupt loss could conversely have introduced a glacial epoch. A 7% drop in atmospheric C02 today would cool world climate by 40C. (70F.). Perhaps the dolomite-to-conglomerate sequence in late- Precambrian rocks all over the world is not a coincidence but represents an Ice-age triggered by one of the greatest advances in the history of life.

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