Aztec Sandstone title bar for LMNRA

The Early Paleozoic
Tropical seas

About 560 million years ago the sea began to slowly invade North America from the west. By 520 million years ago the sea submerged what is now LMNRA. Sand derived from erosion of the older Proterozoic rocks accumulated along the shore. This beach sand would eventually become the Tapeats Sandstone.
Sea level continued to rise, spreading eastward across the continent. As the shoreline moved eastward over what is now LMNRA, the water deepened and the sediment deposited became finer and finer. Eventually the Tapeats beach sand was covered with fine silt and clay that later hardened to siltstone and shale. By the time the beach had migrated to central Arizona, off shore shoals and carbonate reefs teeming with plant and animal life were established at what is now LMNRA.
For millions of years, (520-270) countless calcium carbonate skeletons accumulated layer upon layer on the sea bottom. The gradual sinking of the sea bottom kept pace with skeletal accumulation, so the sea remained shallow while the sediment pile below got thicker and thicker (quote from Bohannon). Burial converted the calcium carbonate skeletons to layered limestone and dolomite rock which is up to 2000 meters (over 6000 feet thick!) at LMNRA.
Sea level rose and fell several times during the Paleozoic. During periods of low sea level, seas completely withdrew from the area. Rocks and sediment that had accumulated on the sea floor were then exposed to erosion, sometimes for millions of years. This completely erased some of the rock layers deposited earlier, creating gaps in the rock record here. In some localities, terrestrial sand dune and river deposits accumulated during times when the seas withdrew. When high sea levels returned, the region was once more covered by shallow seas and marine sediments were deposited again.


For more detail about this time period at Lake Mead NRA click here.

Earth view 458 million years ago
Earth view 425 million years ago
Earth view 390 million years ago
Earth view 356 million years ago

Geology of Lake Mead NRA home | US Geological Survey home
National Park Service home | Lake Mead NRA home
Education resources | Technically speaking
lmnra horizontal bar
| USGS Geology in the Parks home | NPS Park Geology Tour home |

This site is a cooperative project of the
US Geological Survey Western Earth Surface Processes Team
and the National Park Service.

Please share your comments and suggestions with us!
This page was last updated on 4/5/99