Other sediments may have been added but, if they ever existed, were completely removed by erosion.
Such gaps in the geologic record are called unconformities by geologists.
Fluvial gravels initially accumulated in shallow river valleys.
It is 120 to 340 feet (37 to 100 m) thick and grayish in color.Today it is very bright orange-red and gives the Red Canyon its name.Eventually, this sunken region of Laurentia was flooded with a shallow seaway that extended from at least present-day Lake Superior to Glacier National Park in Montana to the Grand Canyon and the Uinta Mountains.The oldest section of the supergroup is the Unkar Group.The new river captured the older drainage to form the ancestral Colorado River, which in turn started to form the Grand Canyon.
Wetter climates brought upon by ice ages starting 2 million years ago greatly increased excavation of the Grand Canyon, which was nearly as deep as it is now, 1.2 million years ago.
Volcanic activity deposited lava over the area 1.8 million to 500,000 years ago.
At least 13 lava dams blocked the Colorado River, forming lakes that were up to 2,000 feet (610 m) deep.
The Bass Formation was deposited in a shallow sea near the coast as a mix of limestone, sandstone, and shale.
Diagenesis later altered the bulk of the limestone into dolomite.
It accumulated in a variety of fluvial, deltaic, tidal, nearshore marine, and offshore marine environments.