THE WILLOW BEACH BEDS – A PRE-COLORADO RIVER AXIAL-BASIN DEPOSIT

The Colorado River below Lake Mead flows through a rugged landscape produced by late Miocene volcanism, extensional tectonism, and erosion. The interior-drained axial basin between Hoover Dam and Cottonwood Valley was created within the Colorado River extensional corridor and filled with sediments from ca. 13 to 6 Ma. The Colorado River entered the basin 6-5 Ma and subsequently removed much of the sedimentary deposits. Remnants of the deposits, preserved along the eastern side of the Colorado River at Willow Beach, are informally called the Willow Beach beds (Twb). The Willow Beach beds unconformably overlie Proterozoic rocks or fanglomerates derived from basement and mid-Miocene volcanic rocks. Sediments consist of moderately lithified matrix-supported conglomerates at the southern end of the basin. Imbrication measurements of the conglomerates show the sediment source to the southeast and southwest.

About 1 km east of Willow Beach is the approximate basin center of the Twb deposit. The basal sediments are mainly weakly lithified sandstone. A 9-m gypsum and gypsiferous sandstone in the middle of the deposit shows the axial-basin contained either a playa or a brackish lake. Above the gypsum are well-bedded, immature sandstones and clayey sandstones. There are several thin- to moderately bedded tuffs and rare pumice clasts throughout the section. A tuff sample from the upper Twb deposit was geochemically correlated with a locally derived but undated tephra from near Oreana, Nevada that suggests a preliminary age of 5.9 to 5.3 Ma.

Twb has been included in the lower Black Mountain conglomerate (Tml), which contains a 13.01 Ma ash to the north, and an 11.72 Ma ash to the south. The correlated age of the tephra suggests that Twb is younger than the Tml fanglomerate, and may instead be correlative with Muddy Creek beds near Frenchman Mountain that contained a vitric ash that was geochemically correlated with the 5.84 Ma tuff of Wolverine Creek or the Lost Cabin beds in the Cottonwood Valley that also contain the Wolverine Creek ash.

Twb also differs lithologically from the Tml deposits, which are primarily fanglomerate. Twb is an axial basin deposit similar to the Lost Cabin beds filling a tectonic sag or topographic low. Additional work is needed in correlating these deposits with other pre-Colorado River deposits.