DEPOSITIONAL HISTORY OF THE BLACK MOUNTAIN CONGLOMERATE, MOHAVE COUNTY, ARIZONA: SEDIMENTARY RESPONSE TO MIOCENE EXTENSION IN THE CENTRAL BASIN AND RANGE PROVINCE

This study documents the Miocene depositional and extensional history for the area west of Wilson Ridge in the Black Mountains, northwestern Arizona. Clastic sedimentary rocks in the basin are linked to the extension of the region. The field area, part of the Basin and Range province, lies within an area that experienced northward migration of intense volcanism several million years prior to significant regional extension. Extension propagated northward from the Colorado River Extensional Corridor to the northern Lake Mead area.

Documentation of the basin fill was conducted with the aim of determining whether sedimentary units can be tied to lithologies that are exposed in the Wilson Ridge and whether there is an unroofing sequence preserved within the basin. Stratigraphic sections from four locations were measured to capture the vertical succession of clastic and volcanic rocks and correlated to determine the lateral variability within the basin. Data collected from these sections included changes in bedding orientations, clast compositions, paleocurrent directions, and provenance.

The clast types in the basin mirror the exposed rock types in the Wilson Ridge. Clasts at the southern most section of the basin are dominantly metamorphic clasts. Age-equivalent conglomerates in the northern most section have an evolving composition that changes from volcanic-dominated at the base of the section to plutonic-dominated at the top of the section. Paleocurrent and grain size measurements indicate that sediment was mainly derived from the east and transferred to the west.

The lower portion of the Black Mountain conglomerate was deposited while deformation was occurring, and is folded into an open, gently southward plunging syncline. Fanning of sediments occurs in some localities adjacent to the Wilson Ridge. The upper portion of the Black Mountain conglomerate is post-tectonic and bedding is undeformed. Age relationships and provenance relationships indicate the Wilson Ridge pluton was unroofed at a rate of ~4mm/yr.