MIDDLE MIOCENE PALEOGEOGRAPHY OF THE EASTERN LAKE MEAD REGION, NORTHWEST ARIZONA, BASED ON CONGLOMERATES IN TWO SMALL EXTENSIONAL BASINS

The Lake Mead region of northwest Arizona and southeast Nevada exposes extensional basins and associated normal and strike-slip faults.  Research in the region has yielded timing constraints on conglomerate and breccia deposition that place basin formation and associated faulting contemporaneous with Middle Miocene regional extension.  Two basins in northwest Arizona positioned across-strike of a detachment system, one in the hanging wall of the detachment in the northern White Hills and one in the footwall of the detachment in the northern Lost Basin Range, are the focus of detailed basin analyses designed to investigate the structural and stratigraphic evolution of extension in 3D. Geochronology and apatite fission-track thermochronology constrain movement on this detachment system to ~18-13 Ma.  Syn-faulting strata deposited after 15 Ma have been documented in the hanging wall of the detachment in the northern White Hills and to the north in the South Virgin Mountains.

Unique relationships between the detachment and the hanging wall basin allow documentation of extension-perpendicular changes recorded in basin conglomerates: (1) position of the basin within a re-entrant along the corrugated detachment fault; (2) decreasing offset and exhumation of footwall rocks to the south along the detachment; and (3) location of distinctive granite and metamorphic footwall basement blocks.  Comparison with a basin in the footwall of the detachment reveals extension-parallel changes in sedimentation and faulting. 

Paleogeographic reconstructions of these two basins incorporate new geochronologic dates and utilize field observations and quantitative sedimentologic data. The record of basin development in the hanging wall demonstrates (1) a changing depositional system from proximal to medial fan processes 15-8 Ma, (2) a growing faulted and folded basin margin, (3) basin-axial flow with dominant sediment input from the east 14-13 Ma, (4) a rapid decrease in basin subsidence as a result of cessation of movement along the detachment ~13 Ma, and (5) change to a southeasterly sediment source 13-11 Ma.  The record of basin development in the footwall demonstrates (1) late-stage basin filling ~15 Ma, with subsequent development of an erosional surface, and (2) renewed sedimentation and lake formation 11-8 Ma.