Summary

The Black Mountains Accommodation Zone is a Miocene conjugate convergent accommodation zone in a highly extended terrain, the northern Colorado River extensional corridor (NCREC). The structure features show that the BMAZ accommodate the polarity reversal of two adjacent half-grabens as commonly indicated in East African Rift Systems. West-dipping normal faults dominate the northern part of the accommodation zone, whereas east-dipping normal faults control the southern part.

Accommodation zones are an important component of rift basins both structurally and sedimentologically. However, not much sedimentologic study of accommodation zones of rift basins exists in the literature at present. Study in the BMAZ shows that extensive volcanic activities occurred in the accommodation zone and its vicinity. Volcanism had strong influence on the composition and distribution of the depositional systems in the accommodation zone and probably in the half-grabens as well.

Five sedimentary facies exist in the BMAZ; they are: debris flow, slump, sheetflood, stream flow, and suspension facies. Facies assemblages indicate that three depositional systems: alluvial fan, river, and ash-fall systems in the BMAZ. Alluvial fan systems are dominant both vertically and spatially. They are further divided, according to their sedimentary sources, into footwall-sourced, volcano-sourced, locally sourced, and fault-tip alluvial fans.