Depositional Settings of the Chalk Hills and Glenns Ferry Formations West of Bruneau, Idaho

Abstract Late Cenozoic deposits in the western Snake River Plain accumulated in a rift-type basin that experienced periodic motion on boundary faults, differential rates of basin subsidence, and episodes of volcanic activity. The late Miocene Chalk Hills Formation and late Pliocene Glenns Ferry Formation represent thick accumulations of lacustrine deposits and subordinate in-channel and floodplain sediments of both low and high sinuosity streams. Facies reconstructions indicate that extensive lakes existed throughout the western plain during both Chalk Hills and Glenns Ferry times. The Chalk Hills is characterized by nearshore to offshore transitions of siliciclastic and volcaniclastic deposits, and the Glenns Ferry Formation is characterized by oolitic carbonate terrace deposits, beach and deeper water deposits, and finer grained siliciclastic and volcaniclastic sequences. Fluvial deposits in both formations are dominated by thick accumulations of overbank fines that contain lenticular, discontinuous channelized sand and mud. Low-sinuosity streams comprise basin margin settings, whereas, higher sinuosity streams are more common along basinal axes. Both levee and crevasse splay deposits occur. Structural adjustments and volcanic events controlled growth and decay of lacustrine environments. Alluvial processes expressed by sand-body geometry, degree of sandbody interconnectedness, and fluvial styles were controlled mainly by structural adjustments along the margins of the Snake River Plain and reflect differential rates of basin subsidence.