CHARACTERIZATION AND INTERPRETATION OF THE MIOCENE HOT SPRINGS LIMESTONE AND ITS SURROUNDING STRATA: SOUTHWESTERN SNAKE RIVER PLAIN, IDAHO

The Miocene Hot Spring limestone is a unique lacustrine algal reef exposed along the southwestern Snake River Plain near Bruneau, Idaho. The morphology and structural detail of this carbonate unit have been previously described, but little is known about the sediment directly above and below it. An in-depth study through mapping, measuring sections, and petrographic analysis of the deposits will help place the Hot Spring limestone into geologic context within the Chalk Hills Formation of the Idaho Group and establish the relationship of the limestone reef within the surrounding sediment.

The western portion of the Snake River Plain is an irregular northwest-trending graben marked by the steep, faulted front of the Owyhee Mountains. Within the Plain is the Chalk Hills Formation, which has been dated as upper Miocene. This formation has been interpreted as a fluvial-lacustrine transgressive sequence and includes evidence of intermittent volcanic activity. The Hot Springs limestone unit belongs to the Chalk Hills Formation, and has been interpreted as having been deposited in a nearshore lacustrine environment. The sediments below the unit have been simply interpreted as floodplain sediments, and sediment above the limestone have been interpreted as lacustrine. Not much attention has been paid to these sediments, however. The processes that lead to the formation of lacustrine carbonate deposits are not fully understood; this study will attempt to develop a better understanding of these processes. It is expected that detailed study of the sediments above, below and including the Hot Springs limestone will help constrain the circumstances that led to the initiation and decline of the reef.