Abstract

The Fish Lake Plateau in south-central Utah is underlain by a thick sequence of mid-Tertiary volcanic rocks that unconformably overlie lower Tertiary to Mesozoic sedimentary rocks. Volcanic rocks include alkali-rich trachyandesite and trachyte ash-flow tuffs erupted from the Marysvale volcanic field as well as younger basalts derived from local sources. Sets of steeply dipping normal faults produced multiple grabens on the Fish Lake Plateau; the oldest grabens are northwest-trending, contain sedimentary fill deposits, and likely formed between 1 and 5 Ma. A younger suite of northeast-trending grabens developed after 500–900 ka, but do not cut late Pleistocene glacial deposits. The total amount of extension associated with graben development is limited. An array of surficial deposits, including two generations of Pleistocene glacial deposits, is present. Fish Lake is a wide, trough-shaped lake with steep margins and includes a flooded moraine complex along its northwest edge.