HEBERTSON, GREGORY F., Brigham Young University, Provo, UT

ABSTRACT: Emplacement and Diagenesis of Exotic Megabreccia Blocks Within an Extensional Tectonic Province: Eastern Basin and Range, Utah

Exotic megabreccia blocks of carbonate and quartzite are perched in the Tertiary Oak City Formation west of the Canyon Range in central Utah. Some blocks are abutting the Canyon Range and others are resting some 5 km from the range. The blocks were believed to be emplaced as gravity driven slides that were seismically triggered at the breakaway zone of the Sevier Desert detachment. This detachment surface is believed to have accommodated much of the Cenozoic extension of western Utah.

Three breccia facies can be identified within each block. Stratigraphically from the base they include (1) a matrix-rich or pulverized facies, (2) a jigsaw or mosaic breccia facies, and (3) a crackle breccia facies.

Field relations and petrographic studies of each facies provides information regarding the timing of emplacement, diagenesis, and economic importance of megabreccia blocks in extended terrains. Preliminary studies have revealed the following sequence of events. First, initial emplacement of blocks during the late Miocene in conjunction with the deposition of the City Formation and the uplift of the Canyon Range. Second, episodic brecciation and complete recementation following emplacement in the meteoric phreatic zone. This event is documented by, moving further into the basin, a decrease in relative block size, an increase of cross-cutting cements, and a decrease in average clast size within similar breccia facies. Third, subsequent dissolution of carbonate cement by undersaturated p re fluids and the development of secondary porosity. Porosity is best developed within the jigsaw breccia facies.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.