HEBERTSON, GREGORY F., and THomas H. Morris, Brigham Young University, Provo, UT

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

Blocks of exotic megabreccia 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 while others are resting some five kilometers from the range. It is believed that the blocks were 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.

Petrographic studies of the paragenetic sequence of cements within each facies provides information regarding the timing of emplacement and diagenesis of megabreccia blocks in extended terrains. Preliminary studies have revealed the following sequence of events. First, initial emplacement of blocks during late Miocene time in conjunction with the deposition of the Oak City Formation and the uplift of the Canyon Range. Second, episodic brecciation and complete re-cementation following emplacement in the meteoric phreatic zone. This is documented by an increase of cross-cutting cements, a decrease in average clast size within similar breccia facies, and an increase in fractures within clasts. Third, subsequent dissolution of carbonate cement by undersaturated pore fluids and the develop ent of secondary porosity. Porosity is best developed within the jigsaw breccia facies.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.