A Tale of Two Breccia Types in the Mississippian Leadville Limestone, Lisbon Field, Paradox Basin, Southeastern Utah
Breccia associated with sediment-filled cavities is relatively common throughout the upper third of the Mississippian Leadville Limestone in Lisbon field, Paradox Basin, southeastern Utah. These cavities or cracks are related to karstification of the exposed Leadville during Late Mississippian time. Infilling of the cavities by detrital carbonate and siliciclastic sediments occurred before deposition of the Pennsylvanian Molas Formation. The transported material consists of poorly sorted detrital quartz grains, chert fragments, and clasts of carbonate and clay. The carbonate muds infilling the karst cavities are very finely crystalline and non-porous dolomites.
Post-burial brecciation is also quite common within the Leadville reservoir at Lisbon field. This brecciation is most generally caused by natural hydrofracturing, creating an explosive-looking, pulverized rock. The result yields an “autobreccia” as opposed to a collapse breccia. Clasts within these autobreccias have remained in place or moved very little. Dolomite clasts are commonly surrounded by solution-enlarged fractures partially filled with coarse rhombic and saddle dolomites. Areas between clasts may exhibit very good intercrystalline porosity or microporosity, or are filled by dolomite cements. Intense pyrobitumen plugging was concurrent with, or took place shortly after, brecciation. “Reike,” or stair-step fractures, are present, reflecting shear and the explosive fluid expulsion from the buildup of pore pressure. The post-burial breccias are associated with the best reservoir development at Lisbon.
Outcrop analogs for both breccia types are present in the stratigraphically equivalent Mississippian Madison Limestone along the south flank of the Uinta Mountains. Examples from cores of both types of breccias are also available.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009