SHAKE, RATTLE, AND ROLL: LATE JURASSIC EARTHQUAKE RECORDED IN MORRISON FORMATION FLUVIAL SEDIMENTS OF WESTERN OKLAHOMA

Recent evaluation of the Morrison Formation in the western panhandle of Oklahoma provided evidence of Late Jurassic seismic activity. The study area is in the southeastern peripheral margin of the Morrison foreland basin near present-day Kenton, OK. Eight meters below the unconformable contact with the Lower Cretaceous Cheyenne Sandstone is a seven meter thick, straight, sandy, braided fluvial channel sandstone bed with numerous soft sediment deformation features. An eastward flow direction was ascertained from trough cross sets. The sandstone is a medium-grained (1.65 Φ), very well sorted (0.31), subrounded, quartz arenite. Thin section analysis demonstrates free floating and point grain contacts, indicating shallow burial.

Soft-sediment deformation structures include convolute and recumbent beds, sills, gas fluidization, fluid escape conduits, and possibly sand volcanoes. Cumulatively, these structures are interpreted to be seismites and have been traced for a total distance of 56 km. These are the first seismites identified for the Morrison Formation.

The sandstone bed consists of two very different lithologic types: a hematitic sandstone and chalcedony-cemented quartzite. A sharp contact always segregates the two lithologic types. The undisturbed hematitic sandstone frequently displays protruding iron nodules and, in thin section, hematite grain rims and intergranular cement. At the top of the sandstone bed are small coalescing hematitic rings, signifying gas fluidization. The chalcedony-cemented quartzite recumbent beds and concordant sills are mostly devoid of hematite. Occasionally, there are remnants of hematitic cement, suggesting the sills were leached by acid. Hydrothermal fluids were likely the source of the silica cements.

Various small tubes indicate fluid flow by liquefaction or fluidization. A 2.5-meter-long, six-centimeter diameter pipe signifies a substantial amount of upward fluid migration. Very well cemented amporhous mounds at the top of the channel may represent sand volcanoes. The existence of seismites and the injection of hydrothermal fluids into the sandstone were caused by a large-scale paleoearthquake likely triggered by regional stresses which activated an intracratonic fault.