MEGASEISMITES IN THE TYPE AREA OF THE LANCE FORMATION (MAASTRICHTIAN, CRETACEOUS; NIOBRARA COUNTY, WYOMING)

In its type area, the Lance Fm. is estimated to be 2400’ thick. Indurated sandstones constitute 37-45% of the sedimentary thickness and 37-47% of them display sedimentary features indicative of seismite. At least 43 distinct megaseismites (deformation greater than 3’) are found in the type Lance Formation—35 in the lower 2000’ (sections SE of Lance Creek) and 8 in the upper 320’ (sections W of Lance Creek)—averaging a megaseismite every 40-50’ up section. The upper part of the underlying Fox Hills Fm. and the lower part of the overlying Fort Union Fm. each contain at least one additional megaseismite. Sedimentary indicators range from undulations in the original bedding in 3-4’ megaseismites, to vertical fluid mobilization and escape structures in 4-15’ megaseismites, to homogenized bedding and shale-conglomerate clastic dikes in 15-30+’ megaseismites. Megaseismites have been correlated over 6 miles east-west in the lower Lance and over 12 miles north south in the upper Lance, and a middle Lance megaseismite has been traced in surface exposure 10 miles north-south on each side of Ridge Road between Lance Creek and Buck Creek.

The Lance megaseismites are an enigma. Their huge size compared to modern seismites suggests superquakes with continent-wide devastation. However, their high frequency in the Lance seems to rule out astronomical impact, leaving no apparent cause for such enormous earthquakes. Furthermore, such earthquakes should have left considerable seismic evidence, but reports of correlative megaseismites are absent. Also, releasing inter-grain water via fluid escape structures and mobilizing sub-sand muds via clastic dikes suggest each earthquake shock wave impacted decimeters of unburied, unlithified, unsettled, water-soaked sediments, as if they had been recently and rapidly buried subaqueously. Yet, all the Lance Fm. fossils are terrestrial, and the sediments are interpreted as (terrestrial) braided stream deposits. How the Lance sediments could have been susceptible to seismic deformation in this manner is unknown.