A PACIFIC MARGIN EXAMPLE OF COASTAL LANDSLIDING AND CATASTROPHIC SEDIMENTATION TRIGGERED BY K/T BOLIDE IMPACT

We report here the first recognized Pacific margin sequence containing evidence for catastrophic landsliding, attributed to bolide impact-related seismic shocking.at the Cretaceous-Tertiary boundary. The Cretaceous-Tertiary boundary is not commonly preserved in stratigraphic sequences of the Pacific margin, but we have discovered it within a coastal paleovalley in Baja California del Norte, Mexico (near El Rosario). This 5 km-wide, >15 km long, and 200 m deep coastal paleovalley formed by massive gravitational collapses, and rapidly filled with coastal (shallow marine to fluvial) gravels and sands, as well as slide sheets of marine mudstone that range from meters to kilometers in length. We infer that seismic shocking caused liquifaction and extremely rapid sedimentation of the gravels and sands, in addition to unleashing the slide sheets. A 20m thick pumice lapilli tuff in the middle of the valley fill is inferred to have been fed directly by an eruption, and deposited from a high-density turbidity current in a shallow marine setting. Laser-heating 40Ar/39Ar data for biotite, hornblende and plagioclase (single crystal and bulk step heating) give an age of 65.5 +/- 0.6 MA for the tuff; this is indistinguishable in age from Haitian tektites dated by the same lab. The K/T coastal paleovalley in Baja California differs from those of standard stratigraphic sequence models, which are carved during lowstands and filled during highstands in sea level. This valley was cut and filled during ongoing gravitational collapse events that must have been controlled by seismicity, on a time scale much faster than sea level change. The incredible size of the initial quake at the impact site (magnitude 13) may have resulted in adjustments on regional fault systems in Mexico, which is an area riddled with lithosphere-penetrating faults related to the assemblage of disparate tectonic microterranes. Our new Pacific margin sequence, like many K/T boundary sequences in the Gulf of Mexico-Caribbean region, provides evidence of giant landslides and catastrophic sedimentation at distances over 1800 km from the bolide impact site.