Paleoseismology          
 
Paleoseismology is the study of the record of ancient earthquakes preserved within sedimentary rocks. Our work is centered on three problems: seismicity associated with the late Paleozoic Alleghanian orogeny, seismicity associated with the late Cretaceous/early Tertiary Laramide orogeny, and seismicity associated with the K/T impact.    
     
     
Seismic shaking can cause liquefaction of unconsolidated sediments, producing sand blows and slumping. These features are sometimes preserved in ancient sedimentary deposits. The photo to the right shows a seismically induced clastic dike cutting through late Cretaceous shale. This dike was the result of Laramide seismicity in the San Juan basin of New Mexico.    
         

Alleghanian orogeny        

Late Mississippian sedimentary rocks in the Appalachian Plateau of Virgina and West Virginia contain a rich record of paleoseismicity. The paleoseismites in this area are centered on the modern-day Giles County, VA seismic zone, indicating that the faults responsible for the current seismicity may have been active over 300 million years ago as well. Photo on the right shows syn-depostional faulting induced by seismic shaking in Mississippian Hinton Formation near Princeton, WV.

Click here to download a copy of a paper we recently published on this topic.

 
 
   
             

Laramide orogeny
T
he timing of Laramide deformation in the northern Rockies comes primarily from apatite fission track ages, dating of unconformities in sedimentary rocks, and dating of synorogenic conglomerates. One problem with these kinds of records is that the orogenic signal does not appear until after there has been significant deformation, uplift, and cooling. Paleoseismites, on the other hand, form instantaneously, and can provide a more accurate record of the true beginning of orogenesis. We have used this technique to show that Laramide deformation began in the southern Rockies in New Mexico during the Late Cretaceous (poster presentation). We are also working near the Beartooth mountains in Montana using the same technique in an attempt to constrain more precisely the onset of Laramide deformation in the northern Rockies.
 
         
     
Syndepostional slump folds in Late Creatceous Menefee sandstone near San Miguel, NM. These folds are at the base of a large slump sheet that slid off the east limb of theSan Pablo anticline as it began to grow during the earliest stages of Laramide deformation in this area.  
     
   
Upwardly injected sand dike within liquefied sands of the Paleocene Fort Union Formation, Red Lodge, MT. These sediments were liquefied by large paleo-earthquakes along the Beartooth fault.  
   

K/T boundary

Perhaps the largest earthquake ever on Earth was that associated with the Cretaceous - Tertiary (K/T) asteroid impact. In addition to the global environmental catastrophe that killed the dinosaurs and lots of other animals, the impact would have also generated a M=~11-12 earthquake. This is over 100 times stronger than the largest tectonic earthquake that has ever been recorded. Structures in the K/T boundary sediments along the Tombigbee River at Moscow Landing, AL are a record of the violence associated with this large impact.

   
Thick sand dike with large chalk clasts injected into Cretaceous chalk
   
   
   
Ductile flow in Cretaceous chalk caused by forceful clastic dike injection  
   
             
Copyright © 2003 Kevin G. Stewart
page last modified: September 5, 2003