Samual R. Teeters¹, John M. Holbrook¹, Craig A. Cox¹, Jason A. Perez¹

(1) Southeast Missouri State University, Cape Girardeau, MO

ABSTRACT: Sedimentary Constraints on the Epicentral Location and Degree of Deformation for the New Madrid 12/16/1811 Dawn Earthquake, and the Role of Splays in Tectonically Driven Floodplain Aggradation

The New Madrid seismic zone of southeast Missouri and environs was the site of four large (M>7.2) earthquakes in the winter of 1811/1812. Epicenters for the first, third, and fourth earthquakes are established. The epicenter of the second earthquake (12/16/1811) is uncertain, but was recently proposed to be associated with the Cottonwood Grove right-lateral strike-slip fault, based on modeling and historical accounts (Johnston and Schweig, 1996). Sedimentary data from this study support this inference, and constrain the locus and magnitude of deformation.

Deformation over the Cottonwood Grove fault is focused 2.5 km northwest of Cottonwood Point, Missouri. Floodplain deposits are down-warped two meters there, resulting in formation of Pemiscot Big Lake sunklands above the down-dropped northern fault block. Silt-dominated splay deposits from the proximal Mississippi River have filled over 1.5 meters of the initial relief. Splay deposits were supplied by small (<2 m deep) channels, which reached the sunklands by passing more than three kilometers over the up-thrown/southern fault block through preexistent topographic lows above underfilled abandoned river channels. Pemiscot Big Lake is historically reported to have formed during the 1811/1812 earthquake sequence; however, it is uncertain if all of the relief recorded accumulated in the 1811 event. Two meters should currently be regarded as a maximum amount of 1811 vertical deformation.

Splay deposits similarly fill sunklands on the down-thrown block of the nearby Reelfoot fault. This suggests a tendency for the Mississippi River to respond to subtle tectonic lows on its proximal floodplain by rapidly filling them with splay deposits.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado