No. 58: Jul-Aug 1988
Geologists are wont to liken the earth's sedimentary strata to pages in a history book. Well, it seems that seismologists may have discovered a previously unread chapter or two deep beneath the continental United States, in the guise of extensive stratified rocks in the Precambrian basement:
"The extent of the layered rocks became evident last summer as the Consortium for Continental Reflection Profiling (COCORP) completed a major deep seismic reflection traverse across Ohio, Indiana, Illinois, and part of Missouri. The survey was conducted partly because industrial seismic data and studies by the Illinois Geological Survey showed basement layering in southern Illinois, partly because earlier COCORP surveys also showed such layering in Oklahoma and Texas, and partly because COCORP's broad program calls for comprehensive exploration of the entire continental basement of the United States.
"Although the composition and precise age of the Precambrian rocks are yet to be determined, their seismic reflection character suggests a sedimentary assemblage, at least in part. These layers occur within the Proterozoic Granite-Rhyolite province, where drilling typically recovers undeformed granite or rhyolite with ages of 1.3 to 1.5 b.y. Such prominent and orderly layering is surprising, given the widespread occurrence of granitic rocks. If the layered rocks are indeed igneous, the volume of silicic volcanic material is spectacular."
(COCORP Research Group; "COCORP Finds Thick Proterozoic (?) Strata under Midcontinent," Eos, 69:209, 1989.)
Some dimensions for these newly discovered pages were given in a report bearing the almost embarrassingly alliterative title indicated in the reference at the end of this item:
"In southern Illinois and Indiana, the layered rocks extend at least 180 kilometers in an east-west direction and average about 6 km in thickness."
(Monastersky, Richard; "Boring Plains Belie Bounty Beneath," Science News, 133:363, 1988.)