Monday, January 30, 2012

BAKING NEW MEXICO SHALE

THE CRETACEOUS PIERRE SHALE INTRUDED BY A DIKE NEAR MP 435-436 ALONG I-25 IN NORTHERN NEW MEXICO.


The Raton Basin is a structural basin--beds dipping toward the center-- straddling the New Mexico-Colorado state line immediately east of the Rocky Mountain front.  The basin is asymmetrical with the basin axis closest to the bounding Sangre de Cristo Range on the west (steeper dipping beds).  The eastern boundary includes the Apishapa Arch and Las Animas Arch (Colorado) and Sierra Grande Arch or Uplands in New Mexico.  Tertiary sedimentary rocks are well exposed in the center of the Basin while the Cretaceous Pierre Shale often crops out along the exterior boundary.
The Basin is probably best known for 1) the dikes associated with the  Spanish Peaks granitic intrusives near Walsenburg, Colorado; 2) the coal mines and coal bed methane wells associated with the Cretaceous Vermejo Formation and the late Cretaceous-early Tertiary Raton Formation near Trinidad, Colorado; 3) the Miocene volcanic rocks capping the mesas at Raton Pass; and 4) exposures of the K-T boundary in the Raton, New Mexico, and Trinidad areas.
Most visitors to the Basin are impressed with the numerous radiating dikes near Walsenburg and La Veta, and they are correct.  These dikes are pictured in hundreds of textbooks and popular reading materials and are recognized by geologists the world over. 

However, the place to get “up close” to an igneous intrusive is to examine the smaller dikes cropping out, generally perpendicular to the basin axis, across the state line in New Mexico. Along I-25 near MP 436 and 435 (south of the city of Raton) the interstate cuts through two dikes and exposes their central core as well as the adjacent baked Pierre Shale.  The metamorphic aureole of the Pierre extends about 12-15 feet on either side of the dike.  The shale has been altered to a hornfel.  The age of the intrusion is either Eocene or Oligocene.

mike
 
File:Raton-Map.jpg
GEOLOGIC MAP OF THE RATON BASIN, NEW MEXICO AND COLORADO.  MAP COURTESY OF USGS GEOLOGICAL SURVEY BULLETIN 2184-B. 
THE DIKE IS HARDER THAN THE SHALE AND THEREFORE IS MORE RESISTANT TO EROSION.  NOTE THE BAKED SHALE ADJACENT TO THE INTRUSION.
 

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