28S darton: peculiar fault in new mexico 



GEOLOGY. — A peculiar fault in southwestern New Mexico^ 

 N. H. Darton, U. S. Geological Survey. 



In a detailed study of the geology of Luna County, New 

 jMexico, I have found a very remarkable overthrust fault which 

 causes a sliver of granite to penetrate a mass of Umestone. At 

 first sight the relations appeared to indicate an intrusion, but 

 closer inspection showed that the relation was caused by complex 

 faulting. 



The region is one of scattered mountain ranges and buttes 

 rising out of wide desert plains or bolsons. Florida Mountain, 

 in which the fault occurs, lies southeast of Deming and is one of 

 the most prominent features in the area. This range consists of 

 pre-Cambrian granite overlain by a succession of sandstone and 

 limestones of Cambrian to late Carboniferous age, with a thick 

 mass of volcanic agglomerate at its northern end. The contact of 

 granite and basal Cambrian sandstone is exposed at various 

 places, showing unmistakable unconformity with well-marked 

 shore-line conditions. Much of the granite is very massive and 

 of coarse texture, similar to the greater part of the pre-Cambrian 

 granite of the Southwest. The range is traversed by numerous 

 faults, many of them of great throw and with planes at various 

 inclinations. The fault to which this paper relates is on the 

 southwestern slope of the range, 18 miles south-southeast of 

 Deming and nearly 2 miles due south of Gym Peak. The lime- 

 stone mass penetrated by the granite is one of three small wedges 

 overthrust onto granite and considerably isolated by erosion. 

 The limestone contains fossils of the Pennsylvanian series of 

 the Carboniferous. Underlying Silurian and Ordovician lime- 

 stones and Cambrian sandstone are believed to be faulted out, 

 for their presence a mile or so north apparently precludes the 

 chance of unconformable overlap. The relations at the over- 

 thrust are represented in the following sections which show that 

 there was a complex movement of such character that wedges 

 of limestone are partly included in the granite and that a sliver 

 of gi-anite projects through one of them, as shown on a larger 

 scale in the section B. 



1 Published with permission of the Director of the U. 8. Geological Survey. 



