Vol. VIII] DUMBLE— GEOLOGY TAMPICO EMBAYMENT AREA 13/ 



"Overlying the Tamasopa limestone and resting uncon- 

 formably (?) upon it is a series of alternating impure thin- 

 bedded limestones and gray, red, and green shales and marls 



The entire formation is somewhat sandy and contains 



locally beds of tuff of variegated colors which contain decom- 

 posed mica and are finely porous With the exception of 



one or two doubtful inliers the outcrop of the formation is 

 confined to a narrow strip adjoining the outcrop of the Tama- 

 sopa limestone in the mountain front. The thickness of the 



formation varies from 600 to 1000 feet The formation 



is apparently Tertiary if one may judge from the few 



fossils which have been secured from drill cuttings. If such is 

 true it is of lower Eocene age. The formation grades imper- 

 ceptibly into the overlying shales series, the limestones becom- 

 ing gradually more argillaceous and impure and grading finally 

 into hard shale and in turn into soft shale." 



His description of his Mendez follows :^* 



"Grading from the underlying San Felipe beds is a thick 

 series of gray to green shales, marls and clays containing rarely 

 thin shaly sandstones and limestones and red shales " 



"This formation outcrops, for the most part, over the entire 

 floor of the Sabanita basin. It is the surface rock of the Fur- 

 bero field proper, extending from the Oligocene hills on the east 

 to the lava flows at the foot of the hills of the Sierra Madre on 

 the west. The thickness of this formation at Furbero is ap- 

 proximately 4000 feet. No fossils have been found in this 

 region." 



"Both the altered and unaltered shales of the Mendez forma- 

 tion, a series of blue and gray, medium soft, fine-grained shales, 

 more or less calcareous in places, and (when not metamor- 

 phosed) a fairly constant lithological character throughout." 



The Sabanita Valley, from which De Golyer describes his 

 Mendez and San Felipe, is 60 miles southeast of Chicontepec. 

 Aquismon is 65 miles northwest of Chicontepec. 



At Aquismon the blue shales and clays, with "practically no 

 change in their lithological character from top to bottom", 

 gradually pass downward into limestone interbedded with simi- 

 lar blue shale. 



" Trans. A. I. M. E. LII, p. 275. 



