Structure 659 



the Trenton and was probably coincident with the formation of the Cin- 

 cinnati arch. Further uplift along the La Salle anticline occurred in 

 post-Pennsylvanian, pre-Pleistocene time, and it is possible that move- 

 ment was effective even during Coal Measures time. 



In southwestern Illinois it is clear that some folding took place after 

 the deposition of the Chester and prior to Pennsylvanian time. The 

 early existence of the La Salle fold appears to be responsible for the thin- 

 ning out of the producing sands to the north along the axis of the anti- 

 cline. Considering the different periods of folding and regional uplifts, 

 it is fortunate that work on surface rocks has disclosed satisfao4:orily the 

 existence of small folds affecting beds as low as the Maquoketa. 



The important diastrophism prior to that time renders impossible any 

 accurate determination of Trenton structure from surface work except 

 along the major structural features. 



RELATION OF FIELDS TO STRUCTURE 



The main oil fields of Clark, Crawford, and Lawrence counties lie at 

 the southeast end of the plunging La Salle anticline, on the east side of 

 the Illinois basin. The other fields of the State occupy isolated positions 

 along the gentle eastward dip in the western part of the State. 



Conditions affecting Accumulation 

 the la salle anticline 



Broadly considered, accumulation has taken place in the main oil fields 

 at the southeast end of the plunging La Salle anticline. The dip of the 

 beds to the southeast along the axis of the anticline is well illustrated by 

 the fact that the lowest producing horizon — namely, the McClosky "sand'^ 

 of the Sainte Genevieve limestone — lies within 350 feet of the surface at 

 the northwest end of the Clark County field,' whereas in the Lawrence 

 County district it ranges in depth from 1,700 to about 1,860 feet. That 

 this sand does not produce throughout the length of the field is probably 

 due to local irregularities in structure and to the variable nature of the 

 producing bed. It ranges from 2 to 10 feet in thickness and averages not 

 more than 10 feet over the entire field. Besides, instead of being a single 

 bed, it is probably a zone in the upper part of the Sainte Genevieve for- 

 mation, the position of the oil being controlled by the porosity of the 

 rocks. Within the zone, which has a maximum reported thickness of 80 

 feet, one to three oil-bearing horizons are reported. 



Above the McClosky oil sand six other sands produce in the main oil 

 fields of the State. Xo sand is everywhere present in such thickness and 



