()90 J. H. GARDNER THE MID-CONTINENT OIL FIELDS 



111 the Peiiiisylvaniaii scries of Kansas limestone constitutes a very 

 large percentage of the whole. Southward in Oklahoma limestone is of 

 rapidly diminishing importance in tlie Pennsylvanian, the same being 

 chiefly sandstone and shale. This is a very noteworthy change in the 

 nature of the sedimentation, when one considers the fact that Kansas lies 

 directly north of Oklahoma along the strike and continuous outcrop of 

 the series. It bears testimony that deeper waters prevailed to the north 

 and northwest in Pennsylvanian time. While some of the sandstones of 

 Oklahoma are of deep marine origin, as shown by both paleontologic and 

 lithologic evidence, a large percentage of them were unquestionably de- 

 posited in brackish or shallow water; they show ripple-marks on bedding 

 planes and contain large casts of fossil trees; moreover, they alternate 

 with beds of shale carrying plant remains and percentages of carbon 

 varying from a trace to sufficient amounts to color the beds black. Such 

 sediments are common in the strata through the oil belt of Kansas, and 

 well logs show that near the Oklahoma line the formations are similar to 

 those farther south. Northward in Kansas, however, the beds of shale 

 contain less carbonaceous matter and more lime than they do in Okla- 

 homa. 



That an ocean lay to the west as well as to the north of Oklahoma and 

 Kansas in Pennsylvanian time is further indicated by the fact that the 

 sediments of the same age in southern Colorado and northern New Mex- 

 ico are almost entirely of marine origin. Some years ago the writer made 

 a stratigraphic section across the massive limestone beds near the town 

 of Glorietta, New Mexico. Fossil Brachiopoda from that thick section of 

 limestone were identified by Dr. George IT. Girty, who pronounced them 

 of Pennsylvanian age, and hence these limestones occupy the same gen- 

 eral time interval as the sandstone and shale beds of the oil-bearing region 

 in southeastern Kansas, eastern Oklahoma, and northern Texas. Other 

 ('lianges in the physiographic conditions during Pennsylvanian deposition 

 arc shown in the stratigraphy of Oklahoma and Texas. 



In the Mid-Continent fields there is strong evidence that petroleum 

 originated in largest quantities in rocks that were deposited in a position 

 intermediate between near-shore sediments and those of the deep sea. 



STRUCTURE 



The topogiapliic relief of the Ozark region east of Kansas is moderate, 

 l)ut the influence of the rise of igneous rocks in that territory was exten- 

 sive. In fact, it now appears that effects of this disturbance can be ob- 

 served in the sedimentary strata for the full length of Kansas east and 

 west. All recks from the Mississippian to the Cretaceous, inclusive, show 



