The load of sediments derived from the Cordilleran Mountains depressed the edge of the craton into a series of basins stretching from the Arctic Ocean to the Gulf of Mexico. Cretaceous parts of this wedge reach thicknesses of over 6000 m in western Wyoming, where sands were deposited close to the Cretaceous Mountain front. The wedge does not thin regularly eastward, but thickens locally into basins along the edge of the platform. The regressive sandstone tongues of the Cretaceous clastic wedge are economically important, not only because they are commonly associated with coral deposits, but also because they serve as reservoirs for oil and gas. Large gas fields in the San Juan Basin of southern Colorado and northern New Mexico have reservoirs in the sandstone tongues of the Mesaverde Formation, which extends out into the Moncos Shale.
Further north the Dakota Sandstone has acted as a reservoir for petroleum in many fields in Colorado and Wyoming. Lower Cretaceous Sandstones, such as the Viking, Bow Island, and "Glauconitic," yield both oil and gas in numerous fields in Montana and Alberta. The Cardium Sandstone lenses out eastward into Upper Cretaceous shales and forms a stratigraphic trap for oil at the Pembina Field, Canada's largest, with reserves of 750 million barrels.
The tectonic elements had diverse histories and not all were active at the same time. The group of uplifts along the TX-OK border and in the TX panhandle have been called the Oklahoma Mountains; those in CO and NM have been called the Ancestral Rockies, or the Colorado Mountains. Periodic uplift of the arches has produced complex facies changes in late Paleozoic sediments, unconformities, and structures, all of which are important in forming traps for oil and gas. The Absaroka rocks of KS, OK, and TX yield a large part of the oil supply of the United States. The search for additional petroleum in these rocks is aided by an understanding of the history of the basins.
The Oklahoma Mountains consist of elongate domes of basement rock. A few of these domes form ridges and hills at the surface, but most are now deeply buried beneath Permian, Mesozoic, and Cenozoic sedimentary rocks and have been discovered by drilling in the search for oil.
The elevation of the Oklahoma Mountains began during the Morrow Age and continued to a climax at the end of the Pennsylvania Period. The first episode of uplift, referred to as the Witchita Orogeny, resulted in the stripping of up to 4500 m of pre-Pennsylvanian sedimentary rocks covering the granite basement of the Arbuckle Mountains. The eroded material was carried to the Ardmore and Anadarko basins, which accumulated 5000 m of sedimentary rocks in this episode.
The progressive stripping by erosion of the crests of the rising anticline exposed older formations and is reflected in the type of sedimentary particles supplied to the basins. Although thin sandstone tongues spread from the mountains as far north as KS, most of the sediments were trapped in the rapidly subsiding basins. The sands from these uplifts, now folded by later movements, form reservoirs for many of the oilfields.
In KS the rise of the Central Kansas Arch and the Nemaha Granite Ridge took place in early Pennsylvanian time. The Nemaha Ridge is an acute uplift of early Paleozoic rocks bounded on one side by a fault and stripped by erosion to its pre-Paleozoic core before it was covered by the basal shales of the transgressing Absaroka Sea.
Many of the most productive oilfields of the mid-continent region derived their oil from what is known as bald-headed structures on the Nemaha Ridge. This type of trap results from the doming of strata containing a porous bed, their erosion to a more-or-less flat surface, and the sealing of the truncated edges of the reservoir bed by an impervious layer deposited over the unconformity. Two of the largest fields in cumulative production in the mid-continent area, the Eldorado and the Oklahoma City fields, are located on bald-headed structures in which the major reservoir rock is the Upper Cambrian and Lower Ordovician Arbuckle Limestone.
The shale that seals the bald-headed structures in much of KS is the Cherokee Formation (of Atoka-Des Moines age). This shale contains long, thin lenses of sand called "shoe-string sands." Most of the Cherokee sands are believed to have been channel fillings. They form the reservoirs for several productive oilfields in KS.
Near the end of Pennsylvanian time (Virgil) and early in the Permian period the Oklahoma Mountain system was strongly uplifted in what has been called the Arbuckle Orogeny. Up to this time the OK structures were simple and had been raised intermittently since late Mississippian time. In this orogeny vertical movements still dominated, but the rocks broke along a series of faults into horsts and graben.
The complexity of the structure along the north front of the Arbuckle and Witchita Mountains has been revealed by deep drilling in southern OK. Anticlines in the basins that were formed in the Early Pennsylvanian Witchita Orogeny and then eroded were refolded and broken along high-angle faults. The complexity of stratigraphy and structures produced by these events is illustrated in the cross-section of the Eola Field. Petroleum has been trapped in many of these anticlines in beds that range in age from Ordovician to Permian, but mostly in Pennsylvania sandstones.
Some of the traps were formed in the crests of anticlines, others were formed where permeable beds had been brought against impermeable ones along faults, and still others were formed where sand tongues pinch out over the tops of buried hills.
In the western part of the Oklahoma Mountains, late Pennsylvanian deformationwas reflected largely as uplift; little folding or faulting took place. The regolith that had been developed by the weathering of the granite basement rocks was washed off into the surrounding basins by the rejuvenated streams. This "granite wash" is one of the most important reservoirs in the Panhandle oil and gas field of northern TX.
The rocks of the Mesozoic have supplied the world with a variety of important economic resources. These include such fossil fuels as coal, petroleum, and natural gas as well as nuclear fuels. Orogenic activity during the Mesozoic resulted in the emplacement of such critical metals as copper, zinc, chromite, gold, silver, lead, and mercury.