The pre-Triassic strata of the Four Corners region are a hemi-cyclic complex of marine and non-marine facies resulting from complicated inter-reactions of broad uplifts from both outside and inside the region, localized uplifts and subsidences within the region, and major shifts of eugeosynclinal deposition within the Paleozoic Cordilleran geosyncline on the west.

The major stratigraphic divisions of the Four Corners region are thus four-fold as a direct result of a complex interplay of epeirogenic and orogenic activities within and surrounding the region. An outline summary of these divisions follows.

1. Widespread erosion of the pre-Cambrian complex during the Lipalian interval over the broad San Luis platform of which the San Loid, Defiance, Kaibab, Navajo, and Apache positive areas were only shelf components of the Cordilleran eugeosyncline. The San Luis platform is thus considered to be a broad region separating the Ouachita geosyncline on the east, the Sonoran geosyncline

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on the south, and the Cordilleran geosyncline on the west during much of Paleozoic time. Subsequent to the widespread pre-Cambrian erosion, the San Luis platform oscillated downward episodically sufficiently to allow the deposition of Cambrian, Ordovician, Devonian, and Mississippian carbonate and sorted clastic strata as shelf sediments grading and thickening westward into basinal equivalents. A major positive oscillation resulted in the stripping-off of virtually all of the Ordovician shelf sediments during Silurian time.

2. Shelf conditions succeeded a widespread erosion interval during which the Molas shale was formed as a complex soil profile of early Pennsylvanian age. The earliest Pennsylvanian marine sediments thus are shelf carbonates which contain a vast pre-Des Moines intraformational disconformity that preceded the basination of the Paradox geosyncline.

3. Localized subsidence of a major part of the former widespread San Luis platform occurred to form the Paradox evaporite basin, which was a southeast-projecting arm of the Cordilleran geosyncline. Local shelves to the Paradox geosyncline, in which shelf carbonates and clastics were Kaiparowits basins. Marine access-ways to the Paradox basin existed on the northwest and on the southeast, with the San Rafael, Kaibab, Uncompahgre-San Luis-Nacimiento, and Zuni uplifts providing localized positive barriers tectonically accentuated during Pennsylvanian time.

4. Rapid rise of the Uncompahgre and Nacimiento uplifts caused coarse clastic arkosic sediments to be dumped into Hermosa seas from the northeast and east and Kaibab-Supai fine clastics from the south and west. Bodily uplift of the entire Four Corners region to continental conditions which existed from latest Pennsylvanian to latest Jurassic time.

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