The sedimentary rocks of a region can be organized into (1) units containing similar fauna, (2) units of equal time span, or (3) units of similar lithology. Locally all three unit types may be nearly identical within a sedimentary sequence, but regionally their bounding planes intersect at appreciable angles.

Methods (1) and (2) depend upon paleontological criteria. A major fraction of rocks in the field (and most well cuttings) yields no diagnostic fossils; of fossils recovered, taxonomy and range are, more often than not, moot questions. Methods (1) and (2) are subject therefore to a fair measure of personal interpretation. Organization of sedimentary rocks on the basis of lithology is concrete. This virtue is recognized in the U.S.G.S. definition of a formation as "a mappable lithologic unit." Facies change introduces complexities, but these can frequently be resolved into operational units by diligent effort. The organizational system in this paper is lithologic.

Cambrian sediments of the Four Corners region can be divided into: (1) a transgressive phase composed of a basal sand (Tapeats-Ignacio) followed and overlain in turn by marine shales (Bright Angel-Ophir) and carbonates (Muav-Hartman-Bowman-Lynch), and (2) a thinner regressive phase composed of retreatal sandstones and dolomites.

Time lines cross lithology; hence, at the close of Lower Cambrian time Tapeats sandstone was being deposited in the west Grand Canyon-San Rafael swell region, Bright Angel-Ophir shales in the St. George-Cedar City belt, and carbonates in Nevada. By Upper Cambrian time Tapeats-Ignacio facies had advanced to the Four Corners-San Juan mountain area; Bright Angel facies had been overlapped by carbonates. Latest Cambrian witnessed a rapid marine retreat and clastic offlap.

Ordovician and Silurian rocks have not been identified on the Four Corners region.

Subaerial denudation during Ordovician, Silurian, and Lower to Middle Devonian time developed at least locally a mature topography on Cambrian sediments. Marine invasion of the Upper Devonian

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differed from that of the Cambrian in that Devonian seas advanced upon a carbonate terrane, except (1) where valley cutting exposed Tapeats, and (2) in the easterly reaches of Cambrian deposition where Cambrian carbonates never covered the Ignacio conglomeratic sandstone and arkose. The lower part of the Upper Devonian is therefore dolomite contaminated with sands and shales except (1) in valleys where clean sandstone lenses are present in Devonian dolomites, and (2) in the eastern reaches of Devonian deposition where it overlapped exposed Tapeats-Ignacio to lie locally on pre-Cambrian.

Brief marine withdrawal gave rise to a thin gray-green clay zone at the top of the Devonian over which Mississippian carbonate-depositing seas advanced. Distribution of the clean carbonate Madison in the Four Corners region is similar to Devonian.

Cambrian-Devonian-Mississippian distribution patterns were replaced in Pennsylvanian time by new tectonic trends. Crustal mobility supplanted earlier Paleozoic stability. Permo-Pennsylvanian sedimentation exhibits great variation in thickness and lithology. Dominant tectonic elements in the region were the Uncompahgre-San Luis uplift, Zuni-Defiance uplift, Paradox basin, and intervening shelf areas. Excessive rate of crustal subsidence in the Paradox basin produced an evaporite lens nearly a mile thick while normal limestones were deposited on surrounding shelves. Contemporaneous moderate uplift of the Uncompahgre element caused delivery of Paradox basin-marginal arkosic flysch. Climactic uplift in later Pennsylvanian and Permian time spread a thick Cutler arkosic fanglomerate and mix d clastic apron over the central Four Corners region. On its periphery the semiterrigenous Abo-Supai-Hermit-Queantoweap redbed terrane accumulated.

Diminishing Permian redbed deposition was augmented and followed by invasion of light-colored, largely aeolian sands and local bolson evaporites, the Toroweap-White Rim-De Chelly-Coconino-Yeso sequence.

Marine Kaibab-San Andres deposition on the western, southwestern and southern margins of the Four Corners region ended Paleozoic stratigraphic history.

Subsurface stratigraphy of the Four Corners region displays a wealth of possible petroleum exploration frontiers. A few of the most promising include: (1) Devonian channel sands of the Sage Plain-Farmington area, (2) linear Mississippian limestone-dolomite facies change belts flanking the Monument upwarp, (3) Paradox basin-marginal bioherms and associated dolomites, (4) Kaibab limestone and local sand lenses in the Castle Valley-Kaiparowitz region. Emphasis in this region should be strongly stratigraphic, preference going to noses developed on interesting stratigraphy rather than to closures in neutral rocks.

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