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Large-scale intertonguing between marine Mancos and nonmarine Mesaverde strata in New Mexico, Arizona, and southwestern Colorado—a region situated on the southwestern margin of the Upper Cretaceous epicontinental sea—was brought about by oscillations of the ancient shore line. These oscillations resulted from variations in the rate of subsidence of the continually sinking trough of deposition and in the rate of supply of detritus. The several withdrawals of the sea apparently were caused by silting-in along its margins—an upward and outward building of the shore —rather than by any relative uplift of the sea floor. As a result of this intertonguing, the Mancos and Mesaverde formations include rocks of widely differing and over-lapping ages in different localities; thus, the names “Mancos” and “Mesaverde” are applied to lithologic units not referable to any particular age.

Detailed field studies in the Mesa Verde-Atarque area show the relations of the several tongues of the Mancos and members of the Mesaverde and indicate that the upper part of the Hosta sandstone of the southern San Juan Basin region is the homo-genetic equivalent of the Point Lookout sandstone, the lowest formation of the Mesaverde group at the type locality. This sandstone, the same lithologic unit throughout, is younger in the north than in the south. The unit sometimes called “Tocito sandstone lentil of the Mancos” in areas adjacent to San Juan River is the correlative of the Gallup sandstone member and part of the Dilco coal member of the Mesaverde of the southern San Juan Basin region; and the nonmarine strata between the Hosta and the Dilco in the southern area grade northward into marine shale of the upper part of the Mancos.

Lateral tracing of the formations southward from southern San Juan Basin through the McCarty-Alamosa Creek area establishes the correlation of the locally named formations in the valley of Alamosa Creek with the several tongues and members of the Mancos and Mesaverde formations.

Details of the stratigraphy worked out in the Mesa Verde-Atarque and McCarty-Alamosa Creek areas provide a “key” that is useful in solving regional correlation problems. Application of this “key” resulted in reasonably certain correlation of Upper Cretaceous remnants in southwestern Colorado, eastern and southern New Mexico, and eastern Arizona. From these correlations one can determine the approximate location of the shore line at various times during the early Upper Cretaceous and can show that the inundation that began in Dakota time reached its maximum extent in early Benton time. The remainder of Upper Cretaceous time was occupied by a slow oscillatory retreat as the sea was forced back gradually by deposition along its margins.

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