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.

Abstract The carbonatite complex at Iron Hill is located southeast of the town of Powderhorn along Cebolla Creek, Deldorado Creek, and Beaver Creek in Gunnison County, Colorado. To reach the complex, travel 9 mi (14.5 km)west of Gunnison on U.S. 50 to the intersection with Colorado 149, turn south on Colorado 149 and travel 15 mi (24 km) to the intersection with Cebolla Creekroad; turn southeast on Cebolla Creek road; after about 2.5 mi (4km), the outcrops on the left are in the carbonatite stock of the Present address: Department of Geology, Adams State College, Alamosa, Colorado 81102 complex. Roads to the east off Cebolla Creek road along Deldorado Creek and Beaver Creek connect with a variety of dirt ranch roads that provide excellent access to the rocks of the complex.These roads are periodically maintained by Humphreys Mineral Industries, Inc., of Denver, Colorado, the property managers.Therefore, most of the complex is easily accessible by short hikes from roads navigable by most vehicles. A number of patented mining claims exist in the area, but permission to access the property has never been denied. The geologic map of the complex(Fig. 1) is located within the Rudolph Hill Quadrangle(Olson, 1974) and within the Powderhorn Quadrangle (Hedlund and Olson, 1975).Access roads are also shown on the Rudolph 325 Hill and Powderhorn 7.5-minute quadrangles (1:24,000). Some of the better localities at which to see specific rock exposures are shown on Figure 1.

Abstract The San Luis Basin encompasses the largest structural and hydrologic basin of the Rio Grande rift. On this field trip, we will examine the timing of transition of the San Luis Basin from hydrologically closed, aggrading subbasins to a continuous fluvial system that eroded the basin, formed the Rio Grande gorge, and ultimately, integrated the Rio Grande from Colorado to the Gulf of Mexico. Waning Pleistocene neotectonic activity and onset of major glacial episodes, in particular Marine Isotope Stages 11–2 (~420–14 ka), induced basin fill, spillover, and erosion of the southern San Luis Basin. The combined use of new geologic mapping, fluvial geomorphology, reinterpreted surficial geology of the Taos Plateau, pedogenic relative dating studies, 3 He surface exposure dating of basalts, and U-series dating of pedogenic carbonate supports a sequence of events wherein pluvial Lake Alamosa in the northern San Luis Basin overflowed, and began to drain to the south across the closed Sunshine Valley–Costilla Plain region ≤400 ka. By ~200 ka, erosion had cut through topographic highs at Ute Mountain and the Red River fault zone, and began deep-canyon incision across the southern San Luis Basin. Previous studies indicate that prior to 200 ka, the present Rio Grande terminated into a large bolson complex in the vicinity of El Paso, Texas, and systematic, headward erosional processes had subtly integrated discontinuously connected basins along the eastern flank of the Rio Grande rift and southern Rocky Mountains. We propose that the integration of the entire San Luis Basin into the Rio Grande drainage system (~400–200 ka) was the critical event in the formation of the modern Rio Grande, integrating hinterland basins of the Rio Grande rift from El Paso, Texas, north to the San Luis Basin with the Gulf of Mexico. This event dramatically affected basins southeast of El Paso, Texas, across the Chisos Mountains and southeastern Basin and Range province, including the Rio Conchos watershed and much of the Chihuahuan Desert, inducing broad regional landscape incision and exhumation.