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Memoir 14—Sedimentology of the Upper Cretaceous rocks of Todilto Park, New Mexico

By M. E. Willard, 1964, 47 pp., 4 tables, 30 figs., 1 index.

This study involves the fundamental sedimentology of the complex transgressive-regressive Upper Cretaceous sediments exposed at Todilto Park, west of Tohetchi, New Mexico. The three measured and sampled sections represent approximately 3,000 ft of strata ranging from the Jurassic Morrison Formation through the Cretaceous sandstones, siltstones, and shales of the Dakota(?), Mancos, and Mesaverde. These sections have been differentiated and correlated on the basis of physical and mineralogical characteristics.

Five principal petrographic zones have been recognized and each zone has been further subdivided on the basis of its clay mineralogy. Each principal zone is named for the distinctive accessory heavy mineral or minerals it contains. The recognized zones in their order of occurrence, starting at the base of the sections, are (1) staurolite, (2) tourmaline-garnet-zircon, (3) biotite-chlorite, (4) sphene-zoisite, and (5) epidote. At Todilto Park, the mineral zones and the formations do not coincide. It appears that any attempt regionally to characterize a formation on the basis of its mineralogy will fail.

Interpretations of the statistical parameters obtained from the size analyses of the Todilto Park samples result in conclusions that are consistent with currently accepted ideas concerning the history of Cretaceous sedimentation in the San Juan Basin. There are no large-scale abrupt changes in the mechanical composition of the sediments, and hence it appears that during their deposition there were no rapid changes in the tectonic framework of this area. As sedimentation proceeded from deposition of the Dakota through the Point Lookout, there was a gradual decrease in the energy level, a decrease in the rate of deposition, and an increase in the amount of reworking. The characteristics of the sediments above the Point Lookout are typical of backshore and transitional sediments. As the basin filled, these deposits encroached on the earlier beach and neritic deposits. Comparisons with recent deposits support these conclusions. Detailed interpretations of the mechanical analyses suggest that many of the Cretaceous formations include both marine and nonmarine sedimentary units. This is especially true for the Dakota(?) and Gallup Sandstones. It appears that formations deposited during a stillstand of the sea are made up of sedimentary units that are mechanically similar and characteristically very well sorted. The sedimentary units that make up a formation deposited during an interval of fluctuating sea level differ greatly and have a wide range of sorting.

Clay and heavy mineral data suggest that the effects of intrastratal solution have not appreciably altered the character of the original mineral assemblages. Most of the diagenetic alterations were produced by acid solutions. The observed products of alkaline reactions are associated with montmorillonite that probably was derived from the alterations of volcanic ash.

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