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^/ The Todilto Limestone of Middle Jurassic age in the Ambrosia Lake uranium mining district of McKinley and Valencia Counties, New Mexico, is the host formation for numerous smallto medium-sized uranium deposits in joints, shear zones, and fractures within smallto large-scale intraformational folds. The folds probably were formed as a result of differential sediment loading when eolian sand dunes of the overlying Summerville Formation of Middle Jurassic age migrated over soft, chemically precipitated, lime muds of the Todilto shortly after their deposition in a regressive, mixed fresh and saline lacustrine or marine environment of deposition. Encroachment of Summerville eolian dunes over soft Todilto lime muds was apparently a local phenomenon and was restricted to postulated beltlike zones which trended radially across the Todilto coastline toward the receding body of water. Intraformational folding is believed to be confined to the pathways of individual eolian dunes or clusters of dunes within the dune belts. During the process of sediment loading by migrating sand dunes, layers of Todilto lime mud were differentially compacted, contorted, and dewatered, producing both smalland large-scale plastic deformation structures, including convolute laminations, mounds, rolls, folds, and small anticlines and 'Abstract, in modified version, published ir\_ American Association of Petroleum Geologists Bulletin, vol. 65, no. 3, p. 560; abstracts of papers presented at the Rocky Mountain Section, AAPG meeting, Albuquerque, New Mexico, April 12-15, 1981. synclines. With continued compaction and dewatering, the mud, in localized areas, reached a point of desaturation at which sediment plasticity was lost. Prolonged loading by overlying dune sands thus caused faulting, shearing, fracturing, and jointing of contorted limestone beds. These areas or zones of deformation within the limestone became the preferred sites of epigenetic uranium mineralization because of the induced transmissivity created by sediment rupture. Along most of the prograding Todilto coastline, adjacent to the eolian dune belts, both interdune and coastal sabkha environments dominated during Todilto-Summerville time. Sediments in coastal areas consisted mainly of clay, silt, sandy silt, and very fine-grained sand, which was apparently derived from the winnowing of the finer grained fraction of sediment from adjacent dune fields during periods of eolian activity. Most of the sabkha sediments were probably carried in airborne suspension to the low-lying, ground-water-saturated coastal areas, where they were deposited as relatively uniform blanket-like layers. Deposition of sabkha deposits was apparently slow and uniform over most of the Todilto coastal areas and crested only small-scale deformation features in underlying Todilto rocks. Large-scale deformation features and uranium deposits are both notably absent in the Todilto where it is overlain by finer textured sabkha deposits in the Summerville. Introduction The Todilto Limestone of Middle Jurassic age in the Ambrosia Lake uranium mining district of McKinley and Valencia Counties, New Mexico, is the host formation for numerous smallto medium-sized epigenetic uranium deposits that occur in joints, shear zones, and fractures within a series of smallto large-scale intraformational folds in the formation. Field evidence presented in this report indicates that the intraformational folds were formed as a result of localized differential sediment loading when eolian sand dunes of the overlying Summerville Formation, also of Middle Jurassic age, migrated over soft, chemically precipitated lime mud flats at marginal areas of the regressive Todilto saline lake or arm of the sea. The area of study is located at the southern margin of the San Juan basin within the Ambrosia Lake uranium mining district of the Grants mineral belt (fig. 1). Todilto uranium deposits are concentrated along the southern edge of the mining district in the area between Haystack Butte on the west and the west flank of Mount Taylor on the east (fig. 2). Isolated areas where intraformational folding has occurred in Todilto rocks are also present west of the Mount Taylor-Haystack area. The area studied is approximately 12 miles (19.3 km) long and three-quarters to one mile (1.2-1.6 km) wide and includes virtually all of the area where the Todilto crops out as an erosion-resistant caprock on top of the Jurassic Entrada Sandstone within the mining district. Stratigraphy and Environments of Deposition p The Todilto Limestone occupies an area of approximately 34,000 mi o (88,060 km ) that includes the present Tertiary San Juan basin and an adjacent region east of the basin in north-central New Mexico. The Todilto correlates with and is considered the age equivalent of the Pony Express Limestone Member of the Wanakah Formation of southwest Colorado and the Curtis Formation of

Origin of intraformational folds in the Jurassic Todilto Limestone, Ambrosia Lake uranium mining district, McKinley and Valencia counties, New Mexico