Upper Cretaceous and Paleogene stratigraphy and sedimentation adjacent to the Nacimiento uplift, southeastern San Juan Basin

Uppermost Cretaceous and Paleogene terrestrial strata in the San Juan Basin record the local disruption and partitioning of the Cretaceous foreland basin by Laramide-aged up lifts. The importance of unconformities within the stratigraphic section, wh ich includes the Fruitland Formation, Kirtland Shale and Ojo Alamo Sandstone, and the nature of the Cretaceous/Tertiary boundary has been controversial, especially in the southeastern San Juan Basin near the Nacimiento uplift . Surface and subsurface mapping has recent ly identified an unconformity-bound sand-rich flu vial unit, here referred to as "unit B," between the Fruitland/ Kirtland and Ojo Alamo formations, which may explain some of the earlier mapping problems . " Unit B" on Mesa Portales contains distinct channel sandstones and interbedded siltstones and mudrocks that show mottling and oxidation profiles typ ical of a high energy fluvial system with well-drained floodplains. Paleocurrent measurements clearly show that deposition of much of "unit B" was related to north-to-south paleoslopes. Detrital lithologies indicate that sandstones of "unit B" and the Ojo Alamo Sandstone were derived from similar bedrock lithologies . North-to-south directed channels, parallel to the Nacimiento up lift, were responsible for depositing "unit B, " the Ojo Alamo Sandstone, the upper Nacimiento Formation and the Cuba Mesa Member of the San Jose Formation . Maintenance of the position of channels parall el and adjacent to the Nacimiento uplift represents continued syntectonic sedimentation in this region from the Late Cretaceous through the early Eocene . INTRODUCTION Uppermost Cretaceous and Paleogene terrestrial strata in the San Juan Basin were deposited during northeast deltaic progradation following withdrawal of the Cretaceous seaway and subsequent Laramide (Late Cretaceous-Eocene) uplift within the southern Rocky Mountains (Baltz, 1967; Fassett and Hinds, 1971; Smith et al. , 1985; Ayers et al. , 1990; Smith, 1991 ). The stratigraphic and temporal significance of erosional contacts near the Cretaceous/Tertiary boundary has been controversial, especially in the southeastern San Juan Basin near the Nacimiento uplift (Fig. I). Arguments for single or multiple unconformities at and near the KIT boundary (Baltz, 1967; Fassett and Hinds, 1971; Sikkink, 1987) have been countered by a hypothesis of nearly continuous sedimentation across the contact (Klute, 1986). Here, I present data that show the stratigraphic posit ions of regional unconformities in the Upper Cretaceous through Lower Eocene section and present a model that describes sedimentation adjacent to the episodically active Nacimicnto uplift. A complete treatment of stratigraphic problems involving the Fruitland, Kirtland and Ojo Alamo formations is beyond the scope of this report. STRATIGRAPHY The stratigraphic sequence of interest includes the Fruitland Formation, Kirtland Shale, Ojo Alamo Sandstone , Nacimiento Formation and San Jose Formation (Fig . I). The Fruitland Formation and Kirtland Shale are each composed of fineto medium-grained sandstone and shale (see Hunt et al., this guidebook). The units are typically distinguished by the occurrence of coal only in the stratigraphically lower Fruit land Formation (Fassett and Hinds, 1971 , p. 19) . A regionally correlative thin shale above the last coal has been a useful marker for picking the contact in subsurface (Ayers et al., 1990). Using either definition, subsurface correlation of the Fruitland Formation and Kirtland Shale from their type areas to the southeastern San Juan Basin has shown that the Kirtland Shale is removed by erosion and the Fruitland Formation is truncated east-southeast of T2 IN , R5-7W, approximately 40-48 km (24-30 mi) due east of the outcrops at Mesa Portales (Fassett and Hinds, 1971; Ayers et al., 1990). Baltz (1967) distinguished the informal units "A" and "B" in the Fruitland and Kirtland (undivided) in the southeastern San Juan Basin based on the greater percentage and coarser grained nature of sandstone in the stratigraphic ally higher "uni t B" (Fig. 2). " Unit A" is composed of silty shale, carbonceous shale, coal and some fine-grained sandstones that locally contain marine fossils. "Unit B" is distinguished by a basal, very coarseto fine-gra ined, crossbedded sandstone, which locally contains large pieces of silicified wood. Multiple coarse-grained, lenticular sandstones in "unit B" arc separated by drab gray-green, red and locally purple mudrocks and siltstones and bentonitic gray clay. Baltz (1967) showed that "unit B" erosionally overlies the lower Fruitland and cuts it out locally where "unit B" overlies the Pictured Cliffs Sandstone . Fassett and Hinds (1971) distinguished in the southeastern San Juan Basin, but d id not map or show correlat ions for, "sandstone beds that seem to be lithologically different from sandstone beds in the Fruitland throughout the res t of the bas in" (p. 17). These sandstones are in the bed that corresponds to "unit B" and were inferred to represent deposition both contemporaneous with the Fruitland Formation and Kirtland Shale, and post-Fruitland deposition in channels on a sub-Ojo Alamo Sandstone unconformity (p. 19). "Unit B" at Mesa Portales is equivalen t , in part, to the "upper Kirtland Shale sandstone facies " of Klute (I 986). The upper contact of the Fru itland Formation with the Ojo Alamo Sandstone in the southeastern San Juan Basin has been contentious as to its position and its conformable or unconformable nature (see discuss ion by Kl ute, 1986 and Fassett et al., 1987). The series of mediumto coarse-grained sandstones and interbedded mudrocks in "uni t B" in the Mesa Portales area are overlain by a thick sandstone that has been un iversally referred to the Ojo Alamo Sandstone, although not all workers believe it to be the basal sandstone of the formation (Fig. 2). An abrupt transition from Cretaceous to Paleocene palynomorphs in a shale sequence adjacent to a lenticular, coarse-grained sandstone body has been interpreted as an unconformable contact with sands tones and shales of the Ojo Alamo Sandstone over the Fru itland Format ion (Fassett and Hinds, 1971; Fassett et al. , 1987). Based on lithologic similarities between the coarse-grained sandstones and apparent intertonguing of the sandstones and shales on Mesa Portales, Klute ( 1986) concluded that the "Kirtland Shale" and Ojo Alamo Sandstone contact is conformable. She did not discuss the evidence for regional truncation of the Kirtland Shale. I propose that strata of "unit B" may be recognized as a separate stratigraphic unit containing one or more unconformities near the Nacimiento uplift (Fig . 2). Details of the significance of the sands tones in the sequence described as "unit B" are discussed in detail below. Mudrocks and fineto coarse-grained sandstones of the Paleocene Nacimiento Formation conformably overlie the Ojo Alamo Sandstone .