Stromatolites in the Todilto Formation?

—The Middle Jurassic Todilto Formation is found in northern New Mexico and southwestern Colorado. In the Ambrosia Lake uranium district, previous workers have identified the large fold-like features in the Todilto Formation as intraformational folds that were produced by loading of the overlying Middle-Upper Jurassic eolian Summerville Formation on the water-saturated sediments of the Todilto Formation or by later structural deformation. A re-interpretation of the outcrops of Green (1982) near Mesa Montañosa, based on preliminary field data, suggests that these features are large, domal stromatolites or bioherms. The composition and morphology of these structures indicate microbial growth rather than loading as the source of the mounds. While similar structures elsewhere in the Todilto Formation have been attributed to intraformational or tectonic deformation, this study suggests that there may be more than one mechanism capable of producing fold-like structures in the Todilto Formation, and a reassessment of previously identified structures may be needed. FIGURE 1. The study area within the Ambrosia Lake uranium district, New Mexico. 381 STROMATOLITES IN THE TODILTO FORMATION? basin (Fig. 2). It is conformably underlain by the eolian, Middle Jurassic Entrada Sandstone and overlain by the lacustrine and sabkha deposits (Kirkland et al., 1995) of the Middle-Upper Jurassic Summerville Formation (Fig. 3). The Todilto Formation ranges in thickness from 10 to almost 40 m and consists of two members: the lower limestone Luciano Mesa Member and the upper gypsum Tonque Arroyo Member (Fig. 3). The age of the Todilto Formation, based on fossil evidence compiled by Lucas et al. (1985), is Middle Callovian (~159 Ma). Berglof (1992) considers the uranium ores to be syndepositional. Isotopic ages for uraninite within the Todilto Formation provide an age of 150 to 155 Ma. Paleogeographic and paleoclimatic reconstructions (Fig. 4, Scotese et al., 2005) for this time period place the Todilto Formation at approximately 20°N latitude in an arid climatic belt. Over the years, a significant debate has existed over the Todilto’s depositional environment. Some authors suggested that the carbonates and evaporates are marine (Baker et al., 1947; Evans and Kirkland, 1988; Harshbarger et al., 1957; Imlay, 1952; Ridgley and Goldhaber, 1983), others proposed a non-marine, lacustrine origin (Anderson and Kirkland, 1960; Rapaport et al., 1952; Tanner, 1970), and yet others proposed a coastal salina that may have been periodically flooded by marine waters (Anderson and Lucas, 1993; Armstrong, 1995; Lucas et al., 1985; McCrary, 1985). Kirkland et al. (1995), based on paleontology, sedimentology and the isotopic data (carbon, strontium and sulfur), concluded that the Lucas et al. (1985) model of a coastal salina with a complex interplay of both marine and freshwaters best explains the Todilto deposits. More recently, Benan and Kocurek (2000), based on the Todilto Formation filling the remnant topography preserved on the Entrada Sandstone in the Ghost Ranch area (northeast of the study area), have called for a “catastrophic flooding” event that buried the Entrada dune forms with minimal reworking and in deep enough water that wave-generated features are minimal to non-existent. Overall, because of the Todilto’s high organic content and the lack of bioturbation and/or ripples or other waveformed features, the Todilto waters had to be relatively deep, poorly-oxygenated and possibly chemically stratified. THE LUCIANO MESA MEMBER The Luciano Mesa Member of the Todilto Formation is a thin (<10 m), micro-laminated, kerogen-rich limestone. Anderson and Kirkland (1960) considered the laminae (alternating layers of calcite, clastics and organics) to be varves and, based on the number of varves, estimated that deposition occurred over a period of FIGURE 2. Map of the aerial extent of Todilto Formation (Lucas and Anderson, 1997). FIGURE 3. The stratigraphy of the Todilto Formation (modified from Lucas and Heckert, 2003).