The nature of limestone-siliciclastic "cycles" in Middle and Upper Pennsylvanian strata, Tejano Canyon, Sandia Mountains, New Mexico
Gary A. Smith
Middle and Upper Pennsylvanian strata exposed in roadcuts on the east flank of the Sandia Mountains, New Mexico, consist of intimately interbedded limestone and siliciclastic facies that do not alternate in a repeated fashion and are not strictly described as cyclic. Nonetheless, facies analysis, with an emphasis on outcrop and petrographic characteristics of thin beds, permits designation of transgressiveregressive facies alternations that are arguably related to glacioeustasy recorded globally in strata of this age. The complexity of the transgressive- regressive cycles, leading to their nonrepetitive facies sequences, is determined by variations in the elastic facies and associated erosion surfaces. Limestone facies typically form upward-shallowing successions, similar to those described from many localities. Each such succession is commonly overlain by marine and/or nonmarine elastic facies that differ considerably from one transgressive-regressive cycle to the next. Erosion prior to deposition of most elastic facies removed variable amounts of underlying limestone adding to the nonrepetitive facies character. Variations in elastic facies relate to how delta distributary channels and related delta-front/delta-plain environments responded to sea-level change, and lateral variability of facies inherent in these environments. Strong asymmetry of most cycles, well-developed ravinement surfaces with paired sandstone and grainstone lag deposits, interpreted storm beds, and rarity of tidal indicators are all suggestive of deposition in a wave-dominated setting. Most transitions from limestone to elastic facies involve subaerial erosion or weathering surfaces and/or evidence of extensive, early meteoric diagenesis of subtidal limestone facies. The lack of intertidal limestone facies implies that carbonate sedimentation did not fill all available accommodation space prior to exposure. Nonmarine elastic facies are, in almost all cases, found resting directly on erosion surfaces cut on limestone and only rarely are found overlying progradational marine elastic rocks. These relationships require relative sea-level change, rather than autogenic mechanisms, to account for transgressive-regressive cycles. Roadcut exposures in Tejano Canyon provide insights into limestone-clastic "cycles" that are rarely appreciated on natural outcrops of such facies in northern New Mexico and serve as an important locality for interpreting Pennsylvanian strata in that region.
- Smith, Gary A., 1999, The nature of limestone-siliciclastic "cycles" in Middle and Upper Pennsylvanian strata, Tejano Canyon, Sandia Mountains, New Mexico, in: Albuquerque Country, Pazzaglia, Frank J.; Lucas, S. G.; Austin, G. S., New Mexico Geological Society, Guidebook, 50th Field Conference, pp. 269-280.