New Mexico Geological Society Annual Spring Meeting — Abstracts

Ongoing field reconnaissance and research of the ichnology of the Lower Cretaceous (Albian) Mesilla Valley Formation (Washita Group) at Cerro de Cristo Rey, Sunland Park, NM, has led to discovery and identification of several additional ichnotaxa as well as characterization of eight ichnoassemblages from the Cruziana and Skolithos ichnofacies. These ichnotaxa and ichnoassemblages are preserved in shallow marine facies as epichnia, endichnia, and hypichnia within thinly bedded sandstones. Sandstone beds are intercalated with black/dark gray shales, and often have sharp (or fluted) erosive bases and are massively bedded, laminar, or have ripple laminae or hummocky cross-strata. These sandstones are interpreted as tempestites (storm deposits) within shales deposited on the upper/middle shelf.

New taxa identified are as follows: Chomatichnus, Gordia, Fuersichnus, Protovirgularadichotoma, Rhizocorallium, Rugalichnusmatthewii, Spongeliomorphaoraviense, Spongeliomorphasublumbricoides, and Treptichnus. Ichnoassemblages are the following: 1) Planolites-Chondrites-Ophiomorpha, 2) Protovirgularia-Arenicolites-Skolithos, 3) the Arenicolites ichnocoenose of the Skolithos Ichnofacies, 4) Palaeophycusstriatus, 5) Palaeophycus-Spongeliomorpha, 6) Thalassinoides/Planolites-Chondrites, 7) Cardioichnus, and 8) Spongeliomorpha-Palaeophycus. Ichnoassemblages 3, 4, and 7 are hypichnial, with the rest being epichnial. The ichnoassemblage with the largest ichnodiversity is 8, with up to eight different ichnotaxa present in convex hyporelief. The ichnoassemblage with the least diversity is 4, although dense, overcrossing, hypichnial, monospecific assemblages of Palaeophycusstriatus are common. Ichnoassemblage 1 has similar ichnotaxa to 6, however the latter has burrow fill consisting mainly of the foraminfera Cribratinatexana and may represent a similar community in a shallower environment. The Cardioichnus assemblage is only found in one bioclastic conglomerate with ammonoid fragments and Peiliniaquadriplicata, and is approximately 10 m below the upper contact of the unit in the western portion of the field area. Ichnoassemblages 2 and 3 were found in the upper 10 m of the Mesilla Valley Formation, within the gradational contact with the overlying Mojado Formation. Both 2 and 3 contain Skolithos burrows, and ichnoassemblage 3 is part of the Skolithos ichnofacies, which coincides with coarser grained delta lobe front deposits of the lower Mojado Formation.

Coarser grained interbeds and burrow fill (often bioclastic) are associated with the western portion of the field area, and finer grained beds without coarse grained burrow fill are associated with the eastern portion. This is evidence that the eastern portion was more seaward during the late Albian.

Based on burrow fill and trace preservation, cross-cutting, and composite traces (ie Chondrites reworking epichnial Planolites), it appears the equilibrium community consisted of Thalassinoides, Planolites, Chondrites, and/or Bergaueria. Different beds reveal different equilibrium communities and further investigation is needed.