New Mexico Geological Society Annual Spring Meeting — Abstracts

Intepretation of sequence stratigraphy in the middle Cenomanian-middle Turonian Tokay Tongue of the Mancos Shale and lower Tres Hermanos Formation, south-central New Mexico, and in the Colorado Shale, Atarque Sandstone, and lower Moreno Hill Formation, southwestern New Mexico, is hampered by widely spaced outcrops that are dominated by offshore marine shale. Our analysis focuses on the four best-exposed outcrops with the highest biostratigraphic resolution, which are, from southwest to northeast, in the Cookes Range (Luna County), at Mescal Canyon (Sierra County), in the Carthage coal field (Socorro County), and at Bull Gap (Lincoln County). In nearshore settings, sequence boundaries and falling stage systems tracts are defined by non-Waltherian, seaward shifts in facies, resulting in shoreline sandstones with erosional bases overlying offshore marine shales. Correlative sequence boundaries in offshore settings are represented by amalgamated storm sands or by sandy inoceramid-shell-rich calcarenites, although locally sequence boundaries are not recognizable within successions of offshore marine shale. Transgressive systems tracts are characterized by non-Waltherian, landward shifts in facies and are almost exclusively composed of offshore marine shale. However, the initial Mancos/Colorado transgression resulted in a thin (<1 m), transgressive lag composed of granular to pebbly sandstone. Condensed sections associated with maximum flooding surfaces are represented by thin, globigerinid wackestones and packstones, some of which are iron-rich and partially phosphatized. Upsection increases in the relative abundance and thickness of storm sands (progradational parasequence sets) define highstand systems tracts. Eight sequence boundaries are recognized in the ~200-m-thick interval. The lower five are dominated by distal shoreline facies and correlative calcarenites or amalgamated storm sandstones. Sequence boundary 6 at Mescal Canyon is at the base of the Atarque Sandstone Member of the Tres Hermanos Formation, which consists of a complete barrier island succession within the lower Turonian Mammites nodosoides Zone and correlates to calcarenites at the top of the Bridge Creek Limestone Beds of the Tokay Tongue of the Mancos Shale at Carthage. Sequence boundaries 7 and 8 at Carthage and Bull Gap are at the base and top of shoreline sandstones of the Atarque Sandstone Member, which at these locations is in the middle Turonian Collignoniceras woollgari woollgari Subzone and correlates to fluvial strata at Mescal Canyon and in the Cookes Range. This analysis is a high-resolution record of relative sea-level change within the previously defined T-1 transgression and R-1 regression of the Late Cretaceous Western Interior Seaway and provides outcrop analogs for evaluating the hydrocarbon potential of the Mancos Shale.