| Abstrakt: |
Using three stratigraphic sections, this paper compares the sedimentologic and stratigraphic characteristics of the Point Lookout (early Campanian, 84-81 Ma) and Gallup (early Coniacian, 90-89.5 Ma) sandstone tongues near Cabezon, New Mexico. Each of these tongues represents a major regression event along the southwestern shoreline of the Western Interior Seaway, exhibiting progressive upward coarsening and shallowing trends. Two differences are noted. First, lenticular and channel-shaped, >20-cm-thick sandstone beds with basal scour features (e.g., toolmarks, gutter casts) are common in the lower Point Lookout Sandstone and its lower transition with the Mancos Shale, but they are very sparse in the Gallup Sandstone. In the Point Lookout Sandstone, the orientation of channel fills and toolmarks in these and overlying, tabular beds range from 340° to 020° (mostly north-northeast), which is orthogonal (and to a lesser extent, left-oblique) to the northwestern-trending coastline and likely reflects periodic storm-related, offshore-directed currents. Second, 5th-order parasequences are more obvious in the Point Lookout Sandstone, where laterally continuous mudstones (up to 3 m thick) transition upwards into sandstones, resulting in mudstone-sandstone couplets that indicate deepening (transgression) followed by shallowing (regression). These mudstone-sandstone couplets are absent in the Gallup Sandstone, and except for the transitional base and lower part of this unit, individual parasequences are more obscure. There is also a lack of upward coarsening in the middle to upper part of the Gallup Sandstone. Differences in shoreline morphology controlled by contrasting sea-level stages that relate to systems tracts, as modeled by sequence stratigraphy, may explain the observed differences. The Point Lookout Sandstone has an aggradation-progradation accommodation (stacking) succession and previously was interpreted as deposited in a highstand systems tract. During transgressions, sand was captured by lagoons and estuaries that resulted in sand-starved, muddy deposition on the shoreface ramp. Organic detritus in marine shales and along sandstone bedding planes is consistent with nearby delta distributary channels, which likely facilitated storm-flood currents and related deposition. In contrast, the middle-upper Gallup at Guadalupe Ruin has a progradation-aggradation stacking succession; its middle-upper part is inferred to have aggraded in a late stage of a lowstand system tract with slowly rising sea level. A corresponding coastline with incised river valleys on the coastal plain, increased shore-parallel transport of sand, and a lack of lagoons and estuaries resulted in a high volume of sand input to the nearshore environment. The high sand flux may have inhibited development of thick, transgressive mudstones during high-frequency (5th-order) transgressive events. Relatively long distances to deltaic distributary channels, not necessarily related to low sea levels but perhaps happenstance, may explain the paucity of paleochannel, gutter casts, and toolmark features in the Gallup Sandstone at Guadalupe Ruin. [ABSTRACT FROM AUTHOR] |
