New Mexico Geological Society Annual Spring Meeting — Abstracts

High frequency (<100,000) transgressive (T-R) shoreline couplets form the building units of the Point Lookout Sandstone. Following cycle hierarchy, these couplets are of fifth order, while the entire point lookout forms the regressive half of a larger third order couplet. Third and Fourth order transgressions are commonly accompanied by development of associated transgressive deposits. Most observations suggest, however, that fifth order cycles are asymmetric, and that a transgressive surface of erosion of non-deposition is followed vertically by a coarsening-upward progradational package. This study demonstrates greater symmetry may exist within these cycles than previously observed. Eight sections were measured at a laminae scale on a transect roughly perpendicular to paleoshoreline in the transition from Mancos Shale to the base of the Point Lookout Sandstone. All sections were lateral equivalents within the same fifth order couplet (selected for its continuous exposure), and all display the same general geometry: fining upward mudrock base, clay-rich "condensed" section, and coarsening upward mudrock to sandstone top. Symmetry is best displayed in distal sections, while coarse shoreface sandstones directly overlie the "condensed" section in proximal localities. Similar detailed measurements must be made for many other fifth order cycles before a depositional model can be constructed; but this study demonstrates that mud deposition accompanied transgression within one high frequency T-R couplet. This observation is significant because transgressive mudrocks may play an important role in fluid migration within shoreline sandstones and because their blanket geometry is very different from their regressive counterparts.