Short-Term Coastal Cycles in Upper Cretaceous Point Lookout Sandstone, Southeastern San Juan Basin, New Mexico: ABSTRACT

The Point Lookout Sandstone offers an excellent opportunity to study short-term (100,000-year) cyclic strandline events in the rock record. Although thin (30 to 60 m in the study area), this sandstone is divisible into numerous genetic strandline sequences separated from one another by distinct transgressive surfaces. Individual progradational sequences do not form major sandstone benches and are distinguished only through correlation of subtle transgressive surfaces. Transgressive surfaces are recognized where offshore mudstones sharply overlie transition or shoreface deposits. Such surfaces trace updip into progressively thinner mudstone partings that separate stacked shoreface sandstones. Presence of these mudstone partings suggest that a physical shift in mud-line position accompanied transgression. A probable explanation for the landward mud-line shift is entrapment of coarse sediment in estuaries and/or aggrading fluvial systems. Knowledge of such partings in subsurface strata is critical to understanding fluid migration through these reservoir sandstones. Delineation of short-term transgressive-regressive couplets in the Point Lookout Sandstone facilitates detailed reconstruction of local paleoshoreline trends. Because of short-term nature of individual cycles, variation in coastal morphology is resolvable at a scale of 100,000 years or less. Paleogeographic maps for each cycle, constructed at points of maximum transgression and maximum preserved progradation, define local shoreline irregularitiesmore » that resulted from migration of a fluvial-dominated deltaic complex along the coastline. The occurrence of the fluvial delta in the area suggests autocyclicity as an important factor in deposition of these strata. Any significant allocyclic component must yet be determined by regional correlation.« less