ABSTRACT: Recognition of Parasequences and Marine-Flooding Surfaces on Gamma-Ray Logs in the Intertongued Dakota Sandstone and Mancos Shale, San Juan Basin, New Mexico and Colorado

Donald E. Owen, Diane K. Sparks

Parasequences and marine-flooding surfaces may be recognized on gamma-ray logs in offshore marine members of the intertongued Dakota Sandstone and Mancos Shale of the San Juan basin. Parasequences are a few tens of feet thick, generally coarsen upward (but blocky and fining-upward patterns occur), and are truncated abruptly at the top by marine-flooding surfaces. High gamma-ray, very low resistivity shales only a few feet thick overlie the marine-flooding surfaces. Individual parasequences and marine-flooding surfaces have been correlated up to 75 mi. They are most apparent in clay- and silt-rich strata deposited seaward of the wedge out of marine sandstones but are recognizable in marine sandstones. In the paleoshoreface, the coarsening-upward pattern and regularity of t e parasequences and marine-flooding surfaces become obscure. Best-developed parasequences are in the Cubero, Whitewater Arroyo, and Twowells, but they are developed in the Clay Mesa, Paguate, and Graneros. Individual parasequences occur locally as sandstones in the subsurface that do not correspond to formal sandstone members on the outcrop. For example, the formal Twowells Sandstone of the outcrop correlates with the middle, dominant one of three parasequences in the Whitewater Arroyo-Twowells-Graneros succession that locally develop sandstones seen on logs. Parasequences and marine-flooding surfaces may be more useful in correlation of gamma-ray logs than on outcrop.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990