ABSTRACT: Characteristics of Parasequences of Transgressive System Tracts Deposited in Foreland Basins--Examples from the Cretaceous Frontier and Dakota Formations, Utah and Wyoming

RYER, THOMAS A., The ARIES Group, Inc., Louisville, CO

The characteristics of parasequences recognized in Cretaceous strata of the Western Interior differ markedly between transgressive system tracts and high-stand and regressive systems tracts. Parasequences are defined on the basis of bodies of predominantly sandy strata deposited on the shoreline during brief phases of progradation. Shoreline sandstone bodies deposited during transgressions are comparatively thin and shaly. These characteristics are the result of the low wave energies that were incident on the shorelines during transgression.

In foreland basins, there are two principal, related causes for low wave energies during transgressions: (1) The seaward dip of the coastal plain across which transgression occurred was low, resulting in a very gentle gradient on the shelf; and (2) uplift of a forebulge produced a shoal area at the hinge of the foredeep basin upon which the energy of waves approaching from the east was dissipated.

The transgressive systems tract of the Greenhorn cycle, represented by the Dakota Sandstone and the Coalville Member of the Frontier Formation, corresponds to a general rise of global sea level. Equally important was an acceleration in the rate of subsidence within the foredeep, presumably because of an increase in the rate of crustal shortening to the west. Differential subsidence caused flattening of the already gentle slope of the coastal plain. A forebulge formed offshore and may have become emergent locally. The result was very low wave-energy conditions along the western shoreline. These relationships are of value in predicting reservoir occurrence in lesser explored parts of foreland basins.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)