Abstract

The isopachous method of study of structural evolution has been applied to the Aneth field. The following Pennsylvanian stratigraphic units, in descending order, are pertinent: the upper member of the Hermosa formation (Honaker Trail formation), the Bluff (Ismay) zone, and the Desert Creek zone.

The Paradox basin was partly bounded by the Emery, Uncompahgre, San Luis, and Defiance uplifts during the time of deposition of the upper member of the Hermosa formation and the underlying Desert Creek zone. Locally, including the Aneth, McElmo Creek, White Mesa, and Ratherford fields, the present structure at the base of the Bluff (Ismay) zone is a northwest-trending, horseshoe-shaped, fold and “reeflike” development, opening northwest. Aneth is a northwest-trending anticline with two highs on the crestal portion.

The major portion of this paper presents the results obtained by isopaching cumulative stratigraphic intervals, while retaining a constant lower datum, and progressively raising the upper datum. Theoretically this procedure effectively portrays the stages in structural growth of the lower datum at the time of deposition of each successively higher (or younger) datum.

The base of the Bluff (Ismay) zone was selected as the lower datum and the younger successive datums—radioactive log correlation points—used in the study were: top of Bluff (Ismay) zone, “main carbonates”, and “upper Hermosa”. The Desert Creek zone was also isopached and ranges from about 122 to 200 feet, attaining its maximum thickness in the southeastern part of the field.

The following conclusions, resulting from interpretation of the isopachous data, outline the structural evolution of the Aneth area during late Hermosa time.

(1) The structure at the end of Bluff (Ismay) zone deposition was possibly anticlinal.

(2) Extremely minor structural growth along with draping and compaction probably continued throughout deposition of the “main carbonates”. Several subsidiary folds were present by the end of “main carbonates” time.

(3) By the end of “upper Hermosa time” the general structural form and all the subsidiary folds were developed. Crustal movement was more pronounced during deposition of the upper 300 feet of the upper member of the Hermosa formation.

The present structural form of the Aneth area results from mild anticlinal warping that apparently began near the end of Desert Creek time, and during the Pennsylvanian continued intermittently throughout upper Hermosa time. Related sedimentary compaction may have been a factor, and probably was responsible for about 30 per cent of the observable structure. However, most of the present structural relief is due to post-upper Hermosa deformation, much of which may have been associated with the Laramide orogeny.