--> Abstract: Sharp-Based Shoreface Sandstones of the Gallup Sandstone (Turonian), Northwestern New Mexico--Implications for the "Forced-Regression" Concept, by D. W. Valasek; #91012 (1992).
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ABSTRACT: Sharp-Based Shoreface Sandstones of the Gallup Sandstone (Turonian), Northwestern New Mexico--Implications for the "Forced-Regression" Concept

VALASEK, DAVID W., Colorado School of Mines, Golden, CO

The Gallup (Turonian) and Tocito (Coniacian) sandstones represent a seaward-stepping and landward-stepping series of Previous HitfourthNext Hit-Previous HitorderNext Hit sequences in the San Juan basin, northwestern New Mexico. The structural setting of this portion of the Cretaceous Western Interior basin was a ramp setting. The Gallup Sandstone prograded across what became the San Juan basin in six Previous HitfourthNext Hit-Previous HitorderNext Hit sequences.

The older, more landward, Previous HitfourthNext Hit-Previous HitorderNext Hit sequences contain shoreface deposits that have an interbedded sandstone and mudstone facies transition. This type of transition produced a gradational shoreface base. The younger, more basinward, Previous HitfourthNext Hit-Previous HitorderNext Hit sequences display a sharp lower shoreface-shelf transition, producing a sharp shoreface base characteristic of forced regressions. However, unlike forced regression shoreface systems, the Gallup Sandstone consists of complete shoreface deposits and shows no shoreface dislocation. Stratal thickness of the underlying mudstone package shows little thinning. Near the seaward pinchout, there is subtle decrease in the angle of clinoforms within sequences. It is unlikely that the Gallup is typical forced regression; however, it does indicate sedime tologic variation associated with a relative sea level fall.

The Previous HitfourthNext Hit-Previous HitorderTop sequences contain smaller fifth- and sixth-order sequences. Depending upon the stacking geometry the terminations of these sequences may develop a different shoreface-shelf transition. Landward units of the Gallup Sandstone are slightly more aggradational than the basinward units and develop a gradational facies transition. Basinward sequences are strongly progradational and have relatively sharp facies transitions. Determination of stacking geometries is a crucial step to determine difference of sedimentological versus stratigraphic features.

 

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