SEQUENCE STRATIGRAPHY OF FOREDEEP BASIN ALLUVIAL DEPOSITS, CONIACIAN THROUGH CAMPANIAN STRATA OF THE KAIPAROWITS BASIN, UTAH

Coniacian through early Campanian deposits of the Kaiparowits Basin of south-central Utah include three tectonically-generated second-order sequences that record the migration of the Western Interior Foredeep axis as the Sevier Thrust Belt approached from the west. Each sequence consists of four parts that demonstrate distinct styles of alluvial architecture associated with varying rates of basin subsidence and together make up an approximately 2 km-thick succession dominated by nearly continuous fluvial deposition. At the base of each sequence is a laterally-restricted sandstone sheet that grades both laterally and vertically into fine-grained deposits. Next is a thick interval of predominantly fine-grained material containing scattered lenses and thin sheets of sandstone. This interval grades upward into extensive multistoried sandstone sheets that include very little fine-grained material. Sequences are capped by laterally extensive multistory sheets of gravely sandstone and sandy conglomerate.

Tectonic and eustatic models based on interpreted relationships between alluvial architecture and rates of accommodation production were developed to try and determine whether or not eustatic and tectonic effects can be differentiated from one another in a foredeep setting. Locally, both tectonically- and eustatically-controlled base level fluctuations are believed to produce similar vertical patterns. The key to distinguishing between them is thought to lie in regional distribution patterns for the coarse-grained sheets that cap each sequence. In tectonically-controlled sequences, these sheets have planar lower and upper boundaries, become progressively thicker and coarser-grained upward through the section, thicken toward the thrust belt but are not continuous back to that region, and step in a progressively basinward direction. In eustatically-dominated sequences, sheets are valley-fill deposits with irregular lower boundaries, there is no vertical grain-size trend for successive sheets, and the sheets are continuous to the thrust belt. However, there may still be a progressive basinward migration of these deposits associated with thrust belt advancement.

It is proposed that the sequences of the Kaiparowits Basin best fit what has been previously termed the Fault Propagation Model.