Abstract

Facies analysis suggests that much of the late Middle Jurassic (Callovian) Entrada Sandstone of south-central Utah is dominated by mudstone-rich intertidal facies. These intertidal deposits interfinger with subtidal ooid shoals as well as supratidal sabkha and erg-margin eolian deposits. Three sections of the Entrada Sandstone in south-central Utah were measured in detail to improve understanding of the lateral and vertical facies transitions away from the understudied, mudstone-dominated portion of the formation. Understanding facies transitions is necessary for constraining the paleogeography and evaluating the economic potential of the Entrada Sandstone as a hydrocarbon reservoir. The thickness of the Entrada Sandstone varies from the southwest (369.1 m), to the north (194.9 m), and to the east (92.1 m), a lateral distance of 193 km. This variation indicates that during Callovian time the rate of developing accommodation space was high along the southwestern shoreline and relatively low along the northeastern shoreline. This is likely the result of the western sections being closer to the Jurassic Western Cordilleran. However, the eastern section displays a more complete range of facies, from the subtidal to supratidal erg-margin. Although accommodation space was highest to the west, sediment supply from the west kept pace with, and eventually outpaced subsidence. To the east, sediment supply was significant but was at one time outpaced by subsidence causing deposition of subtidal ooid shoals and bars. Along this eastern shoreline, erg-margin coastal dunes associated with the larger erg to the east eventually prograded westward. The variation in subsidence, sediment supply, and sediment source makes sequence stratigraphic correlation difficult. We conclude that candidate sequences interpreted for each section cannot be correlated with certainty without the aid of absolute ages.