Evidence for Local Tectonism Related to a Mid-Turonian Unconformity Beneath Upper Cretaceous Turner Sandy Member of Carlile Formation, Northeastern Powder River Basin, Wyoming: ABSTRACT

Abstract

In the northeastern Powder River basin the Upper Cretaceous (upper Turonian) Turner Sandy Member consists of marine sandstone, siltstone, shale, and bentonites. These lithologies comprise several laterally continuous, primarily upward-coarsening sequences. Turner petroleum production (> 11 million bbl of oil) has been largely from the lowermost sandstone. In this part of the basin, the Turner is conformably overlain by the Sage Breaks shale, but the base of the Turner is, in most areas, unconformable with the underlying Greenhorn Formation. This unconformity records a mid-Turonian sea level lowstand accompanied by widespread erosion. Turner sediments were deposited during the subsequent sea level rise and onlap the erosional surface. Detailed correlations of several shaly or bentonitic marker horizons within the Turner and Greenhorn indicate the unconformity typically occurs at the base of an upward-coarsening sequence. In the far northeast the older Turner sequence may be conformable with the underlying Pool Creek shale. However, to the southwest progressively younger basal Turner sequences onlap progressively older Greenhorn strata at the unconformity. Cross sections and isopach maps document several dominantly north-south-oriented Turner and Greenhorn thins, which they interpret to represent areas of structurally controlled paleohighs. As much as 150 ft of pre-Turner erosion occurs over thesemore » paleohighs. Thickness patterns suggest that the greatest uplift immediately preceded the mid-Turonian erosional event. Decreased uplift continued throughout upper Turonian (Turner) time but became much less evident in post-Turner time.« less