Pinch-out style and position of tidally influenced strata in a regressive-transgressive wave-dominated deltaic sandbody, Twentymile Sandstone, Mesaverde Group, NW Colorado

The Upper Cretaceous Twentymile Sandstone of the Mesaverde Group in NW Colo rado, USA, has been analysed with respect to its pinch-out style and the st ratigraphic position of tidally influenced facies within the sandstone tong ue. Detailed sedimentological analysis has revealed that the Twentymile San dstone as a whole is a deltaic shoreface sandstone tongue up to 50 m thick proximally. Facies change character vertically from very fine-grained, stor m wave-dominated shelf sandstones and mudstones to fine-grained, wave-domin ated sandstones and, finally, to fine- to coarse-grained tidally dominated sandstones. The pinch-out style is characterized by a basinward splitting o f the massive proximal sandbody into seven coarsening-upward fourth-order s equences consisting of a lower shaly part and an upper sandy part (sandston e tongue). These are stacked overall to reflect the regressive-to-transgres sive development of the tongue. Each of the lower sandstone tongues 1-3 are gradationally based, very fine-grained and dominated by hummocky cross-str atification and were deposited on the lower to upper shoreface. Sandstone t ongues 4 and 5 prograded further basinwards than the underlying tongues, ar e erosively based, fine- to coarse-grained and mainly hummocky, herringbone and trough cross-stratified. Especially in tongue 5, tidal indicators, suc h as bipolar foresets and double mud drapes, are common. These tongues were deposited as upper shoreface and tidal channel sandstones respectively. Sa ndstone tongues 6 and 7 retrograded in relation to tongue 5, are very fine- to fine-grained and hummocky cross-stratified. These tongues were deposite d in lower shoreface to offshore transition environments. The two lower fou rth-order sequences were deposited during normal regressions during slowly rising or stable relative sea level and represent the highstand systems tra ct. The three succeeding fourth-order sequences, which show succeedingly in creasing evidence of tidal influence, were deposited during falling and low stand of relative sea level and represent the falling stage (forced regress ive) and lowstand systems tracts. The uppermost two fourth-order sequences were deposited during rapidly rising sea level in the transgressive systems tract. The maximum tidal influence occurred during lowstand progradation, in contrast to most other published examples reporting maximum tidal influe nce during transgression.