Differentiating amalgamated parasequences in deltaic settings with the aid of ichnology: An example from the Upper Turonian Wall Creek Member of the Frontier Formation, Wyoming

The Wall Creek Member in the Salt Creek oilfield at first glance appears to be a thick, undifferentiated sandbody, in marked contrast to its nearby outcrop counterpart. The signature of this unit in gamma and SP well logs is “blocky”, superficially resembling incised valley fill deposits. A closer examination, which integrates physical sedimentology and ichnology of 1200m of cores from 40 wells, demonstrates that the apparently blocky depositional unit has no genetic or scale difference with the deltaic parasequences separated by distinct prodelta shales encountered in the outcrop. Clear changes in ichnofacies allow us to subdivide the blocky unit into at least five shoaling upward deltaic parasequences punctuated by subtle expressions of a relatively more distal though sand-prone facies. Subsurface correlation indicates progradation of the parasequences towards the southeast. Cores from fifteen wells were scrutinized in greater detail to accurately record the bioturbation intensity, ichnological diversity, trace fossil size, and ichnological suites. This detailed facies analysis yields 4 facies associations. They are, river-dominated deltaic deposits (Facies Association 1), wave-/storm-influenced deltaic/shoreface deposits (Facies Association 2), tidally influenced deltaic deposits (Facies Association 3) and marine transgression related deposits (Facies Association 4). Of these, facies associations 1-3 correspond to delta lobes and shorefaces that show varying influences of river, waves/storms and tides.