1 DIFFERENTIATING AMALGAMATED PARASEQUENCES IN DELTAIC SETTINGS USING ICHNOLOGY: AN EXAMPLE FROM THE UPPER TURONIAN WALL CREEK MEMBER OF THE FRONTIER FORMATION, WYOMING JUNAID SADEQUE 1 , JANOK P. BHATTACHARYA 1 , JAMES A. MACEACHERN 2 AND CHARLES D. HOWELL 1 1 Department of Geosciences, University of Texas at Dallas, P.O. Box 830688, FO21, Richardson, TX 75080-0688, USA j.sadeque@gmail.com 2 Department of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC, Canada, V5A1S6 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. INTRODUCTION SCOPE OF PRESENT STUDY Core data from the Turonian Wall Creek Member of the Frontier Formation (Fig. 1) in the Salt Creek oilfield (Fig. 2) shows a thick, locally sharp-based sandstone unit devoid of significant mudstone intervals. In corresponding gamma and SP logs, this unit commonly appears to be “blocky” or “box-car” shaped (Fig. 3). Traditionally, such blocky sandstones, particularly in well-log response, are interpreted as incised valley fills, channel-fills or basin floor fan sandbodies (Serra and Abbot 1982; Cant 1992; Rider 1990). However, a parallel study in the nearby outcrop west of the Salt Creek oilfield recognizes seven progradational parasequences separated by distinct prodelta shales (Fig. 4 and 5) that may represent a top-truncated delta system variably influenced by waves and tides (Bhattacharya et al. 2003; Gani and Bhattacharya 2004). Well-log response adjacent to the outcrop also reflects comparable coarsening- upward motifs (Fig. 3). This study conducts a detailed sedimentological and ichnological investigation to evaluate whether the blocky sandstone unit at the Salt Creek oilfield actually represents stacked deltaic parasequences as seen in the outcrop or represents an incised valley. Firmground and/or ‘palimpsest softground’ substrate conditions are the main bases of the stratigraphic analysis, because key stratigraphic surfaces for identifying parasequences in this case are not obvious. In addition, detailed observation of trace fossils in relation to changing sedimentary processes has been conducted to understand the relative influence of waves, tides and river through the successions of the Wall Creek. The study will aid in subsurface correlations farther into the basin where only gamma, SP and electric well logs are available. Likewise, the products of this detailed analysis may serve as a “training set” for a parallel project to calibrate dip-meter data to sedimentary structures and bioturbation in the uncored intervals. Fig. 1- Type log for the Frontier Formation from Sec. 2, T36N, R85 W showing vertical stratigraphic succession and major bentonite horizons (Bhattacharya and Willis 2001).