Recognition and significance of bayhead delta deposits in the rock record: A comparison of modern and ancient systems

Despite advances in estuarine facies models, ancient bayhead delta deposits are not widely recognized or utilized, and their facies characteristics are poorly documented. Sedimentology of three well-exposed, bayhead delta deposits within the well-known stratigraphic framework of the Book Cliffs, Utah, and comparison to modern bayhead deltas provide insight into their variability, and a model for reservoir prediction. Bayhead deltas develop at the innermost part of bays within wave-dominated and mixed-energy estuaries on transgressive coastlines. The deposits of these deltas are valuable because they record the high-frequency turnaround from transgression to regression; in turn, this delineates the long-term transgression trajectory and informs reservoir prediction. This study uses outcrop data and synthesis of modern bayhead delta characteristics to define criteria for bayhead delta recognition, develop a detailed facies model and highlight their utility in a regional context. The following criteria for bayhead delta identification are proposed herein: (i) overall coarsening-up pattern; (ii) small-scale clinoform (5 to 15m height; 100 to 1000m length) showing a down-clinoform decrease in grain size and palaeocurrent energy; (iii) increasing abundance of mud interbeds towards the clinoform toe; (iv) basinward-directed palaeocurrents generated mainly by fluvial input with strong tidal overprint; (v) greatest tidal influence in the intertidal zone of the inactive delta front and better preservation of turbidites at clinoform toes; (vi) brackish trace and body fossils; and (vii) position within an overall backstepping stratigraphic succession. Ancient bayhead deltas described in this study are situated within a moderately rising to flatly rising transgressive interval over a 300km transect. Steeply rising transgressive trajectories sequester sandy, thicker, better connected transgressive deposits in the inner part of the estuary where bayhead deltas tend to stack vertically. Flatter transgressive trajectories sequester sand in disparate, disconnected estuarine sandstone bodies with partial preservation of bayhead deltas. Proposed criteria aid recognition of bayhead delta deposits in other basins to reconstruct transgressive shoreline trajectories that inform reservoir models and volumetrics.